(19)
(11)EP 3 702 738 A1

(12)EUROPEAN PATENT APPLICATION

(43)Date of publication:
02.09.2020 Bulletin 2020/36

(21)Application number: 20159859.6

(22)Date of filing:  27.02.2020
(51)International Patent Classification (IPC): 
G01D 5/20(2006.01)
(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR
Designated Extension States:
BA ME
Designated Validation States:
KH MA MD TN

(30)Priority: 01.03.2019 US 201962812851 P
12.02.2020 US 202016789374

(71)Applicant: Renesas Electronics America Inc.
Milpitas, CA 95035 (US)

(72)Inventors:
  • BERTINI, Giovanni
    85579 Neubiberg (DE)
  • MATHISEN, Monica
    80469 München (DE)
  • BINDA, Ivo
    27058 Voghera (IT)
  • LEONCAVALLO, Ruggero
    8101 Gratkorn (AT)

(74)Representative: Lippert Stachow Patentanwälte Rechtsanwälte Partnerschaft mbB 
Krenkelstraße 3
01309 Dresden
01309 Dresden (DE)

  


(54)METHOD FOR INCREASING THE POSITION MEASUREMENT ACCURACY USING INDUCTIVE POSITION SENSOR


(57) An accurate position sensor that operates over a long range is provided. The position sensor can include a first sensor coil having a first number of periods over a range of motion of a target; and a second sensor coil having a second number of periods over the range, wherein the first number of periods is different from the second number of periods, and wherein the first sensor coil and the second sensor coil are arranged with respect to one another such that the target engages both of them simultaneously. In some embodiments, the first number of periods is one and the second number of periods is greater than one. In some embodiments, the first number of periods is greater than one and the second number of periods is greater than the first number of periods.




Description

Related Applications



[0001] This application claims priority to U.S. Provisional Application 62/812,851, filed on March 1, 2019, which is herein incorporated by reference in its entirety.

Technical Field



[0002] Embodiments of the present invention are related to position sensors and, in particular, to improvements on position sensors.

Discussion of Related Art



[0003] Position sensors are used in various settings for measuring the position of one component with respect to another. Inductive position sensors can be used in automotive, industrial and consumer applications for absolute rotary and linear motion sensing. In many inductive positioning sensing systems, a transmit coil is used to induce eddy currents in a metallic target that is sliding or rotating above a set of receiver coils. Receiver coils receive the magnetic field generated from eddy currents and the transmit coils and provide signals to a processor. The processor uses the signals from the receiver coils to determine the position of the metallic target above the set of coils. The processor, transmitter, and receiver coils may all be formed on a printed circuit board (PCB). However, maintaining accuracy of the position sensor may be difficult over a longer distance measurement (e.g. 124mm vs 19mm distances).

[0004] Therefore, there is a need to develop better methods of designing position sensors that offer better accuracy for position sensing.

Summary



[0005] An accurate position sensor that operates over a long range is provided. The position sensor can include a first sensor coil having a first number of periods over a range of motion of a target; and a second sensor coil having a second number of periods over the range, wherein the first number of periods is different from the second number of periods, and wherein the first sensor coil and the second sensor coil are arranged with respect to one another such that the target engages both of them simultaneously. In some embodiments, the first number of periods is one and the second number of periods is greater than one. In some embodiments, the first number of periods is greater than one and the second number of periods is greater than the first number of periods.

[0006] A method of determining a position with a position sensor includes determining a first value from a first sensor having a first number of periods over a range of motion of a target; determining a second value from a second sensor coil having a second number of periods over the range; and calculating a position value from the first value and the second value.

[0007] These and other embodiments are discussed below with respect to the following figures.

Brief Description of the Figures



[0008] 

Figures 1A and 1B illustrate typical operation of a position sensor.

Figures 2A and 2B illustrate position sensors according to some embodiments of the present invention.

Figure 3A illustrates operation of a position sensor having a one-period geometry and a multi-period geometry according to some embodiments.

Figure 3B illustrates operation of a position sensor having two multi-period geometries according to some embodiments.

Figure 4 illustrates parameters for various sensor geometries according to a particular 140 mm example sensor.

Figure 5 illustrates design parameters for various sensor geometries according to some embodiments.

