Weighing items such as mailpieces in the presence of external vibration

Background of the Invention

The present invention relates to a system and method for rapid weighing of items. More particularly it relates to scale systems used in postage metering systems to weigh, and determine postage for, mailpieces or the like.

Postal scale systems are well known. Such scale systems weigh a mailpiece and determine the appropriate postage for that mailpiece as a function of the weight. Postal mailing systems where a mailpiece is transported onto a postage scale system, the appropriate postage is determined, and the mailpiece is then transported to postage metering system for imprinting with a postal indicium representative of the postage determined are also known. One such system is described in <patcit id="pcit0001" dnum="US4742878A"><text>U.S. Patent Number 4,742,878; issued May 10, 1988</text></patcit>. In such systems there is a constant need to increase the rate at which the scale can determine the weight of a mailpiece in order that the throughput of the system can be increased.

<patcit id="pcit0002" dnum="US4787048A"><text>U.S. Patent Number 4,787,048; issued: Nov. 22, I988</text></patcit>, discloses one approach to decreasing the time required for a postage scale system to determine the weight of a mailpiece. The system disclosed in this patent takes advantage of the structure of postage rate charts, i.e., the function that relates the weight of the mailpiece to the appropriate postage amount. Such rate charts provide a constant postage value for all weights between a pair of predetermined weight breaks. The system of the '048 patent takes advantage of this by use of an algorithm where a first estimate of the weight is made and used to determine the postage amount unless the first estimate is within a predetermined distance of a break point, in which case a second more accurate estimate is made.

A basic cause of the delay in determining weight for a mail piece is the tendency for a scale system to oscillate in response to the arrival of the mailpiece on the system. These oscillations are damped, but only slowly arrive at a new stable output value representative of the weight of the mail piece. Heretofore systems have relied on an averaging process over a number of samples taken over a number of cycles of the oscillations to approximate the weight output. While systems using such averaging type algorithms have generally proved satisfactory in the past, presently they are approaching limits such that it is difficult to increase the throughput of postage metering systems using such algorithms. One approach to increasing the speed with which weights are determined has been to develop algorithms, which determine a weight, at least to a broad approximation, from the initial transient response to a mail piece or other item arriving on a scale platform. One example of such an algorithm is commonly assigned, co-pending <patcit id="pcit0003" dnum="US165161A" dnum-type="L"><text>U.S. application Serial No. 10/165,161</text></patcit> (<patcit id="pcit0004" dnum="US7247801B2"><text>US 7 247 801 B2</text></patcit>); titled: SYSTEM AND METHOD FOR FAST WEIGHING OF ITEMS SUCH AS MAILPIECES; filed Jun. 7, 2002, which attempts to identify mail pieces in the first weight range (&lt;1 oz.) from the initial response to the mail piece.

This problem is exacerbated by the presence of external (e.g. ground) vibrations, which can slow or even prevent the scale system output from converging to a sufficiently accurate approximation of the weight. One approach to the problem of external vibrations is to use sophisticated weighing algorithms that can determine a weight in the presence of external vibrations. One example of such an algorithm is commonly assigned, co-pending <patcit id="pcit0005" dnum="US165532A" dnum-type="L"><text>U.S. application Serial No. 10/165,532</text></patcit> (<patcit id="pcit0006" dnum="US6839694B2"><text>US 6 839 694 B2</text></patcit>); titled: SYSTEM AND METHOD FOR WEIGHING ITEMS SUCH AS MAILPIECES IN THE PRESENCE OF EXERNAL VIBRATION; filed Jun. 7, 2002. Such algorithms are complex and can be relatively slow. Further they rely on assumptions about the vibration spectrum and so may not prove effective in all cases. Still further, since they require time to identify external vibrations, they are not useful with algorithms based on the initial transient response, as discussed above.

