Low Noise Differential Torquer Servo Loop and Circuitry

The invention described herein may employ some of the teachings disclosed and claimed in commonly owned copending application filed on even date herewith by McBrien et al, U.S. Pat. Ser. No. 692,071 entitled DIFFERENTIAL TORQUER.

This invention relates to a servo loop for controlling a differential torquer.

Guidance, navigation, pointing, and tracking systems use gyroscopes, accelerometers, and various other sensors to obtain highly accurate data. A gyroscope, for example, uses a spinning mass to sense rotation rates. One type of gyro is the rate integrating type which balances the input rate against a torque exerted by a current carrying coil in the presence of a magnetic field. The current can be scaled to produce the desired counter-torque, and measured to indicate the input rate, or integrated and measured to indicate the input change in angle.

Servo loops used in such sensor applications have some degree of temperature sensitivity. This sensitivity can be a serious problem depending upon the type of loop designed and the accuracy required. For example, in a rate integrating gyro application, as the current is varied to match the input rate, the thermal effects resulting from this current change produce errors in the data such as scale factor drift and bias drift. Existing ways of dealing with such problems are directed at precise temperature regulation of the servo loop environment. In the rate integrating gyro or any other such device it would therefore be desirable to supply constant power to the device's torquer regardless of the input rate. Existing designs switch the direction of constant current through a torquer to achieve constant power, e.g., a variable duty cycle scheme whereby the current is maintained constant by switching its direction through the torquer coil and modulating its duration, to control the net torque in a desired direction. This approach provides the desired constant power but introduces some effects that could compromise performance for low noise applications. This is due to the bidirectional pulse width modulating technique which introduces noise at harmonics of the modulating frequency.

The object of the present invention is to provide both a low noise torque balance loop and also maintain constant power in a torquer assembly.

According to the present invention a source of DC power is applied to the dual windings of a unique "differential" torquer in such a way that the unidirectional currents flowing within each of the windings cause the torquer to have oppositely directed torquing effects. The current in each of the windings is sensed and the total current flowing in both windings is summed and regulated at a constant value. At the same time, an error signal indicative of torquer position is compared to a difference signal indicative of the difference between the two torquer currents. The difference between the error signal and the difference signal is maintained at zero so that the torques they represent are counter balanced.

In further accord with the present invention, the two torquer currents are passed through a coil assembly with opposing magnetic polarities. Instead of a single winding, a bifilar coil is created for each loop of the torquer assembly. All loops are wired together to produce an integral assembly of torquer coils. The four loose ends are then routed to the loop electronics to control the coil currents.

Thus, a low noise torque balance loop with constant power is achieved. A quiet loop can thus be created with less generation of switching transients and noise. Drift of the switching elements is no longer of any concern. Components affecting accuracy of the system are confined to a small number of low power elements, such as the resistors, op amps and the D/A converter in the rate integrating differential torquer loop. The loop can be easily trimmed by parallel elements across sense resistors as shown in the best mode embodiment. Also, different update rates to the A/D converter can change the bit weight of the output data, which provides a means of dynamically adapting to different inputs.

In still further accord with the invention, having the analog to digital (A/D) converter inside the loop removes A/D conversion errors from the output data.

These and other objects, features, and advantages of the present invention will become more apparent in the light of the following detailed description of an exemplary embodiment thereof as illustrated in the accompanying drawing.

FIG. 1 is a simplified schematic block diagram illustration of a differential torquer servo loop according to the present invention;

FIG. 2 is a cut away view of a precision gyroscope which contains a differential torquer which may be controlled by a servo loop according to the present invention;

FIG. 3 is a more detailed view of one embodiment of a differential torquer;

FIG. 4 is an illustration of an individual coil of the torquer of FIG. 3;

FIG. 5 is an illustration of a pair of coils showing the electrical hook-up;

FIG. 6 is an illustration of a differential torquer in a stationary magnetic field undergoing an incremental rotation; and

FIG. 7 is a more detailed schematic block diagram illustration of the servo loop of FIG. 1.

