A device for measuring physical parameters

Abstract: A device for measuring physical parameters, in particular suitable for measuring temperature, comprising a probe including a sensor element capable of converting the value to be measured into an electrical signal, said probe including a receiving circuit in which an AC voltage can be induced by an electromagnetic interrogation field which, in operation, is generated by a transmitter/receiver forming part of the measuring device, which AC voltage can be modulated depending on the measured value of the physical parameter, and wherein the probe includes a digital code circuit in which an identification code is stored, as well as a substantially digital measuring circuit connected to the sensor element, the arrangement being such that, in operation, the code circuit and the measuring circuit successively, through a switch element, modulate the AC voltage induced in the receiver circuit with a binary signal.

Description: The invention relates to a device for measuring physical parameters, in particular suitable for measuring temperature, comprising a probe including a sensor element capable of converting the value to be measured into an electrical signal. Temperatures are measured in known manner, for example, by means of mercury and alcohol thermometers. Furthermore, electronic thermometers are also known. The disadvantages of the known thermometers are that the device with which the temperature is measured is directly connected to the appurtenant read-out means, such as a reading scale or dial, a display, and the like. When temperatures of human beings, animals or goods are to be measured, there should always be a fixed contact between the measuring device within the body or good, and the read-out device outside the body or good. Furthermore, when the temperature is determined electronically, a power source should be present directly at or within the temperature sensor, which provides the circuit with power. It is accordingly not always possible to use such measuring devices. From the journal PT Elektrotechniek Elektronica 39 (1984) Nr. 7/8 a passive probe is known for wirelessly measuring temperature. This known probe comprises a resonant circuit which can be brought into resonance by a high-frequency interrogation field. Coupled to the resonant circuit is an RC oscillator which comprises a temperature-dependent resistor. The temperature-dependent resistor determines the oscillating frequency of the RC oscillator. If the probe is present within an interrogation field generated by a transmitter/receiver of an interrogation device, the probe is activated. The resonant circuit gets into the resonant state and the RC oscillator begins to oscillate at a frequency depending on the ambient temperature of the temperature-sensitive resistor. The signal from the RC oscillator modulates the high-frequency signal induced in the resonant circuit in phase, which is detected in the transmitter/receiver. One disadvantage of this known technique is that the measured values obtained become available in the form of an analogue signal, which is rather sensitive to interference and also cannot be processed by a digital processor without more ado. There is accordingly a need for a measuring device comprising a wirelessly operating probe providing binary measuring signals Furthermore, there is a need for a probe which, in addition to measuring signals can provide binary identification signals to identify the wearer (human being, animal, article, apparatus or any part thereof) of the probe. This latter is of particular importance if a plurality of probes are used, with each probe being fixedly arranged within or at a different wearer or different part of the same wearer. It is an object of the invention to satisfy the need outlined. To this effect, the object of the invention is met by the device according to claim 1. It is observed that Netherlands patent 176404 discloses a passive electronic responder comprising a receiver circuit and a digital circuit, which in operation receives supply energy from a rectifying circuit connected to the receiver circuit. The digital circuit further provides, in operation, a binary identification signal which, through a switch element, can modulate the AC voltage induced in the receiver circuit, by periodically attenuating the receiver circuit. US-A-4 075 632 relates to a telemetering system comprising a interrogator/generator which generates at least one single rf signal, a transponder for receiving that signal and for amplitude modulating it in accordance with information selected for transmitting it back to the interrogator. Such information can comprise a measuring signal and a digitally coded signal for identifying the transponder whereby said signals are simultaneously superimposed on the rf carrier of the rf