<patcit id="pcit0007" dnum="WO9005285A"><text>WO 90/05285</text></patcit> discloses a maritime weighing system for obtaining accurate product weighing results in ships, in which environment it is not sufficient to effect a correction of the weighing results by means of an accelerometer, which will only roughly compensate for the weighing errors due to the general movements in the sea. Therefore, corrections are also effected for errors from other marked sources of error, such as errors due to tiltings of the ship and due to the mechanical noise produced by the engine of the ship or by working appliances on the ship.

An approach to the problem of external vibration that has been considered by the present inventors is to separately measure the external vibration and subtract it out from the weight signal. While perhaps technically feasible, this approach has proved difficult and ultimately may prove too expensive for applications such as postal scales.

Thus it is an object of the present invention to provide a scale system, and a postage metering system incorporating such scale system, which can determine the weight of a mailpiece or the like even in an environment where periods of external vibrations which would delay or prevent convergence of the system output can occur.

According to one aspect of the present invention there is provided a system for determining a weight for an item according to claim 1.

Further, according to another aspect, the present invention also provides a method for determining a weight for an item according to claim 7. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li><figref idref="f0001">FIG. 1</figref> shows a schematic block diagram of a postage metering system including a scale system in accordance with one embodiment of the present invention.;</li><li><figref idref="f0002">FIG. 2</figref> shows a schematic block diagram of a load cell for producing a response to the force from platform supporting mailpiece, and an accelerometer for detecting external vertical vibrations, and a limit detector for providing a go/no go binary signal ; and</li><li><figref idref="f0003">FIG. 3</figref> shows a flow diagram of the method of one embodiment of the present invention.</li></ul>

In <figref idref="f0001">FIG. 1</figref> postage metering system 10 includes scale system 12, postage meter 16, and printer 18. Scale system 12 includes load cell 20, producing a response to the force from platform 22, supporting mailpiece MP; analog-to digital converter 24, for converting the output of load cell 20 to a sequence of digital values; low pass filter 28, which together comprise transducer 29 for producing a digital output representative of the response of platform 22; and microprocessor 30, for processing the output to determine a weight for mail piece MP and provide an output representative of the postage corresponding to that weight to conventional postage meter 16, which in turn controls printer 18 to print a corresponding postal indicia on mail piece MP. (While filter 28 is shown as a separate element in <figref idref="f0001">FIG. 1</figref> for simplicity of illustration, it should be noted that filter 28 can be, and preferably is, implemented by software running in microprocessor 30.)

Scale system 12 also includes accelerometer 32 for detecting external vertical vibrations which might affect the scale output, and limit detector 34 for providing a go/no go binary signal to microprocessor 30 and which together form external vibration detector 36. Detector 34 provides a no go signal when the absolute value of the output of accelerometer 32 exceeds predetermined limits, as will be described further below.

The acceptable range of external vibration will vary with the particular weighing algorithm used and the acceptable trade off between processing speed and error rate for a particular application. Different algorithms will have differing sensitivities to external vibration and for any particular application (including the anticipated vibration environment) there will be requirements for the error rate for weight determination and for the processing rate. For any particular algorithm, increasing the acceptable range of external vibration will tend to increase both the error rate (as weight determinations can be made in the presence of greater external vibration) and the processing rate (as the algorithm is aborted less often). It is believed that an optimal range of acceptable external vibration balancing these constraints can best be determined by routine experimentation well within the abilities of those skilled in the art. Accelerometer 32 is preferably in two way communication with processor 30 to allow processor 30 to alter the predetermined limits for various applications.

