Method and device for measuring resistance of resistive sensor using an actively controlled resistor network

Abstract

The present disclosure refers to a method of measuring resistance of the resistive sensor (5), where the value of resistance of the resistive sensor (5) is determined from its series connection with actively controlled resistor network (3) with selectable value of resistance and with a periodic waveform voltage source (7), and further a device for measuring resistance of the resistive sensor (5) including a periodic waveform voltage source (7), actively controlled resistor network (3), and a resistive sensor (5), wherein the terminals of the periodic waveform voltage source (7) are connected to the first node (2) and the third node (6), terminals of actively controlled resistor network (3) are connected to the first node (2) and the second node (4), and terminals of the resistive sensor are connected to the second node (4) and the third node (6), thus forming a connection in a resistive voltage divider with an automatic selection of one resistor of the divider, and usage of this method for measuring time-varying resistance of the sensor.

Claims

1. A device for measuring resistance of a resistive sensor including a periodic waveform voltage source having two terminals, an actively controlled resistor network having two terminals, and a resistive sensor having two terminals wherein the first terminal of the periodic waveform voltage source is connected to a first node and the second terminal of the periodic waveform voltage source is connected to a third node, the first terminal of the actively controlled resistor network is connected to the first node and the second terminal of the actively controlled resistor network is connected to a second node, and the first terminal of the resistive sensor is connected to the second node and the second terminal of the resistive sensor is connected to the third node, the device is further comprising an operational amplifier, wherein the non-inverting input of the operational amplifier is connected to the second node and the output of the operational amplifier is connected to an inverting input, the actively controlled resistor network comprises at least one branch comprising at least a switching transistor, a branch resistor connected in series with a switching transistor, one terminal of said branch resistor being connected to the first node, the branch further comprising a first auxiliary switching transistor, a second auxiliary switching transistor, an auxiliary resistor connecting the DRAIN electrodes of the first auxiliary switching transistor and the second auxiliary switching transistor with the GATE electrode of the switching transistor, a second auxiliary resistor connecting the SOURCE electrode of the first auxiliary switching transistor to an auxiliary potential, an auxiliary capacitor connecting the SOURCE electrodes of the first auxiliary switching transistor and the second auxiliary switching transistor, wherein the GATE electrodes of the first auxiliary switching transistor and the second auxiliary switching transistor are at the same potential, wherein the output of the operational amplifier is superimposed on the GATE electrode of the switching transistor, SOURCE electrode of the second auxiliary transistor and via the auxiliary capacitor on the SOURCE electrode of the second auxiliary transistor, wherein the voltage of the periodic waveform voltage source has, throughout the whole duration of the period, lower value than the threshold voltage of switching transistors.

2. The device according to claim 1 wherein said device contains at least two branches of the actively controlled resistor network.

3. The device according to claim 1 wherein the actively controlled resistor network contains a permanently connected branch resistor having two terminals, wherein the first terminal is connected to the first node and the second terminal is connected to the second node.

4. The device according to claim 3 wherein said device contains at least one branch of the actively controlled resistor network connected in parallel to the permanently connected branch resistor.

5. The device according to claim 3 wherein an input impedance of the operational amplifier is higher than the value of resistance of the permanently connected branch resistor.

6. The device according to claim 1 wherein the values of resistance of branch resistors of branches of the actively controlled resistor network form geometric series.

7. The device according to claim 1 wherein the branch of the actively controlled resistor network contains at least two switching transistors connected in series.

8. The device according to claim 1 wherein a second branch of the actively controlled resistor network with the branch resistor is connected in parallel to the branch resistor of the first branch of the actively controlled resistor network.

9. The device according to claim 1 wherein the resistive sensor and the actively controlled resistor network are connected by a shielded cable.

10. The device according to claim 9 wherein the non-inverting input of the operational amplifier is connected with the shielded cable and the output of the operational amplifier is connected to a shielding layer of the shielded cable.

