Application Note
Contents
1. BEMEF zero crossing event detection phase current measurement and complementary independent unipolar PWM switching Commutation Transient time is called T timeZCToff in the software code implementation Figure 19 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 23 Software implementation on the MC9S12ZVML128 if driveStatus bit AfterCMT 0 amp amp driveStatus bit NewZC 0 amp amp driveStatus bit Sensorless 1 Is Torr period elapsed zero crossin current commutation period Close loop modeZ Z2CdetectionAdc ActualCmtSector bemfVoltageOld bemfVoltage driveStatus bit AdcSaved 1 yes Look for Zero cross Save actual Back EMF voltage value zero cross found Switch phase for Back EMF voltage sensing for the following commutation period if driveStatus bit NewZC GDUPHMUX_GPHMX BemfPhase NextCmtSector PTUC_PTULDOK i DCBusCurrentzcC DCBusCurrent DCBusCurrentFiltered tFracl6 long long DCBusCurrentFiltered long DCBusCurrentFiltered long DCBusCurrentFiltered long DCBusCurrentZC gt gt 2 Arm PTU fornext reload if driveStatus bit DisableAdc 1 driveStatus bit DisableAdc 0 driveStatus bit NewZC 0 Enable ADC measurements after first PWM period after commutation event ADCOCONIF 1 Clear flag Figure 192. 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 2 Freescale Semiconductor BLDC Sensorless Control the flat part of the BEMF waveforms this guarantees that the developed torque is constant or ripple free at all times In order to align current conduction in each phase with the flat part of the BEMF the rotor position must be known Figure 1 3 phase BEMF voltages and phase currents of a BLDC motor The position of the rotor can be obtained by a position sensor or a sensorless algorithm Various kinds of position sensors are used however since the rotor is a permanent magnet it is a very simple matter to determine where the physical pole edges are using a simple reliable and inexpensive Hall effect sensor The following techniques are commonly used to estimate rotor position in applications that rely on sensorless control of a BLDC motor e BEMEF zero crossing detection method e Flux level detection method e Various kinds of system state observers e Signal injection methods From a control perspective two logical mechanisms must be employed e Commutation control where the phases are energized according to rotor position with the quasi square current waveforms e Speed torque control where the amplitude of the quasi square current waveform applied to the phases is controlled to achieve the desired speed torque performance The following sections discuss the concept of the BEM
3. 4 2 5 MC Library The application source code uses the Freescale Motor Control Library for the MC9S12ZVML128 dF UpdateCurrentLimitingLed TIMOTFLG1 TIMOTFLG1 C3F MASK Speed evaluation and phase current limitation microcontrollers described in MC9SI2ZVMMCLUG Math and Motor Control Library for MC9S12ZVML128 User s Guide GDFLIB and GMCLIB GFLIB includes basic mathematical functions such as sine cosine ramp and The library contains three independent library blocks GFLIB 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 28 Freescale Semiconductor FreeMASTER user interface so on Advanced filter functions are part of the General Digital Filters Library and standard motor control algorithms are part of the General Motor Control Library 5 FreeMASTER user interface The FreeMASTER debugging tool is used to control the application and monitor variables during runtime For further information see freescale com FREEMASTER and document number MTRCKTSBNZVM128QSG Quick Start Guide for 3 Phase Sensorless BLDC Kit with MagniV MC9S12ZVML128 MCUs Communication with the host PC passes via USB However because FreeMASTER supports RS232 communication there must be a driver for the physical USB interface CP2102 installed on the host PC that creates a virtual COM port from the USB The driver can be installed from silabs com The application configures the SCI module of the MC9
4. Commutation transients processing Where the commutation transient time Tog has not yet expired the zero crossing calculation will not be performed The calculation will also not be performed if the zero crossing point has already been identified in the current commutation period or if the application is running in open loop mode If the above mentioned conditions are not met the zero crossing detection routine will be executed Based on the current commutation sector a slightly different code execution will be performed so therefore this sector number of 0 to 5 is passed to the algorithm When the zero crossing position calculation is finished the BEMF voltage value is stored as the old value as it will be referenced again in the next PWM cycle When a zero crossing point is identified the multiplexer that reads one of the three phase voltages is switched to the phase that is relevant for phase voltage reading in the next commutation cycle Afterwards the glb_Idok of the PTU is set to be prepared for the next reload signal to change the phase multiplexer located in the GDU module The flow chart and the code listing in Figure 20 describe the zero crossing detection routine that was called in the interrupt shown before Depending on the actual detected phase and the active sector a routine is called for either a rising or a falling BEMF voltage detection function The example here shows the routine called in the case of a rising BEMF on
5. After commutation evaluate precision of zero cross findings In close loop mode Calculate Speed PI controller Calculate duty cycle scale PI duty cycle MLIB Mul PIOut PWM MODULO F16 output to 0 PWM modulo value UpdateDutycycle Update duty cycle and set PTU actualSpeed SPEED CALC _NUMERATOR period6ZC speedErr requiredSpeed tFracl6 actualSpeed speedPIOut GFLIB ControllerPIrAW speedErr amp speedPIPrms F16 trigger delays CheckManualInterface UpdateCurrentLimitingLed TIMOTFLG1 TIMOTFLG1 C3F_MASK Figure 21 Speed evaluation ISR software flow 4 2 4 Current limitation controller The current limit controller is located in the same 1 ms timer interrupt as the speed controller because the inputs and outputs of both controllers are linked together When the actual speed has been calculated the current limit PI controller can be called by feeding it with the difference between actual current and the maximum allowed current of the motor The output of the PI controller is scaled to the number proportional to the PWM period After the current PI controller has calculated its duty cycle both the duty cycle output values are compared to each other 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 26 Freescale Semiconductor Software implementation on the MC9S12ZVML128 If the speed PI controller duty cycle output is higher than the current limit PI
6. controller output then the speed PI Controller duty cycle output value is limited to the value of the current limit PI controller otherwise the speed PI duty cycle output will be taken as the duty cycle update value The value of the duty cycle will be used to update the PMF module and the trigger delay values in the PTU unit At the end the integral portion values of both the PI controllers need to be synchronized to avoid one of the controllers increasing its internal value as far as the upper limit If the duty cycle was limited to the current PI duty cycle output then the integral portion of the current PI controller will be copied into the integral portion of the speed controller and vice versa The above described procedure is also described in Figure 22 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 27 Software implementation on the MC9S12ZVML128 INTERRUPT void TIMchan3_ISR void Interrupt TiMchan3 actualSpeed scaled 1 sum of all zero cross periods Evaluation of zero cross findings againstlimits tFracl6 PIoOut EnableInterrupts TIMOTC3 TIMOTC3 TIMER _1MS if driveStatus bit StallCheckReq driveStatus bit StallCheckReg StallCheck if driveStatus bit Sensorless period6Zc periodzC_F_PhA periodzC_F_PhB periodzZC_F_Phc T A periodzZzC_R_PhA periodZC_R_PhB periodZC_R_PhC actualSpeed speedErr speedPIOut SPEED_
