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Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
Pierre Voultoury Digital Signal Processoring Solutions
Abstract
Brushless (BLDC) variable-speed drive widely used because particularly high mechanical power density, simplicity, cost effectiveness. software rotor position generation been implemented Controller family, thus saving mechanical position sensor cost. Based non-fed phase back electromotive force measurement, this softwarecomputed rotor position fully integrated able adapt itself BLDC drive-inherent problem. This application report presents successively sensorless principle theoretical background, implementation rotor position determination, BLDC drive imperfection handling operating system, every necessary experimental result, thus leading adaptable simple cost-effective drive.
Contents
Introduction BLDC Drive Presentation Sensorless Rotor Position Getting Conclusion References.16
Figures
Figure Figure Figure Figure Figure Figure Figure Figure TMS320C242-Based Sensorless BLDC Drive Exhaustive Sensorless Additional Hardware.5 Strategy Operating System Zero Crossing Results Different Running Speeds.9 Computed Commutation Instants Hall Effect Sensor Output.11 Balanced Unbalanced Phases' Use.12 Phases' Imbalance Correction Module Phases' without with Imbalance Corrector.14
Digital Signal Processing Solutions
December 1998
Introduction
trapezoidal control electronically commuted permanent magnet synchronous drive (BLDC drive) widely used because particularly high mechanical power density, simplicity, cost effectiveness. based speed regulation interface performing CPU, current regulation position sensor synchronize both stator rotor fluxes (commutation signals). improve system cost effectiveness, many solutions exist that remove necessary cost-ineffective rotor position sensor. software solution presented here Back measurement-based method commutation BLDC motor requiring rotor position sensor additional silicon (IC, ASIC, MCU). This speed controlled sensorless BLDC drive thus integrates single chip solution only mandatory control processing power software commutation signal generation well. This accomplished using only Texas Instruments digital signal processor (DSP) Controller family resource. Furthermore, this solution uses customizable software resolve BLDC drive inherent problems: electrical behavior commutation points, phase's imbalance selfcorrection, closed-loop control from zero speed. neutral point voltage computation performed every current regulation loop thanks versatile Controller unit. Bemf zero crossing point determination necessary shift between commutation signal detected zero crossing point performed real time thanks high core computational power. phases' imbalance, mostly non-symmetrical behavior three-phase system, corrected thanks software offset (computed function actual rotor speed) Bemf computation. highly disturbing current voltage commutation glitches filtered customizable software that they disturb correct operation software position computation.
BLDC Drive Presentation
Brushless drive been used industrial applications many years because excellent performance, extreme control simplicity, high cost effectiveness. strategy presented this report very close existing method includes same control simplicity, performance, additional system cost reduction.
BLDC Motor
permanent magnet synchronous machine used this drive three-phase, Yconnected motor. There brushes rotor phase's commutation must performed electronically. magnetic non-salient pole pair rotor. stator made ironless windings. stator phase inductance 0.045mH (measured 1kHz) phase resistance 300mW. maximum permissible current 5000rpm 2.9A torque constant equal 11.8mNm/A. considered have trapezoidal back electromagnetic force (Bemf) waveform shapes supplied with direct currents. voltage 18V.
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
Control Algorithm
Brushless motor speed control requires three control layers performed. innermost determines rotor position commute correctly stator flux. Once rotor position known, magnitude stator flux generated controlled. Assuming that stator flux proportional current flowing stator coils, control stator flux magnitude equivalent control input current. most outer control loop speed regulation loop. Most time, mechanical angular position sensors obtain rotor position. wellknown incremental encoders Hall effect sensors represent most chosen solutions obtaining rotor position. Incremental encoders provide very high angular resolution well accurate derived speed feedback speed range. They ideally suited highly precise speed and/or position control. Nevertheless, sensors such those based Hall effect often preferred incremental encoders when speed range (above 50rpm), even they provide performances comparable encoders' ones. reasons this rough position information required brushless drive cost effectiveness this drive type. Even cheap Hall effect sensors removed replaced Controller software with additional glue logic, resulting cost optimized drive. This solution provides Hall-effect sensors output equivalent position information. TMS320C242 algorithm presentation shows high robustness software rotor position detection: handling electrical behavior commutation points, phases' imbalance selfcorrection, closed-loop startup sequence. Depending motor electrical time constant, current regulation might performed either analog, thanks Hysteresis-type regulator realized with operational amplifiers, digital, thanks software. choice analog solution constraining because forces power board support these regulators. This fixed need having high chopping frequency maintain continuous phase current into motor with very small electrical time constant. Assuming that digital solution also supports these high chopping frequencies, current regulators might also numerically performed. This integration shown this report. Regarding speed regulation, digital integration appears once again efficient solution. speed feedback computed based clock signal given software position sensor. Once this feedback computed, speed regulation might performed according speed reference, which acquired sampling analog input serial link. This section showed that complete speed control integrated single Controller, thus avoiding need wire separate external silicon performing features, such PWM, position determination. This integration leads costoptimized board that includes both single chip controller power stage. replacing mechanical position sensor with software avoiding need additional integrated circuit (operational amplifiers, comparators, FPGAs), Controller family further optimizes overall drive cost.
