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Title:
DIFFERENTIAL MODE HEIGHT SENSOR
Document Type and Number:
WIPO Patent Application WO/2019/086367
Kind Code:
A1
Abstract:
A distance measuring device (100) comprises a first sensing module (110), a second sensing module (120), a reference device (130), and an evaluating module (140). The first and second sensing modules are arranged on a common base line (150) and are each configured to detect the strength of a magnetic field (50) in a first and second sensing direction (111, 121), respectively. The reference device is movable with respect to the sensing modules along a movement trajectory (160) and comprises a magnetic field element to emit a magnetic field (50) detectable by the first and second sensing module. Each of the first and second sensing module has a sensing direction pointing towards the other sensing module. The evaluating module is configured to determine the distance between the base line and the reference device based on the strength of the magnetic field in the first and second sensing direction.

Inventors:
MAY, Lutz (Etztalstr. 14, Berg, 82335, DE)
Application Number:
EP2018/079539
Publication Date:
May 09, 2019
Filing Date:
October 29, 2018
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
TRAFAG AG (Industriestrasse 11, 8608 Bubikon, 82319, CH)
International Classes:
G01B7/14; G01B7/02; G01D5/14
Domestic Patent References:
WO2017162538A12017-09-28
Foreign References:
US20140306695A12014-10-16
US20140032163A12014-01-30
EP1464918A22004-10-06
Other References:
None
Attorney, Agent or Firm:
LKGLOBAL | LORENZ & KOPF PARTG MBB PATENTANWÄLTE (Brienner Str. 11, Munich, 80333, DE)
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Claims:
CLAIMS

1. Distance measuring device (100) comprising:

a first sensing module (110),

a second sensing module (120),

a reference device (130),

an evaluating module (140),

wherein the first sensing module and the second sensing module are arranged on a common base line (150),

wherein the first sensing module (110) is configured to detect the strength of a magnetic field (50) in a first sensing direction (11 1),

wherein the second sensing module (120) is configured to detect the strength of a magnetic field (50) in a second sensing direction (121),

wherein the reference device is movable with respect to the first sensing module and the second sensing module along a movement trajectory (160),

wherein the reference device comprises a magnetic field element configured to emit a magnetic field (50) detectable by the first and second sensing module,

wherein the first sensing direction (111) points towards the second sensing module (120);

wherein the second sensing direction (121) points towards the first sensing module (110),

wherein the evaluating module is configured to determine the distance between the base line and the reference device based on the strength of the magnetic field in the first and second sensing direction.

2. Distance measuring device according to claim 1,

wherein the movement trajectory is linear and perpendicular with respect to the base line.

3. Distance measuring device according to claim 1 or 2, wherein the first sensing direction (111) and the second sensing direction (121) are perpendicular with respect to the movement trajectory.

4. Distance measuring device according to any one of the preceding claims,

wherein the distance measuring device is configured to use only a measured value from a single sensing direction of each of the first sensing module and the second sensing module.

5. Distance measuring device according to any one of the preceding claims,

wherein the movement trajectory intersects with the base line at a center point which is equidistant from the first sensing module and the second sensing module.

6. Distance measuring device according to any one of the preceding claims, further comprising:

a third sensing module (410) and a fourth sensing module (420) interconnected by a second base line (460);

wherein the third sensing module (410) is configured to detect the strength of the magnetic field (50) in a third sensing direction that points towards the fourth sensing module,

wherein the fourth sensing module (420) is configured to detect the strength of the magnetic field (50) in a fourth sensing direction that points towards the third sensing module.

7. Distance measuring device according to claim 6,

wherein the second base line (460) intersects the base line (160) at an angle of

90°.

8. Distance measuring device according to any one of the preceding claims, further comprising: a fifth sensing module (430) arranged at a center point between the first, second, third, and fourth sensing modules.

9. Distance measuring device according to claim 8,

wherein the fifth sensing module is configured to detect the strength of the magnetic field emitted by the reference device,

wherein the evaluating unit is configured to determine a distance between the fifth sensing module and the reference device based on the strength of the magnetic field detected by the fifth sensing module.

10. Distance measuring device according to any one of the preceding claims,

wherein the reference device comprises a permanent magnet.

11. Distance measuring device according to claim 10,

wherein the permanent magnet has a magnetic pole axis which coincides with the movement trajectory.

12. Distance measuring device according to any one of claims 1 to 9,

wherein the reference device is an electrically powered magnetic field generator.

