LASER GYRO SMART DIGITAL PLC ACQUISITION CONTROL

Abstract: In the present invention, an A/D converter is utilized to feed the relative positions of the gyro mirrors as well as the gyro body temperature to a digital processor. The processor can then calculate the optimum initial position for the gyro mirrors and direct the analog control loop into that position.

Description: LASER GYRO SMART DIGITAL PLC ACQUISITION CONTROL This invention relates to ring laser gyros in general and more particularly to an improved path length control for a ring laser gyro. BACKGROUND OF THE INVENTION Ring laser gyros are now well known and are similar to those shown and described in U.S. Patent 3,373,650, issued to Killpatrick, and U.S. Patent 3,390,606, issued to Podgorski. In such ring laser gyros, the ring laser can have moveable mirrors that are traditionally used to control the length of the cavity to an integer number of wavelengths of light. Doing so allows the ring laser beams to be operated at maximum intensity. Previously, a random start-up of the analog control circuits could yield an operating position that might be difficult to maintain over a wide temperature range without running out of mirror transducer authority. That is, the mirror position may only be translationally within operating limits of the transducer design, generally only several wavelengths of the laser beam light. For ring laser gyros in general, the path length of a ring laser gyro must be kept constant at an integral number of wavelengths of light for proper gyro operation. Large gyros, however, can mode hop, or change their path lengths by an integral number of wavelengths during a mission without serious degradation in output performance. In some laser gyros having closed-loop optical path length, for example 2 inches on a side or leg, the laser beam can extinguish (stop operating) when its transducer mirrors are shifted from one mode to another. Of course, the laser beams operating within a ring laser gyro must not be allowed to extinguish since loss of data will result and navigation errors will be incurred. Therefore, it is desirable that ring laser beams acquire a "mode" at power up, and maintain that same mode (or path length) throughout a mission. The moveable mirrors on a ring laser gyroscope are traditionally positioned by an analog control loop that acts to stabilize the gyro cavity path length to an integer number of wavelengths of the laser light. After initially setting the gyro laser cavity to a length equal to "n" wavelengths, the control loop must then maintain that exact cavity length, although arbitrarily chosen, throughout the mission. Subsequent temperature changes will expand or contract the material the gyro is made from, so the control loop must position the mirrors to counteract those movements. Manufacturers are finding it difficult to manufacture a gyro that can maintain a constant path length over a wide temperature range. Specifically, the yield is not as high as they would like it to be. Several methods are being used to increase the yield of gyros that can maintain a constant path length over a wide temperature range. They include (1) extending the range of authority of the transducer mirrors to allow a wider range of movement and (2) thermally stabilizing the gyro block so that less movement occurs that needs to be compensated. However, these techniques are generally costly, and also have other undesirable weight and size effects. SUMMARY OF THE INVENTION In the present invention, an A/D converter is utilized to feed the relative positions of the gyro mirrors as well as the gyro body temperature to a digital processor. The processor can then calculate the optimum initial position for the gyro mirrors and direct the analog control loop into that position. The result is that the control loops use a minimum of mirror transducer authority during the start-up period and leave the maximum amount of authority left for subsequent temperature compensation. An object of the present invention is to provide invention gives a third option. The invention uses a computer to calculate the ideal operating point for the gyro path length control (pic) at power up of the laser gyro. Three inputs are used in the calculation, the present PLC position, the present gyro temperature, and a model of the particular gyro's response to temperature. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic block diagram of a path length control system in accordance with the present invention. Figure 2 is illustrates a temperature characteristic for a path length control transducer. Figure 3a and 3b illustrate the behavior of a path length control transducer for a laser beam traveling a single straight line path segment for a change in temperature from room temperature to a raised temperature. Figure 4a and 4b illustrate the behavior of a path length control transducer for a laser beam traveling a single straight line path segment for a change in temperature from a raised temperature to a room temperature. Figure 5a and 5b illustrate the behavior of a path length control transducer for a laser beam traveling a single straight line path segment for a change in temperature from a raised temperature to a room temperature when incorporating the path length control system in accordance with present invention. DETAILED DESCRIPTION OF THE INVENTION The optimum PLC position is based on a