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
Hardware Platform
approaches should considered when focus hardware platforms. These platforms different, depending whether BLDC drive under development about sent production. During development, control hardware used TMS320F240 Evaluation Module (EVM) introduced Texas Instruments. board contains following components:
TMS320F240 controller oscillator JTAG RS232 link Four digital-to-analog converters Connectors that output Controller pin.
board simply wired dedicated power converter BLDC motor [6]. production, possible integrate single board TMS320C242 Controller well power electronic elements. board shown Figure thus provides highly integrated sensorless solution speed-controlled BLDC drives. Notice that glue logic necessary, other than mandatory current-sensing amplifier.
Figure TMS320C242-Based Sensorless BLDC Drive
+18V NTC1 10TQ045 560R 220R 0.5W 120R 1.5W 2200uF 220R 220uF LM7805A 100nF +15V
100nF
100nF
TLP721 100nF
330R
100nF
+15V
10BF40 10BF40
10BF40
IRFIZ24G
IRFIZ24G
IRFIZ24G
100nF 100nF 100nF 560R 220R 560R 220R 330nF 330nF
CMP1 CMP2 CMP3 CMP4 CMP5 CMP6 HIN1* LIN1* HIN2* LIN2* HIN3* LIN3* Fault* FLTCLR* ITRIP 22uF
XINT2 IOPA2 CLKOUT/IOPD0 BIO*/IOPC1 XF/IOPC0
SCITXD SCIRXD
GNDA VREFLO Vsso Test VREFHI Vcca
WDDIS
20pF
Vssdd1 Vssdd2 Vssdd3 Vssdd4 Vssdc1 Vssdc2 Vssdc3 IOPB7 IOPB6 IOPB5 IOPB4 XTAL2
Vccdd1 Vccdd2 Vccdd3 Vccdd4 Vccdc1 Vccdc2
IOPA5 IOPA4 IOPA3
TMS320C242
ADCIN07 ADCIN06 ADCIN05 ADCIN04 ADCIN03 ADCIN02 ADCIN01 EMU1 EMU0 TRST* ADCIN00 IOPC7 IOPC6 IOPC5 IOPC4
IOPC2 PDPINT IOPC3
4.9MHz
IR2131 IRFIZ24G IRFIZ24G IRFIZ24G 560R 220R 330nF
20pF
XTAL1
10uF 1N4148 330R LL4148 100nF
+15V
100K
This board single layer with discrete power elements further increase system cost reduction. board allows accurate sensorless speed control from 100rpm maximum permissible motor speed.
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
TLC277
Figure details exhaustive hardware necessary presented sensorless control. This shows that system cost saving highly efficient mechanical position sensor fully replaced terms hardware extremely cost resistors. Notice also that this hardware might easily freely modified different motor characteristics. This makes this sensorless dedicated hardware upgradable versatile.
Figure Exhaustive Sensorless Additional Hardware
R520 560R R510 560R R511 220R C510 330nF R521 220R C520 330nF
R500 560R R501 220R C500 330nF
resistance bridge sized that maximum bridge output addresses full conversion range. reference voltages zero voltage 18V, bridge ratio should equal 0.27. Note that TMS320C242 Controller Unit requires 260W input impedance analog line conversion unit full speed. filtering capacitor should filter only chopping frequency. sensorless algorithm only based three terminal voltage measurements thus requires only four input lines.
Sensorless Rotor Position Getting
This core section presents software solution able deliver classical BLDC drive control necessary rotor position without help other than C242 Controller capabilities (this means additional Ics, star connection wired motor housing). following section explains succinct theoretical background every practical aspect.
Theoretical Background
so-called BLDC control, phases commuted once every mechanical (number pole pairs equals one) rotation rotor time only phases with direct currents. Furthermore, efficient control implies synchronization between phase Bemf phase supply that Bemf crosses zero once during non-fed sector.
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
formula below depicts motor terminal model, where phase inductance, phase resistance, back electromagnetic force, star connection voltage referenced ground phase voltage referenced ground. voltages measured thanks controller Unit resistor bridge depicted above.
only currents flow stator windings same time, phase currents opposite third equal zero. Furthermore remembering that both three stator currents three Bemf equal zero, neutral point voltage calculated follows:
Regarding non-fed phase (zero current flowing) stator terminal voltage rewritten follows:
Vnonfed Vnonfed
each Bemfs crosses zero times mechanical revolution, Bemfs numerically easy compute thanks Controller, possible necessary commutation information.
