bldc_motorcontrol

Brushless DC Motor Control Using PIC18Fxx31 Microcontrollers

© 2003 Microchip Technology Inc. 1

© 2003 Microchip Technology Incorporated. All Rights Reserved. Brushless DC Motor Control Using PIC18Fxx31 Microcontrollers 1

Brushless DC (BLDC)Motor Control

using

PIC18Fxx31Microcontrollers




Agenda� Overview of motor control solutions from Microchip� PIC18Fxx31 peripherals for motor control� BLDC motor control using PIC18Fxx31

� Open loop control� Closed loop control using Hall sensors� Closed loop control using Quadrature encoder� Sensorless control

The agenda for this discussion is:� First we will discuss an overview of motor control solutions from Microchip� PIC18Fxx31 has many peripherals that are useful for motor control application. We

will discuss the major peripherals that are helpful in developing motor controlapplication.

� Then we will discuss the Brushless DC motor control using PIC18Fxx31 devices inopen loop, closed loop with Hall sensors and closed loop with Quadrature Encoderor also known as optical encoder. Finally we will see how a sensorless control ofBLDC motor can be implemented using PIC18Fxx31

� At the end of this discussion, we will show few references that would be helpful foryou to understand BLDC motors and controls using PIC18Fxx31




Motor Control from Microchip� Complete solutions for Stepper, Brushed DC,

BLDC, ACIM & SR motors utilizing PIC16, PIC18and dsPIC® devices

� Microchip Op Amps and International Rectifierdrivers

� Provide everything a design engineer needs:� Low-risk product development� Lower total system cost� Faster time to market� Outstanding technical support� Dependable delivery & quality

� Visit us at www.microchip.com/motor

Microchip Technology offers a broad product portfolio that provides a completesystem solution for your brushed DC motor, variable speed brushless DC motor,AC induction motor, switched reluctance motor and stepper motor applications.This includes the microcontroller with firmware to drive the motor, analog op ampsand comparators for sensors or feedback and power electronics from Microchip andInternational Rectifier.With our sophisticated development systems and technical documentation,Microchip makes it easy for designers of all experience levels to complete a highperformance electronic motor control design quickly and cost effectively.Microchip provides everything a motor control design engineer needs: low-riskproduct development, lower total system cost, faster time-to-market, outstandingtechnical support and dependable delivery and quality.

For access to Microchip�s complete motor control design resources,visit the Motor Control Design Center at www.microchip.com/motor.




PIC18Fxx31 overview

PIC18Fxx31 family of microcontrollers have 4 parts, having 28-pin and 40-pinpackages with 8Kbytes and 16Kbytes of program memory. The major peripherals that are usefulin motor control are Power Control PWM or PCPWM, High-speed Analog-to-Digital Converterand Motion Feedback module. These peripherals simplify the motor control algorithm to a greatextent.The main features of PCPWM include:� Up to 8 channels output with independent or 4 pairs complimentary outputs� Up to 14 bits PWM of resolution� Center aligned or edge aligned PWM operation. Also known as symmetrical PWM orasymmetrical PWM operation� Programmable dead band control for complementary outputs� Hardware Fault interface pins for fast PWM shut down in the event of fault.Main features of high-speed ADC include:� Up to 9 channels input, with 2 Sample and Hold circuits� Simultaneous and sequential conversion capabilities with multiple channel selection� 4 word deep FIFO with flexible interrupt settings Main features of Motion Feedback Modules are:�Multiplexed Input Capture and Quadrature Encoder Interface (QEI) modules�In QEI, A, B and Index signals interface for measuring, position velocity and direction ofrotation�3 Input Capture pins with multiple modes for pulse width and frequency measurements -Interrupt on change feature used for Hall sensor interface.




