GB2061514A - Inductive Pick-off Devices - Google Patents

Abstract

An inductive pick-off device for use with apparatus, the pick-off device comprising inductive coil means 38 arranged in use to have induced therein a signal representative of a parameter of the apparatus and arranged to have applied to it a control signal for the apparatus, whereby the coil means 38 serve a dual function, the pick-off device further comprising a phase shifter 45 to which one end of the coil means 38 is connected as an input, summation means 42 receiving as one input the output from the phase shifter 45 and as a second input the output from inverting filter means 46 which is connected to the same end of the coil means 38 and is operable to filter the induced signal and invert the control signal, and summing amplifier means 48 having one input at earth potential and the other input connected to the output of the summation means 42, the output from the amplifier means 48 being the induced signal substantially free from degradation by the control signal. <IMAGE>

Description

SPECIFICATION Inductive Pick-Off Devices This invention relates to inductive pick-off devices and more particularly to such devices in which inductive coil means serve a dual function, namely a pick-off function and a control function. The invention has been developed in relation to a combined pick-off and torquing device for a rate gyroscope and will, in the main, be discussed in that context but it is to be understood that the invention is applicable generally to pick-off devices of the type specified above.

Combined pick-off and torquing devices for gyroscopic devices are known in which inductive coil means is used to detect movement of the gyroscope sensitive element relative to a datum and to receive a torquer current in order to precess the gyroscope to correct for that movement. Naturally, interaction of the pick-off (induced) signal and the torquer signal occurs and it is necessary to distinguish the pick-off signal from the torquer signal in order to preserve the accuracy of the former. In some known devices, a relatively high gain differential amplifier has been connected across the pick-off and torquer coil means with passive torquer signal decoupling circuits (typically capacitive/resistive circuits) employed in the amplifier input circuit to prevent the torquer signal being applied to the amplifier.

In the case of a rate gyroscope this technique is satisfactory for low slew rates ot the gyroscope but a problem has been discovered when a gyroscope is slewed at a sinusoidal rate greater than 20 per second in that the differential amplifier saturates with consequential loss of the pick-off signal and hence loss of loop capture. Also it has been discovered that the slew rate giving rise to saturation of the differential amplifier decreases as the frequency increases so that, for example, the amplifier may saturate at a slew rate of 0.060 per second at 60 Hz which is unacceptable in many rate gyroscope applications which can call for 1 500 per second peak slew rate at a 60 Hz loop bandwidth.

In the interests of minimising the number of components, and hence size and weight, of the pickoff and torquing device, it is desirable to retain the combined pick-off and torquing functions of the coil means and, therefore, the object of the present invention is to solve the problem of amplifier saturation.

Of course, one solution would be to enhance the performance of the torquer signal decoupling circuits and another would be to employ some form of transformer isolation technique but in both instances this would be costly and give rise to a more voluminous device and to an unacceptable phase shift in the pick-off signal. Furthermore, it is another object of the present invention to obviate the use of capacitors which do not lend themselves well to hybridisation which technique is gainfully employed to effect the desired minuturisation of ancilliary components of a gyroscope or other apparatus.

According to one aspect of the present invention there is provided an inductive pick-off device for use with apparatus, the pick-off device comprising inductive coil means arranged in use to have induced therein a signal representative of a parameter of the apparatus and arranged to have applied to it a control signal for the apparatus, whereby the coil means serve a dual function, the pick-off device further comprising a phase shifter to which one end of the coil means is connected as an input, summation means receiving as one input the output from the phase shifter and receiving as a second input the output from inverting filter means which is also connected to said one end of the coil means and is operable to filter the induced signal and invert the control signal, and summing amplifier means having one input at earth potential and another input connected to the output of the summation means, the output from the amplifier means being the induced signal substantially free from degradation by the control signal.

If it is required to have at said one end of the coil means the difference between the induced signal and the control signal, the coil means may be in the form of two identical coils wound in antiphase.