Practical Point View
This section presents practical implementation above theory. discusses particular theory limitations practical solutions used overcome them. Each software module also addressed.
Neutral Point Voltage Computation
shown above, neutral point voltage computation requires knowing three instantaneous terminal voltages referenced ground. explain managed perform this real-time task. very small phase electrical time constant, chopping frequency been 80kHz. period then equal 12.5ms. current regulation loop period been 50ms. four period flags occurs current loop period. Controller Core acknowledges them corresponding interrupt subroutine served. Only current period first second (among four available during current control loop), double conversion started software. Conversion interrupt core asked, acknowledged served handle results. following plot depicts interrupt organization.
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
Figure Strategy Operating System
Current Loop Period
Period Conversion Interrupt
Interrupt
12.5
6.6ms
37.5
Time
first conversion gives phase flowing current second phase terminal voltage information. Once these results have been handled, input channels dedicated other phase terminal voltage measurements selected. second Conversion subroutine handles conversion results reload current second terminal voltage channels hand computes neutral point voltage according above formula other hand. This solution assumes that variation phase terminal voltage negligible during period (12.5ms). Note that this real-time sampling structure fits chopping regulation frequencies: motor requiring 20kHz chopping frequency easily imagine current regulation loop frequency 10kHz still having three terminal voltages.
Bemf Zero Crossing Point Computation
Once neutral point voltage available necessary Bemf non-fed phase. This realized subtracting computed neutral voltage from non-fed phase terminal voltage. interest focused zero crossing Bemf possible look only Bemf sign change; this assumes that Bemf scanning loop period much shorter than mechanical time constant. This function computed after three terminal voltage samplings, once every 50ms, during during duty cycle update.
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
Electrical Behavior Commutation Points
phases' commutation instants high dV/dt dI/dt glitches might occur direct current level parasitic inductance capacitor power board. Beyond electromagnetic interference troubles they might generate, these glitches lead misunderstanding computed neutral voltage. solution avoid this problem drastically filter phase voltages before acquiring them. major filtering drawback that necessary filter introduces consequent Bemf zero-crossing detection. This leads phase supply synchronization then poor drive dynamic behavior power conversion efficiency. phase filtering discarding first scans Bemf once phase commutation occurs solves these electrical behavior troubles commutation. discarding, should understood that neutral voltage still calculated zerocrossing detection function disabled, thus avoiding spurious detection. discard duration fully customizable, this software module. duration depends power switches, power board design, phase inductance driven direct current. This parameter system dependent should large value early development stages. Later possible tune very small discard duration order achieve better control.
Some Bemf Zero Crossing Results
each scope picture presented below channel represents phase current, channel corresponding half bridge voltage, channel computed neutral point voltage channel gives peak each time software detects zero crossing scanned Bemf.
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
Figure Zero Crossing Results Different Running Speeds
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
different speeds tested here 2000rpm, respectively. Notice that Bemf zero crossing algorithm been successfully tested down 30rpm. most interesting information given neutral voltage computation: even there very high commutation glitches and/or measurement noises, software understands them glitches, thus allowing detection right Bemf sign change. This achievable because high frequency computing neutral point voltage. This high frequency reached thanks autonomous peripheral high Controller power. channel spikes (representing Bemf zero crossing) located middle non-fed sector; this synchronization between Bemf sector primary reason highest motor performances achieved.
Commutation Instants Computation
efficient sensor control, Bemf zero crossing events displaced from phases' commutation instants. before running sensorless BLDC motor with help zero crossing events necessary compute commutation instants shift. fact according different desired speed ranges angle shift might angle. position interpolation function should realized. this control software implemented follows: time that rotor spent complete previous revolution desired shift angle. dividing multiplying result obtain amount time (let call shift time) spent before commuting phases' pair. question asked regarding transient response system. Assume that motor slows down. does control react? computed shift time will short comparison with actual necessary shift time; this early coming commutation instant will tend accelerate motor. controller will have opposite reaction motor accelerates. kind natural robustness control algorithm added speed control loop. presented software, fixed imagine additional software function giving shift angle output taking mechanical speed input. Figure presents charts showing commutation instant computation compared three Hall-effect sensor outputs. chart shows results speed (400rpm) bottom chart results high speed (2000 rpm).