13. Distance measuring device according to any one of the preceding claims, further comprising:

a calibration sensor (470) and a calibration unit (148),

wherein the calibration sensor is configured to determine the distance between the base line (150) and the reference device (130),

wherein the calibration sensor is a magnetic field sensor with three sensing directions.

14. Air spring comprising:

a first mounting plate being adapted to be mounted to a chassis of a vehicle, a second mounting plate being adapted to be mounted to a wheel suspension, and a distance measuring device according to any one of claims 1 to 12,

wherein the first and second sensing modules (110, 120) are mounted to the first mounting plate, and wherein the reference device (130) is mounted to the second mounting plate,

wherein the air spring further comprises a flexible member, wherein the first mounting plate, the second mounting plate, and the flexible member define a pressurizable chamber, and wherein the first and second sensing modules and the reference device are located within the pressurizable chamber.

Description:
DIFFERENTIAL MODE HEIGHT SENSOR

Technical Field

The present invention relates to a distance measuring device which may be a distance measurement sensor, for example, and an air spring with such a distance measuring device. Technical Background

Height or distance measurement has a wide variety of possible applications. For instance, it is a parameter that frequently needs to be monitored to optimize the performance of various types of machinery and vehicles, such as automobiles, trucks, trains, agricultural vehicles, mining vehicles, construction vehicles, and the like. For instance, monitoring height and various distances can lead to reduced fuel consumption, improved comfort, reduced overall cost, extended product service life, and safety. In any case, the need to monitor such distance parameters may generally increase with sophistication of the devise and the complexity of its features.

Virtually every aspect of complex machinery may need to be tightly monitored and controlled to attain maximum advantages. For instance, constant adaptations may be required to optimize the performances and efficiency of almost every moving part of the machinery. This typically needs to be done while the operational conditions in the environment of the equipment are subject to change and can change significantly over very short time frames. Changing environmental conditions are virtually always encountered by vehicle. In addition to this, vehicles frequently operate under changing conditions which can make monitoring a difficult challenge. For instance, monitoring suspension height by distance measurements between air spring components can yield useful information. However, the environment where the height measurement is being made can present a wide variety of challenges. For example, in measuring the height of a vehicle frame above the surface of a road, challenges are typically presented by road noise, dirt, dust, and vibrations which are normally present in the environment surrounding the vehicle where the measurement is being taken.

For height sensors based on magnetic principle measurements, dealing with

environmental influences like parasitic magnetic fields, especially the earth magnetic field, dynamically changing magnetic fields, and vehicle caused magnetic fields might require particular attention.

Summary of the Invention

There may be a need to reduce the effect of unwanted magnetic stray fields on the distance measurement using magnetic fields.

According to an aspect, a distance measuring device is provided. The distance measuring device comprises a first sensing module, a second sensing module, a reference device, and an evaluating module. The first sensing module and the second sensing module are arranged on a common base line. The first sensing module is configured to detect the strength of a magnetic field in a first sensing direction. The second sensing module is configured to detect the strength of a magnetic field in a second sensing direction. The reference device is movable with respect to the first sensing module and the second sensing module along a movement trajectory, wherein the reference device comprises a magnetic field element configured to emit a magnetic field detectable by the first sensing module and second sensing module. The first sensing direction points towards the second sensing module and the second sensing direction points towards the first sensing module. The evaluating module is configured to determine the distance between the base line and the reference device based on the strength of the magnetic field in the first sensing direction and second sensing direction. In other words, the first sensing module and the second sensing module are laterally offset with respect to the reference device and its movement trajectory or direction of movement. A connection line between the first and second sensing modules defines a base line. The first and second sensing modules are spaced apart from each other along the base line. The lateral spacing between each one of the sensing modules and the reference device is preferably the same, i.e., the movement trajectory or line of movement of the reference device intersects the base line centrically between the first and second sensing module. It should be understood that a movement of the reference device with respect to the sensing modules may be a relative movement between the sensing modules and the reference device and may mean that the reference device is moved while the sensing modules are standing still or that the sensing modules are moved while the reference device is standing still or that both, the sensing modules and the reference device are moved. However, there is no relative movement between the first and the second sensing modules.

According to an embodiment, the distance between the reference device and the sensing modules is determined by a differential measurement approach. This means that the difference between a first sensed signal of the first sensing module and a second sensed signal of the second sensing module is determined. The determined distance is an indicator for the distance between the reference device and the base line.

