Introduction
In making a choice of the correct motor for a particular need, there are a number of factors to be considered; for example, speed of response, accuracy, and dynamic error due to an external disturbance, coupled with capital costs, reliability, and availability. In the previous chapters, dynamic properties of several types of servo motors were discussed and by obtaining the mathematical model the dynamic performance may be studied. In this chapter, a systematic approach is given for making the choice on the basis of a number of simplified graphs of performance.
Servo motors are manufactured in various forms, speed ratings, and weights. In some applications, it is required to use a gearbox for two reasons of increasing the output torque and reducing the effect of load inertia referred to the motor. It is, therefore, difficult to compare them. To help the designer to select a servo motor, this chapter tries to introduce some common variables so that it is possible to com- pare them.
In recent years, considerable effort has gone into improving servo motor performance, and a wide variety of servo motors are available with comparable characteristics over a range of power ratings. Hydraulic servo motors have been common in the past mainly because of their high power to weight or size ratio. Problems such as the effect of contamination and the necessity for a hydraulic power unit have diverted the attention of designers from hydraulic to electric motors. At the same time, electric motors capable of greater overload for reasonable periods of time have resulted in more compact electric drives, and better design and packaging have made them more attractive to the user.
Motors of up to 10 kW power rating of the following type have been compared in this chapter.
1. Ceramic magnet DC motors
2. Rare-earth magnet DC motors
3. Brushless DC motors
4. Stepping motors
5. Variable frequency induction AC servo motors
As discussed in previous chapters, there are also a number of power units available for driving the electrical DC motors and these are thyristors controlled with up to a frequency of 300 Hz and Pulse Width Modulated control unit of up and above 2 kHz pulsations.
It should be noted that only the classical feedback control strategy has been used to compare different types of servo motors. State variable feedback control strategy as presented in Chap. 4 is still the subject of further research even though the theory is well established.
The smallest hydraulic motor available is with power of 10 kW with maximum speed of 5,000 rpm. But electrical motors are available in a wide range of power. Therefore, for small power ratings there are servo motors to meet the required power requirements.
The results of the analysis are given in terms of a number of parameters which have been separated out in a way that is not strictly rigorous, but which allows presentation in a series of graphs which are easy to design purpose. It should be noted that in order to compare servo motors with rated velocity of 1,500 rpm has been used. For each type of motor, the lead-lag network together with an integrator with their linearized mathematical model for small variation of inputs have been opti- mized to get the dynamic behavior for both small input signal and external torque.
When referring to diagrams, the following key has been used:
1. Ceramic magnet DC motors
a. Controlled by 50 Hz thyristor bridge
b. Controlled by 150 Hz three phase thyristor bridge
c. Controlled by pulse width modulated (PWM) with frequency of 2 kHz
2. AC induction motors
3. Stepping servo motors
4. Electrohydraulic servo motors
5. Rare earth magnet DC motors
6. Printed or brushless DC motors