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1、The contral techniques drives and contrals handbookChapter A4Torque, speed and position controlA4.1 General principlesA4.1.1 The ideal control systemMany applications exist where something has to be controlled to follow a reference quantity. For example, the speed of a large motor may be set from a

2、low-power control signal. This can be achieved using a variable-speed drive as described in the following.Ideally, the relationship between the reference and the motor speed should be linear, and the speed should change instantly with changes in the reference. Any control system can be represented a

3、s in Figure A4.1b, with an input reference signal, a transfer function F and an output. For the system to be ideal, the transfer function F would be a simple constant, so that the output would be proportional to the reference with no delay.Figure A4.1 Variable-speed drive and motorA4.1.2 Open-loop c

4、ontrol Unfortunately, the transfer function of many practical systems is not a constant, and so without any form of feedback from the output to correct for the non-ideal nature of the transfer function, the output does not follow the demand as required. Using an induction motor supplied bya simple o

5、pen-loop variable-speed drive as an example, the following illustrates some unwanted effects that can occur in practical systems:Speed regulation. The output of a simple open-loop drive is a fixed frequency that is proportional to the speed reference, and so the frequency applied to the motor remain

6、s constant for a constant speed reference. The speed of the motor drops as load is applied because of the slip characteristic of the motor, and so the speed does not remain at the required level. Instability. It is possible under certain load conditions and at certain frequencies for the motor speed

7、 to oscillate around the required speed, even though the applied frequency is constant. Another major source of instability in rotating mechanical systems is low-loss elastic couplings and shafts. Non-linearity. There are many possible sources of non-linearity. If, for example, the motor is connecte

8、d to a gearbox, the speed at the output of the gearbox could be affected by backlash between the gears. Variations with temperature. Some aspects of the system transfer function may vary with temperature. For example, the slip of an induction motor increases as the motor heats up, and so for a given

9、 load the motor speed may reduce from the starting speed when the motor was cold. Delay. With a simple open-loop inverter and induction motor there can be a delay before the motor speed reaches the demanded level after a change in the speed reference. In very simple applications such as controlling

10、the speed of a conveyor belt, this type of delay may not be a problem. In more complex systems, such as on a machine tool axis, delays have a significant effect on the quality of the system. These are just some of the unwanted effects that can be produced if an open-loop control system is used. One

11、method that improves the quality of the controller is to use a measure of the output quantity to apply some feedback to give closed-loop control.A4.1.3 Closed-loop control The simple open-loop drive of Section A4.1.2 can be replaced with a control system as in Figure A4.2. This control system not on

12、ly provides a means to correct for any error in the output variable, but also enable a stable response characteristic A4.2 Controllers in a driveA4.2.1 GeneralAlthough a modern variable-speed drive includes many features, the basic function of the drive is to control torque （or force）, speed or posi

13、tion. Before proceeding to the specific details of how different types of variable-speed drive function, the theory of control for each of these quantities is discussed. A position control system is shown in Figure A4.5. This includes an inner speed controller, and within the speed controller there

14、is an inner torque controller. It is possible to create a system where the position controller determines the mechanical torque that is applied to the load directly without the inner speed and torque loops. However, the position controller would need to be able to control the complex combined transf

15、er function of the motor windings, the mechanical load and the conversion from speed to position.Therefore it is more usual to use the format shown in Figure A4.5. The other advantage of this approach is that limits can be applied to the range or rate of change of speed and torque between each of th

16、e controllers. When a system is required to control speed only, the position controller is omitted, and when a system is required to control torque only, the position and speed controllers are omitted.A position sensor is shown providing feedback for the system, but this may be replaced by a speed sensor or it may be omitted altogether as follows.Position information is required by the torque controller to function in an