A Feasible Approach to the Evaluation of the Tractions of Vehicle Wheels Driven by DC Motors

Transcription

1 A Feasble Approach to the Evaluaton of the Tractons of Vehcle Wheels Drven by DC Motors Jeh-Shan Young Insttute of Vehcle Engneerng, Natonal Changhua Unversty of Educaton Changhua, Tawan, R.O.C. and Sheng-You Wu Insttute of Vehcle Engneerng, Natonal Changhua Unversty of Educaton Changhua, Tawan, R.O.C. ABSTRACT Ths paper proposes an approach to evaluate the tractons of the vehcle wheels drven by DC motors drectly. The database of the DC motors, such as current versus voltage, PWM and loadng, has to be bult through the automatc data acquston system. Instead of the slp rato measurements, the evaluaton of the tractons can provde the force nformaton of the tracton wheels by usng the nterpolatons of the database wth appled voltages, PWM sgnals and measured currents of the DC motors. In order to verfy the feasblty of ths approach, the automatc data acquston system has been mplemented for a sngle tracton wheel ncludng the database establshments of DC motors. Ths system should assure that the commands and data transmtted are always correct. The detals of the unts n the system are dscussed such as ntalzatons of unts, flow chart of PWM command transmssons, etc. The motor loadngs are also evaluated wth specfc appled voltages, PWM values and measured currents. The result shows that t s feasble to evaluate the loadngs through the nterpolatons of the database of the DC motor. Keywords: Communcaton protocol, DC motors, pulse-wdth modulaton (PWM), slp rato, tracton forces. 1. INTRODUCTION The global envronmental protecton becomes one of the most sgnfcant ssues for the studes and developments of new alternatve products, the vehcles of the transportaton especally. For nstance, the electrc vehcle (EV) drven by drect current (DC) motor s a feasble and potental canddate for the future transportaton vehcles. Comparng wth tradtonal vehcle (nternal combuston engne vehcle, ICV), EV does not need any addtonal costly hardware to acheve the engne control, brake control, or msson control [1]. For example, antlock brakng systems (ABS) of vehcles have receved a great concern and become one of the most common automotve technologes [2-9]. The am of ABS s to control the wheel slp so as to attan the optmum values that can provde maxmum negatve tracton force durng heavy brakng and mantan the suffcent stablty and steerng ablty. For most road surfaces, the maxmum tre-road adhesve coeffcent s at the slp rato ranged from 0.1 to 0.3 [10]. When a vehcle brakes, the ABS wll prevent the wheels from lockng and keep the wheel slp rato wthn a specfc range. Most ABS control wll retan the wheel slp rato at a value 0.2 as compromse [2]. However, the wheel slp rato s dffcult to be measured from the speed of the vehcle. In addton, the optmal wheel slp ratos hghly depend on the dfferent road surfaces and road condtons. Fujmoto and Fuj [11] patented ther slp rato estmatng devce that can measure the slp rato wthout usng the speed of a vehcle body and can be utlzed by the so-called the slp raton control devce. Nakao also proposed a method and program for judgng road surface condtons [12], Ths method and program can determne the slp rato from wheel rotaton speed. The purpose of the measurement of slp rato n ABS s to evaluate the forces between the tre and the road surface. The measurement of slp rato s stll a tough ssue for ICVs snce the exact speed of a vehcle s dffcult to measure although there are some feasble ways to estmate the slp rato[11, 12]. On account of the rapd mprovements and developments n motors and batteres, the EV has been the most potental alternatve to the tradtonal vehcles. In addton, EVs have attracted great nterests as a feasble soluton aganst the problems of envronment protecton and energy depleton. One of the advantages of the EV s that ts motor torques can be measured easly [9]. Instead of the wheel slp rato measurements, the tracton evaluatons of the n-wheel motors can drectly provde the nformaton of the tractons of the vehcle s wheels drven by motors. They can be utlzed to stablze the steerng of the vehcles travelng nto other road surfaces such as a wet or slppery road surfaces. Accordngly, ths paper proposes an approach to measurng the tractons of a sngle wheel based on the database of the permanent magnet (PM) drect current (DC) motors. We set up an automatc data acquston system that can collect the database for a DC motor. The tracton can be evaluated by nterpolatons from the appled voltages, PWM values, and measured currents of the DC motor. The result shows ths approach s feasble. Secton 2 brefly dscusses the force balance and the knematcs of a vehcle. Some related characterstcs of PM DC motors are also addressed n ths secton. Secton 3 states the constructon of the automatc data acquston system. In addton, the mplementaton of the system s also descrbed n ths secton. The detals of the data acquston system are gven n Secton 4 ncludng the flow charts of the ntalzaton and command transmssons. Conclusons are drawn n

