NL06023 DHT/SPB 2007-02-16

MULTITURN ABSOLUTE ENCODING

TECHNICAL FIELD

The invention concerns a device and a method of determining the absolute angular position of a rotating component, and is more particularly directed to determining the absolute angular position of a rotating component through more than one full revolution, i.e. multiturn absolute encoding.

BACKGROUND

Encoders are sensors of mechanical motion. They translate motion, such as speed, direction, and shaft angle, into electrical signals, and can primarily be divided into either an incremental type or an absolute type. Incremental encoders read a position of, for example, a rotating component by counting the number of incremental pulses from a reference position of the rotating component. When restarting encoder operation after a power shut down, an incremental encoder needs to rotate past its reference position to be able to give a position in relation to the reference position and not only give a relative position in relation to the position at restart. Absolute encoders produce a unique output for each detectable position, or angle of for example a rotating component, and consequently do not 'lose ¹ their position in relation to a reference position when power is removed.

There are several applications where it is important to detect whether a rotating component, such as a shaft, has moved during a power shut down, i.e. to know an absolute position in relation to a predetermined reference position at all times. Examples of such applications can for example be found in the field of robotics or in automotive technology, vehicle steering systems in particular.

In vehicle steering systems, it is desirable to be able to measure the steering angle of the road wheels. This information can be used as an input variable in a number of subsystems in the vehicle, such as power-assisted steering, suspension damper control systems, vehicle stability control systems and vehicle lane guidance systems.

A single-turn absolute encoder could be mounted on the steering shaft, but given that more than one full revolution of the steering shaft is usually needed to turn the wheels from one lock position to the other, such an encoder is unable to determine the absolute angle of the road wheels with certainty. A multiturn absolute encoder is needed for this.

Many types of multiturn absolute encoders are known that make use of a variety of methods to measure absolute angular position. These methods include magnetic, optical, capacitive and potentiometric methods. One type of multiturn absolute encoder that is found in vehicle steering systems comprises a first code carrier connected to the steering shaft, with sensors to detect the angular position of the steering shaft over a full turn. The multiturn absolute encoder further comprises a second code carrier with corresponding sensors to detect which turn the steering shaft currently is at, i.e. it keeps track of the number of turns. The second code carrier and corresponding sensors will usually require some sort of mechanical arrangement to enable it to keep track of the number of turns. Thus this type of multiturn absolute encoder is made up of a single turn absolute encoder and a turn tracking encoder. There seems to be room for improvement.

SUMMARY OF THE INVENTION

An object of the invention is to define an encoding method and an encoder that enables absolute multiturn encoding.

Another object of the invention is to define a multiturn absolute encoder suitable for extracting load information and a method of extracting load information from such a multiturn absolute encoder.

A further object of the invention is to define a device combining a rolling element bearing function and a multitum absolute encoder function.

A still further object of the invention is to define a method of utilizing a cage of a rolling element bearing to create a multiturn absolute encoder and to define such a device.

An even further object of the invention is to define a method and a device to determine the absolute angular position over multiple turns by means of one single or sub single turn absolute encoder.

A still even further object of the invention is to define a method and a device to determine the absolute angular position over multiple turns by means of two single or sub single turn absolute encoders.

An additional object of the invention is to define a method and a device to determine load and/or load variations by means of one or more absolute encoders.

A still additional object of the invention is to define a multiturn absolute encoder integrated with a bearing such as a radial bearing.

The present invention is based on the fact that the cage of a radial bearing rotates at a slower angular speed than the rotating bearing ring. The exact ratio is calculable and depends on the geometry of the bearing, i.e. the pitch diameter of the bearing and the diameter of the rolling elements. The ratio also depends on which ring is rotating in relation to the other. For a bearing with a rotating outer ring, the ratio of outer ring speed to cage speed typically lies in

the region of 5:4. For a bearing with a rotating inner ring, the ratio of inner ring speed to cage speed typically lies in the region of 5:2, i.e. the cage makes two revolutions for every five revolutions of the inner ring. By measuring the absolute angular position of the cage by means of a single or part single turn absolute position sensor, the absolute angular position of the rotating bearing ring can thereby be determined through more than one full revolution of the rotating bearing ring.

According to the invention, this is achieved by means of a bearing unit comprising relatively rotatable bearing rings, a plurality of rolling elements arranged between the bearing rings and a cage in which the rolling elements are retained. The invention is further achieved by providing the bearing unit with sensing means that are adapted to produce a first signal dependent on the angular position of the cage in relation to a reference. The invention is further achieved by means of a processing unit that is adapted to process the first signal to produce an angular position signal that is indicative of the angular position of the rotating bearing ring through more than one revolution.

More particularly, the bearing unit is provided with a first absolute position sensor that is adapted to produce the first signal. The first absolute position sensor comprises a suitable code carrier and a corresponding suitable transducer, to detect the relative position of the code carrier and the transducer, and translate this relative position into the first signal.

