Abstract:
A sensor assembly for use in sensor bearings, the sensor assembly comprising at least two sensor units configured to be arranged on a ring of the sensor bearing in different angular positions with regard to the rotation axis of the bearing. Each of the sensor units includes at least one hall sensor plate. At least two hall sensor plates of different sensor units are wired in an antiparallel orientation in order to compensate offset voltages of the hall sensor plates.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a United States National Stage Application claiming the benefit of International Application Number PCT/EP2014/060005 filed on 15 May 2014, which claims the benefit of Europe (EP) Patent Application PCT/EP2013/060486 filed on 22 May 2013 (22.05.2013), both of which are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a sensor assembly for use in sensor bearings and to a method for manufacturing a sensor assembly. 
       TECHNICAL BACKGROUND 
       [0003]    Sensor bearings including absolute angle sensors are widely used to serve motion control, electric motor control and commutation, in particular at high temperatures and/or speeds. Such motor application fields could be mining, cement, minerals, drilling of oil/gas, processes in chemistry and mineral refining, off-road electrically driven vehicles, hybrid and electrical cars and trucks, and motors for auxiliary equipment around internal combustion engines such as sensors for the motors/generators of electrically supported turbocompression, electrical air inlet compressor, double clutch actuator motors, turbo variable geometry actuators, turbo waste valve actuators, air heating, cooling and ventilation compressors, water pumps, start-stop motor-alternators, mild and full hybrid electric motor-generators. 
         [0004]    Absolute angle sensing devices for use in bearings are usually made of a magnetic ring with at least one north and one south pole, and a collection of two of more isotropically distributed analogue Hall sensors with regular spacing. Adding the signals with the correct weight makes it possible to directly present output voltages representing the sine and cosine of the angle of the (usually) rotating magnet ring with respect to the (usually fixed) sensor collection. 
         [0005]    An angle sensing bearings with semiconductor sensors are normally limited to ambient temperatures below the range of 150-175° C. due to the need of highly integrated magnetic sensors as well as analogue and/or digital signal treatment parts produced in silicon (e.g. op-amps, AD converters etc). There are signal processing components available for ambient temperatures beyond 175° C. but they are 100 to 1000 times more expensive than their automotive grade equivalents as used for industrial sensor bearings. 
         [0006]    High temperature magnetic sensors are feasible however, usually in the form of Hall plates with sensor areas (in the shape of maltese cross design) in high temperature semiconductors of the GaAs, GaN or SiC instead of Si materials. These Hall elements are today commercially available in GaAs but without any amplification, buffering or on-board signal treatment. The integration level of all analogue and digital processing in Silicon has entailed the massive success and subsequent multi billion pieces/year volumes and thereby low cost silicon Hall sensors and is not available in high temperature semiconductors today. 
         [0007]    The cost restrictions applicable in the automotive industry impose strong limits on the possible choices of absolute angle sensors for bearings. For applications where high temperatures are likely to occur such as wheel bearings close to brakes or bearings in internal combustion engines, these limitations do not allow for the use of signal processing elements such as amplifiers and only elementary Hall sensors and passive components can be used. 
         [0008]    One of the problems when using high temperature magnetic sensors is that these sensors, e.g. GaAs based Hall elements, have large offset, gain and other variations with respect to the average parameters. 
         [0009]    However, the common manufacturing process has the capability to very accurately reproduce these errors in all such Hall elements spread over the same wafer. This means that the Hall bridge resistances, the Hall voltage offset and the Hall gain of these Hall elements may deviate strongly from the typical specification from batch to batch but within one batch of sensors, these numbers will be very constant. 
       SUMMARY OF THE INVENTION 
       [0010]    In order to provide a arrangement for absolute angle sensing in sensor bearings meeting the requirements of the industry in terms of cost and volume, the invention therefore proposes to use at least two Hall plates in particular cut from one semiconductor wafer and to wire the Hall plates so as to form a compensation circuit compensating the tolerances resulting in variations offset, gain or other parameters of the individual Hall plates. The Hall resistances being nearly identical for individuals from one wafer, it is possible to directly put the sensor output wiring in parallel or anti-parallel according to the needs. 
