Inductive sensor circuit with coil resistance compensation

A sensor circuit has coils 3a, 3b whose inductances change in response to a change in a physical quantity. A drive circuit 4A applies an a.c. drive voltage to the coils. A voltage detector circuit 5 senses the voltage across each coil. A current integration circuit 10 integrates the current through each coil starting at the time of a polarity inversion of the drive voltage, and outputs a control signal C1, C2 until the time the integrated value of the coil current becomes zero. A phase detector circuit 6A detects the coil voltage to generate a detection voltage Vd during the time the current integration circuit outputs the control signal. A smoothing circuit 7 processes the detection voltage to output a mean voltage Vm. The sensor circuit eliminates any adverse effects of the internal resistances of the coils by inverting and offsetting the resistive component of the coil voltage during the detection period.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a circuit that measures a physical quantity by 
detecting minute changes in the inductance of a coil used in such an 
apparatus as a magnetostrictive torque sensor, etc., and in particular, to 
a sensor circuit that can offset the effect of coil internal resistance, 
to perform high precision sensing of a physical quantity. 
2. Description of the Related Art 
An example of a sensor circuit for measuring a physical quantity by 
detecting minute changes in the inductance of a coil is described in the 
paper "Magnetostrictive Torque Sensor" by Yoshihiko UTSUI, et al published 
in the transactions of the magnetics society of the Institute of 
Electrical Engineers of Japan (reference number MAG-88-158, Oct. 11, 1988) 
FIG. 3 is a block diagram of the conventional sensor circuit described in 
the above article (FIG. 4 in the referenced publication), and FIG. 4 shows 
the main parts of FIG. 3 (FIG. 5 in the referenced publication). 
In FIG. 3, torque receiver shaft 1 is a rotating shaft, and first and 
second magnetic substances 2a and 2b are magnetostrictive layers affixed 
to the outer circumferential surface of torque receiver shaft 1. 
As shown, first magnetic substance 2a is formed in multiple strips oriented 
at a fixed angle (=45.degree.) relative to the center axis of torque 
receiver shaft 1. Second magnetic substance 2b is separated from first 
magnetic substance 2a, along the length of the shaft, and is formed in 
multiple strips oriented at a fixed angle perpendicular to that of first 
magnetic substance 2a (=-45.degree.). 
First and second coils 3a and 3b are separately positioned outside of and 
facing first and second magnetic substances 2a and 2b, respectively; and 
have inductances L1 and L2, and internal resistances r1 and r2, 
respectively, as shown in FIG. 4. 
In FIG. 4, the first and second coils 3a and 3b, which are the sensor 
circuit coils, are connected in series with each other. Their inductance 
changes in response to a change in some physical quantity (such as a 
torque applied to torque receiver shaft 1). 
In FIG. 3, drive circuit 4, connected across the series-connected 
combination of coils 3a and 3b, applies drive voltage Va to coils 3a and 
3b to produce the coil current i. 
Voltage detector circuit 5 outputs coil voltage Vc based on the voltages V1 
and V2 generated across coils 3a and 3b by the application of drive 
voltage Va; e.g., the gain G times the differential of the voltages 
(V1-V2). 
In response to a control signal C from drive circuit 4, phase detector 6 
inputs coil voltage Vc, and outputs the result as detection voltage Vd. 
Smoothing circuit 7 performs a smoothing process on detection voltage Vd to 
generate, as the final sensor output signal at the output terminal, a d.c. 
mean voltage Vm, responsive to changes in the inductances L1 and L2 of 
coils 3a and 3b, respectively. 
Next, the operation of the conventional sensor circuit of FIGS. 3 and 4, 
with respect to the sensing of a physical quantity (e.g. torque) , will be 
explained, with reference to the waveforms of FIG. 5. 
FIG. 5 shows the changes, over time, of various signal voltages occurring 
in the conventional sensor circuit in a magnetostrictive torque sensing 
system application. As shown, drive voltage Va is a rectangular waveform 
having an a.c. cycle time T. Va periodically switches between positive and 
negative levels centered around 0 V once each T/2 half cycle, a fixed 
time. 
