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AVR1607: Brushless DC Motor (BLDC) Control

in Sensor mode using ATxmega128A

1 and

ATAVRMC323

Features

• BLDC Motor Basics

• Hardware Implementation

• Code Example

References

[1] ATxmega128A1 Data sheet

[2] AVR430: MC300 Hardware User Guide

[3] AVR1014: MC303 Hardware User Guide

[4] AVR601: Atmel Modular Evaluation Kits for Motor Control Applications

Glossary

BLDC : Brushless DC

PWM : Pulsed Width Modulation

IGBT : Insulated Gate Bipolar Transistor

AWEX : Advanced Waveform Extension

1 Introduction

This application note describes how to implement a control of Brushless DC motor

(BLDC) in sensor mode using the ATxmega128A1, Atmel

AVR

microcontroller

and the ATAVRMC323 development kit.

This application note deals only with BLDC motor control application using Hall

effect position sensors to control commutation sequence.

In this document, we will give a short description of brushless DC motor theory of

operations. We will detail how to control a brushless DC motor in sensor mode and

we will also provide a short description of the ATAVRMC303 and ATAVRMC300

boards used in this application note.

Software implementation is also discussed with software control loop using a PID

filter.

8-bit

Microcontrollers

Application Note

Rev. 8311A-AVR-07/10

Summary of content (19 pages)