The code carrier can, for example, be a magnetized ring and the transducer can, for example, comprise an arrangement of Hall-effect sensors. When there is relative motion between the magnetized ring and the Hall-effect sensors, the output voltage of these sensors varies in response to the corresponding differences in magnetic field density of the magnetized ring. The method according to the invention is not restricted to the use of magnets and magnetic sensors, but is also valid for optical, capacitive, potentiometric and

any other suitable means of determining the absolute angular position of a rotating component.

In a preferred embodiment of the invention, the code carrier is coupled to or forms part of the cage of the bearing. The transducer is coupled directly or indirectly to the non-rotating bearing ring or another reference in relation to which the absolute angular position is to be measured. In other words, the code carrier rotates and the transducer has a fixed spatial relationship to the non-rotating bearing ring or reference. Other embodiments are envisaged where the transducer is coupled to the rotating cage and the code carrier has a fixed spatial relationship to the non-rotating bearing ring or reference.

Consequently, there are a variety of different sensing methods and possible configurations that can be applied in order to obtain the first signal, which is dependent on the angular position of the cage. In whatever way the first signal is obtained, a processing unit is suitable to process the signal to thereby determine the angular position of the cage in relation to the non-rotating bearing ring or reference. This processing unit can be outside the bearing housing, mounted within the bearing housing, or for example be mounted on or integrated with a seal of the bearing.

The determination of cage angular position can be made using a lookup table of stored values and corresponding angles, or by some other suitable method including, but not restricted to, Fourier transformation, curve fitting or the Arc- tangent method. Once the angular position of the cage has been determined, the processing unit or some other processing unit then determines the angular position of the rotating bearing ring that is associated with the measured angular position of the cage. This can also be done on the basis of a lookup table of cage position and corresponding rotating bearing ring position, or some other suitable method as for example mentioned above.

According to this embodiment of the invention, in which the absolute angular position of the cage is measured, it is possible to determine more than one complete revolution of the rotating bearing ring. However, the maximum number of absolute revolutions of the rotating bearing ring that can be determined, known as the multitum range, is limited to an absolute angle that corresponds to just less than one full revolution of the cage. The unique mapping of cage position to rotating bearing ring position can only be determined through one revolution of the cage. But the cage and the rotating bearing ring will usually have unique relative positions through more than one revolution of the cage.

In order to be able to identify the full range of unique relative positions of the cage and the rotating bearing ring, a further embodiment of the invention comprises a second absolute position sensor that is adapted to produce a second signal dependent on the angular position of the rotating bearing ring in relation to a reference. The first and second absolute position sensors are connected to the processing unit, which is then adapted to process the first and second signals to produce an angular position signal indicative of the angular position of the rotating bearing ring through more than one complete revolution.

This second absolute position sensor comprises a second code carrier and appropriate corresponding second transducer. Just as the first absolute position sensor, the second absolute position sensor can be an optical, magnetic, potentiometric or some other suitable type of absolute sensor. As described for the first absolute position sensor, the second code carrier can be coupled to or form part of its corresponding rotating component, the rotating bearing ring in this case. The second transducer can have a fixed spatial position with respect to the non-rotating bearing ring or a reference. Likewise, the second transducer can be coupled to or form part of the rotating bearing ring and the second code carrier can have a fixed spatial relationship with respect to the non-rotating bearing ring or the reference.

The addition of the second absolute position sensor means that a second signal is generated. Due to the difference in relative angular speed of the cage and rotating bearing ring, the first and second signals are out of phase with each other. They come back into phase with each other on a cyclical basis after a first whole number of revolutions of the cage and a second whole number of revolutions of the rotating bearing ring. The cyclicity is governed by the higher of the first and second whole numbers, which for a bearing will be the number of revolutions of the rotating bearing ring. The cyclicity is therefore the multiturn range attainable with this embodiment.

When only the absolute angular position of the cage is measured, the cyclicity of the signal produced is a maximum of the multiturn range than can be determined from one revolution of the cage. With the exception of the case where the ratio of cage angular speed to rotating bearing ring angular speed is 1 :2, the cyclicity of the first and second signals will be greater than the cyclicity of the first signal alone.

A further advantage of the second embodiment of the invention is that it enables improved accuracy of the determination of absolute angular position of the cage, and thus the rotating bearing ring. During operation, the rolling elements when subjected to load will likely slip somewhat on the bearing raceway, causing a small drift of the position of the cage relative to the actual motion of rotating bearing ring. Consequently, the measured angle of the cage may not be the correct (theoretical) angle with respect to the measured angle of the rotating bearing ring (which is not subject to drift and therefore known with greater certainty). To prevent the cage drift from accumulating to a degree that would cause the cage movement to lose its relationship to the movement of the rotating bearing ring, active drift compensation is applied. This means that during operation, the angular position of the cage and the rotating bearing ring are regularly measured at intervals of, for example, a few seconds.

