input of the velocity control amplifier (KV). The velocity control system makes the system 'stiffer' when presented with torque variations such as wind, movement about the second axis and torque ripple from the motor. Position control loop The outermost loop, the main loop, regulates load position. In this example, position feedback of the actual load position is presented by a Rotary encoder that produces a binary output code. The actual position is compared to the desired position by a digital subtracter that drives a DAC (Digital-to-analog converter) that drives the position control amplifier (KP). Position control allows the servo to compensate for sag and for slight position ripple caused by gears (not shown) between the motor and the telescope Synthesis The usual design procedure is to design the innermost subsystem (the current control loop in the telescope example) using local feedback to linearize and flatten the gain. Stability is generally assured by Bode plot methods. Usually, the bandwidth is made as wide as possible. Then the next loop (the velocity loop in the telescope example) is designed. The bandwidth of this sub-system is set to be a factor of 3 to 5 less than the bandwidth of the enclosed system. This process continues with each loop having less bandwidth than the bandwidth of the enclosed system. As long as the bandwidth of each loop is less than the bandwidth of the enclosed sub-system by a factor of 3 to 5, the phase shift of the enclosed system can be neglected, i.e. the sub-system can be treated as simple flat gain. Since the bandwidth of each sub-system is less than the bandwidth of the system it encloses, it is desirable to make the bandwidth of each sub-system as large as possible so that there is enough bandwidth in the outermost loop. The system is often expressed as a Signal-flow graph and its overall transfer function can be computed from Mason's Gain Formula. References External links Li, Yunfeng and Roberto Horowitz. "Mechatronics of Electrostatic Microactuators for Computer Disk Drive Dual-Stage Servo Systems." IEEE/ASME Transactions on Mechatronics, Vol. 6 No. 2. June 2001. Dawson, Joel L. "Feedback Systems." MIT. Large Telescope Conference 1971, contains full text of Dittmar's presentation. Control theory