P6.1. Magnetic position sensor with low coercivity material

Transcription

1 P6. Mgnetic position sensor with low coercivity mteril Jernce N., Frchon D. Moving Mgnet Technologies rue Christin Huygens, 25 Besnçon, Frnce. Introduction Mgnetic position sensors re widely used, especilly in utomotive pplictions. This is due to their low cost, bsence o riction nd insensitivity to dirt. Modern mgnetic position sensors use mesurement o mgnetic ield direction []. These sensors hve dimetriclly mgnetized mgnet which is ttched to the extremity o rotting sht, nd n ASIC mesures the mgnetic ield ngle in the plne below the sht. The rotting prt o the sensor is n ssembly o two prts: the rotting sht itsel nd the mgnet. These mterils, in generl cse, hve dierent therml expnsion coeicients. In order to improve the mechnicl robustness o the sensor nd to simpliy the production process, we propose to use directly the sht (mde o steel) lredy in the ppliction (to guide the rotting prt) s the source o mgnetic ield or low coercivity mgnetized steel press itted t the end o the sht. A prticulr geometry o the mgnetized prt is proposed in order to obtin suicient induction in the sensor. In this pper, the properties nd the choice o the low coercivity mterils re briely exposed, the prcticl reliztion o prototypes nd mesurement results re given. Mesurements o induction ter temperture cycles nd presence o demgnetizing ield re presented. 2. Low coercivity mterils As we lredy sid the gol is to use mterils hving good mechnicl properties nd signiicnt mgnetiztion. In certin cses it is diicult to use modern permnent mgnets in the utomotive pplictions becuse o their mechnicl properties such s: rgility, criticl use t high tempertures, diiculty to ssemble with mechnicl prts. Most o mechniclly good mterils re not erromgnetic t ll, or hve very low mgnetiztion. Mgnetic steels re good trde-o: very good mechnicl properties nd coercivity (Hc) up to 6-7 Oe [2]. Our choice re crbon steels, which re low cost mterils with suicient Hc (round 55 Oe). Z3C3 (.3% C nd 3% Cr) is the mteril which is used or the mesurements nd prototypes described below. It is importnt to mention tht the high coercive ield is chieved ter quenching, s it is shown in Figure. Figure : Mesured B-H curve or Z3C3 steel beore nd ter quenching 3. Prcticl reliztion o the sensor A new genertion o Hll sensors, bsed on mesuring the mgnetic ield direction, is shown in Figure 2. These sensors hve some dvntges over clssicl Hll sensors, mesuring the ield strength: no need to compenste or mgnet over temperture, better tolernces, simple structure.

2 Figure 2: Hll sensor mesuring the direction o the mgnetic ield In the proposed sensor, the dimetriclly mgnetized mgnet is replced by erromgnetic prt, hving good mechnicl properties, but coercivity signiicntly lower thn modern permnent mgnets. The min problem when using low coercivity mterils is to hve enough mgnetic ield in the system. The system geometry should be redesigned in order to chieve suicient induction mplitude or the probe (ew hundreds o guss). It is possible with U shped prt, s it is shown in Figure 3. Figure 3: Shpe o the erromgnetic prt nd its mgnetiztion pttern Two rms re mgnetized in opposite directions, s it is shown by the mgnetiztion vector M. This shpe increses the pth length through the mgnetized mteril. We cn write Ampere s lw on the pth shown in Figure 3: H l H l where: H - mgnetic ield in the ir l - length o the pth through the ir H - mgnetic ield in the erromgnetic mteril l - length o the pth through the erromgnetic mteril The vlue o H is intrisic to the selected mteril, so one cn esily notice tht the mgnetic ield in the ir is higher i the pth through the mgnetized mteril is longer. 4. Mgnetizing method The erromgnetic prt cn be esily mgnetized by winding conductor round one rm o the erromgnetic prt nd by closing the mgnetic circuit with nother erromgnetic prt, s it is shown in Figure 4. Figure 4: Method o mgnetizing The current nd number o conductors re clculted by mens o the Ampere s lw:

3 H l H where: H - mgnetic ield in the lux pth closing prt l - length o the pth through the lux pth closing prt H - mgnetic ield in the erromgnetic mteril l - length o the pth through the erromgnetic mteril N - number o conductors I - mgnetizing current l NI With H being equl to the sturtion ield o the erromgnetic mteril, low coercivity mteril is esily mgnetized with 5- A current (N is generlly low, or 2), or common sensor dimensions. 5. Dimensions o the erromgnetic prt The vlue o mgnetic ield s unction o dimensions nd distnce to the probe is studied. We will give 3D inite element study or three prmeters (Figure 5) o U-shped erromgnetic prt: slot thickness, length nd dimeter o the mgnetized prt. The results re given or point locted on the rottion xis t mm below the mgnetized prt. Figure 5: Simultions o the induction s unction o the erromgnetic prt dimensions We notice tht the slot thickness is the most importnt prmeter it should be s low s possible, however, it is limited by mgnetizing conductor thickness. The rms should be s long s possible, nd the dimeter s lrge s possible, but these two prmeters re limited by the vilble spce or the sensor. 6. A mesurement exmple An exmple o mesurement result or 36 stroke is given in Figure 6 nd the dimensions o the erromgnetic prt re here below: E = 2 mm, D = 6 mm, H = 9 mm. A mesurement o induction is done with 2SA- Hll probe [4] (probe ble to mesured 2 components o the induction in single point) nd the ngle mesurement is done with MLX936 Hll probe [5].

4 induction (G) ngle (deg) voltge (V) ngle (deg) Figure 6: 36 stroke with erromgnetic prt - mesurement result A good linerity (.4%) is chieved nd the induction t the mesurement point is 2 G which is enough or the considered Hll probe. The xil distnce between the erromgnetic prt nd the point o mesurement is mm pproximtely nd it is kept identicl or ll the prototypes mentioned below. For most utomotive pplictions shorter stroke is required. Such exmples o prototypes re presented in the next prgrphs. 7. Prototypes or utomotive pplictions In ll prototypes described below the Hll probe MLX936 [5] is used. In Figure 7 prototype o sensor or n EGR ctutor is shown. The sensor prt dimensions re E=.5 mm, H=9. mm nd D=2 mm. The non-linerity over 8 stroke is given in Figure n o n -lin e rity (% ) ngle (deg) Figure 7: Sensor prototype or n EGR ctutor nd non-linerity on 8 stroke The induction on the mesurement point is bout 7 G. A very good non-linerity (close to. %) is chieved or 8 stroke. Suspension sensor A sensor tht its in suspension sensor housing hs been mde. The dimensions o the erromgnetic prt re: D = 2 mm, H = 6mm, E = mm. The sensor nd the mesurement results or 2 strokes (66 nd 86 ) re shown in Figure 8. Figure 8: Suspension sensor: prototype nd mesurement result Induction on the mesurement point is bout 8 G nd the non-linerity is below.4% or both 66 nd 86 degree strokes (two possible strokes or this ppliction).

5 Pedl sensor For short stroke ppliction, pedl sensor hs been mde. The dimensions o the erromgnetic prt re E = mm, D = 4 mm, H= mm. It is shown in Figure 9, with the mesurement result. Figure 9: Pedl sensor with mesurement result We ind bout.2% non-linerity or 2 stroke with bout 25 G on the mesurement point. For ll prototypes very good linerity is chieved with high enough mgnetic ield or the unctioning o the probe. 8. Stbility issues Stbility o mgnetiztion or these sensors should be creully exmined. Externl ield, high tempertures nd geing cn be criticl or mgnetized steel [2]. In this type o sensor, the induction t the mesurement point should be kept bove the criticl vlue or the probe used. A set o tests is done or stbility in temperture, with temperture cycling rom -4 to 35 C (see Figure ). The mximum induction in the sensor s unction o the number o temperture cycles is shown in Figure. temperture cycles B ter temperture cycles temperture ( C) tim e (m in ) induction (G) temperture cycles B Figure : Temperture cycles Figure : Mesured induction s unction o the number o temperture cycles Stbility in presence o n externl ield ws lso studied. In Figure 2 mesurement result or 4 G externl ield pplied times is given induction (G) number o demgnetizings Figure 2: Chnge o induction in presence o demgnetizing ield The mesured induction decreses ter the irst externl ield exposure, ter it remins quite stble (rom 34 to 3 G).

6 9. Sensor in presence o strong externl ield Ater irst mesurement without externl ield, permnent mgnet ( cube o pproximtely xx mm nd pproximtely T o remnent induction) is brought close to sensor. A mesurement is done in presence o externl mgnet, then nother mesurement is done when externl mgnet is tken wy. A strong mgnetic ield will disturb ny unshielded mgnetic sensor, thereore we present only the cse when the mgnet is tken wy, knowing tht the mgnet ws t 5 mm miniml distnce in ll directions. The result is shown in Figure Vout voltge (V) 3 2 NL sensor without mgnet NL mgnet removed ngle (deg) Figure 3: Inluence o strong externl ield The erromgnetic prt hs kept suicient mgnetiztion, but we remrk tht there is n eect on the mesurement even when the permnent mgnet is tken wy (the non-linerity is now.8% insted o.% with respect to the initil stroke). This is the criticl cse or this type o sensor nd should be tken into ccount or the inl sensor design.. Conclusion A simple nd robust sensor or up to 36 strokes is presented. The low coercivity mterils such s mgnetic steels represent trde-o between mgnetic nd mechnicl properties. With n dpttion o sensor geometry, the mgnetic ield in the rnge o 5 G 25 G cn be obtined, which is suicient or the probes which re normlly used in mgnetic sensors (exmples: MLX936, HMC52 [6]). The ield mplitude tht cn be obtined is limited by sensor size. It ws shown, however, tht this type o sensor cn it in the sensor housings or the utomotive pplictions, still producing enough mgnetic ield or the probe. Stbility o mgnetiztion in presence o externl ield is exmined nd tests over wide temperture rnge re done. Very good stbility in temperture is shown. Moderte externl demgnetizing ield (4 G, or exmple) cn decrese the sensor ield once, but the mgnetiztion then remins stble i the sme demgnetizing ield is pplied. Strong externl ields (permnent mgnets) cn demgnetize steel prt or remgnetize it in wrong direction. Long term stbility o the mgnetiztion in such sensors should be creully exmined.. Literture. V. Hiligsmnn - 36 Degree Rotry Position Sensing with Novel Hll Eect Sensors, Sensors, Mrch R.M. Bozorth Ferromgnetism, IEEE Press, P.Gndel, D.Frchon, N.Jernce Cpteur mgnétique de position sns contct, French ptent FR SA- dtsheet, Sentron, 5. MLX936 dtsheet, Melexis, 6. HMC52 dtsheet, Honeywell,