The processing unit determines the angular position of the rotating bearing ring from the second signal and determines the angular position of the cage from the first signal. Both determinations are made, for example, with the aid of look-up tables of stored values and corresponding angles. Other methods are possible, as described above for the determination of the cage angular position in the first embodiment of the invention.

The processing unit then calculates possible cage positions for the given rotating bearing ring position within the multiturn range using the last known combination of rotating bearing ring position and cage position. The closest match is then found between the measured cage position and the calculated cage positions. This closest match is the correct angular position of the cage. The multiturn position of the rotating bearing ring can then be determined with the aid of, for example, a lookup table of cage position and rotating bearing ring position combinations with corresponding multiturn position.

Active compensation for cage drift, as enabled by the second embodiment of the invention, has further advantages. By determining the magnitude of cage drift, information on load conditions can be obtained. The magnitude of cage drift is obtained by comparing the nominal rotation of the cage (calculated from the measured rotation of the rotating bearing ring) with the measured rotation of the cage. If there is a deviation then cage drift has occurred. Cage drift is influenced by the amplitude and direction of the load on the bearing. The second embodiment of the invention therefore enables the determination of load and/or variations in the load on the bearing. This information can, for example, be used for condition monitoring purposes.

If the rotating bearing ring moves while the sensors and processing unit are inactive, it is important that cage drift is minimized. This can in most cases be accomplished by applying a small preload to the bearing. If any cage drift occurs when power is removed, in most cases it will be small enough to have

no effect on the accuracy of the determination of absolute angular position of the rotating bearing ring.

In some applications, it may not be possible to minimize the cage drift to a sufficient degree. The second embodiment of the invention can then be further provided with, for example, a rechargeable battery. This battery powers the sensors and processing unit, so that cage drift can be determined when the system, such as a steering system, is offline. The frequency of this determination can, for example, be once a minute or once a second, depending on the application and expected cage drift.

A further advantage of the invention is its flexibility. As mentioned before, the multiturn range is governed by the cyclicity of the first signal (in the first embodiment of the invention) and by the cyclicity of the first and second signals (in the second embodiment of the invention). The cyclicity depends on the type of absolute position sensors used. For example, if half-turn absolute position sensors are used, this halves the cyclicity with respect to the cyclicity obtainable using single-turn absolute position sensors. However, the cyclicity also depends on the ratio of cage angular speed to rotating bearing ring angular speed, which ratio is governed by the geometry of the bearing. An appropriately dimensioned bearing can therefore be selected for a particular application on the basis of the multiturn range that is needed.

As an example there is a need for a multiturn range of four revolutions of the rotating bearing ring. One way of attaining this is through the second embodiment of the invention, in which single-turn absolute position sensors are used in combination with a bearing that has, for example, a 2:5 ratio of cage angular speed to rotating bearing ring angular speed. Another way of attaining this multiturn range is through the second embodiment of the invention, in which half-turn absolute position sensors are used in combination with a bearing that has, for example, a 3:8 ratio of cage angular speed to rotating bearing ring angular speed. The use of half-turn absolute position sensors may

be desirable for reasons of improved signal resolution or reduced power consumption. A multiturn absolute encoder according to the invention also allows the use of a single turn absolute position sensor in combination with a sub-single turn absolute position sensor.

The invention is extremely flexible as it enables the required multiturn range to be attained in a variety of different ways, allowing the optimal solution to be selected. In summary, a multiturn absolute encoder according to the invention has a number of advantages over previously known multiturn absolute encoders. A more compact design is possible to attain in relation to other multiturn absolute encoders, given that the rotating components of a multiturn absolute encoder according to the invention have one and the same axis of rotation. Further a multiturn absolute encoder according to the invention is multifunctional in that it is both a bearing and a multiturn absolute encoder and also given that it can also be used to determine load and/or variations in the load on the bearing. For example, a shaft, such as steering shaft, is typically supported in some manner, and in many cases a radial bearing is used for this purpose. Such a bearing can be replaced with a bearing unit/multiturn absolute encoder according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail for explanatory, and in no sense limiting, purposes, with reference to the following figures, in which

Fig. 1 illustrates a steering column arrangement where the shaft is supported by a bearing unit according to the invention,

Fig. 2 illustrates a typical bearing,

Fig. 3A-3C illustrates a section of a bearing unit according to a first embodiment of the invention and examples of signals generated,

Fig. 4 illustrates an example of part of a look-up table suitable for the first embodiment of the invention,

Figs. 5 illustrates an example of a flowchart of an algorithm to determine an absolute multiturn position of a rotating bearing ring according to the invention,

Figs. 6A-6C illustrates a second embodiment of the invention and examples of signals generated,

Fig. 7 illustrates an example of part of a look-up table suitable for the second embodiment of the invention,

Fig. 8 illustrates an example of a flowchart of an algorithm to determine an absolute multiturn position of a rotating bearing ring according to the invention,

Fig. 9 illustrates an example of a flowchart of an algorithm to compensate for cage drift in a bearing according to the invention,

Fig. 10 illustrates an example of an absolute position sensor,

Fig. 11 illustrates an example of part of a look-up table suitable for an absolute position sensor as shown in Fig. 10.

