Motion Control
Precision MicroControl Corp.
66
Manually rotate the motor/encoder in either direction, the position reported should increment or
decrement accordingly. Refer to the
Troubleshooting Guide
later in this manual if the controller does not
report an appropriate change of position.
Setting the Allowable Following Error
Following error is the difference between where an axis
‘is’
and where the controller has
‘calculated it
should be’
. Most all servo systems require ‘some’ position error to generate motion. When a servo axis is
turned on, if a position error exists, the PID algorithm will cause a command voltage to be applied to the
servo to correct the error.
While an axis is executing a move, the following error will typically be between 1 and 1000 encoder
counts. Very high performance systems can be ‘tightly tuned’ to maintain a following error within 1 to 10
encoder counts. Systems with low resolution encoders and/or high inertial loads will typically maintain a
following error between 150 and 5000 encoder counts during a move.
The controller supports ‘hard coded’ following error checking. If at anytime the difference between the
optimal position and the current position exceeds the user defined ‘allowable following error’, an error
condition will be indicated. The axis will be disabled (Amplifier Enable output turned off, output command
signal set to 0.0V) and the axis status word will indicate that an error has occurred. The
MCEnableAxis( )
function is used to clear a following error condition. To disable ‘hard coded’ following error checking set
the allowable following error to zero.
i
The three conditions that will typically cause a following error are:
1) Improper servo tuning (Proportional gain
too low
)
2) Velocity profile that the system cannot execute (moving too fast)
3) The axis is reversed phased (positive command results in
negative motion)
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