PIC18Fxx31 peripherals

� Power control PWM� Up to 8 channels with up to 14-bit resolution� Center aligned and edge aligned operations

� High-speed ADC� Up to 9 channels with up to 200 Ksps� 4 word FIFO buffer with flexible interrupts

� Motion feedback module� Quadrature Encoder Interface (QEI) - QEA, QEB, Index

� Position and velocity measurement modes� 3 Input Capture (IC) pins

� Pulse width and frequency measurement modes

PIC18Fxx31 family of microcontrollers have 4 parts, having 28-pin and 40-pin packages with8Kbytes and 16Kbytes of program memory.The major peripheral that are useful in motor control are Power Control PWM or PCPWM, high-speed Analog-to-Digital Converter and Motion Feedback module. These peripherals simplify themotor control algorithm to a great extent.Main features of PCPWM include:� Up to 8 channels output with independent or 4 pairs complimentary outputs� Up to 14 bits PWM of resolution� Center aligned or edge aligned PWM operation. Also known as symmetrical PWM orasymmetrical PWM operation� Programmable dead band control for complementary outputs� Hardware Fault interface pins for fast PWM shut down in the event of fault.Main features of high-speed ADC include:� Up to 9 channels input, with 2 Sample and Hold circuits� Simultaneous and sequential conversion capabilities with multiple channel selection� 4 word deep FIFO with flexible interrupt settings Main features of Motion Feedback Modules are:� Multiplexed Input Capture and Quadrature Encoder Interface modules�In QEI, A, B and Index signals interface for measuring, position velocity and direction ofrotation�3 Input Capture pins with multiple modes for pulse width and frequency measurements -Interrupt on change feature used for Hall sensor interface.


© 2003 Microchip Technology Inc.


Drive topologyVDC+

VDC-

(1)(4)

(5)(2)

(3)(6)

H1

L1

H2

L2

H3

L3

Phase A

Phase B

Phase C

A brushless DC motor is a synchronous motor that finds numerous applications inmotion control. Appliance, automotive, aerospace, industrial automation, automationare few industries to list. These come with different torque and voltage ratings. ABLDC motor has windings on stator and alternate permanent magnets on rotor. BLDCmotors are electronically commutated based on the rotor position with respect to thestator winding. This means, to run a BLDC motor an electronic drive is required.Normally 3 Hall effect sensors mounted on the stator are used to determine the rotorposition. The Hall effect sensors give a combination of high and low signals when theypass next to the rotor poles. With this combination, the commutation sequence isdetermined.A typical control circuit with 3 phase winding connection is shown in the figureH1,H2, H3 and L1, L2,L3 make 3 phase voltage source inverter connected across thepower supply indicated by VDC+ and VDC-. Stator windings A, B and C areconnected in star to the inverter.In coming slide we will see how these stator windings are energized in synchronouswith the Hall sensor inputs.




Switching Sequence(1) (2) (3) (4) (5) (6) (1) (2) (3) (4) (5) (6)

Phase A

Phase B

Phase C

H3L2

H1L2

H1L3

H2L3

H2L1

H3L1

H3L2

H1L2

H1L3

H2L3

H2L1

101 001 011 010 110 100 101 001 011 010 110Step

Hall sensorHall A

Hall B

Hall C

HallA

HallB

HallC

PhaseA

PhaseB

PhaseC

1 0 1 NC Vdc- Vdc+1 0 0 Vdc+ Vdc- NC1 1 0 Vdc+ NC Vdc-0 1 0 NC Vdc+ Vdc-0 1 1 Vdc- Vdc+ NC0 0 1 Vdc- NC Vdc+

High switchLow switch

Sequence table example

With 3 Hall sensors on the motor, every 60 degrees of electrical cycle, a Hall sensormakes transition either from low to high or from high to low. With this everyelectrical cycle has 6 steps to complete one full cycle. The energizing sequence willhave 6 combinations of turning ON and Off of the 6 switches that we saw in theprevious slide. A typical switching sequence is shown on the table here.The Hall sensor inputs are at 120 degrees phase shift to each other. Every sequencehas two windings connected across the power supply and third winding left open.Considering the first step where the Hall sensor input is 101, Phase C is connectedto positive DC bus and phase B is connected to negative DC bus and phase A is leftopen. In order to achieve this switch H3 and L2 should be closed and all otherswitches should be open. This will turn the rotor by 60 degree electrical in the givendirection. This will make the Hall sensor to make another transition triggering thenext point on the sequence table and so on.The electrical cycle and shaft rotation have a definite relation. The electrical cyclerepeats with every rotor pole pairs. So to complete one rotation on the shaft, these 6steps should be repeated as many times as rotor pole pairs.If the sequence is followed with rated motor voltage across the motor windings,motor will run at rated speed. Speed can be controlled using PWMs, by PWMingeach switch according to the speed required.The phase current waveform looks trapezoidal in shape with each phase currenthaving 120 degrees phase shift to each other.




