Elevator Subsystems
In FRC an elevator is a subsystem that moves something up and down in a linear fashion. Usually this is accomplished by one or more motors driving a spool through gears or pulleys. The ratio of motor revolutions to spool revolutions is the gear ratio G. In this arrangement the spool winds up and lets out a cable to raise or lower the mechanism. The linear movement is π * D, where D is the spool diameter plus the thickness of the strap or cable on the spool. Another approach would be to have the motor drive a gear meshed with a linear gear to directly raise and lower the mechanism.
An elevator motor is typically commanded though a feedback controller that reads the position (height) of the mechanism so it can be moved to the specified position and held there, even if the mass changes due to holding or releasing an object. This can be done with a PID Controller or Profiled PID Controller. A Feedforward can be used along with the PID controller to provide a better response.
With an elevator the effect of gravity is constant over the range of movement, so the control approach is simpler than for an arm which has angular movement.
References:
WPILib: Introduction to DC Motor Feedforward: Elevator Feedforward
https://docs.wpilib.org/en/stable/docs/software/advanced-controls/introduction/introduction-to-feedforward.html#elevator-feedforward
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