FIG. 1 is a simplified schematic block diagram illustration of a differential torquer servo control loop 10, according to the present invention. The loop 10 includes a rotary variable differential transformer (RVDT) circuit 12 which monitors the position of a unique differential torquer 14 which is further disclosed and claimed in copending application U.S. Ser. No. 692,071, invented by McBrien and Swarts and assigned to the assignee of this invention. The RVDT circuit 12 is responsive to a sinusoidal excitation signal on a line 16. The sinusoidal signal on the line 16 is used for exciting the RVDT primary and for demodulating the signal picked up on the RVDT secondaries. The RVDT circuit 12 provides a position output signal on a line 18 to an analog to digital converter 20 which converts the analog signal on the line 18 to a digital signal on a line 22. The digital signal on the line 22 is provided as data for use elsewhere within the guidance and/or navigation system and is also provided to a digital to analog converter 24 for reconversion to analog signal form. A reconverted signal on a line 26 is provided to a control circuit 28 which includes the torquer 14, sensing resistors 30, 32, summing junctions 34, 36, 38, and 40, a total current controller 42, and an amplifier 44. The circuit 28 of FIG. 1 is shown in simplified form for illustrative purposes.

The sensing resistors 30, 32 sense the separate currents in each of the torquer 14 windings. Signals from each of the sensing resistors are presented to the summing junction 38 where they are summed to provide a summed total current signal on a line 46 for comparison with a reference current signal on a line 48 at a junction 40. A total current control signal on a line 50 is provided to the current controller 42 for controlling the total current through the torquer windings at a constant power level. The same sensed signals on lines 52, 54 are summed at the junction 34 and a difference signal on a line 56 indicative of the difference between the two currents in the two torquer windings is provided to the junction 36. The difference signal on the line 56 is subtracted from the position signal on the line 26 and a signal indicative of their difference is provided on line 58 to amplifier 44 where it is amplified with positive and negative polarities for presentation on lines 60, 62 to the torquer windings. Thus, as long as a difference exists between the position signal on the line 26 and the difference signal on the line 56, the signal on the line 58 will continue to attempt to rebalance the position of the torquer until the signals on the line 26 and 56 are equal.

The A/D converter 20 and the D/A converter 24 are inserted into the servo loop 10 in order to remove A/D conversion errors from the output data. Of course, different update rates to the A/D converter can change the bit weight of the output data, which provides a means of dynamically adapting to different inputs.

FIG. 2 is an illustration of a precision gyroscope for which the differential torquer servo loop of the present invention may be utilized. A rotating member 70 is sealed within a hollow cylinder 72 which forms the inner gimble of the gyroscope. The rotating member rotates about a spin axis 73. An input rate about an input axis 74 is translated into an output torque about an output axis 75. The cylinder has a plurality of coils (not shown in FIG. 2; but see FIG. 3) mounted around its sides which are excited externally and carry currents which interact with a permanent magnetic field provided by permanent magnets 76, 77. The cylinder 72 is mounted in a gyro case 78 with a fluid surrounding the cylinder. The density of the fluid is chosen to exactly float the cylinder, thus removing the weight of the inner gimble from bearings 80, 82 and greatly relieving the bearing frictional load. An input rate to the gyro produces a proportional rate about the output axis of the instrument. Similarly, in a torque balance servo loop, an input torque about the input axis of the gyro produces a proportional torque about the output axis. The sensor loop creates an equal and opposite torque with the current carrying coils, thus maintaining the float fixed relative to the case. It is the current to the torquer that is used as the measure of angular rotation of the gyro and hence of the vehicle in which it is mounted.

FIG. 3 is a more detailed illustration of the inside of the cylinder 72 showing a plurality of coils 88, 90, 92, 94 mounted around its sides. Each of the coils is made up of a multi-loop bifilar winding as shown in FIG. 4. FIG. 5 shows two of the loops connected electrically.

