Mitsubishi Electric FR-A820-00046 (0.4K) Instruction Manual page 311

The following formula shows the relationship between the motor speed and internal command pulse frequency.
NOTE
• Set the electronic gear ratio in the range of 1/50 to 20. Note that, if the setting value is too small, the speed command will also
be too small; while if it is too large, the speed ripple will be too large.
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 ∆ s = 10 (mm)
when ∆ = 0.01 (mm) and Pf = 4000 (pulses/rev) is set as the number
of feedback pulses. Based on this, use the following formula:
Δ
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 s for the initial value Kp = 25 s
The accuracy of positioning ∆ε is (5 to 10) × ∆ = ∆ε [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.
310
5. PARAMETERS
5.5 Position control under vector control and PM sensorless vector control
Pr.420
fo × = Pf ×
Pr.421
Setting example 1
Δs
Pr.420
= ×
Pf Pr.421
Pf
Δ ×
=
Δs
4000 4
= 0.01 × =
10 1
1 1
) × Δε
Δ
< ( to
5 10
204800
[pulse] (with the rated motor speed)
2 5
No
fo: internal command pulse frequency [pps]
No: motor rotation speed [r/min]
60
Find the internal command pulse frequency for the rated motor speed
of the dedicated motor.
However, the command pulse ratio is Pr.420/ Pr.421 = "1".
If the number of encoder pulses is 2048 (pulses/rev), (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 285. Pulses as much as the motor speed delay are accumulated
-1
.
-1
.
Setting example 2
No Pr.421
×
60 Pr.420