Command: Move Velocity (37)

Supported Axes:	Position or Velocity Control Axes
Supported Control Modes:	Position PID, Position I-PD, Velocity PID, Velocity I-PD

See the Commands Overview topic for basic command information and how to issue commands from PLCs, HMIs, etc.

Command Parameters

#	Parameter Description	Range
1	Requested Speed (position-units/s)	any
2	Acceleration Rate (position-units/s2)	>0
3	Deceleration Rate (position-units/s2)	>0
4	Direction Positive (1): Moves at a positive velocity. Speed must be positive. Negative (-1): Moves at a negative velocity. Speed must be positive. Current (2): Moves at the current Target velocity. If the Target Velocity is zero when the command is issued, it will move with a positive velocity. Speed must be positive. Signed (4): The direction of the move is determined by the sign of the Speed command parameter.	a valid integer as described

Description

This command ramps the axis velocity to the Requested Speed. The velocity is ramped at the rates specified by the Acceleration Rate and Deceleration Rate. The Acceleration Rate is used when ramping the speed away from zero and Deceleration Rate is used when ramping the speed toward zero.

The behavior of the motion depends on the control mode:

Position PID, Position I-PD

In these modes, the Move Velocity performs Continuous Position Control. The Move Velocity command does not actually generate a Target Velocity. Instead, it generates a moving Target Position. The Target Position will move at the specified rate until commanded otherwise. Therefore, if the axis falls behind, it will not try to catch up to the Target Velocity; it will actually attempt to catch up to the Target Position. This may cause the axis to move considerably faster than the Target Velocity while it catches up.

In these control modes, the Move Velocity command works well for applications where it is important that the Target Position is always moving at a certain rate. If it is very important that the velocity remain constant, and the position itself is not so important, you may wish to use the Velocity PID control mode instead.

Velocity PID, Velocity I-PD

In these modes, the Move Velocity command ramps the Target Velocity to the RequestedSpeed. The axis will move at that speed until commanded otherwise.

Special Notes

Move Velocity vs. Move Absolute on a Position Axis

On a position axis, the Move Velocity command is sometimes used unnecessarily. The user perhaps assumes the Move Velocity command is the best choice because it moves the axis at a certain speed. However, the Move Absolute (20) command will also move the axis at a specific speed, with the advantage that it stops at the requested position. The constant speed control portion of the move is identical for both of these commands.

An example is an injection molding press. When the press is injecting, the speed should be constant, until it reaches the stopping position. In this case, the Move Absolute (20) is a better choice than the Move Velocity command.

As rule of thumb, use the Move Velocity command when you want the axis to move at a certain velocity indefinitely. If you want the axis to eventually stop at some position, use the Move Absolute command.

Target Generator State Bits

The Target Generator bits in the Status Bits register indicate which portion of the move the axis is currently in. These bits are useful when programming complex motion sequences.

Target Generator Done bit

This bit indicates the move is complete, which occurs when the Target Velocity has reached the Requested Speed.

Target Generator State A and B bits

B	A	Description
0	0	Stopped
0	1	Accelerating (away from zero velocity)
1	0	Constant Velocity
1	1	Decelerating (toward zero velocity)