Rockwell Automation 1756-HYD02 ControlLogix Hydraulic Servo Module - Manual (Page 55)

Publication 1756-UM525A-EN-P - June 2003 55
Using the 1756-HYD02 Module Features Chapter 4
Master Delay
Compensation Master Delay Compensation helps a slave approach zero tracking error when gearing or camming to the
actual position of a master axis. When camming or gearing off of actual feedback, some delay exists
between when the master position data is generated and when the controller acts on that data for a slave.
This delay causes a slave response to lag behind a master. You can compensate for this delay with the
Master Delay Compensation.
The Position Camming and Gearing functions, when applied to a slave axis, perform Master Delay
Compensation to compensate for the delay time between reading the master axis command position and
applying the associated slave command position to the output of the slave’s axis loop. When the master axis
is running at a fixed speed, this compensation makes sure that the slave axis command position accurately
tracks the actual position of the master axis.
The Master Delay Compensation algorithm extrapolates the position of the master axis at the predicted
time when the command position will be applied to the slave’s axis loop. The total position update delay
created by the Coarse Update Period of the motion group, and, if the master or the slave involves an
AXIS_SERVO_DRIVE data type, it also includes the delay term that is proportional to the SERCOS Update
Period.
The motion planner implements a 1st order extrapolation algorithm that results in zero tracking error while
the master axis is moving at constant velocity. If the master axis accelerates or decelerates the tracking
error is non-zero and proportional to the acceleration or deceleration rate and also proportional to the
square of the total position update delay time.
If your application does not require zero tracking error between the master and the slave axis, disable the
Master Delay Compensation feature to eliminate the disturbances the extrapolation algorithm introduces to
the slave axis. When the Master Delay Compensation feature is disabled (bit cleared), the slave axis
appears to be more responsive to the master’s movements, and runs generally smoother than when Master
Delay Compensation feature is enabled (bit set). However, when the master axis is running at a constant
velocity, the slave lags the master by a tracking error that is proportional to the speed of the master.
Since the master axis position is measured in discrete feedback counts and is inherently noisy, the
extrapolation process amplifies that noise according to the total position update delay. The greater the
delay, the greater the noise introduced by the extrapolator.
This feature also includes an extrapolation filter to filter the noise introduced by the extrapolation process.
The time constant of the filter is fixed at 4x the total position update delay (independent of the Master
Position Filter Bandwidth), which again is a function of the Coarse Update Period and the SERCOS Update
Period, if a AXIS_SERVO_DRIVE data type.
Note that Master Delay Compensation, even if explicitly enabled, is not applied in cases where a slave axis
is gearing or camming to the master axis’ command position. Since the ControlLogix controller generates
the command position directly, there is no intrinsic master position delay to compensate for.
Enable Master Position
Filter When checked, this field enables the Master Position filter to eliminate high frequency oscillations or spikes
from the master position signal being sent to the 1756-HYD02 module. High speed axis oscillation typically
causes the oscillations and spikes in the signal. By removing the high frequency noise, the application may
be able to accurately operate on true variation of the master position signal.
Caution should be used when applying filtering since, by definition, it makes the system less responsive.
Master Position Filter
Bandwidth Bandwidth used on the Master Position filter. The servo position loop is also a filter. To be effective, the
master position filter bandwidth should be less than or equal to position loop bandwidth. However,
remember that this filter adds propagation delay.
Feature: Definition: