AN857 BLDC Motor Control Made Easy (Microchip)

AN857

INTRODUCTIONThis application note discusses the steps of developingseveral controllers for brushless motors. We cover sen-sored, sensorless, open loop, and closed loop design.There is even a controller with independent voltage andspeed controls so you can discover your motors char-acteristics empirically.The code in this application note was developed withthe Microchip PIC16F877 PICmicro Microcontroller, inconjuction with the In-Circuit Debugger (ICD). Thiscombination was chosen because the ICD is inexpen-sive, and code can be debugged in the prototype hard-ware without need for an extra programmer oremulator. As the design develops, we program the tar-

get device and exercise the code directly from theMPLAB environment. The final code can then beported to one of the smaller, less expensive, PICmicro microcontrollers. The porting takes minimaleffort because the instruction set is identical for all PICmicro 14-bit core devices. It should also be noted that the code was bench testedand optimized for a Pittman N2311A011 brushless DCmotor. Other motors were also tested to assure that thecode was generally useful.

Anatomy of a BLDCFigure 1 is a simplified illustration of BLDC motor con-struction. A brushless motor is constructed with a per-manent magnet rotor and wire wound stator poles.Electrical energy is converted to mechanical energy bythe magnetic attractive forces between the permanentmagnet rotor and a rotating magnetic field induced inthe wound stator poles.

FIGURE 1: SIMPLIFIED BLDC MOTOR DIAGRAMS

Author: Ward BrownMicrochip Technology Inc.

N

S

A

C

a

a

bb

c

c

B

com

com

com

N

N

S

S 110

010

011

101

100

001

N

S

S

N

63 4

1

2

5

A

CB

c b

a

com

Brushless DC Motor Control Made Easy 2002 Microchip Technology Inc. DS00857A-page 1

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In this example there are three electromagnetic circuitsconnected at a common point. Each electromagneticcircuit is split in the center, thereby permitting the per-manent magnet rotor to move in the middle of theinduced magnetic field. Most BLDC motors have athree-phase winding topology with star connection. Amotor with this topology is driven by energizing 2phases at a time. The static alignment shown inFigure 2, is that which would be realized by creating anelectric current flow from terminal A to B, noted as path1 on the schematic in Figure 1. The rotor can be madeto rotate clockwise 60 degrees from the A to B align-ment by changing the current path to flow from terminalC to B, noted as path 2 on the schematic. The sug-gested magnetic alignment is used only for illustrationpurposes because it is easy to visualize. In practice,maximum torque is obtained when the permanent mag-net rotor is 90 degrees away from alignment with thestator magnetic field. The key to BLDC commutation is to sense the rotorposition, then energize the phases that will produce themost amount of torque. The rotor travels 60 electricaldegrees per commutation step. The appropriate statorcurrent path is activated when the rotor is 120 degreesfrom alignment with the corresponding stator magneticfield, and then deactivated when the rotor is 60 degreesfrom alignment, at which time the next circuit is acti-vated and the process repeats. Commutation for therotor position, shown in Figure 1, would be at the com-pletion of current path 2 and the beginning of currentpath 3 for clockwise rotation. Commutating the electri-

cal connections through the six possible combinations,numbered 1 through 6, at precisely the right momentswill pull the rotor through one electrical revolution.In the simplified motor of Figure 1, one electrical revo-lution is the same as one mechanical revolution. Inactual practice, BLDC motors have more than one ofthe electrical circuits shown, wired in parallel to eachother, and a corresponding multi-pole permanent mag-netic rotor. For two circuits there are two electrical rev-olutions per mechanical revolution, so for a two circuitmotor, each electrical commutation phase would cover30 degrees of mechanical rotation.

Sensored CommutationThe easiest way to know the correct moment to com-mutate the winding currents is by means of a positionsensor. Many BLDC motor manufacturers supplymotors with a three-element Hall effect position sensor.Each sensor element outputs a digital high level for 180electrical degrees of electrical rotation, and a low levelfor the other 180 electrical degrees. The three sensorsare offset from each other by 60 electrical degrees sothat each sensor output is in alignment with one of theelectromagnetic circuits. A timing diagram showing therelationship between the sensor outputs and therequired motor drive voltages is shown in Figure 2.

FIGURE 2: SENSOR VERSUS DRIVE TIMING

A

+V

-V

Float

B

+V

-V

Float

C

+V

-V

Float

H

LH

LH

L

Sensor A

Sensor B

Sensor C

6 5 4 3 2 1 6......1

Code 101 001 011 010 110 100 101 001DS00857A-page 2 2002 Microchip Technology Inc.

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The numbers at the top of Figure 2 correspond to thecurrent phases shown in Figure 1. It is apparent fromFigure 2 that the three sensor outputs overlap in sucha way as to create six unique three-bit codes corre-sponding to each of the drive phases. The numbersshown around the peripheral of the motor diagram inFigure 1 represent the sensor position code. The northpole of the rotor points to the code that is output at thatrotor position. The numbers are the sensor logic levelswhere the Most Significant bit is sensor C and the LeastSignificant bit is sensor A. Each drive phase consists of one motor terminal drivenhigh, one motor terminal driven low, and one motor ter-minal left floating. A simplified drive circuit is shown inFigure 3. Individual drive controls for the high and lowdrivers permit high drive, low drive, and floating drive ateach motor terminal. One precaution that must betaken with this type of driver circuit is that both high sideand low side drivers must never be activated at thesame time. Pull-up and pull-down resistors must beplaced at the driver inputs to ensure that the drivers areoff immediately after a microcontoller RESET, when themicrocontroller outputs are configured as high imped-ance inputs.Another precaution against both drivers being active atthe same time is called dead time control. When an out-put transitions from the high drive state to the low drivestate, the proper amount of time for the high side driverto turn off must be allowed to elapse before the low sidedriver is activated. Drivers take more time to turn offthan to turn on, so extra time must be allowed to elapseso that both drivers are not conducting at the sametime. Notice in Figure 3 that the high drive period andlow drive period of each output, is separated by a float-ing drive phase period. This dead time is inherent to thethree phase BLDC drive scenario, so special timing fordead time control is not necessary. The BLDC commu-tation sequence will never switch the high-side deviceand the low-side device in a phase, at the same time.

At this point we are ready to start building the motorcommutation control code. Commutation consists oflinking the input sensor state with the correspondingdrive state. This is best accomplished with a state tableand a table offset pointer. The sensor inputs will formthe table offset pointer, and the list of possible outputdrive codes will form the state table. Code developmentwill be performed with a PIC16F877 in an ICD. I havearbitrarily assigned PORTC as the motor drive port andPORTE as the sensor input port. PORTC was chosenas the driver port because the ICD demo board alsohas LED indicators on that port so we can watch theslow speed commutation drive signals without anyexternal test equipment.Each driver requires two pins, one for high drive andone for low drive, so six pins of PORTC will be used tocontrol the six motor drive MOSFETS. Each sensorrequires one pin, so three pins of PORTE will be usedto read the current state of the motors three-outputsensor. The sensor state will be linked to the drive stateby using the sensor input code as a binary offset to thedrive table index. The sensor states and motor drivestates from Figure 2 are tabulated in Table 1.

FIGURE 3: THREE PHASE BRIDGE

To A

-VM

+VM

A Highcontrol

A Lowcontrol

To B

-VM

+VM

B Highcontrol

B Lowcontrol

To C

-VM

+VM

C Highcontrol

C Lowcontrol 2002 Microchip Technology Inc. DS00857A-page 3

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TABLE 1: CW SENSOR AND DRIVE BITS BY PHASE ORDER

Sorting Table 1 by sensor code binary weight results in Table 2. Activating the motor drivers, according to a state tablebuilt from Table 2, will cause the motor of Figure 1 to rotate clockwise.

