A touch sensing apparatus and method of operating the same

(54) Title: Touch sensitive apparatus and method for operating the same (56) Publications cited: - (47) Abstract:

A touch sensitive device is controlled to determine the position of one or more objects (7) that interact with a touch surface (1). The apparatus comprises a group of emitters (2) arranged to emit light for illuminating at least a portion of the touch surface (1), a light detector (4) arranged to receive light from the group of emitters (2), and a processing element (7). Each emitter (2) is controlled to transmit a code via the emitted light so that the code identifies the respective emitter (2). The codes can be transmitted at least partially simultaneously. The codes can be selected such that a value for an autocorrelation for each code is significantly greater than a value for a cross correlation between each set of two codes for different emitters (2). The processing element processes an output of the light detector (4) to separate the light received from the individual emitters (2) on the basis of the transmitted codes, and to determine a position of the object (s) (7) on the basis of that of the individual emitters (2) received light.

<img file="SE533704C2_D0001.tif" />

<img file="SE533704C2_D0002.tif" />

533 704

SUMMARY

A touch sensitive device is controlled to determine the position of one or more objects (7) that interact with a touch surface (1). The apparatus comprises a group of emitters (2) arranged to emit light for illuminating at least a portion of the touch surface (1), a light detector (4) arranged to receive light from the group of emitters (2), and a processing element (7). Each emitter (2) is controlled to transmit a code via the emitted light so that the code identifies the respective emitter (2). The codes can be transmitted at least partially simultaneously. The codes may be selected such that a value for one autocorrelation for each code is significantly greater than a value for a cross correlation between each set of two codes for different emitters (2). The processing element processes an output of the light detector (4) to separate the light received from the individual emitters (2) on the basis of the transmitted codes, and to determine a position of the object (s) (7) on the basis of that of the individual emitters (2) received light.

533 704

CONSENSIVE DEVICE AND PROCEDURE FOR DRIVING ITSELF

Technical area

The present invention relates to techniques for determining the position of one or more objects on a touch surface.

Background

Touch sensitive screens are increasingly used for data entry in computers, electronic measuring and test equipment, gaming devices etc.

In a category of touch sensitive screens known from, for example, US 3673327, a plurality of optical emitters and optical receivers are arranged around the periphery of a touch surface to create a grid pattern of intersecting light paths above the touch surface. Each light path extends between a respective emitter / detector pair. An object that touches the surface will block some of the light paths. Based on the identity of the receivers detecting a blocked light path, a processor can determine the position of the intersection between the blocked light paths. This type of system is only capable of detecting the position of an object (single touch detection). In addition, the required number of emitters and detectors increases rapidly, and thus the cost and complexity of the system, with an increased surface size and / or spatial resolution for the touch screen.

In a variant, for example shown in WO2006 / 095320, each optical emitter emits a light beam that diverges across the touch surface and each beam is detected by more than one optical receiver located around the periphery of the touch surface. Thus, a large number of light paths are created by sequential activation of various emitters around the periphery of the touch surface and detection of the received light from each emitter by means of a plurality of Optical receivers. In this way, it is possible to reduce the number of emitters and receivers for a given surface size or spatial resolution, or to enable simultaneous position detection for more than one touching object (multi touch detection). However, this is accomplished at the expense of a reduced time resolution since the emitters are activated sequentially. This can be a considerable disadvantage when the number of emitters is large. To increase the time resolution, each emitter can be activated for a shortened time period. However, this can result in a significant decrease in signal-to-noise ratio (SNR).

533 704

Summary of the Invention

It is an object of the invention to at least partially overcome one or more of the above-identified limitations with the prior art.

This and other objects, which will become apparent from the description below, are achieved, at least in part, by a touch-sensitive device, a method of operating a touch-sensitive device and a computer-readable medium according to the independent claims, the independent claims defining embodiments thereof.

In a first aspect, there is provided a touch sensitive apparatus comprising: a touch surface; a group of emitters arranged to emit light for illumination of at least a portion of the touch surface; a light detector arranged to receive light from the group of emitters; and a processing element designed to process an output from the light detector for determining the position of one or more objects interacting with the touch surface; wherein each emitter is controlled to transmit a code via the emitted light such that the code identifies the emitter, and wherein the processing element is designed to separate the light received from individual emitters on the basis of the transmitted codes.

According to a second aspect, there is provided a method for operating a touch sensitive apparatus comprising a touch surface, a group of emitters arranged to emit light for illumination of at least a portion of the touch surface, and a light detector arranged to receive light from the group of emitters, wherein said method comprising: controlling each emitter to transmit a code via the emitted light such that the code identifies the emitter; processing an output of the light detector to separate the light received from individual emitters on the basis of the transmitted codes; and determining a position for one or more objects interacting with the touch surface based on the light received from the individual emitter frame.

According to a third aspect, a computer-readable medium is provided which stores processing instructions which, when executed by a processor, execute the process of the second aspect.

Further objects, features, aspects and advantages of the present invention will become apparent from the following detailed description, from the appended claims and from the drawings.

Brief description of the drawings

Exemplary embodiments of the invention will now be described in greater detail with reference to the accompanying schematic drawings.

533 704

Fig. 1 is a top plan view of a touch sensitive apparatus with detection of light rays above a touch surface.

Fig. 2 is a side view of the apparatus of Fig. 1.

Fig. 3 (A) -3 (C) are top plan views of another embodiment, where Fig. 3 (A) shows light paths between a single emitter and a plurality of detectors; Fig. 3 (B) shows a detection network consisting of all light paths. , and Fig. 3 (C) shows the light paths affected by an affected object.

Fig. 4 (A) -4 (E) are top plan views of the apparatus of Fig. 1, to show: activation of emitters in a time interval sequence during a code generation cycle.

Fig. 5 is a timing diagram for activating the individual emitters of Fig. 4 (A) -4 (E)

Fig. 6 is a top plan view of an alternative embodiment.

Fig. 7 is a top plan view of a touch-sensitive apparatus with detection of light rays propagating inside a light transmitting panel.

Fig. 8 is a side view of the apparatus of Fig. 7.

Fig. 9 is a side view of another touch-sensitive apparatus with detection of light rays propagating inside a light transmitting panel:

Fig. 10 is a top plan view of the apparatus of Fig. 9.

Fig. 11 is a top plan view of a touch-sensitive apparatus with the detection of light scattered from a touching object.

Figures 12-15 are top plan views of exemplary arrangements of emitters and detectors. around the periphery of the touch surface.

Fig. 16 is a side view of an exemplary arrangement of an emitter and a panel.

Fig. 17 is a flow chart of an exemplary method for determining touch positions.

Figures 18-19 are timing diagrams to illustrate alternative ways of embedding codes by modulating light.

Figs. 20-21 are top plan views of various embodiments utilizing frequency modulation for embedding codes.

Detailed description of exemplary embodiments

The description begins with a presentation of an embodiment of a touch-sensitive apparatus which creates a grid pattern of light rays above the touch surface. Then follows a description of codes to be transmitted by a group of emitters in a touch-sensitive apparatus according to embodiments of the invention, together with criteria for selecting and optimizing the codes and combining codes between

533 704 different groups of emitters. Subsequently, embodiments of alternative types of touch-sensitive apparatus, as well as exemplary arrangements of emitters and detectors, are described. The description concludes with a data processing example and a general discussion of components for a touch-sensitive apparatus according to embodiments of the invention. Throughout the description, reference numerals are used to denote corresponding constituents.

