Chapter 4
What You Need to Know about Moves
©
National Instruments Corporation
4-5
NI 73
xx
Time Base
Velocity and acceleration values are loaded in counts/s, RPM, RPS/s,
steps/s, and so on, which are all functions of seconds or minutes. However,
the trajectory generator updates target position at the Trajectory Update
Rate, which is programmable with the Enable Axes function. This means
that the range for these parameters depends on the update rate selected, as
shown in the following examples.
Table 4-1 lists minimum and maximum update rates for acceleration and
velocity in various units.
NI 73
xx
Velocity in RPM
Velocity values in RPM are converted to an internal 16.16 fixed-point
format in units of counts (steps) per sample period (update period) before
being used by the trajectory generator. NI-Motion can control velocity to
1/65,536 of a count or step per sample.
Table 4-1 shows the minimum and maximum velocity in counts/min. Use
the formula shown in the
Calculation Based on Units
column to determine
the counts/min value to RPM.
Table 4-1.
Velocity in Counts/Min
Update Rate
MIN
MAX
Calculation Based
on Units
62.5 µs
14.648438 counts/min
For servo motors, the
maximum counts/min is
1.2 billion independent
of the update rate.
±RPM
max
= MAX×1/
r
where
r
= counts/revolution
125 µs
7.324219 counts/min
187.5 µs
4.882813 counts/min
250 µs
3.662109 counts/min
312.5 µs
2.929688 counts/min
For stepper motors, the
maximum counts/min
value is dependent on the
controller:
• 480 million
counts/min for 7350
• 240 million
counts/min for 7330,
7340, and 7390
±RPM
min
= MIN×1/
r
where
r
= counts/revolution
375 µs
2.441406 counts/min
437.5 µs
2.092634 counts/min
500 µs
1.831055 counts/min
