User's Manual

PMAC User Manual
220 Synchronizing PMAC to External Events
Changing Ratios on the Fly
To vary the following ratio in the middle of an application, change Ix07 alone. Ix08 is involved in the
scaling of servo feedback calculations, and so should not be changed in the middle of an application.
There can be tradeoffs between the resolution of on-the-fly changes and the servo performance of the
system. The higher the Ix08 scale factor, the finer the resolution of the changes can be. However, the
higher Ix08 is, the lower proportional gain Ix30 can be before internal saturation occurs, and the lower the
maximum velocity can be before there is internal saturation of those registers. In general, Ix08 should be
kept below 1000.
Superimposing Following on Programmed Moves
In addition, this following function can be superimposed on calculated trajectories. This permits, for
instance, shapes to be cut out of a moving web, where the shape program can be written without regard to
the web movement, and a master signal from the web compensates for the movement.
Bit 16 of Ix05 determine whether the following occurs in offset mode, where the reported position of the
following motor does not reflect the change due to following, or normal mode, in which it does. Usually
to superimpose following and calculated moves, use offset mode in which bit 16 is 1 (if the master
position is taken from X-register $0721, the parameter value would be, for example, I105=$10721).
Time-Base Control (Electronic Cams)
A more sophisticated method of coordination to external axes is time-base control, in which the input
signal frequency controls the rate of execution of moves and programs. Time-base control operates on an
entire coordinate system together. Specify which encoder register is receiving the input frequency, and
the relationship between the input frequency and the program rate of execution. This not only varies the
speed of moves in proportion to the input frequency (all the way down to zero frequency), but also keeps
total position synchronization. This permits operations such as multi-pass screw threading.
What Is Time-Base Control?
The PMAC motion language expresses the position trajectories as functions of time. Whether the moves
are specified directly by time, or by speed, ultimately the trajectory is defined as a position-vs.-time
function.
This is fine for a great number of applications. However, in many applications the PMAC axes is slaved
to an external axis not under PMAC control (or occasionally, an independent axis under PMAC control in
a different coordinate system). In these applications, define the PMAC trajectories as functions of master
position, not of time.
Encoder Table
$0720-$0739
Ix07
Ix05
D
+
-
PI
FECP
AP
Ix05 Master (HW) Position Address & Mode
Ix06 Following Enable
Ix07 Master Scale Factor
Ix08 Position Loop (Load) Scale Factor
Ix09 Velocity Loop (Motor) Scale Factor
+
-
HW
Ix06
+
Ix08
Ix09
Ix08
Trajectory
Command
Cannot be
changed on
the fly
To DA C
A
ctual
Motor
Position
Actual
Load
Position
Can be
changed on
the fly
Figure 52 Position Following Parameters