Fig. 1 is a top plan view of a touch surface 1 illuminated by a plurality of emitters 2. The emitters 2 are located around the periphery of the touch surface 1. Each emitter 2 can be actuated by a controller 3 to generate a divergent light beam above the touch surface 1. A plurality of optical detectors 4 are located around the periphery to detect the light emitted from the emitters 2, and a processing element 5 is electrically coupled to the detectors 4 to receive a respective output or measurement signal representing the light energy received by each detector 4 .

Thus, light paths are created between each emitter 2 and that number of detectors 4. The light paths, indicated by dashed lines, together define a detection network. As shown in Figure 1, each detector 4 receives light from a group of emitters 2 along a plurality of light paths, each light path here having a given angle of incidence toward the detector 4.

An object 7 moved near the touch surface 1 within the detection network can at least partially block one or more light paths, as indicated in the side view of FIG.

2nd Whenever object 7 at least partially blocks two or more light paths, i.e. when the object 7 is moved near any intersection of dotted lines in Fig. 1, it is possible to determine the position of the object 7. The processing element 5 processes the outputs of the detectors 4 to identify blocked light paths. Each blocked light path corresponds to an angle of incidence to a specific detector 4, and thus the processing element 5 can determine the position of the object 7 by triangulation.

The position of the object 7 is determined during a so-called sensing event and the time resolution of the apparatus in Fig. 1 is given by the duration of each sensing event. The duration of a sensing event is determined by the time required to generate a complete detection network and / or the time required to sample the output of all detectors 4 with an acceptable signal to noise ratio (SNR).

The spatial resolution of the touch-sensitive device in Fig. 1 depends on the density of the detection network. For example, it may be desirable to achieve a high and possibly uniform density of light road intersections. It may be accomplished by appropriate selection of the number of emitters 2 and detectors 4 and their location, and by appropriate selection of the beam angles of the emitter 2 and detector 4.

533 704 field of view (ie, the angle range within which the detector is capable of receiving incoming light).

As noted above, each detector 4 receives light from a group of emitters

2nd Thus, the output of each detector 4 will represent the received light energy on a number of light paths. The apparatus is designed to allow the processing element 5 to distinguish between the contributions of different emitters 2 to the output of a specific detector 4. For this purpose, each emitter 2 is controlled to transmit a code via the emitted light so that the code identifies the respective emitter 2 for the detector 4, and the processing element 5 is designed to separate the light received by the detector 4 from individual emitters 2 based on the transmitted codes. . This allows, as will be described in more detail below, for two or more emitters 2 to simultaneously generate a beam, even if these beams overlap on one and the same detector 4. This in turn allows the time resolution and / or SNR to be improved, compared to a scenario where the individual emitters 2 are activated sequentially one at a time during a sensing event.

Within the scope of the present application, a "code" denotes any time-varying function that can be embedded in the transmitted light. For example, the code may be a sequence of discrete values, e.g., binary values. Alternatively, the code may be a periodic function, for example, a cosine function.

Each code is typically emitted during a code generation cycle. The code generation cycles for different emitters 2 may coincide or not coincide in time. It should be appreciated that a detection network for a sensing event has been created when all emitters 2 in the touch-sensitive apparatus have completed a code generation cycle.

The code is typically embedded in the emitted light through modulation. Thus, the processing element 5 can distinguish between modulated light transmitted simultaneously from different emitters 2 based on the time-resolved output of a single detector 4. Thus, the processing element 5 can identify each of them. the emitters 2 in the output signal and measure the energy of the modulated light from the identified emitter 2 in the output signal.

In one embodiment, the codes are selected such that an autocorrelation value for each code is significantly higher than a cross correlation value between any codes for two different emitters 2. The processing element 5 can, for example, measure the energy of the individual emitters 2 by autocorrelating the output signal with a set of known signal pattern representing available codes.

533 704

If the code is a sequence of discrete values, the code generation cycle can. include a sequence of time intervals, each time interval comprising a value in the code.

Prior to the detailed discussion of code selection, some general benefits of using wide-angle beams should be briefly discussed. Fig. 3 shows an embodiment in which a large number of emitters 2 and detectors 4 are alternately arranged around the periphery of the touch surface 1. Fig. 3 (A) shows the light paths created between one of the emitters 2 and a plurality of detectors 4 when an emitter emits a light beam. FIG. 3 (B) shows the complete detection network generated during a sensing event when all emitters 2 have been activated. Obviously, a dense grid pattern is generated, which allows for a high spatial resolution.

Fig. 3 (C) shows an example of the light paths affected by an object 7 which is moved near or touching the touch surface 1 during a sensing event. The large number of affected light paths creates redundancy for the determination of the point position. This redundancy enables high precision at the specified position. Alternatively or additionally, it may enable the processing element 5 to determine the size and / or shape of the object 7. In addition, the redundancy can enable the processing element 5 to determine the position of more than one touching object 7 during a sensing event. Conventional touch sensing using an orthogonal grid pattern of light paths above the touch surface is normally limited to the detection of a single touching object 7, since the touching object shadows a portion of the touching surface 1, thereby preventing detection of another touching object in this shaded portion. However, it should be apparent from Fig. 3 that a high density non-orthogonal grid pattern can be generated so that even if an object 7 blocks the number of light paths in the detection network, the remaining (unblocked) detection network allows the processing element 5 to determine the position for a further touching object.

CODE SELECTION

In general, the following discussion analyzes various criteria for selecting codes to be emitted by each emitter in the touch-sensitive device. The following discussion is given in connection with an embodiment in which the codes, at least insofar as they are transmitted simultaneously, are linearly independent. It should be noted that linearly independent codes are also characterized by the fact that an autocorrelation value for each code is significantly higher than a cross correlation value between any two codes.

As will be shown below, the use of linearly independent codes generally enables efficient processing of the output signal for measuring the energy

533 704 received by the individual emit frames. The linearly independent codes can form a multiplex matrix, and the processing element can separate the energy from different emitters by operating the inverse of the multiplex matrix on the output signal.

In the upcoming discussion, each code is a sequence of binary values ​​generated by on / off modulation of the emitter at the time intervals within the aforementioned code generation cycle.

Thus, the amount of light emitted from the emitters is modulated by linearly independent functions according to a multiplexing scheme. In one example, the amount of light detected by a detector having five light paths to different emitters is given by η = ΜΈ, i.e.

<td> 71'</td><td></td><td>Ί 1 0 0 0 '</td><td> '&lt;</td>

<td> 7₂</td><td></td><td> 0 110 0</td><td></td>

<td>th</td><td> =</td><td> 0 0 110</td><td> *3</td>

<td> 7₄</td><td></td><td> 0 0 0 1 1</td><td></td>

<td> .75.</td><td></td><td> 1 0 0 0 1_</td><td>Λ.</td>

where η i is the light detected at the detector at a given time interval during the code generation cycle, M is the multiplex matrix, and e_k is the amount of light that can reach the detector from emitter k.