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
Figure Computed Commutation Instants Hall Effect Sensor Output
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
These charts show that very good results achieved high speed. slight shift between Hall sensor output computed commutation noticeable; phases' imbalance corrector will compensate this. This commutation instant algorithm based time spent rotor achieve previous revolution. noise system dynamics troubles possible design some slightly more complex algorithms: example, pass filter speed feedback could improve transient behavior. simply implemented saving revolution time stage revolution time stage k-1, then computation
gives smoother change shift time, thus improving transient behavior. Another possible algorithm that improves dynamic behavior computes time spent rotor achieve third revolution. This last algorithm quicker reaction speed variation much more sensitive measurement noises.
Phases' Imbalance Self Correction
Some imbalance phases' might present when simply apply above algorithm (detect zero crossing point wait shift time elapsed before commuting phases). imbalance understand that instead having equivalent sectors revolution have wide wide sectors, shown Figure
Figure Balanced Unbalanced Phases'
This imbalance created non-symmetrical behavior three-phase system terminal voltage measurement resistor bridges. imbalance applicationdependent approximated linear function mechanical speed corrected thanks software offset Bemf computation. following implemented structure shown Figure solves this trouble.
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
Figure Phases' Imbalance Correction Module
Position
Corrected Bemf
Neutral Computation Bemf Computation
Imbalance Corrector
scope pictures Figure show phases' with (right chart) without this additional correction module (left chart).
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
Figure Phases' without with Imbalance Corrector
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
These charts make necessity Bemf computation correction obvious.
Startup Procedure
first step start sensorless drive initial rotor position. position determination strategy differs depending whether motor reluctance variation around not. there significant reluctance variation around (for example, salient poles rotor) measure each phase pair current response voltage pulse applied each winding short constant period time. evaluating relative magnitude current flow through phase windings possible initial rotor position thus determine proper starting phase motor. This solution ensures start rotation direction. When there reluctance variation around motor case with motor used this drive), might impossible estimate initial rotor position. startup necessary energize arbitrary phase pair wait rotor aligned with created stator flux. Nevertheless, algorithm must address following questions: long should algorithm wait shaft stop oscillating prior starting control loops? applied torque enough load move? this application counter that magnetic stall performed long enough shaft move stop oscillating. current flowing supplied phase pair regulated. further improve this sensorless drive, above questions might answered scanning non-fed phase interpreting results follows: oscillating measured voltages means back forward rotor moves, constant measured signal means rotor move. Such interpretation leads adaptive startup phase current level time optimized initial rotor position obtaining function. second step consists accelerating motor thirty rpm, which point this sensorless algorithm able closed loop. Here also solutions possible depending whether load unknown not. load characteristics unknown, then shift time value must that enables first revolution. This implies maximum motoring torque long shift time. every first revolution Bemf zero crossing comparison between time elapsed between commutation zero-crossing shift time gives software good model system dynamics. first shift time might adapted within first revolution. effective improvement this method might thus implemented when first revolution shift time priori calculated. This requires knowing motor torque constant braking torque (including friction losses load) startup. fundamental dynamic principal applied rotational systems gives
where
system inertia angular position represents torque gives double integral equation.
solve this equation needs know system torque inertia. torque might considered constant known control flowing current know torque constant ([Nm/A]). system inertia calculated from load characteristics. This equation's solution time necessary motor perform revolution. This first shift time then given relation:
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
This calculated value should then divided Bemf scan loop period result should stored into software shift time variable. Once these first steps have been performed, initial rotor position adjacent sector with Direct Current Bemf zero crossing detection algorithm immediately started. After complete revolution, computed shift time replaces initial one.
Conclusion
This document discussed software calculation rotor position sensorless Brushless drive. This single-chip solution based TMS320x24x Controller family capabilities: high core power highly versatile peripherals. This solution replaces position sensor cost with software. Furthermore, this solution increases drive reliability, position information will temperature- vibration-sensitive longer. This makes sensorless solution more cost effective more reliable. Assuming that initial rotor position detectable knowing that this solution runs closed loop from quasi zero speed, this sensorless drive advantageously replace sensor solution wide range speed control applications.
References
TMS320C24x Controllers Reference Set: Vol.1, Texas Instruments Inc, 1997. TMS320C24x Controllers Reference Set: Vol.2 Texas Instruments Inc, 1997. Implementation Sensorless Speed Controlled Brushless Drive using TMS320F240, Texas Instruments Inc., 1997 part #BPRA072. Solutions BLDC Motors, Texas Instruments Inc.,1997 part #BPRA055. 12-24V Three Phase Power Hardware PMSM Induction Machine, Texas Instruments Inc.,1997 part #BPRA071.
TMS320C24x Controllers Evaluation Module, Texas Instruments Inc., 1997 #SPRU248A.
Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
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Sensorless Speed Controlled Brushless Drive using TMS320C242 Controller
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