7. cycle of the PWM period both the Phase A and Phase C bottom switches are on Therefore phase current circulates through Phase A Phase B and two bottom switches back During this cycle the phase current is unable to reach the DC bus shunt resistor and the phase current cannot be measured The 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 9 BLDC Sensorless Control star point of the motor as well is connected to ground and the zero crossing cannot precisely be measured in that cycle commutation SAtop SAtop PhaseA SAbot SAbot SBtop SCtop PhaseB SBbot SCbot Phase Phase Phase Phase Phase Phase SCtop PhaseC SCbot oc aa solga GND Figure 6 BEMF zero crossing detection with complementary independent unipolar PWM switching Following on from the discussion above phase current and BEMF voltage measurements must be performed in the active phase of the PWM cycle Moreover right after the commutation event there is a commutation transient pulse measured on the phase voltages which is produced by the current recirculation when the flyback diodes are conducting The duration of this transient is dependent on the motor inductance phase current and speed The BEMF voltage should not be measured during this time The software must take care of this interval 3 3 3 BEMF voltage measurement As we learned earlier the BEMF vol
8. on the MC9S12ZVML128 The quality of the zero crossing periods is evaluated in a function StallCheck and compared against the min and max limits with error stored if those limits are exceeded In the closed loop mode the speed error between the actual speed and the required speed is calculated This calculated speed error is fed into the PI controller function The PI controller is part of the Freescale Motor Control Library for further information see MC9S12ZVMMCLUG Math and Motor Control Library for MC9SI2ZVML128 User 5 Guide Inputs to the PI controller function include the speed error and the PI controller s parameters such as the proportional and integral constants The output of the PI controller is the duty cycle which is scaled to the PWM resolution At the end of the speed control function the duty cycle is assigned to the PMF module and based on the duty cycle new trigger points are also calculated and updated in the PTU module Interrupt TiMchan3 INTERRUPT void TIMchan3_ ISR void EhracleeE Louie EnableInterrupts TIMOTC3 TIMOTC3 TIMER _1MS if driveStatus bit StallCheckReq 1 Setnext ISR after1 ms driveStatus bit StallCheckReq 0 StallCheck actualSpeed scaled 1 sum of all zero cross periods if driveStatus bit Sensorless 1 PSI OCIWAC jorssealieyslC E BVA ap jersestoysVAC_ R PIWA ar tees OSA 1 J2AeVE Gr IPE C07 REENE E periodZC_F_PhC periodZC_R_PhC
9. phase voltages the DC bus current and DC bus voltage The DC bus current is amplified by the current sense amplifier which is part of the Gate Drive Unit GDU and then routed together with the DC bus voltage to one of the two ADCs for measurement acquisition The phase voltages are multiplexed by the GDU and then used by the second ADC From a control perspective the block diagram is divided into two logical parts e Commutation control where the phase voltages and DC bus voltage are used to calculate the actual position of the shaft According to the identified position the next commutation event can be prepared e Speed torque control where the required shaft velocity is compared to the actual measured speed and regulated by the PI controller The output of the speed PI controller is the duty cycle The duty cycle is limited by the current PI controller and assigned to the PWM 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 19 Software implementation on the MC9S12ZVML128 The application is controlled by FreeMASTER a real time debugging tool FreeMASTER communicates with the S12ZVM device by means of BDM or an SCI peripheral 4 2 1 Application flow speed Alignment i Open Loop Start Run Desired i E nat ee eh ee eS Speed l Vp PWM I I s Lp Starting 4 I Vector i Open Loop Closed Loop S Zoa Commutation time 1 Commutation time calc
10. phase motor the sum of all BEMF voltages is zero therefore Eqn 5 ec t ep t e 0 e lep t e The unpowered phase has the following voltage equation since there is no current flowing Eqn 6 un Uc eec By substituting Equation 5 and Equation 6 into Equation 4 the phase voltage on the unpowered phase can be derived as Eqn 7 Upce 3 2 12 Uc At the time of the BEMF zero crossing the BEMF voltage e in this case is zero as the name implies Therefore by measuring voltage at the unpowered phase e and comparing it to half of the DC Bus voltage a the BEMF zero crossing can be accurately identified 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 8 Freescale Semiconductor BLDC Sensorless Control 3 3 2 BEMF zero crossing event detection phase current measurement and complementary independent unipolar PWM switching BEMF evaluation window Commutations phase voltage eene ered enenneter Ce Se eee asad al i 4 ar oy oes fees were j Or p 1 We i 1 E aprssesaaas ee er Pat t i 1 Pit gt a ae time Commutation Zero cross Zero cross period period events Figure 5 BEMF zero crossing events commutations and their relationship to complementary independent unipolar PWM switching The exact position of the rotor can be sensed by measuring the BEMF voltage induced by the rotating perman
11. reload event The PMF reload event restarts the PTU timebase If the PTU is enabled the reload is immediately passed on as a ptu_reload event to the ADC and GDU modules The PMF generates the reload event at the required PWM reload frequency The PMF reload event causes the PTU timebase to restart to acquire the first trigger time from the list and to generate the ptu_reload signal for the ADCx to start loading the ADC conversion command from the Command Sequence List CSL When the trigger time is encountered the corresponding PTU trigger generates the trigger_x signal for the associated ADC For simultaneous sampling the PTU generates two simultaneous trigger_x signals one for each ADC At the trigger_x signal assertion the ADC starts the first conversion of the next conversion sequence in the CSL the first A D command is already downloaded For further information see MC9S12ZVMRMV1 MC9S12ZVM Family Reference Manual 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor Software implementation on the MC9S12ZVML128 In this way the PTU module serves as a delay block that can schedule several acquisitions of state variables relative to the start of the PWM period and within one PWM period 4 1 2 Module involvement in BLDC sensorless software control loop This section discusses measurements on the MC9S12ZVM128L and the internal hardware features to support the measurement of
12. startup During the alignment state all three phases are powered in order to get the best performance behavior in either direction of shaft rotation Phase A and Phase B are connected to the positive DC bus voltage and Phase C is grounded The alignment time depends on the mechanical constant of the motor including load and also on the applied motor current In this state the motor current torque is controlled by the PI controller 3 5 2 Startup In the startup state motor commutation is controlled in an open loop mode without any rotor position feedback The commutation period is controlled with an open loop starting curve The open loop start is required only until the shaft speed is high enough approximately 5 of nominal motor speed to produce an identifiable BEMF voltage 3 5 3 Run The block diagram of the run state is represented by Figure 10 and includes the BEMF acquisition with zero crossing detection in order to control the commutations The motor speed is estimated based on zero crossing time periods The difference between the demanded and estimated speeds is fed to the speed PI controller The output of the speed PI controller is proportional to the voltage to be applied to the BLDC motor The motor current is measured and filtered during the BEMF zero crossing event and used as feedback into the current controller The output of the current PI controller limits the output of the speed PI controller The limitation of the speed P