The first and second sensed signals are proportional do a magnetic field strength measured or sensed in a direction along the base line. That is, although the reference device moves perpendicular with respect to the base line, the magnetic field intensity emanated by the reference device is measured or sensed in a direction that is

perpendicular to the movement of the reference device. Thus, the number of relevant sensing directions is reduced and also the possible effect of magnetic stray fields is reduced. For example, any magnetic stray fields that are directed in other directions than the sensing directions of the first and second sensing modules does not substantially affect the first and second sensed signals. Furthermore, even if a magnetic stray field acts in a direction of the first and second sensing direction, this effect can be eliminated by the differential measurement approach.

According to a further embodiment, the movement trajectory of the reference device is perpendicular or substantially perpendicular to the base line.

Thus, when the reference device moves towards or away from the first and second sensing modules, it moves along a linear movement trajectory. However, the movement trajectory might be shaped other than linear. The first and second sensing modules will determine the linear distance (shortest distance) between the base line and the actual position of the reference device.

According to a further embodiment, the first sensing direction and the second sensing direction are perpendicular with respect to the movement trajectory. In other words, the sensing directions are parallel to the base line interconnecting the first and second sensing modules. Thus, the magnetic field intensity of the magnetic field emanated by the reference device is measured in a direction that is perpendicular with respect to the direction of movement of the reference device. The magnetic field intensity increases the closer the reference device gets to the base line. This applies at least for an effective or desired measurement range (corresponds to the effective moving range of the reference device; the effective moving range might be limited by physical characteristics of the system in which the distance measuring device is implemented).

For example, if the distance measuring device is used within an air spring, the sensing modules and the reference device are arranged at opposite ends of the air spring. Limitation of movement of the air spring may be provided by support surfaces or any kind of limiting mechanisms. The minimum working stroke of the air spring corresponds to the minimum distance between the sensing modules and the reference device. This minimum working stroke does not necessarily correspond to a zero distance between the reference device and the base line. Typically, the minimum working stroke corresponds to a distance that is greater than zero.

According to a further embodiment, the distance measuring device is configured to use only a measured value from a single sensing direction of each of the first sensing module and the second sensing module.

For example, the first and second sensing modules are unidirectional magnetic field sensing modules. However, the first and second sensing modules might be sensing modules having more than one sensing direction. In that case, however, the sensed values of only one sensing direction are acquired from the sensing modules and used for determining the distance between the reference device and the base line.

The first sensing module senses the magnetic field strength in a direction parallel to the base line and so does the second sensing module. However, the sensing directions of the first and second sensing modules face toward each other.

According to a further embodiment, the movement trajectory intersects with the base line at a center point which is equidistant from the first sensing module and the second sensing module.

Thus, the distance between each of the sensing modules and the reference device is same, given that the reference device moves along the movement trajectory. Assuming that the magnetic field emanated by the reference device is symmetric, any differences between the first and second sensed signals arise from stray magnetic fields and the effect of the stray magnetic fields can be eliminated by applying a differential measurement approach (subtracting the first sensing signal from the second sensing signal, or vice versa, and using the result of this operation for determining the distance between reference device and base line). According to a further embodiment, the distance measuring device further comprises a third sensing module and a fourth sensing module. The third and fourth sensing modules are interconnected by a second base line. The third sensing module is configured to detect the strength of the magnetic field in a third sensing direction that points towards the fourth sensing module, and the fourth sensing module is configured to detect the strength of the magnetic field in a fourth sensing direction that points towards the third sensing module.

The third and fourth sensing modules are arranged with respect to each other in a similar manner than the first and second sensing modules. In other words, the first and second sensing module may be referred to as first sensing unit and the third and fourth sensing module may be referred to as second sensing unit. Each of these sensing units can work independent of the other sensing unit.

According to a further embodiment, the second base line intersects the first base line at an angle of 90°.

Using two sensing units that are arranged such that their base lines intersect at an angle of 90° may be preferred. The distance may be measured even if the reference device moves to a position that is lateral offset with respect to its intended movement trajectory.

Independent of the angle the base lines intersect each other, these two sensing units provide redundancy and it is possible to compare the distance measured by the first sensing unit with the distance measured by the second sensing unit. If the values of the measured distances deviate from each other by more than a given threshold value, it may be assumed that the distance measuring device must undergo check or inspection. According to a further embodiment, the distance measuring device further comprises a fifth sensing module arranged at a center point between the first, second, third, and fourth sensing modules.

