Encoderless Operation - Siemens SINAMICS S120 Function Manual

5.14

Encoderless operation

Overview
Both encoderless and mixed operation (without/with encoder) is possible in the servo control.
Encoderless operation with the motor model allows a higher dynamic response and greater
stability in the servo control than a standard drive with U/f control. Compared with drives with
an encoder, however, speed accuracy is lower and the dynamic response and smooth running
features deteriorate.
Function description
Since the dynamic response in operation without an encoder is lower than in operation with an
encoder, acceleration torque precontrol is implemented to improve the control dynamic
performance. It controls, knowing the drive torque, and taking into account the existing torque
and current limits as well as the load moment of inertia (motor moment of inertia: p0341 · p0342
+ load torque: p1498), the required torque for a demanded speed dynamic performance
optimized from a time perspective.
Note
If the motor is operated with and without an encoder (e.g. p0491 ≠ 0 or p1404 < p1082), the
maximum current during operation without an encoder can be reduced via p0642 (reference
value is p0640) in order to minimize interfering, saturation-related motor data changes during
operation without an encoder.
A torque smoothing time can be parameterized via p1517 for the torque precontrol. The speed
controller needs to be optimized for operation without an encoder due to the lower dynamic
response. This can be achieved via p1470 (P gain) and p1472 (integral time).
In the low-speed range, the actual speed value, the orientation, and the actual flux can no
longer be calculated during operation without an encoder due to the accuracy of the measured
values and the parameter sensitivity of the procedure. This is the reason that the system
switches over to current/frequency control (I/f-controlled operation), where only the current and
frequency are impressed. The switchover threshold is parameterized via p1755 and the
hysteresis via p1756.
To accept a high load torque even in the open-loop controlled range, the motor current (current
setpoint) can be set using p1612. To do so, the drive torque (e.g. friction torque) must be known
or estimated. An additional reserve of approx. 20% should also be added. In synchronous
motors, the torque is converted to the current via the motor torque constant (p0316). For
induction motors, the magnetizing current (r0331) must be additionally taken into account in
p1612. In the lower speed range, the required current cannot be measured directly on the Motor
Module. The default setting is 50% (synchronous motor) or 80% (induction motor) of the motor
rated current (p0305). When parameterizing the motor current (p1612), you must take into
Drive functions
Function Manual, 06/2019, 6SL3097-5AB00-0BP2
Servo control
5.14 Encoderless operation
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