13. BEMF voltages and phase current Each commutation event first gets triggered by a timer interrupt This interrupt can be defined as a PWM reload signal The PWM reload signal itself also triggers the reload of the trigger list in the PTU module and restarts the PTU counter When the PTU counter reaches the predefined value in the trigger list T1 and T2 the PTU triggers an ADC measurement At the time T1 measurement of the DC bus current is triggered Two other simultaneous measurements are triggered at time T2 one for BEMF voltage and one for the DC bus voltage The ADC conversion results are automatically stored into a predefined queue in memory The principles of phase voltage DC bus voltage and DC bus current measurement are described in Section 3 3 3 BEMF voltage measurement and Section 3 4 DC bus current measurement The CPU is triggered by the ADC conversion complete interrupt service routine Based on the stored ADC values it calculates the zero crossing event Based on the zero crossing period the time of the next commutation event is calculated Async TIM commutation D event PWM releoad M 7 PWMAtop PTUreleoad i to T PTUtriggers T T2 Measured Back EMF ADC1 voltage voltage uU U n ADCO ADCO H tigger Measured DCBUS current average value time CPU Zea tnssdeecion Figure 12 Module involvement in BLDC sensorless software control lo
14. CALC_NUMERATOR period6ZC requiredSpeed tFracl6 actualSpeed GFLIB ControllerPIrAW speedErr amp speedPIPrms F16 Calculate Current Limit PI controller Calculate current limitduty cycle scale to PWM Calculate Speed Pl controller Calculate duty cycle scale to PWM Speed PI duty cycle output compared to Current Limit PI duty cycle output IfSpeed PI output gt CurrentPI output duty cycle CurrentPI output else duty cycle Speed PI output Update PMF duty cycle and PTU trigger delays currentErr DCBusCurrentLimit DCBusCurrentFiltered currentPIOut GFLIB_ ControllerPIrAW currentErr amp CurrentPilPrms ELG if currentPIOut gt speedPIOut currentPIPrms f32Acc ErracsZ currentPIPrms f16InErrKk1 OF PIOut speedPIOut driveStatus bit CurrentLimiting else speedPIPrms f32Acc tFrac32 speedPIPrms f1l6InErrKkl cS PIOut currentPlOut driveStatus bit CurrentLimiting speedPIOut lt lt 16 OF CUrrentrlouty lt lt Wo 1 Update PMF duty cycle and PTU trigger delays duty_cycle MLIB Mul PIOut UpdateDutycycle PWM MODULO F16 else if appState APP START appState APP ALIGNMENT currentErr PIOut ALIGN CURRENT SCALED DCBusCurrent GFLIB ControllerPIrAW currentErr AlignCurrentPIPrms Eve duty_cycle MLIB Mul PIOut PWM MODULO F16 UpdateDutycycle CheckManualInterface Figure 22
15. DC sensorless control based on BEMF zero crossing detection 0 eeee eee 12 Software implementation on the MC9S12ZVML128 13 4 1 MC9S12ZVM configuration 13 4 2 Software architecture 0 00000 18 FreeMASTER user interface 005 29 CONCUSSION 6326 55 acciec div ater teat eidateceees 29 RCIENESS Lan cacd ane soda teas aeteenes een awe 29 K freescale BLDC Sensorless Control e Runs on the hardware of the 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM for further information see document number MTRCKTSBNZVM128 Details about the MC9S12ZVML128 evaluation board can be found in MC9S12ZVM128MCBUG MC9S12ZVML128 Evaluation Board User s Manual e Incorporates a control technique with Six step commutation control of a 3 phase BLDC motor Position sensing by means of BEMF zero crossing detection technique Closed loop speed control Motor current limitation Alignment at startup e FreeMASTER software control interface motor start stop speed setup e DC bus over voltage under voltage over current and overload protection 3 BLDC Sensorless Control 3 1 Brief overview of brushless DC motors The BLDC motor is a rotating electric machine with a classic 3 phase stator similar to an induction motor The phases mounted on the stator are connected to form a way or delta connection The rotor has surface mounted permanent magnets The motor ca
16. F zero crossing detection method as well as the methods and conditions for its correct evaluation 1 Ibid 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 3 BLDC Sensorless Control 3 1 1 Electronic commutation control The commutation process provides a mechanism to energize phases according to the rotor position with the quasi square current waveforms Since only six discreet outputs per electrical cycle are required as shown in Figure 1 six semiconductor power switches are sufficient to create quasi square current waveforms for the phases Six semiconductor power switches form a 3 phase power inverter designed using IGBT or MOSFET switches The power for the system is provided by the DC Bus voltage Upcp The semiconductor switches and diodes are modeled as ideal devices in Figure 2 Figure 2 Power stage and motor topology Six step commutation is a very common method for driving a 3 phase way connected BLDC motor In six step commutation control the BLDC motor is operated in a two phase model Two phases are energized while the third phase is disconnected as the space between the magnet poles passes over it and produces a zero BEMF voltage Selection of the two energized phases is carried out by a position sensor or a position observer Table 1 shows the output current waveforms for a 3 phase inverter and the switching devices that conduct during the six switching inte
17. Freescale Semiconductor Application Note Document Number AN4704 Rev 1 08 2013 3 phase Sensorless BLDC Motor Control Kit with the 12 MagniV MC9S12ZVM by Branislav Zigmund Petr Staszko and Anita Maliverney 1 Introduction This application note describes the design of a 3 phase brushless DC BLDC motor control drive using a sensorless algorithm The design is targeted at automotive applications This cost effective solution is based on the Freescale Semiconductor MC9S12ZVML128 chip which is dedicated to automotive motor control The design exhibits the suitability and advantages of the MC9S12ZVML128 microcontroller for motor control It serves as an example of a BLDC control design using Freescale s 16 bit S12 MagniV mixed signal MCUs 2 System Concept The system is designed to drive a 3 phase BLDC motor The application meets the following system concept e Targets the MC9S12ZVML128 microcontroller Freescale Semiconductor Inc 2013 All rights reserved Contents MPCAUCHON 5 odin dar iccieae toe addan deena ba 1 System CONCEP sedcia aan biroi edd irradia 1 BLDC Sensorless Control 00000e aes 2 3 1 Brief overview of brushless DC motors 2 3 2 Complementary independent unipolar PWM modulation technique 0 cc eee eee 6 3 3 Position estimation based on BEMF zero crossing deton assenaar ae ace foment ahaa it E 7 3 4 DC bus current measurement 11 3 5 States of BL