2 Secton FORCE BALANCE AND KINEMATICS OF VEHICLE Recently, the studes of the relatons between frcton coeffcent and slp rato, so called the μ - λ curves (frcton coeffcent versus slp rato), become one of man streams n the vehcle stablty control such as slp rato estmaton, control of ABS, etc [11-13]. In Fgure 1, ω and r respectvely denote the rollng angular velocty and radus of the specfed wheel ndexed. The slp rato s defned as follows. ( rω vx )/ rω ( rω > vx ); s = ( rω vx )/ vx ( rω < vx ), where s s ranged from -1 to 1. means the vehcle s > 0 gans the tracton from correspondng wheel, whle s < 0 as the correspondng wheel s n brake stuaton. The slp rato s regarded as one of the most sgnfcant parameters for the qualty of vehcle steerng control. There are at least two ssues to be concerned. One s that the road surfaces are manfold and tres are, too. It s hard to characterze all the condtons between tres and road surfaces. The other s that the speed of the vehcle s dffcult to be measured actually. That s, evaluatng the slp rato s qute mpossble for ICV n real tme. However, the ams of the sensors to measure the slp rato, of algorthms to evaluate the slp rato, or of the estmatons of the slp rato are to provde the nformaton of tracton forces for wheels. Furthermore, the dynamcs and knematcs of a vehcle can be analyzed f the forces of the tracton wheels are known. (a) Sde vew. (b) Top vew. Fgure 1. Knematc model of a wheel. By the Newton s Law, the force balances of the tracton wheels govern the dynamcs and knematcs for a vehcle as shown n Fgure 2. The dynamcs of the vehcle hghly depends on the tracton forces provded by the wheels such postons, veloctes, and acceleratons of lnear and rotatonal motons. In case the forces of all wheels are well evaluated, the measurements of slp rato are not necessary and the steerng capablty and stablty of vehcles can be acheved ntutvely. So far, EV s entrely drven by the electrcal motors (such as PM DC motors) whose torques generated can be controlled much more quckly and precsely than those of nternal combuston engnes [14]. For nstance, the relatonshp between torque and current s very smple for a PM DC motor. It can be expressed n the followng equaton. T = KI A, where T s the mechancal torque, K s the motor constant, and I A s the armature current. The torque can not only drve ts armature and shaft but also provde the appled loadng torques and result n the angular acceleraton of wheel. The torques from PM DC motor can be evaluated wthout any addtonal sensors f the database of ths PM DC motor s establshed such as the current-pwm-torque data wth specfed appled voltage. That s, the tractons can be evaluated from nterpolatons wth the PWMs, currents, and voltages appled to the motor. Fgure 2. Knematcs and Force model of a vehcle. 3. THE AUTOMATIC DATA ACQUISITION SYSTEM OF TARGET DC MOTOR Motors are classfed as ether alternatve current (AC) or DC, whch depends on the type of power appled to them. DC motors have well speed-control capablty whle AC motors generally run at a fxed speed determned by the frequency of power. The feld flux of a PM motor, a knd of DC motor, retans constant regardless of speed. Ths fact makes for a very lnear torque-speed curve for ths knd of motors. Ths s proftable n control applcatons because of ts smplcty n control. In addton, PM DC motor wll become a feasble and potental canddate of the EV tracton wheel drvers. In steady of analog-drve system of a DC motor control, the pulse-wdth modulaton (PWM) s a completely dfferent approach to controllng the torque and speed of a DC motor. It s dgtal nherently and can be controlled from a computer wth a sngle pn of mcro control unt such as sngle chp, etc The speed of a DC motor depends on the percentage duty cycles of PWM sgnals. The armature current of a DC motor wll ncrease as the torque ncreases. The force nformaton for a DC motor can be obtaned from the relatons between the percentage PWMs, currents, and voltages appled to ths DC motor. It s feasble to develop the force evaluaton algorthm from those above data. In general, the characterstcs of a PM DC motor are tme varant although they are not qute dfferent durng a short nterval. The database of a PM DC motor wll become more relable f t s updated n short perods when the vehcle needs mantenance servces. Therefore, a quck data acquston system for motors s ndspensable. In ths paper, we construct an automatc data acquston system for PM DC motors. The system wll facltate the data acqustons for PM DC motors. Fgure 3 shows the functon block dagram of the automatc data acquston system for PM DC motors. PWM commands and the torque settngs of loadngs can be generated by the dedcated program n personal computer (PC) sequentally and automatcally. When the motor control unt (MCU) receves