TABLE 2: CW SENSOR AND DRIVE BITS BY SENSOR ORDER

Counter clockwise rotation is accomplished by driving current through the motor coils in the direction opposite of thatfor clockwise rotation. Table 3 was constructed by swapping all the high and low drives of Table 2. Activating the motorcoils, according to a state table built from Table 3, will cause the motor to rotate counter clockwise. Phase numbers inTable 3 are preceded by a slash denoting that the EMF is opposite that of the phases in Table 2.

TABLE 3: CCW SENSOR AND DRIVE BITS

The code segment for determining the appropriate drive word from the sensor inputs is shown in Figure 4.

Pin RE2 RE1 RE0 RC5 RC4 RC3 RC2 RC1 RC0

Phase Sensor CSensor

BSensor

AC High Drive

C Low Drive

B High Drive

B Low Drive

A High Drive

A Low Drive

1 1 0 1 0 0 0 1 1 02 1 0 0 1 0 0 1 0 03 1 1 0 1 0 0 0 0 14 0 1 0 0 0 1 0 0 15 0 1 1 0 1 1 0 0 06 0 0 1 0 1 0 0 1 0



BSensor

AC High Drive

C Low Drive

B High Drive

B Low Drive

A High Drive

A Low Drive

6 0 0 1 0 1 0 0 1 04 0 1 0 0 0 1 0 0 15 0 1 1 0 1 1 0 0 02 1 0 0 1 0 0 1 0 01 1 0 1 0 0 0 1 1 03 1 1 0 1 0 0 0 0 1



BSensor

AC High Drive

C Low Drive

B High Drive

B Low Drive

A High Drive

A Low Drive

/6 0 0 1 1 0 0 0 0 1/4 0 1 0 0 0 0 1 1 0/5 0 1 1 1 0 0 1 0 0/2 1 0 0 0 1 1 0 0 0/1 1 0 1 0 0 1 0 0 1/3 1 1 0 0 1 0 0 1 0DS00857A-page 4 2002 Microchip Technology Inc.

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FIGURE 4: COMMUTATION CODE SEGMENT#define DrivePort PORTC#define SensorMask B00000111#define SensorPort PORTE#define DirectionBit PORTA, 1

Commutatemovlw SensorMask ;retain only the sensor bitsandwf SensorPort ;get sensor dataxorwf LastSensor, w ;test if motion sensedbtfsc STATUS, Z ;zero if no changereturn ;no change - return

xorwf LastSensor, f ;replace last sensor data with currentbtfss DirectionBit ;test direction bitgoto FwdCom ;bit is zero - do forward commutation

;reverse commutationmovlw HIGH RevTable ;get MS byte to tablemovwf PCLATH ;prepare for computed GOTOmovlw LOW RevTable ;get LS byte of tablegoto Com2

FwdCom ;forward commutationmovlw HIGH FwdTable ;get MS byte of tablemovwf PCLATH ;prepare for computed GOTOmovlw LOW FwdTable ;get LS byte of table

Com2addwf LastSensor, w ;add sensor offsetbtfsc STATUS, C ;page change in table?incf PCLATH, f ;yes - adjust MS byte

call GetDrive ;get drive word from tablemovwf DriveWord ;save as current drive wordreturn

GetDrivemovwf PCL

FwdTableretlw B00000000 ;invalidretlw B00010010 ;phase 6retlw B00001001 ;phase 4retlw B00011000 ;phase 5retlw B00100100 ;phase 2retlw B00000110 ;phase 1retlw B00100001 ;phase 3retlw B00000000 ;invalid

RevTableretlw B00000000 ;invalidretlw B00100001 ;phase /6retlw B00000110 ;phase /4retlw B00100100 ;phase /5retlw B00011000 ;phase /2retlw B00001001 ;phase /1retlw B00010010 ;phase /3retlw B00000000 ;invalid 2002 Microchip Technology Inc. DS00857A-page 5

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Before we try the commutation code with our motor, letsconsider what happens when a voltage is applied to aDC motor. A greatly simplified electrical model of a DCmotor is shown in Figure 5.

FIGURE 5: DC MOTOR EQUIVALENT CIRCUIT

When the rotor is stationary, the only resistance to cur-rent flow is the impedance of the electromagnetic coils.The impedance is comprised of the parasitic resistanceof the copper in the windings, and the parasitic induc-tance of the windings themselves. The resistance andinductance are very small by design, so start-up cur-rents would be very large, if not limited. When the motor is spinning, the permanent magnetrotor moving past the stator coils induces an electricalpotential in the coils called Back Electromotive Force,or BEMF. BEMF is directly proportional to the motorspeed and is determined from the motor voltage con-stant KV.

EQUATION 1:

In an ideal motor, R and L are zero, and the motor willspin at a rate such that the BEMF exactly equals theapplied voltage. The current that a motor draws is directly proportionalto the torque load on the motor shaft. Motor current isdetermined from the motor torque constant KT.

EQUATION 2:

An interesting fact about KT and KV is that their productis the same for all motors. Volts and Amps areexpressed in MKS units, so if we also express KT inMKS units, that is N-M/Rad/Sec, then the product of KVand KT is 1.

EQUATION 3:

This is not surprising when you consider that the unitsof the product are [1/(V*A)]*[(N*M)/(Rad/Sec)], which isthe same as mechanical power divided by electricalpower.If voltage were to be applied to an ideal motor from anideal voltage source, it would draw an infinite amount ofcurrent and accelerate instantly to the speed dictatedby the applied voltage and KV. Of course no motor isideal, and the start-up current will be limited by the par-asitic resistance and inductance of the motor windings,as well as the current capacity of the power source.Two detrimental effects of unlimited start-up currentand voltage are excessive torque and excessive cur-rent. Excessive torque can cause gears to strip, shaftcouplings to slip, and other undesirable mechanicalproblems. Excessive current can cause driver MOS-FETS to blow out and circuitry to burn. We can minimize the effects of excessive current andtorque by limiting the applied voltage at start-up withpulse width modulation (PWM). Pulse width modulationis effective and fairly simple to do. Two things to con-sider with PWM are, the MOSFET losses due to switch-ing, and the effect that the PWM rate has on the motor.Higher PWM frequencies mean higher switchinglosses, but too low of a PWM frequency will mean thatthe current to the motor will be a series of high currentpulses instead of the desired average of the voltagewaveform. Averaging is easier to attain at lower fre-quencies if the parasitic motor inductance is relativelyhigh, but high inductance is an undesirable motor char-acteristic. The ideal frequency is dependent on thecharacteristics of your motor and power switches. Forthis application, the PWM frequency will be approxi-mately 10 kHz.

BEMF

Motor

R L

RPM = KV x VoltsBEMF = RPM / KV

Torque = KT x Amps

KV * KT = 1DS00857A-page 6 2002 Microchip Technology Inc.

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We are using PWM to control start-up current, so whynot use it as a speed control also? We will use the ana-log-to-digital converter (ADC), of the PIC16F877 toread a potentiometer and use the voltage reading asthe relative speed control input. Only 8 bits of the ADCare used, so our speed control will have 256 levels. Wewant the relative speed to correspond to the relativepotentiometer position. Motor speed is directly propor-tional to applied voltage, so varying the PWM dutycycle linearly from 0% to 100% will result in a linearspeed control from 0% to 100% of maximum RPM.Pulse width is determined by continuously adding theADC result to the free running Timer0 count to deter-mine when the drivers should be on or off. If the addi-tion results in an overflow, then the drivers are on,otherwise they are off. An 8-bit timer is used so that theADC to timer additions need no scaling to cover the fullrange. To obtain a PWM frequency of 10 kHz Timer0must be running at 256 times that rate, or 2.56 MHz.The minimum prescale value for Timer0 is 1:2, so weneed an input frequency of 5.12 MHz. The input toTimer0 is FOSC/4. This requires an FOSC of 20.48 MHz.That is an odd frequency, and 20 MHz is close enough,so we will use 20 MHz resulting in a PWM frequency of9.77 kHz.

