Digitax HD M753 Control User Guide
81
Issue Number: 3
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
.
NOTE: In sensorless mode, the speed controller bandwidth may need to be limited to 10 Hz or less for stable operation.
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-S Sensorless mode, it is unlikely that the integral
gain can be increased much above 0.50.
Differential Gain
(Kd), Pr
00.009
{
0 3.012
} 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 six 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.
Speed demand
Insufficient proportional
gain [
]
00.007
Excessive proportional
gain [
00.007
]
Excessive integral gain
[
00.008
]
Ideal response
6. Pr
03.017
= 7
If
Speed Controller Set-up Method
(03.017) = 7 then
Speed Controller
Proportional Gain Kp1
Pr
00.007
{
03.010
},
Speed Controller Integral Gain
Ki1
Pr
00.008
{
03.011
} and
Speed Controller Differential Feedback Gain
Kd1
Pr
00.009
{
03.012
} are set up to give a closed-loop speed controller
response that approximates to a first order system with a transfer function of
1 / (st + 1), where t= 1/wbw and wbw = 2p x
Bandwidth
(03.020). In this case
the damping factor is meaningless, and
Damping Factor
(03.021) and
Compliance Angle
(03.019) have no effect.
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
Pr
00.007
{
03.010
} and
Speed
Controller Integral Gain Ki1
Pr
00.008
{
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.
Pr 03.017
Performance
Bandwidth
4
Low
5 Hz
5
Standard
25 Hz
6
High
100 Hz
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