Bifilar windings were selected for each of the loops in order to provide the best possible symmetry of coils, thereby insuring symmetry of magnetic fields for this unique "differential torquer". A single wire winding for alternate loops could also be used. However, the bifilar method is superior.

FIG. 6 is an illustration of the torquer 14 of FIG. 1 under the influence of a magnetic field from the permanent magnets 76, 77 of FIG. 2. Lines of magnetic induction 100 are shown penetrating the torquer 14. These lines of induction are stationary with respect to the case 78 of FIG. 2 and the torquer 14 rotates to a very slight degree about axis 75 under the influence of an externally applied rate about the input axis 74 of FIG. 2. Individual coils 102, 104 within the torquer 14 are provided with unidirectional DC as indicated by currents I.sub.1 and I.sub.2. These currents create opposing magnetic fields and are controlled by the servo loop to produce exactly the correct amount of magnetic torque to counterbalance the applied input rate. In other words, the applied input rate will cause a slight rotation of the torquer through an angle theta (.theta.) as shown. The servo loop will immediately act to restore the torquer to its original position and the relationship between I.sub.1 and I.sub.2 will be a measure of the input rate magnitude and direction.

FIG. 7 is a more detailed schematic block diagram illustration of the circuitry of FIG. 1. The RVDT circuit 12 is shown having a moveable iron core 110 which is attached to the torquer 14 in a manner which permits the measurement of rotary motion. A primary winding 112 is provided with a sinusoidal voltage from an amplifier 114 which amplifies an input sinusoid signal on the line 16. Two secondary windings 116, 118 are connected in series opposition so that the transformer output is the difference of the two secondary voltages. When the core is centered, the two secondary voltages are equal and the transformer output is zero. This is the balance or null position. When the core is displaced from the null point, the two secondary voltages are no longer alike and the transformer produces an output voltage. The output voltage is designed to vary linearly with core position over a small range. Motion of the core in the opposite direction produces a similar effect with 180.degree. phase reversal of the alternating output voltage. The output voltage is applied to an amplifier 120 and a demodulator 122. The demodulator 122 demodulates the amplified pickoff using a demodulation reference signal on a line 124 which has a frequency equal to that of the signal on the line 16. A buffer 126 responds to the demodulated signal on a line 128 and provides the position output signal on the line 18 to the A/D converter 20. After A/D conversion the digital data on the line 22 is provided to the navigation/guidance system and is then reconverted in the D/A converter 24 to an analog signal on the line 26. The position signal on the line 26 is summed at a junction 130 with a signal on a line 132 indicative of the difference (I.sub.1 -I.sub.2) between the currents in the torquer 14. The summed voltage on a line 134 is presented to the inverting input of a difference amplifier 136 configured as an integrator 138. Whenever a difference exists between the position signal on the line 26 and the difference signal on the line 132 the integrator 138 attempts to zero its input on the line 134 and it does this by readjusting the magnitudes of the currents (I.sub.1, I.sub.2) through the individual windings of the torquer 14. This is done with the integrator controlling the turn-on and turn-off times of the steer-through transistors 140, 142.

The total current through the torquer is sensed in a resistor 144 and the sensed voltage is provided to a differential amplifier 146 which provides an output voltage on a line 148 to a second differential amplifier 150 which provides a means of controlling the total current through the torquer at a constant value by controlling a transistor 154.

A pair of sensing resistors 155, 156 are used to sense the individual currents through the individual torquer coils. Parallel trim resistors 157, 158 are provided to fit the individual characteristics of individual circuits. The voltages across these sensing resistors are conditioned by a network of resistors 160-166 and a differential amplifier circuit 168 which provides the signal on the line 132 indicative of the difference in the two currents (I.sub.1 -I.sub.2)

Although the differential torquer servo loop has been described in an application involving a gyroscope, it should be understood by those skilled in the art that the differential torquer servo loop according to the present invention could as easily be used in other sensor applications including accelerometer applications.

Although the invention has been shown and described with respect to illustrated embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, ommissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.