AN1083, Sensorless BLDC Control with Back-EMF …

1 2007 Microchip Technology 1 AN1083 INTRODUCTIONThis application note describes a Sensorless brushlessDC ( bldc ) motor Control algorithm, implemented usingthe dsPIC digital signal controller (DSC). The algorithmworks by digitally filtering the back-Electromotive Force( Back-EMF ) on each phase of the motor and determin-ing when to commutate the motor windings based on thefiltered Back-EMF signals. This Control techniqueprecludes the need for discrete, low-pass filteringhardware and off-chip motors are used in a variety of applications. Thealgorithm described in this application note targetsBLDC motors that operate in the 40k to 100k electricalRPM range. Some bldc motor applications that run inthis RPM range are model RC motors, fans, harddrives, air pumps and dental algorithm described in this application note can beimplemented on two Microchip development boardplatforms: PICDEM MC LV Development Board dsPICDEM MC1 Development BoardThe PICDEM MC LV Development Board includes adsPIC30F3010 DSC.

2 The described algorithm wasimplemented on this device because it is included withthe PICDEM MC LV Development Board. However,for cost reduction, you can use the dsPIC30F2010 as asubstitute its default configuration, this board includes a 5 MHzcrystal. A MHz crystal was used during the testingof this resources used on the PICDEM MC LVDevelopment Board are:The resources used on the dsPICDEM MC1 Development Board are:These specifications are common to both hardwareplatforms: Maximum motor Speed: 100,000 Electrical RPM Tunable PID Speed Control Loop Configurable Open-Loop Start-up Ramp Supports the DMCI Tool (see the Implementing the Algorithm section of this document)FIGURE 1:PICDEM MC LV DEVELOPMENT BOARDFIGURE 2:dsPICDEM MC1 DEVELOPMENT BOARD WITH ATTACHED POWER MODULEP rocessor Type:dsPIC30F3010 or dsPIC2010 MIPS: 21 MIPSP rogram Memory.

3 2000 24-bit instruction words RAM: 280 MHzAuthor:Reston ConditMicrochip Technology : dsPIC30F6010 AMIPS: 21 MIPSP rogram Memory: 2089 24-bit instruction wordsRAM: 280 bytes(1)Note 1:With signal buffers enabled, RAM usageis 4400 bldc Control With Back-EMF FilteringAN1083DS01083A-page 2 2007 Microchip Technology motor CONSTRUCTIONThis algorithm has been tested on these motors: 4-pole, 12V, 10A fan motor with a maximum speed of 29,000 RPM 14-pole, 12V model airplane motor with a maximum speed of 13,000 RPM 4-pole, 24V, 1A Hurst bldc motor model DMB0224C10002 These motors have been tested with loads that areproportional to speed they have light loads at lowspeed and full loads at high the Y-connected bldc motor (see Figure 3), themotor has three leads, each connected to a winding (or series of windings) is connected to acommon point shared by all three windings, as shownin the upper portion of Figure 3.

4 The basic constructionof a simple bldc motor is shown in the lower portionof Figure 3:Y-CONNECTED bldc MOTORThe outer layer of the motor , which houses the motorwindings, is the stator. The inner part of the motor is therotor. The rotor consists of opposing magnetic poleslocated around the circumference of the 4 shows a rotor with only two poles, north andsouth. In reality, most motors have more than one setof magnetic poles on the motor turns when a current is passed throughmotor windings, as shown by the arrow (1) in Figure this example, a positive potential is applied to the red(R) lead, and a negative potential is applied to thegreen (G) lead. Charging the motor windings in thismanner generates the magnetic field in the stator,noted by the N and S designations.

5 The rotor then turnsso that the north pole in the rotor aligns with themagnetic south generated in the stator. Likewise, thesouth pole in the rotor aligns with the magnetic northgenerated in the 4:CURRENT FLOW THROUGH WINDINGSRGB100 NSRB rrggbbGcomcomcom110010011101001Y-Connect ed bldc motor SchematicBLDC motor ConstructionCommonRGB1100 NSRB rrggbbGcomcomcomNNSSNNSS110010011101001 Current FlowCurrent flow through windings generatesmagnetic field (designated by N and S) inthe stator, causing the north and southpoles of the rotor to align with the southand north poles, respectively, of the stator. 2007 Microchip Technology 3AN1083 SIX-STEP (TRAPEZOIDAL) COMMUTATIONThe method for energizing the motor windings in thesensorless algorithm described in this application noteis six-step trapezoidal or 120 commutation.