If a torque is applied to torque receiver shaft 1, a principal stress will 
be created in torque receiver shaft 1 in the direction of the two fixed 
angles (=.+-.45.degree.). 
Due to the Villari effect, when this stress is created, the permeability of 
the magnetic strips 2a and 2b changes, with the permeability change being 
in one direction for tensile stress, and in the opposite direction for 
compression stress. 
Accordingly, the inductance of one of the coils 3a and 3b will increase, 
and that of the other coil will decrease. This change in inductances L1 
and L2 will cause a corresponding change in coil voltage Vc, and 
ultimately, in detection voltage Vd and mean voltage Vm. Thus the 
magnitude of the externally applied torque can be known by sensing the 
mean voltage Vm of detection voltage Vd. 
In other words, as shown in FIG. 5, drive circuit 4 generates drive voltage 
rectangular waveform Va across the series combination of first and second 
coils 3a and 3b, causing coil current i to flow. 
Now, if r1 and r2, the internal resistances of coils 3a and 3b, 
respectively, are assumed to be extremely low, then i, the coil current 
flowing in coils 3a and 3b, can be expressed in terms of L1 and L2, the 
inductances of coils 3a and 3b, respectively, by equation (1), below: 
EQU i=.intg.Va.multidot.dt/(L1+L2) (1) 
And voltages V1 and V2 across the terminals of coils 3a and 3b, 
respectively, are given by equations (2) and (3), below: 
##EQU1## 
Voltage detector circuit 5 takes the difference between the above V1 and V2 
voltages (=V1-V2), and outputs a coil voltage Vc equal to the voltage 
differential times the gain G, as expressed in equation (4), below: 
##EQU2## 
As is evident from equation (4), when L1 and L2, the inductances of coil 3a 
and 3b, respectively, are equal, the coil (differential) voltage Vc output 
by voltage detector circuit 5 will be zero, regardless of the value of 
drive voltage Va. If the inductances differ, however, the coil voltage Vc 
derived from the differential voltage will assume some non-zero level. If 
L2 is greater than L1, for example, the phase of the coil voltage Vc 
waveform will be opposite to that of the drive voltage Va, which is the 
case shown in FIG. 5. 
At the same time as coil voltage Vc is being output by voltage detector 
circuit 5, drive circuit 4 outputs control signal C, which is synchronous 
with drive voltage Va (FIG. 5), and is also provided as an input to phase 
detector 6. 
When control signal C is at a low level, phase detector circuit 6 outputs 
coil voltage Vc, as is, as a detection voltage Vd of the same polarity; 
and when control signal C is at a high level, phase detector 6 inverts 
coil voltage Vc, and outputs it as a detection voltage Vd of opposite 
polarity. 
As a result, detection voltage Vd is a d.c. voltage that is directly 
proportional to the difference in the inductances of coils 3a and 3b 
(L2-L1). 
Accordingly, if L2&gt;L1, then detection voltage Vd&gt;0; and if L2&lt;L1, then 
detection voltage Vd&lt;0. In other words, the magnitude and polarity of 
detection voltage Vd are functions of the magnitude and direction, 
respectively, of the physical quantity being sensed. 
Now, since noise is superimposed on detection voltage Vd during phase 
detection, smoothing circuit 7 first performs a smoothing process on 
detection voltage Vd, after which it is output as d.c. mean voltage Vm. 
The above describes the operation of an ideal sensor circuit, where r1 and 
r2, the internal resistances of coils 3a and 3b, are extremely small. 
In actual sensor circuits, however, where small diameter wire is used in 
coils 3a and 3b to reduce physical size, the internal resistances r1 and 
r2 easily become too large to ignore. 
Operation will now be described for a conventional sensor circuit having 
larger internal resistances r1 and r2, referring this time to FIG. 6. 
In FIG. 6, Va, the drive voltage applied to the coils (=V1+V2), is 
separated into its resistive and inductive voltage components, Vr and VL. 
Thus, as shown in FIG. 6, along with drive voltage Va, coil current i, and 
control signal C, we also have Vr, the voltage developed due to the 
internal resistance of the coils (r1+r2), and VL, the voltage developed 
due to the inductance of the coils (L1+L2). 