18. I controller output protects the motor current from exceeding the maximal allowed motor current 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 12 Freescale Semiconductor 4 4 1 Software implementation on the MC9S12ZVML128 3 phase Inverter Commutation Control Measurement Zero Crossing Period amp Position Recognition Figure 10 Speed control with current limitation Software implementation on the MC9S12ZVML128 MC9S12ZVM configuration The BLDC sensorless application framework is designed to meet the following technical specification Targeted at the MC9S12ZVML128 evaluation board for further information see MC9S12ZVM128MCBUG MC9S12ZVML128 Evaluation Board User s Manual and MTRCKTSBNZVM128QSG Quick Start Guide for 3 Phase Sensorless BLDC Kit with Magni V MC9S12ZVML128 MCUs PWM output frequency 20 kHz Current loop sampling period 1 ms Speed loop sampling period 1 ms Linear approximation of the ADC samples for accurate BEMF zero crossing detection Internal dividers and multiplexer of the MC9S12ZVML128 are used for phase voltage measurement DC bus voltage measurement at the HD pin by the ADC The internal voltage dividers of the MC9S12ZVML128 are employed Internal operational amplifier AMPO is used for DC bus current measurement The MC9S12ZVML128 device includes modules such as the Pulse Width Modulator with Fault Protection PMF a Programmabl
19. Phase A 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 24 Freescale Semiconductor Software implementation on the MC9S12ZVML128 void ZCdetectPhAraisingAdc void tFrach6 delta if bemfVoltage gt 0 Raising approximation delta bemfVoltage bemfVoltageOld Function ZC Detect PhaseA Raising ADC if driveStatus bit AdcSaved 1 amp amp delta gt bemfVoltage timeBEMF MLIB Mul MLIB Div bemfVoltage delta F16 BEMF voltage gt A BeBe timeBEMF timeOldBEMF F16 es else middle of previous ADC sensing events timeBEMF timeBEMF timeOldBEMF gt gt 1 Save currentand previous zero cross time Calculate zero cross period filter value Calculate and setnext commutation time lastTimeZC timeZC timeZC timeBEMF timeZC lastTimeZC actualPeriodZC periodZC_R_PhA gt gt 1 periodZC_R_PhA actualPeriodZc NextCmtPeriod MLIB Mul actualPeriodZC advanceAngle F16 TIMOTCO timeZC NextCmtPeriod driveStatus bit NewZC 1 Figure 20 Zero crossing detection routine In the case of a negative BEMF voltage the zero crossing point has not been passed and the zero crossing point is not detectable The software exits the zero crossing detecting routine and leaves the zero crossing status bit as still unmarked In the case of a positive BEMF voltage the zero crossing point was passed and E
20. S12ZVML128 for a communication speed of 9600 bit s Therefore the FreeMASTER user interface also needs to be configured respectively 6 Conclusion The design described in this document shows the simplicity and efficiency of using the MC9S812ZVML128 microcontroller for sensorless BLDC motor control and introduces it as an appropriate candidate for various low cost applications in the automotive area 7 References Document number Title Availability MC9S12ZVMRMV1 MC9S12ZVM Family Reference Manual freescale com MC9S12ZVM128MCBUG MC9S12ZVML128 Evaluation Board User s Manual MTRCKTSBNZVM128 3 phase Sensorless BLDC Motor Control Kit with the MagniV freescale com AutoMCDevkits MC9S12ZVM MTRCKTSBNZVM128QSG Quick Start Guide for 3 Phase Sensorless BLDC Kit with MagniV MC9S12ZVML 128 MCU MC9S12ZVMMCLUG Math and Motor Control Library for MC9S12ZVML 128 User s freescale com AutoMCLib Guide FREEMASTER Runtime Debugging Tool freescale com FREEMASTER 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 29 How to Reach Us Home Page freescale com Web Support freescale com support Document Number AN4704 Rev 1 08 2013 Information in this document is provided solely to enable system and software implementers to use Freescale products There are no express or implied copyright licenses granted hereunder to design or fabricate any integr
21. aintain the high side driver gate source voltage ee when PWM is running at a 100 duty cycle The clock for the charge pump is set to be Sous GDUCLK2_GCPCD 2 e Desaturation error integrates three desaturation comparators for the low side FET pre drivers and three desaturation comparators for the high side FET pre drivers The desaturation level is set to be 1 35 V GDUDSLVL 0x77 for both low side and high side FETs A blanking time during the FET transients needs to be employed The blanking time is set to be approximately 8 us GDUCTR 0x13 e Phase muxing used to select the phase voltage GDUPHMUX to be routed internally to ADC1 Channel 2 e Current sense amplifier Internal current sense amplifier 0 GDUE_GCSE0 1 is used to measure motor phase current The output of the current sense amplifier 0 is routed internally to ADCO Channel 0 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 17 Software implementation on the MC9S12ZVML128 4 1 7 ADC The MC9S12ZVML128 uses two independent Analog to Digital Converters ADC Both ADCs are n channel multiplexed input successive approximation analog to digital converters The List Based Architecture LBA provides a flexible conversion sequence definition as well as flexible oversampling Both ADC conversion command lists are stored inside the global memory map that is inside the system memory as two dimensional arrays of byt
22. ated circuits based on the information in this document Freescale reserves the right to make changes without further notice to any products herein Freescale makes no warranty representation or guarantee regarding the suitability of its products for any particular purpose nor does Freescale assume any liability arising out of the application or use of any product or circuit and specifically disclaims any and all liability including without limitation consequential or incidental damages Typical parameters that may be provided in Freescale data sheets and or specifications can and do vary in different applications and actual performance may vary over time All operating parameters including typicals must be validated for each customer application by customer s technical experts Freescale does not convey any license under its patent rights nor the rights of others Freescale sells products pursuant to standard terms and conditions of sale which can be found at the following address freescale com SalesTermsandConditions Freescale and the Freescale logo are trademarks of Freescale Semiconductor Inc Reg U S Pat amp Tm Off MagniV is a trademark of Freescale Semiconductor Inc All other product or service names are the property of their respective owners 2013 Freescale Semiconductor Inc e Po Z freescale
23. e 8 it can be seen that the crossing of the BEMF voltage and e level can take place wherever between two following BEMF voltage measurements For accurate position estimation an exact zero crossing point has to be identified This exact zero crossing point identification is done by an approximation based on the interpolation of two following BEMF measurements Assuming that the shaft is not accelerating actual BEMF voltage was measured at time Tapc with the voltage level of e7 and the previous measurement was taken at the time of Tapco Tpwy with the voltage level of e7_ then the equation to calculate the exact time of the zero crossing event could be derived as follows Eqn 8 er ery er Upcr 2 er Upce 2 i i ae gt Tze Tapc gt T wm PWM ADC ZC er eT 1 This formula is calculated in the commutation period when two following comparisons of the BEMF voltage to half of the DC bus have the opposite signs In order to enhance the accuracy of the zero crossing event even further the DC bus voltage and BEMF voltage need to be measured simultaneously 3 4 DC bus current measurement ADC samp le ee value o j DCBUS current Sampling inthe middle Of PWM ean yey d Sampling closet fe closet beginning theend Of PWM OfPWM amam DCBUS current DCBUS current ase current phase current Se DCBUS not measurable by DC BUS shunt shunt Figure 9 DC bus curr
