Open Loop Control

Open Loop Control is the simplest form of control. A signal is given to a system, for example, 2 volts of Control Output to a valve, causing a cylinder to extend at a speed determined by the system characteristics (valve, cylinder and load, or drive, motor and load). 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.

Sometimes, improperly tuned closed loop control may cause a system to oscillate or exhibit other erratic behavior. Usually, issuing an open loop command with 0 volts of Control Output will stop the system safely. See Auto Stops for details on halting a system.

Some of the RMC open loop commands have a ramp rate parameter that specifies the rate at which the Control Output ramps up to the Requested Output. The ramp avoids jerking the system.

Open Loop Commands

The RMC has the following open-loop commands:

Open Loop Rate (10)
This is the basic open-loop command. It ramps the Control Output from the current value to the Requested Output at the rate specified by the Ramp Rate parameter. Use this command for normal open-loop moves.
Direct Output (9)
This command is similar to the Open Loop Rate command, except it disables all the safety features of the RMC. It is intended only for testing the Control Output. Use the Open Loop Rate command for all other purposes.
Open Loop Halt (2)
This halt ramps the Control Output from its current value to zero. It also performs other halt actions.
Open Loop Absolute (11) and Open Loop Relative (12)
These are specialty open loop commands. They specify the Control Output as a function of distance. As such, they do require position feedback on the axis.

Partial Open Loop Commands

The following commands use open-loop control for part of the motion. As the axis reaches the requested position, the axis switches to closed loop control, decelerates, and holds position. These commands are useful for fast motion.