Parker PDHX-E Series User Manual page 46

Position Maintenance
By attaching an encoder to the load, Position Maintenance can be used to correct position
errors between motor and encoder at the end of a move. The controller will detect the
difference between the number of steps the motor was commanded to move and the number
of steps reported by the encoder. This position error is used, at the end of a move, to drive
the motor in a direction to give the correct encoder reading.
To make use of Position Maintenance an encoder will need to be fitted at the load end of the
system and connected to the controller. If the encoder were connected directly to the stepper
motor shaft the stepper motor would need to have a step resolution at least 4X that of the
encoder. For example, using the PDHX-E ministepping drive set to 4000 steps/rev will
require the use of a 250 step/rev quadrature encoder, which after processing will generate
1000 steps/rev (with CCS0 i.e. normal x4 decode). In a practical application the motor may
drive the load via a gearbox or the encoder may sense the load in a manner that alters the
number of steps being generated. The aim is to achieve at least 4 motor steps for every
encoder edge, thereby maintaining the minimum X4 step resolution of the motor over the
encoder.
Position Maintenance is enabled using the FSC command and can be operated in Encoder
Step mode or Motor Step mode using the FSB1 command. Note: If motor steps are chosen
the position resolution will still be determined by the encoder.
Position Maintenance Using The PDHX-E
The PDHX-E can use Position Maintenance to correct for positional errors at the end of a
move. If a 4,000-step move is made, the motor will attempt to go 4,000 steps (or 1 rev). At
the end of the move the controller will read the encoder to check it reads 1,000 steps or one
revolution. If not, it will correct the motor's final position by commanding the motor to move
until the encoder reads 1,000 steps. In practice, mechanical alignment errors and lost motion
within the system will not allow precise matching of motor and encoder, consequently an
error band (Dead Band Window) is defined using the DW command. With DW10 the motor
may have an error of 10 mini steps before Position Maintenance will attempt to correct the
error. If the number is set too small, the motor may oscillate about its correct position.
If a Dead Band Window has been set and Position Maintenance is used to correct a position
error the motor will only be moved by an amount that just re-positions the motor back within
the Dead Band Window. Greater re-positioning accuracy will only be achieved by making the
Dead Band Window smaller.
Speed Of Correction
Once a move has been completed and the controller decides position maintenance
correction is required, it will move the motor at a fixed speed, set by the Maintenance Velocity
(MV) command.
Use of Position Maintenance
Used with a ministepping drive, Position Maintenance is generally used to correct for load
positioning errors at the end of a move. A typical application would be the correction of
slippage in pinch rollers. A stepper motor is used to drive the pinch rollers and the encoder
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