Section 5. Basic Motion Control Concepts - Parker PDHX-E Series User Manual

Section 5. BASIC MOTION CONTROL CONCEPTS

Chapter Objectives
This chapter is preliminary to the programming section and describes some of the basic
concepts of motion control systems in general and the PDHX-E in particular.
Motion Profiles
In any motion control application the most important requirement is precise shaft rotation,
whether it be with respect to position, time or velocity. The type of motion profile needed will
depend upon the motion control requirement. The following sections describe the basic
types of motion profiles.
Preset Moves
A preset move referred to in this manual is a move of a specified distance (in user steps).
Preset moves allow the user to position in relation to the motor's previous stopped position
(incremental moves) or in relation to a defined zero reference position (absolute moves).
Preset moves are selected by putting the positioner into incremental mode using the MPI
command and absolute moves are made using the MPA command.
Incremental Preset Moves
If the positioner is in the incremental mode (MPI command), a preset move will move the
shaft of the motor the specified distance (in user steps) from its starting position. For
example, to move the motor shaft 1.5 revolutions, a preset move with a distance of +6000
steps would be specified, assuming a 4000 step per rev encoder resolution setup. Every
time this move is executed, the motor shaft will move 1.5 revolutions positive from its current
position. The direction of the move can be specified at the same time as the distance by
using the optional sign (D+6000 or D-6000), or it can be defined separately with the H
command (H+ or H-).
Absolute Preset Moves
A preset move in absolute mode (MPA command) will move the shaft of the motor the
specified distance (in user steps) from the absolute zero position. The absolute position can
be set to zero with the PZ or SP commands, for instance at the end of a GO HOME move
(GH command). The absolute zero position is initially the power-up position, and will remain
that way until changed with a PZ command. Any preset move performed while in the
absolute mode will position the motor shaft the defined distance (in user steps) from the
absolute zero position. For example, with the positioner at the absolute zero position a move
with a distance of +4000 will cause the motor shaft to turn 1 revolution in the positive
direction. If a move with the same defined distance is executed immediately after this move,
the motor shaft will not turn, since it is already +4000 steps from the absolute zero position.
The direction of an absolute preset move will depend upon the shaft position at the beginning
of the move and the position that it is being commanded to move to. For example, if the
motor shaft is at absolute position +12,800, and position commanded is +5000, the motor
shaft will move in the negative direction a distance of 7800 steps to absolute position +5000.
The positioner saves the mode that it was in at power down and powers up again in the same
mode. Issuing the MPA command will set the mode to absolute. Issuing the MPI command
will switch the mode from absolute to incremental. The positioner retains the absolute
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