AN1905 ST Microelectronics, Inc., AN1905 Datasheet - Page 57

• Manufacturer Part Number: AN1905
• Description: St7mc Three-phase BLDC Motor Control Software Library
• Manufacturer: ST Microelectronics, Inc.

C.Software demagnetization
All demagnetization events are simulated and the end of the demagnetization occurs after a
pre-programmed amount of time (see ‘MTC_Settings_Sensorless.h’ file, ‘Setting of demag-
netization time in running mode’ section). The demagnetization time is an arbitrary value that
has to adjusted according to the motor specifications; it has to be kept in mind that very induc-
tive motors will require longer demagnetization time, and therefore will require longer step
times. This solution might be preferred when the ‘Hardware with software backup’ demagnet-
ization solution can’t provide reliable motor operation.
7.2.3 The 4 Z event sampling methods (Sensorless)
Below is the description of the 4 sampling types that can be used for Z event detection while
running motors in sensorless mode (for further information on these methods, refer to the ap-
plication note AN1946).
Figure 37. Z event sampling methods as shown in the GUI
7.2.3.1 At the end of the PWM low state
This is the ST patented method; it provides very good sensitivity on the full speed range,
without the usage of any external components. This solution requires an OFF time during
each PWM period in order to detect the zero-crossing event (PWM low state). Samples are
taken after a time window configured with ZWF[3:0] bits in MZFR register. Therefore the duty-
cycle can’t be set to its maximum. The maximum duty cycle will depend of the minimum PWM
off time needed by the system in order to detect the Z event.
Figure 38. Sampling during OFF time, at PWM frequency
ST7MC THREE-PHASE BLDC MOTOR CONTROL SOFTWARE LIBRARY
PWM signal
T
Sampling
ZWF[3:0]
PWM OFF
time
ZWF[3:0]
Current
sample
sample
New
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