1.5 BLDC Sensorless Block Commutation using BEMF Zero Crossing Detection (ZCD)

Brushless Direct Current (BLDC ) motor is controlled using Block Commutation of Sensorless BEMF Zero Crossing Detection (ZCD). Motor start/stop operation is controlled by the switch and motor speed can be changed by the on-board potentiometer. Waveforms and variables can be monitored runtime using X2CScope.

Key features enabled in this project are:

• BLDC Commutaton

• ADC0-ADC1 Peripheral:
  • ADC0 and ADC1 are setup to operate in Master - Slave mode with ADC0 acting as a Master.
  • Both ADCs convert single ended inputs. DC link current is sampled and converted by ADC0 which can be used for current control. Potentiometer is sampled and converted by ADC1
  • Both ADCs are hardware triggered simultaneously by an event generated from TCC0 at the end of each PWM cycle
  • Conversion Ready interrupt is generated by ADC0. Since both ADCs are triggered simultaneously and have same resolution and sampling time, both ADCs complete conversion at the same time.
• TCC0 Peripheral:
  • Configured to generate 3 pairs of single PWM signals at frequency of 20 kHz in Edge Aligned Mode
  • Event output is enabled which is generated for counter overflow
  • Non-recoverable Fault is enabled on EV0. When an event is detected on EV0, all PWM channels are held low.
• TC0, TC2-TC4 Peripherals:
  • TC0 peripheral:
    • The TC0 peripheral is used as a timer.
    • The timer period is set to 0.003 ms.
    • The one-shot mode and the interrupt is enabled for the timer.
    • The input clock frequency for the TC0 peripheral is 48 MHz.
    • TC0 ISR is used to generate the Blank end timing after the commutation. It sets the blanking time to start the zero-crossing detection once the new sector is switched.
  • TC2 peripheral:
    • The TC2 peripheral is used as a timer.
    • The timer period is set to 1 ms.
    • The one-shot mode and the interrupt is enabled for the timer.
    • The input clock frequency for the TC2 peripheral is 8 MHz.
    • TC2 ISR is used for Block commutation.
  • TC3 peripheral :
    • The TC3 peripheral is used as a timer.
    • The timer period is set to 131.072006 ms.
    • The input clock frequency for the TC3 peripheral is 48 MHz.
    • TC3 peripheral is use to measure the period between the two zero crossing points.
  • TC4 peripheral:
    • The TC4 peripheral is used as an internal timer counter.
    • The timer interval is set to 1 ms.
    • The input clock frequency for the TC4 peripheral is 8 MHz.
    • The 1 ms timer interrupt handler is used to excute the rotor alignment mode, open-loop start-up mode, and closed-loop BMEF ZCD mode.
• EIC:
  • External Interrupt Controller detects hardware over-current fault input and generates a non-recoverable fault event for TCC0, thereby shutting down the PWM in the event of an over-current fault
• EVSYS:
  • Event System acts as an intermediary between event generator and event users.
  • Event generated by the TCC0 when the counter reaches ZERO, is used by the ADC0 as a hardware trigger source via the Event System.
  • Event generated by the EIC upon over-current fault, is used by the TCC0 as a non-recoverable fault event via the Event System.
• Analog Comparator (AC):
  • The analog comparators allow comparison of analog voltages against a known reference voltage to determine their relative levels.
  • Analog comparators for zero-crossing detection, one terminal of the comparator input is configured to monitor the relevant analog signal of U V W phases, and the other terminal is configured to virtual ground.
• SERCOM3 Peripheral:
  • SERCOM3 is configured in USART mode and is set to operate at 115200 bps.
  • This USART channel is used by the X2CScope plugin to plot or watch global variables in run-time. Refer to X2C Scope Plugin section for more details on how to install and use the X2CScope.

Brushless Direct Current electric motors, or BLDC motors for short, are electronically commutated motors powered by a DC electric source via an external motor controller. The electronic commutation to rotate the motor is achieved by a three phase inverter. The commutation technique are broadly classified as block Commutation and Sinusoidal commutation.The block commutation has reduced system complexity as compared to the sinusoidal commutation. Hence, the block commutation technique is quite popular for the low cost applications where control precision, reduced efficiency, and higher acoustic noise are permitted.

The key of the sensorless control method is to determine the appropriate moment when the motor winding should be commutated. This moment is determined by the rotor position. With sensored control, the rotor position is well known. However, in sensorless control, rotor position must be determined by the detection of the BEMF zero-crossing.

The method for energizing the motor windings in the sensorless method is the six-step commutation. Each step, or sector, is equivalent to 60 electrical degrees. Six sectors make up 360 degrees, or one electrical revolution. For every sector, two windings are energized and one is not energized. The fact that one of the windings is not energized during each sector is an important characteristic for the use of sensorless control.

Pulse-width modulation is used to apply a variable voltage to the motor windings. Changing the PWM duty-cycle of the transistors (T1 to T6) signals, changes the amount of current flowing through the windings of the motor.

Below diagram shows the high side modulated PWM:

The BEMF zero-crossing signal is used to provide a position feedback to estimate the right commutation instant. In this implementation, the method employed to measure the zero-cross point is by using a comparator with a variable reference consisting of the sum of all the phase voltages.