System for detecting the level of liquid in a tank

Description of equivalent WO 2004008082 A2

SYSTEM FOR DETECTING THE LEVEL OF A LIQUID IN A TANK

0002Technical field

0003This invention relates to a system for detecting the level of a liquid in

0004a tank, and more particularly the level of ink contained in the cartridge of

0005an ink jet printhead.

0006Brief description of the state of the art

0007Various devices and relative methods are known in the sector art,

0008and in particular in the field of ink jet printers, for detecting the residual ink

0009quantity, or the level of ink in the cartridge of the printhead, the most

0010widely used of which may be divided into the following categories:

0011devices for detecting the residual quantity of ink by means of float

0012systems;

0013detecting devices that use the measurement of a rapid variation of

0014the electrical resistance of the ink between two electrodes, when one of

0015these remains uncovered, of the ON - OFF type;

0016finally, detecting devices that use the measurement of the variation of

0017the resistance of the ink between two electrodes placed in a porous body

0018impregnated with ink.

0019These devices have drawbacks of different types, such as lack of

0020precision, mechanical failures and jamming for all the float detector types,

0021lack of precision and incorrect measurement of the resistance of the ink for all the other types of detectors, mainly on account of the fact that the

0022resistance is highly dependant on the temperature and on the

0023composition of the ink.

0024What is more, the residual ink quantity detecting devices known in

0025the sector art are unsuitable for working in the presence of electrically

0026non-conducting liquids, such as for instance oily liquids and liquids

0027derived from crude oil.

0028Summary description of the invention

0029One object of this invention, therefore, is to overcome the drawbacks

0030found in the type of detectors known in the art and classified above.

0031Another object of the invention is to produce a system for detecting

0032the level of a liquid in a tank in such a way that the indication of the level

0033of the liquid is independent of the influence of parameters, such as

0034temperature, composition of the liquid, in particular an ink, and any errors

0035in the dimensions and position of the detecting electrodes.

0036Finally a further object of this invention is to produce a system for

0037detecting the level, or quantity of liquid in a tank, filled with an electrically

0038non-conducting liquid.

0039Therefore in accordance with the stated objects of this invention, a

0040system is proposed for detecting the level of a liquid in a tank, the

0041detecting system comprising: at least two electrodes (5, 6) extending into the inside of said tank

0042(2), in contact with said liquid, said electrodes (5, 6) being separated by a

0043volume of liquid (8) presenting an own electrical resistance (Rs) variable

0044in function of the level of the liquid in said tank, and influenced by

0045environmental conditions and by physical properties of said liquid,

0046detecting means (1 ) electrically connected to said electrodes (5, 6),

0047and

0048a control unit (CPU) suitable for controlling said detecting means,

0049characterized as defined in the main claim.

0050These and other characteristics of the invention will appear more

0051clearly from the following description of a preferred embodiment, provided

0052by way of non-restrictive example, with reference to the figures of the

0053accompanying drawings.

0054Brief description of the drawings

0055Figure 1 represents a wiring diagram of the^' detecting system

0056according to the invention;

0057figure 2 depicts schematically one of the arrangements of the

0058electrodes inside the ink cartridge;

0059figure 3 shows a diagram of the standard variation of resistivity of the

0060ink with ambient temperature; figure 4 represents a different embodiment of the detecting system of

0061fig. 1 ;

0062figure 5 represents a version of the device of fig. 1 adapted for

0063dielectric liquids;

0064figure 6 represents schematically the disposition of a pair of

0065capacitive electrodes for dielectric liquids;

0066figure 7 shows a variant of the diagram of fig. 5.

0067Detailed description of the invention

0068Although the discovery according to this invention may be used for

0069measuring the level of any liquid contained in a tank, the description that

0070follows focuses, for simplicity's sake, on an application, certainly non-

0071restrictive, of the system for detecting the level of a liquid used for

0072measuring the ink level of a cartridge 2 (fig. 2) of an ink jet printhead 3,

0073integral with the cartridge 2.

0074As a non-restrictive, illustrative example of the above-mentioned

0075application, reference is made to use of the printhead 3 on slip printing

0076equipment, used at points of sale (POS) in shopping centres, such as that

0077described in the Italian patent application No. TO 2002 000428, filed by

0078the Applicant, which may be consulted for greater details about using the

0079system for detecting the level of a liquid according to this invention. The ink jet printhead 3 is of a type widely known in the sector art, and

0080is therefore not described in detail, the reader being referred instead to

0081Italian patent no. 1.233.061 , for example.

0082The cartridge 2 contains a spongy body 4 of electrically insulating

0083material and provided with high porosity which, when the cartridge is new,

0084is completely impregnated with ink, which is held on the inside by

0085capillarity.

