Control Modes Overview

This topic describes various modes of control. For details on motion types, see the Controller Features Overview.

Open Loop Control

Open Loop Control is the simplest form of control. A control output is given to a system, for example 2 volts, causing the motion system to move at some approximate speed. Open loop control does not use the feedback to determine how much Control Output should be given. Therefore, in open loop control, there is no way of commanding the system to go exactly at a specific speed or go to an exact position. See the Open Loop Control topic for more details.

Closed Loop Control

Closed Loop control uses feedback from the system being controlled. For example, a command is issued to go to 20 inches. The RMC computes a target (a motion path) to get to 20 inches. For each control loop, the controller (the RMC) uses the feedback and the gains to compute the amount of Control Output that should be given to the system so that it follows the profile. The system will automatically go to 20 inches. See the Closed Loop Control topic for more details.

The RMC offers several different algorithms for closed-loop control, depending on the feedback type:

Feedback Type	Closed-Loop Control Algorithms
Position	Position PID Advanced: Position I-PD Velocity PID Velocity I-PD
Velocity	Velocity PID Advanced: Velocity I-PD
Pressure	Pressure/Force Control, Pressure/Force Limit
Force	Pressure/Force Control, Pressure/Force Limit

High-Order Control

The RMC also supports Acceleration Control and Active Damping, for difficult-to-control systems such as pneumatic cylinders.

Other

Cascaded Loops

Gain Scheduling

Unidirectional Mode

Unidirectional Mode, also known as Absolute Mode, is specifically designed for systems that require a unipolar control signal. Some common cases are:

Two Hydraulic Valves
Some hydraulic systems have two valves, one for direction control, the other for flow control. Unidirectional mode is for controlling the flow valve. The directional valve must be controlled by other means, but Unidirectional Mode does provide for easy switching of the control direction based on the directional valve setting.
Unidirectional Belt
A belt that must always move in the same direction.

For more details, see the Unidirectional Mode axis parameter.

Velocity and Torque Drives

Most actuators, together with their power source and/or drive electronics, can be classified as velocity mode or torque mode. Which type it is affects the tuning procedure and how the actuator handles certain RMC commands.

A velocity mode actuator produces a speed proportional to the Control Output. A torque mode actuator produces a torque or force proportional to the Control Output.

For details, see the Velocity and Torque Drives topic.