         [0011]    It is possible to combine such Hall plates in pairs with anti-parallel wiring, such that the circuit of two such sensors cancels in particular the offset without further calibration 
         [0012]    In an embodiment of the invention, it is proposed to distribute a multitude of sensor units each including one pair of Hall plates, where the individuals per pair are set up as “differential” magnetic field sensors with antiparallel wiring, and distribute the sensor units around the magnetic ring circumference in a meaningful geometric pattern to have a sine and cosine output while also suppressing or attenuating unwanted harmonic content of the magnetic field of the magnetic ring. 
         [0013]    In a preferred embodiment of the invention, it is proposed that the Hall plates are GaAs plates. The Offset voltage Voffset is found to be highly reproducible in GaAs based cells, in particular when stemming from one wafer unlike in Silicon based cells. Then, these cells can be arranged around the magnetic ring, in a way such that they sample the field variation due to the magnetic ring being sensed. The cancellation of the offset is preferably done by using a pair of sensors electrically connected in anti-parallel fashion. 
         [0014]    If no magnetic ring is feasible, which might be the case when the object is rotating with a speed where the centrifugal force prohibits the use of magnetic rings based on our commonly available materials, then a variable reluctance solution is in principle possible too. This solution would require sensor pairs forming a differential bridge, a bias magnet system and a ferritic steel ring shaped to vary the magnetic field direction and strength such that it is easy to observe by the two closely spaced hall elements in anti-parallel wiring. The offset thus being cancelled, as well as the total bias field result being cancelled, only the modulation of the size and direction of the field will be visible. Combining a number of regularly spaced pairs of sensors will then allow a variable reluctance based absolute angle sensing system which not only can run at higher than ordinarily feasible temperature but also achieve this at high speed. 
         [0015]    The invention preferably uses a diametrical magnetic ring to provide a (reasonable) magnetic field and pairs of Hall sensors arranged around this ring following optimization rules layed out in this text. The Hall sensors provide in fact a voltage output which is the in-product of the magnetic vector from the ring with the sensitivity axis of the sensors. This voltage is thus a function of the angle that can be described by a Fourier sum of functions representing the (required) sine of the angle a and its (generally unwanted) higher harmonics i.e. sin(n*a), wherein n≧2. For a well behaved magnetic ring, the field is symmetrical thereby the even harmonics are generally very small. 
         [0016]    The ring may also have more than one pole pair. For the purpose of explaining the invention, we assume one pole pair. When the invention is generalized to rings with more than one pole, the phase delays given below relate to phase angles in relation to the period of the magnetic pattern of the ring, wherein the phase angles are to be taken modulo the period. 
         [0017]    The known silicon based analogue Hall sensors are massively employed, and contain all qualities that the CMOS and mixed signal manufacturing in standard silicon wafers can bring except for performance at high temperatures. Beyond 175 degrees C. practically the leakage currents dominate the signals and the sensors are no longer usable. 
         [0018]    The sensors that can be used at high temperature are generally made of a semiconductor material with a sufficient band gap between valence and conduction bands so that the thermally generated leakage currents at high temperatures are sufficiently small. Devices that have become recently available include Hall sensors made in GaAs. The Hall elements thus made consist of just the Hall plates in monolithic semiconductor material, and it does generally not contain active electronic components like operational amplifiers, multiplexers etc. However, such components might become available at reasonable costs for high-temperature applications in the future. The voltage is available as a differential output. These sensors behave like resistor bridges and voltage sources described in detail below. 
