154 CHAPTER 4. FUZZY CONTROL
Figure 4.12. Simulink diagram for DC servomotor control using
the transfer function model of the DC servomotor
Using the prototype membership functions, the scope outputs for Position
Error, Error Velocity (shown as the error derivative), the actual Rotor Position,
and the Fuzzy Controller output, namelythe required Motor Voltage, are shown
in Figure 4.13.
Figure 4.13. Simulated output of DC servomotor controlAs in the previous example, it is clear from the simulated outputs that rapid
prototyping of the controller is possible, but the membership functions have to
be ìtunedî to yield the desired output characteristics. We see that for a step
change of 1 radian (the desired position), it takes in excess of 100 milliseconds to
reach the desired value. If we focus on the membership functions for the applied
voltage, we observe that for a large initial error we generate a large voltage. As
the error progressively becomes small, we apply a smaller and smaller voltage,
ultimately reducing the voltage to zero upon reaching the desired position. We
could therefore adjust the membership function for the applied voltage so that
we apply a large voltage until the rotor has reached very close to its desired