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11.SELECTION
11-4 Stopping characteristics
(
1) Droop pulses (ε)
When a pulse train command is used to run the servo motor, there is a relationship between the
command pulse frequency and servo motor speed as shown in the figure. The difference between
the command pulses and feedback pulses during acceleration are called droop pulses, which
are accumulated in the servo amplifier's deviation counter. Equation 11-7 defines a relationship
between the command pulse frequency (f) and position control gain 1 (Kp).
f
o
ε
[pulse] ............................................................................................................................................... (11-7)
K
p
Supposing that the value of position control gain 1 is 70 [rad/s], the droop pulses during operation
will be as follows at the command pulse frequency of 200 [kpps] according to Equation 11-7:
3
200 x 10
ε
2858[pulse]
70
[r/min]
(2) Settling time (ts) during linear acceleration/deceleration
Since droop pulses still exist when there are no command pulses, settling time (ts) is required until the
servo motor stops. Set the operation pattern in consideration for the settling time.
The ts value is obtained according to Equation 11-8:
t
3 • T
s
p
1
= 3 •
[s] .................................................................................................................................................. (11-8)
K
p
*When K
=70 [rad/s], ts
p
Note: The settling time (ts) indicates the time required for the servo motor to stop in the necessary
positioning accuracy range. This does not always mean that the servo motor has stopped
completely. Thus, especially when the servo motor is used in high-duty operation and
positioning accuracy has no margin for travel per pulse (∆R), the value obtained by Equation
11-8 must be increased.
ts will vary with the moving part conditions. Especially when the load friction torque is large,
movement may be unstable near the stopping position.
Command Droop pulses
Servo motor
speed
f
0
[pps]
Tpsa
0.04 [s]. (Refer to the above diagram.)
11– 5
Time
Settling time
0.04s
Tpsd
ts
1
ts
3 x
70
0.04
11
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