Rockwell Automation 1398-PDM-xxx IQ Master Version 3.2.4 for IA-2000 and IQ-5000 Positioning Drive Modules, IQ-55 - Manual (Page 393)

Application Examples • Continuous Web with Registration 371
Publication 1398-PM601A-EN-P — October 2000
APPENDIXES
usually desirable to limit the position around the target that an interrupt will be responded to. This will
eliminate most spurious registration marks (due to ink spillage, material blemishes, etc.) and reduce the
effect of those that do get through because only those marks close to the correct mark will be seen. This
process is called windowing; there is a window around the target position in which registration inter-
rupts will be allowed. The window radius is set by the user.
With a “stable” process there are measures that can be taken to reduce the effects of noise. For example,
averaging position error over several cycles can reduce the effect of reader noise. Obviously the more
cycles, the less noise there will be; but the system will be slower in responding to a real error. If the pat-
terns per revolution are not evenly spaced, averaging over the number of patterns on the axis will elimi-
nate this “noise.” For these two reasons it is recommended that the number of cycles an offset is
averaged over should be a multiple of the number of patterns/revolution.
Program Parameters
The following program has several parameters that control its behavior. These parameters are the num-
ber of patterns to average over, the gear ratio, detector delay, capture window radius, and the pattern
increment.
The number of patterns to average over determines how many measurements the program should make
before calculating any offset error and performing a corresponding correction. In the following program
this is set using the X1 key.
The second parameter is the gear ratio. This is determined by the mechanics of the machine. The gear
ratio is how many times the controlled axis should turn for each turn of the master encoder. The ULTRA
Plus or IQ can compensate for position errors, even errors due to incorrect gear ratio, but the fewer
errors it has to correct the better a system will run. For this reason it may be necessary to “tweak” the
gear ratio to compensate for dimensional tolerances, web thickness, etc., to achieve maximum perfor-
mance from a system.
The next parameter is the detector delay. All position detectors have a finite response time. Some photo
detectors can have several milliseconds of delay between the time a photo mark is in a position to be
sensed and the time the photo detector output is activated. This delay (in mS) is called PDelay in this
program. If the web speed ever changes this can be a real problem since the amount of web passing by
in this time will be determined by the speed of the web. Without some sort of compensation, what will
happen is that as the web speed changes the ULTRA Plus or IQ will see that the photo-mark position has
changed by (web speed1 - web speed2) * PDelay. To compensate for this problem this program uses the
position the axis was in PDelay milli-seconds before the detector signal was seen. This works providing
that real PDelay is known and repeatable.
The next parameter is the capture window radius (windRad). In a stable web process (this cannot be
used with a random position process), once the system is running, it is possible to closely predict where
the next photo signal will be and signals that are too far from this spot can be assumed to be noise and
ignored. The windowing system in this program does this. Once a measured error is smaller than win-
dRad, only signals that are within windRad from the predicted spot will be used to compute the next
position error.
The final parameter is the pattern repeat increment (incr). This is the distance, in motor encoder counts,
between patterns and is the distance the target and window positions will be incremented for each pat-
tern. This can be any number of counts, including fractional counts. This is usually easy to calculate.
For example, if the slave axis is printing 6 patterns per rev and there is a 4:1 reduction from the motor to
the axis, and 8000 counts/rev of the motor, there would be 4 x 8000 counts/rev of the slave axis and 4 x
8000 / 6 = 5333 1/3 counts per pattern. In this case pattern repeat increment should be set to 1600/3.
This is hard coded in the program, but if need be the program could be modified to allow the operator to
change this number at run-time.
I/O Connections
There are four I/O connections in this program. The only critical connection is the sensor connection,
which must be connected to input 11. The other connections may be omitted or moved to other inputs
by modifying the code. I13, when turned off, will inhibit corrections from being made. This can be use-
ful in determining the action the correction is having on print registration. I15 and I16 are used for man-