24. e ADC1 and ADCO The flowchart and interrupt routine in Figure 18 describe the steps of processing the measurements Interrupt ADCO Done Save ADC results of DC Bus Current Voltage Phase voltage measurements INTERRUPT void ADCOdone_ISR void DCBusCurrent ADCOResultList 0 DCBusCurrentOffset DCBusVoltageHalf MLIB Mul ADCOResultList 1 gt gt 1 13653 F16 phaseVoltage ADC1ResultList 0 gt gt 1 bemfVoltage phaseVoltage DCBusVoltageHalf if driveStatus bit AfterCMT 1 if timeBEMF timeCommutation gt timeZCToff driveStatus bit AfterCMT 0 Scale phase and DC Bus voltages according to their resistor divider ratio Calculate Back EMF voltage Check Torr period Figure 18 Processing measurements in the ADC Done ISR Before the ADCO Done ISR routine is executed the ADC stores the results into predefined memory space also known as the ADCx result list by use of the DMA The DC bus current and the DC bus voltage measurements are stored in the ADCO result list and the phase voltage is stored in the ADC1 result list In the ADCO Done ISR routine the ADC results are scaled according to the hardware setup and saved into the appropriate variables The BEMF voltage is calculated as the difference between the phase voltage and voce The BEMF voltage value is the signed number The software checks whether the commutation transient time has already passed see Section 3 3 2
25. e MTRCKTSBNZVM128QSG Quick Start Guide for 3 Phase Sensorless BLDC Kit with MagniV MC9SI2ZVML128 MCUs 3 Phase Inverter MC9S12ZVML128 o Power Evaluation Board i 3 Phase Line o Bii Motor Phase Voltage FTE ADC Module 1 4 Ph Comp Triggers la DC Bus Current Mux GDU Re ADC Module 0 4 DC Bus Voltage Current Amp mo Zero Crossing Period Commutation cael A aon A pMmF H PTU and Position Recognition Control Required DC Bus _ Current Limit 1 T Actual Trigger Points On Chip Debugging Current PI Controller AA Freemaster peta Required Limitations Superior i Speed Q Speed System Module PI Controller Sia Figure 13 System block diagram The block diagram shown in Figure 13 is the overview of the control blocks The top box shows the power stage the lower parts in the figure are the functions implemented in the S12ZVM The power stage uses a one shunt based system to measure the phase current The light green blocks are implemented in hardware on the S12ZVM device the light blue blocks are implemented in software and benefit from the Motor Control Library MCLib for further information see MC9SI2ZVMMCLUG Math and Motor Control Library for MC9SI2ZVML128 User 5 Guide The inputs of the control loop are the measured voltages and current on the power stage in particular the
26. e Trigger Unit PTU an Analog to Digital Converter ADC a Timer module TIM and a Gate Drive Unit GDU suitable for control applications in particular motor control applications These modules are directly interconnected and can be configured to meet various motor control application requirements Figure 11 shows module interconnection for a typical BLDC sensorless application The modules are described below and a detailed description can be found in MC9S12ZVMRMV1 MC9S12ZVM Family Reference Manual 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 13 Software implementation on the MC9S12ZVML128 4 1 1 Module interconnection The modules involved in output actuation data acquisition and the synchronization of actuation and acquisition form the so called control loop This control loop consists of the PMF GDU ADC and PTU modules The control loop is very flexible in operation and can support static dynamic or asynchronous timing The PTU and ADC operate using lists stored in memory These lists define the trigger points for the PTU commands for the ADC and results from the ADC zero crossing comparators async_event commutation dc_bus_current0 dc_bus_current1 trigger_1 ptu_reload Figure 11 Block diagram of sensorless BDLC configuration Each control loop cycle is started by a PMF
27. ent magnet in the unpowered phase In Figure 5 the green windows mark the time periods in which the respective phase is unpowered The voltage measured in this time window is the BEMF voltage At the BEMF zero crossing event the permanent magnet is right in front of a coil and the rotor field is positioned 90 versus the stator field This event happens in the middle of a commutation period and is marked as the black circles in the green BEMF window At this time the phase voltage is equal to half of the DC bus voltage as described in Section 3 3 1 BEMF zero crossing principle In the case of a constant shaft velocity the period between two following zero crossing events is equal to the commutation period Figure 6 zooms in closer to one of the PWM cycles At the top of the figure is the PWM pattern where Phase A is controlled by PWM and Phase C is grounded for the entire PWM period During the PWM On cycle the top switch of Phase A is turned on and the bottom switch of Phase C is grounded Current flows from the DC bus into Phase A and back through Phase C and the DC bus shunt resistor In this cycle the waypoint of the motor shows a voltage level of ae The BEMF voltage in the unpowered phase changes relatively to 2 in the positive and negative directions which means that the zero crossing is detectable when the phase voltage on the unpowered phase is equal to 2i Also the phase current is measurable on the DC bus shunt During the Off
28. ent measurement 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 11 BLDC Sensorless Control As mentioned in the previous section the DC bus current has to be measured in the active cycle of the PWM period due to the fact that the DC bus current equals the phase current only in the active cycle as illustrated in Figure 9 During the active cycle of the PWM period the phase current is rising The slope of the rising current is defined by the motor phase coil inductance the lower the phase inductance the larger the gradient of the rising current It is important to measure the current in the middle of the PWM active cycle to obtain the average phase current 3 5 States of BLDC sensorless control based on BEMF zero crossing detection In order to start and run the BLDC motor the control algorithm has to go through the following states e Alignment initial position setting e Startup forced commutation or open loop mode e Run sensorless running with BEMF acquisition and zero crossing detection 3 5 1 Alignment As mentioned previously the main task for sensorless control of a BLDC motor is position estimation Before starting the motor however the rotor position is not known The aim of the alignment state is to align the rotor to a known position This known position enables starting the rotation of the shaft in the desired direction and generating the maximal torque during
29. equation can be rewritten to the following form Eqn 3 360 A A 1 Ager 1 6 mech p AT p Tag 1 360 p T 330 gt 30 T 30 gt 90 T90 gt 150 T 150 gt 210 T 210 gt 270 T 270 gt 330 _ 360 E P Din 1 Tem 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 5 BLDC Sensorless Control Where e pis the number of pole pairs e Temis the time between two following commutations Tc is the time between commutations in sector n 1 2 3 4 5 6 e 1 1s the electrical position 3 2 Complementary independent unipolar PWM modulation technique There are different methodologies for powering and switching the phases The unipolar PWM control technique combines commutation control and torque control While the state of the switches is determined by commutation control the torque is controlled by the applied duty cycle An application with BLDC control where the unipolar PWM control technique is employed benefits from a reduction in the MOSFET switching losses and an improvement in the system s EMC robustness The unipolar PWM control means that the motor phase sees only the positive polarity of the voltage To achieve the unipolar PWM pattern one phase is in complementary PWM mode while the second phase is grounded and the third phase stays unpowered as shown in Figure 3 This PWM pattern can be seen every 60 electrical deg