DETAILED DESCRIPTION

In order to clarify the method and device according to the invention, some examples of its use will now be described in connection with Figures 1 to 11.

Fig. 1 illustrates a steering column arrangement for a vehicle in which a steering shaft 100 is cooperatively connected at one end to a steering wheel 102 and supported by a unit 104 according to the invention. The unit 104 comprises a multiturn absolute encoding function as well as a bearing function, and is connected to a processing unit 106. The processing unit 106 can be integrated with the bearing unit 104 or, as illustrated, be physically separated. Alternatively the processing unit 106 can be a part of one or more of the processing units that exist in modern vehicles. The non steering-wheel end of the steering shaft 100 can, for example, be mechanically coupled to a pair of road wheels of the vehicle through, for example, a rack and pinion (not shown). The steering wheel 102 is adapted to rotate the steering shaft 100, which in turn displaces the rack and eventually the road wheels.

In vehicle systems, steering shaft angle (and the corresponding road wheel angle) is an input variable used by many vehicle subsystems, such as power- assisted steering. In these systems, it is important that when the steering wheel has been rotated while the system is shut down, the system is able to detect this rotation when it is turned on again.

Alternatively, the steering column arrangement is part of a steer-by-wire system, in which case the non steering-wheel end of the steering shaft 100 has no mechanical connection, and the output of the processing unit 106 is used to steer the vehicle. The road wheels are then oriented by an electrically controlled motor, which operates in response to detected values of various steering parameters, such as steering shaft angle and vehicle speed. Steering shaft angle is thus an important input parameter for steer- by-wire systems.

The range of movement permissible for the steering wheel 102 between end stops (known as "turns for lock to lock") exceeds one complete revolution in most cases. A device that can detect steering shaft position over a range of more than one revolution is therefore needed. This is achieved by means of

the unit 104 and the processing unit 106 according to the invention. The unit 104 is provided with sensing means to measure an absolute angular position of a cage of a bearing of the unit 104. The processing unit 106 processes the information from the measurement to determine the absolute multiturn position of a rotating ring of the bearing, which in this application corresponds to the multiturn position of the steering shaft 100.

Fig. 2 illustrates an example of a bearing. The bearing comprises an inner ring 210 and an outer ring 220, one of which is rotatable in relation to the other. In some applications both rings rotate in relation to each other and an external reference. The bearing further comprises rolling elements 230 arranged between the bearing rings 210, 220. A cage 240 retains the rolling elements. In this example, the bearing is mounted on a shaft 200 and located in a housing 250.

According to the invention, the calculable relationship between the angular movement/speed of the cage and the angular movement/speed of the rotating bearing ring is used to determine an absolute angle of the rotating bearing ring over more than a single turn of the rotating bearing ring. Using standard equations of bearing kinematics, it is possible to calculate the angular speed of the cage 240 from the angular speed of the rotating bearing ring, whether this is the inner ring 210 or the outer ring 220 and whether the rolling elements 330 are balls, as shown, or rollers, needle rollers or another type of roller. The exact ratio of cage angular speed to rotating ring angular speed depends on which ring is rotating in relation to the other, and is determined by the pitch diameter 260 of the bearing and the diameter 270 of the rolling elements.

As an example with reference to Fig. 2, the ball bearing has a rotating inner ring 210 mounted on a shaft 200 rotating at an angular speed ω. The inner ring 210 rotates at the same speed as the shaft 200. The angular speed ω _c of the cage 240 is calculated as follows:

where D _n , is the pitch diameter 260 of the bearing, D _b is the diameter 270 of the rolling elements 230 and α is the contact angle between the areas of rolling element and ring contact and a line perpendicular to the bearing axis. Assuming purely radial load, the contact angle α for a ball bearing is zero.

For example, with rolling elements with a diameter of 12 mm and a bearing pitch diameter of 36 mm, the cage angular speed is then 3/8 that of the inner ring angular speed. In other words, the rotating inner ring 210 (and shaft 200) makes eight revolutions for every three revolutions of the cage 240.

By measuring the absolute angular position of the cage 240, it is therefore possible to calculate the angular position of the inner ring 210 through a range that corresponds to just less than one full revolution of the cage. In the example given above, the absolute angle of the inner ring 210 can be calculated through a range of up to 8/3 revolutions. This is attained through a first embodiment of the invention as illustrated in Fig. 3A.