There are several ways to modulate the motor drivers.We could switch the high and low side drivers together,or just the high or low driver while leaving the otherdriver on. Some high side MOSFET drivers use acapacitor charge pump to boost the gate drive abovethe drain voltage. The charge pump charges when thedriver is off and discharges into the MOSFET gatewhen the driver is on. It makes sense then to switch thehigh side driver to keep the charge pump refreshed.Even though this application does not use the chargepump type drivers, we will modulate the high side driverwhile leaving the low side driver on. There are threehigh side drivers, any one of which could be activedepending on the position of the rotor. The motor driveword is 6-bits wide, so if we logically AND the driveword with zeros in the high driver bit positions, and 1sin the low driver bit positions, we will turn off the activehigh driver regardless which one of the three it is.We have now identified 4 tasks of the control loop: Read the sensor inputs Commutate the motor drive connections Read the speed control ADC PWM the motor drivers using the ADC and Timer0

addition resultsAt 20 MHz clock rate, control latency, caused by theloop time, is not significant so we will construct a simplepolled task loop. The control loop flow chart is shown inFigure 6 and code listings are in Appendix B. 2002 Microchip Technology Inc. DS00857A-page 7

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FIGURE 6: SENSORED DRIVE FLOWCHART

Initialize

ADCReady

?Read new ADC

Set ADC GO

Add ADRESH to TMR0

Carry?

Mask DriveWord

Output DriveWord

Sensor Change

Save SensorCode

Commutate

Yes

No

No

Yes

No

YesDS00857A-page 8 2002 Microchip Technology Inc.

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Sensorless Motor ControlIt is possible to determine when to commutate themotor drive voltages by sensing the back EMF voltageon an undriven motor terminal during one of the drivephases. The obvious cost advantage of sensorlesscontrol is the elimination of the Hall position sensors.There are several disadvantages to sensorless control: The motor must be moving at a minimum rate to

generate sufficient back EMF to be sensed Abrupt changes to the motor load can cause the

BEMF drive loop to go out of lock The BEMF voltage can be measured only when

the motor speed is within a limited range of the ideal commutation rate for the applied voltage

Commutation at rates faster than the ideal rate will result in a discontinuous motor response

If low cost is a primary concern and low speed motoroperation is not a requirement and the motor load is notexpected to change rapidly then sensorless controlmay be the better choice for your application.

Determining the BEMFThe BEMF, relative to the coil common connectionpoint, generated by each of the motor coils, can beexpressed as shown in Equation 4 through Equation 6.

EQUATION 4:

EQUATION 5:

EQUATION 6:

FIGURE 7: BEMF EQUIVALENT CIRCUIT

Figure 7 shows the equivalent circuit of the motor withcoils B and C driven while coil A is undriven and avail-able for BEMF measurement. At the commutation fre-quency the L's are negligible. The R's are assumed tobe equal. The L and R components are not shown inthe A branch since no significant current flows in thispart of the circuit so those components can be ignored.

BBEMF = sin ( )

2pi

3

CBEMF = sin -

4pi3

ABEMF = sin -

BBEMF

CBEMF

ABEMF

V

R

L

R

L

COM A

B

C 2002 Microchip Technology Inc. DS00857A-page 9

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The BEMF generated by the B and C coils in tandem,as shown in Figure 7, can be expressed as shown inEquation 7.

EQUATION 7:

The sign reversal of CBEMF is due to moving the refer-ence point from the common connection to ground.Recall that there are six drive phases in one electricalrevolution. Each drive phase occurs +/- 30 degreesaround the peak back EMF of the two motor windingsbeing driven during that phase. At full speed theapplied DC voltage is equivalent to the RMS BEMFvoltage in that 60 degree range. In terms of the peakBEMF generated by any one winding, the RMS BEMFvoltage across two of the windings can be expressedas shown in Equation 8.

EQUATION 8:

We will use this result to normalize the BEMF diagramspresented later, but first lets consider the expectedBEMF at the undriven motor terminal. Since the applied voltage is pulse width modulated, thedrive alternates between on and off throughout thephase time. The BEMF, relative to ground, seen at theA terminal when the drive is on, can be expressed asshown in Equation 9.

EQUATION 9:

Notice that the winding resistance cancels out, soresistive voltage drop, due to motor torque load, is nota factor when measuring BEMF.The BEMF, relative to ground, seen at the A terminalwhen the drive is off can be expressed as shown inEquation 10.

EQUATION 10:

BEMFBC = BBEMF - CBEMF

BEMFRMS = sin () - sin - d 3pipi2

pi6

2

BEMFRMS = + 3pi

pi

2pi3

4

BEMFRMS = 1.6554

2pi3

BEMFA = [V - (BBEMF - CBEMF )]R C + ABEMFBEMF

BEMFA = V - BBEMF + CBEMF CBEMF + ABEMF

2R

2

-

-

BEMFA = ABEMF - CBEMFDS00857A-page 10 2002 Microchip Technology Inc.

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Figure 8 is a graphical representation of the BEMF for-mulas computed over one electrical revolution. Toavoid clutter, only the terminal A waveform, as wouldbe observed on a oscilloscope is displayed and isdenoted as BEMF(drive on). The terminal A waveformis flattened at the top and bottom because at thosepoints the terminal is connected to the drive voltage orground. The sinusoidal waveforms are the individualcoil BEMFs relative to the coil common connectionpoint. The 60 degree sinusoidal humps are the BEMFsof the driven coil pairs relative to ground. The entiregraph has been normalized to the RMS value of the coilpair BEMFs.

FIGURE 8: BEMF AT 100% DRIVE

Notice that the BEMF(drive on) waveform is fairly linearand passes through a voltage that is exactly half of theapplied voltage at precisely 60 degrees which coin-cides with the zero crossing of the coil A BEMF wave-form. This implies that we can determine the rotorelectrical position by detecting when the open terminalvoltage equals half the applied voltage.What happens when the PWM duty cycle is less than100%? Figure 9 is a graphical representation of theBEMF formulas computed over one electrical revolu-tion when the effective applied voltage is 50% of thatshown in Figure 8. The entire graph has been normal-ized to the peak applied voltage.

BLDC Motor Waveforms

-1

-0.5

0

0.5

1

1.5

-30 30 90 150 210 270 330Electrical Degrees

Vollt

s (N

orm

aliz

ed to

D

C Dr

ive)

BCAABS(B-C)ABS(C-A)ABS(A-B)BEMF(drive on)

(PWM at 100% Duty Cycle) 2002 Microchip Technology Inc. DS00857A-page 11

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FIGURE 9: BEMF AT 50% DRIVE

As expected the BEMF waveforms are all reduced pro-portionally but notice that the BEMF on the open termi-nal still equals half the applied voltage midway throughthe 60 degree drive phase. This occurs only when thedrive voltage is on. Figure 10 shows a detail of the openterminal BEMF when the drive voltage is on and whenthe drive voltage is off. At various duty cycles, noticethat the drive on curve always equals half the appliedvoltage at 60 degrees.

BLDC Motor Waveforms

-1

-0.5

0

0.5

1

1.5

-30 30 90 150 210 270 330Electrical Degrees

Vollt

s (N

orm

aliz

ed to

D

C Dr

ive)

BCAABS(B-C)ABS(C-A)ABS(A-B)BEMF(drive on)

(PWM at 50% Duty Cycle)DS00857A-page 12 2002 Microchip Technology Inc.

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FIGURE 10: DRIVE ON VS. DRIVE OFF BEMF

How well do the predictions match an actual motor?Figure 11 is shows the waveforms present on terminalA of a Pittman N2311A011 brushless motor at variousPWM duty cycle configurations. The large transients,especially prevalent in the 100% duty cycle waveform,are due to flyback currents caused by the motor wind-ing inductance.

