Mitsubishi Electric FR-A800 Series Instruction Manual page 327

In a driving system whose ball screw pitch is PB = 10 (mm) and the
reduction ratio is 1/n = 1, the electronic gear ratio is Δ s = 10 (mm)
when Δ = 0.01 (mm) and Pf = 4000 (pulses/rev) is set as the number
of feedback pulses. Based on this, use the following formula:
Pr.420
Pr.421
Thus, set the parameters as follows: Pr.420 = "4", Pr.421 = "1".
 Relationship between the position resolution and system accuracy
The system accuracy (the positioning accuracy of the machine) is the sum of electric deviation and mechanical deviation.
Normally try to prevent the total deviation from being affected by the electronic deviation. Refer to the following relationship as
a reference.
 Motor stop characteristics
When running the motor by the parameter settings, the relationship between the internal command pulse frequency and the
number of motor rotations is as shown in the figure on
in the deviation counter. These pulses are called droop pulses (ε). The relationship between the command frequency (fo) and
position loop gain (Kp: Pr.422) is shown in the following formula.
fo
ε =
[pulse]
K p
The number of droop pulses (ε) is 8192 with the initial value Kp = 25 s
Since the inverter has droop pulses during operation, a stop settling time (ts), which is the time between the zero command
output and the motor stop, is required. Set the operation pattern taking into the account the stop setting time.
1
ts = 3 × [s]
Kp
The stop settling time (ts) is 0.12 second for the initial value Kp = 25 s
The accuracy of positioning Δε is (5 to 10) × Δ = Δε [mm]
 Position command acceleration/deceleration time constant (Pr.424)
• If the electronic gear ratio is large (1:10 or larger) and the rotation speed is slow, the rotation is not smooth and the rotation
shape becomes like a pulse.
Set this option in such a case to smoothen the rotation.
• If the command pulse frequency varies rapidly when no acceleration time can be assigned to the command pulse,
overshoot or excessive error alarms may occur. Set this option in such a case to set the acceleration/deceleration time.
Normally it is set to 0.
Parameters referred to
Pr.422 Position control
326
5. PARAMETERS
5.5 Position control under vector control and PM sensorless vector control
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Setting example 1
's
Pr.420
'
×
=
Pr.421
Pf
Pf
= '
×
's
4000 4
= 0.01 × =
10 1
1 1
) × Δε
Δ
< ( to
5 10
204800
[pulse] (with the rated motor speed)
2 5
gainpage 328
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Find the internal command pulse frequency for the rated motor speed
of the dedicated motor.
Where the command pulse ratio (Pr.420/ Pr.421) = 1,
the number of encoder pulses = 2048 (pulses/rev), and the feedback
pulse pf = 2048 × 4.
fo = 2048 × 4 ×
= 204800
The internal command pulse is 204800 (pulses/s) in accordance with
the above formula.
Δε: positioning accuracy
page 299. Pulses as much as the motor speed delay are accumulated
-1
.
-1
.
Setting example 2
No Pr.421
×
60 Pr.420