User's Manual

HARSFEN0602
Hall A
Hall B
HallC
270
deg
330
deg
30
deg
90
deg
150
deg
210
deg
270
deg
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.
9.3.2 Shaft 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 =
⋅⋅
revolutionshaftpercounts
pairspoleofnumber
360 mod(p1,
pairspoleofnumber
revolutionshaftpercounts
)
9.3.3 Combination of direct magnetic field sensors and shaft angle sensor
9.3.3.1 Initializing 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.