Electromechanical Servomechanism

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.

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 servosystems are frequently encountered in practice. They can be used as positioning systems or drive systems. At the core of these systems one typically finds a DC (direct current) motor connected to a shaft which drives the load, usually via a gear box.

The DC (direct current) motor is an actuator that converts electrical energy into rotational mechanical energy that is then applied to the load. DC motors are widely used in many control applications including robots, machine tools, valve actuators and also tape, disk and CD drives.

The goal of this laboratory is to study the dynamics and control of a DC electromechanical servosystem. The virtual laboratory is based on a typical ‘class room’ style apparatus found in many University laboratories. Indeed, this virtual laboratory has been designed to closely emulate a physical laboratory located at the University of Newcastle, Australia.

The physical laboratory setup that this virtual lab is modeled on is shown in Figure 1.2. The setup consists of a servo kit, signal generator, power supplies, power amplifiers and a PC that contains an analog I/O card and runs control software. The summation block, controller block, gain blocks and oscilloscope are all implemented via the control software.

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

The physical servo kit itself is illustrated in Figure 1.3. The tachometer is directly connected to one end of the motor shaft, the other end of the motor shaft has an inertia weight clamped on to it and connects to a gearbox. The gearbox has a reduction ratio in the order of 30:1. The gearbox then drives both an angular marker disc and a potentiometer.

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