Fig. 3A illustrates an example of part of a bearing unit according to one embodiment the invention. The bearing unit comprises a rotating inner ring 310 and a non-rotating outer ring 320, with rolling elements 330 arranged between the bearing rings 310, 320. A cage 340 retains the rolling elements 330. The bearing unit is suitably located in a housing 350. The bearing unit is provided with a first sensing means, comprising a first code carrier 365 and a corresponding first transducer 375, which is suitably arranged to detect the relative angular position of the first code carrier 365 and first transducer 375. In this example, the first code carrier is coupled to or forms part of the cage 340 and the corresponding first transducer is spatially fixed in relation to the non-rotating bearing ring 320 or another reference. The first transducer 375

generates a signal that is dependent on the angular position of the cage 340 and unique over a single turn, as shown in Fig. 3B, or part of a single turn, as shown in Fig. 3C. In this example the first transducer 375 is connected to a processing unit 360 via a PCB 355 that is itself attached the housing 350. Alternatively, the processing unit can be located outside of the bearing housing, or be mounted on or integral with a seal of the bearing. Other alternatives are also possible.

Fig. 3B and Fig. 3C illustrate examples of signals generated by sensing means according to the first embodiment of the invention, where the x-axis 390 represents cage rotation in radians and the y-axis represents signal amplitude. Fig. 3A illustrates an example of a signal generated when the sensing means is a single-turn absolute position sensor. In this case, the signal has a unique value through one full revolution (2π radians) of the cage 340. Fig. 3B illustrates an example of a signal generated when the first sensing means is a half-turn absolute position sensor. In this case, the signal has a unique value through one half revolution (π radians) of the cage 340. The processing unit 360 processes the unique cage angular position value to obtain the corresponding angular position of the rotating bearing ring 310 in the multitum range. With reference to the previous example, with a bearing with a 3:8 ratio of cage angular speed to inner ring angular speed, the multiturn range is 8/3 revolutions when a single-turn absolute position sensor is used. The use of a half-turn absolute position sensor halves the multiturn range to 4/3 revolutions. This still represents more than one full revolution of the rotating bearing ring.

One method by which the processing unit 360 can calculate the multiturn position of the rotating bearing ring is by means of a lookup table, such as shown in Fig. 4. This is a table of stored values for cage angular position 410 and the corresponding multiturn position 420 of the rotating bearing ring. An example of an algorithm that could be used to determine the multiturn

angular position of the rotating bearing ring according to the invention is as follows:

In a first step 500, the transducer output values are measured.

In a second step 510, the processing unit calculates the angular position based on the measured transducer values. One way of doing this is by matching stored transducer values with the measured transducer values and finding a best fit. The result of the second step 510 is the angular position of the cage.

In a third step 520, the processing unit calculates the corresponding angular position of the rotating bearing ring. One way of doing this is by means of a lookup table of cage angular position and rotating bearing ring angular position combinations through one revolution of the cage. The result of the third step 520 is the multiturn angular position of the rotating bearing ring.

In applications where a multiturn encoder according to the first embodiment of the invention may be subject to axial load, the use of a bearing like a cylindrical roller bearing would be preferred. This is because the contact angle α of the rolling elements in such a bearing does not change when axial load is applied and consequently, the rotation of the cage is not affected by changes in the direction of the load acting on the bearing.

In the first embodiment of the invention, the use of a single turn absolute position sensor is preferred, as it allows the multiturn position of the rotating bearing ring to be determined through a range of up to one revolution of the cage. It is likely, however, that the cage and rotating bearing ring will have unique relative angular positions through more than one revolution of the cage. A second embodiment of the invention is therefore provided, which enables the multiturn position of the rotating bearing ring to be determined through more than one revolution of the cage.

This is attained according to the invention by providing the bearing unit according to the first embodiment with a second sensing means arranged to produce a second signal uniquely dependent on the angular position of the rotating bearing ring over a single turn or part of a turn. According to the invention, combining the information from the first and second signals makes it possible to maximize the multiturn range. This will be explained with reference to Fig. 6A - 6C.

Fig. 6 illustrates an example of part of a bearing unit according to the second embodiment of the invention, which comprises a rotating inner ring 610 and a non-rotating outer ring 620, with rolling elements 660 arranged between the bearing rings 610, 620. A cage 640 retains the rolling elements 630, and the bearing unit is suitable located in a housing 650. Just as the first embodiment, the second embodiment comprises a first sensing means. The first sensing means again comprises a first code carrier 665 and a corresponding first transducer 675, which is suitably arranged to detect the relative angular position of the first code carrier 665 and first transducer 675. In this example, the first code carrier is coupled to or forms part of the cage 640 and the corresponding first transducer is spatially fixed in relation to the non-rotating bearing ring 620 or another reference. The first transducer 675 generates a first signal that is dependent on the angular position of the cage 640 and unique over a single turn, as shown in Fig. 6B, or part of a single turn, as shown in Fig. 6C.