11. A method of measuring resistance of a resistive sensor with the device according to claim 1 wherein the value of resistance of the resistive sensor is determined by applying a periodical voltage waveform between the first node and the third node by the periodic waveform voltage source, wherein the voltage of the periodic waveform voltage source has, throughout the whole duration of the period, lower value than the threshold voltage of switching transistors, measuring a potential at the second node, wherein the measured potential is a function of a resistance of the resistive sensor.

12. The method of measuring resistance of the resistive sensor according to claim 11 wherein the voltage of the periodic waveform voltage source is in the form of a harmonic signal.

13. The method of measuring resistance of the resistive sensor according to claim 11 wherein the voltage of the periodic waveform voltage source is in the form of a rectangular signal or sawtooth signal or another periodical signal waveform.

14. The method of measuring resistance of the resistive sensor according to claim 11 wherein the output of the operational amplifier is filtered by analog or digital filter passing only the frequency of the periodic waveform voltage source.

Description

DESCRIPTION OF THE DRAWINGS

(1) The invention is further clarified by means of exemplary embodiments, which are described by means of the attached drawings, where:

(2) FIG. 1—is a schematic view of the connection of the resistive sensor into the voltage divider,

(3) FIG. 2—is a dependency of a minimal detectable percentage change of resistance of the sensor on the ratio of resistances of the divider resistor and the sensor according to the relation (6),

(4) FIG. 3—is a schematic view of the embodiment of the invention,

(5) FIG. 4—is a schematic view of possible mutual arrangement of two branches of the actively controlled resistor network with a resistor (R) and a unipolar transistor (T),

(6) FIG. 5—is an electric circuit diagram of a possible embodiment of the invention,

(7) FIG. 6—is an electric circuit diagram of another possible embodiment of the invention,

(8) FIG. 7—is a schematic view of the connection of the actively controlled resistor network branch.

EXEMPLARY EMBODIMENT OF THE INVENTION

(9) The basis of the invention is a voltage divider formed by the connection of the actively controlled resistor network 3 with two terminals between the first node 2 and the second node 4, and a sensor 5 between the second node 4 and the third node 6, see FIG. 3. The actively controlled resistor network 3 includes an arrangement of two or more branches 10 that contain an element with defined resistance (resistor) and a switching element, which is a unipolar transistor. Preferably, the switching element is formed by series connection of multiple switching transistors T1. Two ways of mutual arrangement of two branches 10 in the actively controlled resistor network 3 are apparent from FIG. 4. If a variable arrangement of the resistor and the transistor is possible, it is marked in the diagram by a two-way arrow. Other branches 10 can be added to the actively controlled resistor network 3 in an analogous manner. Connection of a given branch resistor into the divider is carried out by switching on the switching transistors, which are between the branch resistor and the first node 2, or the second node 4. The immediate value of resistance between the first node 2 and the second node 4 is further referred to as R.sub.ACRN.

(10) It is preferred that the values of resistances of branch resistors in individual branches 10 form a geometric series which can provide, by appropriate connection of individual resistors, sufficient resolution of measurement in case there is a change of resistance of the sensor 5 by several orders of magnitude. Further, it is preferred that one branch 10 containing branch resistor R0 with the highest value of resistance (R.sub.ACRN,max) does not contain a switching element and is thus included into the divider permanently. The resistance of switching elements in a switched-off state should be many times higher than the highest value of resistance of resistors in the actively controlled resistor network 3. For this purpose, it is possible to use series connection of multiple switching transistors T1.

(11) Periodic waveform voltage u.sub.in(t) is applied between the first node 2 and the third node 4. It is preferred to define one of the nodes 2, 6 as a common node, or a reference conductor, to consider its potential (V.sub.A, V.sub.C respectively) as being constant or zero (V.sub.ref=V.sub.A=0, or V.sub.ref=V.sub.C=0) and to relate all other potentials to that potential. The waveform of u′ (t) can be harmonic (sine) or other (rectangular, sawtooth, etc.). By means of the operational amplifier 9 connected as voltage follower, the potential waveform in the second node 4 is followed. The input impedance of the operational amplifier 9 should be many times higher than the maximum value of resistance of actively controlled resistor network 3 to prevent undesired load of the divider. The potential waveform at the output of the operational amplifier 9 (v.sub.B(t)) then corresponds to the superposition of (i) applied voltage u.sub.in(t) divided by the divider formed by the resistance of the sensor R.sub.s and the resistance of the actively controlled resistor network 3 R.sub.ACRN and (ii) of potential induced as a result of EMI (v.sub.EMI(t)):