signal. The known responder does not, however, comprise a sensor element and a digital measuring circuit and accordingly can not be used as a measuring probe. The invention will be described in more detail hereinafter with reference to the accompanying drawings. Fig. 1 diagrammatically shows a measuring device according to the invention; Fig. 2 shows diagrammatically the electrical diagram of one embodiment of a probe for a measuring device according to the invention; Fig. 3 shows, in greater detail, an example of an electrical diagram of a measuring section of a measuring sensor for a measuring device according to the invention; Fig. 4 illustrates in a few diagrams the variation in the time of a plurality of signals occurring in the measuring sensor of Fig. 3; and Fig. 5 and Fig. 6 diagrammatically show a detail of the measuring section of a temperature measuring sensor according to the invention. Fig. 1 shows diagrammatically an example of a measuring device according to the invention with a transmitter/receiver 1 and a measuring probe 2. In operation, the transmitter/receiver generates, through a transmitting coil 3, an electromagnetic interrogation field. If a probe 2 is present in the interrogation field, which probe has a receiving circuit 4 with a coil L, a mutual inductive coupling is established, as indicated by a double arrow M. In the example shown, the receiver circuit 4 comprises a resonant circuit LC tuned to the frequency of the interrogation field. The receiver circuit can draw energy from the interrogation field in the manner described, for example, in Netherlands patent 176404, which, through a rectifying circuit 5 can be supplied as supply power to an active digital circuit 6 of the passive probe. Furthermore, a clock signal generator 9 is shown, which in its simplest form may be a one-sided rectifier, but also, for example, a flipflop. In operation, the digital circuit provides a binary output signal which controls a switch element 7. In this way the switch element 7 attenuates and/or detunes the resonant circuit LC in the rhythm of the binary output signal, which can be detected at the transmitter/receiver end. The operating principle described above is described in more detail in Netherlands patent 176404. According to the invention, a sensor element 8 is connected to digital circuit 6, the electrical value of which depends upon a physical parameter to be measured. The sensor element may, for example, be a temperature-dependent resistor, such as an NTC resistor (= negative temperature coefficient resistor) or a pressure-dependent resistor, or a light-dependent resistor (LDR) and the like. The choice of the sensor element depends upon the nature of the physical parameter to be measured. In the embodiment of Figs. 2 and 3, the sensor element comprises an NTC resistor, so that the probe is suitable for measuring the temperature. The temperature or other physical parameter measured is converted by the digital circuit into a binary signal, which can be supplied as a control signal to switch element 7. The digital circuit further preferably comprises a pre-determined binary identification code which, before or after the binary measuring signal, is supplied to the switch element 7 as a control signal. If identification is not necessary or not desirable, a code consisting exclusively of noughts or ones can be programmed, so that the signal ultimately generated by the probe and received by the transmitter/receiver only represents the measured value. With regard to the use of an NTC resistor, the following is further observed. The resistance of an NTC resistor depends according to an exponential function upon the temperature. When a temperature-independent resistor (R off = offset resistor) is series-connected to an NTC resistor, a substantially linear relationship between temperature and resistance can be obtained through a limited temperature range of about 15°C. The error resulting in this temperature range is negligible if the error permitted is ± 0.1°C, which in practice is mostly the case. As a result of the linear relationship between temperature and the combination of the two resistors, it is possible for the absolute temperature to be accurately determined in the limited temperature range. When this (absolute) temperature is measured, the whole of R off and R ntc is compared to a reference resistance R ref . When the NTC resistance and the reference resistance are properly selected, the measuring range of the thermometer is adjustable throughout a large temperature range. In the desired measuring range, preferably in the middle thereof, the NTC resistance value equals the reference resistance at a temperature x. Furthermore, the conversion factor B from temperature to resistance should be accurately fixed. B is a material constant selected during the manufacture of