Ratioed Gains

Position or velocity control axes that behave differently in either direction of motion are called asymmetric systems. Such systems require different gains in each direction. The RMC can ratio the gains such that the system controls identically in both directions. A single-rod hydraulic cylinder is an asymmetrical system. A motor with the same load in both directions is typically symmetrical.

A related concept is using different gains for different control types. See the Gain Sets Overview topic for details.

To apply different directional gains to an asymmetrical system, the RMC ratios the gains. To ratio the gains, set the Symmetrical/Ratioed parameter to Ratioed. This creates two Velocity Feed Forwards, one for each direction. Once the Velocity Feed Forwards have been tuned, the gains in the direction of the highest Velocity feed Forward will be applied as they are. The gains in the direction of the lowest Velocity feed Forward will be decreased by the ratio of the Velocity Feed Forwards. This matches the PID output to the system dynamics in each direction, resulting in very good control of the cylinder.

The RMC uses the Target Velocity and Position Error to determine whether to use the forward or reverse gains. If there is a non-zero Target Velocity, then its sign determines which directional gains to use. If the Target Velocity is zero, then the Position Error (or Velocity Error for Velocity Control) determines the intended direction of movement.

Note:
Ratioed gains are automatically selected by the Tuning Wizard if moves in both directions are used.

Example

Consider a single-rod hydraulic cylinder that moves at 3 in/sec when a Control Output of 1V is applied and -2.3 in/sec when a Control Output of -1V is applied. Since it moves faster in the positive direction, those gains need to be smaller.

The correct Positive Velocity Feed Forward for this system is:

1 [V] x 10 [%/V] / 3 [in/s] = 3.33 [%/in/s]

The correct Negative Velocity Feed Forward is:

-1 [V] x 10 [%/V] / -2.3 [in/s] = 4.35 [%/in/s]

By choosing Ratioed gains, the gains in the negative direction will be applied as they are. The gains in the positive direction will be multiplied by 3.33/4.35, which is 0.766.

(NOTE: The RMC200 Control Output is in percentage in RMCTools vs. Voltage for the RMC75/150.)

Choosing Ratioed Gains

Use the Symmetrical/Ratioed parameter to choose ratioed gains. This parameter is located in the Axis Tools, Axis Parameters pane, on the Tune tab.