Servo System

The ZY servo system's function is to position a beam steering mirror in one of three modes (see figure 2.1):

1. illuminate a target retroreflector with the instrument's laser beam
2. aim a retroreflector on the back of the mirror mount toward another ranger
3. aim the main mirror toward another ranger.

Each axis of the ZY servo system consists of a DC servo motor with a Heidenhain Corporation 100,000 count per revolution (interpolated) incremental encoder, a 21 kHz pulse width modulated servo amplifier, and a 6 MHz National Semiconductor LM628 Precision Motion Controller[11][12]. The ZY controls the two LM628 axis controllers using 2 software objects of class LM628 (an ``azimuth'' object and an ``elevation'' object). Class LM628 encapsulates all the functionality of the LM628 controller. Earlier versions of the ZY software would monitor the operation of the LM628s by polling them. Since early prototypes of the ZY hardware used the faster 8 MHz version of the LM628 and relatively slow 10 MHz i286 Single Board Computers (SBC), this presented no problems. But later, because of cost considerations, we switched to slower 6 MHz LM628 controllers, and as the SBCs became faster, polling was found to be detrimental: despite the use of the LM628 busy bit[12], the LM628 was so busy responding to the polling requests that it would miss encoder counts, thus losing its zero point. To remedy this, completion events for the LM628 objects are now handled by ISRs, one for each object. When an LM628 event occurs, the IRQ for that controller asserts, and the ISR then sets event flags in the object whose axis asserted the IRQ. This eliminates polling for all but a couple of functions. (These are: finding home and checking for settling. In these cases, a 1 mS delay is introduced to avoid over-polling). Before the servos can be used to aim the laser beam to a target, the ZY (or its client) must ensure that the following steps are taken:

1. Reset and initialize the LM628 motion controllers.
2. Load the PID and Acceleration/Velocity values for the LM628 motion controllers.
3. Turn on the axis amplifiers.
4. Find home.

Once these steps have been performed, the servo system is ready to perform its function.