Allowable Repeatability; Allowable Repeatability As Limited By The Servomotor - YASKAWA SGM Series User Manual

Servodrive Characteristics

3.3.1 Allowable Repeatability as Limited by the Servomotor

3.3 Allowable Repeatability

3.3.1 Allowable Repeatability as Limited by the Servomotor
3
Running and stopping repeatability are limited by the Servomotor.
Running and stopping repeatability vary with motor conditions, such as the load conditions and
running time. A typical example is given below (See 3.2 "Starting and Stopping Time" for de-
tails on symbols.).
J With Motor Idling or Stopped
The most common example is the operating cycle shown in Fig. 3.3 where rms frequency for
motor armature current is lower than the rated motor current. If we assume that T is the operating
cycle, then the range for T will satisfy the following equation.
2
(tr + tf) + I
I
P
T ²
2
I
R
: Motor rated current (A)
I
R
α = I : Acceleration/deceleration current coefficient
/I
P R
[I : Acceleration/deceleration current (acceleration/deceleration current α times larger than mo-
P
tor rated current) (A)]
β = I : Load current coefficient
/I
L R
[I : Current equivalent to load torque (load current β times larger than motor rated current) (A)]
L
Find I , tr, and tf that satisfy the equation above when cycle time (T) is already known.
P
Motor current
Motor speed
Figure 3.3 Motor Current − Motor Speed Timing Chart
J With Motor Stopped without Idling Except during Acceleration or
Deceleration
The timing chart for motor armature current and motor speed is shown in Fig. 3.4. If we assume
that allowable repeatability is n, then n can be found using the equation given below.
T
R
n = 286.5 ×
(J
N + J
R M
2
ts
L
(s)
(1∕α − β 2 3 )
∕α
[times per minute]
)
L
3 -4
Time
Time