In this example, each of the emitter frames' codes is given a five-bit sequence. For the first emitter, the bit sequence is 10001, which corresponds to the first emitter being turned on, off, off, on.

As described in detail below, SNR can be improved if each emitter is controlled to emit light for extended periods of time within each code generation cycle, i.e. for more than one time interval. In the example above, two emitters emit light during each time interval. Each emitter will then emit light twice during a code generation cycle.

To separate the detected signal into measured energy from each emitter, the multiplex matrix M is inverted, and the resulting inverse M⁴ operated on the light detected at the detector according to: E = M⁴ 'Η.

In this example, the inversion process is:

<td> ------------------1 __________________1</td><td></td>

<td>^e2</td><td> 1</td>

<td></td><td>1 II</td>

<td>^e4</td><td></td>

<td>Λ.</td><td></td>

-1

<img file="SE533704C2_D0003.tif" />

533 704

In this way, the processing element can calculate the amount of light (energy) reaching the detector from each individual emitter.

In another example, the code for the first emitter is modified to contain only one light pulse:

<td></td><td> '1</td><td> 1</td><td> 0</td><td> 0</td><td> 0</td>

<td></td><td> 0</td><td> .1</td><td> 1</td><td> 0</td><td> 0</td>

<td>M =</td><td> 0</td><td> 0</td><td> 1</td><td> 1</td><td> 0</td>

<td></td><td> 0</td><td> 0</td><td> 0</td><td> 1</td><td> 1</td>

<td></td><td> 0</td><td> 0</td><td> 0</td><td> 0</td><td> 1</td>

Thus, matrix M may be easier to invert. For this multiplexing scheme, the inversion process is:

<td> «1’</td><td></td><td>T</td><td> -1</td><td> 1</td><td> -1</td><td> 1 ‘</td><td>V</td>

<td></td><td></td><td> 0</td><td> 1</td><td> -1</td><td> 1</td><td> -1</td><td> 72</td>

<td></td><td> =</td><td> 0</td><td> 0</td><td> 1</td><td> -1</td><td> 1</td><td>7a</td>

<td></td><td></td><td> 0</td><td> 0</td><td> 0</td><td> 1</td><td> -1</td><td> 74</td>

<td> «5.</td><td></td><td> 0</td><td> 0</td><td> 0</td><td> 0</td><td> 1</td><td>7s.</td>

The idea of ​​controlling multiple emitters to simultaneously emit light can be extended to three emitters simultaneously, etc. An example of a matrix M for a multiplexing scheme with activation of three emitters during each time interval is:

<td></td><td>T</td><td>T</td><td> 1</td><td> 0</td><td> 0</td>

<td></td><td> 0</td><td> 1</td><td> 1</td><td> 1</td><td> 0</td>

<td>M =</td><td> 0</td><td> 0</td><td> 1</td><td> 1</td><td> 1</td>

<td></td><td> 1</td><td> 0</td><td> 0</td><td> 1</td><td> 1</td>

<td></td><td> 1</td><td> 1</td><td> 0</td><td> 0</td><td> 1</td>

Figs. 4 (A) - (E) show the embodiment of Fig. 1 at five sequential time steps during a code generation cycle according to the latter multiplexing scheme (detectors are not shown, emitters are designated el-e5, and activated emitters are illustrated as emitting a divergent beam Each time step represents a code value for each emitter. Fig. 5 is a timing diagram showing

533 704 time steps in the code generation cycle (CGC) with on / off modulation of each emitter or e5.

In the example of Figures 4-5, each emitter is controlled to emit light simultaneously with at least one of its neighbors. At present, however, it is considered that a better SNR can be achieved by such control of the emitters that the simultaneously activated emitters are more scattered along the periphery of the touch surface. Such an arrangement can result in a multiplex matrix with more optimal properties, which is explained in more detail below.

Basically, the multiplexing scheme can be based on any invertible multiplex matrix. However, there are certain criteria that, when achieved, can be used to design a multiplex matrix that allows further improvement of the SNR. Such a matrix can be useful when the code generation cycle is to be time-limited, for example to achieve a high time resolution. For a detector having N light paths to different emitters, these criteria allow an increase of SNR by up to a factor \ N / 2 for a given duration of a sensing event (compared to a sensing event with sequential activation of the emitters, hereafter referred to as the "non-multiplexed lighting scheme"). "), Or a reduced duration of a sensing event while retaining SNR.

These criteria are described and justified below.

However, it should be emphasized that these criteria are only examples of ways to improve or "optimize" multiplex matrices for a specific purpose. There may be other ways of improving the multiplex matrices, for this or other purposes. Also, any choice of a multiplex matrix with linearly independent columns will allow an improvement of SNR compared to a non-multiplexed illumination scheme.

It should also be noted that although the following discussion refers to on / off modulation, it is also applicable to other types of emitter modulation.

optimization criteria

Consider a system with N emitters and a single detector (d_k). Each emitter can contribute the following intensity to the detected detector:

E = (e ,, e₂e₃.., e_N )^r. We want to find a multiplex matrix M, with size Nx N, that maximizes SNR. The measured signals, η (a measured value of the detector, d_k, for each time interval), then: η = ΜΈ + e, where e is the noise level in the measurements. Each column in the multiplex matrix, M = [m_y m₂ ... m_N ], is the multiplex base for an individual emitter, e_k.

To find the energy received from each emitter, we multiply the measured signal by the inverse of the multiplex matrix: M^A * η = Ε + M^A · Ε

533 704

We see that we can calculate the measured energy from each emitter that:

É = M⁴ · Η. The resulting noise in the measured energy from the emitters is then given by ε = M⁴ · Ε. Since this algorithm uses the inverse of the multiplex matrix, we see that we want to use a multiplex matrix that has a low condition number.

The fitness number of the matrix can be calculated as:

κ (λ /) = || µ-^;|| · || λ / ||

The condition number of the matrix measures the stability / sensitivity of the solution of a linear equation system. In this context, it essentially means how errors in the inversion process affect the result of the de-multiplexing of the signals. When choosing multiplex matrix it may be desirable that the norm for its inverse is small. With l₂-standard becomes the fitness number:

k (M) =

<img file="SE533704C2_D0004.tif" />

<img file="SE533704C2_D0005.tif" />

where a_max and o_my is the largest and smallest singular value for the matrix. It may be desirable to select a multiplex matrix that has such a small condition number so as not to increase the noise level during the inversion process. If we let M be a normal matrix (M^T· MMM^T) the fitness condition number is calculated as k (M) =

IaI i 'max | Λ | . '

In Imin where | X |_max and | X |_mj_n is the largest and smallest size of the matrix's eigenvalues.

To estimate how noise spreads in the inversion process, we can look at the expected mean square error (M5E) estimator:

MSE = # ((E - É)²) = cov (E).

The variance of the matrix is ​​the diagonal elements of the covariance matrix given by: cqn (E) = σ² {M^T -My '

It can be shown that the individual contributions to the noise from different measurements are uncorrelated. Accordingly, we can overlook the elements outside the diagonal of the covariance matrix. The sum of the squared error (SSE) is thus the sum of all diagonal elements in the covariance matrix, ie. the individual variances for the estimated parameters.

