AN957 Sensored BLDC Motor Control Using …

1 2004 Microchip Technology 1AN957 INTRODUCTIONThe dspic30f2010 is a 28-pin 16-bit MCU specificallydesigned for embedded Motor Control applications. ACInduction Motors (ACIM), brushless DC ( bldc ) andDC are some typical Motor types for which thedsPIC30F2010 has been specifically designed. Someof the key features on the dspic30f2010 are: 6 independent or 3 complementary pairs of dedicated Motor Control PWM outputs. 6 input, 500 Ksps ADC with up to 4 simultaneous sampling capability. Multiple serial communications: UART, I2C and SPI Small package: 6 x 6 mm QFN for embedded Control applications DSP engine for fast response in Control this application note we discuss how thedsPIC30F2010 is used to Control a Sensored bldc Motor .

2 Please refer to AN901 for details on how bldc motors operate and general information on what needsto be done to run and Control bldc motors. Thisapplication note discusses the specific implementationusing the dspic30f2010 . It touches only briefly onBLDC Motor detailsBLDC MOTORSBLDC motors are basically inside-out DC motors. In aDC Motor the stator is a permanent magnet. The rotorhas the windings, which are excited with a current. Thecurrent in the rotor is reversed to create a rotating ormoving electric field by means of a split commutatorand brushes. On the other hand, in a bldc Motor thewindings are on the stator and the rotor is a permanentmagnet.

3 Hence the term inside-out DC Motor . To make the rotor turn, there must be a rotating electricfield. Typically a three-phase bldc Motor has threestator phases that are excited two at a time to create arotating electric field. This method is fairly easy toimplement, but to prevent the permanent magnet rotorfrom getting locked with the stator, the excitation on thestator must be sequenced in a specific manner whileknowing the exact position of the rotor information can be gotten by either a shaftencoder or, more often, by Hall effect sensors thatdetect the rotor magnet position. For a typical three-phase, Sensored bldc Motor there are six distinctregions or sectors in which two specific windings areexcited.

4 These are as shown in Figure 1: bldc COMMUTATION DIAGRAMBy reading the Hall effect sensors, a 3-bit code can beobtained with values ranging from 1 to 6. Each codevalue represents a sector on which the rotor ispresently located. Each code value, therefore, gives usinformation on which windings need to be excited. Thusa simple lookup table can be used by the program todetermine which two specific windings to excite and,thus, turn the that state 0 and 7 are invalid states for Halleffect sensors. Software should check for these valuesand appropriately disable the Notification InputsTaking this technique a step further, the Hall effectsensors can be connected to dspic30f2010 inputsthat detect a change (Change Notification (CN) inputs).

5 An input change on these pins generates an the CN Interrupt Service Routine (ISR) the userapplication program reads the Hall effect sensor valueand uses it to generate an offset in the lookup table fordriving the windings of the bldc : Stan D SouzaMicrochip TechnologyFIRING60oHALL RHALL YHALL BHALL STATERYB546231546Q1,Q5Q1,Q6Q3,Q5Q1,Q5Q1, Q6Q2,Q6Q2,Q4Q3,Q4Q3,Q5 Sensored bldc Motor Control Using dsPIC30F2010AN957DS00957A-page 2 2004 Microchip Technology Control PULSE WIDTH MODULATION (MCPWM) Using the above method, you can get full speedrotation of the bldc Motor . However, to get variablespeed of the bldc Motor , you must apply a variablevoltage to the terminals of the windings.

6 Putting this indigital terms, the variable voltage can be obtained bydifferent duty cycles of a PWM signal going to thewindings of the bldc Motor . The dspic30f2010 has six PWM outputs that can bedriven with the PWM signal. As shown in Figure 2, thethree windings can be driven ON High, driven ON Lowor not driven at all by Using six switches, IGBTs orMOSFETs. When one leg of the winding is connectedfor example, to the high side, the variable duty cyclesignal PWM can be injected on the low side driver. Thishas the same effect as having a PWM signal on thehigh side and connecting the low side to VSS or driving the PWM signal, low side drivers arepreferred over high side 2:BEMF SENSING HARDWARE EXAMPLEPWM is provided by the dspic30f2010 s dedicatedMotor Control (MC) PWM.

7 The MCPWM module hasbeen designed specifically for Motor controlapplications. (Please refer to Figure 3 as you follow thisdiscussion of the MCPWM module.) The MCPWM has a dedicated 16-bit PTMR time baseregister. This timer is incremented by a user definedclock tick, which can be as low as TCY. The user alsodecides the period required for the PWM by selecting avalue and loading it in the PTPER registers. The PTMRis compared to the PTPER value at every TCY. Whenthere is a match, a new period is started. The duty cycle is controlled similarly, by loading a valuein the three duty cycle registers. Unlike the period com-pare, the value in the duty cycle register is compared atevery TCY/2 interval ( , twice as fast as the periodcompare).

8 If there is a match between the PTMR valueand the PDCx value, then the corresponding duty cycleoutput is driven low or high as dictated by the PWMmode selected. The three outputs from the duty cyclecompare are channeled to a complementary outputpair where one output is high while the other is low, andvice versa. The two outputs can also be configured asindependent outputs. When driven as complementaryoutputs, a dead time can be inserted between the timethe high level goes low and the low level goes dead time is hardware configured and has aminimum value of TCY. Dead time insertion preventsinadvertent shoot-thru in output 2004 Microchip Technology 3AN957 FIGURE 3.

9 PWM BLOCK DIAGRAM PDC3 PDC3 BufferPWMCON1 PTPER BufferPWMCON2 PTPERPTMRC omparatorComparatorChannel 3 Dead TimeGenerator and PTCONSEVTCMPC omparatorSpecial Event TriggerFLTACONOVDCONPWM Enable and Mode SFRsFLTA Pin Control SFRPWM ManualChannel 2 Dead TimeGenerator and Channel 1 Dead TimeGenerator and PWM Generator#2 PWM Generator#1 PWM Generator #3 SEVTDIRPTDIRDTCON1 Dead Time Control SFRS pecial EventPostscalerFLTAPWM1 LPWM1 HPWM2 LPWM2 HPWM3 LPWM3H16-bit Data BusOverride LogicOverride LogicOverride Logic Control SFR PWM time baseOutputDriverBlockNote: Details of PWM Generator #1 and #2 not shown for 4 2004 Microchip Technology are several modes in which the MCPWM modulecan be configured.

10 Edge aligned output is probably themost common mode. Figure 4 depicts the operation ofan edge aligned PWM. At the start of the period, theoutputs are all driven high. As the PTMR increments, amatch with the duty cycle registers causes the corre-sponding duty cycle output to go low thereby markingthe end of the duty cycle. The PTMR match withPTPER register caused a new period to start and alloutputs go high to start a whole new 4: EDGE-ALIGNED PWMThe other modes that the MCPWM can be set up forare center-aligned PWM and single-shot PWM. Thesemodes are not discussed here because they were notused for controlling the bldc Motor . For details onthese modes, please refer to the dsPIC30F FamilyReference Manual (DS70046).