In addition to the first sensing means, the second embodiment further comprises a second sensing means. The second sensing means comprises a second code carrier 670 and a corresponding second transducer 685, which is suitably arranged to detect the relative angular position of the second code carrier 670 and second transducer 680. In this example, the second code carrier is coupled to or forms part of the inner ring 610 and the corresponding second transducer is spatially fixed in relation to the non-

rotating bearing ring 620 or another reference. The second transducer 680 generates a second signal that is dependent on the angular position of the rotating bearing ring 610 and unique over a single turn, as shown in Fig. 6B, or part of a single turn, as shown in Fig. 6C. Both transducers 675, 680 are connected to a processing unit 660 via a PCB.

Fig. 6B and Fig. 6C illustrate examples of signals generated by the sensing means according to the second embodiment of the invention, where the x- axis 390 represents bearing ring rotation in radians and the y-axis represents signal amplitude. Fig. 6B illustrates an example of the first 693 and second signal 694 generated when the first and second sensing means are both single-turn absolute position sensors. In this example, a bearing with a 3:8 ratio of cage angular speed to inner ring angular speed is once again assumed. It can be seen from Fig. 6B that the signal 693 from the cage transducer and the signal 694 from the rotating bearing ring transducer are out of phase with each other, and come back into phase with each other after eight revolutions of the rotating bearing ring (16π radians). For the duration of these eight revolutions, the signals from the transducers have unique value combinations. It is therefore possible to determine the multiturn position of the rotating bearing ring through eight revolutions. Note that with the first embodiment of the invention, the maximum multiturn range is 8/3 revolutions of the rotating bearing ring.

Fig. 6C illustrates an example of the first 695 and second signal 696 generated when the first and second sensing means are both half-turn absolute angular position sensors. A bearing with a 3:8 ratio of cage angular speed to inner ring angular speed is once again assumed. It can be seen from Fig. 6C that the signal 695 from the cage transducer and the signal 696 from the rotating bearing ring transducer are out of phase with each other, and come back into phase with each other after four revolutions of the rotating bearing ring (8π radians). For the duration of these four revolutions, the signals from the transducers have unique value combinations. It is

therefore possible to determine the multiturn position of the rotating bearing ring through four revolutions. Note that with the first embodiment of the invention, the maximum multiturn range (using one half-turn absolute position sensor) is 4/3 revolutions of the rotating bearing ring.

One way of determining the multiturn position of the rotating bearing ring is to use a lookup table, such as illustrated in Fig. 7. In this table, the multiturn position 730 of the rotating bearing ring corresponds to a unique combination of rotating bearing ring angular position 710 and cage angular position 720.

With the second embodiment of the invention, the processing unit 660 applies an algorithm such as depicted by the flowchart in Fig. 8.

In a first step 800, the transducer output values for the cage and rotating bearing ring are measured.

In a second step 810, the processing unit calculates the angular position of the cage and rotating bearing ring from the measured transducer values. One way of doing this is by matching stored transducer values with the measured transducer values and finding a best fit. The result of the second step 810 is a combination of cage angular position and rotating bearing ring angular position.

In a third step 820, the processing unit determines the multiturn position of the rotating bearing ring. Again, this can be done by searching in a lookup table of cage position and rotating bearing ring position combinations through the multiturn range, such as shown in Fig. 7. The result of the algorithm is the multiturn position of the rotating bearing ring.

In applications where a multiturn encoder according to the second embodiment of the invention may be subject to axial load, the use of a bearing like a cylindrical roller bearing would once again be preferred. As

stated before, the cage rotation of such a bearing is not affected by changes in the direction of the load acting on the bearing

In an advantageous further development of the second embodiment of the invention, the processing unit is adapted to improve the accuracy of the determination of the angular position of the cage, and thus of the rotating bearing ring. This is attained by comparing the measured movement of the cage and rotating bearing ring with the expected movement of the cage and rotating bearing ring, and adjusting for any deviations.

During operation, the rolling elements 630 when subjected to load will likely slip somewhat on the bearing raceway, causing a small drift of the position of the cage 640 relative to the actual motion of the rotating bearing ring 610. Consequently, the measured angle of the cage relative to the non-rotating bearing ring 620 may not be the correct (theoretical) angle with respect to the measured angle of the rotating bearing ring 610 relative to the non-rotating bearing ring 620. The rotating bearing ring is not subject to drift, and this measurement is therefore known with greater certainty. To prevent the cage drift from accumulating to a degree that would cause the cage movement to lose its relationship to the movement of the rotating bearing ring, active drift compensation is applied. This means that during operation, the angular position of the cage 640 and the rotating bearing ring 610 are regularly measured at intervals of, for example, a few seconds, depending on the application.

To obtain a more accurate determination of the cage angular position, the processing unit 660 processes the first and second signals using an algorithm such as depicted by the flowchart in Fig. 9.

