Rockwell Automation 1398-PDM-xxx IQ Master Version 3.2.4 for IA-2000 and IQ-5000 Positioning Drive Modules, IQ-55 - Manual (Page 392)

370 Application Examples • Continuous Web with Registration
Publication 1398-PM601A-EN-P — October 2000
APPENDIXES
Continuous Web with Registration
Description
Controlling motion relative to a continuous web with registration is common in printing, die-cutting,
and labeling processes. The application shown is slicing a printed web into strips. The printed patterns
have been staggered to optimize web usage, so the slicing axis needs to cut with a zigzag pattern. This
pattern must be registered to the printed pattern.
Requirements
This axis is required to follow the web speed and correct for any offset differences between the pattern
already printed on the web and the pattern the axis is cutting on the web. The controller will need to be
able to work in a noisy environment with finite sensor response time.
Description of the ULTRA Plus or IQ Solution
A registration mark is printed on the web to ensure proper phase alignment. The slave (slicing) axis will
follow a printing axis encoder, preferably the axis printing the registration mark. Since the ULTRA Plus
or IQ drive knows where the cutter should be each time a registration mark signal is generated, it can
measure the difference between where it is and where it should be, and correct for any errors.
Design Considerations
• Since registration of the slave axis is to the print, not the web, the master encoder (the encoder the
slave is following) should be tied to a printing axis, not the web itself. If the print pattern length is
truly fixed to a certain amount of web then this distinction is meaningless. In most applications this
is not true, however, and since stretch or slippage errors are likely to always be in the same direc-
tion, the error introduced by these effects will accumulate. So even though, for example, a print pat-
tern is only 1/64th of an inch longer at the end of a roll that it was at the beginning, this error is
cumulative so that each cycle adds another 1/64th of an inch to the error.
• As speeds increase, a significant source of error is the finite response time of registration mark
readers, typically a photo eye. If the delay time is known and fixed, its effects can be minimized
through software. If high speed precision is crucial, a high-speed reader should be considered
because the response of a slow reader will vary more than a high speed reader.
• To avoid having to manually re-align the print each time the machine is started up, the axis must be
able to home itself. This need not imply that the axis needs to be able to move freely to complete a
traditional home cycle. A home routine can be written so that the axis homes itself when the
machine is first powered up and the product starts moving. However, if there is more than one
encoder marker pulse per revolution of the axis (if there is any reduction from the motor to the axis)
then a switch will need to be placed on the axis so that the ULTRA Plus or IQ has some way to
know which encoder index it should home to. Also, any motor/axis reduction ratios need to be
whole integers, i.e., 6:1, 3:1, 4:1.
Correction Algorithm
This note deals with web processes that are stable but where small errors (due to web stretch, slippage,
etc.) can be introduced and need to be corrected in a manner that will not damage the process. The algo-
rithm is simple: in any given cycle the ULTRA Plus or IQ drive knows the ideal position where it should
receive a registration pulse interrupt. This position is called the target. When an interrupt happens, the
ULTRA Plus or IQ captures the actual position the ULTRA Plus or IQ drive is in. The difference
between the target and the actual position is the Offset Error. Once the pre-determined number of cycles
has been measured and the average taken, a correction move is done and the process starts over.
If the machine is homed and the offset known, the initial target position can be computed. If this is not
known, the axis must be manually aligned using inputs I15 and I16 to advance and retard the axis. Once
aligned, the offset is measured for future reference.
After the initial error is corrected, the subsequent errors should be quite small. If this is the case it is