The fifth sensing module may be a hall effect sensor and may be configured to measure an intensity of the magnetic field of the reference device. Especially when the distance between the reference device and the base line is low, the distance measuring device may have a low sensitivity, i.e., the signal value measured by the sensing modules changes only slightly although the amount of distance changes is high.

Once the sensing modules indicate that the distance between the reference device and the base line has reached a predetermined minimum distance value, the evaluating unit may consider the value measured by the firth sensing module for further distance measurement. In contrast, when the actual distance is below the predetermined minimum distance value and the distance again increases, the evaluating unit used the first and second (and optionally the third and fourth) sensing modules for determining the distance when the predetermined minimum distance value is exceeded. According to a further embodiment, the fifth sensing module is configured to detect the strength of the magnetic field emitted by the reference device, wherein the evaluating unit is configured to determine a distance between the fifth sensing module and the reference device based on the strength of the magnetic field detected by the fifth sensing module.

Generally, measuring the intensity of a magnetic field is a reliable parameter for indicating the distance to the source of the magnetic field if the source of the magnetic field is in close proximity to the sensing device. This approach has a low sensitivity for large distances. Thus, combining the first and second (and third and fourth) sensing modules with the fifth sensing module and using the fifth sensing module for low distances one the one hand, and the first and second (and third and fourth) sensing modules for larger distances on the other hand will provide high accuracy measurement results.

According to a further embodiment, the reference device comprises a permanent magnet.

A permanent magnet is a passive device and does not require any substantial

maintenance. Even if the permanent magnet is subject to aging and the intensity of its magnetic field decreases, the distance measuring device described herein can handle these changes of the magnet ' s characteristics and it will not have any major effect on the measurement results because of the differential measurement approach.

According to an alternative embodiment, the reference device is an electrically powered magnetic field generator and the emitted magnetic field is either alternating or is static (i.e., has a fixed field strength). For example, the reference device is a coil that is driven by electric power to generate a magnetic field.

According to a further embodiment, the permanent magnet has a magnetic pole axis which coincides with the movement trajectory.

In other words, the magnetic pole axis is perpendicular to the base line. This orientation of the permanent magnet provides the highest sensitivity of the distance measuring device to changes of the magnetic field intensity in a direction parallel to the base line (perpendicular to the movement trajectory and also perpendicular to the magnetic pole axis).

The magnetic pole axis is a line that interconnects the poles of the magnet. For example, the north pole is positioned towards the base line (faces towards the base line) and the south pole faces in the opposite direction, i.e., away from the base line. According to a further embodiment, the distance measuring device further comprises a calibration sensor and a calibration unit. The calibration sensor is configured to determine the distance between the base line and the reference device. The calibration sensor is a magnetic field sensor with three sensing directions.

In order to determine the effect of aging of the permanent magnet to the values sensed by the sensing modules, the distance measuring device comprises the calibration unit. The calibration unit may have an increased precision of distance measurement because it uses three sensing directions.

However, once the distance measuring device is calibrated, it is sufficient for a reliable distance measurement to use the sensing units with a single sensing direction. This may be preferred because the required computational power is less compared to distance measurement with a sensor having three sensing directions.

According to a further aspect, an air spring is provided. The air spring comprises a first mounting plate being adapted to be mounted to a chassis of a vehicle, a second mounting plate being adapted to be mounted to a wheel suspension, and a distance measuring device as described herein.

Preferably, the first and second sensing modules are mounted to the first mounting plate, and the reference device is mounted to the second mounting plate. The air spring further comprises a flexible member (for example an air spring bellows), wherein the first mounting plate, the second mounting plate, and the flexible member define a

pressurizable chamber, and wherein the first and second sensing modules and the reference device are located within the pressurizable chamber.

The air-spring design to which this description is applicable includes a flexible member (an elastic rubber bellows) that is mounted in an air-tight manner onto top and bottom plates to define an air tight (pressurizable) chamber. By pumping pressured air into the pressurizable chamber the air spring will expand, and by releasing the air from the pressurizable chamber the air spring will begin to collapse. Usually, mechanically controlled or electrically controlled pneumatic valves are used to change the amount of air within the pressurizable chamber of the air spring.

The total maximum distance that needs to be measured is equivalent to the working stroke range of the air spring. The total working stroke of an air-spring is the difference in distance between when the air-spring is fully expanded (the maximal working length of the air spring) and when the air spring is fully contracted (the shortest possible working length of the air spring). In other words, this working stroke is the changes in length of the air spring when fully pumped-up (maximum practical air volume within the air spring bellows) and when almost all of the air inside the air spring has been pumped- out (lowest practical air volume within the air spring belly).