Floating Terminal Back EMF

0

0.5

1

30 90Electrical Degrees

Volta

ge (N

orm

aliz

ed to

D

C D

rive)

BEMF(drive on)BEMF(drive off)

(PWM at 100% Duty Cycle)Floating Terminal Back EMF

0

0.5

1


Volta

ge (N

orm

aliz

ed to

D

C D

rive)


(PWM at 60% Duty Cycle)

Floating Terminal Back EMF

0

0.5

1


Volta

ge (N

orm

aliz

ed to

D

C D

rive)


(PWM at 75% Duty Cycle)Floating Terminal Back EMF

0

0.5

1


Volta

ge (N

orm

aliz

ed to

D

C D

rive)


(PWM at 10% Duty Cycle) 2002 Microchip Technology Inc. DS00857A-page 13

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FIGURE 11: PITTMAN BEMF WAVEFORMS

The rotor position can be determined by measuring thevoltage on the open terminal when the drive voltage isapplied and then comparing the result to one half of theapplied voltage.Recall that motor speed is proportional to the appliedvoltage. The formulas and graphs presented so far rep-resent motor operation when commutation rate coin-cides with the effective applied voltage. When thecommutation rate is too fast then commutation occursearly and the zero crossing point occurs later in thedrive phase. When the commutation rate is too slowthen commutation occurs late and the zero crossingpoint occurs earlier in the drive phase. We can senseand use this shift in zero crossing to adjust the commu-tation rate to keep the motor running at the ideal speedfor the applied voltage and load torque.

100% Duty Cycle 50% Duty Cycle

10% Duty Cycle75% Duty CycleDS00857A-page 14 2002 Microchip Technology Inc.

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Open Loop Speed ControlAn interesting property of brushless DC motors is thatthey will operate synchronously to a certain extent. Thismeans that for a given load, applied voltage, and com-mutation rate the motor will maintain open loop lockwith the commutation rate provided that these threevariables do not deviate from the ideal by a significantamount. The ideal is determined by the motor voltageand torque constants. How does this work? Considerthat when the commutation rate is too slow for anapplied voltage, the BEMF will be too low resulting inmore motor current. The motor will react by accelerat-ing to the next phase position then slow down waitingfor the next commutation. In the extreme case themotor will snap to each position like a stepper motoruntil the next commutation occurs. Since the motor isable to accelerate faster than the commutation rate,rates much slower than the ideal can be tolerated with-out losing lock but at the expense of excessive current. Now consider what happens when commutation is toofast. When commutation occurs early the BEMF hasnot reached peak resulting in more motor current and agreater rate of acceleration to the next phase but it willarrive there too late. The motor tries to keep up with thecommutation but at the expense of excessive current.If the commutation arrives so early that the motor cannot accelerate fast enough to catch the next commuta-tion, lock is lost and the motor spins down. This hap-pens abruptly not very far from the ideal rate. Theabrupt loss of lock looks like a discontinuity in the motorresponse which makes closed loop control difficult. Analternative to closed loop control is to adjust the com-mutation rate until self locking open loop control isachieved. This is the method we will use in our applica-tion.When the load on a motor is constant over its operatingrange then the response curve of motor speed relativeto applied voltage is linear. If the supply voltage is wellregulated, in addition to a constant torque load, thenthe motor can be operated open loop over its entirespeed range. Consider that with pulse width modula-tion the effective voltage is linearly proportional to thePWM duty cycle. An open loop controller can be madeby linking the PWM duty cycle to a table of motor speedvalues stored as the time of commutation for each drivephase. We need a table because revolutions per unittime is linear, but we need time per revolution which isnot linear. Looking up the time values in a table is muchfaster than computing them repeatedly.

The program that we use to run the motor open loop isthe same program we will use to automatically adjustthe commutation rate in response to variations in thetorque load. The program uses two potentiometers asspeed control inputs. One potentiometer, well call it thePWM potentiometer, is directly linked to both the PWMduty cycle and the commutation time lookup table. Thesecond potentiometer, well call this the Offset potenti-ometer, is used to provide an offset to the PWM dutycycle determined by the PWM potentiometer. An ana-log-to-digital conversion of the PWM potentiometerproduces a number between 0 and 255. The PWM dutycycle is generated by adding the PWM potentiometerreading to a free running 8-bit timer. When the additionresults in a carry the drive state is on, otherwise it is off.The PWM potentiometer reading is also used to accessthe 256 location commutation time lookup table. TheOffset potentiometer also produces a number between0 and 255. The Most Significant bit of this number isinverted making it a signed number between -128 and127. This offset result, when added to the PWM poten-tiometer, becomes the PWM duty cycle threshold, andcontrols the drive on and off states described previ-ously.

Closed Loop Speed ControlClosed loop speed control is achieved by unlinking thecommutation time table index from the PWM duty cyclenumber. The PWM potentiometer is added to a fixedmanual threshold number between 0 and 255. Whenthis addition results in a carry, the mode is switched toautomatic. On entering Automatic mode the commuta-tion index is initially set to the PWM potentiometerreading. Thereafter, as long as Automatic mode is stillin effect, the commutation table index is automaticallyadjusted up or down according to voltages read atmotor terminal A at specific times. Three voltage read-ings are taken.

FIGURE 12: BEMF SAMPLE TIMES 2002 Microchip Technology Inc. DS00857A-page 15

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The first reading is taken during drive phase 4 when ter-minal A is actively driven high. This is the applied volt-age. The next two readings are taken during drivephase 5 when terminal A is floating. The first reading istaken when of the commutation time has elapsedand the second reading is taken when of the commu-tation time has elapsed. We'll call these readings 1 and2 respectively. The commutation table index is adjustedaccording to the following relationship between theapplied voltage reading and readings 1 and 2: Index is unchanged if Reading 1 > Applied Volt-

age/2 and Reading 2 < Applied Voltage/2 Index is increased if Reading 1 < Applied Voltage/

2 Index is decreased if Reading 1 > Applied Volt-

age/2 and Reading 2 > Applied Voltage/2The motor rotor and everything it is connected to has acertain amount of inertia. The inertia delays the motorresponse to changes in voltage load and commutationtime. Updates to the commutation time table index aredelayed to compensate for the mechanical delay andallow the motor to catch up.

Acceleration and Deceleration DelayThe inertia of the motor and what it is driving, tends todelay motor response to changes in the drive voltage.We need to compensate for this delay by adding amatching delay to the control loop. The control loopdelay requires two time constants, a relatively slow onefor acceleration, and a relatively fast one for decelera-tion.Consider what happens in the control loop when thevoltage to the motor suddenly rises, or the motor loadis suddenly reduced. The control senses that the motorrotation is too slow and attempts to adjust by makingthe commutation time shorter. Without delay in the con-trol loop, the next speed measurement will be takenbefore the motor has reacted to the adjustment, and

another speed adjustment will be made. Adjustmentscontinue to be made ahead of the motor response untileventually, the commutation time is too short for theapplied voltage, and the motor goes out of lock. Theacceleration timer delay prevents this runaway condi-tion. Since the motor can tolerate commutation timesthat are too long, but not commutation times that aretoo short, the acceleration time delay can be longerthan required without serious detrimental effect.Consider what happens in the control loop when thevoltage to the motor suddenly falls, or the motor load issuddenly increased. If the change is sufficiently large,commutation time will immediately be running too shortfor the motor conditions. The motor cannot tolerate this,and loss of lock will occur. To prevent loss of lock, theloop deceleration timer delay must be short enough forthe control loop to track, or precede the changing motorcondition. If the time delay is too short, then the controlloop will continue to lengthen the commutation timeahead of the motor response resulting in over compen-sation. The motor will eventually slow to a speed thatwill indicate to the BEMF sensor that the speed is tooslow for the applied voltage. At that point, commutationdeceleration will cease, and the commutation changewill adjust in the opposite direction governed by theacceleration time delay. Over compensation duringdeceleration will not result in loss of lock, but will causeincreased levels of torque ripple and motor current untilthe ideal commutation time is eventually reached.

Determining The Commutation Time Table ValuesThe assembler supplied with MPLAB performs all cal-culations as 32-bit integers. To avoid the roundingerrors that would be caused by integer math, we willuse a spreadsheet, such as Excel, to compute the tableentries then cut and paste the results to an include file.The spreadsheet is setup as shown in Table 4.

TABLE 4: COMMUTATION TIME TABLE VALUESVariable Name Number or Formula Description

Phases 12 Number of commutation phase changes in one mechanical revolution.