Figures 6A and 6B illustrate printed circuit board layouts for position sensor configurations according to some embodiments.

Figure 6C illustrates a sensing system using the position sensor configurations according to some embodiments.

Figure 7 illustrates electrical data and pre-measurement data for sensor geometries according to some embodiments.

Figures 8A, 8B, and 8C illustrate testing of printed-circuit boards having sensor geometry layouts according to some embodiments.

Figures 9A, 9B, 9C, and 9D illustrate error data for operation of position sensors with geometries according to some embodiments.



[0009] These and other aspects of embodiments of the present invention are further discussed below.

Detailed Description



[0010] In the following description, specific details are set forth describing some embodiments of the present invention. It will be apparent, however, to one skilled in the art that some embodiments may be practiced without some or all of these specific details. The specific embodiments disclosed herein are meant to be illustrative but not limiting. One skilled in the art may realize other elements that, although not specifically described here, are within the scope and the spirit of this disclosure.

[0011] This description illustrates inventive aspects and embodiments should not be taken as limiting--the claims define the protected invention. Various changes may be made without departing from the spirit and scope of this description and the claims. In some instances, well-known structures and techniques have not been shown or described in detail in order not to obscure the invention.

[0012] Embodiments of the present invention provide for accuracy over a long-range position sensing situation by using multiple sets of sensor coils having different periods. In that fashion, the multiple sets of sensor coils provide multiple measurements of the position of the target, which can be processed to provide an accurate measurement of the position of the target.

[0013] Figures 1A and 1B illustrate operation of a typical positioning system 100. Although Figures 1A and 1B illustrate operation of a linear position sensor, rotational position sensors are operationally the same. As illustrated in Figure 1A, the positioning system includes transmit/receive control circuit 102 that is coupled to drive a transmitter coil 106 and receive signals from receive coils 104. In most configurations, receive coils 104 are located within transmitter coil 106, however in Figure 1A they are illustrated separately for clarification purposes. Receive coils 104 are generally physically located within the border of transmit coil 106. Embodiments of the present invention can include a transmitter coil 106, two receiver coils 104, and an integrated circuit (IC) 102 driving the transmitter coil 106 and measuring the signals originated in the receiver coils 104 all formed on a printed circuit board (PCB).

[0014] Figure 1B illustrates a configuration of transmit coils 106 and receive coils 104 in a linear position locating system. As is shown in Figure 1B, a conductive metallic target 124 can be positioned over the transmitter coil 106 and the two receiver coils 104.

[0015] As is illustrated in Figure 1A, transmit coil 106 is driven to form magnetic field 108. Transmit coil 106 can be driven at a range of frequencies or at particular frequencies. In Figure 1A, magnetic field 108, with the positive current illustrated by the arrows, is circular around each wire and in a direction that points out of the page inside coil 106 and into the page outside of coil 108 with the current direction as illustrated in Figure 1A. Receive coils 104 are located inside coil 106, as is illustrated in Figure 1B. Transmit coil 106 is driven at any frequency that can produce electromagnetic field 108 to induce voltages in receiver coils 104. In general, there can be any number of receiver coils, however, for ease of discussion, a system with two receiver coils is discussed below.

[0016] Figure 1B illustrates the arrangement of sensor receive coils (RX) 104 within transmit coil (TX) 106 for a linear position sensor. As illustrated in Figure 1B, sensor receive coils 104 includes a sine-wave oriented coil RXSIN 112 and a cosine-oriented signal coil RXCOS 110. Sine-wave oriented coil RXSIN 112 includes sine loops 114, 116, and 118 where coil 112 is wound in in-phase or anti-phase directions, here depicted as clockwise or counter clockwise depictions, to result in the production of voltages in the loop of opposite sign as a result of the presence of electro-magnetic field 108. As is illustrated, the wiring of sine- wave oriented coil 112 provides a clockwise rotation in loops 114 and 118 resulting in a nominally positive voltage and a counterclockwise rotation in loop 116 resulting in nominally negative voltages. Similarly, cosine-oriented coil 110 may include a first loop 120 with a clockwise orientation and a second loop 122 with a counterclockwise orientation. Figure 1B illustrates a possible electromotive force reference direction, as indicated by the arrows, that is consistent with the magnetic fields produced by transmitter coil 106 as illustrated in Figure 1A.