In a first step 900, the output values for the cage transducer and the rotating bearing ring transducer are measured.

In a second step 910, the processing unit calculates the closest match for both the angular position of the cage and the angular position of the rotating bearing ring. One way of doing this is with the aid of a lookup table of stored values with corresponding angles for both the cage and rotating bearing ring.

In a third step 920, possible cage positions are calculated for the measured position of the rotating bearing ring within the multiturn range using the last- determined combination of rotating bearing ring position and cage position.

In a fourth step 930, the processing unit determines the closest match between the measured cage position and the calculated cage positions. The result of the fourth step 930 is the determined correct angular position of the cage.

In a fifth step 940, the multiturn position of the rotating bearing ring is determined. Once again, this can be done using a lookup table of cage position and rotating bearing ring position combinations through the multiturn range. The result of this algorithm is the determined correct multiturn position of the rotating bearing ring.

In applications where a ball bearing is used in a multiturn absolute encoder according to this advantageous further development and the bearing is subject to axial load, variations in the movement of the cage due to variations in the contact angle α can be compensated for by measuring the rotation of the cage and rotating bearing ring with sufficient frequency.

Active compensation for cage drift, as enabled by the advantageous further development of the second embodiment of the invention, has further advantages. By determining the magnitude of cage drift, information on load conditions can be extracted. The magnitude of cage drift is obtained by comparing the nominal rotation of the cage (calculated from the measured rotation of the rotating bearing ring) with the measured rotation of the cage.

Cage drift is expressed as the relation of the measured ratio of cage rotation to bearing ring rotation to the theoretical value, and is calculated from the following formula:

Cage drift J l - ^{Wc / w} ' ^imeaSUred) ) . l00 % w _c I w _r (theoretical) J

where ω _c represents the rotation of the cage and ω _r represents the rotation of the rotating bearing ring. Consequently, the processing unit used in combination with the inventive multiturn absolute encoder can be further adapted to calculate and/or monitor the cage drift based on the measured signals from the first and second transducers.

When, for example, a cylindrical roller bearing is used in the inventive multiturn encoder, the theoretical ratio of cage rotation to bearing ring rotation can be calculated with greater accuracy, since the contact angle remains constant. The magnitude of cage drift can be calculated. Cage drift is dependent on the radial load on the bearing, and so with a cylindrical roller bearing, the magnitude of cage drift may be used by the processing unit to generate a signal that is indicative of the magnitude of the radial load on the bearing.

With a ball bearing, the theoretical rotation of the cage can be calculated less accurately from the measured rotation of the rotating bearing ring. However, the processing unit could be adapted to monitor changes in the ratio of measured cage rotation to measured bearing ring rotation and produce a corresponding signal that is indicative of variations in the load on the bearing. Moreover, because the angular movement of the cage of a ball bearing is dependent on contact angle, which in turn is dependent on axial load, the processing unit could be further adapted to produce a signal that is indicative of the axial load acting on the ball bearing. The advantageous further development of the

invention thus enables the load and/or variations in the load on the bearing to be determined.

An example of a preferred type of code carrier and transducer that can suitably be used in the embodiments of the invention described, and other embodiments, is illustrated in Fig. 10. The code carrier in this example is a diametrically magnetized bipolar ring 1010, which comprises a first half being the North pole 1015 of the bipolar ring and a second half being the South pole 1020 of the bipolar ring. In a preferred embodiment of the invention, the bipolar ring 1010 is mounted on a rotating part of the bearing, the cage for example. This means that the magnetic field from the bipolar ring rotates as the cage rotates. With the aid of a suitable transducer, the angular position of the magnetic field (and therefore the cage) can be uniquely identified.

The suitable transducer in this example comprises a ring-shaped magnetic field concentrator 1025 made of a magnetically conducting material. The magnetic field concentrator 1025 is split into three equal sections by air gaps 1070. The transducer further comprises Hall-effect sensors 1031 , 1032, 1033, which are disposed in the air gaps 1070. As the angular position of the bipolar ring changes, the output voltage of the Hall-effect sensors 1031 , 1032, 1033 varies in response to the corresponding changes in magnetic field density. The Hall-effect sensors 1031 , 1032, 1033 are connected to a processing unit 1060, which processes the sensor output voltages to obtain a signal indicative of the angular position of the cage.

One method by which this can be done is with the aid of a lookup table of stored values and corresponding angular positions. An example of part of such a lookup table is shown in Fig. 11. The unique combination of values 1110, 1120 and 1130 from the first 1031 , second 1032, and third Hall-effect sensor 1033 correspond to a unique angular position 1140 of the cage.

The preferred type of code carrier and transducer can likewise be applied to determine the absolute angular position of the rotating bearing ring. In this case the processing unit would, for example, make use of a lookup table like the one shown in Fig. 11 , but where the unique combination of values 1110, 1120 and 1130 from the first 1031 , second 1032, and third Hall-effect sensor 1033 correspond to a unique angular position of the rotating bearing ring.

