Microprocessor control of electro-mechanical actuators
This thesis is concerned with the use of all-electric systems for the closed loop position control of mechanical (valve) actuators. It embraces a wide range of topics including * the use of 3-phase induction motors and their speed/torque control using Pulse Width Modulation techniques * implementation of both analogue and digital (PID) controllers * Using computer simulation methods for the development of digital control algorithms and tuning techniques * the use of Computer Assisted Tuning methods for tuning up the position control loop. The major hardware activities described here are concerned with the design, development and construction of a 3-phase 115 volt inverter unit, an analogue controller, and interfaces to a single board microcomputer (SBC). The construction and test of the SBC is also described in the text. Details of the use of an analog controller to study and determine the transfer function of the inverter/actuator system is presented. Digital implementation of PID control (for actuator's position) by microcomputer is also described, together with the theoretical development of the control algorithm. Software activities consist of two major parts, plant simulation and software development for the microprocessor (embedded) controller. The derivation of a plant model from the results of on-line testing is given; from this a computer simulation is developed to study the effects of controller tuning parameters on the loop performance. Software development for the embedded controller covers Man-Machine Interfacing, tuning, and control functions. A new approach to the tuning of control systems is developed here, that of computer assisted tuning. Test results are given showing the effectiveness of CAT techniques for the tuning of the actuator position control loop; these tests also demonstrate the performance achieved using a digital PID controller. It is concluded that, provided plant parameters can be established, Computer Aided Tuning enables plant tuning to be carried out to meet specific performance targets (e.g. rise time, overshoot) set by the plant operator. Furthermore this can be carried out by a relatively unskilled operator.