[0017] In the system illustrated in Figure 1B, the transmitter coil (TX) 106 is stimulated by the circuit 102, which may be an integrated circuit, to generate a variable Electromagnetic field (EMF), illustrated as magnetic field 108. The magnetic field 108 couples with the receiver coils (RX) 104. If a conductive metallic target 124 is placed on the top of the receiver coils 104 as illustrated in Figure 1B, an eddy current is generated in the metallic target 124. This eddy current generates a new electromagnetic field that is ideally equal and opposite of field 108, canceling the field in receiver coils 104 directly under metallic target 124. The receiver coils (RX) 104 capture the variable EMF field 108 generated by the transmit coils 106 and those induced by metallic target 124, resulting in sinusoidal voltages generated at the terminals of receiver coils 104.

[0018] In absence of metallic target 124, there will be no voltage at the terminals of the RX coils 104 (labeled RxCOS 110 and RXSin 112 in Figure 1B). When metallic target 124 is placed in a specific position with respect to the RX coils 104, the resultant electromagnetic field on the area covered by the metallic target 124 is ideally zero and therefore the voltages at the terminals of the RX coils 104 will have different characteristic depending on the location of metallic target 124 relative to receiver coils 104. The RX coils 104 are designed in a way that a sine voltage is created at the terminals of one RX coil (RxSin 112) and a cosine voltage is created at the terminals of the other RX coil (RxCos 110) as metallic target 124 is swept across receiver coils 104. The position of the target with respect to the RX coils 104 modulates the amplitude and the phase of the voltage at the terminals of the RX coils 104.

[0019] As illustrated in Figure 1A and discussed above, transmitter coil 106, receive coils 104, and transmit/receive circuit 102 can be mounted on a single PCB. Further, the PCB can be positioned such that metallic target 124 is positioned above receive coils 104 and spaced from receive coils 104 by an air gap (AG). The position of metallic target 124 relative to the PCB on which receive coils 104 and transmitter coil 106 is mounted can be determined by processing the signals generated by sine-oriented coil 112 and cosine-oriented coil 110. Below, the determination of the position of metallic target 124 with respect to receive coils 104 is described in a theoretical ideal condition.

[0020] In Figure 1B, metallic target 124 is located at a first location. In this example, Figures 1B and depicts operation of a linear position locator system. The principle of operation is the same in both linear and circular locators. In the discussion below, the position is given in relation to the construction of cosine-oriented coil 110 and sine-oriented coil 112 by providing the angular relations with respect to the sine operation of sine-oriented coil 112 which results from the position of the leading edge of metallic target 124 and coils 110 and 112. The actual position of metallic target 124 in such a system can be derived from the angular position as measured by the output voltages of receive coils 104 and the topology of receive coils 110 and 112. Furthermore, as illustrated in Figure 1B, the topology of coil 110 and the topology of coil 112 are coordinated to provide indication of the location of metallic target 124 relative to the receive coils 104.

[0021] Figure 1B illustrates a one-period receiver coil 104. However, the accuracy of this arrangement can be improved by the addition of a second receiver coil having multiple periods along the traveling distance of target 124.

[0022] Embodiments of the present invention can achieve higher accuracy on long-range measurements using similar technologies. Some embodiments are directed towards achieving high accuracy on long range measurements using the same silicon devices that applies for short ranges. For example, getting the same accuracy over 124 mm of travel as can be achieved over a 19 mm range. Applications are currently using long measurement ranges and better accuracy than the short-range measurement ranges are capable of producing. The accuracy of long range measurement positions sensors according to some embodiments are increased by a factor of M over the short-range single period systems currently available.

[0023] Accordingly, some embodiments of the present invention implement sensors with a double scale provided on the printed circuit board (PCB). One scale can be for raw positioning while one scale is for fine positioning. A physical implementation of a system using two sensors simultaneously working in parallel. One of the two sensors can have on period over the range of the position sensor while the other sensor has multiple periods over the range. Both sensors use the same target and have the same travel range. An algorithm for analyzing the position sensor signals from both sensors provides an accurate position of the target.