0086Two metallic electrodes 5 and 6 are attached to a lid 7 of the

0087cartridge 2, located at the opposite end with respect to the printhead 3,

0088and extend inside the spongy body 4; the electrodes 5 and 6 are

0089therefore maintained in contact with the ink contained in the cartridge 2.

0090The two electrodes 5 and 6 are arranged at a distance from one

0091another, functionally pre-established so as to define between them a

0092volume 8 of ink, which presents a determined electrical resistance Rs to

0093the passage of an electrical current of constant and defined intensity.

0094Gradually as the ink is consumed by the head 3 for printing, the

0095spongy body 4 empties, the ink inside the spongy body 4 recedes starting

0096from the lid 7, in the direction of the printhead 3. The volume 8 of ink

0097between the electrodes 5 and 6 diminishes, and as a result the resistance

0098Rs increases in value. In practice, however, the resistance Rs is influenced by other

0099parasitic parameters that modify its value, causing errors during the

0100detection of the level of ink in the cartridge 2, if they are not taken into

0101due consideration. These parasitic parameters are first and foremost the

0102environmental conditions that the cartridge 2 is in at the time the level of

0103ink is measured, and the chemical composition of the ink. The

0104environmental condition that most influences the resistance Rs is

0105temperature. Figure 3 shows a typical trend of the variation of the value of

0106the resistance Rs, or resistivity, upon variation of the ambient

0107temperature. It may be noted that, for the most commonly used types of

0108ink in ink jet printers, in the ambient temperature range from 10°C to

010930°C, resistivity of the ink varies on average from 1.6 to 0.8

0110approximately, once the resistivity at 20°C has been fixed at 1.

0111The main object of this invention, therefore, is to be able to detect the

0112level of the ink in the cartridge 2, regardless of the influence of the

0113parasitic parameters mentioned above.

0114With reference to figure 1 , the numeral 1 is used to indicate as a

0115whole a detecting circuit, comprising the detecting means according to

0116the invention. The circuit 1 comprises a transistor T1 , configured as a

0117current generator, polarized by means of two fixed resistances Ra and

0118Rb, fed with power by a voltage source +V; the point 9 of common connection of the resistances Ra and Rb is connected to the base 10 of

0119transistor T1.

0120The emitter 11 of T1 is connected to one end B of the resistance Rs,

0121i.e. it is connected to the corresponding electrode 6 (fig. 2) of the

0122cartridge 2, whereas the other end A of the resistance, Rs, corresponding

0123in turn to the electrode 5, is directly connected to the source +V.

0124In order to render measurement of the level of ink independent of the

0125influence of the parasitic parameters mentioned earlier, according to the

0126invention a capacitor 13, of known and constant capacitance Cx, has

0127been included in the circuit of figure 1. The capacitor 13 is connected

0128between the collector 12 of T1 and the common earth M.

0129An FET transistor T2 is connected in parallel to the ends of the

0130capacitor 13, its function being to discharge the capacitor 13, after each

0131measurement, in order to re-establish its starting conditions for a new

0132measurement. For this purpose, the transistor T2, normally inactive, is

0133activated by a pulse applied in a known way to the gate 15.

0134A third transistor T3 is placed in series with the resistances Ra, Rb,

0135between the resistance Rb and the earth M. The transistor T3 is normally

0136inactive, but is activated by means of a signal S generated by the CPU, of

0137predefined duration, as will be described later, and applied to the base 14 of the transistor T3, only when it is necessary to make the transistor T1

0138conducting, i.e. when the level of ink has to be measured.

0139Inclusion of the capacitor 13 permits generation of a signal

0140representing the level of liquid, namely of ink in the cartridge 2, no longer

0141proportional to the current flowing through a reference resistance in

0142series with the resistance offered by the ink, as was known in the sector

0143art, for example as in a device described in USA patent no. 5.162.817,

0144but proportional to the integral of the current. This leads to a different

0145form of behaviour of the circuit 1 , when it is powered with a constant

0146current I.

0147In fact, if we indicate with Vp the voltage drop measured on the

0148terminals A, B of the resistance Rs, with Vx the voltage on the terminals

0149of the capacitor 13 of capacitance Cx, we obtain the relation 1):

0150Vx = Tp Vp / Cx Rs 1)

0151in which Tp indicates the duration of the time to charge the capacitor

015213 up to a voltage Vx, i.e. the driving time of the circuit 1. However the

0153voltage drop Vp, being proportional to the resistance Rs, will also suffer

0154the variations due to the above-mentioned parasitic parameters, and will

0155not give a true indication if used as a quantity representing the level of

0156ink. If the relation 1) is interpreted in another way, where the voltage Vx is

0157considered to be constant, the following relation 2), derived from 1), is

0158obtained:

0159Vp = Vx Cx Rs Tp, 2)

0160in which the driving time Tp is now the quantity proportional to Rs.