30. es ADCOCommandList ADC 1CommandList The exact location of the ADC conversion commands in the system memory is given by the linker command file and linked to the respective ADC module during the initialization phase The same strategy is used for the ADC results The conversion results are stored in an array of shorts ADCOResultList ADC1ResultList located in system memory ADCO Command Base Pointer ADCOCBP_0 uint8_t long ADCOCommandList gt gt 16 ADCOCBP_1 uint8_t long ADCOCommandList gt gt 8 ADCOCBP 2 uint8 t long ADCOCommandList ADCO Result Base Pointer ADCORBP_0 uint8 t long ADCOResultList gt gt 16 ADCORBP_1 uint8_t long ADCOResultList gt gt 8 ADCORBP_2 uint8_t long ADCOResultList ADC1 Command Base Pointer ADCICBP_0 uint8_t long ADC1CommandList gt gt 16 ADCICBP_1 uint8_t long ADC1CommandList gt gt 8 ADCICBP_2 uint8_ t long ADC1CommandList ADC1 Result Base Pointer ADCIRBP_0 uint8_t long ADC1ResultList gt gt 16 ADCIRBP 1 uint8 t long ADC1ResultList gt gt 8 ADC1RBP 2 uint8 t long ADC1ResultList The ADC conversion clocks are set to be 6 25 MHz ADCOTIM 0 ADCITIM 0 The results are stored in memory as 12 bit ADCOFMT_SRES 4 ADCIFMT_SRES 4 left justified data ADCOFMT DJM 0 ADCIFMT DM 0 Conversion flow of both ADCs is controlled by internal signals ge
31. ftware control mode PMFOUTC PMFOUTB and the third phase stays unpowered by masking the output PMFCFG2 MSK5 MSK0 as shown in Figure 3 The registers PMFOUTC PMFOUTB PMFCFG2 MSK5 MSK0 used to control the unipolar PWM pattern are in double buffered mode PMFCFG1_ ENCE 1 The double buffered signals are swapped at a commutation event A PWM pulse width PMFVAL registers are also double buffered and are swapped when GLDOK is set and the PWM Reload signal occurs The GLDOK is an external signal generated by the PTU module The GLDOK is enabled at the PWM module PMFENCA_GLDOKA 1 For example in the first cycle Phase A is powered by the complementary PWM signal while the bottom transistor of Phase B is grounded PMFOUTC 0x0C PMFOUTB 0x2A and Phase C is unpowered PMFCFG2 MSK5 MSK0 0x34 After the commutation event at 90 electrical degrees Phase A is still powered by the complementary PWM signal Phase B is now unpowered PMFCFG2 MSK5 MSKO 0x1C and Phase C becomes grounded instead PMFOUTC 0x30 PMFOUTB 0x2A 4 1 4 PTU The Programmable Trigger Unit PTU is intended to completely avoid CPU involvement in the time acquisitions of state variables during the control cycle The PTU module consists of two trigger generators TG For each TG a separate enable bit is available so that both TGs can be enabled independently TGO is connected to ADCO and TG1 is connected to ADC1 The trigger generation of the PTU module is s
32. h enough to reliably detect the zero crossing After a defined number of commutation cycles the state changes from the open loop start state to the run state From here on the commutation process based on the BEMF zero crossing measurement takes place and the control enters the closed loop mode 4 2 2 Application timing and interrupts PTU 1 done ISR an J1 l l l Save time of Back EMF voltage measurement Lt s MAO Une unenunduangueneg ADCO done ISR i 550 Zero cross detection ak i ADC conversion time 3f i i i H t R m a a a i A p Timer channel 0 2 A a Commut tion peridd gt 200us Se eib i i i Timer channel 3 1m Speed Control Loop merger Bilan 9 il Pa rons teeny es CPSP Pees Phase voltages 0 i Figure 15 Application timing and interrupts Figure 15 shows the application timing and the associated interrupts used for the commutation zero crossing and speed control The grey boxes show the executed interrupt routines versus the phase voltage measurement The top row shows the interrupt that is activated when all the triggers in the PTU have been executed which happens the moment the BEMF measurement is triggered by the Trigger Unit at the end of each active PWM cycle In this interrupt the timer value as a BEMF measurement reference point is saved The second row in the figure shows the interrupt that is activated when all the ATD measurements wit
33. hin a PWM cycle have been executed At that time all the necessary values for the zero crossing detection have been stored automatically into memory by the ATD converter and the detection of the zero crossing event and calculation of the zero crossing period can then take place After the zero crossing detection the 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 21 Software implementation on the MC9S12ZVML128 multiplexer for the phase voltage measurement is switched to the phase on which the BEMF will be sensed in the following commutation period The third row shows the interrupt that is the actual trigger for the commutation event triggered by a timer channel interrupt The time between each call is dependent on the actual speed of the motor The last row shows the interrupt routine used for the speed control loop This is handled in a timer channel interrupt which is set every millisecond The actual speed information together with the variable of the required speed is an input value to the speed controller 4 2 3 Zero crossing detection processing 4 2 3 1 Getting the time of the BEMF measurement PTU triggers T1 T2 4 carnai Gaa nR gt SAtop Measured Back EMF voltage ADC1 gt iagert 1 ADC 087 Measured i cojersionsime DC Bus voltage i Measured DC BUS current Interrupt PTU1 Trigger Done Interrupt ADCO Done Figure 16 Measurement of DC b
34. n have more than one pole pair per phase The pole pair per phase defines the ratio between the electrical revolution and the mechanical revolution The BLDC motor is equivalent to an inverted DC brushed motor where the magnet rotates while the conductors remain stationary In the DC brushed motor the commutator and brushes reverse the current polarity in such a way that stator and rotor magnetic fields are perpendicular However in the brushless DC motor a power transistor which must be switched in synchronization with the rotor position performs the polarity reversal This process is also known as electronic commutation The displacement of the magnets on the rotor creates a trapezoidal back electromotive force BEMF shape when the rotor is spinning Neglecting the higher order harmonic terms the BEMF in the motor phase epep c is as indicated in Figure 1 Each BEMF has a constant amplitude for 120 electrical degrees followed by a 60 electrical degree transition in each half cycle The developed torque at any instance is given by the following equation Eqn 1 eala epip ecic Wel Where e 1 1S electrical angular velocity The ideal current waveforms in each phase i i need to be quasi square waveforms of 120 electrical degrees of conduction angle in each half cycle The conduction of current in each phase must coincide with 1 Rashid Muhammad H ed Power Electronics Handbook 2nd ed London Academic Press 2007
35. nerated by the PTU and by the data bus ADCOCTL_0 ACC _CFG 3 ADCICTL_0 ACC CFG 3 The results are stored in system memory even if commutation occurs when conversion is ongoing ADCOCTL_0 STR SEQA 1 ADCICTL_0 STR_SEQA 1 ADCO schedules the end of list interrupt ADCOCONIE_1_ EOL_IE 1 to calculate the BEMF zero crossing algorithm The BLDC sensorless algorithm uses ADCO to measure the motor phase current and DC bus voltage The ADC is used to measure the respective motor phase voltage 4 2 Software architecture This section describes the software design of the BLDC sensorless algorithm based on zero crossing Figure 13 shows the conceptual system block diagram The application is optimized for MC9S12ZVM motor control peripherals to achieve the least possible core involvement in state variable acquisition and output action application The motor control peripherals PMF PTU ADC TIM and GDU modules are internally linked set up together to work independently 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 18 Freescale Semiconductor Software implementation on the MC9S12ZVML128 from the core and to achieve deterministic sampling of analog quantities and precise commutation of the stator field The software part of the application consists of different blocks which are described below The entire application behavior is controlled from a PC with the FreeMASTER tool For further information se