the NTC resistor. If, during the manufacturing process, the absolute value of both the NTC resistance and the reference resistance only has a slight tolerance, the resistors need not be adjusted. The electronic circuit of the measuring probe has the task of comparing the NTC resistance and the reference resistance with each other. The circuit can function with the available power of about 10 µW, which is withdrawn from the interrogation field. The offset resistance R off is in practice only a calculating magnitude and R off is not present in the actual thermometer. During a reset period, by means of the reference resistor an offset voltage is generated and stored in a memory. This voltage has the same value as the voltage across an imaginary offset resistor during the measuring period. Subsequently this voltage is electronically added to the voltage across the NTC resistor. The result ultimately is that the remainder of the circuit responds equally to a real series circuit of NTC and offset resistors. Fig. 2 shows diagrammatically in the form of a block diagram the construction of a temperature measuring probe according to the invention. If the probe is present in an interrogation field, the receiver circuit 4 again provides, through a rectifying circuit 5, supply voltage V DD for the active components of the probe. Furthermore, there is again shown a clock signal generator 9, which provides clock signals to code circuit 10, which contains the identification code of the probe. A measuring circuit 11, which will be described in more detail hereinafter, is connected to an NTC resistor R ntc for measuring the temperature and with a constant current source I o , which through the measuring circuit 11 is alternately connected to the NTC resistor and to a reference resistor R ref . The measuring circuit 11 receives control signals, such as synchronization signals, reset signals, clock signals, and the like, from a control signal generator 12. The control signal generator also controls a switch element 14, whose position determines whether the output of code circuit 10 or the output of measuring circuit 11 provides the control signal for the switch element 7. It is noted that Fig. 2 is highly schematical. Parts of different blocks, for example, may actually be combined. On the other hand, in practice, the circuits shown as one block in Fig. 2 may consist of a plurality of scattered parts. After completion of the measurement, the temperature measured is shown on the data outputs D o of a binary counter IC 1 , indicated in Fig. 3, in binary form. The 7 bits Q 1 to Q 7 indicated may form 2 7 = 128 different figures, so that, with an accuracy of 0.1°C, the measuring range equals 128/10 = 12.8°C. The binary temperature code is subsequently combined in a binary or other suitable code form with a binary identification code in the responder memory. The complete code can then be transmitted through field modulation to a processing unit (transmitter/receiver). The circuit begins to operate after the reset input R has assumed the logic value "0". Under the control of a clock signal CL, the binary counter (IC 1 ) begins to count from the reset value 0. Electronic switches ES4 and ES5 are now closed via AND gate N5, inverters N7, N8 and AND gate N10. Counter outputs Q 8 and Q 9 are in fact "0". Electronic switches ES11 and ES12 are now closed because the two inputs of OR gate N4 are "0", so that the output of inverter N9, which controls switches ES11, ES12 is "1". There is now a current I o , provided by the contstant current sources kI o and (1-k)I o through resistor R ref . This causes a voltage across this resistor, which is transmitted to capacitor C1. Switches ES1 and ES2 are opened and closed in the rhythm of clock CL one after the other through OR gate N3 and inverter N2. As a result capacitor C1 is each time charged through ES1 and discharged through ES2. The charge which thus flows away is Stored in a capacitor C2, across which the voltage increases. Through differential amplifier A2, the voltage across capacitor C2 is copied across a capacitor C3 until counter IC 1 has reached the position 384. In this situation, Q 8 and Q 9 are "1" and ES5 is opened because the output of AND gate N5 becomes "0". The voltage across C3 can then no longer change. Through OR gate N4, AND gate N6, inverter N7 and OR gate N10, the circuit is reset. Capacitors C1 and C2 are thereby discharged through switches ES7, ES8 and ES2. Through switches ES10 and ES13, a circuit is put into operation, with which, by means of a fraction kI o of the total current source I o , a capacitor C4 is charged to a voltage equalling the offset of an amplifier A1 plus R off x I o . The output of OR gate N4 is at that moment "1", so that switches ES10 and ES13 are closed. At the same time, switches ES11 and ES12 are opened. Accordingly, only