[0024] Embodiments of the present invention can be formed with existing technologies. For example, the silicon device ZMID520x made by Integrated Device Technologies can be used. Such a device has a performance of about 0.2% accuracy. Such accuracy performance is sensor shape dependent. For purposes of explanation, a linear geometry is discussed below. A linear geometry does not have the best accuracy in comparison of other geometries however it may have a larger market potential for a linear long-stroke position sensor.

[0025] Some embodiments use two algorithms: a) a Pseudo Vernier (Double scale: one for raw positioning and one for fine positioning); and b) A Vernier algorithm. Some embodiments employ the use of at least two sets of sensor coils working in parallel (i.e. simultaneously), each one having a different period with respect to the full travel of the target over the sensor coils. For example, in a first exemplary system, two sensor coil geometries are used (each sensor geometry, for example, including a sine and a cosine coil) where a first sensor coil geometry includes a one-period geometry and the second sensor coil geometry includes a multiperiod geometry. In a second example, both sensor geometries are multi-period geometries that include different periods, e.g. a first board having a sensor coils of N1 periods and a second board having sensor coils of N2 periods. In either example, both sensor geometries have the same travel range and the same target characteristics. These two examples are discussed further below.

[0026] Figures 2A and 2B illustrate relative relationships of sensor coils according to some embodiments of the present invention. As illustrated in Figure 2A, configuration 200 includes a first coil 202, including periods 202-1 through 202-N, having N periods and a second coil 204 having a single period. Both sets of receive coils are arranged to have the same target travel range. Furthermore, each coil determines a target position based on the values of the sine and cosine voltages. Coil 202 determines a position XA based on VAcos and VAsin while coil 204 determines a position XB based on VBcos and VBsin.

[0027] Figure 3A illustrates a position recovery geometry from a multi-period position sensor combined with a single period sensor as illustrated in Figure 2A. Response from the single period sensor geometry as a function of target position X is provided as response 302 while response from the multi-period sensor geometry as a function of target position X is provided as response 304. At an actual position Xs, single period geometry from sensor coils 204 resulting in response 302 provides a response value of Ys while multi-period geometry from coils 202 giving response 304 provides a response value of Ym. As is further illustrated in Figure 1, the single period length of multi-period geometry 204, with response 304, is given by L. As is illustrated, the single period response value Ys provides locates which period of the multiperiod sensor 204 (K=0, 1, 2, 3, ...) is active and the multi-period provides the fine location within that period. Consequently, the position value can be provided by a rough value Xs and a fine value Xm as follows:



where Xs is the rough position given by response 302, Xm is the fine position given by response 304, δ represents the angle of the line representing line 302 (given in terms of counts/mm), Ys is the response from response 302 with the coil at the actual position of target, Ym is the response from response 304of the multi-period geometry, and k is the period of response 304 as determined by rough value Ys. As indicated in Figure 3, a response Ym of response 304 can correspond to positions Xma, Xmb, Xmc, and Xmd. However, the response Ys locates target in this particular example within the k=2 period of response 304. This can be determined by calculating the following:





and

And determining which of them fulfills the relationship
|Xs-Xmj|<L/2, where j=0,1,2,3.

[0028] Figure 2B illustrates a configuration 210 with a first receive coil 212 having N1 periods (212-1 through 212-N1) and a second receive coil 214 having N2 periods (214-1 through 214-N2). As discussed above, coil 212 and 214 are positioned relative to each other to measure the same travel of the target. Further, receive coil 212 provides voltages VCcos and VCsin from which a position of the target can be determined relative to coil 212 while coil 214 provides voltages VDcos and VDsin from which a position of the target can be determined relative to coil 214. As discussed above, the positions measured by each of coils 212 is periodic depending on the one period segment where the target is located.

[0029] Figure 3B illustrates a situation where two multi-period geometries are used such as that illustrated in Figure 2B. In this illustrates, N1 is the number of periods of the first geometry and N2 is the number of periods in the second geometry. It is sufficient, as in this case, that N1=N2+1, however any relationship between N2 and N1 such that N2 is not equal to N1 can be used. Using this as an example, if OUTD is the output value from coil 214, with depicted response 308 , OUTC is the output value 306 from coil 212, and the absolution position value is provided as line 310, then the position can be calculated as follows:

If OUTC>OUTD: absolute measured position is OUTC-OUTD;

IF OUTC<OUTD: absolute measured position is OUTC-OUTD+Full Scale Position.