36. op The TIM PTU GDU and ADC peripherals are based on the bus clock To achieve a higher resolution of PWM the PMF module is supplied by a core clock The core clock is double that of the bus clock The clock module uses a 4 MHz external crystal and is configured to generate a 12 5 MHz bus clock and 25 MHz core clock 4 1 3 PMF The Pulse Width Modulator with Fault Protection PMF module is configured to generate an edge aligned PMFCFGO_EDGEx 1 PWM with a frequency of 20 kHz PMFMODA 1250 In order to protect the MOSFET devices in the same leg of the inverter dead time is set to approximately 0 5 us 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 15 Software implementation on the MC9S12ZVML128 PMFDTMA 13 PWM generator A runs as the master and generates the Reload signal as a synchronization signal for the other submodules PMFCFG2_REV 0 1 1 The Reload signal is generated at every PWM opportunity PMFFQCA 0 Pair A Pair B and Pair C PWMs are synchronized to PWM generator A PMFCFG0O_MTG 0 All three pairs use value register zero PMFCFG3_VLMODE 1 the value written into value register zero is also written into value registers 1 to 5 to set the duty cycle The PWM generator is restarted with every commutation event PMFENCA_RSTRIA 1 To achieve a unipolar PWM pattern one phase is in complementary PWM mode while the second phase is grounded by the so
37. pplied on the shaft Such a technique aligns the shaft with Phase C which is perpendicular to the two flux vectors generated by the stator windings and therefore ensures the same startup torque in both directions of rotation The duration of the alignment state depends on the motor s electrical and mechanical constants the applied current meaning duty cycle and the mechanical load When the alignment timeout expires the application software moves to the open loop start state At a very low shaft velocity the BEMF voltage is too low to reliably detect the zero crossing Therefore the motor 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 20 Freescale Semiconductor Software implementation on the MC9S12ZVML128 has to be controlled in an open loop mode for a certain time period The very first vector generated by the stator windings needs to be set to a position 90 relative to the position of the flux vector generated by magnets mounted on the rotor The alignment and first start up vector are shown in Figure 14 The duration of the open loop start state is defined by the number of open loop commutations The number of open loop commutations depends on the mechanical time constant of the motor including load and also on the applied motor current The shaft velocity after an open loop startup is approximately 5 of nominal velocity At a velocity approximately 5 of nominal velocity the BEMF voltage is hig
38. quation 8 is calculated meaning that the BEMF voltage is divided by the delta of the two measured points and multiplied by the measured PWM period BEMF measurement period After this calculation the old zero crossing time and the new one are saved into the appropriate variables The zero crossing period is then calculated based on the calculated time of zero crossing and the time of the zero crossing in the previous commutation cycle The zero crossing period is also filtered to improve reliability At the end of the routine the new commutation time is calculated Here some motor characteristics have to be taken into account Instead of just adding half of a zero crossing period to the actual zero crossing time a so called advance angle factor is taken into account which actually activates the commutation a bit earlier than calculated This is usually a constant and depends on the motor characteristics Finally the zero crossing point is marked as found and in that way the calculation does not take place anymore in current commutation cycle 4 2 3 3 Speed evaluation and control The speed controller in Figure 21 is executed in a timer interrupt every ms First of all the actual speed is calculated from all of the last six zero crossing periods and this is stored in a scaled format as the actual speed 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 25 Software implementation
39. rees and they differ only in phase order The phase order is determined according to the shaft position by commutation control 30 90 150 210 270 330 30 Figure 3 Complementary independent unipolar PWM switching 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 6 Freescale Semiconductor BLDC Sensorless Control For example in the first cycle Phase A is powered by the complementary PWM signal while the bottom transistor of Phase B is grounded and Phase C is unpowered After the commutation event at 90 electrical degrees Phase A is still powered by the complementary PWM signal Phase B is unpowered and Phase C becomes grounded instead The control described in this application note is based on the complementary independent unipolar PWM modulation technique The following section explains sensorless position estimation by means of BEMF zero crossing detection for commutation control purposes 3 3 Position estimation based on BEMF zero crossing detection Figure 4 shows ideal BEMF waveforms and depicts a commutation event occurring at a position of 30 electrical degrees after the point where a BEMF zero crossing arises The BEMF zero crossing happens at a position of 30 electrical degrees after the point of the last commutation event Let us assume that the motor is spinning at a constant velocity in this case the motor needs the same amount of time to travel from the po
40. rvals per cycle 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor BLDC Sensorless Control Table 1 Six step switching sequence Rotor Sector Switch Phase current position number closed A B P 330 30 1 Sar Sbg R 7 off eee 2 Sar Scp Off 90 150 3 Spt ScB off 150 210 4 Spr Sap 3 Fi off 210 270 5 Sct Sap B off 270 330 6 Scr Sp off 7 3 1 2 Speed torque control Commutation ensures the proper direction of the phase current according to the rotor position of the BLDC motor while the motor torque speed only depends on the amplitude of the quasi square current waveform Continued control of the amplitude of the quasi square current waveform for each phase of the motor is ensured by hysteresis or PWM control PWM control is commonly used in applications where microcontrollers are employed The duty cycle for the PWM modulator is obtained by the speed PI controller The speed PI controller amplifies the error between the required and actual speeds and its output appropriately scaled is assigned to the PWM modulator The actual mechanical speed can be calculated as a time derivative of the shaft position pech Eqn 2 2i APmecn 1dgei z 1Ager meat dt pdt p AT Since the shaft travels exactly 1 6 of one electrical revolution 2x in radians between two commutations the above
