Position Control with Speed Feedbac

Position Control with Speed Feedback

When the gain is raised in a position control system to minimize the deadband effect, the closed loop system responded with an overshoot which resulted in undesired system oscillation. One way to mitigate oscillation is to add a brake which is proportional to the speed to the output shaft. The brake method may produce a satisfactory result. However, it consumes a significant power and makes acceleration of the load difficult.

Better way of preventing oscillation is to add a speed control loop to the position control loop. The speed control loop provides a negative feedback signal from the output of the Tachogenerator which is proportional to the speed of the motor.

The effect of adding a speed loop is illustrated in Figure 1 (a), (b), and (c). An optimum control of the speed feedback loop produces a system response as shown in (b).

Figure 1: Effect of a speed feedback loop to the system response

An actual system with both the speed and position control loops is shown in Figure 2. As it can be seen in the Figure 2, this system is essentially the same system as experimented in the previous two sections, except that one more loop which is consisted of the Tacho circuit and VR2 is added. To obtain the waveforms in Figure 1, it is needed to replace the input potentiometer with a squarewave input and connect Po signal to an oscilloscope.

Figure 2: Speed control loop placed inside a position control loop

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