In this case, the drive voltage Va is the sum of Vr and VL. 
If r1 and r2 (the internal resistances of the coils) can be ignored, as 
they were in the above description (FIG. 5), then the drive voltage Va 
waveform will exactly match the inductive component voltage VL waveform. 
If coil internal resistances r1 and r2 are too large to be ignored, 
however, a resistive component voltage Vr, proportional to coil current i, 
is also developed, and a mismatch will exist between drive voltage Va and 
the inductive component voltage VL. 
The larger r1 and r2 (the coil internal resistances), and the longer the 
a.c. cycle period T of the drive voltage Va, the more conspicuous will be 
this VL/Va mismatch. 
If the a.c. cycle period T is made extremely long, for example, a voltage 
proportional to the coil internal resistance difference (r2-r1) will 
appear in the differential coil voltage Vc, and the sensor circuit will 
thus be incapable of accurately measuring the coil inductances L1 and L2. 
If the internal resistances of the coils are precisely matched (r1=r2), 
this effect of internal resistances r1 and r2 will not appear in the 
differential coil voltage Vc. In a sensor circuit that measures physical 
quantity by sensing variations in inductances L1 and L2, where small 
diameter coil wire is used as discussed earlier, however, significant 
variances in internal resistances r1 and r2 are unavoidable. 
Accordingly, where coils 3a and 3b are mass-produced, it is extremely 
difficult to maintain precisely matched r1 and r2 internal resistances in 
all of the parts produced. 
Also, since the resistance of the coil wire varies with temperature, it is 
extremely difficult to eliminate the effect of internal resistances r1 and 
r2 on the differential coil voltage Vc over the entire temperature range. 
Consequently, an error is produced in the mean voltage Vm obtained as the 
final output, due to the effect of temperature induced variations of the 
internal resistances r1 and r2. 
Conventional sensor circuits such as the one discussed above simply 
detected a differential coil voltage Vc in response to a control signal C 
synchronous with the timing of polarity inversions in drive voltage Va to 
obtain a detection voltage Vd, and also outputted a mean voltage Vm. 
Therefore, when coils having large internal resistances r1 and r2 were 
used, the circuits became incapable of accurately measuring the coil 
inductances L1 and L2, due to the presence of voltage Vr (FIG. 6) 
proportional to coil current i. This problem rendered the circuits 
incapable of accurately sensing a physical quantity. 
The above problem is especially prominent when small diameter coil wire is 
used to reduce physical size. When small wire is used, variances in the 
internal resistances r1 and r2 become larger. Also, resistance varies with 
temperature. These combined factors make it extremely difficult to 
eliminate the effect of internal resistances rl and r2 on coil voltage Vc, 
and an error is therefore produced in the mean voltage Vm, making the 
circuit incapable of performing accurate sensing of a physical quantity. 
SUMMARY OF THE INVENTION 
The present invention was devised to solve the above problem. Its object is 
to obtain a sensor circuit in which there is no error in the mean voltage 
that corresponds to the sensor output, and in which high-precision sensing 
of a physical quantity can therefore be performed even when the coils have 
high internal resistances. 
The foregoing object is achieved by providing a sensor circuit comprising 
coil means whose inductance changes in response to a change in a physical 
quantity; a drive circuit that applies an a.c. drive voltage to the coil 
means; a voltage detector circuit that senses a coil voltage produced at 
the coil means due to the application of the drive voltage; a current 
integration circuit that integrates the current flowing in the coil means 
due to the application of the drive voltage, starting at the time of a 
polarity inversion of the drive voltage, and outputs a control signal 
until the time the integrated value of the coil current becomes zero; a 
phase detector circuit that detects the coil voltage to generate a 
detection voltage during the time the current integration circuit outputs 
the control signal; and a smoothing circuit that performs a smoothing 
process on the detection voltage to output a mean voltage, whereby the 
physical quantity is sensed based on the mean voltage in response to a 
change in the inductance of the coil means. 