The code carrier and corresponding transducer described above constitute a single-turn absolute position sensor. Alternatively, the sensors disposed in the air gaps 1070 of the magnetic field concentrator 1025 can be anisotropic magneto-resistive sensors. Such an arrangement generates an output signal that is dependent on the angular position of the bipolar ring 1010 over one half revolution. As described earlier, this halves the maximum obtainable multiturn range. But if the multitum range that can be obtained using anisotropic magneto-resistive sensors is sufficient, their application may be desirable for reasons of improved signal resolution or reduced power consumption.

The invention is not restricted to magnetized code carriers and magnetic transducers; any type of absolute angular position sensor can be used. Neither is the invention limited to a particular type of bearing; the principle of the invention can be applied to any type of rolling element bearing with a cage. In short, the invention is not restricted to the above-described embodiments, but may be varied within the scope of the appended claims.

NL06023 DHT/SPB 2007-02-16

FIGURE 1 - illustrates a steering column arrangement where the shaft is supported by a bearing unit according to the invention,

100 steering shaft,

102 steering wheel,

104 bearing unit according to the invention,

106 processing unit.

FIGURE 2 - illustrates a bearing,

200 shaft,

210 inner ring,

220 outer ring,

230 rolling elements,

240 cage,

250 housing,

260 pitch diameter of bearing,

270 diameter of rolling element.

FIGURE 3A - 3C - illustrate a section of a bearing unit according to a first embodiment of the invention and examples of signals generated

310 inner ring,

320 outer ring, 330 rolling elements,

340 cage,

350 housing,

355 PCB,

360 processing unit, 365 code carrier

375 transducer.

390 x axis: cage rotation in radians,

391 , 393 signal

392 y axis: signal amplitude

FIGURE 4 - illustrates an example of part of a look-up table suitable for the first embodiment of the invention, 410 absolute angular position of cage (in radians)

420 multiturn position of rotating bearing ring (in radians)

FIGURE 5 - illustrates an example of a flowchart of an algorithm to determine an absolute multiturn position of a rotating bearing ring according to the invention, 500 Measure transducer output values. 510 Calculate angular position of cage from measured transducer values.

520 Calculate corresponding angular position of rotating bearing ring e.g. by means of a lookup table of cage position and rotating bearing ring position combinations through one revolution of the cage.

FIGURE 6A - 6C - illustrate a second embodiment of the invention and examples of signals generated,

610 inner ring,

620 outer ring,

630 rolling elements,

640 cage,

650 housing,

655 PCB,

660 processing unit,

665 code carrier on cage,

660 code carrier on rotating bearing ring,

665 transducer for code carrier on cage,

680 transducer for code carrier on rotating bearing ring.

690 x axis: bearing ring rotation in radians,

691 y axis: signal amplitude 693, 695 signal from cage transducer,

694, 696 signal from rotating bearing ring transducer.

FIGURE 7 - illustrates an example of part of a look-up table suitable for the second embodiment of the invention, 710 Angular position in radians of rotating bearing ring,

720 Angular position in radians of cage,

730 Multiturn position of rotating bearing ring.

FIGURE 8 - illustrates an example of a flowchart of an algorithm to determine an absolute multiturn position of a rotating bearing ring according to the invention, 800 Measure transducer output values for cage and rotating bearing ring.

810 Search for best fit in database of stored values with corresponding angles for both cage and rotating bearing ring.

820 Find closest match to this combination in a lookup table of cage position and rotating bearing ring position combinations through multiturn range.

IFIGURE 9 - illustrates an example of a flowchart of an algorithm to compensate for cage drift in a bearing according to the invention, 900 Measure transducer output values for rotating bearing ring and for cage.

910 Search for best fit in database of stored values with corresponding angles for both cage and rotating bearing ring.

920 Calculate possible cage positions for the given rotating bearing ring position within multiturn range using the last known rotating bearing ring position/cage position combination.

930 Find closest match between the measured cage position and the calculated cage positions.

940 Find closest match to combination of cage position and rotating bearing ring position in a lookup table of cage position and rotating bearing ring position combinations through multiturn range.

FIGURE 10 - illustrates an example of an absolute position sensor,

1010 diametrically magnetized bipolar ring,

1015 North pole of diametrically magnetized bipolar ring,

1020 South pole of diametrically magnetized bipolar ring,

1025 magnetic field concentrator, 1031 Hall-effect sensor 1 ,

1032 Hall-effect sensor 2,

1033 Hall-effect sensor 3, 1060 processing unit, 1070 air gap.

FIGURE 11 - illustrates an example of part of a lookup table suitable for an absolute position sensor as shown in Fig. 10.

1110 Sensor 1 output in Volts,

1120 Sensor 2 output in Volts, 1130 Sensor 3 output in Volts,

1140 Angular position in radians of cage.