According to another aspect of the present invention there is provided an inductive pick-off device for use with gyroscopic apparatus, the pick-off device comprising inductive coil means comprising a pair of identical coils wound in anti-phase and arranged in use to have induced therein a signal representative of displacement of a sensitive element of the apparatus relative to a sensitive axis thereof and arranged to have applied to it a torquer signal for the gyroscopic apparatus, whereby the coil means serve a dual function and the signal appearing at one end is the difference between the displacement signal and the torquer signal, the pick-off device further comprising a phase shifter to which said one end of the coil means is connected as an input, first summation means receiving as one input the output from the phase shifter and receiving as a second input the output from inverting filter means which is also connected to said one end of the coil means and is operable to filter the displacement signal and invert the torquer signal, and first summing amplifier means having one input at earth potential and another input connected to the output of the summation means, the output from the first amplifier means being the displacement signal substantially free from degradation by the torquer signal.

In the context of gyroscopic apparatus, it is desirable to provide a reference for the induced signal and, therefore, according to a preferred feature of this aspect of the invention, which could also be employed with the broader aspect of the invention, there is provided a reference channel for said sensitive axis comprising second summation means having two inputs connected to respective ends of the coil means, second summing amplifier means having one input at earth potential and another input connected to the output of the second summation means, third summation means having one input connected to the output of the second amplifier means and a second input connected to a centre tap of the coil means, third summing amplifier means having one input at earth potential and another input connected to the output of the third summation means, whereby the output signal from the third amplifier means is phase-coherent with the output from the first amplifier means, square wave generator means connected to the output signal of the third amplifier means, and demodulator means for demodulating the output signal of the first amplifier means and in use receiving as inputs the output signal from the first amplifier means and the output signal from the square wave generator means.

Preferably, the output signal from the first amplifier means is connected to the demodulator means via a high pass filter.

It will be appreciated that if the gyroscopic apparatus has more than one sensitive axis, then each axis may have associated therewith a pick-off device in accordance with the present invention.

Greater accuracy of the output signal of the pick-off device is obtained if the other end of the coil means is at, or substantially at, earth potential.

A gyroscope embodying a pick-off device constructed in accordance with the present invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an elevational view of the gyroscope in cross section, Figure 2 is a fragmentary cross-sectional view taken at ninety degrees to the section of Figure 1, Figure 3 is a circuit diagram of the pick-off device, and Figures 4, 5 and 6 are explanatory diagrams.

Referring to Figures 1 and 2, the gyroscope is indicated at 1 and is of the flexure-suspended, free-rotor type and comprises a housing 2 within which are mounted anti-friction bearings 3 and 4 which journal a hollow drive shaft 5 about its geometrical spin axis. The drive shaft 5 is rotated by a generally conventional hysteresis or synchronous spin motor 6 having a stator 7 and a rotor 8. The gyroscope has a sensitive element in the form of a wheel 9 radially suspended by a flexure support spider 11 having four equiangularly disposed resilient, thin, flat arms 12,1 2, 13,13 which flexibly support the wheel 9 for rotation about the normal spin axis.The wheel 9 is supported in the spin axis direction by a further flexure support or strut 14 extending through an aperture 1 5 in the spider 1 The flexure support 14 has a cylindrical extension 1 6 received by the hollow shaft 5, whereby it is affixed within the latter.

The flexure support 14 consists of a machined cylindrical rod having three flat flexure elements 18, 19 and 21 milled in its active flexure region. The cylindrical portion 16 of the rod is affixed within the shaft 5 in the central bore 23, while its opposite cylindrical portion 24 is fixed to a spoke 25 (Figure 1 ) of the gyroscope wheel 9 through a tubular projection 26. The end flexures 18 and 21 lie in the same diametral plane of the support 14, a plane perpendicular to the diametral plane of the intermediate flexure 19. The fiexure element 1 9 is preferably twice as long as either of the equal length end flexure elements 18 and 21.

The suspension system for the gyroscope wheel 9 provides translational rigidity along three mutually perpendicular axes and a low torsional restraint in a simple, low cost configuration having an inherently low sensitivity to twice-rotor-speed vibration. The use of the series of three flat flexure elements 18, 1 9 and 21 results in a desirable and significant reduction in the flexural rigidity of the support 14. The use of the three flat flexure elements 18, 1 9 and 21 also advantageously keeps the centre of flexing constant, no matter what the direction of deflection of the gyroscope wheel 9. The intermediate flat flexure element 19 is centred in the aperture 1 5 in the spider 11.

