Position variable overview, Figure 38 commanded position, Commanded position – Parker Hannifin 88-021610-01G User Manual
Page 80: Actual position
Parker Hannifin
80 Aries User Guide
Position Variable Overview
In a servo system, the controller uses two types of position information:
commanded position and actual position. As these positions change with
time, you can use the position values to determine if the system is positioning
as you expect.
Commanded Position
The commanded position is calculated by the motion profile routine from the
controller and it is updated every servo sampling period. Therefore, the
commanded position is the intended position at any given point of time.
To view the commanded position, use the TPC (Transfer Commanded
Position) command; the response represents the commanded position at the
instant the command is received.
Figure 38 Commanded Position
Actual Position
The actual position of the motor/load is the drive’s response to the
commanded position, and is measured with the feedback device. The profile
resulting from the actual position across time is the position response (for
more information, see “Servo Response Overview” on page
To view the actual position, use the TPE (Transfer Position of Encoder)
command; the response represents the actual position at the instant the
command is received.
The difference between commanded and actual positions is called position
error
. To view the position error, use the TPER (Transfer Position Error)
command; the response represents the position error at the instant the
command is received.
If the motor is not moving, the position error is called a steady-state position
error
. If a position error occurs when the motor is moving, it is called a
position tracking error
.
Even when the system is properly tuned, the position error can still be quite
significant due to a combination of factors such as the desired profile, the
motor's limitation, the dynamic characteristics of the system, etc. For
example, if the commanded velocity is higher than the maximum velocity the
motor can physically achieve, the actual position will always lag behind the