Abstract
Phase-locked loop technique permits the precise control of motor speed. In the PLL speed control system, the speed accuracy of 0.002% may be achieved. This represents almost a hundredfold improvement over a conventional method of speed regulation.
This paper presents a microprocessor-based digital PLL speed control system and describes several methods to analyse the system performance. In this system, digital techniques provide all the control functions such as detection of speed and phase error of the rotor, signal processing and the determination of the control signal of the power converter. They can be achieved with a simple circuit architecture by using a microprocessor. The completely digital scheme is free from drift and offset error which are inherent in an analog circuit.
Sampling, time delay and quantization of phase error detection affect respectively the stability, dynamics and speed error of the system. For the analysis of system performance, the digital simulation based on the state-space method is used. The stability, dynamics and speed error of the system are additionally analysed with the simplified models.
An experimental apparatus including a 2.2kW DC motor is built and the system is tested. The experimental results cofirm the validity of the analytical methods mentioned above. The speed accuracy observed is 0.028% or less and the possibility to reduce the figure to 0.0018% is shown.