For other values of N2 and N1, other calculates and arrangements will apply. However, these calculations are apparent from the geometric layout.

[0030] Coil configurations 200 and 210 as depicted in Figures 2A and 2B can be formed on a single printed circuit board (PCB) or can be formed on separate PCBs and arranged relative to each other. In some embodiments, coil 204 and 202 illustrated in Figure 2A are arranged to overlap, e.g. one being over-laid on the other. In some embodiments, the end period of one or both of the coils may include only a partial period.

[0031] Figure 4 shows a table with various PCB configurations to facilitate an example of configuration 200 illustrated in Figure 2A or 2B formed on different PCB boards. This example is for a target travel distance of 140 mm. For example, a one period PCB (PCB_1P) has a receive coil length of 140 mm a transmitter coil width of 16 mm. With a target size of 16 mm X 16mm, the target is located with 11.4%. With a three-period receive coil of length 140 mm, the 1-period length can be about 46.7 mm. With a four-period receive coil of length 140 mm, the 1 period length is 35.0 mm. As is further illustrated, with a total travel range of 124 mm, the PCB_3P one-period travel range is 30.7 while in the PCB-4P configuration, the one-period travel range is 19.0 mm. Figure 5 illustrates results of various geometric characteristics with three-period, four-period, and one-period configurations according a particular example of the present invention. One skilled in the art will recognize that these operation parameters can be obtained for any particular configuration according to some embodiments.

[0032] Figures 6A and 6B illustrate the layout of sensor coils on PCB boards according to some embodiments. As shown in Figures 6A and 6B, two arrangement of sensor coils are arranged on separate PCBs or on separate sides of a single PCB. The target can travel over both sensor coils at the same time. Figure 6A, for example, illustrates a geometry 600 with a first sensor coil 602 being a single period construction and a second sensor coil 604 with a multi-period construction. As is further illustrated in Figure 6A, electronics 606 drives transmit coils 610 and receives sine and cosine signals from receive coils 604 while electronics 608 drives transmit coils 612 and receives sine and cosine signals from receive coils 602. Consequently, electronics 606 provides a position indication from receive coils 604 while electronics 608 provides a position indication from receive coils 602, which was discussed above with respect to Figure 3A. Consequently, Figure 6A illustrates a 4P-1P embodiment.

[0033] Figure 6B illustrates a geometry with both sensor coils being multi-period construction, where the periods are different. In particular, Figure 6B illustrates a 4P-3P implementation. As shown, implementation 620 includes a 4P receive coil 624 and a 3P receive coil 622 implemented side by side such that the target can overlay both of them. Electronic circuit 626 can drive transmit coils 630, receive signals from receive coils 624, and provide location data indicating the location of a target over receive coils 624. Similarly, electronic circuit 638 can drive transmit coils 632, receive signals from receive coils 622, and provide location data indicating the location of a target over receive coils 622.

[0034] Figure 6C illustrates a block diagram of a system 650 further illustrating the arrangement illustrated in Figures 6A and 6B. As shown in Figure 6C, system 650 includes a processor 652 coupled to a configuration 654 according to some embodiments. Configuration 654 includes first coils 656, which is a N1P coil, and second coils 658, which is a N2P coil. Consequently, configuration 654 represents an N1P-N2P configuration. Circuit 660 drives transmit coils, receives signals from receive coils, and provides location data to processor 652 related to first coils 656. Circuit 662 drives transmit coils, receives signals from receive coils, and provides location data to processor 652 related to second coils 658. The location data in each of first coil 656 and second coils 658 indicates the location of target 664 as it is traversed along the length. Processor 652 includes memory, microprocessors, and other components sufficient to store and execute instructions for calculating the accurate positions as described in this disclosure. In some embodiments, processor 652 may be included on the PCB of configuration 654 to form an integrated package.

[0035] Figure 7 illustrates electrical data and pre-measurements for various coil configurations according to some embodiments. The boards being investigated are 1P, 3P, 4P and combinations of these boards. These boards are further tested as shown in Figures 8A, 8B, and 8C.

