Page 62 - Anatomy of a Robot

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CONTROL SYSTEMS 47

FIGURE 2-18 A baseball pitching robot trying for the Cyborg Young Award

If the robot is susceptible to oscillations at specific frequencies, consider altering
v to a frequency that might work better inside the system.
Unknown oscillations Sometimes robots will just not follow the model and
behave properly. That’s okay. Kids behave the same way and it’s all part of the joy
of living. The result is that instabilities might develop with severe vibrations or
even wild behavior. (This sounds more like my family by the minute.) With the
kids, we can experiment with cutting down on the sugar. With robots, we can con-
sider taking two actions:
Perform the actions mentioned earlier to get rid of severe ringing.
Look for design flaws in the mechanics and control system that would make it
more complex than the second-order system we’re trying for. Look for places
energy might be stored that we didn’t expect. Change the design to compensate
for it.
What happens when we take a second-order system and try to put it in a closed-loop
feedback system? Well, consider the following closed-loop feedback control system
(see Figure 2-19).
Let’s assume the actuator is a second-order system such as the one we have studied.
As we’ve seen, it will not react immediately to a step input function. It goes through
some delay, a rise time, and then a settling time. Suppose we wildly put inputs into the