(12) $\begin{array}{cc}{v}_B\left(t\right)=\frac{R_{\mathrm{SET}}}{R_s+R_{\mathrm{SET}}}{u}_{in}\left(t\right)+v_{\mathrm{EMI}}\left(t\right)\mathrm{for}{V}_{ref}=V_A,& \left(7a\right)\\ V_B\left(t\right)=\frac{R_s}{R_s+R_{\mathrm{SET}}}{u}_{in}\left(t\right)+v_{\mathrm{EMI}}\left(t\right)\mathrm{for}{V}_{ref}=V_C.& \left(7b\right)\end{array}$

(13) Parasitic capacitances between GATE and DRAIN or SOURCE electrodes of transistors in the actively controlled resistor network 3 and the capacitance of the shielded cable 8, in case of its usage between the second node 4 and the sensor 5, together with resistors in the divider, form a low-pass filter which can decrease amplitude of the potential v.sub.B(t) depending on the frequency of u.sub.in(t). The elimination of this low-pass filter can be achieved by superposition of v.sub.B(t) on the potential applied on GATE electrodes of switching transistors of the actively controlled resistor network 3 and by applying v.sub.B(t) on the shielding layer of the shielded cable 8, if used. Thus, it is ensured that the parasitic capacitors are charged by a source with low output impedance (operational amplifier 9) and that they do not load the voltage divider. If the potential of the GATE electrode causing switching on of a given transistor is referred as V.sub.G, potential of the GATE electrode applied for the purpose of switching the transistor T1 off and on is referred to as V.sub.off and V.sub.on, respectively, it is necessary to ensure that

(14) V.sub.on+V.sub.B(t)>V.sub.G{circumflex over ( )}V.sub.off+v.sub.B(t)<V.sub.G for transistors with N-channel

(15) V.sub.on+v.sub.B(t)<V.sub.G{circumflex over ( )}V.sub.off+v.sub.B(t)>V.sub.G for transistors with P-channel.

(16) Conditions mentioned above can be fulfilled by the selection of sufficiently low amplitude of u.sub.in(t) and, in case of strong interference, the output of operational amplifier 9 can be subjected to filtration by an analog filter passing fully the frequency of u.sub.in(t) and suppressing EMI frequencies.

(17) A necessary prerequisite for determining the resistance of the sensor is elimination of the effect of EMI. The output of the operational amplifier 9 filtered by an analog or digital filter (v.sub.B,f(t)) corresponds to the potential waveform in the second node 4 modified by the transfer function H of a given filter, i.e. after being introduced to the relations (7a) or (7b):

(18) $\begin{array}{cc}{v}_{B,f}\left(t\right)=\frac{R_{\mathrm{ACRN}}}{R_s+R_{\mathrm{ACRN}}}H\left(u_{in}\left(t\right)\right)+H\left(v_{\mathrm{EMI}}\left(t\right)\right)v_{B,f}\left(t\right)=\frac{R_{\mathrm{ACRN}}}{R_s+R_{\mathrm{SACRN}}}H\left(u_{\mathrm{in}}\left(t\right)\right)+H\left(v_{\mathrm{EMI}}\left(t\right)\right)\mathrm{for}{V}_{ref}=V_A& \left(8a\right)\\ v_{B,f}\left(t\right)=\frac{R_S}{R_s+R_{\mathrm{ACRN}}}H\left(u_{in}\left(t\right)\right)+H\left(v_{\mathrm{EMI}}\left(t\right)\right)v_{B,f}\left(t\right)=\frac{R_S}{R_s+R_{\mathrm{ACRN}}}H\left(u_{\mathrm{in}}\left(t\right)\right)+H\left(v_{\mathrm{EMI}}\left(t\right)\right)f\mathrm{or}{V}_{ref}=V_C& \left(8b\right)\end{array}$
it is preferred to select the filter so that H(v.sub.EMI/(t))=0 (elimination of effect of EMI); the following then applies:

(19) $\begin{array}{cc}{v}_{B,f}\left(t\right)=\frac{R_{\mathrm{ACRN}}}{R_s+R_{\mathrm{ACRN}}}H\left(u_{in}\left(t\right)\right)\mathrm{for}{V}_{ref}=V_A& \left(9a\right)\\ v_{B,f}\left(t\right)=\frac{R_S}{R_s+R_{\mathrm{ACRN}}}H\left(u_{in}\left(t\right)\right)\mathrm{for}{V}_{ref}=V_C& \left(9b\right)\end{array}$

(20) On the basis of the relation (9a) or (9b), the knowledge of transmitting function of the filter H, and the currently set value of resistance of the actively controlled resistor network 3 R.sub.ACRN, it is possible to determine the resistance of the sensor 5 R.sub.s.

(21) An example of an embodiment of the invention is in FIG. 5. A harmonic voltage waveform with the amplitude of 300 mV and the frequency of 55 Hz is applied on the voltage divider formed by the actively controlled resistor network 3 and the resistor R.sub.s (it simulates resistance of the sensor 5). The potential between the actively controlled resistor network 3 and R.sub.s (with all potentials referred to the common node) is followed by the operational amplifier 9 with FET inputs connected as a voltage follower. The actively controlled resistor network 3 contains branch resistors R0-R3 connected in the way according to FIG. 4b. Triplets of switching MOSFET transistors T1-T3 connect the corresponding branch resistors R1-R3 in parallel to the permanently connected branch resistor R0 and thus change the resistance of actively controlled resistor network 3 of parallel resistors. The potential applied to GATE electrodes of switching transistors T1 is selected by the potential applied on GATE electrodes of the auxiliary switching transistors TX1N and TX1P. If the common potential G1 of GATE electrodes of the first and the second auxiliary switching transistor TX1N and TX1P is higher than the threshold voltage of these auxiliary switching transistors (e.g. G1=+5 V), the second auxiliary switching transistor TX1N is opened and the first auxiliary switching transistor TX1P is closed. Then, the potential on GATE electrodes of switching transistors T1 has directly the output value of the operational amplifier 9 which can take up the value between +150 and −150 mV (voltage applied on the divider). These values are lower than the threshold voltage of switching transistors T1, which are thus closed, and the resistance of actively controlled resistor network 3 is determined by the value of resistance of the permanently connected resistor R0.

(22) In case that the common potential G1 of GATE electrodes of the first and the second switching transistor TX1N and TX1P is lower than the threshold voltage of these auxiliary switching transistors (e.g. 0 V), the second auxiliary switching transistor TX1N is closed and the first auxiliary switching transistor TX1P is opened. Then, the potential on GATE electrodes of switching transistors T1 has a DC component UP1 and an AC component determined by the output of the operational amplifier 9 which is provided by the second auxiliary resistor RX1 and the auxiliary capacitor CX1; an appropriate potential UP1 is selected (e.g. +5 V) so that the potential on GATE electrodes of switching transistors T1 has, in every time point, a higher value than the threshold value of the switching transistors T1 which are therefore opened. The resistance of actively controlled resistor network 3 is determined by a parallel connection of the permanently connected branch resistor R0 and the branch resistor R1.

(23) In an entirely analogous way, the branch resistors R2 and R3 are connected through common potentials of GATE electrodes of auxiliary switching transistors G2 or G3. To connect the branch resistor R2, it is necessary to connect the branch resistor R1 as well; analogously, to connect the branch resistor R3, it is necessary to connect the branch resistors R1 and R2 as well. The resistance of actively controlled resistor network 3 for values of resistance of branch resistors R0-R3 shown in FIG. 5 depending on common potentials of GATE electrodes G1-G3 of the auxiliary switching transistors is shown in Table 1.