FOSC 20 MHz Microcontroller clock frequencyFOSC_4 FOSC/4 Microcontroller timers source clockPrescale 4 Timer 1 prescaleMaxRPM 8000 Maximum expected speed of the motor at full

applied voltageMinRPM (60*FOSC_4)/Phases*Prescale*65535)+1 Limitation of 16-bit timer

Offset -345 This is the zero voltage intercept on the RPM axis. A property normalized to the 8-bit A to D converter.

Slope (MaxRPM-Offset)/255 Slope of the RPM to voltage input response curve normalized to the 8-bit A to D converter.DS00857A-page 16 2002 Microchip Technology Inc.

AN857

The body of the spreadsheet starts arbitrarily at row 13.Row 12 contains the column headings. The body of thespreadsheet is constructed as follows: Column A is the commutation table index number

N. The numbers in column A are integers from 0 to 255.

Column B is the RPM that will result by using the counter values at index number N. The formula in column B is: =IF(Offset+A13*Slope>MinRPM,Off-set+A13*Slope,MinRPM).

Column C is the duration of each commutation phase expressed in seconds. The formula for col-umn C is: =60/(Phases*B13).

Column D is the duration of each commutation phase expressed in timer counts. The formula for column D is: =C13*FOSC_4/Prescale.

The range of commutation phase times at a reasonableresolution requires a 16-bit timer. The timer counts from0 to a compare value then automatically resets to 0.The compare values are stored in the commutationtime table. Since the comparison is 16 bits and tablescan only handle 8 bits the commutation times will bestored in two tables accessed by the same index. Column E is the most significant byte of the 16-bit

timer compare value. The formula for column E is: =CONCATENATE("retlw high D',INT(D13),').

Column F is the least significant byte of the 16-bit timer compare value. The formula for column F is: =CONCATENATE(retlw low D',INT(D13),').

When all spreadsheet formulas have been entered inrow 13, the formulas can be dragged down to row 268to expand the table to the required 256 entries. Col-umns E and F will have the table entries in assemblerready format. An example of the table spreadsheet isshown in Figure 13.

FIGURE 13: PWM LOOKUP TABLE GENERATOR 2002 Microchip Technology Inc. DS00857A-page 17

AN857

Using Open Loop Control to Determine Motor CharacteristicsYou can measure the motor characteristics by operat-ing the motor in Open Loop mode, and measuring themotor current at several applied voltages. You can thenchart the response curve in a spreadsheet, such asExcel, to determine the slope and offset numbers.Finally, plug the maximum RPM and offset numbersback into the table generator spreadsheet to regener-ate the RPM tables.To operate the motor in Open Loop mode: Set the manual threshold number (ManThresh)

to 0xFF. This will prevent the Auto mode from tak-ing over.

When operating the motor in Open Loop mode, start by adjusting the offset control until the motor starts to move. You may also need to adjust the PWM control slightly above minimum.

After the motor starts, you can increase the PWM control to increase the motor speed. The RPM and voltage will track, but you will need to adjust the offset frequently to optimize the voltage for the selected RPM.

Optimize the voltage by adjusting the offset for minimum current.

To obtain the response offset with Excel, enter thevoltage (left column), and RPM (right column) pairs inadjacent columns of the spreadsheet. Use the chartwizard to make an X-Y scatter chart. When the chart isfinished, right click on the response curve and selectthe pop-up menu add trendline. . . option. Choose thelinear regression type and, in the Options tab, checkthe display equation on chart option. An example ofthe spreadsheet is shown in Figure 14.

FIGURE 14: MOTOR RESPONSE SCOPE DETERMINATIONDS00857A-page 18 2002 Microchip Technology Inc.

AN857

Constructing The Sensorless Control CodeAt this point we have all the pieces required to controla sensorless motor. We can measure BEMF and theapplied voltage then compare them to each other todetermine rotor position. We can vary the effectiveapplied voltage with PWM and control the speed of themotor by timing the commutation phases. Some mea-surement events must be precisely timed. Other mea-surement events need not to interfere with each other.The ADC must be switched from one source to anotherand allow for sufficient acquisition time. Some eventsmust happen rapidly with minimum latency. Theseinclude PWM and commutation. We can accomplish everything with a short main loopthat calls a state table. The main loop will handle PWMand commutation and the state table will schedulereading the two potentiometers, the peak applied volt-age and the BEMF voltages at two times when theattached motor terminal is floating. Figure A-1 throughFigure A-10, in Appendix A, is the resulting flow chartof sensorless motor control. Code listings are inAppendix C and Appendix D. 2002 Microchip Technology Inc. DS00857A-page 19

AN857

APPENDIX A: SENSORLESS CONTROL FLOWCHART

FIGURE A-1: MAIN LOOP

Sensorless Control

Initialize

Is Timer1Compare Flag

Set?

Call Commutate

Is Full OnFlag Set?

Add PWM Threshold to

Timer0

Carry?

Set Drive-OnFlag

Yes

No

Yes

Yes No

Clear Drive-OnFlag

Call DriveMotor

Call LockTest

Call StateMachine

NoDS00857A-page 20 2002 Microchip Technology Inc.

AN857

FIGURE A-2: MOTOR COMMUTATION

Commutate

Is Timer1Clear on Compare

Enabled?

DecrementPhaseIndex

IsPhaseIndex

=0?

PhaseIndex = 6

Drive Word =Table Entry@PhaseIndex

DriveMotor

Commutate End

Yes

No

Yes

No 2002 Microchip Technology Inc. DS00857A-page 21

AN857

FIGURE A-3: MOTOR DRIVER CONTROL

FIGURE A-4: PHASE DRIVE PERIOD

DriveMotor

Get StoredDriveWord

IsDriveOnFlag

Set?

AND DriveWordwith OffMask

OR DriveWordwith SpeedStatus

Output DriveWordto motor drive port

DriveMotor End

No

Yes

SetTimer

High byte of Timer1 compare=High byte Table@RPMIndex

Low byte of Timer1 compare=Low byte Table@RPMIndex

SetTimer EndDS00857A-page 22 2002 Microchip Technology Inc.

AN857

FIGURE A-5: MOTOR SPEED LOCKED WITH COMMUTATION RATE

LockTest

Is PWMcycle startflag set?

Which halfof PWM cycle

is longest?

Is DriveActive?

Clear PWMcycle start flag

DecrementRampTimer

IsRampTimer

Zero?

IsADCRPM > Manual

Threshold?

Reset AutoRPMFlag

Set AutoRPMFlag

LT2LT3

No

On Cycle

No Yes Off Cycle

No

Yes

No Yes

Yes 2002 Microchip Technology Inc. DS00857A-page 23

AN857

FIGURE A-6: MOTOR SPEED LOCKED WITH COMMUTATION RATE (CONT.)