PAGE 1
AVR1607: Brushless DC Motor (BLDC) Control in Sensor mode using ATxmega128A1 and ATAVRMC323 Features • BLDC Motor Basics • Hardware Implementation • Code Example 8-bit Microcontrollers Application Note References [1] ATxmega128A1 Data sheet [2] AVR430: MC300 Hardware User Guide [3] AVR1014: MC303 Hardware User Guide [4] AVR601: Atmel Modular Evaluation Kits for Motor Control Applications Glossary BLDC : Brushless DC PWM : Pulsed Width Modulation IGBT : Insulated Gate Bipolar Transistor AWEX : Advanced Wa
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Brushless DC motor Theory of Operation 2.1 Simplified Model of a BLDC Motor A simplified model of a three phases BLDC motor stator consists of three coils arranged in three directions U, V and W (Figure 1). A permanent magnet forms the rotor. The rotor in a BLDC motor consists of an even number of permanent magnets. The number of magnetic poles in the rotor also affects the step size and torque ripple of the motor. More poles provide smaller steps and less torque ripple.
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AVR1607 Figure 2. Model of a BLDC Motor with current flows versus rotor positions Figure 3. Commutation sequence Notes : - Open phase is the one shown with dotted line. - Hall states are detailed in 2.4 section 2.3 Power Stage The commutation pattern is controlled with a 3-phase bridge (see Figure 4). The 3 half bridges have 6 power switches (IGBT or MOSFET transistors) which are switched according to the defined commutation pattern. Figure 4.
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2.4 Hall sensors For the estimation of the rotor position, the motor is equipped with three hall sensors. These hall sensors are placed every 120°. With these sensors, 6 different commutations are possible. Phase commutation will depend on these hall sensor values. Figure 5 shows the three Hall sensors signals H1,H2,H3 as motor turns using sensor control. Hall states are the combination result of H1,H2,H3 signals. Figure 5.
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AVR1607 Reading hall sensors values (HS_xxx variable) indicates which new scheme should be switched.(see following table) Table 1-1.
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motor. Then Phase U is unpowered and Phase W is connected to the positive DC bus, resulting in a new stator flux vector ‘Step 4’. Following the commutation schematic Figure 5 and Table 1-1, we get six different stator flux vectors corresponding to the six commutation steps. These six steps provide one electrical revolution.
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AVR1607 o 8x4 differential inputs with selectable 1/2/4/8/16/32/64x gain • Four Analog Comparators with Window compare function, with selectable comparison levels, and interrupts on pin change I/Os. The ATxmega128A1 includes independent positive and negative comparator inputs available for over-current detection.
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4.1 Hardware implementation The block diagram of the sensor closed loop is following : Figure 8. System block diagram The outputs UH, UL, VH, VL, WH & WL of AVR303 are used to control the power bridge (see Table 1-2). As previously seen, they depend on the Timer0 and AWEX which generates PWM signals. An external comparator on MC303 board provides a fault signal (Fault_overcurrent) connected to PE4 input.: The compared inputs are the shunt_pos and AVCC/ 34. Shunt_pos is the voltage monitored across the 0.
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AVR1607 PA3 PA4 PA5 PA6 PA7 PORTB PB0 PB1 PB2 PB3 98 99 100 1 2 V_conditioned W_conditioned Vn_conditioned Vm W_cond_neg = V / 6 (Not used) = W / 6 (Not used) Neutral voltage / 34 (Not used) (Not used) W Reference for zero-crossing (Not used) 5 6 7 8 Speed_ref Speed Potentiometer (MC303) Current_reference Shunt_pos PB4 PB5 PB6 PB7 PORTC PC0 PC1 PC2 PC3 PC4 PC5 PORT E PE0 PE1 PE2 PE4 PE5 PORT H PHO 9 10 11 12 Shunt_neg Shunt_U Shunt_V Shunt_W = AVCC / 34 0.
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Table 1-4. ATAVRMC303 jumper settings Jumper Position Comment J15 Open Shunt_neg (Jumpers to be removed to enable JTAG Programming) J16 Open Shunt_U (Jumpers to be removed to enable JTAG Programming) J17 Open Shunt_V (Jumpers to be removed to enable JTAG Programming) J18 Open Shunt_W (Jumpers to be removed to enable JTAG Programming) 4.4 Power-supply This firmware example has been configured according to a power-supply Vm=12V. This power-supply must be able to provide up to 4A output current.
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AVR1607 4.6.2 Ground and Power Wirings One design its own board has to take care of the ground wiring and power wiring. The power supply of the processor and additional signal conditioning components (e.g. additional fast comparators, operational amplifiers) has to be decoupled from the motor power supply. The ground connection has to be of low resistance and low inductance to prevent against voltage drop and noise due to high currents.
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Figure 9. Main flow chart The tasks are scheduled thanks to the g_tick produced each 1.024ms with Timer1.
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AVR1607 5.2 INIT : Initialisation functions The initialization functions are following 5.2.1 clock_init() • Oscillator : 32MHz RC oscillator and OSC oscillator • PLL : clock source=32MHz and factor =16 • Prescalers: PSADIV=1 and PSB and PSCDIV=2 : means clk_PE R4 = 128MHz, clk_PE R2 = 64MHz and clk_PE R= 32MHz 5.2.
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duty_cycle = mc_control_current(mc_get_potentiometer_value()) 5.4 Commutation The phase commutation has to be achieved according to the 3 Hall sensors. Commutation between steps is achieved when a rising or falling edge occurs on one of the 3 Hall sensor signals. Hall sensor signals H1/H2/H3 are connected to PE0/PE1/PE2 which are configured as interrupt sources. (Another solution could be using internal comparators to generate interrupts.
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AVR1607 • and secondly, a switch of the power bridge according to next expected position and the direction of the rotation. AWEX feature, described previously is configured on PortC. Port C0 to C5 are the UH/UL/VH/VL/WH/WL outputs.
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Table 1-5. List of commands Command Action ru Run motor st Stop Motor help Gives help fw Set direction to Forward bw Set direction to Backward ss Set Speed (followed with speed value) gi Get ID g0 Get Status 0 g1 Get Status 1 6.2.2 Motor Control Center The User Guide is available in Install directory at URL :C:\Program Files\Atmel\Motor Control Center\help\Overview.
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AVR1607 7 USB communication Communication can be achieved from PC to USB connector of MC303 board. The AVR1014, MC303 Hardware User Guide details the configuration to be achieved. Communication port becomes a Virtual Com port which must be forced in RS232 Settings window. Same tools and commands as described in section 6 can be used through this Virtual Com port.
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