Delta RMC101 - Manual (Page 601)

Quadrature with Stepper Output 6.4
Transducer Interface Modules 6-57
We can improve both of these problems by using hundredths of an inch as our position units.
Since we have about 1571 hundredths of an inch per revolution, we could use the following
parameters.
Parameter Value
Steps/Rev 5000
Position Units/Rev 1571
Quad Counts/Rev 4000
This eliminates the first problem if a hundredth of an inch is an adequate resolution. It also
reduces the position error to 0.013% or 0.002” per revolution. This is a major improvement and
may be adequate for many applications.
We can further improve our scale by maximizing the dynamic range of these three parameters.
That is, since these parameters are used as ratios, we can multiply each by a constant without
changing the effective scales. Any multiplier—integer or non-integer—is acceptable as long is at
lowers the scaling error and doesn’t exceed the parameter limits: Steps/Rev and Position
Units/Rev are limited to 65535, and Quad Counts/Rev is limited to ±32767. A good value to
multiply by is 5. The Position Units/Rev parameter becomes 15.70796 x 100 x 5, which is
7853.98 and gets rounded to 7854. Our new parameters are:
Parameter Value
Steps/Rev 25000
Position Units/Rev 7854
Quad Counts/Rev 20000
Because we multiplied these parameters by 5, the Position Units/Rev parameter is equivalent to
7854/(5 x 100) or 15.70800 inches per revolution. This gives a scaling error of 0.0002% or
0.000037” per revolution. This is effectively the best we can do with our scaling precision for this
application.
Two questions remain: What is the effect of this remaining scaling error? What are the limitations
of the 16-bit positions? Each is discussed in turn below.
What is the effect of the remaining scaling error?
The remaining scaling error must be carefully evaluated for your system. In many cases it may be
adequate, or perhaps the mechanical error overshadows the remaining scaling error. For
example, the tolerance of the radius of the driving wheel may result in a greater error than the
scaling error.
However, in many indexing applications the error for a single cycle is acceptable, but over many
cycles, the error accumulates until it becomes significant. If this is the case, then you may need to
use the home input or index pulse to reset the positions between—or even during—each cycle or
set of cycles. See Quadrature Homing for details on this topic.
If the scaling error still appears too large, contact a Delta Computer Systems Application
Specialist to discuss your system. See Technical Support for details on contacting Delta.
What are the limitations of the 16-bit positions?
Because positions are stored in 16-bits, the RMC can neither display positions greater than
65535 position units nor receive commands to move more than 65535 position units. In our
application, we have position units equal to a hundredth of an inch, so this limitation is 655.35
inches (over 54 feet). This won’t limit most linear applications, but applications requiring longer
distances will need to use a lower position unit resolution. For example, using tenths of an inch