room temperature power up of the gyro. At the factory, gyros are powered on at room temperature and then varied between hot and cold. Data taken during the transitions is then the basis for the calculation. Figure 2 shows a typical plot of a gyro's PLC mirror position versus temperature in terms of the laser beam wavelength. The object of the invention is to force the PLC control to follow this same curve at every power cycle, rather than allowing it to jump to some parallel but offset curve. Figures 3 and 4 show how the prior art handles both a room temperature and a high temperature power up of a one legged gyro. (Invention applies to three legged gyro, but a simple one legged gyro was drawn for illustrative purposes). The top half of Figure 3 shows that the prior art always positions the gyro mirrors near the center of their travel at power up. As the laser block or body of gyro (e.g., a low temperature coefficient glass or glass ceramic block) expands or shrinks with temperature, the PLC will move the mirrors in relation to the body in order to maintain the absolute distance between the mirrors at a constant. If it is assumed that this example follows the temperature curve in Figure 2 and that the mirrors can be moved either in or out one full wavelength, then it can be seen that this example could power up at room temperature and operate properly out to either temperature extreme. Figure 4 shows the shortcomings of the prior art. If the example gyro were to power up at high temperature, its mirrors would reach the limit of their movement at room temperature, and hit their stops before getting to the cold end of the curve. Figure 5 graphically illustrates how the present invention provides a distinct advantage in the same situation as shown in Figure 4. At the time of power up at a high temperature, the mirror is "preset" to an extreme position of its travel in anticipation of shrinkage of the gyro body. By the time the gyro body has reached room temperature and shrunk by one wavelength, the mirror is in its middle position with a full wavelength of travel still available to compensate for additional body shrinkage. The invention is not limited to the simple case illustrated by the Figures. The invention is implemented by storing the full curve, and presetting the PLC to match the curve at any temperature that the gyro was factory tested at. A unique curve is stored for each gyro, allowing individual gyros to have wildly different responses, but still able to take full advantage of the invention. The preset itself is accomplished by injecting a measure amount of charge into the PLC integrator from the computer. A digital signal is summed into the feedback signal from the gyro, and synchronized with the demodulator. In this manner, no digital to analog (D/A) converters are needed, and the implementation is kept simple. Figure 1 is a block diagram of one embodiment in accordance with the present invention. Dashed block 10 includes a block diagram for path length control circuit for ring laser gyro as particularly shown and described in U.S. Patent 4,152,071, issued to Podgorski. Dashed block 10 includes a ring laser gyro 12 including path length control actuators 14 and 16 for affecting the position of ring laser mirrors 14m and 16m, respectively. Ring laser gyro 12 includes a partially transmissive mirror 18m for providing output light representative of the intensity of one of the counter- propagating laser beams of ring laser block 15 travelling in a closed-loop path. Photodetector 20 provides an output signal to amplifier 22 for providing an electrical signal output 23 representative of the intensity of one of the counter- propagating laser beams 5a or 5b. The output signal 23 is provided as an input to demodulator 30 through a summing means 40. In turn, the output of demodulator 30 is integrated by integrator 45 whose output is amplified by high voltage amplifier 50. The output of high voltage amplifier 50 is presented to actuator 14, and to actuator 16 through inverting amplifier 52. The path length control circuit, dashed block 10, also includes an oscillator 60 having the output thereof as one input to demodulator 30 and as one input to summing means 62 which sums the output of oscillator 60 and the output of integrator 45. It is well understood, that the oscillator 60 provides a dither signal to provide a closed- loop servo control for maintaining the intensity of one of the laser beams 5 a or 5b at maximum intensity. Also shown in Figure 1 are additional components including an analog-to- digital converter 210, computer 220 and gyro temperature history data block 230. The output of integrator 45 is converted to a digital signal by A/D converter 210, which in turn provides one data input to computer 220. Also included in the circuit of Figure 1 is a gyro temperature sensor 230 having an output thereof electrically connected to A/D converter 210, which in turns provides a second input into computer 220. It should be noted that actuators 14 and 16 including mirrors 14m and 16m may be provided by a variety of techniques, similar to that shown and described in U.S. Patent 3,581,227, issued to Podgorski. The operation of computer 220 will now be described. Computer 220 receives look up table information from gyro temperature history data block 230. Temperature history data is substantially characterized as that illustrated in Figure 2. That