3 PWM commands from PC, t wll follow these commands and send the correspondng PWM sgnals to drve the motor n approprate actons wth power supply. MCU also measures the voltages and the currents appled to the motor and the rotaton speeds (revolutons per mnutes, RPM) of the motor. The other motor whch can be set by PC and s able to be controlled n order to keep constant torques provdes the specfed torque loadngs. In addton, the torque sensor feeds back the measured torques to PC. The data acquston unt can acqure and save the data of PWM commands, voltages, currents, RPMs, and torques as well as establsh a database of the measured motor. Fgure 5. The characterstc curves for IG42GM Motor n 12-V[16]. Fgure 3. The functon block dagram of the data acquston system for DC motors. A low-cost experment test bed s employed so as to verfy the feasblty of the proposed approach. The test bed ncludes a personal computer (PC) as the data acquston unt and the command generator wth a dedcated program, a MCU (as shown n Fgure 4) as the motor control unt, a dgtal tachometer to measure the RPM of motor, a PM DC Motor wth ts stand and controller, a current sensor chp (INA138)[15], and the test weghts. Ths test bed utlzes a PM DC motor (IG42GM Motor)[16] and ts motor controller (Pololu SMCO4A)[17] as show as Fgure 5 and Fgure 6, respectvely. Ths motor s used to smulate the motor of the EV tracton wheels although t provdes a small maxmal torque. Some approprate weghts ranged from 0.2-kg to 2-kg are hung on a 5-cm radus pulley whch s mounted on the motor shaft. Substtutng the motor for constant torque rotaton and torque sensor, the weghts smulate the loadngs, or tractons, on the motor though the frcton coeffcent between the pulley and the rope s unknown. The loadngs could be proportonal to the weghts. That s, the database of the motor for the loadngs, or tractons, wll be those proportonal ndces nstead of real loadngs. Besdes, the frcton coeffcent may be varyng a lttle n operaton. However, t wll not cause any sgnfcant problem because of the lght weghts. Fgure 4. The motor control unt (MCU) for the data acquston system. Fgure 6. The motor controller (Pololu SMCO4A)[17]. Fgure 7 shows the mplementaton of the automatc data acquston system test bed of the PM DC motor. The dedcated program n PC can automatcally generate the PWM commands sequentally and transmt these commands to MCU whch can transfer these commands to correspondng PWM sgnals. The motor controller mounted on the MCU prnt crcut board can receve the PWM sgnals va ts TTL seral nput and drve the motor at specfed speeds accordng to the PWM sgnals. The current sensor and the dgtal tachometer measure the appled currents of the motor controller and the RPMs of the motor rotaton speed, respectvely, for dfferent loadngs, dfferent PWM sgnals, and dfferent appled voltages. The analog sgnals of the appled currents measured can be converted nto dgtal sgnals by the analog to dgtal (A/D) converter embedded n the mcro control unt. MCU can receve the dgtal RPM sgnals va seres port, or RS232, through ts seres port (RS232). MCU wll also transmt the data of the currents and RPMs to PC wth the specfed weghts after the currents settle to steady states. PC acqures these data and automatcally sends another PWM command to MCU n order to acqure another correspondng data from the target PM DC motor untl the full range of the specfed PWM sgnals s fulflled. The RPMs of the motor hghly depend on the PWM sgnals. They are hardly affected by dfferent loadngs. Therefore, we take down the values of appled currents, PWMs, and torques as database whle the correspondng values of RPM are as reference. The mesh dagram of the database whch s measured for the PM DC motor by the automatc data acquston system test bed s llustrated n Fgure 8 wth 12-V appled voltage. The unknown loadng hung on the pulley of the motor can be evaluated from the database by nterpolaton. For nstance, an unknown weght s appled whle the percentage PWM sgnal and the appled current measured are 63% and A, respectvely. Then, the weght can be evaluated as kg through nterpolaton from the PM DC motor database.