0161In other words, by using a certain cartridge 2, and by driving the

0162circuit 1 with a current I until a prefixed voltage Vx is obtained, depending

0163for instance on the dynamics of the A/D conversion equipment 30, a

0164driving time Tp1 is obtained, that can easily be measured by means of a

0165digital counter 29 and stored in a memory, for instance in a memory 16

0166(fig. 1 , 2) located on board the cartridge itself.

0167Afterwards, as the ink in the cartridge 2 is gradually used for printing,

0168by subsequently driving the circuit 1 with a pulse of current I of duration

0169equal to the time Tp1 , corresponding values of the voltage drop Vp may

0170be measured, which will give a true representation of the pattern of the

0171level of the ink as they are no longer affected by the influences of the

0172parasitic parameters, now automatically compensated for by the value of

0173Tp1. Therefore in this mode of operation of the circuit 1 , just described,

0174the voltage drop on the terminals of the resistance Rs is truly

0175representative of the level of ink contained in the cartridge 2.

0176Method of measuring the level of liquid The method of measuring the level of a liquid contained in a tank,

0177and in particular the level of ink in a cartridge 2 of an ink jet printhead 3, is

0178conducted according to the following steps:

0179step 1): each newly produced cartridge, filled with a certain type of

0180ink, black or colour, is connected to the circuit 1 , which is powered with a

0181current I;

0182step 2): the ambient temperature surrounding the new cartridge is

0183measured;

0184step 3): the counter 29 is used to measure the driving time Tp used

0185to reach a voltage Vx that is fixed in advance and suitable for the

0186characteristics of the converting/measuring device 30 used for digitizing

0187and measuring the voltages;

0188step 4): the time Tp measured in step 3) is associated with a

0189standard ambient temperature of 20° and is stored in the non-volatile

0190memory 16 fixed on board the cartridge 2; this measured time Tp

0191represents the maximum value of the level of ink contained in the new

0192cartridge 2;

0193step 5): in use with the cartridge 2, mounted on printing equipment

0194and connected to the detecting circuit 1 , still controlled by the CPU, the

0195ambient temperature of the cartridge during the operating step is

0196measured; step 6): the detecting circuit 1 is powered with a pulse of current I of

0197duration equal to the driving time Tp, taken from the memory 16, and the

0198voltage drop Vp on the terminals of the resistance Rs is measured, before

0199being converted by the converter 30, connected to the control unit or

CPU;

0201step 7): the measurement of Vp is associated with the temperature of

020220°, in such a way as to obtain the corresponding value of the level of ink,

0203converted according to a suitable scale;

0204step 8): the measurement is repeated as in steps 6) and 7), each

0205time the value of level of the ink in the cartridge 2 needs to be known;

0206step 9): the cartridge 2 is replaced when a voltage drop Vp is

0207detected on the resistance Rs equal to or less than a previously

0208calculated limit value, indicative of a situation of cartridge empty.

0209Figure 3 shows the influence of the ambient temperature on the

0210resistance of various inks; the diagram of figure 3 is drawn in

0211dimensionless and normalized form, in which the value 1 on the ordinates

0212corresponds to the resistance of the ink at 20°C.

0213Figure 4 represents a different embodiment 17 of the detecting circuit

02141 of fig. 1 , suitable for use with digitizing and measuring equipment

0215having a floating input, i.e. not referred to the earth; in the circuit 17 the

0216resistance Rs and the capacitor 13 are changed over and the transistor T3 is connected between the +V pole and the resistance Ra; a transistor

0217T4 has been added together with the charging 18 and polarizing 19

0218resistance of T3; this addition does not affect global operation of the

0219circuit 17.

0220Figure 5 shows a detecting circuit 20 equivalent to that of figure 1 ,

0221and which therefore works in the same way, suitable for non-conducting,

0222or dielectric, liquids, such as for example liquids used in the chemical

0223industry, liquids derived from crude oil, mineral oils, or vegetable oils, etc.

0224The capacitor 22 represents the capacitance Cx of a pair of

0225electrodes 23 and 24 (fig. 6), facing one another and immersed in the

0226liquid 25, the level of which in a tank 26 has to be measured. The

0227capacitance Cx varies depending on the level of the liquid in the tank 26,

0228as a portion of the electrodes remains uncovered and therefore the

0229dielectric constant which defines the capacitance Cx varies as a result.

0230Finally the circuit 27 of fig. 7 is a detecting circuit equivalent to that of

0231figure 5, in which the resistance Re and the capacitor 22 are changed

0232over, should the voltage drop Ve be measured by floating input

0233equipment, without earth reference.

0234It remains understood that changes, additions or component part

0235substitutions may be made to the system for detecting the level of a liquid in a tank, according to this invention, without departing from the scope of

0236this invention.