533 704

In one embodiment, the SÄE parameter is used as an optimization parameter for the multiplex matrix: SSE = a²grooves ((M^T · M)¹), where σ² is the variance of the noise in a non-multiplexed lighting scheme. The resulting variance (noise) in a solitary estimated value, é_k, is then its corresponding diagonal element in the covariance matrix. The diagonal elements of the covariance matrix give the decrease in noise level (noise variance) in the system.

To find an optimized solution, we try to minimize the function above. For a system where the noise is not dependent on the light incident on the detector and if the number of emitters is fixed, we can simplify this minimization problem to:

minimizeSSE) = minimize (track) (M^T M)¹).

It can be shown that the optimum number of emitters that are on simultaneously is close to N / 2. Thus, this value is likely to provide near optimal inversion properties for the multiplex matrix.

Furthermore, it can be shown that Hadamard and Sylvester matrices fulfill the desired properties of a multiplex matrix as previously described. The use of codes forming Hadamard / Sylvester multiplex matrices can increase SNR by a significant factor (N + 1) / 2vN, which for large N becomes λ / ν / 2.

Generally, the multiplex matrix can contain any values ​​as long as its determinant is zero, i.e. its columns are linearly independent.

The above-mentioned Hadamard matrix is ​​a matrix that only contains values ​​of 1 or -1 and whose columns are linearly independent. For example, a Hadamard matrix can be constructed by the following recursive definition:

-HRS_m-\ ^0=+1

A Hadamard matrix satisfies Η Ή^τ = Η^τ Ή = N² · · Z, where Z is the identity matrix. From the recursive definition above, it is clear that Hadamard matrices of order N = 2^P exists, where p is a non-negative number. It can be shown that Hadamard matrices of order N = 1.2 and N = 4 p exist.

The absolute intrinsic values ​​of a Hadamard matrix (including its transposate and inverse) are all equal. This means that the condition number for the multiplex inversion is 1, which gives low noise in the inversion process.

<img file="SE533704C2_D0006.tif" />

533 704

In the example of on / off modulation, it may be difficult to produce negative signals. One would think that such modulation would be limited to binary multiplex values, i.e. 0 (no light) and 1 (full strength). However, it is possible to set zero signal level to half the maximum signal level and consider -1 as no light and 1 as full strength.

To achieve the same multiplexing properties as the Hadamard matrix but with only zeros and ones in the multiplex matrix, we can construct a Sylvester matrix by removing the first row and column of the Hadamard matrix (creating a 77 matrix) and then changing 1 (in Hadamard) to 0 (in Sylvester) and 1 (in Hadamard) to 1 (in Sylvester), S '= (1-77) / 2. An example of a Sylvester matrix is:

<img file="SE533704C2_D0007.tif" />

10 1

0 11

110

1111

10

110 0

0 1

The Sylvester version of multiplex matrices are normal matrices, ie. S^T · S = S · S^T. All absolute eigenvalues ​​for a Sylvester matrix (including its transposate) are equal except for a single eigenvalue which is greater. The value of the largest intrinsic value is C, which is the number of emitters that are on simultaneously. All eigenvalues ​​for the inverse of the Sylvester matrix are equal except for a eigenvalue lower (1 / C). Thus, the Sylvester matrices have good condition numbers and are useful in the multiplex inversion process.

MULT1PLEXING OF OTHER ORDERS

The use of Hadamard / Sylvester multiplexing requires that the number of emitters be a multiple of 4, N = 4p for Hadamard and N = 4p-1 for Sylvester. In a rectangular multitouch system, it is quite possible to arrange emitters and detectors such that each detector receives light from a multiple of 4 emitters. However, it may be desirable to be able to perform multiplexing with any number of emitters. Since the Sylvester matrix needs 4p-l emitters, we may have to use a Hadamard / Sylvester matrix that is slightly larger than

533 704 is actually required by the actual number of emitters, ie. we may need to add a number of fictitious emitters.

One way to construct an optimal multiplex matrix may be to use graph theory for Strongly Regular Graphs, srg, for example as described by RC Bose in "Strongly regular graphs, partial geometries, and partially balanced designs", Pacific J. Math., Vol. 13, no. 2 (1963), p. 389-419. This type of graph can be defined as follows. G = (V, E) is a regular graph with V nodes, E edges, and degree k (the number of edges exiting each node). If there are two integers λ and μ such that every two neighboring nodes have λ common neighbors, and every two non-neighboring nodes have μ common neighbors, then this graph is strongly regular and is called srg (o, k, λ, μ). It can be shown that the neighbor matrix of a srg (N, C, a, d), where C is the number of emitters that are on simultaneously and a = C- (C-1) / (V-1), constitutes an optimal multiplex matrix. The properties of the resulting multiplex matrices are consistent with those of the Hadamard / Sylvester matrices.

In a Hadamard or Sylvester matrix, as in other optimal or near optimal multiplex matrices, about half of the emitters are below each. time interval. If saturation of the detectors is expected to be a problem, it may be desirable to reduce the number of emitters simultaneously activated. Reduction of the energy detected by a detector can be done by decreasing the order, C, of ​​the expression srg (N, C, a, d) used to calculate the neighboring matrix of the graph. The order is the number of connections each node has to other nodes, which is equivalent to the number of emitters that are on during each time interval.

MULTIPLEXING OF MULTIPLE DETECTORS

If we have several different detectors in the system, the output signals can be multiplexed using the inverse of one and the same multiplex matrix. In this way, the multiplex matrix can be designed to take into account all emitters in relation to all detectors in the system.

However, if each detector receives light only from a subset of the emitters, it may be advantageous to use the inverse of several multiplex matrices, e.g., one for each detector.

Such an embodiment will be further exemplified with reference to Fig. 6, which is a top plan view of a touch-sensitive apparatus with six emitters (designated el-e6) and six detectors (denoted dl-d6). The light paths between the emitters and detectors are shown in broken lines. In this

533 704 examples, the touch surface 1 is circular, but any other shape is possible, for example rectangular.

When an emitter emits light that can be detected by a subset of the detectors, and another emitter emits light that can be detected by another subset of the detectors, the multiplex matrix can be reduced to a set of multiplex matrices that are permutated in a circular manner. From Fig. 6, it is clear that there are only light paths between emitter electrical and detectors d3, d4 and d5, and that there are only light paths between emitter e2 and detectors d4, d5 and d6, and so on.

Instead of using a 6x6 matrix, it is possible to use a set of 3 * 3 matrices based on a master matrix S. For example, the master matrix can be given by:

<img file="SE533704C2_D0008.tif" />

which is based on the linearly independent codes: Si - [1 0 1]^T, S2 = [0 1 1]^T, Sa = [1 1 0]^T. Thus, the main matrix can be written as a combination of three individual codes: S = [S; S2 S3]. In this example, the main matrix is ​​a Sylvester matrix. In a Hadamard / Sylvester matrix, the columns or rows can change order (different row / column permutations) without changing the properties of the matrix. In this way, the emitters can be assigned a respective code by the codes S_; - S3, so that a 3&gt; &lt;3 multiplex matrix is ​​created for each detector. In one example, emitters electricity and e4 are modulated with S_hrs emitters e2 and e5 are modulated with S₂, and emitters e3 and e6 are modulated with S3.1 in this example, the respective output of the detectors dl - d6 is:

<td></td><td>V</td>

<td>&lt;* 5, η = ^S2 S] ·</td><td> ¢2</td>

<td></td><td> /3.</td>

<td></td><td>V</td>

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533 704

<img file="SE533704C2_D0009.tif" />

<img file="SE533704C2_D0010.tif" />

This kind of simple circular construction of multiplex matrices is possible when the ratio between the total number of emitters and the number of light paths for each detector is an integer&gt; 2. If the quota is not such an integer, an arital fictitious emitter can be added to make the quota an integer. Furthermore, it may be desirable that the ratio between the total number of emitters (including any fictitious emitters) and the size of the main matrix is ​​an integer&gt; 2, and thus the number of bits in the codes in the main matrix may need to be increased.