(24) TABLE-US-00001 TABLE 1 G1 (V) G2 (V) G3 (V) R.sub.ACRN +5 +5 +5 R.sub.0 = 100 MΩ 0 +5 +5 R.sub.0||R.sub.1 = 9,09 MΩ 0 0 +5 R.sub.0||R.sub.1||R.sub.2 = 1,06 MΩ 0 0 0 R.sub.0||R.sub.1||R.sub.2||R.sub.3 = 91,4 kΩ
Another option of the connection is shown in FIG. 6. Actively controlled resistor network 3 contains branch resistors R0-R3 connected according to FIG. 4a. Connection of the branch resistors is achieved by the method described above by means of common potentials of GATE electrodes of the auxiliary switching transistors G1-G3. Unlike in the previous arrangement (FIG. 5), it is possible to connect individual branch resistors independently of the connection of the other branch resistors. Resistance of actively controlled resistor network 3 depending on the common potentials of GATE electrodes of the auxiliary switching transistors G1-G3 is, similarly as in the previous case, determined as the resistance of parallel connection of the connected branch resistors.

(25) The shielded cable 8 is used to connect non-inverting input of the operational amplifier 9 and the resistive sensor 5. The output of the operational amplifier 9 is applied on the shielding layer of the shielded cable 8 for the purpose of suppressing the effect of capacitance of the shielded cable 8.

LIST OF REFERENCE SIGNS

(26) 1—Reference Conductor 2—First Node 3—Actively controlled resistor network 4—Second Node 5—Resistive Sensor 6—Third Node 7—_Periodic Waveform Voltage Source 8—Shielded Cable 9—Operational Amplifier 10—Branch of Actively Controlled Resistor Network RD—Resistance of Divider R0—Permanently Connected Branch Resistor R1—Branch Resistor of the First Branch of actively controlled resistor network R2—Branch Resistor of the Second Branch of actively controlled resistor network R3—Branch Resistor of the Third Branch of actively controlled resistor network T1—Switching Transistor MOSFET of the First Branch of actively controlled resistor network T2—Switching Transistor MOSFET of the Second Branch of actively controlled resistor network T3—Switching Transistor MOSFET of the Third Branch of actively controlled resistor network RP1—First Auxiliary Resistor of the First Branch of actively controlled resistor network RP2—First Auxiliary Resistor of the Second Branch of actively controlled resistor network RP3—First Auxiliary Resistor of the Third Branch of actively controlled resistor network TX1P—First Auxiliary Switching Transistor MOSFET of the First Branch of actively controlled resistor network TX1N—Second Auxiliary Switching Transistor MOSFET of the First Branch of actively controlled resistor network TX2P—First Auxiliary Switching Transistor MOSFET of the Second Branch of actively controlled resistor network TX2N—Second Auxiliary Switching Transistor MOSFET of the Second Branch of actively controlled resistor network TX3P—First Auxiliary Switching Transistor MOSFET of the Third Branch of actively controlled resistor network TX3N—Second Auxiliary Switching Transistor MOSFET of the Third Branch of actively controlled resistor network G1—Common Potential of GATE Electrodes of the First and Second Auxiliary Switching Transistors of the First Branch of actively controlled resistor network G2—Common Potential of GATE Electrodes of the First and Second Auxiliary Switching Transistors of the Second Branch of actively controlled resistor network G3—Common Potential of GATE Electrodes of the First and Second Auxiliary Switching Transistors of the Third Branch of actively controlled resistor network RX1—Second Auxiliary Resistor of the First Branch of actively controlled resistor network RX2—Second Auxiliary Resistor of the Second Branch of actively controlled resistor network RX3—Second Auxiliary Resistor of the Third Branch of actively controlled resistor network CX1—Auxiliary Capacitor of the First Branch of actively controlled resistor network CX2—Auxiliary Capacitor of the Second Branch of actively controlled resistor network CX3—Auxiliary Capacitor of the Third Branch of actively controlled resistor network UP1—Auxiliary Potential of the First Branch of actively controlled resistor network UP2—Auxiliary Potential of the Second Branch of actively controlled resistor network UP3—Auxiliary Potential of the Third Branch of actively controlled resistor network