In one version of the invention, the drive voltage has a rectangular 
waveform in which the voltage switches between a positive value and a 
negative value at set time intervals; and wherein the control signals 
comprise: a first control signal having a first output period that extends 
from the time at which the drive voltage switches from the positive to the 
negative value to the time at which the integrated current value reaches 
zero; and a second control signal having a second output period that 
extends from the time at which the drive voltage switches from the 
negative to the positive value to the time at which the integrated current 
value reaches zero; and wherein the phase detector outputs, as the 
detector voltage, the coil voltage with its polarity unchanged during 
either one of the first and second output periods, and with its polarity 
inverted, during the other one of the first and second output periods. 
In another version of the invention, the coils comprise: a first coil whose 
inductance changes in response to the change in the physical quantity; and 
a second coil connected in series to the first coil and having an 
inductance which changes in an opposite relation to the change of the 
first coil in response to a change in the physical quantity; wherein the 
voltage detector circuit outputs, as the coil voltage, an amplified 
differential value between the voltages produced on the first and second 
coils. 
In yet another version of the invention, the coils of the sensor circuit 
comprise: a first coil whose inductance changes in response to a change in 
the physical quantity; and a second coil connected in series with the 
first coil and having an inductance which remains constant regardless of 
changes in the physical quantity; wherein the voltage detector circuit 
outputs, as the coil voltage, an amplified differential value between the 
voltages produced on the first and second coils.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment 
The first embodiment of the invention is explained below, with reference to 
the drawing. FIG. 1 is a block diagram of this embodiment of the 
invention, and FIG. 2 is a waveform diagram showing variations over time 
of various signal voltages occurring in the circuit of FIG. 1. 
In FIG. 1, drive circuit 4A and phase detector circuit 6A correspond to the 
above drive circuit 4 and phase detector circuit 6, respectively. 
Similarly, coils 3a and 3b, voltage detector circuit 5, and smoothing 
circuit 7 also correspond to their respective counterparts in FIGS. 3 and 
4. 
Also, although not illustrated, torque receiver shaft 1 with its magnetic 
substance strips 2a and 2b (the object being measured) is configured as 
shown in FIG. 3. 
Resistors 8 and 9, which have equal resistance values, are inserted at the 
ends of the series-connected first and second coils 3a and 3b to form a 
series string comprising resistor 8, first coil 3a, second coil 3b, and 
resistor 9, in that order. The purpose of resistor 9 is to maintain the 
symmetry of the series circuit comprising first and second coils 3a and 
3b. 
This series circuit comprising coils 3a and 3b and resistors 8 and 9 is 
connected between the terminals of drive circuit 4A, which outputs drive 
voltage Va. 
Connected across resistor 8 is current integration circuit 10. 
Current integration circuit 10 takes Vi, the voltage developed across 
resistor 8 (which corresponds to the coil current i), as its input. It 
then derives from Vi, an integrated current value, and outputs first and 
second control signals C1 and C2 based on this integrated current value. 
Phase detector circuit 6A detects the differential coil voltage Vc output 
by voltage detector circuit 5 over output periods P1 and P2 (FIG. 2) of 
control signals C1 and C2 output by current integration circuit 10, to 
output detection voltage Vd. 
Next, the operation of this first embodiment of the invention will be 
explained, with reference to the waveform diagrams shown in FIG. 2. 
For explanation purposes, the integrated current value of coil current i 
will be made 0 in order to clearly show only the effect. Also, for the 
sake of simplicity, the example described will be that for the case where 
voltage detector circuit 5 detects, as coil voltage Vc, not a differential 
voltage, but rather the total coil voltage. Accordingly, the waveforms for 
coil voltage Vc and detection voltage Vd will differ from those shown in 
FIG. 2, which are for differential voltages. 
First, drive voltage Va from drive circuit 4A is applied, causing a.c. coil 
current i to flow in coils 3a and 3b, as described above. This results in 
an inductive component voltage VL and resistive component voltage Vr being 
developed due to the total inductance L and internal resistance r, 
respectively, of the coils. 
Since voltage detector circuit 5 cannot isolate the inductive and resistive 
components of the voltage (VL and Vr) for detection, it must of necessity 
detect their composite voltage as coil voltage Vc. 