         [0019]    A further problem in a number applications is that the signals are essentially the subtle DC signals as output from the arrangements shown in the figures, which may not be feasible in a high electromagnetic noise level environment such as an electrical motor with vector drive. According to one aspect of the invention, it is proposed that the Hall sensor arrangement is therefore driven with an AC supply with a carrier frequency generated by the controller unit that needs to interpret it. The AC drive can be applied under the condition that the substrate of the Hall sensors is biased with DC to form a good non-conducting and low capacitance barrier, avoiding current leakage between Hall element and substrate. The differential AC signals can be filtered and amplified and then using synchronous detection of the amplitude response can be demodulated back to the representative values that are proportional to the that the angle is calculated. In this way, the semiconductors are effectively mimicking more complex and bulky inductive units such as commonly used RVDTs and Resolvers, while staying backwards compatible with these already known and designed-in components. 
         [0020]    The above description of the invention as well as the appended claims, figures and the following description of preferred embodiments show multiple characterizing features of the invention in specific combinations. The skilled person will easily be able to consider further combinations or sub-combinations of these features in order to adapt the invention as defined in the claims to his or her specific needs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1 . is a schematic diagram of a wired high temperature Hall sensor plate for use in an arrangement of the present invention. 
           [0022]      FIG. 2  is an equivalent circuit of the wired high temperature Hall sensor plate of  FIG. 1 . 
           [0023]      FIG. 3  is a schematical representation of a first embodiment of the invention. 
           [0024]      FIG. 4  is a schematical representation of a wiring scheme of the sensor arrangement according to  FIG. 3 . 
           [0025]      FIGS. 5   a,    5   b,  and  5   c  are graphs showing output voltages of the individual Hall sensor plates in  FIGS. 3 and 4  and the differential output of the circuit in  FIG. 4 . 
           [0026]      FIG. 6  is a schematical representation of a second embodiment of the invention. 
           [0027]      FIG. 7  is a schematical representation of a wiring scheme of the sensor arrangement according to  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0028]      FIG. 1  illustrates a schematic diagram of a wired high-temperature Hall sensor plate  10  for use in an arrangement of the present invention. The Hall sensor plate  10  basically consists of a GaAs die  10   a  as a semi-conductor substrate and a magnetic field detecting portion  10   b  formed as a Greek cross or Maltese cross with a suitable sequence of doping layers. 
         [0029]    Four connector terminals are provided, one at the end of each leg of the cross-shaped active Hall region  10   b.    
         [0030]    Electrical supply current flowing from one connector terminal to another connector terminal opposite to the one connector terminal is biased by a magnetic field oriented perpendicular to the plain of the drawing and to the plain of the plate  10   a  due to the magnetic forces acting on the charge carriers such that a Hall voltage can be measured at the terminals provided on the legs oriented perpendicular to the legs provided with the one connector and the connector on the opposite side. 
         [0031]    In the situation illustrated in  FIG. 1 , the supply current would flow from the connector on the bottom to the connecter on the top of the Hall sensor plate  10  and a Hall voltage could be measured between the connectors labeled with out+ and out−. 
         [0032]      FIG. 2  illustrates an equivalent circuit for the Hall plate of  FIG. 1 . The four legs of the Hall plate including the connectors are equivalent to four resistors connected as illustrated in  FIG. 2 , wherein the resistance values RH 2 A, RH 2 B, RH 1 A, RH 1 B depend on the magnetic field component perpendicular to the plate  10  in  FIG. 1 . The Hall voltage is illustrated as VHDIFF. 
         [0033]      FIG. 3  illustrates an arrangement of four Hall sensors IC 1 , IC 2 , IC 11  and IC 12  each constituting one sensor unit  12   a - 12   d  provided with a housing (not shown) and mounted on a bearing ring. The four Hall sensor units  12   a - 12   d  are connected in pairs, the first pair comprising the Hall sensor IC 1  and the Hall sensor IC 2  and the second pair comprising the Hall sensor IC 11  and the Hall sensor IC 12 . 
         [0034]    Within each pair, the Hall sensors are connected in an identical way, which is illustrated in  FIG. 4 . The Hall sensors within one pair are identical Hall sensors according to  FIG. 1 , wherein the sensor plates  10  are made in the same manufacturing process and are cut from the same semi-conductor wafer such that the properties thereof are highly similar. The differences in resistances, offset voltages etc. are smaller than the differences which would be measured when comparing sensor plates  10  taken from different batches manufactured from different semi-conductor wafers. 