In operation, all radial and drive motor torque loads on the rotating system are carried by the spider 11 which accommodates tilt of the gyroscope wheel 9 with respect to the drive shaft 5 by twisting deflection of its crossed arm 12, 13. In fact, the central part of the spider 11 may be likened to the intermediate gimbal of a Hooke's universal joint. Because this effective gimbal is formed from extremely thin metal sheet, it is inherently mass-balanced with respect to the two mutually perpendicular pivot axes and the gyroscope thus has a low inherent sensitivity to twice-rotor-speed vibration.

In operation, all axial loads on the rotating system are carried by the triple flexure support 4. As previously noted, the support 14 is proportioned with the flat intermediate flexure element 19 twice as long as each of the two flat and flexure elements 18 and 21. Such a configuration has equal flexural stiffness in any deflection direction, as well as equal columnar strength. Although the axial support of the effective central gimbal portion of the radial suspension is soft, the extremely low mass of the effective gimbal prevents an excessive anisoelastic acceleration sensitivity.

It is seen that the axially disposed triple flexure support 14 is affixed at one end through the tube 26 to the spoke 25, and fixed at its opposite end in the bore 23 in the hollow drive shaft 5. The opposite end of the hollow shaft 5 is provided with a screw 28 mating with a thread internally of the hollow shaft 5. The races of the ball bearings 3 and 4 are thus confined between a flanged portion 29, from which a bridge or yoke 31 extends, and the head of the screw 28 when the latter is tightened. There may be used a magnetic suspension spring compensation system (not shown) of the type disclosed in British Patent Specification No. 722492.

The gyroscope wheel 9 includes a ring-shaped or annular channel 32 at its periphery. The open end of the annular channel 32 faces the hysteresis motor 6 and provides an air gap region generally indicated at 33, the channel 32 being constructed of soft iron and having integrated sides or legs 34, 35 and 36 for providing a magnetic circuit, including the air gap 33. Within the air gap 33 and affixed by a conventional adhesive to the inner surface of the outer leg 34 of the annular channel 32 for rotation therewith is a ring-shaped permanent magnet 37, which magnet may be constructed as a flat cylinder of a conventional magnetic ailoy such as a platinum-cobalt or other permanent magnetic alloy having similar characteristics.The magnetic material of the ring 37 is permanently magnetised in the radial direction, for example, at eight equiangularly spaced sites all of which are polarised in the same radial sense. Between adjacent poles, the magnetisation of the ring 37 falls to a low value or preferably even to zero. Thus a uni-directional magnetic field resides in the air gap 32 between the ring magnet 37 and the second or inner leg 36 of the annular channel 32, the amplitude of the field varying in a generally sinusoidal or undulating manner around the air gap 33.

Whilst eight permanently magnetised sites are provided on the permanent magnet 37, they are arranged to cooperate with four identical, equiangularly spaced air core pick-off coils 38 (only two being seen in Figures 1 and 2) disposed in a cylindrical shell 39 of electrically-insulating material such as a conventional synthetic plastics composition. The coils 38 are disposed generally conformally within the cylindrical shell 39, so that they may be supported by the shell partly in the annular air gap 33. In this manner, the four air core coils 38 are mounted in the shell 39 for fixed support with respect to the housing 2, the edge of each coil being inserted into a sector of the air gap 33, between the permanent magnet 37 and the inner leg 36 of the soft iron channel 32.In view of the use of four coils 38 and of the eight permanently magnetised sites in the magnet 37, the angular length of each coil 38 along the air gap 33 is approximately equal to the angular distance between the centres of the magnetised sites in the permanent magnet 37. It will be understood that the number of magnetised sites in the magnet 37 was chosen merely by way of example, and that this number may be changed as circumstances dictate.

As will be seen from Figure 3, one opposed pair of coils 38 form part of the pick-off device 39 associated with one sensitive axis of the gyroscope, which axis has been arbitrarily designated as the A axis in Figure 3. The other opposed pair of coils 38 is associated with the sensitive axis B of the gyroscope and forms part of a pick-off device 41 which is identical to that associated with the A axis whereby only the latter will be described in detail.

The pick-off device 39 comprises displacement channel having a first summation device 42 with two input leads 43 and 44 connected respectively to the outputs of a unity gain phase shifter 45 and a two-pole, low pass, inverting filter 46, both the phase shifter and the filter being in the form of an operational amplifier. The input to the phase shifter 45 is connected to one end (point A) of the inductive coil means formed by the pair of identical coils 38, the input to the filter 46 being connected to the same point. The output from the summation device 42 provides an input on lead 47 to a first summing amplifier 48 which has another input 49 at earth potential. The output of the amplifier 48 is connected to a demodulator 51 for the A axis via a high pass filter 52.The output of the demodulator on lead 50 representing the displacement of the gyroscope relative to the associated axis is used in the normal manner.

