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62 Unidrive SP Advanced User Guide
www.controltechniques.com Issue Number: 10
To analyze the performance of the speed controller it may be represented as an s-domain model as shown below.
Where:
Kc is the conversion between the speed controller output and the torque producing current. A value of unity at the output of the speed controller gives
a torque producing current equal to Kc. The drive automatically compensates the torque producing current for flux variations in field weakening, and
so Kc can be assumed to have a constant value even in field weakening. See menu 4 for the value of Kc each drive size).
Kt is the torque constant of the motor (i.e. torque in Nm per amp of torque producing current). This value is normally available for a servo motor from
the manufacturer, however for induction motors the value must be calculated from
Kt = Motor rated torque / Motor rated torque producing current
= Motor rated torque / √(Motor rated current
2
- No load current
2
)
L(s) is the transfer function of the load.
The s-domain system above may be used to determine the performance of systems with a relatively low bandwidth. However, the real drive system
also includes non-ideal delays due to the torque controller response, and speed measurement and control delays. These delays, which can be
approximated with a simple unity gain transport delay (T
delay
) as shown below, should be taken into account for more accurate results.
The speed controller gains used in previous Unidrive products were in internal drive units. Conversion between the previous internal units and the SI
units used in this product are given in the table below.
The user may enter the required speed controller gains into Pr 3.10 to Pr 3.15. However, if the load is predominantly a constant inertia and constant
torque, the drive can calculate the required Kp and Ki gains, provided a value of motor plus load inertia (Pr 3.18) and the motor torque per amp
(Pr 5.32) are set-up correctly. The gain values are calculated to give a required compliance angle or bandwidth. The calculated values for Kp and Ki
are written to Pr 3.10 and Pr 3.11 once per second when one of these set-up methods is selected (i.e. Pr 3.17 = 1 or 2). The values are calculated
from a linear model assuming a pure inertia load, not including unwanted delays in the speed and current controllers. The Kd gain is not affected. If
Pr 3.17 is set to 3 automatic gain set up is not active, but Kp is boosted by a factor of 16.
Gain Conversion from previous internal units to new SI units
Kp Kp_old / 17103
Ki Ki_old / 94.41
Kd Kd_old / 46376
3.17
Speed controller set-up method
Drive modes Closed-loop vector, Servo
Coding
Bit SP FI DE Txt VM DP ND RA NC NV PT US RW BU PS
111
Range Closed-loop vector, Servo 0 to 3
Default Closed-loop vector, Servo 0
Update rate Background (1s) read
+
_
+
+
w*(s)
rads
-1
w(s)
Speed controller
Ki.Kd
_
rads
-1
Kp+Ki/s
Ki.Kd
Kc.Kt L(s)
+
_
+
_
w*(s)
w(s)
T
delay
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