Phase detector 6A phase-detects coil voltage Vc output by voltage detector 
circuit 5 during output periods P1 and P2, respectively, of the first and 
second control signals C1 and C2. (Periods P1 and P2 represent the time 
required for the integrated current value to reach 0 after current 
integration circuit 10 starts integrating the coil current.) 
The integrated current value over the output period P1 starting at the 
falling edge of drive voltage Va, for example, is expressed by equation 
(5), below. 
EQU .intg.i.multidot.dt=0 (5) 
Also, the resistive component voltage Vr developed due to the total 
internal resistance r of the coil, is expressed by equation (6), below. 
EQU Vr=r.times.i (6) 
Therefore, if the coil voltage Vc from the voltage detector circuit is 
detected during output period P1, and the resulting detection voltage Vd 
is averaged by smoothing circuit 7, the resulting mean voltage Vm during 
output period P1 will be as given by equation (7), below. 
##EQU3## 
As is evident from equation (7), the effect of the resistive component 
voltage Vr can be offset by making the integrated current value 
.intg.i.multidot.dt=0. It then becomes clear that we thereby obtain a mean 
voltage Vm that is made up of inductive component voltage VL only. 
By so doing, we can eliminate the measurement error due to internal 
resistance r from measurements of the inductance L, to realize a high 
precision sensor circuit. 
It is clear, then, that even for the case where the detection voltage for 
the total coil is detected as coil voltage Vc, a high precision sensor 
circuit can be obtained. 
If, then, instead of the total coil voltage, the amplified differential of 
voltages V1 and V2 (V1-V2), developed across series connected first and 
second coils 3a and 3b, respectively, is detected as coil voltage Vc (as 
shown in the waveforms of FIG. 2), then by offsetting the effect of 
internal resistances r1 and r2 as described above, a sensor circuit of 
even greater precision can be realized. 
Also, drive circuit 4A generates a rectangular waveform that switches 
between positive and negative voltage levels centered around 0 V at set 
intervals (T/2), as shown in FIG. 2. Current integration circuit 10, then, 
determines period P1, from the point at which drive voltage Va switches 
from positive to negative (Va falling edge) to where the integrated 
current value of coil current i reaches 0, and period P2, from the point 
at which voltage Va switches from negative to positive (Va rising edge) to 
where the integrated current value of coil current i reaches 0, and 
alternately outputs control signals C1 and C2 at set intervals (T/2). 
During either one of the first control signal C1 output period P1 or the 
second control signal C2 output period P2 (P1 is used in the example of 
FIG. 2), phase detector circuit 6A outputs coil voltage Vc from voltage 
detector circuit 5, as is, as detection voltage Vd, without changing its 
polarity; and during the other control signal output period (P2 in the 
example of FIG. 2), it inverts the polarity of coil voltage Vc prior to 
outputting it as detection voltage Vd. 
As a result, since two pulses of detection voltage Vd are obtained during 
one cycle time T, a sensor circuit of still greater precision can be 
realized. 
Also, by configuring the system so that, as in the magnetostrictive torque 
sensing system of FIG. 3, when a torque external to the system is applied 
to torque receiver shaft 1, the inductance of the first coil 3a or the 
second coil 3b increases, and the inductance of the remaining coil 
decreases, a physical quantity, e.g., torque, can be sensed, based on 
changes in inductances L1 and L2. 
Next, the operation of a sensor circuit according to Embodiment 1 of the 
sensor circuit in a magnetostrictive torque sensor application will be 
described more concretely, with reference to FIG. 1 and FIG. 2. 
First, as described above, coil current i is caused to flow in the series 
circuit comprising resistor 8, first coil 3a, second coil 3b and resistor 
9, connected in that order, by the rectangular wave drive voltage Va 
output by drive circuit 4A. 
Now, since the series circuit that includes coils 3a and 3b includes an 
inductance L, although drive voltage Va switches from positive to negative 
at time 0 on time axis t, as shown in FIG. 2, the direction of coil 
current is still positive after that point, decaying thereafter, and 
eventually flowing in the negative direction. 