It should be understood that the above is just one example and that there are other ways to enable the use of individual multiplex matrices for different detectors.

APPLICATION IN ALTERNATIVE TEMPERATURE TECHNOLOGIES

However, the techniques described above for emitting codes and separating the light received by a detector based on the codes are applicable in other touch detection concepts. A number of different concepts are described below. Although not explicitly expressed in connection with each arrangement, it should be understood that all included arrangements may include a processing element and a controller operating in accordance with the above discussion of the embodiment of Figures 1-2.

In an alternative touch-sensitive apparatus, the detection network, and thus a touch surface 1, is created at an interface of a light transmitting panel by propagating light inside the light transmitting panel. Such an embodiment is shown in

533 704 FIGS. 7-8, wherein a number of emitters 2 are arranged circumferentially around a light transmitting panel 8 to inject a respective light beam into the panel, typically via the edges of the panel 8 or via one or more of the upper or lower surface 9, 10 of the panel 8. wedges (not shown). One or more detectors 4 are arranged around the periphery of the panel 8 to measure the energy of received light. Thus, each detector 4 receives light from a group of emitters 2 along a set of light paths. Panel 8 delineates two opposite and substantially parallel surfaces 9,10 and may be flat or curved. A radiation propagation channel is arranged between the two interfaces 9, 10 of the panel 8, at least one of the interfaces permitting the propagating light to interact with a touching object 7. Typically, the light propagates through total internal reflection (TIR) ​​in the radiation propagation channel. In this interaction, part of the light can be scattered by object 7, part of the light is absorbed by object 7 and part of the light continues to propagate unaffected. As shown in the side view of Fig. 8, the total internal reflection is frustrated and the energy of the transmitted light decreases as the object 7 touches the interface of the panel (e.g., the upper surface 9). The position of the moving object 7 can be detected by measuring the energy of the light transmitted through the panel 8 from a variety of directions.

Thus, it will be appreciated that the above described techniques for controlling the emitters 2 for transmitting codes and for separating the received light from individual emitters 2 based on the transmitted codes can be used to identify any light paths to each detector 4 affected by the affected object 7.

It should be noted that, unlike the embodiments of FIGS. 1-2, the light is not blocked by the affected object 7. Thus, the light will interact with both objects if two objects accidentally are placed one after the other along a light path from an emitter 2 to a detector. 4. Provided that the light energy is sufficient, the remaining light will reach the detector 4 and generate a measurement signal that allows identification of both interactions. This means that the creation of the detection network within the panel 8 can improve the ability of the apparatus to detect the positions of a plurality of touching objects during a sensing event.

Normally, each touch point has p_n a transmission t_n, which is in the range of 0-1, but usually in the range of 0.7 - 0.99. The total transmission 7}, along a light path Sy is the product of the individual transmissions t_n for the touch points p „on that light path: Ty =. For example, give two.

touch points pj and p2, with respective transmissions 0.9 and 0.8, on a light path Sy, a total transmission Ty = 0.72.

As in Fig. 1, each of the emitters 2 can emit a divergent light beam, and one or more detectors 4 can receive light from a plurality of emitters. The

533 However, 704 need not be necessary for emitters 2 to inject divergent rays into the panel. If sufficient scattering occurs in the panel, the injected rays will widen in the plane of the panel 8 as they propagate from the injection site through the panel 8. For each internal reflection, some radiation is led away from the main direction of the beam, and the center of the beam loses energy with the distance. Scattering is particularly noticeable if one or both interfaces 9, 10 are provided with an anti-glare structure or anti-glare layer. The anti-glare structure / layer provides a diffusing structure which can increase the scattering of the beam for any internal reflection, and which may also cause the radiation to leave the surface 9,10 for each internal reflection. Thus, the presence of an anti-glare structure / layer generally leads to increased widening of the jet with the distance from the injection site.

The use of an anti-glare structure / layer may be advantageous for reducing glare from external lighting on the touch surface 1. of the panel 8 In addition, when the touching object 7 is a naked finger, the contact between the finger 7 and the panel 8 normally leaves a fingerprint on the touch surface 1. On a perfectly flat surface, such fingerprints are clearly visible and usually undesirable. By applying an anti-shine structure / layer to the surface, the visibility of the fingerprints is reduced. In addition, the friction between the finger and the panel decreases when anti-glare is used, thereby improving the user experience.

Fig. 9 is a side view of an alternative arrangement where light also propagates inside a light transmitting panel 8. Here, emitters 2 are arranged below panel 8 to inject a respective light beam into panel 8 through the lower interface 10. The injected light beam propagates via total internal reflection between the interfaces 9,10, and the propagating light is captured by a number of detectors 4. These detectors 4 are also arranged below the panel 8, typically involved with the emitters 2. An example of such an arrangement of mixed emitters 2 and detectors 4 is shown in the plan view from above in Fig. 10. It will be appreciated that a number of light paths may be defined between each emitter 2 and a number of adjacent detectors 4, thereby creating a detection network at the upper level. interface 9.

Fig. 9 shows a respective light path defined between two different pairs of emitters 2 and detectors 4. When an object 7 touches the upper surface 9 of the panel 8, one or more of the propagating rays are frustrated, and the detector 4 will measure one decreased energy in the received light. It should be appreciated that if the detection grid is known and if the measured energy at each detector 4 can be separated into different light paths, it is possible to determine a position for a touching object 7 based on the light paths that experience a decrease in measured light energy.

533 704

Thus, it will be appreciated that the above described techniques for controlling the emitters 2 to transmit codes, and to separate the light received from individual emitters 2 based on the transmitted codes, can be used to identify all light paths to a detector 4 affected by the object concerned. 7th

As can be seen in Fig. 9, some of the propagating light is scattered by the touching object 7. This scattered light can also be detected by one or more detectors 4. However, the energy of the scattered light is usually much less than the energy attenuated by the interaction. thus, the scattered light will generally not make any significant contribution to the energy measured by the detectors 4 in the apparatus.

Typically, each emitter 2 generates a divergent beam in such a way that at least a portion of the beam has an angle of incidence toward the upper boundary surface 9 normal which exceeds the critical angle. The emitter 2 may be arranged to emit a beam having a beam angle of at least 90 ° and preferably at least 120 °. In one embodiment, the beam angle is close to 180 °, such as at least 160 °. The beam may or may not have a principal direction orthogonal to the upper interface 9. When using divergent rays, a significant portion of the emitted radiation can pass through panel 8 instead of being reflected internally. For this purpose, an element (not shown) may be provided between each emitter 2 and the lower interface 10 to block a portion of the emitted beam to only pass beams at an angle of incidence at the upper interface 9 maintaining total internal reflection. Alternatively, the element may be designed to reorient the beams in said portions of the beam to cause these beams to have at least the necessary angle of incidence at the upper interface 9.