Control using Hall Sensors

/MCLR 1 28

2

3

4

5

6

7

8

9

11

12

13

14

10

27

26

25

24

23

22

21

20

18

17

15

19

16

AN0

AN1/IC1AN2/IC2

AN3/IC3

AN4

AVdd

AVss

OSC1

OSC2

RC0

/FLTA/CCP2

/FLTB/CCP1

RC3/INT0

PWM2

PWM1

PWM0

Vdd

Vss

RC7/RX/DT

RC6/TX/CK

RC5/INT2

RC4/INT1

PWM3

PWM5

RB6/PGC

RB7/PGD

PWM4

R-LowR-High

Y-Low

Y-HighB-High

B-Low

Motor current

Potentiometer

+

Motor current

Reference-

Hall sensors

PIC

18F2

431

BLDCMotor

3 phase Inverter bridge

AC in

+ - DC bus

Hallsensors

Motorcurrent

This slide shows the hardware interface used for controlling a BLDC motor. Hallsensors are used for commutation. Hall sensors can be connected to either Interrupt onchange pins that are on port C or they can be connected to Input capture pins onMotion feedback module.The input capture module has a mode in which the module generates an interrupt ontransition on each pin, making it ideal for Hall sensor interface. Optionally, Timer5 canbe captured on each transition. This Timer5 value can be used for determining thespeed at which motor is running. This gives a low resolution speed measurement.For the applications where high resolution of speed measurement is required,quadrature encoder is used. The QE gives speed, position and direction of rotation.




OVDCOND v/s PWM output(1) (2) (3) (4) (5) (6)Step

101 001 011 010 110 100Hall sensor

PWM5 - H3

PWM4 - L3

PWM3 - H2

PWM2 - L2

PWM1 - H1

PWM0 - L1

00100100 00000110 00010010 00011000 00001001 00100001OVDCOND

Now let us see how to control the speed using PCPWM module. PCPWM modulehas a feature of over riding the PWM outputs based on the value in OVDCONDregister. When the corresponding value in OVDCOND is set to 1, the PWM outputbecomes active and vise versa. With this we can efficiently allow the required PWMto appear on the pin, when required as per the sequence, and inhibit when notrequired.Speed variation is achieved by varying the duty cycle of each PWM. To increase thespeed, the active part on duty cycle needs to be increased and to reduce the speed,the active time is reduced. By doing this the average voltage supplied across themotor winding varies, thus controlling the motor speed.This slide shows the relationship between the OVDCOND register and the PWMoutput. PWM0 to PWM5 control on and off of 6 switches. The PWMs are passedand inhibited according to the sequence discussed earlier.




Control Flow ChartInitialization

Load new step sequence

to the OVDCOND

register from Table

Hall sensorchange?

Yes

No

Key scan/

Other application?

ChangeSpeed?

Calculate new PWM

duty cycle

Yes

No

HallA

HallB

HallC

PhaseA

PhaseB

PhaseC

1 0 1 NC Vdc- Vdc+1 0 0 Vdc+ Vdc- NC1 1 0 Vdc+ NC Vdc-0 1 0 NC Vdc+ Vdc-0 1 1 Vdc- Vdc+ NC0 0 1 Vdc- NC Vdc+

This shows a simplified control flow chart for BLDC motor control. The sequencetable dictates the PWM channels that should be in active or inactive states, based onthe Hall sensor states. On every transition on Hall sensor input, new step from thesequence is loaded to the OVDCOND register.Any change in the speed command is read either using a potentiometer connected toone of the AD channel or a digital value from another controller calculate a newPWM duty cycle and load to the appropriate registers.




