This invention is directed to gyroscopes and, more particularly, to control systems for controlling the rate of rotation of the spinning mass (wheel) of a gyroscope such that the spin rate remains constant in inertial space.
Gyroscopes are widely used in navigation and control systems to provide information about the rate of movement of a vehicle with which they are associated about three orthogonal axes, normally referred to as the yaw, roll and pitch axes. Depending upon the accuracy required, gyros vary from relatively inexpensive uncomplicated mechanisms to relatively expensive, complicated mechanisms. Regardless of their expense and complication, most presently available gyros include a spinning mass. In order for a gyro to provide accurate rate information, the rotational speed of the spinning mass (wheel) in inertial space must either be known very accurately in time or, preferably, held very constant at a known speed. In a typical gyro used in an aircraft inertial navigation system, for example, the gyro wheel forms the rotor of a synchronous motor. More specifically, an AC source connected to field coils mounted in the gyro case sets up a rotating field which is followed by the gyro wheel. As a result, the gyro wheel rotates at the same speed as the speed of movement of the rotating field. Stated another way, because a relative angular position between the rotor and the field is maintained and because the field rotates, the rotor rotates at the same speed as the field. Since the speed of the rotating field is relative to the field coils, which are mounted in the gyro case, and since the rotating field speed is proportional to the frequency of the AC source, if the frequency of the AC source is maintained highly stable at a known value, the speed of the gyro wheel is maintained at a highly stable known value in inertial space. This result, however, is only true if the gyro case does not rotate about its spin axis.
In inertial guidance and navigation systems that mount gyros on a platform, gyro case rotation about the gyro's spin axis is prevented because the position of the platform remains fixed in inertial space. This solution, however, cannot be used in a strapdown gyro system. In a strapdown gyro system, the gyro cases are attached to the body of a vehicle. As a result, the gyro spin axes are forced to follow the movements of the vehicle. Consequently, when the vehicle rotates about the spin axis of a particular gyro, the speed of the rotating magnetic field of the gyro no longer remains constant with respect to inertial space. Rather, the rotational motion of the vehicle about the spin axis of the gyro will either increase or decrease the speed of the magnetic field and, thus, the speed of the gyro wheel relative to inertial space.
Since vehicles move in a random fashion as a result of many external conditions, strapdown gyros rotate about their spin axes in a random manner. Because the movement is random, the static angular position and speed relationships between the spin vector of the gyro wheel and the rotating magnetic field vector vary in a random manner.
In the general case, the instantaneous magnitude of the spin vector of a gyro wheel is the sum of: (1) the appropriate component of the vehicle angular rate relative to inertial space (i.e., the rotational movement of the vehicle about the spin axis of the gyro); (2) the speed of the rotating field relative to the gyro case; and, (3) a varying factor which depends upon the inertia of the spinning mass and the "stiffness" of the motor (damping, etc.). The second term (relative speed of the rotating field), which is by far the largest of the three terms, can be kept highly constant by controlling the frequency of the AC source that causes the gyro wheel to spin, as discussed above. While the first and third term contributions to the instantaneous magnitude of the gyro wheel spin vector are substantially lower than the second term, they are high enough to create unacceptable errors in precision navigation systems, such as required on aircraft, if ignored. The first term (vehicle movement about the spin axis of the gyro) can be readily determined by mounting a second gyro such that it senses movement about the spin axis of the first gyro. Contrariwise, the third term is a highly dynamic term and its continuous measurement is extremely difficult.
In the past, various proposals have been made to overcome or eliminate the errors created by the first and third terms described above. These proposals can be grouped under two approaches. The first approach maintains the frequency of the gyro motor supply power constant and measures the first and third terms at frequent intervals. The measured data is then used to correct the rate data produced by the gyro. The second approach varies the frequency of the gyro motor AC power supply in such a way that the second term counteracts exactly the variations in field rotation speed caused by vehicle movements about the spin axis of the gyro. If this compensation is done correctly, the gyro wheel is not subject to angular accelerations and decelerations about its spin axis. As a result, the dynamic effects which create the third term vanish. In summary, the first approach measures the first and third terms and modifies the gyro rate data accordingly; and, the second approach modifies the wheel speed relative to the gyro case whereby the resulting data does not include errors caused by the first and third terms.
One proposal for implementing the first approach described above comprises mounting pins on the gyro wheel. The pins are used to induce pulses in a coil affixed to the gyro case. The time interval between pulses is measured and used to determine the time it takes for the wheel to make one complete revolution relative to the position of the coil, i.e., the measured time interval is used to determine wheel speed. These measurements are then used to form the basis for determining the third term. The first term, to an adequate degree of accuracy, is obtained from another gyro mounted so as to sense rotation about the spin axis of the gyro whose wheel induces pulses in the coil. The second term, of course, is known.