Alternatively, each emitter 2 may emit collimated light at a suitable angle to the normal of upper interface 9.

When light is propagated inside the transmitting panel 8, the resulting signal levels at the detectors 4 may be lower compared to when light propagates above the touch surface 1. Thus, the above-described optimization criteria may need to be revised so that they also take into account so-called "shot noise" when optimizing of the multiplex matrix. In this case, we want to minimize a modified SSE function:

SSE = (σ² + Approx_s² ) Trace ((M^T · M)¹), where σ² is the variance of the signal independent noise, o_s² is the variance of the signal dependent noise, and C is the number of emitters that are on simultaneously.

533 704 ¹⁹

When shot noise is a significant factor, we can begin by finding an optimal or near optimal multiplex matrix Af without taking into account shot noise (but we can take into account saturation, see above). Once we have found the matrix Af, we can calculate the SNR improvement using the modified SSE function. Then we can calculate the optimal set of C (below the saturation limit) to obtain an optimal or near optimal multiplex matrix with regard to shot noise, ie. the matrix that provides the best SNR improvement.

The alternative detection concepts presented above are based on the detection / measurement of an attenuation of propagating light caused by one or more touching objects. According to yet another alternative detection concept, touch positions are determined based on the light scattered by a touching object. Fig. 11 shows an exemplary embodiment in which light is injected for propagation within a light transmitting panel 8, as described, previously. In the example of FIG. 11 emitters 2 are arranged along two opposite sides of panel 8 to emit a respective light beam (only two beams are shown). The light beam from each emitter 2 preferably has a small beam angle and can be collimated. Thus, each emitter 2 generates a light path across panel 8 in this example. In the example shown, the detectors 4 are arranged along the other two sides of the panel 8, perpendicular to the emitters 2, typically for receiving light through the side edges of the panel 8, or via one or more wedges arranged on the upper or lower surfaces of the panel 8 (not shown). An object 7 touching the panel 8 will cause the light to spread in all directions inside the panel 8. A number of detectors 4 will detect the scattered light, but due to bulk absorption in the plate, radially intensity dependent and possibly surface scattering, the detector 4 positioned on the same x-coordinate as the affected object 7 will detect the greatest intensity of scattered light. Thus, an x ​​coordinate of the affected object 7 can be determined from the total energy measured by the respective detector 4. To increase the precision, the detectors 4 can be configured with a limited field of view, so that only light scattered at the x coordinate of a detector, or adjacent x coordinates, can be detected by that detector. This can be accomplished with any combination of lenses, apertures, etc. between the panel 8 and the detector 4. Alternatively or additionally, an air gap may be provided between the panel 8 and the detectors 4, whereby total reflection of the scattered light at the side edge of the panel will limit the field of view of the detectors.

The y-coordinate of the object concerned is determined by determining the emitter (s) 2 which generated the scattered light measured by one or more detectors 4. It is thus clear that the techniques described above for controlling the emitters 2

533 704 to transmit codes and to separate received light from individual emitters 2 based on the transmitted codes can be used to identify all light paths affected by the touching object 7.1. 7 y coordinate is given by the y coordinate (s) of the emitter (s) 2 which generate the identified path (s). In an alternative arrangement, the light paths could be non-parallel to the x-axis. As long as the directions of the light paths are known, and the x-coordinate is obtained, the y-coordinate can be calculated when an affected light path has been identified.

In the arrangement above, a position coordinate (γ) is determined based on the affected light rays, which are identified by separation of the light received by the detector (s).

In an alternative (not shown), both position coordinates (x, y) can be determined by identifying light paths based on the light received by the detector (s). In such an arrangement, the emitters 2 are arranged to generate light paths crossed within the touch surface 1. Thus, both the x and y coordinates can be determined by separating the light received by the detector / detectors, by identifying light from at least one emitter in the separated light, and by reconstructing the intersection (s) between the light paths of the emitter frames thus identified.

It should be appreciated that the detection concept discussed above in connection with Fig. 11 is nevertheless applicable to light emitted for propagation above a touch surface 1.

In addition, this detection concept is not limited to the shown arrangement of emitters 2 and detectors 4. For example, emitters 2 and / or detectors 4 could be arranged along only one side of the touch surface 1. Alternatively, emitters 2 and detectors 4 may be alternately arranged at a or more of the points of the touch surface 1. In fact, it can be advantageous to combine the detection of attenuation with the detection of scattered light. For example, if the embodiment of FIG. 7-8 are implemented with the detection network of Fig. 3 (B), the detectors 4 which receive no direct light from the emitters 2 can be used to detect the light scattered from object 7 touching the panel 8. Thus, whenever a specific detector does not receive any direct light so it can be used for scatter detection. The scattered light can be used to improve the precision of the position determined for the affected object 7.

533 704

PERFECT ARRANGEMENTS OF DETECTORS AND EMITORS

The following relates to potential advantages of using various arrangements of emitters and detectors in the embodiments shown in Figures 1-2 and Figures 6-8, i.e. when emitters 2 and detectors 4 are arranged around the periphery of the touch surface 1 to define a light path detection network.

In one variant, the emitters 2 and the detectors 4 may be alternated around the periphery of the touch surface 2 (see Fig. 3). This can, for example, result in a more uniform detection network.

In this and other variants, the number of emitters 2 may be the same as the number of detectors 4.

Alternatively, the number of emitters 2 may be greater than the number of detectors 4, for example, as shown in Fig. 12. An increased number of emitters 2 can be used to reduce the number of detectors 4 and thereby reduce the cost. The spatial resolution mainly depends on the number of light paths, and emitters 2 can be cheaper than detectors 4 and any additional detector equipment such as lenses, A / D converters, amplification circuits or filters.

In yet another alternative arrangement, the number of detectors 4 is greater than the number of emitters 2. Examples of such arrangements are shown in Figures 13 (A) - (B). An advantage of such arrangements may be to reduce the size of the multiplex matrix and thus the sampling frequency, i.e. the frequency of sampling the outputs of the detectors 4.

In these and other variants, the emitters 2 and detectors 4 may be equidistantly arranged around the periphery of the touch surface 1, for example as shown in Figures 3, 12 and 13. Alternatively, as shown in Fig. 14, the distances between each emitter 2 and / or detector 4 to be random. For example, random distances between emitters 2 can be used to reduce the interference phenomenon that can occur when a number of light sources inject light of the same wavelength into the panel.

Fig. 15 shows yet another embodiment, in which emitters 2 near or at the point of the touch surface 1 are positioned to emit light with a wide beam directed towards the center of the touch surface 1 to spread the emitted light over as large a part of the touch surface 1 as possible. If home-close emitters 2 are positioned to emit light centered perpendicular to the periphery of the touch surface 1, a large portion of the emitted beam will reach a detector 4 after only propagating a short path across the touch surface 1. Thus, the resulting light paths between home-close emitters 2 and detectors 4 may cover only a small area of ​​the touch surface 1. It may therefore be advantageous to position home-close emitters, and in addition home-emitters if present, so that they point towards the touch surface. center. This embodiment is generally applicable

533 704 whenever the touch surface is a polygon, and at least one emitter is provided at a point of the touch surface. In one variant, all emitters are positioned to be directed towards the center of the touch surface, thereby ensuring that as much as possible of the emitted light is used for touch detection.