The pick-off device also comprises a reference channel to enable demodulation of the output signal of the amplifier 48 to be effected, the reference channel comprising a second summation device 53 having two input leads 54 and 55 respectively connected to said one end of the coils means (point A) and the other end of the coil means (point C). The output of the summation device 53 provides an input on a lead 54 to a second summing amplifier 55, the output of which forms an input on lead 56 to a third summation device 57. Another input to the summation device on lead 58 is taken from the centre of the coil means (point B), i.e. from the common junction of the two coils 38. The output from the summation device 57 provides an input on lead 59 to a third summing amplifier 61, a second input 62 of which is an earth potential.The output of the amplifier 61 is connected to the demodulator 51 via a square wave generator 63.

A torque generator indicated generally at 64 is associated with the pick-off device 39 (a similar generator 65 being associated with the pick-off 41) to apply, when required, a unidirectional torquer current to the coils 38 to effect precession of the gyroscope. The torque generator 64 comprises an adjustable voltage source 66 the signal from which is applied to an input lead 67 to a further summation device 68. A second input to the summation device 68 is applied on lead 69 from the other end of the coil means (point C) via a feed back resistor RF and the output from the summation device is passed through an operational amplifier 71 to said one end (point A) of the coil means to apply a torque signal to the latter. The other end (point C) of the coil means is earthy in the sense that it is connected to ground via a low value resistor Rs.

In operation of the gyroscope 1, the shaft 5 is driven at a frequency of 200 Hz. As the spatially modulated magnetic field around the air gap 32 passes each air-core coil 38, an alternating voltage is induced in each such coil at a frequency eight times the gyroscope wheel frequency, i.e. at a frequency of 1.6 KHz. When the gyroscope wheel 9 remains at its mechanical null position with its spin axis coincident with the axis of the drive shaft 5, the alternating voltages induced in each coil are substantially equal, and the phases of the voltages in diametrically opposite coils are substantially the same. Whenever the gyroscope wheel 9 tilts with respect to the coils 38, the equality of the voltages induced in the diametrically opposed pair of pick-off coils 38 is disturbed.The difference in the outputs of the two opposed pick-off coils 38 is, accordingly, a measure of the angular displacement of the gyroscope wheel about an input or sensitive axis such as axis A, for example.

The desired pick-off output signals are obtained, as shown in Figure 3, by connecting diametrically opposed pick-off coils 38 in series opposition or anti-phase. The phase reference frequency needed for demodulation of the pick-off signals is obtained from the reference channel described above so that output from the demodulator 51 is a D.C. signal representative of the displacement of the sensitive element or wheel 9 of the gyroscope relative to the axis A or B, as appropriate.In order to prevent the pick-off or displacement signal being degraded by the torque signal, the latter is substantially removed, in accordance with the present invention by feeding forward the unwanted torquer signal and inverting it, using the filter 46, to the summation device 42 which also receives the difference between the unwanted torquer signal and the induced displacement signal from the top, or point A, of the coil means. Thus the output of the summation means is the displacement signal substantially free of the torquer signal. A unity gain phase shifter 45 is employed to avoid introducing a mismatch of the inputs to the summation device 42 but a phase shifter other than of unity gain may be employed, provided a similar gain is imposed on the signal on lead 44.In the reference channel a similar feed forward technique to that used in the displacement channel is employed, again to remove the unwanted torquer signal from the reference signal.

Ideally, the other end (point C) of the coil means would be at earth potential as this provides a greater accuracy of the displacement and reference signals but in the present embodiment the resistance Rs is required for the torque generator to operate effectively although the value of this resistance is kept low (for example 3.3 ohms) so as to maintain point C earthy, i.e. substantially at earth potential.By recognising that if Rs is made small then the signal at point A of the coil means is the difference between the voltage (Vcl) across the upper coil 38 (as seen in Figure 3) and the voltage (Vc2) across the lower coil 38 (since the two coils constitute back-to-back generators), it became possible to use the feed forward cancellation technique for the torque signal with a virtual earth summing amplifier (48 in the displacement channel and 61 in the reference channel). The mathematical justification for connecting the pick-off coils 38 with the amplifiers 48 and 61 in the manner shown in Figure 3 will be provided hereinafter.