Current integration circuit 10 determines coil current i from Vi, the 
voltage developed across resistor 8, and at the time, the point (t=0), at 
which drive voltage Va switches from positive to negative (its falling 
edge), starts integrating coil current i. It then determines period P1, 
the time it takes the integrated current value of coil current i to reach 
0 (the time it takes negative and positive shaded areas in FIG. 2 to 
become equal). Then it takes the first control signal C1 from a low level 
to a high level for the duration of period P1 (the first control signal C1 
output period). 
Next, at the point (t=T/2) where drive voltage Va switches from negative to 
positive (rising edge), coil current i remains negative after that point, 
as was true at the falling edge (t=0) , increasing thereafter, to 
eventually flow in the positive direction. 
Now, current integration circuit 10 starts integrating coil current i at 
the rising edge of Va (t=T/2). It then determines period P2, the time it 
takes the integrated current value of coil current i to reach 0. Then it 
takes the second control signal C2 high for the duration of period P2 (the 
second control signal C2 output period). 
Voltage detector circuit 5 detects and differentially amplifies voltages V1 
and V2 developed across coils 3a and 3b, respectively, to output a 
differential output voltage as coil voltage Vc. 
FIG. 2 shows an example of the coil voltage Vc that would be developed from 
the differential voltage that would exist if inductances L1 and L2 of 
coils 3a and 3b, respectively, satisfied the condition L2&gt;L1. 
During the first output period P1, when the first control signal C1 is at a 
high level, phase detector circuit 6A outputs coil voltage Vc as is 
(without changing its polarity), as detection voltage Vd. 
During the second output period P2, when the second control signal C2 is at 
a high level, it inverts the polarity of coil voltage Vc prior to 
outputting it as detection voltage Vd. At all other times, it sets the 
output signal in the high impedance state. 
Smoothing circuit 7 performs a smoothing process on the detection voltage 
Vd during output periods P1 and P2 of control signals C1 and C2, 
respectively, to output d.c. mean voltage Vm. 
Now, from equations (5) through (7), the effect of the resistive component 
voltage Vr is seen to have been offset due to fact that the value of the 
integrated current over the output periods P1 and P2 is 0. 
Also, since the polarity of the inductive component voltage VL is always 
the same as that of the drive voltage Va, by sensing the polarity in 
response to control signals C1 and C2, ultimately, a mean voltage Vm 
proportional to (L1-L2)/(L1+L2) can be output by smoothing circuit 7. 
Thus by eliminating, in this manner, the measurement error due to internal 
resistance incurred in the measurement of inductance L, a high precision 
sensor can be obtained. 
Second Embodiment 
Although in the first embodiment, the characteristic of the change in 
inductance L2 (the inductance of second coil 3b) with respect to the 
physical quantity was made to be the opposite of the inductance change 
characteristic of L1 (the inductance of first coil 3a), L2 (the inductance 
of second coil 3b) could have been a set value, unaffected by changes in 
the physical quantity. 
Also, although a pair of coils, 3a and 3b, was used, a single coil whose 
inductance changes with changes in the physical quantity, could have been 
used instead. 
Thus as described above, through this invention, by detecting coil voltage 
Vc such that the integrated current value of coil current i is 0, the coil 
resistive component voltage Vr is offset during the phase detection 
period, the effect of internal resistances r1 and r2 does not appear in 
the mean voltage Vm that is finally output, and therefore, even if coils 
having high r1 and r2 internal resistances are used, the inductance 
measurement error due to internal resistance is eliminated, and as a 
result, a high precision sensor circuit is obtained. 
Also, according to this invention, voltages V1 and V2 developed across 
series connected first and second coils 3a and 3b, respectively, are 
differentially amplified to further offset the effect of internal 
resistances r1 and r2, the result being that an even higher precision 
sensor circuit is obtained. 
In addition, according to this invention, coil voltage Vc is phase-detected 
such that it is polarity-inverted depending on the states of control 
signals C1 and C2 output at set intervals (T/2), and two pulses of 
detection voltage Vd are obtained during one cycle time T of drive voltage 
Va, the result being that a still higher precision sensor circuit is 
obtained.