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Motion Control

So you want to build a robot? A good place to start is with the servo control systems - the robot's muscles!

"What is servo control?" Imagine a simple motor. If you connect it to a battery, it will start spinning. If you connect  two batteries, it will spin faster. Now imagine you tell the motor to turn precisely 180 degrees (1/2 revolution) and stay there no matter how many batteries there are. That's servo control.

Central to the task of servo control is the concept of negative feedback. As an example of negative feedback, consider what happens when you are hungry. Hopefully, you will be able to get something to eat. As you eat, you become less and less hungry until you eventually stop eating. This is the idea of negative feedback. Imagine if the opposite were true and eating made you hungrier. You would eat until you exploded - out of control! Please don't build a robot that acts like that.

Negative feedback in a servo control system proceeds in a similar fashion. There is a desired position for the motor and a feedback sensor that tells the motor it is not at the correct position. In effect, it is "hungry" to get to that position and begins turning towards it. As the motor gets closer to the desired position, the feedback device tells the motor that it is becoming "less hungry" and the motor responds by turning more slowly. In a perfect control system, the motor will get to exactly the right position and then will turn no more until it is commanded to a new desired position.

Motion Control includes much more than servo control. Machine setup, for example, involves a very important set of motion control operations. Machine setup includes operations such as setting feed rates, setting offsets, setting limits and writing files. Similarly querying for feed rates, querying for offsets, querying for limits and reading files are important motion control operations. Sending motion start and motion stop commands are further examples of motion control operations. Querying for machine state is another example of an important motion control operation.

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