IsBEMF1 ADCsRPMRead equ sRPMSetup+1 ; Wait for ADC nDONE, Read ADC->RPMsOffsetSetup equ sRPMRead+1 ; Wait for Phase2, Set ADC GO, RA3->ADCsOffsetRead equ sOffsetSetup+1 ; Wait for ADC nDONE, Read ADC->ADCOffsetsVSetup equ sOffsetRead+1 ; Wait for Phase4, Drive On, wait 9 uSec, Set ADC GOsVIdle equ sVSetup+1 ; Wait for Drive On, wait Tacq, set ADC GOsVRead equ sVIdle+1 ; Wait for ADC nDONE, Read ADC->VsupplysBEMFSetup equ sVRead+1 ; Wait for Phase5, set Timer1 compare to half phase timesBEMFIdle equ sBEMFSetup+1 ; Wait for Timer1 compare, Force Drive on and wait 9 uSec,

; Set ADC GO, RA0->ADCsBEMFRead equ sBEMFIdle+1 ; Wait for ADC nDONE, Read ADC->VbemfsBEMF2Idle equ sBEMFRead+1 ; Wait for Timer1 compare, Force Drive on and wait 9 uSec,

; Set ADC GO, RA0->ADCsBEMF2Read equ sBEMF2Idle+1 ; Wait for ADC nDONE, Read ADC->Vbemf

;*********************************************************************************

;*

;* The ADC input is changed depending on the STATE;* Each STATE assumes a previous input selection and changes the selection;* by XORing the control register with the appropriate ADC input change mask;* defined here:;*

ADC0to1 equ B00001000 ; changes ADCON0 from 000 to 001ADC1to3 equ B00010000 ; changes ADCON0 from 001 to 011ADC3to0 equ B00011000 ; changes ADCON0 from 011 to 000

;*********************************************************************************

;**************************** PROGRAM STARTS HERE ********************************;*********************************************************************************

org 0x000nopgoto Initialize

org 0x004bsf Tmr0Ovf ; Timer0 overflow flag used by accel/decel timerbsf Tmr0Sync ; Timer0 overflow flag used to synchronize code executionbcf INTCON,T0IFretfie ;

Initializeclrf PORTC ; all drivers offclrf PORTB 2002 Microchip Technology Inc. DS00857A-page 37

AN857banksel TRISA; setup I/O

clrf TRISC ; motor drivers on PORTCmovlw B00001011 ; A/D on RA0 (PWM), RA1 (Speed) and RA3 (BEMF)movwf TRISA ; movlw B11111110 ; RB0 is locked indicatormovwf TRISB

; setup Timer0movlw B11010000 ; Timer0: Fosc, 1:2movwf OPTION_REGbsf INTCON,T0IE ; enable Timer0 interrupts

; Setup ADCmovlw B00000100 ; ADC left justified, AN0, AN1movwf ADCON1

banksel PORTAmovlw B10000001 ; ADC clk = Fosc/32, AN0, ADC onmovwf ADCON0

; setup Timer 1movlw B00100001 ; 1:4 prescale, internal clock, timer onmovwf T1CON

; setup Timer 1 comparemovlw 0xFF ; set compare to maximum countmovwf CCPR1L ; LS compare registermovwf CCPR1H ; MS compare registermovlw B00001011 ; Timer 1 compare mode, special event - clears timer1movwf CCP1CON

; initialize RAM

clrf PWMThreshmovlw D6movwf PhaseIndxclrf Flagsclrf Status ;clrf STATE ; LoopIdle->STATEbcf INTCON,T0IF ; ensure Timer0 overflow flag is clearedbsf INTCON,GIE ; enable interrupts

MainLoop;*****************************************************************

;

; PWM, Commutation, State machine loop;

;*****************************************************************

btfsc PIR1,CCP1IF ; time for phase change?call Commutate ; yes - change motor drive

PWMbsf DriveOnFlag ; pre-set flagbtfsc FullOnFlag ; is PWM level at maximum?goto PWM02 ; yes - only commutation is necessary

movf PWMThresh,w ; get PWM thresholdaddwf TMR0,w ; compare to Timer0btfss CARRY ; drive is on if carry is setbcf DriveOnFlag ; timer has not reached threshold, disable drive

call DriveMotor ; output drive wordPWM02

call LockTestcall StateMachine ; service state machinegoto MainLoop ; repeat loopDS00857A-page 38 2002 Microchip Technology Inc.

AN857

StateMachine

movlw SMTableEnd-SMTable-1 ; STATE table must have 2^n entriesandwf STATE,f ; limit STATE index to state tablemovlw high SMTable ; get high byte of table addressmovwf PCLATH ; prepare for computed gotomovlw low SMTable ; get low byte of table addressaddwf STATE,w ; add STATE index to table rootbtfsc CARRY ; test for page change in tableincf PCLATH,f ; page change adjustmovwf PCL ; jump into table

SMTable ; number of STATE table entries MUST be evenly divisible by 2goto RPMSetup ; Wait for Phase1, Set ADC GO, RA1->ADC, clear Timer0 overflowgoto RPMRead ; Wait for ADC nDONE, Read ADC->RPMgoto OffsetSetup ; Wait for Phase2, Set ADC GO, RA3->ADCgoto OffsetRead ; Wait for ADC nDONE, Read ADC->ADCOffsetgoto VSetup ; Wait for Phase4goto VIdle ; Wait for Drive On, wait Tacq, set ADC GOgoto VRead ; Wait for ADC nDONE, Read ADC->Vsupplygoto BEMFSetup ; Wait for Phase5, set Timer1 compare to half phase timegoto BEMFIdle ; When Timer1 compares force Drive on, Set ADC GO after Tacq,

RA0->ADCgoto BEMFRead ; Wait for ADC nDONE, Read ADC->Vbemfgoto BEMF2Idle ; When Timer1 compares force Drive on, Set ADC GO after Tacq,

RA0->ADCgoto BEMF2Read ; Wait for ADC nDONE, Read ADC->Vbemf

; fill out table with InvalidStates to make number of table entries evenly divisible by 2

goto InvalidState ; invalid state - reset state machinegoto InvalidState ; invalid state - reset state machinegoto InvalidState ; invalid state - reset state machinegoto InvalidState ; invalid state - reset state machine

SMTableEnd

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

RPMSetup ; Wait for Phase1, Set ADC GO, RA1->ADC, clear Timer0 overflow

movlw Phase1 ; compare Phase1 word...xorwf Drive,w ; ...with current drive wordbtfss ZERO ; ZERO if equalreturn ; not Phase1 - remain in current STATE

bsf ADCON0,GO ; start ADCmovlw ADC0to1 ; prepare to change ADC inputxorwf ADCON0,f ; change from AN0 to AN1incf STATE,f ; next STATEbcf Tmr0Sync ; clear Timer0 overflowreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

RPMRead ; Wait for ADC nDONE, Read ADC->RPM

btfsc ADCON0,GO ; is ADC conversion finished?return ; no - remain in current STATE

movf ADRESH,w ; get ADC resultmovwf ADCRPM ; save in RPM

incf STATE,f ; next STATEreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2002 Microchip Technology Inc. DS00857A-page 39

AN857

OffsetSetup ; Wait for Phase2, Set ADC GO, RA3->ADC

movlw Phase2 ; compare Phase2 word...xorwf Drive,w ; ...with current drive wordbtfss ZERO ; ZERO if equalreturn ; not Phase2 - remain in current STATE

bsf ADCON0,GO ; start ADCmovlw ADC1to3 ; prepare to change ADC inputxorwf ADCON0,f ; change from AN1 to AN3incf STATE,f ; next STATEreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

OffsetRead ; Wait for ADC nDONE, Read ADC->ADCOffset


movf ADRESH,w ; get ADC resultxorlw H80 ; complement MSB for +/- offsetmovwf ADCOffset ; save in offsetaddwf ADCRPM,w ; add offset to PWM resultbtfss ADCOffset,7 ; is offset a negative number?goto OverflowTest ; no - test for overflow

btfss CARRY ; underflow?andlw H00 ; yes - force minimumgoto Threshold ;

OverflowTestbtfsc CARRY ; overflow?movlw Hff ; yes - force maximum

Thresholdmovwf PWMThresh ; PWM threshold is RPM result plus offsetbtfsc ZERO ; is drive off?goto DriveOff ; yes - skip voltage measurements

bcf FullOnFlag ; pre-clear flag in preparation of comparesublw 0xFD ; full on thresholdbtfss CARRY ; CY = 0 if PWMThresh > FullOnbsf FullOnFlag ; set full on flagincf STATE,f ; next STATEreturn ; back to Main Loop

DriveOffclrf Status ; clear speed indicatorsmovlw B11000111 ; reset ADC input to AN0andwf ADCON0,f ;clrf STATE ; reset state machinereturn

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

VSetup ; Wait for Phase4

movlw Phase4 ; compare Phase4 word...xorwf Drive,w ; ...with current Phase drive wordbtfss ZERO ; ZERO if equalreturn ; not Phase4 - remain in current STATE

call SetTimer ; set timer value from RPM tableincf STATE,f ; next STATEreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~DS00857A-page 40 2002 Microchip Technology Inc.