a current kI o flows through R ref , and capacitor C4 can be charged to a voltage equalling kI o R ref plus the offset voltage of amplifier A1. For a proper operation of the circuit, amplifier A1 must not have any offset. The offset compensation of A1 and the offset generation for proper linearity are combined in one circuit, as a result of which the maximum error in the ultimate value measured will be minimal. To clarify the offset compensation and generation, the following is further observed. During a reset period, capacitor C1 is discharged through switches ES2 and ES7, while capacitor C2 is discharged through switches ES7 and ES8. Capacitor C3 is simultaneously charged through switch ES5 to the offset voltage V off A2 of amplifier A2. Finally, capacitor C4 is also charged through switch ES7, amplifier A1, amplifier or inverter A3, switch ES10, and switch ES13 through the section kI o of the constant current source. The section (1-k)I o of the current source has been put out of operation by switch ES11 at that moment. The part of the measuring circuit just described is shown in Fig. 5. The circuit of Fig. 5 corresponds to that of Fig. 6. Fig. 6 shows that capacitor C4, at the end connected to amplifier A1, carries the offset voltage of amplifier A1V offA2 ,and at the other end a voltage -kI o R ref , so that the voltage across C4 is accordingly V offA1 + kI o R ref . After the reset period, when switch ES1 is closed, capacitor C1 is charged to a voltage V = I o .R ref . Subsequently, ES2 is closed and ES1 opened, so that capacitor C1 is discharged to a voltage V c4 - V offA1 . The voltage across capacitor C2 is increased in each switching cycle of switches ES1 and ES2 by ΔV = V c4 - V offA1 + I o R ref . As during the reset period V c4 was brought to the value V c4 = V offA1 + kI o R ef the offset voltage of amplifier A1 drops in ΔV, so that we have ΔV = (k + 1) I o .R ref . For p switching cycles, we have V c2 = p. C1 C2 (k + 1) I o R ref . For practical reasons, the value 384 has been selected for p, which corresponds to a counter position in which Q 8 and Q 9 are "1". When switch ES5 is closed, the voltage across capacitor C3 becomes equal to V c3 = V c2 + V offA2 . In the next reset period, V c3 remains constant, and capacitor C4 is again charged to V c4 = V offA1 + k.I o R ref . During the measuring period, R ref is replaced by R ntc . In that period, a voltage V' c2 is built up, for which we have V' c2 = V c2 + V offA2 . As soon as V' c2 has become equal to V c3 ,the measurement is terminated. When V' c2 is compared to V c3 , the offset voltage V offA2 is again cancelled. Ultimately, a comparison is made of p. (R ref + k R ref ) with n. (R ntc + k R ref ) as a result of which, in spite of an exponential relationship between the resistance value of an NTC resistor and the temperature, there is yet obtained a substantially linear relationship between n (the counter position at the end of the measurement) and T through a sufficiently large temperature range around a reference temperature T ref , with R ntc = R ref . In position 512 of counter IC 1 , the reset period terminates, and the actual measurement starts. Switches ES3 and ES6 are now closed, while switch ES4 is opened. Switch ES11 is again closed. As a consequence, the total current I o travels through R ntc instead of R ref , as a result of which the voltage across C1 may have a different value than during the counter position 0-384. This has consequences for the velocity at which capacitor C2 will be charged. For temperatures above T ref , R ntc is lower than R ref . As a result V c1 is lower, and more than 384 clock pulses are needed before V c2 = V c3 . If now V c2 = V c3 , the amplifier A2, which is connected as a comparator, reverses, and via switch ES6 stops the counter. The counter position now reached depends on the temperature. After deducting the starting value of 512, at temperatures lower than T ref the counter position will be lower than 384, while for temperatures upwards of T ref the counter position will exactly be higher than 384. In the following table, the course of the measuring process and the relevant counter and switch positions are represented in full and in sequence. The process begins with a reset of counter IC 1 . The position of the electronic switches is indicated by "1" for closed and "0" for open. Condition Position IC 1 Electronic Switch Nr. 1 2 3 4 5 6 7 8 9 10 11 12 13 RESET 0 0 1 0 1 1 0 1 1 0 1 0 0 1 1 0 1 0 0 1 1 0 0 0 1 0 1 1 0 1a 0 0 1 0 1 1 0 0 0 1 0 1 1 0 2 1-383 1 0 0 1 1 0 0 0 1 0 1 1 0 2a 1-383 0 1 0 1 1 0 0 0 1 0 1 1 0 3 384-511 0 1 0 1 0 0 1 1 0 1 0 0 1 3a 384-511 0 1 0 1 0 0 1 1 0 1 0 0 1 4 512-... 1 0 1 0 0 1 0 0 1 0 1 1 0 4a 512-... 