Continuing in general vein for the time being, when the gyroscope is being precessed or torqued by the application of a torque signal VT to the coils 38, there is present at point A both VT and (Vci Vc2) and if this combined signal were amplified without modification, the amplifier would soon run into saturation and hence cause temporary failure of the displacement channel with consequential malfunction of the gyroscope. However, this problem is avoided by the present invention. The combined signal at point A is passed through the two-pole, low pass, inverting filter 46 which takes out substantially all of the signal (Vc1Vc2) and provides an additive signal VT at point D, i.e. on the input lead 44 for the summation device 42.With the existence of an induced gyroscope displacement signal of 1.6 KHz in the coils 38, the two poles of the filter 46 are set to approximately 800 Hz. The signal VT will have an inherent phase lag dependant on the effective "frequency" of the D.C. signal VT which in the present embodiment will be in the range of O to 100 Hz.

In addition to the signal VT at point D there will be another signal of magnitudek (VcrVc2), where k is the attenuation effected by the filter 46. In the illustrated embodiment the value of k was set at a nominal value of 0.2 so that virtually all of the signal (Vc1Vc2) is preferentially filtered, leaving in effect substantially only the signal VT at point D for use in cancelling the portion VT of the signal appearing at point E, i.e. on the other input lead 43 of the summation device 42.

As already stated, the signal VT appears at point D with an inherent phase lag. For maximum cancellation of Vthe signal at point E and the signal at point D must be in phase to within +0.10 and to achieve this, the unity gain phase shifter 45 is employed with its pole-zero set to a carefully chosen frequency to ensure accurate phase tracking over the required 0--100 Hz frequency band. A secondary influence over the setting of the pole-zero of the phase shifter 45 involves the requirement to set the phase of the reference output signal from the amplifier 61 to +900 which is readily realised with the illustrated embodiment.The signals present at point D and E are therefore summed very accurately by the amplifier 48 which should cancel VT completely leaving a slightly attenuated displacement voltage (Vc1Vc2) to be processed without the unwanted saturation problem in attendance. However, due to a number of physical effects normally beyond the designer's control, perfect cancellation is not achieved and additional assistance is given by cascading the amplifier 48 with the high pass filter 52 which has a nominal gain of 1 7.7 db at the angular frequency c9C of vc, and VC2 and a gain of unity at 100 Hz. The roll-up in frequency versus gain is 20 db/decade of frequency.

As explained, the reference signal is derived from one coil 38 of the pair associated with the A or B axis of the gyro and amplified to provide a suitable, phase-coherent, signal in order that the displacement signal may be demodulated. More specifically, the reference signal is derived from the common-mode voltage Vc2 generated between point B and ground and is added to a signal which is a proportion of VT, in the ratio of the resistance RC1 of the upper coil 38 (as seen in Figure 3) divided by the sum of the resistances Rs and Rc2' where RC2 is the resistance of the lower coil 38 (also as seen in Figure 3).Due to the large gain of the amplifier 48 (approximately 26 db's in this embodiment), the reference output signal will suffer from the same saturation problem as the displacement channel if no steps are taken to prevent it, hence the use of a feed forward cancellation technique similar to that of the displacement channel as already explained. It is also desirable to keep the added component due to the torque current (T), to a minimum to minimise errors induced into the reference switching circuit (not shown) downstream to the reference output signal.

Referring now to Figure 4, this is an explanatory diagram showing the two coils 38 as back-toback generators of voltages Vc1 and Vc2, as already considered, and which is helpful in justifying the treatment of part of the signal at point A as being (Vc1-Vc2).

Vo=VA+VB where VA and VB are the voltages across the respective coils 38 taking into account their internal resistances R01 and RC2 at the carrier frequency.

Z1 VA = VC1 cos(#t+&alpha;1) (1) Z1 + RC1 VB = VC2 cos(#t+&alpha;2) (2) Z2 + RC2 where a1=00 &alpha;2=180 angular frequency of the induced signals Vci and Vc2 Z,=impedance seen across both coils 38 Z2=impedance seen across the lower coil 38.

