Operation Mode

There are four position modes and one velocity mode, as described below:

Absolute Position

In absolute position mode, target positions are interpreted with respect to an origin, reference, or zero position. The origin is typically set at a home switch, end of travel limit switch, or encoder index position. An absolute position move uses the preprogrammed values of acceleration, deceleration, s-curve, and velocity to complete a trajectory profile with an ending position equal to the loaded absolute target position.

Caution  Any single move is limited to between –231 and 231–1 counts or steps. An error is generated if you exceed this limit by loading a target position too far from the current position.

The length of an absolute move depends upon the loaded position and the current position when the move is started. If the target position is the same as the current position, no move occurs.

Relative Position

In relative position mode while motion is not in progress, loaded target positions are interpreted with respect to the current position at the time the value is loaded. A relative position move uses the preprogrammed values of acceleration, deceleration, s-curve and velocity to complete a trajectory profile with an ending position equal to the sum of the loaded relative target position and the starting position.

If a relative move is started while motion is in progress, the new target position is calculated with respect to the target position of the move already in progress (considered to be the new starting position), as if that move had already completed successfully. Motion continues to the new relative position, independent of the actual position location when the new move is started.

In relative mode, the new target position is calculated and double-buffered when you enter a new Target Position. You must reload the relative target position each time before clicking the Start or Blend button.

Velocity

In velocity mode, the axis moves at the loaded velocity until you stop motion, a limit is encountered, or a new velocity is loaded and you start motion again. Loaded target positions have no effect in velocity mode. The direction of motion is determined by the sign of the loaded velocity.

You can update velocity at any time to accomplish velocity profiling. Changes in velocity while motion is in progress uses the preprogrammed acceleration, deceleration, and s-curve values to control the change in velocity. You can reverse direction by changing the sign of the loaded velocity and clicking the Start button.

Note  Clicking the Blend button in velocity mode has no effect because the move in process never normally stops. You must always click the Start button to update velocity in velocity mode.

Relative To Capture

The relative-to-capture position mode is very similar to absolute position mode, except that the zero position reference (origin) is the last captured position for the axis or axes. A relative-to-capture position move uses the preprogrammed values of acceleration, deceleration, s-curve and velocity to complete a trajectory profile with an ending position equal to the sum of the loaded target position and the last captured position.

In relative mode, the new target position is calculated and double-buffered when you enter a new Target Position. You must reload the relative target position each time before clicking the Start or Blend button.

In relative-to-capture mode, the new target position is calculated and double-buffered when you enter a new Target Position. These functions use existing values in the position capture register(s). You must load the target position after the capture event has occurred and before clicking the Start or Blend button.

This mode is typically used in registration applications. Refer to the Gearing section of the NI-Motion Help for information about using superimposed moves/registration applications. Also, refer to the Rotating Knife section of the NI-Motion Help for example code that includes superimposed moves.

Modulus Position

In modulus position mode, the loaded target position is interpreted within the boundaries of a modulus range and the direction of motion is automatically chosen to generate the shortest trajectory to the target.

Modulus position mode is typically used with rotary axes or for other similarly repetitive motion applications.

Note  Multiple revolution moves cannot be accomplished by indicating target positions greater than the modulus value. All moves are resolved to one modulus range.