4 (a) Top vew of the test bed. (b) Front vew of the test bed. sgnal from PC when PC s on. The perodcal transmsson s used to prevent the data passed errors when PC s not ready to receve any sgnal. MCU s ready to receve any command form PC as long as t receves OK sgnal but Error. PC wats for the Ready sgnal from MCU as t s ntalzed. Besdes, t wll check whether the sgnal s correct. If the sgnal s ncorrect, PC wll respond Error sgnal to MCU. Otherwse, PC wll respond OK sgnal to MCU and wll be ready to transmt other commands to MCU. (c) The man program n PC. Fgure 7. The automatc data acquston system test bed of the DC motor. Fgure 9. The flow chart of the communcaton protocol n system ntalzaton. Fgure 8. The sketch of Current versus PWM (%) and Loadng (Kg) for IG42GM PM DC Motor. 4. THE DETAILS OF THE AUTOMATIC DATA ACQUISITION SYSTEM The automatc data acquston system ncludes two man unts. One s the PC (wth the dedcated program) whch generates the PWM commands and saves the measured correspondng data of the target motor. The other s the MCU whch receves the PWM commands, transfers the PWM commands to the compactable PWM sgnals to control the target motor, transmts the measured data to PC. The communcaton protocol between these two unts becomes more sgnfcant. We adopt the shake-hand mechansm so that the data acquston system can work wthout consderng the order of turnng on these two unts. In addton, ths system can measure the correspondng data automatcally and sequentally by the approprate commands of the motor. The bref logc of communcaton protocol wll address as follows. Fgure 9 shows the flow chart of the communcaton protocol as these two unts are ntalzed. In ths fgure, MCU transmts Ready sgnal to PC perodcally untl t receves the OK The procedure of data measurements can be undertaken by the data acquston system wth the PWM commands transmssons after the ntalzaton of PC and MCU. However, ths acquston system should assure that the transmssons of the measured data are always correct. The shake-hand mechansm has been also adopted n the data acquston system so as to prevent the measured data from the error transmssons. Fgure 10 shows the flow chart of the communcaton protocol n commands transmssons from PC and the actons as well as the reactons between PC and MCU. Frst of all, PC transmts the approprate PWM command to MCU and wats for the response from MCU. If the PWM command s correctly receved by MCU, MCU wll transmt OK response to PC, decode the PWM command and send the correspondng PWM sgnals to the motor controller va the TTL nput of the motor controller. Otherwse, PC wll receve Error response from MCU. In case that PC receves Error response from MCU, t wll transmt the same PWM command agan untl t receves OK response from MCU. Then, PC wll wat for the measured data from MCU. In the meantme, MCU wll measure the appled current of the motor from the current sensor after t sends the PWM command to motor controller. When the current settles down, MCU swtches the seres communcaton, or RS232, to the dgtal tachometer n order to read the measured RPM of the motor snce MCU has only one seres communcaton port. MCU swtches the RS232 to PC f the RPM data are acqured from the dgtal tachometer. Then, MCU transmts the data ncludng the current and RPM of the motor to PC. PC wll checks whether the current and RPM of the motor are correct. If no, PC wll request MCU to send the same data agan untl these data transmtted are correct. If yes, PC wll save these data and transmt another PWM command f necessary.