The terms Z,/Z,+Rc, and Z2/Z2=RC2 in equations (1) and (2), respectively, may be disregarded as Z1 and Z2 are typically of the order of one thousand timer greater than Rc1 and RC2@ Thus VA-VC1 cos ot (3) VB=VC2 cos (cot+1 800) (4) Equations (3) and (4) are more conveniently expressed as VA = \QJ VC1 (cos #t+j sin #t) (5) and VB = VC2#ej (#t+180 ) = VC2#[cos (#t+180 )+j sin (#t+180 )] (6) Substituting #t=0 in equations (5) and (6) VA=Vc1 and Vb=-Vc2 # Vo=Vc1-Vc2 i.e. the difference between the two pick-off coil coltages and hence the displacement of the gyro wheel 9.

Turning now to Figure 5, this shows a system block diagram of the displacement channel of the pick-off device in terms of transfer function. As regards of low pass, inverting filter 46, the reactive gain G51 is given by: #o2 Gs1= (7) S2=2# #oS+#o2 where #o=2# fo with fo=800 Hz #=damping factor=0.6 S=Laplace operator.

The reactive gain Gs2 of the phase shifter 45 is given by: 1-ST Gs2=- (8) 1+ST where 2Xfo The gain G of the summing amplifier 48 is given by: G1=-A1=22.27 db,(volts/volt) (9) Finally, the reactive gain Gs3 of the high pass filter 52 is given by: 1 + as GS3 = 2.5 x 10-4 # # (10) 1 + bs - cs2 where a=7.7 x 10-6 b=1.5x10-9 c=2.45x10-18 In the ideal. simolified case. the voltades V@@ V@@ V@ and V@ appearing at the point A.D.E. and the output of the filter 52, respectively, can be represented as follows::- VA = VT sin @Tt + (VC1-VC2)sin #Ct (11)

where VT=voltage across the coils 38 at point A #T=angular frequency of VT Vc1=common mode voltage of one coil 38 Vc2=common mode voltage of other coil 38 #c=angular frequency of Vcr and Vc2 #T=phase shift of VT through filter 46 #C=phase shift of (VC1-VC2) through phase shifter 45 K=attenuation factor at #c of filter 46 =0.24 at #c=1.005x104 Now VO=(VD+VE)(-A1 # GS3) (14) Substituting equations (12) and (13) in equation (14) gives VO=-[(VC1-VC2) sin (#Ct+#C)][1-K][A1][GS3] Thus it is shown that the output signal from the high pass filter 52 is representative of the gyroscope displacement signal since it is a function of the displacement signal (Vc1-Vc2).

Figure 6 is a system block diagram, again in terms of transfer function, of the reference channel of the pick-off device. The torquer current iT flows through the elements Rc1, Rc2 and rs and thus an analysis can be performed in terms of voltages generated by this current.

VF= -[VA + VC + (XC1-XC2)] GS4 (15) VB = VA (K@)-XC2 (16) where VB=voltage at point B VF=voltage at point F Xc1=Vc1 sin #t Xc2=Vc2 sin cot Rc2+Rs Rc1+Rc2+Rs Gs4=reactive gain of amplifier 55 VOR Equating VF+FB=O= A2 where VOR=voltage output of amplifier 61 -A2=gain of amplifier 61 then-GS4[VA+V#+(XC1-XC2)]+XC2+VAKR=0 (17) thus-GS4 (VA+VC)+VAKR-GS4 (XC1-XC2)+XC2=0 (18) RS Now VC=VA # # RC1+RC2+RS = VA (RS/RO) (19) where Ro=Rc1+Rc2+Rs Substituting equation (19) in equation (18) and equating to Vo/-A2

Rearranging equation (20) gives::-

Taking part Y of equation (21) R5 KR=Gs4(1 +) (22) Ro Since Rc2+Rs KR R0 then RC2+Rs GS4= RC1 +RC2+2Rs In the special case of Rc,=Rc2=X (i.e. when the coils 38 of a pair are identical) then X+Rs GS4= =1/2 (23) 2X+2R5 Thus if G54 is set to a value of one half, then for any value of X, Vo=0 for part Y of equation (21).