6 Figure 5shows how six-step commutation works. Each step, orsector, is equivalent to 60 electrical degrees. Six sec-tors make up 360 electrical degrees, or one arrows in the winding diagram show the directioncurrent flows through the motor windings in each of thesix steps. The graph shows the potential applied at eachlead of the motor during the six steps. Sequencingthrough these six steps moves the motor one 5:SIX-STEP COMMUTATIONFor every sector, two windings are energized and onewinding is not energized. The fact that one of the wind-ings is not energized during each sector is an importantcharacteristic of six-step Control that allows for the useof a Sensorless Control application note uses these terms to describemotor speed: Electrical revolutions per minute (RPMElec) Electrical revolutions per second (RPSElec)It is easier to discuss motor speed in these terms ratherthan mechanical RPM because when talking about elec-trical RPM, the number of motor poles does not have tobe factored in.

7 The relationship between mechanicaland electrical RPM is seen in these equations:EQUATION 1:MECHANICAL/ELECTRICAL RPM RELATIONSHIP To keep the magnetic field in the stator advancingahead of the rotor, the transition from one sector toanother must occur at precise rotor positions foroptimal torque. The next section discusses how rotorposition is determined in a sensored bldc 123456 Blue WindingGreen WindingRed WindingSector+VBUS-VBUS+VBUS-VBUS-VBUS+V BUSStepCommutation1 Red winding is driven positive. Green winding is driven winding is not winding remains winding is driven winding is not winding is driven winding is driven winding is not winding is driven winding is driven winding is not winding is driven winding is driven winding is not winding is driven winding is driven winding is not =(2 * RPMElec)(No.)

8 Of motor Poles)RPMElec =(RPMMech * No. of motor Poles)2 RPSElec = RPMElec 60AN1083DS01083A-page 4 2007 Microchip Technology bldc CONTROLIn most sensored bldc Control applications, Hall effectsensors are used to determine rotor position. Hall effectsensors are positioned in the motor housing in such away that each sensor output changes state every180 electrical degrees (see Figure 6). The rising edgeof Sensor B is offset by 120 electrical degrees withrespect to Sensor A. The rising edge of Sensor C is off-set by 120 electrical degrees with respect to Sensor sensors are positioned so that they change statewhen it is time for motor commutation to 6:SENSORED CONTROLA basic drive circuit for a bldc motor is shown inFigure 7.

9 Each motor lead is connected to a high-sideand a low-side switch. The correlation between thesector and the switch states is noted by the drive circuitfiring shown in Figure 7: bldc DRIVE CIRCUITDRIVE CIRCUIT FIRINGG reen WindingQ1,Q5 Q1,Q6 Q2,Q6 Q2,Q4 Q3,Q4Q3,Q560 HALL EFFECT SENSOR AHALL EFFECT SENSOR BHALL EFFECT SENSOR CQ1,Q5 Q1,Q6Q3,Q5 Sector6 Hall States12354624351651426 Blue WindingRed WindingBRGQ1Q3Q2Q4Q6Q5 2007 Microchip Technology 5AN1083 WHY Sensorless Control ?In sensored Control , sensors determine the position ofthe motor rotor with respect to the motor stator. Thismakes for fairly simple Control of the motor . A processorneed only wait for a Hall effect sensor to change state,determine which sector the rotor is in based on the out-put of the three Hall effect sensors and commutate themotor windings Control has several drawbacks: Sensors cost money.

10 In addition to the sensor itself, there is the further cost of mounting the sensors to the motor during manufacturing as well as the cost of sensor wires. Sensors add another potential failure point to the motor . If a sensor fails, the motor fails. In some environments it is just not practical to use sensors. For instance, in an environment where the motor is flooded (like a compressor or pump), the sensors may be subject to failure before the rest of the these and other reasons, Sensorless bldc controlis desirable in many applications. The next sectionintroduces the theory behind the Sensorless bldc Control technique described in this application bldc Control The key to determining the appropriate moment whenthe motor winding should be commutated, or transitionfrom one sector to another, is the rotor position.