5 The objectve of the automatc data acquston system for PM DC motors s to facltate the procedure to acqure the data of tracton motors of vehcles so that the updated database can be utlzed n the steerng control, ABS, etc. For nstance, we can utlze these databases of the motors to smulate the dynamcs of a vehcle wth ths knd of the tracton motors, to control the steerng of ths vehcle, or to equp the ABS for the vehcle wth the forces feedback. (wth dedcated program) and MCU can assure the correctons of the command transmssons. We also mplemented a low-cost experment test bed to verfy the feasblty of the concept. It shows that ths approach can be mplemented. ACKNOWLEDGEMENT Ths research work s supported by Natonal Scence Councl under Grand NSC S REFERENCES Fgure 10. The flow chart of the communcaton protocol between PC and MCU. 5. CONCLUSIONS The result shows the forces of the tracton wheels between tres and road surface can be evaluated based on the database of the specfed motors. It s ndependent of what knds of tres or road surfaces they are. In addton, the slp rato measurements for the tracton wheels are not necessary snce the force nformaton can be evaluated by the nterpolatons of the database drectly. However, ths approach s not possble for the vehcles wth nternal combuston engne but for EVs wth tracton PM DC motors on account of the quck responses of appled currents, voltages, and PWMs for PM DC motors. Furthermore, the steerng capablty and stablty of vehcles can be mproved by feedback or logcal control ntutvely. We also provded the confguraton of the automatc data acquston system n order to facltate the establshments of the database for PM DC motors. The system wll make rapd upgrades of database for tracton wheels of vehcles feasble n the near future. The communcaton protocols between PC [1] Y. Hor, Y. Toyoda, and Y. Tsuruoka, "Tracton Control of Electrc Vehcle: Basc Expermental Results Usng the Test EV UOT Electrc March," IEEE Transactons on Industry Applcatons, vol. 34, pp [2] Y. S. Kueon and J. S. Bed, "Fuzzy-seural-sldng mode controller and ts applcatons to the vehcle ant-lock brakng systems," n Proceedngs of IEEE/IAS Conference on Industral Automaton and Control: Emergng Technologes, 1995, pp [3] P. Khatun, C. M. Bngham, P. H. Mellor, and N. Schofeld, "Dscrete-tme ABS/TC test faclty for electrc vehcles," n Proceedngs EVS-16, Chna, [4] P. Khatun, C. M. Bngham, and P. H. Mellor, "Comparson of control methods for electrc vehcle antlock brakng/tracton control system," n Proceedngs at the SAE 2001 World Congress, Detrot, MI, 2001, p. Paper [5] X. He and J. W. Hodgson, "Modelng and Smulaton for Hybrd Electrc Vehcles Part I: Modelng," IEEE Transactons on Intellgent Transportaton Systems, vol. 3, pp , [6] X. He and J. W. Hodgson, "Modelng and Smulaton for Hybrd Electrc Vehcles Part II: Smulaton," IEEE Transactons on Intellgent Transportaton Systems, vol. 3, pp , [7] R. Saeks, C. J. Cox, J. Nedhoefer, P. R. Mays, and J. J. Murray, "Adaptve Control of a Hybrd Electrc Vehcle," IEEE Transactons on Intellgent Transportaton Systems, pp , [8] P. Khatun, C. M. Bngham, N. Schofeld, and P. H. Mellor, "Applcaton of Fuzzy Control Algorthms for Electrc Vehcle Antlock Brakng/Tracton Control Systems," IEEE Transactons on Intellgent Transportaton Systems, vol. 52, pp , [9] Y. Hor, "Future Vehcle Drven by Electrcty and Control Research on Four-Wheel-Motored UOT Electrc March II," IEEE Transactons on Intellgent Transportaton Systems, vol. 51, pp , [10] A. K. Baker, Vehcle Brakng. London: Pentch Press, [11] H. Fujmoto and K. Fuj, "Slp Rato Estmatng Devce and Slp Rato Control Devce." vol. WO/2008/029524, W. I. P. Organzaton, Ed. Japan: Yokohama Natonal Unversty [12] Y. Nakao, "Method for Judgng Road Surface Condton and Devce Thereof, and Program for Judgng Road Surface Condton." vol. 7,512,473, U. S. Patent, Ed. Japan: Sumtomo Rubber Industres, [13] W. Y. Wang, I. H. L, M. C. Chen, S. F. Su, and S. B. Hsu, "Dynamc Slp Rato Estmaton and Control of Antlock Brakng Systems Usng an Observer Based Drect Adaptve Fuzzy Neural Controller," IEEE Transactons on Industral Electroncs, vol. 56, pp , [14] T. Furuya, Y. Toyoda, and Y. Hor, "Implementaton of Advanced Adheson Control for Electrc Vehcle," n Proceedngs of th Internatonal Workshop Conference on Advanced Moton Control, 1996, pp [15] Burr-Brown, Hgh-Sde Measurement Current Shunt Montor: Burr-Brown, [16] S. Y. Ye, IG-42 Geared Motor Seres. Hsnchu: PlayRobot, [17] Pololu, Hgh-Power Motor Controller wth Feedback User's Gude (perlmnary). Las Vegas: Pololu Robotcs and Electroncs, 2005.