Taking part Z of equation (21) -Vo/A2=-GS4Xc1+Xc2(GS4+1) substituting GS4=0.5 from equation (23) VaR =-0.5Xc1+0.5Xc2-Xc2 A2 Thus VOR=-A2(-0.5Xc1-0.5Xc2) -A2(Xci +Xc2)O.S The output signal VOR is, therefore, a suitable signal for reference purposes relative to V0 as it contains half the sum of the common mode signal across the two coils 38.

It will be seen that the problem of amplifier saturation existing in known pick-off devices is overcome by the present invention in a manner using a minimum of components and those components necessary are readily available and comparatively inexpensive. Furthermore, the required components lend themselves well to hybridisation so that the overall size of the pick-off can be made very small making it, in the illustrated embodiment, compatible with the gyroscope which has a diameter of approximately 25 mm and a length of approximately 25 mm. As already explained, the invention is applicable to any pick-off device having a dual function and is not restricted to the use with the rate gyroscope of the illustrated embodiment. In the context of gyroscopic apparatus, the pick-off device may be used with other than rate gyroscopes and also with inertial apparatus.

In one embodiment, the pick-off package has a width of approximately 25 mm, a length of approximately 25 mm and a depth of approximately 8 mm.

Claims (11)

Claims

1. An inductive pickoff device for use with apparatus, the pick-off device comprising inductive coil means arranged in use to have induced therein a signal representative of a parameter of the apparatus and arranged to have applied to it a control signal for the apparatus, whereby the coil means serve a dual function, the pick-off device further comprising a phase shifter to which one end of the coil means is connected as an input, summation means receiving as one input the output from the phase shifter and receiving as a second input the output from inverting filter means which is also connected to said one end of the coil means and is operable to filter the induced signal and invert the control signal, and summing amplifier means having one input at earth potential and another input connected to the output of the summation means, the output from the amplifier means being the induced signal substantially free from degradation by the control signal.

2. A pick-off device according to claim 1, wherein the coil means comprise two.identical coils wound in anti-phase, whereby the signal appearing at said one end of the coil means is the difference between the induced signal and the control signal.

3. An inductive pick-off device for use with gyroscopic apparatus, the pick-off device comprising inductive coil means comprising a pair of identical coils wound in anti-phase and arranged in use to have induced therein a signal representative of displacement of a sensitive element of the apparatus relative to a sensitive axis thereof and arranged to have applied to it a torquer signal for the gyroscopic apparatus, whereby the coil means serve a dual function and the signal appearing at one end is the difference between the displacement signal and the torquer signal, the pick-off device further comprising a phase shifter to which said one end of the coil means is connected as an input, first summation means receiving as one input the output from the phase shifter and receiving as a second input the output from inverting filter means which is also connected to said one end of the coil means and is operable to filter the displacement signal and invert the torquer signal, and first summing amplifier means having one input at earth potential and another input connected to the output of the summation means, the output from the first amplifier means being the displacement signal substantially free from degradation by the torquer signal.

4. A pick-off device according to any of the preceding claims and further comprising a reference channel operable to provide a reference signal for the induced signal, the reference channel comprising second summation means having two inputs connected to respective ends of the coil means, second summing amplifier means having one input at earth potential and another input connected to the output of the second summation means, third summation means having one input connected to the output of the second amplifier means and a second input connected to a centre tap of the coil means, third summing amplifier means having one input at earth potential and another input connected to the output of third summation means, whereby the output signal from the third amplifier means is phasecoherent with the output from the first amplifier means, square wave generator means connected to the output signal of the third amplifier means, and demodulator means for demodulating the output signal from the first amplifier means and in use receiving as inputs the output signal from the first amplifier means and the output signal from the square wave generator means.

5. A pick-off device according to claim 4, wherein the output signal from the first amplifier means is connected to the demodulator means via a high pass filter.

6. A pick-off device according to any of the preceding claims, wherein the other end of the coil means is at, or substantially at, earth potential.

7. A pick-off device according to any of the preceding claims, wherein the phase shifter is of unity gain.

8. A pick-off device according to any of the preceding claims, wherein the inverting filter means is in the form of a low pass filter.

9. A pick-off device according to any of the preceding claims, wherein the inverting filter means and the phase shifter are each in the form of an operational amplifier.

1 0. An inductive pick-off device substantially as herein particularly described with reference to the accompanying drawings.

11. Gyroscopic apparatus fitted with a pick-off device constructed in accordance with any of the preceding claims.