
B-63003EN-2/01 Chapter 2 Configuring the 16i/18i/21i 2 - 13
2
The Equation
CMR and
N
M
are calculated according to the following equation:
number of command pulses
distance commanded
CMR = Pulse Count =
number of feedback pulses
distance moved
N
M
××eq. 1
This equation might seem intimidating at first, but if we look at it carefully, it can be broken
down and easily understood.
To make things clearer, let’s say that we are working with millimeters (instead of inches or
degrees). This is set by parameter 1000 bit #0. The left side of the equation represents the
mm
pulses
which are entering the big black dot in Figure 2-3 from the positive side. The right side
of the equation then, represents
mm
pulses
w
hich are entering from the negative side.
The goal of this calculation is to use CMR and
N
M
to make the number of
mm
pulses
entering the
black dot equal on both sides.
The Left Side
If we ignore the right hand side of the equation for now, we have this remaining:
number of command pulses
distance commanded
CMR Pulse
×=
Count
eq. 2
The first thing we can do is figure out the pulse count. The pulse count is what we referred to
above as the
mm
pulses
entering the black dot. Now, there is a value which the customer must
specify known as the detection unit. This value determines “this is how much precision I need
for my control” and it represents the smallest movement the CNC can detect. Since this value
is specified in units of
pulse
mm
, all we have to do is flip this number to get the pulse count.
Already then, we have one given out of the three quantities in this equation.
Next, let’s look at what “number of command pulses per distance commanded” is. There is a
parameter (1004 bits #0 & #1) known as the least command increment (LCI) which the
customer must also specify. This is the smallest unit of movement the CNC can command and
it has units of
pulse
mm
. Just like we did above, if we flip the LCI over as well, we have the
quantity we are looking for.
You can see then, that in order to calculate the value for CMR, you need to find the LCI and
detection unit specified by the customer and just flip them over. The rest is just taking eq. 2
and plugging and chugging.