Elmo HARmonica, HARSFEN0602 software manual 106, Shaft Angle Sensors

HARSFEN0602ElmoHARmonicaSoftwareManual				PRELIMINARY		DRAFT
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Hall

Hall B

HallC

Figure 6: Digital Hall sensors readout

The crude division to six draws rough torque from the motor, and requires hard switching of the motor winding currents.

The digital Hall sensors are used, in many applications, together with higher resolution position sensors. Combined with high-resolution sensors, the Hall sensors serve to initialize the field direction, and add redundant position sensing for increased reliability.

9.3.2Shaft Angle Sensors

The second group consists of shaft angle sensors like encoders of all types (Incremental digital and analog, absolute digital and analog), resolvers, capacitive sensors, and more. The Harmonica can interface directly only digital and analog incremental encoders.

The shaft angle sensors normally have good resolution, but they need to be homed (referenced absolutely) with respect to the rotor electrical angle.

From the above discussion it is clear that for the sake of commutation, it is important to know how many bits the shaft sensor counts per one electrical cycle.

The number of counts per electrical may be not an integer number. If the number of counts per electrical cycle is not an integer, then after enough movement the calculated commutation angle may accumulate numerical errors and the motor shall lose torque.

The numerical commutation error is not a big issue with linear motors, since the limited travel limits also the commutation calculation error.

With rotary motors, we observe that each mechanical shaft rotation involves an integer number of encoder counts, per an integer number of electrical cycles. This means that the commutation can be kept accurate as

p1 = mod(encoder, counts per shafts revolution) and

Electrical rotor angle =

360o⋅	number of polepairs	⋅ mod(p1,	counts per shaft revolution	)
	counts per shaft revolution
			number of polepairs

9.3.3Combination of direct magnetic field sensors and shaft angle sensor

9.3.3.1Initializing encoder based commutation

When starting a motor, a rough estimate of the electrical angle can be taken from the digital Hall sensors. At most of the time, the digital Hall sensors read the electrical angle to +/-30 electrical degrees. The exception is the instance at which the Hall sensor reading switches.