Fig. 16 is a side view of an embodiment where emitters 2 (one shown) are arranged at the periphery for injecting a respective light beam into a light transmitting panel 8. A V-shaped light deflector 11 is located between each emitter and the panel 8. The light deflector 11 is designed to, by means of angled mirror surfaces 12, 13, direct beams emitted substantially parallel to opposite surfaces 9, 10 of panel 8. In particular, the beams are directed toward either of the interfaces 9, 10 at an angle which ensures propagation with total internal reflection. In another embodiment (not shown), the diverter 11 is exchanged or supplemented with an element that prevents rays from reaching the detector without being reflected at least once in the touch surface 1. Any portions of the light propagating through the panel 8 without being reflected in the touch surface 1 do not contribute to the touch detection signal since this light cannot be frustrated by a touching object. Such a blocking element may be an absorbent or reflective element / layer which may be arranged between the emitter 2 and the side edge of the panel 8 and / or between the side edge of the panel 8 and the detector 4. For example, the blocking element may be attached to the side edge of the panel 8.

Similar deflecting elements or blocking elements may be provided between the emitters 2 and the panel 8 when the emitters 2 are arranged below the panel, as discussed above in connection with Figures 9-10.

In all embodiments described herein, a lens (not shown) may be inserted between panel 8 and detector 4 to focus light on the detector surface. This can increase SNR.

Whenever light propagates inside a transmitting panel 8, it may be advantageous to create an air gap between the panel 8 and the detectors 4. The air gap will result in a reduced field of view of the detectors 4, which in turn can reduce shot noise in the detection.

DATA PROCESSING

In all of the above described embodiments, designs, arrangements, alternatives and variants, the processing element 5 (see Figures 1 and 3) can be designed to calculate the contact positions based on the output or measurement signals obtained from the detectors 4. Those skilled in the art will recognize that there are several methods of determination

533 704 of touch positions. Fig. 17 is a flow chart of an exemplary method.

In step 20, measurement signals are collected from the system detectors. Each measurement signal represents the sum of light received from k different angles (i.e. k different emitters), sampled at n time intervals during a sensing event.

In step 21, each measurement signal is separated into a set of emitter signals, using the multiplex inversion scheme. Thus, each emitter signal represents received light energy along one of the available light paths to the relevant detector. The measurement / emitter signals can also be processed. For example, the measurement / emitter signals can be processed for noise reduction using standard filtration techniques, such as low pass filtering, median filter, Fourier plane filter, etc. Furthermore, if the energy of the emitted rays is measured in the system, the meter / emitter signals can be compensated for temporal energy fluctuations in radiant energy. In addition, the touch surface may be a sub-region of the detection network, and certain emitter signals may thus originate from light paths outside this sub-region. Therefore, the preprocessing may involve removing such emitter signals from further processing. In addition, the emitter signals can be rectified, which essentially means that the emitter signals for each detector are interpolated to provide the same mutual angle between all incoming light paths to the detector. The emitter signals for each detector are thus interpolated with a non-linear angle variable, resulting in a complete set of emitter signals evenly distributed across the panel. The rectification is optional but can simplify subsequent calculation of touch positions.

In step 22, the emitter signals are processed to identify light paths that are affected by the affected objects.

If the light propagates above the touch surface, then these light paths are blocked or obstructed by the object (s) concerned and thus identified by the absence of corresponding emitter signals.

If the light is propagated inside the panel, these light paths are identified based on attenuation of the emitter signals. Conveniently, a transmission signal for each preprocessed emitter signal is calculated by dividing the emitter signal by a background signal representing the emitter signal without any objects touching the touch surface. The background signal may be unique, or not unique, for each detector or for each emitter signal. The background signal may be predetermined, applied during a separate calibration step, or applied from the same emitter signal collected during one or more previous sensing occasions. The

533 704 ²⁴the resulting transmission signals indicate the light paths affected by the affected objects.

In step 23, the contact positions are determined based on the identified light paths.

If light is propagated above the touch surface or inside a panel, the contact positions can be determined by determining intersections between the identified light paths.

Alternatively, if the light is propagated inside the panel, touch positions may be determined using the collection of identified light paths and the corresponding transmission signals. For example, the touch-sensitive system can be modeled using known algorithms developed for transmission tomography with a spring-shaped beam geometry. Thus, the touch positions can be reconstructed with any available image reconstruction algorithm operated on the transmission signals for the light path collection.

The accuracy and / or calculation speed of step 23 can be increased by using prior (a priori) knowledge of touch positions, for example, using information about the touch positions identified during the previous sensing event (s).

In step 24, the determined touch positions are output, and the process returns to step 20 for processing an upcoming sensing event.

The data processing may also involve determining the shape and / or size of the object (s), for example, using the algorithms described in the aforementioned WO2006 / 095320, which is incorporated herein by reference.

GENERALLY

The touch surface 1 can be any shape, for example, multilateral, elliptical or circular.

The emitter 2 can be any suitable light source, such as an LED (LED), a light bulb, a halogen lamp, a diode laser, a VCSEL (vertical cavity surface emitting laser), etc. All rays can be generated with identical wavelength. Alternatively, some or all of the rays may be generated in different wavelength ranges, allowing separation of the rays based on wavelength. The emitters 2 can generate divergent or collimated rays.

The energy of the rays can be measured by any radiation detector 4 capable of converting radiation to an electrical signal. For example, the detectors 4 may be simple 0-dimensional detectors, but they may alternatively be 1-dimensional or 2-dimensional detectors.

533 704

The above-described panel 8 may be made of any material that transmits sufficient light in the relevant wavelength range to allow a reasonable measurement of transmitted energy. Such materials include glass, polymethylmethacrylate (PMMA) and polycarbonates (PC).

The processing element 5 and the controller 3 may be implemented by program instructions executed by a processor. The processor may be a commercially available microprocessor such as a Central Processing Unit (CPU), a Digital Signal Processor (DSP) or any other programmable device such as a Field Programmable Gate Array (FPGA). Alternatively, the processing element or controller may be implemented by a dedicated circuit, such as an Application-Specific Integrated Circuit (ASIC), discrete analog and digital components, or any combination of the above. It should be noted that the controller 3 and the processing element 5 may be implemented by processes in the same device.

The invention has been described above mainly with reference to relatively fatal embodiments. However, those skilled in the art will recognize that embodiments other than those described above are equally possible within the scope of the invention. The various features of the invention could be combined in combinations other than those described. The scope of the invention is defined and limited only by the appended claims.

For example, the above linearly independent codes may be any length. Thus, the resulting multiplex matrix does not have to be square (i.e., have the same number of rows as columns). Instead, the linearly independent codes can define an overdetermined system of linear equations, which means that the multiplex matrix is ​​not square and therefore cannot be inverted analytically. However, it is nevertheless possible to calculate an approximate inverse to such an overdetermined multiplex matrix, for example, by extracting and solving corresponding normal equations, as is well known to those skilled in linear algebra and numerical methods.

