ElmoMC SimplIQ Digital Servo Drives-Bell Command Reference - Manual (Page 215)

3.4. Unit Mode 3: Open & Closed Loop Stepper
Control
This chapter describes how a SimplIQ device works with one of the following options:
• Classical open-loop sensorless stepper drive.
• Stepper motor with position feedback.
The PN[9] parameter determines which option is used.
The motor may be two-phase or three-phase: the contents of this chapter apply equally.
For both of the modes, the motor current level is a fixed level with additions. For the
speed and the acceleration of position reference waveform – see the HT[N] command.
The motor steering is made by micro-stepping the electrical angle of the stator field. The
rotor is supposed to track the stator field angle. This is possible if the electrical field angle
varies slowly enough.
The benefit of this unit mode is that it does not require a commutation sensor. In the open
loop mode, no position sensor is required at all.
3.4.1 Open Loop Drives
The benefits of an open loop drive are:
- It does not require any position sensor.
- It does not require complex motor tuning.
- It provides smooth motion at low speed.
The main drawbacks are:
- Low efficiency, since the holding current exists regardless of the mechanical
output torque.
- Tendency to oscillate. The stepper torque is proportional to the position error, so
that it generates a harmonic oscillator with the natural frequency of T
r
K I
J
ω = .
Here
T
K is the motor torque sensitivity, I is the current, J is the motor and load
inertia, reflected to the motor.
- When the field is driven too fast, the stepper stalls. This means that it does not
generate DC torque or power anymore – it simply vibrates in place.
3.4.2 Closed Loop Drives
In the closed loop mode, a position sensor is used to close position feedback.
The benefits of closed loop stepper drive are:
SimplIQ for Steppers Application Note Unit Modes
MAN-STECR (Ver. 1.1)
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