         [0035]    The position of the second pair of Hall sensors IC 11 , IC 12  correspond to the positions of the Hall sensors IC 1 , IC 2  rotated by 90°. 
         [0036]    The positions of the Hall sensors within one pair differ by 120° with respect to the rotation axis of the bearing. 
         [0037]    The Hall sensors are mounted on an inner ring or an outer ring of the bearing and interact with a magnetic ring mounted on the other ring, preferably the rotating ring of the bearing (not illustrated). 
         [0038]    As illustrated in  FIG. 4 , a first output Vout− is connected to a negative output out− of the first Hall sensor plate IC 1  and to the positive output out+ of the second Hall sensor plate IC 2  of the pair. In a similar way, the positive output Vout+ of the circuit comprising one pair is connected to the positive output out+ of the first Hall plate IC 1  and to the negative output out− of the second Hall plate IC 2 . 
         [0039]    The output of a sensor according to  FIG. 1  can be expressed mathematically as the inproduct of the magnetic field vector with a normal vector to the surface of the plate  10  multiplied by a constant plus an offset value. Since the sensor plates IC 1 , IC 2  in  FIG. 4  stem from the safe wafer, the offset voltages are highly similar and are compensated due to the anti-parallel connection illustrated in  FIG. 4 . Due to the different positions of the sensor plates IC 1 , IC 2  with a phase difference of 120°, the magnetic field variations are phase-delayed by 120° such that the residual difference of the signals is a sign-wave without offset as illustrated in  FIG. 5   c.    
         [0040]      FIG. 5 a    illustrates the output voltages at the terminals out+, out− of the different sensor plates IC 1 , IC 2  and  FIG. 5 c    is the signal without offset which is achieved by the circuit illustrated in  FIG. 4 . 
         [0041]    Arrangement I in  FIG. 3 , has the sensors more than a quarter of the rotation of the ring from each other. The sensors have the same offset voltage, have the same internal resistances, but will observe a different field strength. The angular distance is between 90 degrees and 180 degrees, wherein the endpoints of this interval are excluded. 
         [0042]    One such pair, sensors dubbed IC 1  and IC 2  can observe the principal Cosine term of the angle, while a 90 degrees shifted pair may observe the Sine term. 
         [0043]    With the resistor bridge nature of these Hall elements we can now make a circuit working at high temperature, unlike with known silicon Hall sensors that include amplifiers and other active elements by nature. Now it only needs wiring between the bridges to create a differential voltage that represents the difference in field at the locations sensed around the ring, while the circuit can be arranged such that the offset voltage of the sensors are canceling each other. 
         [0044]    The differential output voltage V+out minus V−out is the difference of the Hall voltages generated by the B field inproduct with the normal vectors at the different locations around the magnetic ring. However the offset voltage is the same by the nature of the reproducibility in GaAs, so they will be in anti-parallel and therefore will cancel each other. The requirement to be able to do this electrically anti-parallel connection is that the effective values of the resistors in the Hall elements are quite accurately the same. Also this has been fulfilled already in the manufacturing process of these cells. 
         [0045]    The waveforms from IC 1  and IC 2  are shown in  FIGS. 5 a    and  5   b,  together with the result if they are switched electrically in anti-parallel in  FIG. 5   c.    
         [0046]      FIG. 6  illustrates a second embodiment of the invention which basically corresponds to the Arrangement I of  FIG. 3 , wherein only each sensor unit  12   a - 12   d  comprises pair of sensor plates connected in anti-parallel way as illustrated in  FIG. 4  and provided in one housing or separate housings arranged closed to one another (angular distance preferably less than 60°) and wherein the two pairs of sensor plates are connected as illustrated in  FIG. 7 . In other words, each sensor unit  12   a - 12   d  mounted on a ring of the bearing comprises two sensor plates wired as in  FIG. 4  which are preferably arranged in one housing and originate from the same batch/wafer. Preferably, all of the four sensor plates contained in one pair of sensor units are made from the same semi-conductor wafer. 