The problem with the foregoing proposal is that it is difficult to implement in practice. First, complex computations, which are time consuming, must be made. Second, it is very difficult if not impossible to obtain relative wheel speed data, with sufficient resolution and accuracy, and with a sufficiently small delay, at intervals which are frequent enough to satisfy the computation requirements. Even if obtainable, the circuitry necessary to obtain this information is complex and, therefore, expensive. In this regard, mounting more than one pin about the circumference of the wheel and measuring the time interval between the consecutive pulses produced by a fixed coil so as to determine wheel speed based on measuring only a fraction of revolution cannot be done accurately enough. Such measurements cannot be made accurately enough because the pins cannot be positioned accurately enough. As a result, only the time between pulses induced by the same pin can be used. However, measuring the time needed for a full revolution to occur cannot, in a practical way, be done such that the necessary resolution is achieved. In this regard, in order to meet accurate navigation requirements, revolution measurements must be taken such that a resolution of one part in 800,000 or better is achieved. A typical navigation gyro wheel makes 100 revolutions per second. Consequently, the measurement of a single revolution with a resolution of one part in 800,000 requires an 80 MHz counter setup. An 80 MHz counter requires corresponding very fast reading and reset circuitry and is complex and expensive. Furthermore, only an average value over a full revolution is obtained. While several counters, each started at different, staggered times, could be used to produce wheel speed information that is recent enough, such an arrangement still produces only an average value over a full revolution. Further, such an arrangement is expensive due to the inclusion of several counters and their related subsystems.
In view of the foregoing discussion, it will be readily appreciated that prior art proposals to keep the frequency of the gyro power supply constant and measure data sufficient for the first and third terms to be determined are generally unsatisfactory. Not only is the equipment needed to make the required measurements difficult to design, such equipment is substantially more expensive than desirable.
Prior proposals for implementing the second approach generally described above, i.e., modulating the frequency of the gyro motor AC power supply in such a way that the second term counteracts exactly the variations in field rotational speed (in inertial space) about the gyro spin axis caused by vehicle movements, have also been generally unsatisfactory. In this regard, one prior proposal for implementing the second approach includes a phase-lock loop. A divider connected between the output of the phase-locked loop signal source and the phase comparator of the phase-locked loop has a modulus (division factor) that is controlled by the angular rate signal derived from a gyro mounted so as to detect vehicular movement about the rotational axis of the gyro whose wheel speed is being modulated. The division factor varies by a factor .delta. from a nominal value m.
A major problem with this implementation is that it is impossible, in a practical system, to obtain wheel speed values having sufficient resolution for them to be usable in a fast response phase-locked loop of this type. In this regard, wheel speed resolution is given by the equation 1/(m+.delta.). If it is assumed that the wheel rotates at 100 revolutions per second (36,000.degree./sec) and that vehicle rates up to .+-.10.degree./second must be compensated for, .delta. will be very small compared to m. As a result, wheel speed resolution is essentially equal to 1/m. As noted above, a resolution equal to one part in 800,000 or better must be obtained. Hence, m must be equal to 800,000 or more. As will be readily understood by those familiar with phase-locked loops, the frequency level of the output of phase-locked loop signal source (such as a voltage controlled oscillator) must be equal to m (the divider value) times the frequency level of a reference signal for lock to be achieved. The reference signal is externally produced and is compared with the frequency divided down signal produced by the loop signal source is a phase comparator. The result of the comparison is used to control the frequency of the loop signal source so that the loop becomes locked. At the present time crystal controlled voltage controlled oscillators produce signals that are stable up to the 20 MHz range. As will be readily appreciated by those skilled in the art, the implementation of variable modulus counters above 20 MHz is very difficult. As a result, signal sources having frequency values above this range are generally unusable. Now if the foregoing resolution requirement (one part in 800,000) is applied and the 20 MHz limitation is met, the reference frequency must have a value of 25 Hz, or less (20 MHz/800,000=25 Hz). This resulting low reference frequency value means that the phase-locked loop will be slow acting, unless some highly complex phase comparator is used. The phase-locked loop is slow acting, of course, because the variable inputs of the phase comparator can only be updated 25 times per second at most. The end result is that a loop with a bandwidth well below 1 Hz must be used; otherwise, limit cycling, excessive VCXO modulation due to the reference frequency component of the control signal and other undesirable effects will occur. Because the loop filter bandwidth is below 1 Hz, this arrangement will only be able to compensate accurately for vehicle rate signals whose frequency spectrum does not exceed a small fraction of a Hz. While this limitation may be acceptable in some environments, it is not acceptable in other environments, such as an aircraft environment, where gyro case frequency components of 5 to 20 Hz and above occur and must be compensated for. In this regard, it should be noted that above these frequencies, the inertia of the wheel will keep its speed in inertial space constant because the third term cancels out the first term. Hence, it is frequencies below 5-20 Hz (down to about 1/10 Hz) that must be compensated for by modulating the wheel speed of the gyro. With the physical limitations of practical systems noted above, such compensation cannot be accomplished using the foregoing implementation of the second approach.
In summary, implementations of neither of the proposed approaches discussed above is satisfactory. While the present invention falls generally within the second approach described above, it implements that approach in a manner substantially different than the manner described above. As will be better understood from the following discussion, the invention avoids the results of the latter implementation, whereby it is useful in environments wherein gyro case frequency components lie substantially beyond the passband of the loop filter of the phase lock loop.
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