[0036] Figures 8A, 8B, and 8C illustrate implementations of configurations of sensor coils according to some embodiments placed in a test instrument. As illustrated, each of the sensor coils may be formed on a separate coil PCB boards and placed in relation to each other such that a target, which has the same configuration over each of the coils, is moved along the linear path. Figure 8A illustrates a testing apparatus 802 that is coupled to move target 664 relative to coils configurations 656 and 658. Figure 8A illustrates a first view of the testing apparatus. Figure 8C illustrates a second view of the apparatus, with electronic circuits 660 and 662 coupled to an external processor. Figure 8B illustrates the target 664 along with the four specific boards (1P, 3P and 4P) discussed above. As is illustrated in Figures 8A, 8B, and 8C, target 664 may include two conducting plates separated by an insulator or may be a continuously formed metallic plate.

[0037] Test data was obtained using the geometries illustrated in Figures 8A, 8B, and 8C, with an air gap (distance between the sensor coils and the target) being 1mm, a fixed gain of 10, and a target position resolution of 0.1mm. For a Pseudo-Vernier method, calibration, calibration and linearization for coil A is used on average of SPAs. Using the Vernier method calibration and linearization for coil C and average of SPAs along with calibration and linearization for coil D along with average SPAs was used.

[0038] Figure 9A illustrates the full-scale error for measurement error with a single period sensor coil. Figure 9B illustrates the measurement error using a pseudo-Vernier algorithm. Figure 9C illustrates the measurement error using a Vernier algorithm. Figure 9D illustrates a comparative table of errors. As is illustrates, the Pseudo Vernier method offers the best performance with a factor of 4 improvement in accuracy in comparison to the single coil sensor. An accuracy of 0.12 mm (0.1 %) versus 0.52 mm (0.42%) of the single period coil over a 124mm measurement range is observed.

[0039] The above detailed description is provided to illustrate specific embodiments of the present invention and is not intended to be limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the following claims.


Claims

1. A position sensor, comprising:

a first sensor coil having a first number of periods over a range of motion of a target; and

a second sensor coil having a second number of periods over the range,

wherein the first number of periods is different from the second number of periods, and

wherein the first sensor coil and the second sensor coil are arranged with respect to one another such that the target engages both of them simultaneously.


 
2. The position sensor of claim 1, wherein the first number of periods is one and the second number of periods is greater than one.
 
3. The position sensor of claim 2, wherein the first sensor provides a course position location and the second sensor provides a fine position location.
 
4. The position sensor of claim 3, wherein the second number of periods is three.
 
5. The position sensor of claim 3, wherein the second number of periods is four.
 
6. The position sensor of claim 3, wherein the second number of periods is greater than four.
 
7. The position sensor of claim 1, wherein the first number of periods and the second number of periods is greater than one.
 
8. The position sensor of claim 7, wherein the first number of periods is greater than the second number of periods.
 
9. The position sensor of claim 8, wherein the first number of periods is one more than the second number of periods.
 
10. A method of determining a position of a target with a position sensor, comprising
arranging the target over a first sensor having a first number of periods over a range of motion of the target and a second sensor having a second number of periods over the range of motion of the target;
determining a first value from a first sensor;
determining a second value from the second sensor coil; and
calculating a position value from the first value and the second value.
 
11. The method of claim 10, wherein the first number of periods is one and the first sensor provides a coarse location while the second sensor provides a fine location.
 
12. The method of claim 10, wherein the first number of periods and the second number of periods are both greater than one and the position value is determined by comparing the first value and the second value.
 
13. A position sensing system, including
a first sensor coil having a first number of periods over a range of motion of a target;
a second sensor coil having a second number of periods over the range, wherein the first number of periods is different from the second number of periods, the first sensor coil and the second sensor coil are arranged with respect to one another such that the target engages both of them simultaneously;
a first electronic circuit coupled to drive a first transmit coil and receive signals from the first sensor coil, the first electronic circuit providing first data;
a second electronic circuit coupled to drive a second transmit coil and receives signals from the second sensor coil, the second electronic circuit providing second data; and
a processor coupled to receive the first data and the second data and determine a location of the target based on the first data and the second data.
 
14. The position sensing system of claim 13, wherein the first number of periods is one.
 
15. The position sensing system of claim 13, wherein the second number of periods is one more than the first number of periods.
 




Drawing








































Search report









Search report




Cited references

REFERENCES CITED IN THE DESCRIPTION



This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description