<figref idref="f0002">FIG. 2</figref> shows a more detailed representation of load cell 20, which functions as a transducer to convert the force applied by platform 22 and mail piece MP to generate an analog signal for further processing. Load cell 20 comprises vertical, substantially rigid, members 20-1 and 20-2 that are connected by flexible members 20-3 to form a conventional four-bar linkage. Member 20-1 supports platform 22 and is free to deflect vertically in response to the force applied by platform 22, while member 20-2 is fixed to mechanical ground. Strain gauges 20-4 are fixed to flexible members 20-4 and interconnected to form a bridge circuit (not shown). The output of the bridge circuit is proportional to the instantaneous displacement of member 20-1; and thus, after the transient response has died out, the weight of platform 22 and mail piece MP. It will be apparent that external vibration, typically ground vibration, can delay or even prevent generation of an output which accurately represents the weight since the vertical acceleration of mechanical ground caused by external vibration is indistinguishable by load cell 20 from a force applied by platform 22.

To overcome this, accelerometer 32 is fixed to mechanical ground, and preferably to member 20-2, so that the vertical component of any acceleration of mechanical ground is sensed. (Horizontal components of external vibration can apply a torque to load cell 20 and also cause an inaccurate response. However, the vertical component is dominant and it is believed that it is sufficient, in general, to monitor only the vertical component. In particular applications it may be desirable to monitor the total magnitude of acceleration caused by external vibration, and the use of additional accelerometers to measure horizontal accelerations is within the contemplation of the present invention.) The output of accelerometer 32 is input to limit detector 34, which generates a binary go/no go signal. The output of detector 34 latches to the no go value if the absolute value of the output of accelerometer 32 exceeds a predetermined limit; which is preferably determined by experimentation as described above. The go/no go signal is used by microprocessor 30 to validate a weight which is determined by a weighing algorithm, as will be described further below.

<figref idref="f0003">Figure 3</figref> shows a flow diagram of the operation of scale system 12 in accordance with one embodiment of the present invention. At step 40 microprocessor 30 loops until a mail piece MP is present on platform 22. Then at step 42 it initializes limit detector 34 to a go state, and at step 44 starts a weighing algorithm to call a weight for mail piece MP. (Calling a weight, as used herein, means microprocessor 30 uses the current weight value to determine the postage for a mailpiece.)

Any convenient weighing algorithm based upon the initial transient response to the presence of mail piece MP, such as the algorithm described in the above described U.S. application Serial No. 10/165,532; titled: SYSTEM AND METHOD FOR WEIGHING ITEMS SUCH AS MAILPIECES IN THE PRESENCE OF EXERNAL VIBRATION, or other initial transient based algorithm, is used since it is believed that such algorithms are more reliable if used in the absence of external vibration.

Then at step 46, microprocessor 30 determines if a weight has been called. If so, it exits. Otherwise at step 50, microprocessor 30 determines if a predetermined time, typically 100 to 120 milliseconds, has elapsed. If not, at step 52 microprocessor 30 determines if the external vibration limits have been exceeded, i.e. if limit detector 34 has latched into a no go state. Thus a weight called within the predetermined time and without external vibrations exceeding predetermined limits is assumed valid. Otherwise, if the time or vibration limits are exceeded, microprocessor 30 exits at step 54 to an alternative weighing method.

Any convenient alternative weighing method can be used in the present invention and the choice of weighing algorithm forms no part of the present invention in its broadest contemplation (unless, and to the extent, specifically claimed). In some preferred embodiments a weighing algorithm which is less sensitive to external vibration than the algorithm used in step 44 can be used. For example, the algorithm described in the above mentioned <patcit id="pcit0008" dnum="US165161A" dnum-type="L"><text>U.S. application Serial No. 10/165,161</text></patcit>; titled: SYSTEM AND METHOD FOR FAST WEIGHING OF ITEMS SUCH AS MAILPIECES, or other vibration resistant algorithms, can be used. In some simple embodiments the original algorithm is simply restarted one or more times in the hope that the external vibration will naturally die down.

The embodiments described above and illustrated in the attached drawings have been given by way of example and illustration only. From the teachings of the present application those skilled in the art will readily recognize numerous other embodiments in accordance with the subject invention. Accordingly, limitations on the subject invention are to be found only in the claims set forth below.