Command: Track Position (57)

Supported Axes:	Position Control Axes
Supported Control Modes:	Position PID, Position 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	Master Register Note: See Specifying aRegister Address below.	Valid RMC register
2	Velocity Limit (position-units/s)	>0
3	Acceleration Limit (position-units/s2) This value should typically be at least 10 times the Velocity Limit.	>0
4	Jerk Limit (position-units/s3) This value should typically be at least 20 to 100 times the Acceleration Limit.	>0
5	Deadband (position-units)	≥0

Description

This command continuously tracks the specified Master Register. The axis Target Position is limited by the specified Velocity Limit, Acceleration Limit, and JerkLimit and the Positive and Negative Travel Limits. This command is useful for smoothly tracking a signal containing noise or step-jumps, or for gearing to another position while not exceeding specified motion limits. The Deadband can be used to prevent the axis from following small amounts of noise while the master signal is for all practical purposes stopped.

Velocity, Acceleration, and Jerk Limits

It is very important that the Velocity Limit, Acceleration Limit, and JerkLimit be set correctly. Acceleration is the rate at which the velocity increases. Jerk is the rate at which the acceleration increases. If the Acceleration Limit or JerkLimit are set too low, the axis can significantly overshoot the master value. The larger the Acceleration Limit and JerkLimit values are, the closer the axis will track. The JerkLimit will especially affect how the axis tracks all the quick, small changes of the master.

Typically, the Acceleration Limit should be at least 10 times the Velocity Limit, and the Jerk Limit should be at least 20 to 100 times the Acceleration Limit.

For example, if Axis 0 should track the Axis 1 Actual Position and be limited to 10 in/sec, then the Track (57) command could be sent as follows:

Track Position (57)
Master Register	_Axis[1].ActPos
Velocity Limit	10
Acceleration Limit	100
Jerk Limit	10000
Deadband	0

Setting the velocity, acceleration, and jerk limits to properly follow a highly dynamic signal can be challenging. To minimize overshoot, the acceleration and jerk limits should be significantly larger than the acceleration and jerk components of the master. Delta recommends that you test this command in the expected scenarios of your machine to make sure it will work as expected during normal operation.

Deadband

If you wish to use the deadband, enter a value that is large enough to reject the expected amount of noise on the Master Register. The deadband will begin to apply whenever the axis is stopped—as defined by the axis Target Velocity being less than the Stop Threshold parameter—for more than 10 loop times.

When the axis is stopped, the deadband will cause the axis to not follow the master until the master moves the specified deadband value from its (the master's) stopped location. When that occurs, the axis will begin to track the master. If the velocity, acceleration, and jerk limits are large, this initial motion can cause the axis to jerk suddenly. To minimize the initial sudden jerk, use the smallest Deadband value necessary.

Rotary Axes

When this command is sent to a rotary axis, the direction of motion will be in the Absolute Direction.

When the Master Register is a position from a rotary axis, the slave axis Target Position will continue in its current direction when the master position wraps. If you need the opposite behavior, that is, the slave axis attempts to always be at the same value as the rotary position, and moves back when the master wraps, you can make a user program that continuously copies the rotary position to a variable, then use the variable as the Master Register.

Other Notes

Changes in the master value can effect the Target Position, even if the Target Position is far from the master value. Reducing the jerk and acceleration limits can help minimize this should it be a problem. Or, use the Track Position (I-PD) (58) command.

If you do not need any velocity, acceleration, or jerk limits, consider the Gear Absolute (25) command.

Specifying a Register Address

When issuing this command from anywhere other than RMCTools, the addresses in the Master Register command parameter must be entered as an integer value.

RMC addresses are represented in IEC format as:

%MDfile.element, where file = file number, and element = element number.

Use the following equation to convert a register address to integer format, N:

N = file * 4096 + element

Example:

Status Bits

In Position Bit

Indicates the axis is in position. This bit is set when the Target Generator Done bit is set and the Actual Position is within the In Position Tolerance of the Target Position. This bit does not apply to virtual axes.

Target Generator 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

Indicates that the Target Position is stopped and stable. This bit is set if the slave axis Target Position is stopped—as defined by the slave axis Target Velocity being less than the Stop Threshold parameter—and the slave axis Target Position is within the In Position Tolerance of the master value. This bit is not valid for virtual axes.

Target Generator State A Bit

Indicates the Target Position is closely matched to the Master Register. This bit is set if the slave axis Target Position is within the In Position Tolerance of the master value, regardless of what the velocities are. This bit is not valid for virtual axes.

Target Generator State B Bit

Indicates the Target Position and Target Velocity are closely matched to the Master Register. This bit is set if the slave axis Target Position is within the In Position Tolerance of the master value and the slave axis Target Velocity is within the Stop Threshold of the master velocity. This bit is not valid for virtual axes.