AN1913 Motorola / Freescale Semiconductor, AN1913 Datasheet - Page 38

• Manufacturer Part Number: AN1913
• Description: AN1913 3-phase BLDC Motor Control with Sensorless Back-EMF ADC Zero Crossing Detection using DSP 56F80x
• Manufacturer: Motorola / Freescale Semiconductor

Software Design
7.2.3 Process ADC Zero Crossing Checking
7.2.4 Process Zero Crossing Offset Setting
7.2.5 Process Speed PI Controller
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(Omega_Desired)
This process is based on the ADC Zero Crossing feature. When the free (not energized) phase branch
voltage changes the sign comparing previous conversion results, the Zero Crossing interrupt is
initiated. Then the BLDC motor commutation control is performed in the Zero Crossing ISR.
In order to assure proper behavior of the ADC Zero Crossing Checking, the ADC (Zero Crossing)
Offset Registers are initialized the way that the zero Back-EMF voltage is converted to zero ADC
value. The initialization is provided in two steps:
The ADC Offset Registers for all free phase voltages are set to U_Dc_Bus_Half.
The phase Zero Crossing calibration coefficient is obtained during the Alignment state (to reflect the
unbalance of the sensing circuitry) from non-fed phase branch voltage measurements (average value)
and DC-Bus voltage measurements (average value).
During motor running (and starting) the U_Dc_Bus_Half is continuously updated based on the
following formula:
The general principle of the speed PI control loop is illustrated in
The speed closed loop control is characterized by the feedback of the actual motor speed. This
information is compared with the reference set point and the error signal is generated. The magnitude
and polarity of the error signal corresponds to the difference between the actual and desired speed.
Based on the speed error, the PI controller generates the corrected motor voltage in order to
compensate for the error.
Reference
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Speed
Calibration of Phase Voltage Coefficients
Setting of the ADC Offset Registers (U_Dc_Bus_Half) according to measured DC-Bus
voltage
Coef_Calibr_U_Phx = (U_Dc_Bus_Half + U_Phx)/U_Dc_Bus
U_Dc_Bus_Half = Coef_Calibr_U_Phx * U_Dc_Bus
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BLDC control with BEMF Zero Crossing using ADC
Figure 7-6. Closed Loop Control System
Speed
Error
Controller
PI
(U_Desired)
Corrected
Speed
Figure 7-6.
Controlled
System
(Omega_Actual)
Actual Motor
Speed