Velocity and Torque Mode

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

Definition

The actuator type is defined by its response to the Control Output voltage applied by the RMC:

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.

In practice, this means that if you issue an Open Loop Rate command with a certain amount of Control Output to a velocity mode actuator, the actuator will move at a speed roughly proportional to the Control Output voltage. If you issue the Open Loop Rate command to a torque mode actuator, the actuator will provide a torque proportional to the voltage. The actuator speed will keep increasing until the torque is equal to the friction in the system. The final drive speed for a torque mode actuator is not necessarily proportional to the voltage.

Note:
It is not recommended that you issue Open Loop commands to a torque mode system during normal operation, because you cannot predict the final speed. Use closed loop commands instead.

Examples

Velocity mode actuators:

Hydraulic cylinder
Motors with a velocity drive

Torque mode actuators:

Motors with an amplifier (without an inner velocity loop)

Effect on Tuning

Whether an actuator is velocity or torque mode affects the tuning of a system in the following ways:

Damping
Velocity mode systems typically have high damping because they use a velocity-mode drive, or for hydraulic cylinders, they have high damping during motion, and when stopped, don't move because of the low compressibility of oil. In these cases, the system has, or appears to have, significant damping. Torque mode systems, however, typically have little damping. That means that if given some Control Output, they will coast after the Control Output goes back to zero. When tuning a torque drive, some damping must be provided initially with the Differential gain. This is the primary difference between the tuning methods of velocity drives and torque drives.
Feed Forwards
On a velocity mode system, the Velocity Feed Forwards often provide most of the drive required to move the axis and may therefore be large. On a torque mode system, the Velocity Feed Forwards are basically only for overcoming friction and are often small. On torque mode systems, the Accel Feed Forwards do a lot of the work.

Effect on Commands

The Open Loop Absolute (11) and Open Loop Relative (12) commands are intended only for velocity drives. The Open Loop Rate (10) command can be used on either, but the Control Output voltage should be kept very low for torque mode systems to keep them from running away.