41. sition of the last commutation event to a BEMF zero crossing and from the BEMF zero crossing to the following commutation event In the time domain a BEMF zero crossing is right in the middle of two commutation events Therefore the BEMF zero crossing event with help of a timer can simply be used to estimate the right commutation point as well as the velocity of the rotor o Figure 4 Zero crossing detection and commutation diagram 3 3 1 BEMF zero crossing principle To explain and simulate the idea of BEMF sensing techniques this document provides a simplified mathematical model based on the basic circuit topology see Figure 4 The goal of the mathematical model is to identify dependencies between the measurable motor waveforms and a BEMF zero crossing The BEMF zero crossing in turn helps to identify the commutation event 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 Freescale Semiconductor 7 BLDC Sensorless Control The mathematical model is based on the fact that only two phases of a motor are energized and the third is disconnected The natural voltage level of the whole model is referenced to half of the DC bus voltage which simplifies the mathematical expressions The mathematical model assumes that the motor phases are symmetrical see Figure 4 Eqn 4 l dip un Unce Rly L gp ep _ Upee ep a gt u lea dig ig p 2 2 Uy Rig Ter For a symmetrical 3
42. t of the respective phase voltage The PTU module generates the LDOK signal used to inform other modules that the double buffered registers were updated by software 4 1 5 TIM The Timer Module TIM is a basic scalable timer that consists of a 16 bit software programmable counter driven by a flexible programmable prescaler The BLDC sensorless algorithm employs two timer channels in output compare mode TIMOTIOS_IOSO 1 TIMOTIOS_IOS3 1 Timer Channel 0 serves to identify the commutation event The output compare signal is internally routed to the PMF module as an async_ event in order to perform commutation of the PWM pairs When the timer counter reaches the value in the channel registers of an output compare channel the timer toggles TIMOTCTL2_ OLO 1 TIMOTCTL2_OMO 0 the channel output This channel also schedules the interrupt TIMOTIE_COI 1 where some of the application flags are controlled Timer Channel 3 is employed to control the torque of the motor by a software task Timer Channel 3 schedules periodic interrupts TIMOTIE_C3I 1 with a period of 1 ms TIMOTC3 781 The timer s prescaler is equal to 4 TIMOTSCR2_PR 4 and all of the timers output compare pins are disconnected TIMOOCPD Oxff 4 1 6 GDU The Gate Drive Unit GDU is a Field Effect Transistor FET pre driver designed for 3 phase motor control applications The following GDU features are used in BLDC sensorless control e Charge pump used to m
43. tage can only be measured during the active phase of the PWM Importantly this is measured towards the end of the active cycle due to switching noises In Figure 7 the green marked area shows the window in which the BEMF should be measured zero cros cea p ms I Tm EmN mm fo EN i S i i N EAA TAMU F E va j N 1 N l 1 N kal i v4 is Mil i motor Apg Back EMF voltage m ARM UUN hamiwa E Lemna phase sample measure window 7 Ton P ea i Time of Back EMF Back EMF voltage sample point is used to calculate exacttime of the BEMF tot zero cross 2 rernnnrra E i ec 1 lt Switching i 1 noise _ BEMF DCBUS 2_ Se eee spikes i ze anc E PWM H Measured Back EMF BEMF BEM i voltage __ PWM to ADC j e ee delaybyPTU 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 10 Freescale Semiconductor BLDC Sensorless Control Figure 7 BEMF measurement Figure 8 Precise BEMF zero crossing identification It should be noted that depending on the motor and power stage parameters the amplitude period and damping of the voltage ringing vary As a result it is recommended that you measure BEMF voltage close to the end of the window The time of this sample point also needs to be stored as it is used to enhance zero crossing detection If we zoom out again and look at the BEMF voltage cycles see Figur
44. ulated bai i calculated base on base on Back EMF zero cross Fa rd igtinGh i acceleration equation I 1 1 Start speed T i in sensorless Required speed close loop Realspeed time Appinit B eeu 4 AppAlignment N AppStart AppRun called in main wu ETEA AppStopToAlignment AppAlignmentToStart AppStartToRun lt transientfunctions Figure 14 Application state flow Figure 14 explains the different application states The figure consists of two interconnected parts e The speed over time characteristic e The blocks in the lower part of the picture which show the states of the application and the transitions between respective states The application software has three main states the alignment state the open loop start state and the run state In the run state the BLDC motor is fully controlled in closed loop sensorless mode After the initialization of the peripheral modules has completed the software enters the alignment state In alignment the rotor position is stabilized into a known position in order to create the same startup torque in both directions of rotation This is achieved by applying a PWM signal to Phase A and Phase B The duty cycle is calculated by the alignment PI controller Phase C is assigned with a duty cycle equal to zero that is Phase C is connected to the negative pole of the DC bus The value of the duty cycle on Phase A and Phase B depends on the motor inertia and load a
45. us current voltage and BEMF voltage INTERRUPT void PTUTriggerlDone ISR void timeOldBEMF timeBEMF timeBEMF TIMOTCNT PTUIFL PTUIFL TGIDIF MASK Clear flag Figure 17 Obtaining the time of the BEMF measurement in the PTU Trigger Done ISR The PTU Trigger Done interrupt service routine ISR is activated when all triggers in the PTU trigger list have been scheduled The PTU PTU trigger list and ADC command lists are configured such that the PTU Trigger Done interrupt service routine is executed when the BEMF measurement takes place Figure 16 In the ISR the time of the BEMF measurement taken during the previous cycle is backed up and the counter value of the free running timer is stored as shown in Figure 17 Both timestamps are needed for the calculation of the zero crossing point 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 22 Freescale Semiconductor Software implementation on the MC9S12ZVML128 4 2 3 2 State variable acquisition and zero crossing detection processing For state variable acquisition and zero crossing detection processing the ADCO Done interrupt is used The interrupt service routine is executed when the ADCO and ADCI reach the end of the ADC command list measurements of the phase voltage and the DC bus voltage are the last commands in the ADC1 and ADCO command lists respectively The phase voltage and the DC bus voltage are measured simultaneously on th
46. ynchronized to the incoming reload event This reload event resets and restarts the internal time base counter and makes sure that the first trigger value from the actual trigger list is loaded Furthermore the corresponding ADC is informed that a new control cycle has started If the counter value matches the current trigger value then a trigger event is generated In this way the reload event is delayed by the number of bus clock cycles defined by the current trigger value All acquisition time values are stored inside the global memory map that is inside the system memory as a three dimensional array of integers PTUTriggerEventList The exact location of the acquisition time values PTUTriggerEventList in the system memory is given by the linker command file and linked to the PTU module during the initialization phase 3 phase Sensorless BLDC Motor Control Kit with the S12 MagniV MC9S12ZVM Rev 1 16 Freescale Semiconductor Software implementation on the MC9S12ZVML128 Each TG uses only one list to load the trigger values from the memory The pointers for the primary TGOLOIDX TG1LOIDX and alternate TGOLIIDX TG1ILIIDX lists are equal The TG uses only one physical list of trigger events even if the TG logic is switching eee both pointers At the end of the trigger list the interrupt for TG1 is enabled PTUIEL_TGIDIE 1 This interrupt is used to store the time of the ADC measuremen
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