AN857

VIdle ; Wait for Drive On, wait Tacq, set ADC GO

btfss DriveOnFlag ; is Drive active?return ; no - remain in current STATE

call Tacq ; motor Drive is active - wait ADC Tacq timebsf ADCON0,GO ; start ADCincf STATE,f ; next STATEreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

VRead ; Wait for ADC nDONE, Read ADC->Vsupply


movf ADRESH,w ; get ADC resultmovwf Vsupply ; save as supply voltageincf STATE,f ; next STATEbcf Tmr0Sync ; clear Timer0 overflowreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

BEMFSetup ; Wait for Phase5, set Timer1 compare to half phase time

movlw Phase5 ; compare Phase5 word...xorwf Drive,w ; ...with current drive wordbtfss ZERO ; ZERO if equal

return ; not Phase5 - remain in current STATE

btfss Tmr0Sync ; synchronize with Timer0return ;

btfss PWMThresh,7 ; if PWMThresh > 0x80 then ON is longer than OFFgoto BEMFS1 ; OFF is longer and motor is currently off - compute now

btfss DriveOnFlag ; ON is longer - wait for drive cycle to startreturn ; not started - wait

BEMFS1bcf CCP1CON,0 ; disable special event on comparemovf CCPR1H,w ; save current capture compare statemovwf CCPSaveH ; movwf CCPT2H ; save copy in workspacemovf CCPR1L,w ; low bytemovwf CCPSaveL ; savemovwf CCPT2L ; and save copybcf CARRY ; pre-clear carry for rotaterrf CCPT2H,f ; divide phase time by 2rrf CCPT2L,f ; bcf CARRY ; pre-clear carryrrf CCPT2H,w ; divide phase time by another 2movwf CCPR1H ; first BEMF reading at phase T/4rrf CCPT2L,w ; movwf CCPR1L ;

incf STATE,f ; next STATEreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 2002 Microchip Technology Inc. DS00857A-page 41

AN857

BEMFIdle ; When Timer1 compares force Drive on, Set ADC GO after Tacq, RA0->ADC

btfss PIR1,CCP1IF ; timer compare?return ; no - remain in current STATE

bsf DriveOnFlag ; force drive on for BEMF readingcall DriveMotor ; activate motor drivebsf ReadIndicator ; Diagnosticcall Tacq ; wait ADC acquisition timebsf ADCON0,GO ; start ADCbcf ReadIndicator ; Diagnostic

; setup to capture BEMF at phase 3/4 T

movf CCPT2H,waddwf CCPR1H,f ; next compare at phase 3/4 Tmovf CCPT2L,w ; addwf CCPR1L,f ; set T/2 lsbbtfsc CARRY ; test for carry into MSbincf CCPR1H,f ; perform carrybcf PIR1,CCP1IF ; clear timer compare interrupt flagincf STATE,f ; next STATEreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

BEMFRead ; Wait for ADC nDONE, Read ADC->Vbemf


rrf Vsupply,w ; divide supply voltage by 2subwf ADRESH,w ; Vbemf - Vsupply/2

movwf DeltaV1 ; save error voltageincf STATE,f ; next STATEreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

BEMF2Idle ; When Timer1 compares force Drive on, Set ADC GO after Tacq, RA0->ADC

btfss PIR1,CCP1IF ; timer compare?return ; no - remain in current STATE

bsf DriveOnFlag ; force drive on for BEMF readingcall DriveMotor ; activate motor drivebsf ReadIndicator ; Diagnosticcall Tacq ; wait ADC acquisition timebsf ADCON0,GO ; start ADCbcf ReadIndicator ; Diagnosticmovlw ADC3to0 ; prepare to change ADC inputxorwf ADCON0,f ; change from AN3 to AN0

; restore Timer1 phase time and special event compare mode

movf CCPSaveH,wmovwf CCPR1H ; next compare at phase Tmovf CCPSaveL,w ; movwf CCPR1L ; set T lsbbcf PIR1,CCP1IF ; clear timer compare interrupt flagbsf CCP1CON,0 ; enable special event on compareincf STATE,f ; next STATEreturn ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~DS00857A-page 42 2002 Microchip Technology Inc.

AN857

BEMF2Read ; Wait for ADC nDONE, Read ADC->Vbemf


rrf Vsupply,w ; divide supply voltage by 2subwf ADRESH,w ; Vbemf - Vsupply/2

movwf DeltaV2 ; save error voltage

clrf STATE ; reset state machine to beginning return ; back to Main Loop

;~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

InvalidState ; trap for invalid STATE indexmovlw B11000111 ; reset ADC input to AN0andwf ADCON0,f ;clrf STATEreturn

;________________________________________________________________________________________________

Tacq;*****************************************************************

;

; Software delay for ADC acquisition time; Delay time = Tosc*(3+3*xCount);

;*****************************************************************

movlw D14 ; 14 equates to approx 9 uSec delaymovwf xCount ;decfsz xCount,f ;goto $-1 ; loop here until time completereturn

LockTest;*****************************************************************

;

; T is the commutation phase period. Back EMF is measured on the; floating motor terminal at two times during T to determine; the approximate zero crossing of the BEMF. BEMF low means that; the measured BEMF is below (supply voltage)/2.; If BEMF is low at 1/4 T then accelerate.; If BEMF is high at 1/4 T and low at 3/4 T then speed is OK.; If BEMF is high at 1/4 T and 3/4 T then decelerate.;

; Lock test computation is synchronized to the PWM clock such; that the computation is performed during the PWM ON or OFF; time whichever is longer.;

;*****************************************************************

; synchronize test with start of Timer0

btfss Tmr0Ovf ; has Timer0 wrapped around?return ; no - skip lock test

btfss PWMThresh,7 ; if PWMThresh > 0x80 then ON is longer than OFFgoto LT05 ; OFF is longer and motor is currently off - compute now

btfss DriveOnFlag ; ON is longer - wait for drive cycle to startreturn ; not started - wait 2002 Microchip Technology Inc. DS00857A-page 43

AN857

LT05

bcf Tmr0Ovf ; clear synchronization flagdecfsz RampTimer,f ; RampTimer controls the acceleration/deceleration ratereturn

; use lock results to control RPM only if not manual mode

bsf AutoRPM ; preset flagmovf ADCRPM,w ; compare RPM potentiometer...addlw AutoThresh ; ...to the auto control thresholdbtfss CARRY ; CARRY is set if RPM is > auto thresholdbcf AutoRPM ; not in auto range - reset flag

btfss BEMF1Low ; is first BEMF below Supply/2goto LT20 ; no - test second BEMF

LT10; accelerate if BEMF at 1/4 T is below Supply/2

movlw B10000000 ; indicate lock test resultsmovwf Status ; status is ORd with drive word latermovlw AccelDelay ; set the timer for acceleration delaymovwf RampTimer ;

btfss AutoRPM ; is RPM in auto range?goto ManControl ; no - skip RPM adjustment

incfsz RPMIndex,f ; increment the RPM table indexreturn ; return if Index didnt wrap around

decf RPMIndex,f ; top limit is 0xFFreturn

LT20btfsc BEMF2Low ; BEMF1 was high...goto ShowLocked ; ... and BEMF2 is low - show locked

; decelerate if BEMF at 3/4 T is above Supply/2

movlw B01000000 ; indicate lock test resultsmovwf Status ; status is ORd with drive word latermovlw DecelDelay ; set the timer for deceleration delaymovwf RampTimer ;

btfss AutoRPM ; is RPM in auto range?goto ManControl ; no - skip RPM adjustment

decfsz RPMIndex,f ; set next lower RPM table indexreturn ; return if index didnt wrap around

incf RPMIndex,f ; bottom limit is 0x01return

ShowLockedmovlw B11000000 ; indicate lock test results movwf Status ; status is ORd with drive word latermovlw DecelDelay ; set the timer for deceleration delaymovwf RampTimer ;

btfsc AutoRPM ; was RPM set automatically?return ; yes - were doneDS00857A-page 44 2002 Microchip Technology Inc.