0 1 1 0 0 1 0 0 1 0 1 1 0 Condition RESET: Start of each measuring cycle. Capacitors C1, C2 and C3 are discharged. Capacitor C4 is charged to the voltage described before, which is the sum of the offset voltage of amplifier A1 and the voltage across the imaginary offset resistor. Condition 1: This is the condition immediately after the reset signal R drops off. The position of switches ES1 and ES2 is dependent upon the phase of the clock signal, so that, just as at counter positions 1 to 383, there are alternate switches from condition 1 to 1a and from 1a to 1. The same applies to conditions 2 and 2a. Condition 2: This is the condition at which voltages across C2 and C3 are generated by means of the voltage generated across R ref and R off . When the counter position has passed value 383, C3 will retain the voltage across R ref + the imaginary R off 384 times. Condition 3: This is the condition in which a condition resembling the RESET situation is reached with the only difference that the C3 is not discharged. All switches, except ES5, are reset. The counter, however, is not reset. The voltage across C3 will be used during the second part or the measuring cycle (512-...) as a comparative value for the voltage across C2. The voltage across C2 in that period will be obtained as the result of repeated addition of the voltage across R ntc + R off instead of that across R ref + R off . Condition 4: This is a condition which largely corresponds to condition 2. Instead of R ref , however, resistor R ntc is connected to the constant current source of value I o . Switch ES5 is still open and switch ES6 is closed, so that in the condition: V c2 = V c3 comparator A2 will reverse and further counting will become impossible. In that final situation, depending on the temperature, the counter position will range between 832 and 959. The counter position minus position (512) at the beginning of condition 3 now forms a binary representation of the temperature of the NTC resistor, and, with or without further processing, may be transmitted to the transmitter/receiver similarly to the identification code. It is noted that the voltage at the node: ES1, ES3 and ES4, which is the result of I o x R ntc or I o x R ref is negative in this example relative to mass! In Fig. 4, some voltage configurations are shown as a function of the time of a number of important signals in the temperature meter. Fig. 4a shows the position of counter IC 1 which increases by increments of 250 µs from the starting value 0 to a certain value, which value depends on the temperature. The numbers presented in decimals in Fig. 2a are actually binary figures indicated by outputs Q 1 to Q 10 of IC 1 . Fig. 4b shows the reset signal provided by external logic. The value of the reset signal determines the moment when measurement starts. In this figure, the measurement begins at the transition of reset value 1 to 0. Fig. 4c shows the voltage, and the variation therein, across capacitor C1. At R ref , this voltage is temperature-independent. At R ntc , the voltage is a function of the temperature. Fig. 4d shows the voltage variation across capacitor C2 as a function of the position of counter IC 1 . The slope of the voltage curve in the period associated with counter positions between 0 and 384 is determined by the reference resistance and is hence independent of the temperature. The voltage across C3, shown diagrammatically in Fig. 4e, follows V c2 from counter position 0 to 384, whereafter the voltage is fixed. The NTC resistor provides curve V c2 between counter position 512 and counter position 832....959. The moment when the measurement stops and the counter position between 832 and 959 then reached, depend upon the NTC resistance value and hence upon the temperature. The only criterion applying to termination is that voltages V c2 and V c3 should be equal. When the voltages V c2 and V c3 become equal, the amplifier A2 operating as a comparator provides a positive output signal, as a result of which AND in Fig. 2f assumes a high level and the counter stops. The counter position reached between 832 and 959 is a measure for the temperature measured. When the counter does not stop in the range indicated, the value of the counter indicated is left out of account as a temperature. As an example for an electronic transducer with a measuring circuit, an NTC resistor has been used for determining temperatures. The exponential character of this resistor is used in the measuring circuit. If a transducer is desired for, for example, measuring light intensity, a light-dependent resistor (LDR) may be used. Other non-linear resistors may be used as well. It is then necessary for the measuring circuit to be adapted, without, however, changing the essential features of the apparatus. This is the case, for example, with pressure-dependent resistors or electrolytic solutions which begin to behave as a resistor. The circuits described can be realized by the low-voltage MOS IC technique.

