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Startup Guide - May 2000
B-
63003EN-2/01
2
The Right Side
We’ll concentrate on the right side of the equation now. The right side of the equation looks
like this:
Pulse Count =
number of feedback pulses
distance moved
N
M
× eq. 3
The fraction
N
M
in the equation is also known as the flex feed gear. The flex feed gear is the
scaling factor on the right side of the equation just like CMR was on the left side. Since we
already figured out the pulse count when we solved for CMR, if we figure out how to calculate
the “number of feedback pulses per distance moved”, we’ll have the flex feed gear as well.
The first thing we have to do here is find out what kind of feedback you have and its resolution.
When using motor feedback, you have to check the motor number to find out which encoder it
has built-in. You can find this information in Chapter 2, Section 2 of the GE Fanuc Servo
Description Manual (publication no. B-65002E). This will give you a quantity with units of
pulses
revolution
; that is, how many pulses of feedback per revolution of the motor.
The next thing you need to do is to come up with a value measured in units of
mm
srevolution
. You
can see then, that you can multiply the two values and cancel units to get
mm
pulses
which we are
looking for. The method to calculate
mm
srevolution
will vary from setup to setup. It will depend
upon things such as screw pitches and gearbox ratios, and if you are using separate feedback,
other coupling factors will have to be taken into consideration as well. The easiest way to
approach this calculation is to find out the mechanics of the system and line up the units so that
they cancel to give
mm
srevolution
.
Once you have that value, you can multiply it with your motor resolution to get
mm
pulses
. The
rest involves just taking the values you have and plugging them into eq. 3 to calculate
N
M
.
This is all illustrated in the following example.