Emerson unidrive sp, User Manual

Page: 61 / 420

Parameter
structure
Keypad and
display
Parameter
x.00
Parameter
description format
Advanced parameter
descriptions
Macros
Serial comms
protocol
Electronic
nameplate
Performance RFC mode
Menu 3
Closed-loop
Unidrive SP Advanced User Guide
61
Issue Number: 10 www.controltechniques.com
The following diagram shows a generalised representation of the speed controller. The 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
3.16 . If Pr 3.16 = 0, gains Kp1, Ki1 and Kd1 are used, if Pr 3.16 = 1, gains Kp2, Ki2 and Kd2 are used. Pr 3.16 may be changed
when the drive is enabled or disabled.
Proportional gain (Kp)
If Kp has a value and Ki 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 quantisation (using digital encoders, resolvers, etc). becomes unacceptable, or the closed-loop stability limit is
reached (using SINCOS encoders).
Integral gain (Ki)
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. The integral term is implemented in the form of
∑
(Ki x error), and so the integral gain can be changed when the controller is active without
causing large torque demand transients.
Differential gain (Kd)
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. It should be noted that the differential term is
limited internally so that it is ineffective if speed in rpm x Kd x Ki is greater than 170.
3.12
Speed controller differential feedback gain (Kd1)
3.15 Speed controller differential feedback gain (Kd2)
Drive modes Closed-loop vector, Servo
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
5 1 1 1
Range Closed-loop vector, Servo
0.00000 to 0.65535 s
-1
/rad s
-1
Default Closed-loop vector, Servo 0.00000
Second motor
parameter
Closed-loop vector, Servo Pr 21.19
Update rate Background read
3.16
Speed controller gain select
Drive modes Closed-loop vector, Servo
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
1 1 1
Default Closed-loop vector, Servo 0
Update rate 4ms read
Kp
Ki
Kd
Speed
reference
(wr*)
Speed
feedback
(wr)
Torque
reference
(Te*)
-
+
+
+
+
+