Ignition Strategies

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Ignition Strategies are the strategies to excite a motor from idle state to rotational motion. As our main goal in Open-BLDC is to use a sensorless (no Hall sensors, no encoders) control approach, the motor needs to be in motion before a closed loop control can be applied.

When, for example, using Back EMF as input to the motor controller, it is necessary that the motor turns and current is being induced in the stator coils before the controller can do it's job. Thus, it is necessary to start turning the motor using an open loop controller before the closed loop controller can take over.

The first approach we took was fine spin-up after coarse with no smooth transition:

Fine spin-up after coarse with no smooth transition ignition scheme

We were using a counter counting the overflows of the longest commutation time and using it for a coarse spin-up commutation timing. When this counter was equal to one we used the actual commutation timer for further speed-up.

The disadvantage of this approach was that the transition from coarse to fine was difficult: As there was no constant acceleration, such as in a ramp, the discontinuity in acceleration resulted in an unreliable ignition.