         [0047]    As illustrated in  FIG. 7 , the principle of anti-parallel connection as illustrated in  FIG. 4  is repeated on the level of the sensor units in that the positive output of a first sensor unit of one pair of sensor units is connected with a negative output of a second sensor unit of the same pair of sensor units and vice versa. 
         [0048]    Arrangement II can be made using the electric diagram illustrated in  FIG. 7 . 
         [0049]    Arrangement II in  FIG. 6  uses anti-parallel oriented sensors IC 1  and IC 2  in pairs to measure nearly the same vector at one location and the other pair IC 3  and IC 4  the other location. The distance between the second pair can again be anywhere from 90 to 180 degrees from the first pair. In practice the package will limit how close the two can be fitted together in a pair, but it should effectively not exceed about ⅙th of a circumferential length of the pole pair on a magnetic ring. In multi pole rings therefore the pairs may be distributed with a distance of any integer of the pole pair size to overcome this limitation. 
         [0050]    Note that the appropriate Sine component of the angle of this example ring in the drawing can be produced with Sensor pairs IC 11 , IC 12  and IC  13 , IC  14 . 
         [0051]    The orientation of IC 1  is mechanically opposite to IC 2 , while the B field locally is nearly the same. Therefore the IC 1  will give a positive output equal in size but of opposite electric sign as IC 2 . The offset voltage is however equal both in size and sign. Putting the two electrically in anti-parallel, the offset is canceled while the mean B measurement of IC 1  and IC 2  is preserved. 
         [0052]    The rest of the diagram is a further copy of arrangement I where IC 1  of Arrangement I is replaced by IC  1  and IC 2  in arrangement II, and IC 2  of arrangement I is replaced with the combination of IC 3  and IC 4  in arrangement II. 
         [0053]    The expansion into more phases is now logical too. Instead of a sine-cosine orthogonal pair of electrical signals, it is possible to copy the circuit in more locations to provide N=3, 4, 5 or any more vectors with regular electrical phase spacing. The signals can be combined into an angle by calculating the product of the sine of the angle and the vector values measured in this way with N positions. 
         [0054]    In an alternative embodiment of the invention (not illustrated), the Hall sensor arrangement is driven with an AC supply with a carrier frequency generated by the controller unit that needs to interpret it. The differential AC signals are filtered and amplified and then using synchronous detection the amplitude response is demodulated back to the representative values that are proportional to the that the angle is calculated. 
         [0055]    Measurements have been done with an available device, HE144, from ASensor AB, who produce and market GaAs products. The difference in offset between individually packaged dice has a worst case variation of about +/−0.0001 Volt under 5 volt (equivalent to 5 mA drive current) supply conditions, with a typical offset voltage of individual cells of 0.050 V under these circumstances. 
         [0056]    Measurements show an offset reduction of at least a factor 50. Selected pairs from the factory may reduce this another factor 50, leading to realistic offsets of 1 microvolt when used in anti-parallel circuits as described above. 
         [0057]    The invention proposes to distribute the pairs of offset canceling chips over the circumference of the magnetic ring/bearing. It is proposed to use a distance between ¼ to ½ of “wavelength” between the pairs. The cancelation is not achieved by adding or subtracting but directly putting the chips in (anti) parallel. As they are resistor bridges, which are highly reproducible (between individuals from one and the same wafer) we can make in this way a sensor bearing that needs no active elements nor any additional passive elements in the electronic circuit. This decreases the cost price for high temperature circuitry with orders of magnitude. 
         [0058]    Additionally it is in principle possible to use an AC voltage as supply instead of a DC voltage. The output is then also AC voltage, proportional to the local normal field strength difference at the two locations of the individuals in the bridge. In principle this can run at a frequency normally used by the RVDT units as supplied by TT/Tyco and Tamagawa, and treated by the same kind of resolver to digital chipset. In other words, a semiconductor equivalent of a resolver is then feasible.