Speed Loop Gains - Emerson Unidrive M702 User Manual

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Speed Loop Gains

(Pr 00.007 {03.010}, Pr 00.008 {03.011}, Pr 00.009 {03.012})
The speed loop gains control the response of the speed controller to a change in speed demand. The speed controller includes proportional (Kp)
and integral (Ki) feed forward terms, and a differential (Kd) feedback term. The drive holds two sets of these gains and either set may be selected for
use by the speed controller with Pr 03.016. If Pr 03.016 = 0, gains Kp1, Ki1 and Kd1 (Pr 00.007 to Pr 00.009) are used, and if Pr 03.016 = 1, gains
Kp2, Ki2 and Kd2 (Pr 03.013 to Pr 03.015) are used. Pr 03.016 may be changed when the drive is enabled or disabled. If the load is predominantly
a constant inertia and constant torque, the drive can calculate the required Kp and Ki gains to give a required compliance angle or bandwidth
dependant on the setting of Pr 03.017.
Speed Controller Proportional Gain (Kp), Pr 00.007 {03.010} and Pr 03.013
If the proportional gain has a value and the integral gain is set to zero the controller will only have a proportional term, and there must be a speed
error to produce a torque reference. Therefore as the motor load increases there will be a difference between the reference and actual speeds. This
effect, called regulation, depends on the level of the proportional gain, the higher the gain the smaller the speed error for a given load. If the
proportional gain is too high either the acoustic noise produced by speed feedback quantization becomes unacceptable, or the stability limit is
reached.
Speed Controller Integral Gain (Ki), Pr 00.008 {03.011} and Pr 03.014
The integral gain is provided to prevent speed regulation. The error is accumulated over a period of time and used to produce the necessary torque
demand without any speed error. Increasing the integral gain reduces the time taken for the speed to reach the correct level and increases the
stiffness of the system, i.e. it reduces the positional displacement produced by applying a load torque to the motor. Unfortunately increasing the
integral gain also reduces the system damping giving overshoot after a transient. For a given integral gain the damping can be improved by
increasing the proportional gain. A compromise must be reached where the system response, stiffness and damping are all adequate for the
application. For RFC-A Sensorless mode, it is unlikely that the integral gain can be increased much above 0.50.
Differential Gain (Kd), Pr 00.009 {03.012} and Pr 03.015
The differential gain is provided in the feedback of the speed controller to give additional damping. The differential term is implemented in a way that
does not introduce excessive noise normally associated with this type of function. Increasing the differential term reduces the overshoot produced
by under-damping, however, for most applications the proportional and integral gains alone are sufficient.
There are three methods of tuning the speed loop gains dependant on
the setting of Pr 03.017:
1. Pr 03.017 = 0, User set-up.
This involves the connecting of an oscilloscope to analog output 1 to
monitor the speed feedback.
Give the drive a step change in speed reference and monitor the
response of the drive on the oscilloscope.
The proportional gain (Kp) should be set up initially. The value
should be increased up to the point where the speed overshoots and
then reduced slightly.
The integral gain (Ki) should then be increased up to the point where
the speed becomes unstable and then reduced slightly.
It may now be possible to increase the proportional gain to a higher
value and the process should be repeated until the system response
matches the ideal response as shown.
The diagram shows the effect of incorrect P and I gain settings as
well as the ideal response.
2. Pr 03.017 = 1, Bandwidth set-up
If bandwidth based set-up is required, the drive can calculate Kp and
Ki if the following parameters are set up correctly:
Pr 03.020 - Required bandwidth,
Pr 03.021 - Required damping factor,
Pr 03.018 - Motor and load inertia.
The drive can be made to measure the motor and load inertia by
performing an inertia measurement autotune (see Autotune
Pr 00.040, earlier in this table).
3. Pr 03.017 = 2, Compliance angle set-up
If compliance angle based set-up is required, the drive can calculate
Kp and Ki if the following parameters are set up correctly:
Pr 03.019 - Required compliance angle,
Pr 03.021 - Required damping factor,
Pr 03.018 - Motor and load inertia The drive can be made to
measure the motor and load inertia by performing an inertia
measurement autotune (see Autotune Pr 00.040, earlier in this
table).
4. Pr 03.017 = 3, Kp gains times 16
If Speed Controller Set-up Method (03.017) = 3 the selected
proportional gain used by the drive is multiplied by 16.
Unidrive M702 User Guide
Issue Number: 3
Electrical Getting Basic
installation started parameters the motor
Running NV Media Card
Optimization
Operation
Speed demand
Insufficient proportional
gain [
00.007
Excessive proportional
gain [00.007]
Excessive integral gain
[00.008]
Ideal response
5. Pr 03.017 = 4 - 6
If Speed Controller Set-up Method (03.017) is set to a value from 4 to 6
the Speed Controller Proportional Gain Kp1 (03.010) and Speed
Controller Integral Gain Ki1 (03.011) are automatically set up to give
the bandwidths given in the table below and a damping factor of unity.
These settings give low, standard or high performance.
Speed Controller
Set-up Method
(03.017)
4
5
6
Onboard Advanced Technical
PLC parameters data
]
Performance Bandwidth
Low 5 Hz
Standard 25 Hz
High 100 Hz
UL listing
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