HALL EFFECT SENSORS AND COMMUTATION

Transcription

1 OEM770 5 Hall Effect ensors H P T E R 5 Hall Effect ensors The OEM770 works wth three-phase brushless motors equpped wth Hall effect sensors or equvalent feedback sgnals. In ths chapter we wll explan how Hall effect sensors are used n brushless motors, and how the OEM770 uses Hall effect outputs from ompumotor servo motors for commutaton. If you are usng a motor from another vendor, obtan nformaton about your motor's Hall sgnals and commutaton sequence. Then use the nformaton n ths chapter to help you connect your motor to the OEM770. HLL EFFET EOR D OMMUTTIO To move the rotor n the commanded drecton, the drve wll send current through two of the motor s stator cols. Ths current produces electromagnetc felds that develop a torque on the rotor, and the rotor turns. The rotor wll stop f t can reach a poston where ts permanent magnets are next to the magnetc felds that attract them. efore the rotor can get to ths poston, though, the drve swtches the current to a new combnaton of stator cols, and creates a new set of electromagnetc felds that cause the rotor to contnue ts movement. The process of contnually swtchng current to dfferent motor cols to produce torque on the rotor s called commutaton. If the drve knows the poston of the rotor s permanent magnets, t can set up magnetc felds n the stator that have the correct locaton and polarty to cause the rotor to turn. How can the drve know rotor poston? Three Hall effect sensors 107

2 5 Hall Effect ensors OEM770 located n the motor are affected by the rotor s permanent magnets. The three sensors transmt a unque pattern of sgnals for each rotor poston. The drve uses these sgnals to determne the poston of the rotor. THE HLL EFFET Electrcally charged partcles movng through a magnetc feld experence a deflectng force perpendcular to both the drecton of ther moton and the drecton of the magnetc feld. The Hall effect s a phenomenon whch shows that f a magnetc feld s perpendcular to a thn strp of conductve materal, and an electrc current flows lengthwse through the strp, the moble charges that carry the current wll drft to one edge as they move along the strp. In the example shown n the next drawng, assume that the conductve strp s metal. Electrons are the moble charges. Wth a current as shown n the drawng, the electrons wll move upwards through the strp. In the presence of the magnetc feld, shown n the drawng, the electrons wll drft toward the rght edge of the strp Magnetc Feld - Volts Measurng the Hall Effect Voltage The Hall Effect ecause electrons are concentrated along one edge, there s a potental voltage dfference across the strp. Ths voltage s known as the Hall effect voltage. The drawng shows a voltmeter connected across the strp to measure Hall effect voltage. 108

3 OEM770 5 Hall Effect ensors If the magnetc feld s removed, the Hall effect voltage dsappears. If the magnetc feld s reversed, the Hall effect voltage wll also be reversed. HLL EFFET EOR Many types of sensors use the Hall effect to sense the presence of magnetc felds. The next fgure s a conceptual drawng of a Hall effect sensor. Ground +5VD Output ensor shown magnfed, wth nternal components vsble Hall trp +5VD Rotatng magnetc feld affects Hall strp n sensor Ground Dgtal rcutry ctual ze (approx.) Hall Effect ensor constant current runs through a conductve Hall strp nsde the sensor. The drawng shows a rotatng magnet near the sensor. The alternatng feld from ths rotatng magnet wll cause an alternatng Hall effect voltage to be generated across the strp. Ths alternatng voltage waveform s fed nto crcutry that shapes the waveform. The output of the crcutry s a dgtal sgnal that s ether +5VD or ØVD. ensors are avalable wth a varety of output voltages and polartes. In the followng dscusson, we assume that the sensor s turned O by a south magnetc pole, and remans on after the south pole s removed. When a north magnetc pole approaches, the north pole wll turn the sensor OFF. 109

4 5 Hall Effect ensors OEM770 ote from the drawng that the sensor requres power connectons for ts nternal crcutry (+5VD and Ground). lso note that although the actual Hall effect voltage generated nsde the sensor s an analog sgnal, the output from the sensor s a dgtal sgnal that s ether O or OFF. HLL EFFET EOR UED IIDE RUHLE MOTOR There are three Hall effect sensors nsde of a motor. The next fgure shows a conceptual drawng of the nsde of the motor, and the three sensors. Hall ensor Output and Power Wres Hall #2 Output Hall #3 Output Hall #1 Output +5 VD Ground Hall ensor tator shown wthout col wndngs Rotor wth Permanent Magnets Hall ensor Locaton (hown Mounted bove tator Pole Faces) For clarty, the stator s depcted n smplfed form, wthout ts col wndngs. The Hall effect sensors are located at one end of the stator, near the pole faces of the rotor. They are postoned approxmately as shown n the fgure. Fve wres are shown for makng connectons to the Hall sensors. Three wres are for ndvdual outputs. The fourth and ffth wres are for +5VD and Ground, whch are nternally connected to all three sensors. ote that Hall #3 s postoned between Hall #1 and Hall #2. 110

5 OEM770 5 Hall Effect ensors Do ompumotor Motors Have Hall Effect ensors? Most ompumotor servo motors do not use Hall effect sensors. Instead, the motor's encoder has an extra commutaton track, wth three outputs. These outputs mmc sgnals that would be obtaned from Hall sensors; n fact, the outputs are called Hall outputs. For conceptual reasons, n the dscusson that follows we assume the motor contans Hall sensors. Keep n mnd that no matter how the orgnal sgnals are generated from sensors or from an encoder the result s the same: three output wres that delver commutaton nformaton to the drve. WIDIG I THREE PHE RUHLE MOTOR The next drawng depcts an end vew of the motor, wth the separate phase wndngs shown n ther relatve postons around the stator. The three phases share a center connecton, as the detal wthn the dotted lne shows. Phase Phase Phase Hall Effect ensor Mounted bove tator Pole Face Equvalent Motor ol chematc 3-Phase ervo Motor wth Hall Effect ensors The physcal spacng of the Hall effect sensors s very mportant. otce that one pole of the rotor can affect two sensors at 111

6 5 Hall Effect ensors OEM770 the same tme. In ths drawng, the rotor s north pole s adjacent to both Hall 2 and Hall 3. nce south turns a sensor O and north turns t OFF, the Hall outputs n ths drawng would be 1ØØ. (In ths example, 1 = O and Ø = OFF. 1ØØ, therefore, means that Hall 1 s O, Hall 2 s OFF, and Hall 3 s OFF.) The OEM770 wll send current nto one phase and out of another the thrd phase receves no current. When current flows through a phase, two magnetc poles of the same sgn are formed on opposte sdes of the motor. We wll use the conventon n these drawngs that when current flows from the drve nto a col, t wll produce a north pole. When t flows from a col to the drve, t wll form a south pole. For example, suppose current goes nto the motor through Phase, and exts through Phase. (Phase has no current n t.) The current wll flow through the wndngs n and form north magnetc poles on opposte sdes of the stator. The current flows through the center connecton, and enters s wndngs, where, because of the drecton of the current, south magnetc poles are formed on opposte sdes of the stator. (Refer to the prevous drawng.) From ths example, notce that, although the stator has sx locatons for pole faces, there are only four poles at any one tme. The other two pole faces have wndngs that carry no current therefore no magnetc poles are formed by those wndngs. THE IX POILE HLL TTE The next fgure llustrates that, as the rotor turns, sx dfferent Hall states wll be produced n a predctable and repeatable sequence. Ths drawng shows the rotor, stator, phase cols, and Hall sensors. small black dot has been drawn next to one of the south poles, to help show the moton of the rotor as t turns. (The two south poles n the rotor are actually ndstngushable from each other, as are the north poles.) 112

7 OEM770 5 Hall Effect ensors Hall ensor tates Hall 3 Hall 1 Hall = O = OFF Hall 1 = O Hall 2 = OFF Hall 3 = OFF 113

8 5 Hall Effect ensors OEM770 For each of the sx dfferent rotor postons n the drawng, a current s shown that wll cause the rotor to rotate n a clockwse drecton. The stator s labeled wth or, to show the magnetc felds the current produces. These felds exert the torque on the rotor that causes t to move. Each rotor poston s labeled wth ts correspondng Hall state (100, 101, 001, etc.). These numbers represent the three Hall sensors, and whether they are on or off. The frst dgt corresponds to Hall 1, the second to Hall 2, and the thrd to Hall 3. What voltage levels correspond to on and off? We use the followng conventon: 1 = O = +5VD Ø = OFF = ØVD Voltage s measured at the OEM770 s Hall nput, wth the Hall wre connected to the nput, and the drve turned on. If no drve s avalable, connect the Hall wre to a 1KΩ pullup resstor. onnect the resstor to +5VD. onnect Hall +5 and Hall Gnd to your power supply. Measure the voltage at the pont where the Hall wre s connected to the resstor. To understand ths drawng, examne the rotor poston at Hall state 100. The south pole turns Hall 1 on. The north pole turns off Hall 2 and Hall 3. The Hall state, therefore, s 100. (Hall 1 = O, Hall 2 = OFF, Hall 3 = OFF) If current flows nto phase and out of phase, north and south poles form n the stator. These poles exert a strong torque on the rotor s north pole, and t wll turn clockwse. If the rotor could turn far enough so that ts north pole was algned wth the south pole n the stator, the rotor would stop. However, mmedately before the rotor reaches ths poston, the Hall state changes. The south pole (wth a dot on t, n ths fgure) moves nto poston next to Hall 3 and turns t on. The Hall state s now 101 (Hall 1 = O, Hall 2 = OFF, Hall 3 = O. Remember, Hall 3 s located between Hall 1 and Hall 2. ee the detal at the bottom of the drawng.) 114

9 OEM770 5 Hall Effect ensors If current s now drected nto phase and out of phase, a new set of magnetc felds forms n the stator that exert a strong torque on the rotor s south pole. The rotor moves further n a clockwse drecton, and when t turns far enough, the Hall state changes to 001. t ths pont, drectng current nto phase and out of phase wll keep the rotor turnng to state 001. The next Hall states the rotor wll pass through are 010 and 110. When the south pole wthout the dot reaches state 100, a complete electrcal cycle has occurred, and the rotor has rotated through 360 electrcal degrees. (Physcally, t has rotated through 180 mechancal degrees.) t ths pont, the same sequence of Hall states begns agan. otce that the Hall states are not determned by the current flowng n the stator. They smply report nformaton about the poston of the rotor. Whether you turn the rotor by hand, or cause t to turn by drectng current through the motor s cols, the Hall effect sensors are nfluenced only by the magnetc felds of the rotor. The Hall effect outputs n ompumotor servo motors dvde the electrcal cycle nto three equal segments of 120 (electrcal degrees, not mechancal degrees). Outputs used n ths arrangement are called 120 Hall effect outputs. The Hall states 111 and 000 never occur n ths confguraton. nother arrangement, rarely used n modern servo motors, uses a 60 Hall effect sensor confguraton, n whch the states 111 and 000 can occur. Do not attempt to use such a motor wth the OEM770. It wll not operate properly. OMMUTTIO ED O HLL TTE The OEM770 montors ts three Hall nputs. It uses nternal logc crcutry to assgn a rotor poston to each of the sx Hall states, and then drect a motor current that results n rotor movement n the commanded drecton. The three Hall sgnals produced by clockwse shaft rotaton are shown at the top of the next drawng. The Hall states are also lsted, along wth the table of phase currents the OEM770 uses for each Hall state. 115

10 5 Hall Effect ensors OEM770 5V Hall 1 0 5V Hall 2 0 5V Hall lockwse haft Rotaton (as vewed from faceplate end of motor) PHE - + URRET ommutaton for lockwse haft Rotaton ased on Hall tates 116

11 OEM770 5 Hall Effect ensors For counterclockwse rotaton, two changes are made. Frst, as the rotor moves counterclockwse, t passes through the same Hall states, but n the opposte order. (In ths drawng, read the Hall states from the bottom up for counterclockwse rotaton.) The drve sends currents through the same cols shown n ths pcture, but the drecton of the current s reversed from that shown. s a result, a torque s produced n each state that causes the rotor to turn counterclockwse. OETIG MOTOR FROM OTHER VEDOR The prevous dscusson descrbed ompumotor servo motors, and how the OEM770 drve operates them. If you use a motor from another vendor, obtan nformaton from the motor s manufacturer about ts sequence of Hall states, commutaton scheme, etc. Use the above nformaton about ompumotor motors for gudance on how to connect your motor to the OEM770. IMPROPER WIRIG REULT I POOR PERFORME ssume that you arbtrarly connect your motor s three Hall wres to the OEM770 s Hall nputs. For any partcular Hall wrng pattern, there are sx dfferent ways you can connect wres to Phase, Phase, and Phase. Of these sx possble phase wrng combnatons, only one wll work properly. Three wll not work at all. The other two deserve partcular attenton: f the motor s wred n one of these two confguratons, the motor wll turn, but ts performance wll be severely mpared. How can you tell f your motor s wred mproperly? If t s n one of the two poor-performance confguratons, ts torque wll be much lower than the torque level of a properly wred motor. lso, torque rpple wll be very pronounced as the motor turns. The best way to determne whether or not your motor s wred correctly s to fnd the three wrng confguratons that enable the motor to turn. ompare the motor s torque n each confguraton. The confguraton wth the most torque wll be the proper confguraton. 117

12 5 Hall Effect ensors OEM770 TRIL D ERROR METHOD You can use a tral and error method to connect your motor to the OEM770. Follow these steps: 1. rbtrarly assgn numbers to your motor s three Hall output wres, and connect them to Hall 1, Hall 2, and Hall 3 on the OEM onnect Hall +5V and Hall GD. 3. rbtrarly assgn letters (,, ) to your motor s phase wres, and connect them to Phase, Phase, and Phase on the OEM If the motor turns, fnd the best phase wrng confguraton: Move each phase wre over one poston ( ). ompare torque and torque rpple. Move each phase wre one poston further ( ). ompare torque and torque rpple. Use the wrng confguraton that gves hghest torque and lowest torque rpple. 5. If the motor does not turn, exchange two of the phase wres. The motor should now turn. Go to tep 4, compare the three wrng confguratons that make the motor turn, and use the proper one. 6. If your motor turns n the opposte drecton than you want, you can reverse t usng one of several methods. Reverse the command nput wres. Reverse the approprate encoder connectons. Exchange two Hall nput wres, then follow teps 2 5 above. 118