Electromechanical Servomechanism - Classical Design Virtual Laboratory

Summary

Electromechanical servosystems are frequently encountered in practice. The goal of this laboratory is to study the dynamics and control of a DC electromechanical servosystem using classical control design techniques.  Ultimately the student is guided to implement both proportional feedback control and with and without tacho feedback.

The laboratory is based on this typical “classroom” style apparatus found in many University laboratories.

The non-ideal features of the physical setup have been replicated in this virtual laboratory including the power amplifier output limits, potentiometer “wrap-around” and signal noise is included.

Figure 1.1: Screenshot of Program
Figure 1.1: Screenshot of Program

The Physical Apparatus

Electromechanical servo-mechanisms are frequently encountered in industry. In this laboratory, the servo-mechanism will be used as a positioning system. Fundamentally the system comprises of a simple Direct Current (DC) motor, a gearbox and two metal discs rigidly coupled.

The DC motor is an actuator that converts electrical energy into rotational mechanical energy.  The motor itself has a permanent magnet field and external terminals connecting to the armature winding. Direct Current motors are widely used in many control applications including robotics, machine tools, valve actuators and also tape, disk and CD drives.

Shaft position is measured by a sensor located on the disc and shaft speed is provided by a tacho-generator connected directly to the motor.

The aim of this laboratory is to implement a controller to position a disc mounted on the servomechanism

shaft at a desired angle.

The physical system emulated by this virtual laboratory is shown in Figure 1.2. The setup

consists of a servo-mechanism, signal generator, power supply, power amplifiers and a personal computer that contains an analog input/output card that runs the control software.

Figure 1.2: The Physical Laboratory Setup
Figure 1.2: The Physical Laboratory Setup

The physical servo-mechanism is shown in Figure PS.14. The tachometer is directly connected to one end of the motor shaft, whilst the other end of the motor shaft is connected to an inertial mass and gearbox. The gearbox has a reduction ratio in the order of 30:1. The gearbox output drives both an angular marker disc and a potentiometer. Note that non-ideal features of the physical system have been replicated in the virtual laboratory which include the power amplifier output limits, potentiometer “wrap-around” and signal noise. A schematic of the proposed position control system for this laboratory is shown in Figure 1.3.

Figure 1.3: The Servo Kit
Figure 1.3: The Servo Kit

Note that the non ideal features of the physical setup have been replicated in this virtual laboratory including the power amplifier output limits, potentiometer ‘wraparound’ and signal noise.

A simplified representation of the system is given in Figure 1.4. In describing the system it is not necessary to keep track of the pre and power amplifier gains or the gear box reduction ratio. We will refer to the combined gain, Km, as the ‘DC gain of the motor’, even though it involves all of the above gain terms including the DC gain of the motor itself. Likewise we will refer to θ as the “motor shaft angle” even though it is actually the angular displacement of the gearbox output shaft.

Figure 1.4: Block Diagram of Open Loop Servo System
Figure 1.4: Block Diagram of Open Loop Servo System

In this virtual laboratory, we will design control systems which will enable the motor shaft angle to be controlled, using the feedback configuration shown in Figure 1.5. Before we do this we will first determine the values of Km, Kt, Kp and τ via simple experiments.

Figure 1.5: Closed Loop Position Control System
Figure 1.5: Closed Loop Position Control System

Prerequisites

This laboratory would be suitable, for a first course in control. The assumed knowledge includes:

  • step responses
  • second order dynamics
  • transfer functions
  • proportional feedback
  • tacho feedback

Learning Objectives

The learning objectives of this virtual laboratory include:

  • physical modelling
  • estimation of model parameters using simple physical measurements
  • proportional feedback
  • proportional plus tacho feedback