is, the desired path length control position of mirrors 14m and 16m is substantially as illustrated in Figure 2. For example, at temperature Tχ 5 it is intended that the path length control position should be H-λ (1 wavelength) away from its neutral position. Similarly, at near -40°F, the path length position should be -λ away from its neutral position. A computer 220 responds to information representative of the gyro temperature of the sensor, and information representative of the actual path length control position in order to determine the difference between the actual path length control position and the desired path length control position, and provide the difference thereof. Computer 220 provides output signals A and B which are inputs to switching means 282 and 284, respectively, of signal generating means 280. Signal generating means 280 includes oscillating signal 281 which is provided as one input to switching means 282 and to switching means 284 through inverting amplifier 285. The second output of both switching means 284 and 282 are electrically connected as the second input to summing means 40. The function of computer means 220 in combination with signal generating means 280 is intended to provide a "preset" condition of the path length control device by injecting a measured amount of charge into the path length control integrator. One method of providing this preset input into the integrator is by applying a 50 millisecond oscillating signal into the input of demodulator 30, either in phase or out of phase in order to effect a ±lλ path length control position change of mirrors 14m and 16m in total. For example, consider a situation when the temperature is Tx desiring a path length control of lλ, and that the actual path length position is 2λ. Computer means 220 is operative to determine that the difference between the actual path length position and the actual path length position for the gyro temperature T differs by lλ and provides selectively an output signal on signal lines A or B for a period of 50 milliseconds in order to advance the position of the mirrors by an effective path length change of lλ. On the other hand if the opposite was true where the difference was -lλ, the other of signal lines A and B would be actuated for a period of 50 milliseconds to provide the proper preset to integrator 45 to effect a -lλ path length position change. This process would be repeated periodically, for example, once a second until the optical path length position of mirrors 14m and 16m were within, say for example, .5λ, at which time no further preset charge would be applied to the integrator 45, and the path length control would operate in a manner well known in the prior art. Of course, signal generating means 280 may be incorporated into computer means 220 and has been illustrated in order to enhance understanding of the present invention. More, specifically, computer means may provide an oscillating signal change for a prescribed length of time for achieving the desired preset for integrator 45. Those skilled in the art will recognize that only a preferred embodiment of the present invention has been disclosed herein and that the embodiments thereof may be altered and modified without departing from the true spirit and scope of the invention as defined in the accompanying Claims.

Claims: CLAIMS The embodiments of an invention in which an exclusive property or right is claimed are defined as follows: L A ring laser gyro path length position control comprising: a ring laser including a plurality of mirrors for establishing a pair of oppositely traveling laser beams about an optical closed-loop path, wherein at least a first mirror of said plurality of mirrors is coupled to a mirror transducer for controlling the position of said first mirror; a path length control circuit responsive to at least one of said laser beams for providing a transducer control signal operative for controlling the position of said first mirror to maintain the intensity of said one of said laser beams at a preselected condition; temperature sensing means for providing an output signal representative of the temperature of said ring laser; and computer means for operating on data representative of the position of said first mirror, said temperature of said ring laser, and data characteristic of the desired relative position of said first mirror as a function of the gyro temperature, so as to affect said path length control circuit in a manner to cause the position of said first mirror to approximate said characteristic data at said gyro temperature. 2. The apparatus of Claim 1 wherein: said path length control circuit includes, demodulator means for operating on the sum of a first signal representative of the intensity of one of said laser beams, and a second signal, integrating means for integrating the output of said demodulator means, and first means for operating on said integrating means output signal and providing said computer said data representative of the position of said first mirror; and said computer means provides said second signal as a function of the difference between said position of said first mirror and the desired position of said first mirror at said first mirror temperature.

Inventors: HROVAT ALBERT C, HROVAT, ALBERT, C.
Applicants: HONEYWELL INC, HONEYWELL INC.
Application year: 1993
Main classification: G01C
IPC codes: G01C19, H01S3