Velocity Feed Forward (Negative)

Type:	Axis Parameter Register
Address:	RMC75: Gain Set #0: %MDn.68, where n = 12 + the axis number Gain Set #1: %MDn.135, where n = 12 + the axis number RMC150: Gain Set #0: %MDn.68, where n = 24 + the axis number Gain Set #1: %MDn.135, where n = 24 + the axis number RMC200: Gain Set #0: %MDn.207, where n = 384 + the axis number Gain Set #1: %MDn.237, where n = 384 + the axis number
System Tag:	Gain Set #0: _Axis[n].VelFFwdN Gain Set #1: _Axis[n].VelFFwdN_2 where n is the axis number
How to Find:	Axes Parameters Pane, Tune tab: Position/Velocity Gains
Data Type:	REAL
Units:	%/(pu/s)
Range:	≥ 0
Default Value:	0

Description

The Negative Velocity Feed Forward is one of the gains. It causes the controller to give an amount of Control Output proportional to the speed of the velocity of the axis.

The Negative Velocity Feed Forward applies when the Control Output is negative. The Positive Velocity Feed Forward applies when the Control Output is positive. The Velocity Feed Forwards are only applied when the axis is in Closed Loop control. They help the axis maintain the Target Position while the axis is moving at constant velocity.

The Positive and Negative Velocity Feed Forwards are available only on position or velocity axes with ratioed gains. Position or velocity axes with symmetrical gains have only one Velocity Feed Forward.

All the gains of the axis are ratioed by the Positive Negative Velocity Feed Forwards. Once the Positive and Negative 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 results in very good control of the cylinder in both directions.

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 3.33. The correct Negative Velocity Feed Forward is 4.35. 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.

The Velocity Feed Forward Term is the portion of the Control Output resulting from the Velocity Feed Forwards, which are gains.

I-PD

In Position I-PD and Velocity I-PD control, the Velocity Feed Forwards are used, only if the gains are set to Ratioed. In that case, the Velocity Feed Forward (Positive) and Velocity Feed Forward (Negative) do not contribute to the Control Output. They are used only to ratio the gains for either direction of travel.

If either Feed Forward is set to zero, or they are identical, the gain ratio will be one. Otherwise, for the direction of the largest Feed Forward, the gains will be applied as they. In the direction of the smallest Feed Forward, the gains will be multiplied by the ratio of the smallest Feed Forward divided by the largest feed forward.

Definition

The Velocity Feed Forward units are: percent of the maximum Control Output per position-units per second (pu/sec). The maximum Control Output is 10V, but can be changed using the Output Scale parameter.

The Velocity Feed Forward Term is calculated from the Velocity Feed Forward as follows:

DVFFn

KVFFVTargetn

where

DVFFn	=	Velocity Feed Forward Term at sample n [% of maximum Control Output]
KVFF	=	Velocity Feed Forward [%/(pu/s)]
VTargetn	=	Target Velocity at sample n [pu/s]

Tuning

The Velocity Feed Forwards help the axis maintain the Target Position while the axis is moving. This makes it "easier" for the other gains to control the position of the axis. See the Tuning topic for details on how to properly adjust the Velocity Feed Forward.