Motor Tuning Overview
Motor tuning is perhaps the most difficult aspect of scenic automation. By tuning a motor, you are describing to a Stagehand how it should regulate the motor speed during a cue using mathematical formulae. We won’t get too deep into the intricacies of that math, but it is hard to discuss motor tuning without getting a little involved in the nuts and bolts of motion control theory.
First, it helps to understand the problem that motor tuning solves. The best analogy for motor tuning is cruise control on a car. When you set your cruise control to 55 mph, the car has to adjust the amount of fuel going to the engine to keep moving at that speed. If you are going up hill, the car lags for a second and then more gas rushes into the engine and the car speeds up to get back to the desired speed. Similarly, the car needs to regulate the amount of gas when going down a hill.
Motion control for scenery is very similar. We program the Stagehands with a speed and target position, and the Stagehand has to adjust power to the motor to achieve the desired speed and target. If there’s a bump in the floor, the winch has to have a little more power to maintain speed. If the winch has too much power, it will lurch over the bump and then quickly reduce power so that it doesn’t go too fast once it is over the bump.
The Stagehand is constantly analyzing where the motor is versus where it should be and then adjusting motor power to minimize the difference between where the motor should be and where it really is. It does this analysis a few million times per second. When it wants to apply power to correct for the error in position, it looks to us for guidance.
By entering in some tuning values, we are giving the Stagehand that guidance. In a confounding abstract way, we are specifying how much power to give the motor when it needs to make a correction. If the values that we enter gives the motor too much power during correction, the motor will be jerky as it over-corrects and then has to pull back (remember, this happens millions of times per second). If the values we enter do not provide enough power to the motor to correct position, it will never reach the cue’s target position since it will run out of power and be unable to muscle the load onto the target.