User's Manual

PMAC User Manual
Setting Up a Coordinate System 137
SETTING UP A COORDINATE SYSTEM
Coordinating Multiple Motions
Once the motors have been set up and tuned and they are doing controlled jogging and homing search
moves, assemble one or more coordinate systems so that motion programs can be run.
PMAC has several methods of coordinating multiple motions, whether they are all under the PMAC
direct control or not. Depending on needs, one of the coordination strategies below can be implemented.
What is a Coordinate System?
A coordinate system in PMAC is a grouping of one or more motors for the purpose of synchronizing
movements. A coordinate system (even with only one motor) can run a motion program; a motor cannot.
PMAC can have up to eight coordinate systems, addressed as &1 to &8, in a very flexible fashion (e.g.
eight coordinate systems of one motor each, one coordinate system of eight motors, four coordinate
systems of two motors each, etc.).
In general, if certain motors should move in a coordinated fashion, put them in the same coordinate
system. To have them move independently of each other, put them in separate coordinate systems.
Different coordinate systems can run separate programs at different times (including overlapping times),
or even run the same program at different (or overlapping) times.
A coordinate system must first be established by assigning axes to motors in Axis Definition Statements.
A coordinate system must have at least one motor assigned to an axis within that system, or it cannot run
a motion program, even non-motion parts of it. When a program is written for a coordinate system, if
simultaneous motions are desired of multiple motors, their move commands are simply put on the same
line, and the moves will be coordinated.
What is an Axis?
An axis is an element of a coordinate system. It is similar to a motor, but not the same thing. An axis is
referred to by letter. There can be up to eight axes in a coordinate system, selected from X, Y, Z, A, B, C,
U, V, and W. An axis is defined by assigning it to a motor with a scaling factor and an offset (X, Y, and
Z may be defined as linear combinations of three motors, as may U, V, and W). The variables associated
with an axis are scaled floating-point values.
One-to-One Matching
In the vast majority of cases, there will be a one-to-one correspondence between motors and axes. That
is, a single motor is assigned to a single axis in a coordinate system. Even when this is the case, however,
the matching motor and axis are not completely synonymous. The axis is scaled into engineering units,
and deals only with commanded positions. Except for the PMATCH function, calculations go only from
axis commanded positions to motor commanded positions, not the other way around.
Multiple-Motor Axes
More than one motor may be assigned to the same axis in a coordinate system. This is common in gantry
systems, where motors on opposite ends of the crosspiece are always trying to do the same movement.
By assigning multiple motors to the same axis, a single programmed axis move in a program causes
identical commanded moves in multiple motors. Typically, this is done with two motors but up to eight
motors have been used in this manner with PMAC. Remember that the motors still have independent
servo loops, and that the actual motor positions will not necessarily be the same.