The emitter frames' codes may be embedded in the emitted light by some type of amplitude modulation, which is not limited to on / off modulation. For example, any number of different code values ​​can be encoded by any different intensity values ​​for the light emitted.

Fig. 18 shows yet another type of modulation where different pulse lengths of the emitters are used to represent different code values ​​of the associated code. Thus, the emitter's duty cycle is modulated by changing the length of

533 704 the activation intervals relative to a constant time interval ΔΤ in the code generation cycle. In the example of Fig. 18, the pulse length t represents a code value 0, while the pulse length t₂ represents a code value 1, and the resulting code is 0100.

Fig. 19 shows a further type of modulation, where the delays for activating the emitter are used to represent different code values ​​for the associated code. Thus, the emitted light is modulated by changing the pulse delays within a constant time interval ΔΤ of the code generation cycle. In the example of Fig. 19, the pulse delay Δΐι represents a code value 0, while the pulse delay Δί₂ represents a code value 1, and the resulting code is 0100.

It is also possible to combine any of the above modulations to embed the codes in the emitted light.

In another variant, the codes are embedded in the emitted light by modulating the amplitude of the emitted light according to various functions, selected in such a way that a value for an autocorrelation of each function is significantly greater than a cross-correlation between any two functions. different emitters. In such an example, the different functions of different modulation frequencies © k are given a periodic base function (carrier). Preferably, the base function has a well-defined frequency spectrum around its modulation frequency. For example, the base function may be a cosine function or a sine function such as:

_r l-cos -O ^ek = -------- This means that the functions (codes) of the different emittrams are orthogonal because:

<img file="SE533704C2_D0011.tif" />

i = ki * k

As in the previously described embodiments, each detector generates a measurement signal, which is a time-resolved representation of the light received along a set of light paths, ie. from various emitters. There are different approaches for separating such a measurement signal into a set of emitter signals. The code generation cycle is generally selected to include at least one period of the lowest modulation frequency.

533 704

In one approach, the measurement signal is processed with a frequency spectrum analyzer to identify the light energy received from the various emitters. Such an approach is further exemplified in Fig. 20, which shows five emitters 2, all of which are amplitude modulated with a cosine function, but at separate frequencies ω1-ω5. A detector 4 receives the light from the emitters 2. The detector 4 is sampled at a frequency that is at least twice the highest coding frequency, i.e. according to the Nyquist sampling theorem, to generate a measurement signal. The measurement signal is processed with a frequency spectrum analyzer 14 to generate an intensity spectrum, for example, by calculating the Fourier transform of the measurement signal, for example by using an FFT (Fast Fourier Transform) algorithm. A value of the received light energy from each emitter 2 is then given by the intensity of the intensity spectrum at each frequency. In this coding scheme, it may be advantageous to select modulation frequencies ω1-ω5 that correspond to the actual frequencies that the FFT will measure, so that the frequencies are given by co._k= 2Tm / N, with n = [1, total number of emitters] and N is the total number of sampling points during a code generation cycle. The frequency spectrum analyzer 14 may be implemented as part of the processing element 5 or be a separate unit.

In a second approach, the measurement signal is passed through a set of bandpass filters, each adapted for a respective emitter frequency. Such an approach is further exemplified in Fig. 21. As in the embodiment of Fig. 20, a detector 4 is sampled to generate a measurement signal representing the received light from five emitters 2. A set of bandpass filters 15 is arranged to operate on the measurement signal so that each bandpass filter removes frequencies outside a passband around the modulation frequency ω1-ω5 for each emitter 2. The output of each bandpass filter 15 will represent the received light energy from the respective emitter 2. The output is then forwarded. to an amplitude detector or an integrator 16, which provides an emitter signal representative of the light energy. The bandpass filters 15 and the amplitude detector / amplifier 16 can be implemented by digital signal processing in the processing element 5, or by dedicated electronics circuits operating on analog measuring signals from the detector. The processing of analog signals obviates the need for sampling, and can thus enable the use of higher modulation frequencies. Use of higher modulation frequencies may allow for shorter code generation cycles or increased SNR.

533 704

The use of frequency modulation has the additional advantage that signal effects from ambient light or other noise sources can be eliminated, provided that the modulation frequencies are well separated from the frequencies of such noise sources.

In a further variant, the codes are embedded in the emitted light by phase modulation, so that different code values ​​are represented by different phase shifts for a carrier wave, which may have any suitable waveform including cosine / sine, square, triangle, saw tooth, etc.

In one embodiment, all emitters emit light that is modulated by a common carrier at a common frequency ω, and the phases of the group of emitters are modulated according to a multiplexing scheme. In the following example, the multiplexing scheme uses the code values ​​-1 and 1, where -1 is given by a 180 ° phase shift for the carrier. Thus, the phase modulation is called BPSK (Binary Phase Shift Keying). The light emitted from an emitter e_k during a time interval in a code generation cycle can then be given by:

^ek₄ =^EkQ + T »k, · cos (®-0) / 2, where m_k is the code value for an emitter e_k Thus, the code for each emitter is given by a vector m_k consisting of code values ​​m_k. As explained above, a multiplex matrix M can be formed by the vectors m_k for N emitters: M = [/ Hy m₂ ... m_N], and the codes for the different emitters can be linearly independent, or even orthogonal. In this example, the multiplex matrix may be a Hadamard matrix, as described above.

The signal detected on a detector during a time interval is the sum of light reaching the detector. The light is demodulated by multiplying by a reference signal, typically the original carrier:

<img file="SE533704C2_D0012.tif" />

<img file="SE533704C2_D0013.tif" />

By selecting the integration time T so that it is an even multiple of the carrier frequency ω, all terms containing cos (o »· ή and cos (2co i) disappear). Furthermore, the integration time is chosen to equal an interval of time in the code generation cycle. thus:

533 704 η_ι⁼ ? Σ ^ '^mk, 'k

The multiplication and integration (demodulation) above is performed during each of the time intervals in the code generation cycle, resulting in a measurement signal η. As previously described, the measurement signal can be separated into a set of emitter signals using a multiplex inversion scheme: É = M⁴ · Η. If the codes are orthogonal, this operation can be further simplified since M^T = M⁴ for an orthogonal (orthonormal) multiplex matrix.

The demodulation can be implemented through digital signal processing in the processing element or by dedicated electronics circuits operating on analog measuring signals from the detector. The processing of analog signals obviates the need for sampling, and can thus enable the use of a higher modulation frequency. Use of higher modulation frequencies may allow for shorter code generation cycles or increased SNR. The use of phase modulation has the additional advantage that signal effects from ambient light or other noise sources can be eliminated, provided that the modulation frequency is well separated from the frequencies of such noise sources.

It should be noted that code values ​​-1/1 are given by way of example only, and that other types of code values ​​can be embedded in the emitted light using phase modulation. Furthermore, other types of phase modulation can be used, including but not limited to MSK (Minimum Shift Keying), Quadrature Phase-Shift Keying (QPSK) and Differential Phase-Shift Keying (DPSK).

533 704