AN857

ManControl

movf ADCRPM,w ; get RPM potentiometer reading...movwf RPMIndex ; ...and set table index directlyreturn

Commutate;*****************************************************************

;

; Commutation is triggered by PIR1 flag.; This flag is set when timer1 equals the compare register.; When BEMF measurement is active the compare time is not; cleared automatically (special event trigger is off).; Ignore the PIR1 flag when special trigger is off; because the flag is for BEMF measurement.; If BEMF measurement is not active then decrement phase table; index and get the drive word from the table. Save the; drive word in a global variable and output to motor drivers.;

;*****************************************************************

btfss CCP1CON,0 ; is special event on compare enabled?return ; no - this is a BEMF measurement, let state machine handle this

bcf PIR1,CCP1IF ; clear interrupt flag

movlw high OnTable ; set upper program counter bitsmovwf PCLATHdecfsz PhaseIndx,w ; decrement to next phasegoto $+2 ; skip reset if not zeromovlw D6 ; phase counts 6 to 1movwf PhaseIndx ; save the phase indexaddlw LOW OnTablebtfsc CARRY ; test for possible page boundaryincf PCLATH,f ; page boundary adjustcall GetDrivemovwf Drive ; save motor drive word

DriveMotormovf Drive,w ; restore motor drive wordbtfss DriveOnFlag ; test drive enable flagandlw OffMask ; kill high drive if PWM is offiorwf Status,w ; show speed indicatorsmovwf DrivePort ; output to motor driversreturn

GetDrivemovwf PCL ; computed goto

OnTableretlw Invalidretlw Phase6retlw Phase5retlw Phase4retlw Phase3retlw Phase2retlw Phase1retlw Invalid

SetTimer 2002 Microchip Technology Inc. DS00857A-page 45

AN857

;*****************************************************************

;

; This sets the CCP module compare registers for timer 1. ; The motor phase period is the time it takes timer 1; to count from 0 to the compare value. The CCP module; is configured to clear timer 1 when the compare occurs.; Get the timer1 compare variable from two lookup tables, one; for the compare high byte and the other for the low byte.;

;*****************************************************************

call SetTimerHighmovwf CCPR1H ; Timer1 High byte presetcall SetTimerLowmovwf CCPR1L ; Timer1 Low byte presetreturn

SetTimerHighmovlw high T1HighTable ; lookup preset valuesmovwf PCLATH ; high bytes firstmovlw low T1HighTable ;addwf RPMIndex,w ; add table indexbtfsc STATUS,C ; test for table page crossingincf PCLATH,f ;movwf PCL ; lookup - result returned in W

SetTimerLowmovlw high T1LowTable ; repeat for lower byte movwf PCLATH ; movlw low T1LowTable ;addwf RPMIndex,w ; add table indexbtfsc STATUS,C ; test for table page crossingincf PCLATH,f ;movwf PCL ; lookup - result returned in W

#include "BLDCspd4.inc"

endDS00857A-page 46 2002 Microchip Technology Inc.

Note the following details of the code protection feature on PICmicro MCUs.

The PICmicro family meets the specifications contained in the Microchip Data Sheet.rs is itionsreacher oul proprned r can

e. com

ct theInformation contained in this publication regarding deviceapplications and the like is intended through suggestion onlyand may be superseded by updates. It is your responsibility toensure that your application meets with your specifications.No representation or warranty is given and no liability isassumed by Microchip Technology Incorporated with respectto the accuracy or use of such information, or infringement ofpatents or other intellectual property rights arising from suchuse or otherwise. Use of Microchips products as critical com-ponents in life support systems is not authorized except withexpress written approval by Microchip. No licenses are con-veyed, implicitly or otherwise, under any intellectual propertyrights.

Microchip believes that its family of PICmicro microcontrollewhen used in the intended manner and under normal cond

There are dishonest and possibly illegal methods used to bedge, require using the PICmicro microcontroller in a mannThe person doing so may be engaged in theft of intellectua

Microchip is willing to work with the customer who is conce Neither Microchip nor any other semiconductor manufacture

mean that we are guaranteeing the product as unbreakabl Code protection is constantly evolving. We at Microchip are

our product.If you have any further questions about this matter, please conta 2002 Microchip Technology Inc.Trademarks

The Microchip name and logo, the Microchip logo, KEELOQ,MPLAB, PIC, PICmicro, PICSTART and PRO MATE areregistered trademarks of Microchip Technology Incorporatedin the U.S.A. and other countries.

FilterLab, microID, MXDEV, MXLAB, PICMASTER, SEEVALand The Embedded Control Solutions Company areregistered trademarks of Microchip Technology Incorporatedin the U.S.A.

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one of the most secure products of its kind on the market today, . the code protection feature. All of these methods, to our knowl-tside the operating specifications contained in the data sheet. erty.about the integrity of their code. guarantee the security of their code. Code protection does not

mitted to continuously improving the code protection features of

local sales office nearest to you.DS00857A - page 47

MPLINK, MPSIM, PICC, PICDEM, PICDEM.net, rfPIC, SelectMode and Total Endurance are trademarks of MicrochipTechnology Incorporated in the U.S.A. and other countries.

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All other trademarks mentioned herein are property of theirrespective companies.

2002, Microchip Technology Incorporated, Printed in theU.S.A., All Rights Reserved.

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Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999 and Mountain View, California in March 2002. The Companys quality system processes and procedures are QS-9000 compliant for its PICmicro 8-bit MCUs, KEELOQ code hopping devices, Serial EEPROMs, microperipherals, non-volatile memory and analog products. In addition, Microchips quality system for the design and manufacture of development systems is ISO 9001 certified.

DS00857A-page 48 2002 Microchip Technology Inc.

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08/01/02

WORLDWIDE SALES AND SERVICE

IntroductionAnatomy of a BLDC

FIGURE 1: Simplified BLDC Motor DiagramsSensored Commutation

FIGURE 2: Sensor Versus Drive TimingFIGURE 3: three Phase BridgeTABLE 1: CW Sensor and Drive Bits by Phase OrderTABLE 2: CW Sensor and Drive Bits By Sensor OrderTABLE 3: CCW Sensor and Drive BitsFIGURE 4: Commutation code SegmentFIGURE 5: DC Motor Equivalent CircuitFIGURE 6: Sensored Drive FlowchartSensorless Motor ControlDetermining the BEMF

FIGURE 7: BEMF Equivalent CircuitFIGURE 8: BEMF at 100% DriveFIGURE 9: BEMF at 50% DriveFIGURE 10: Drive On vs. Drive Off BEMFFIGURE 11: Pittman BEMF WaveformsOpen Loop Speed ControlClosed Loop Speed Control

FIGURE 12: BEMF Sample TimesAcceleration and Deceleration DelayDetermining The Commutation Time Table Values

TABLE 4: Commutation Time Table ValuesFIGURE 13: PWM Lookup Table GEneratorUsing Open Loop Control to Determine Motor Characteristics

FIGURE 14: Motor Response Scope DeterminationConstructing The Sensorless Control Code

Appendix A: Sensorless Control FlowchartFIGURE A-1: Main LoopFIGURE A-2: MOtor CommutationFIGURE A-3: MOtor Driver ControlFIGURE A-4: Phase Drive PeriodFIGURE A-5: Motor Speed Locked with Commutation RateFIGURE A-6: Motor Speed Locked With Commutation Rate (Cont.)FIGURE A-7: MOtor Control STate MachineFIGURE A-8: Motor Control STate Machine (Cont.)FIGURE A-9: MOtor Control STate Machine (Cont.)FIGURE A-10: MOtor Control State mACHINE (cONT.)

Appendix B: SchematicsFIGURE B-1: Schematic A - mOTOR dRIVERSFIGURE B-2: Schematic B - cONTROLLER

Appendix C: Sensored CodeAppendix D: Sensorless CodeTrademarksWorldwide Sales

AN857 BLDC Motor Control Made Easy (Microchip)

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