Control using Quadrature Encoder

1 402

34

56

7

9

12

1516

1718

14

39

38

37

36

35

34

3332

3029

26

31

28

INDX

Vss

OSC1

FLTA/CCP2

INT0/RC3

PWM2

PWM1

PWM0VddVssRD7RD6RD5RD4

PWM3

PWM5

PWM4RB6/PGC

RB7/PGD

8

10

13

Vdd

OSC2

1920

RD0RD1

11

27RC7/RX/DT

25 RC6/TX/CK

24 RC5/INT2

23 RC4/INT1

22 RD3

21 RD2

/MCLR

AN0

QEAQEB

AN4

AN5AN6AN7AN8

RC0

FLTB/CCP1 BLDCMotor


R-LowR-High

Y-Low

Y-HighB-High

B-Low

AC in

+ - DC bus

Hallsensors

INT0

QE

QE interface

Motor current

Potentiometer

Temp. sensor

+

Motor current

Reference-

Power factor correction

PIC

18F4

431

Motorcurrent

With QE for speed, position and direction feedback and Hall sensors for commutationcan be interfaced as shown here.Hall sensors connected to INT pins, and QE connected to motion feedback module.




Sensorless control

1 402

34

56

7

9

12

1516

1718

14

39

38

37

36

35

34

3332

3029

26

31

28

IC1

Vss

OSC1

FLTA/CCP2

INT0/RC3

PWM2

PWM1

PWM0VddVssRD7RD6RD5RD4

PWM3

PWM5

PWM4RB6/PGC

RB7/PGD

8

10

13

Vdd

OSC2

1920

RD0RD1

11

27RC7/RX/DT

25 RC6/TX/CK

24 RC5/INT2

23 RC4/INT1

22 RD3

21 RD2

/MCLR

AN0

IC2IC3

AN4

AN5AN6AN7AN8

RC0

FLTB/CCP1


R-LowR-High

Y-Low

Y-HighB-High

B-Low

AC in

+ - DC bus

Back EMF ZC detect

Motor current

Potentiometer

Temp. sensor

+

Motor current

Reference-

+

-Back EMF ZC detect

BLDCMotor

Power factor correction

PIC

18F4

431

Motorcurrent

Sensorless control of BLDC motor gives many advantages, most importantly loweredsystem cost. Every phase develops a voltage called Back EMF, that opposes the powerapplied to the phase. During the non energized phase of the sequence, this back EMFcrosses from positive voltage to negative voltage. From the the zero cross over point,rotor position can be determined, and used for commutation. This method eliminatesthe requirement of sensors for commutation.However, this may require additional hardware on the drive side and additionalfirmware overhead on the microcontroller.




Sensorless control schemes1) Compare with DC bus/2 2) Compare with virtual neutral

3) Using ADC Channels

A

C B

DC+

DC-

DC/2

+_

Back EMF

To IC2BZ_B

+_ To IC1

BZ_A

+_ To IC3

BZ_C A

CB

DC+

DC-

VirtualNeutral

+_

Back EMF

+_ To IC1

BZ_A

+_ To IC3

BZ_C

To IC2BZ_B

A

C B

AN0

AN1

AN2

Back EMF

We will see three different methods of determining the Back EMF zero cross point.1) Comparing WRT center point of DC bus: Every sequence has two windingsconnected across power supply and third winding left open. The BEMF generated inthe non energized winding is compared with respect to the half of DC bus. This gives afairly good result, when the motor terminal voltage is approximately equal to the DCbus voltage. If the DC bus voltage is disproportionately high, the cross over point maydrift away, making it difficult to determine a workable commutation sequence at allspeeds.2) Comparing with a virtual neutral: A virtual neutral point can be generated usingresistor ladders as shown and the BEMF in the non-energized winding can becompared with this neutral point. This makes it comparatively easy to determine thezero cross point at all measurable speeds.3) Using High speed ADCs: The BEMF signals are attenuated and read directly usingan on chip ADC on a PIC18Fxx31 can give a great flexibility in determining the zerocross over point.

However, at low speeds, the BEMF developed is very low in amplitude, which makesit difficult to determine a zero cross over point.




Summary� Overview of motor control solutions from Microchip� PIC18Fxx31 peripherals for motor control� BLDC motor control using PIC18Fxx31

� Open loop control� Closed loop control using Hall sensors� Closed loop control using Quadrature encoder� Sensorless control




Resources� Application notes:

� AN885 : Brushless DC motor fundamentals� AN899 : Brushless DC motor control using PIC18Fxx31� AN857 : Brushless motor control made easy

� Demo and development board� PICDEMTM MC

� Completely isolated, debug tools can be connectedwhen the board is �live�

� Low cost design� Web design center: www.microchip.com/motor

These are additional resources for more information on Microchip�s Motor Control solutions.

bldc_motorcontrol

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