Elmo HARSFEN0602, HARmonica software manual 129, Dual feedback mode UM=4

HARSFEN0602ElmoHARmonicaSoftwareManual PRELIMINARYDRAFT

The stepper unit mode enables the rotation of a motor without feedback control. The motor field is rotated to the desired direction, and the rotor magnet is believed to follow. The user must not rotate the field too abruptly in order that the rotor will be able to track its desired direction. If the rotor misses a full electrical revolution, it will be attracted to a wrong electrical equilibrium, with no feedback to correct that.

Moreover, if the field rotation is stopped abruptly, the motor will not properly brake. If the rotor misses half an electrical revolution, it will start accelerating – and the net braking torque sum over an electrical revolution is zero.

Specifying higher motor current enables larger accelerations and decelerations, but also lose more power at the steady state.

In the stepper mode, most of the time the stator field and the rotor magnet are near equal, and the motor efficiency is very low.

The Harmonica uses the stepper mode mainly for safe testing and controller tuning before any feedback control is tuned. The Harmonica can serve as an advanced micro-stepper driver, with 1024 micro-steps per electrical revolution. The main limitation is that the Harmonica is a 3-phase driver, while most the stepper motors in the market are two-phased.

The block diagram of the micro-stepper mode UM=3 is depicted below:

129

Auxiliary position

command

Software

position

com m and

Smoother

Enable if

RM==1

Stepper angle command W S[20]

Stop	Current
manager	controller

1024 count

per electrical rev.

Σ

Stop and limit

switches

Torquecommand

Torque

(DV[1], Amp)

command

Figure 21: Stepper mode (UM=3)

The stepper angle command is generated by:

The position software command generator

The position auxiliary command generator

•The position Stop Manager

The generation of the stepper angle command is similar to the position command generation in UM=5, and will be described in "Stop management" below.

The torque command is simply given by the command TC.

11.4 The Dual feedback mode: UM=4

The dual feedback mode is used when different sensors are used for speed/commutation and for position. This is a common situation if the motor drives the load through a reduction gear. The controlled position is that of the load. The load position, however, may not be good for commutation or speed feedback, since: