Ferranti approached Pan Controls with a problem requiring exceptionally high accuracy. They needed to rotate a mirror to within 4 arc seconds of a set of predetermined points for a laser system. To complicate matters they could not tolerate any friction. It was therefore necessary to use a motor mounted on air bearings and without any tacho feed-back. Without tacho feed-back such a motor is inherently unstable. The stability must therefore be achieved solely by the control software. A Pan controller was tuned to provide both the exceptional accuracy and necessary stability without overdamping. The system response turned out to be well within Ferranti's specification.
This project illustrates the very high level of control which can be achieved by a purely digital system using an optical encoder of suitable resolution and an efficient control algorithm. The Pan control algorithm actually has five terms:
1. Proportional positional error feedback - to achieve basic error correction.
2. Differential positional error feedback - useful for damping in systems with very high accelerations.
3. Integral positional error feedback - to eliminate permanent offsets in stationary systems subject to steady offsetting forces. This term is specified by both a magnitude and a time constant for maximum flexibility.
4. Velocity feed-back - similar effect to differential position error feed-back - usually the best method of achieving damping and stability of unstable systems.
5. Velocity feed-forward to achieve accurate following of predetermined profiles.