Control System Basics

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Driver Station.jpg
In the FIRST Robotics Competition, the robot control system is unique because teams have a set list of parts that they must use to power and control their robot.  There is variation in how teams use the permitted parts, but at their core, every robot’s electrical system is the same.  Despite this mandated consistency, assembling a reliable and robust electrical system is one of the most challenging and confusing aspects of building a robot.

This tutorial will teach you about each of the components in a FRC control system.  For each component, I will describe its role in a robot’s electrical system and how it connects to other components.

This tutorial was made through the Autodesk FIRST High School Intern program.

A willingness to learn

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Step 1: Terminology

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  • Digital Signal: A digital signal has only two states: on or off.  Robot components that communicate with a digital signal generally do so by varying the frequency at which they alternate between on and off.
  • Analog Signal: Unlike a digital signal, which has discrete values, an analog signal’s output has a continuous range of values.  Robot components that use analog signals communicate by varying the voltage of the signal.
  • PWM Cable: A Pulse-Width Modulation (PWM) cable is a cable commonly used to allow robot components to communicate with each other, using either digital or analog signals.  Most commonly, they have three wires, but there exist versions with anywhere from one to 5 cables.
  • CAN: In FRC, a Controller Area Network (CAN) is used to control motor controllers.  Several motor controllers are connected serially to the same network and then controlled from the central processor.
  • Encoder: An encoder is a sensor used to measure rotation.  It uses a digital signal to communicate that the shaft it is attached to has rotated a certain amount – “1 count.”  The number of counts per full revolution varies between encoders, but can be anywhere from one to one thousand per revolution.
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kelseymh1 year ago
Excellent writeup. It's more along the lines of a "theory of operation" document, rather than a how-to for building a specific system, but it's still great.

I notice that in Step 5 you use the term "CAN bus" several times. Besides it's nearly universal use with passenger-car computer systems, you might be interested to know that CAN bus is also used as part of the environmental control systems for some particle physics experiments and accelerators.

On BaBar, we used CANbus for environmental monitoring of pressures, temperatures, gas and fluid flow rates, and so on, and also to control power supplies and valves on the detector. The SSRL accelerator at SLAC also uses CANbus as an integral part of the accelerator monitoring and control system.