Rockwell Automation 1398-PDM-xxx IQ Master Version 3.2.4 for IA-2000 and IQ-5000 Positioning Drive Modules, IQ-55 - Manual (Page 366)

344 System Programs • System Program 25 - Home
Publication 1398-PM601A-EN-P — October 2000
APPENDIXES
System Program 25 - Home
To move to an absolute position a motion controller must first know where it is. But, when incremental
encoders are used in the system, when the motion controller is first powered up, it does not know where
the axis is. The motion controller must run a routine that will give it some absolute position reference.
This is the function of a home program: to precisely locate a position on a machine. For example, if a
motor is turning a ballscrew that is in turn moving the cutting tool of a lathe, the motion controller must
know where the cutting tool is before it can make a move.
A home program needs to fit the application. In the ballscrew example just given, the normal procedure
is to locate the edge of a Home Switch and then find the next Encoder Index Mark. (An Index Mark is a
signal that comes from an encoder exactly once per revolution.) Since the switch edge locates one out of
many possible index marks, and the index mark locates one pulse out of thousands of pulses/rev, this
type of routine will precisely locate the tooling to within one pulse of the encoder. If the motor is not
tightly coupled to the load (for example, a rubber wheel running on a slide) it would probably be better
to just home the axis to the edge of a switch because the distance between the index mark and the edge
of the switch will not always be the same and the final position would vary by one revolution.
After the ULTRA Plus or IQ loses main and auxiliary power, some sort of a home routine will need to
be run if absolute moves are to be done. (Incremental moves are allowed before a home sequence is
complete.) If the ULTRA Plus or IQ has been supplied with auxiliary power while the main power has
been removed, another home sequence is not needed.
The ULTRA Plus or IQ has several built-in functions for running a home program: an input can tell the
ULTRA Plus or IQ to run the home program, the home switch active sense (home switch active open or
closed) can be selected, outputs can tell if the home program has been run and also if the axis is pres-
ently at home. The home program itself is completely user programmable.
Standard ULTRA Plus or IQ Personality Modules come with a “standard” home program that is docu-
mented below. There are additional home programs that have been written to support special cases to
home an axis. The standard home program is a general purpose program that will handle most homing
requirements; the other home programs are considerably smaller and are easier to understand, but do not
have all of the features of the standard program. If memory space is a consideration, choosing an alter-
native home program that will do what you want should work well. If you need a custom home pro-
gram, it may be better to start with an alternative home program that has the basic features you want and
add to it. The finished custom home program will probably be smaller and easier to follow than if you
started with the standard home program.
When a program is compiled to save to the ULTRA Plus or IQ as the HOME program, the Compiler
Options must be set for Program type - “Main” and Add Debug Information - cleared (off). See Part 2 •
IQ Master Environment, for a complete description of compiler options.