Claims: A device for measuring physical parameters, in particular suitable for measuring temperature, comprising a probe (2) including a sensor element (8) capable of converting the value to be measured into an electrical signal and a digital code circuit (6,10), in which an identification code is stored, said probe including a receiving circuit (4) in which an AC voltage can be induced by an electromagnetic field which, in operation, is generated by a transmitter/receiver(1) forming part of the measuring device, which AC voltage can be modulated depending on the measured value of the physical parameter and a digital identification code generated by the digital code circuit, characterised in that the receiving circuit (4) comprises a resonant circuit tuned to the frequency of the electromagnetic field, the probe (2) further comprises a substantially digital measuring circuit (6) connected to the sensor element (8) for generating a binary signal representing the value to be measured and the digital identification code generated by the digital code circuit is a binary identification code, wherein, in operation, the code circuit (6,10) and the measuring circuit (6,11) successively, through a switch element (7), modulate the AC voltage induced in the resonant circuit (4) with the binary signal and the binary identification code respectively. A measuring device as claimed in claim 1, in which the sensor element (8) as a resistance value depending on the physical parameter to be measured, characterized in that the digital measuring circuit (6,11) comprises first controllable switches (ES4,ES3) which, in operation, successively connect a predetermined reference resistor (Rref) and the sensor element (8) with a constant current source (lo); that the digital measuring circuit (6,11) has second controllable switches (ES1,ES2) via which a first capacitor (C1), is repeatedly charged a predetermined number of times from the voltage provided over the reference resistor (Rref) in a first period when the reference resistor (Rref) is connected to the current source, which charges are stored in a second capacitor (C2) in order to charge said second capacitor (C2) in a predetermined number of increments corresponding to said predetermined number of times and in a second period, through the first capacitor (C1), in a number of corresponding increments, charge the second capacitor (C2) from the voltage provided by the sensor element (8) when the sensor element (8) is connected to the current source (lo), wherein the second capacitor (C2) is discharged between the first and the second period, and the number of increments needed in the second period for charging the second capacitor (C2) to the same voltage as in the first period is counted by means of a counter (IC1) and provides a measure for the parameter to be measured. A measuring device as claimed in claim 2, characterized in that the digital measuring circuit (6,11) comprises a binary counter (IC1) to which, in operation, clock pulses (CL) are supplied continuously, and whose outputs (Q8-Q10) control the controllable switches, while also at least a number of outputs (PO) of the binary counter indicate the number of increments of the second period. A measuring device as claimed in claim 2 or 3, characterized by a third capacitor (C3) which during the first period is connected to the second capacitor (C2) and takes over the charge from said second capacitor (C2), and which by means of a controllable switch (ES5) is uncoupled from said second capacitor (C2) at the end of the first period. A measuring device as claimed in claim 4, characterized in that the second and third capacitors (C2,C3) are each connected during the second period to an input of a comparator (A2), the output of which is connected to the counter (IC1), said comparator providing an output signal as soon as the voltage at the second capacitor (C2) equals the voltage at the third capacitor (C3) which output signal causes the binary counter (IC1) to stop. A measuring device as claimed in any of claims 2-5, adapted to measure temperatures by means of an NTC resistor (Rntc), characterized by a fourth capacitor (C4) which before the beginning of the first and between the first and the second period is connected to a part (klo) of the constant current source (lo) through controllable switches (ES12,ES13), the voltage generated across the fourth capacitor (C4) increasing the measuring accuracy and linearising the NTC resistor (Rntc).

Inventors: STEKELENBURG JAN CORNELIS, STEKELENBURG, JAN CORNELIS
Applicants: NEDAP NV, N.V. NEDERLANDSCHE APPARATENFABRIEK NEDAP
Application year: 1990
Main classification: G01K1
IPC codes: G01K7, G01K1