AMD2000 Series - Servo Drive - User Manual
90
DS619-0-00-0019 - Rev 0
ANCA Motion
10.1.12
Backlash Compensation
Description
The ‘end effector’ or ‘working position’ of a machine may have any number of backlash or tolerance stacks
located in the drivetrain between itself and the driving torque of a motor (e.g. gaps in mating gear teeth, splines
etc.). Consequently the position of the end effector is not always directly proportional to motor position. This is
particularly true when the drive train is reversing direction, and consequently has to traverse this region of
backlash before all components re-engage and the motor can drive the end effector in a positive fashion. High
performance machines generally minimise backlash, but cannot always eliminate it, especially with wear and
differential temperatures between components.
Backlash compensation allows the user to adjust the estimate internal to the drive for the end effector position,
when only motor position is measured. It is generally not necessary when the end effector position is measured
directly. The compensation algorithm monitors the
controller’s internal velocity demand to determine when a
reversal in direction of motion is likely to occur. The user then has the opportunity to use three key parameters to
adjust position estimation.
IDN
Description
S-0-058 / 58
Backlash Compensation Clearance
P-0-847 / 33615
Backlash Compensation Min Speed
P-0-849 / 33617
Backlash Compensation Slew Limit
Clearance is the fixed value of likely backlash by which the end effector position estimate adjusts the raw position
determined from motor feedback and scaling. It is possible for this value to be either positive or negative with
respect to its reference value of 0.0. See below for more details concerning the setting of clearance and homing
the machine.
Min Speed sets the lowest value of speed above which compensation will be applied (it defaults to 0.0)
Slew Limit is a fractional value between 0.0 and 1.0 that determines how fast the fixed clearance value will
accrue to the position estimate. For example, 0.1 indicates that for every Task 1 update of the position controller
at 250 us intervals, the clearance value will increase by 10% of its final value. After 10 such intervals it will apply
the full fixed Clearance value, and this value is maintained from then onwards. Setting this parameter to 1.0 will
result in the immediate application of the full Clearance value at the next position estimate update. Similarly
setting this to 0.0 means the Clearance value is never applied, and no adjustment to position is made.
The direction of homing is important for the successful application of this compensation technique, as it relates
directly to the correct sign (+/-) of the applied clearance value and will vary depending on the particular machine
setup. When the machine moves in the direction in which it was homed, effectively no clearance offset is
applied, as positions generated when moving in this direction are taken as the reference for backlash. When
moving in the opposite direction, however, the full clearance value will be applied after appropriate accrual due to
slew limits
Figure 10-3
and
Figure 10-4
illustrate the algorithm’s implementation.
Software/Hardware Requirements
AMD firmware v02.036 or above
Signal:
Reference
Velocity
Algorithm:
Backlash
Compensation
Parameter:
Min. Command
Velocity
Parameter:
Slew Limit
X
Signal:
Backlash
Displacement
Parameter:
Min. Command
Velocity
Figure 10-3 Backlash Compensation Data Flow