The term "air" as used in this context includes any fluids, in particular gas or mixtures of gasses which is inert to the air spring and includes air, nitrogen, helium, other Noble gases, nitrogen enhanced air and helium enhanced air, for example. In particular, the term "air" when referring to an air spring may be understood as a synonym for any compressible gas.

These and other aspects of the present invention will become apparent from and elucidated with reference to the exemplary embodiments described hereinafter. Brief Description of the Drawings

Fig. 1 schematically shows a side view of a distance measuring device.

Fig. 2 schematically shows a side view of a distance measuring device. Fig. 3 schematically shows a top view of a distance measuring device.

Fig. 4 schematically shows an isometric view of a distance measuring device. Fig. 5 schematically shows a top view of two different examples of a distance measuring device.

Fig. 6 schematically shows a distance measuring device implemented within an air spring.

Fig. 7 schematically shows a top view of a distance measuring device.

Fig. 8 schematically shows a value of a differential measurement signal of a distance measuring device.

Fig. 9 schematically shows a linearized measurement signal of a distance measuring device.

Fig. 10 schematically shows a value of a differential measurement signal of a distance measuring device.

Fig. 11 schematically shows an air spring with a distance measuring device.

Fig. 12 schematically shows an evaluating module of a distance measuring device.

Fig. 13 schematically shows a top view of a distance measuring device. Fig. 14 schematically shows an air spring with a distance measuring device. Fig. 15 schematically shows a wheel suspension with an air spring with a distance measuring device.

Detailed Description of the Invention

For purposes hereof it should be understood that in referring to distances between two points the points are a first point (base point, or point from where the measurement will start, may be the first and second sensing modules as referred to above and hereinafter) and a second point (target point, may be the reference device as referred to above and hereinafter) to which the distance is measured. When aiming for a non-contact distance measurement solution, and when placing the distance sensing system at the base point, then the used measurement system has to be able to physically detect or sense the target point in some way. There may be a multitude of fundamentally different ways to accomplish this purpose. Some of these solutions can be optically based (such as visible light, and invisible light), sound based (for instance, audible and non-audible sounds) or physical based measurements. The measurement solution which is best suited for a specific application may depend on many factors, including: environmental conditions (interfering lights, interfering sound, changing ambient pressure, temperature, dust, and humidity), space availability for the measurement system, the targeted measurement range (millimeters, meters, kilometers), required measurement resolution and absolute accuracy, cost limitations, and the like.

In one embodiment, the herein described distance measurement solution is specifically directed to pneumatic powered, air-spring applications. It is applicable to the air springs which are employed in a wide variety of applications including, but not limited to machinery and vehicles, such as automobiles, trucks, trains, agricultural vehicles, mining vehicles, construction vehicles, and the like. Even though the following description refers to the specific application of air springs, it should be noted that this field of application is provided for purpose of example only and that the distance measuring device may also be used in connection with other applications. The air-spring design to which this description is applicable includes a flexible member (an elastic rubber bellows) that is mounted in an air-tight manner onto top and bottom plates to define an air tight (pressurizable) chamber. By pumping pressured air into the pressurizable chamber the air-spring will expand and by releasing the air from the pressurizable chamber the air-spring will begin to collapse. Usually mechanically controlled or electrically controlled pneumatic valves are used to change the amount of air within the pressurizable chamber of the air spring. The total maximum distance that needs to be measured is equivalent to the working stroke range of the air-spring. The total working stroke of an air-spring is the difference in distance between when the air-spring is fully expanded (the maximal working length of the air-spring) and when the air-spring is fully contracted (the shortest possible working length of the air-spring). In other words, this working stroke is the changes in length of the air-spring when fully pumped-up (maximum practical air- volume within the air-spring bellows) and when almost all of the air inside the air-spring has been pumped-out (lowest practical air-volume within the air-spring bellows). The term "air" as used in this context includes any fluids, in particular gas or mixtures of gasses which is inert to the air spring and includes air, nitrogen, helium, other Noble gases, nitrogen enhanced air and helium enhanced air, for example. In particular, the term "air" when referring to an air spring may be understood as a synonym for any compressible gas.

For purposes hereof, the targeted distance measurement is typically within the range of a few millimeters to around 500 millimeters or even more. Preferably, the measurement range is between 80 mm and 350 mm. For lower distances, the fifth sensing module may be used. The targeted measurement resolution and measurement repeatability is typically within the range of about tenth of mm up to 1 mm.

The sensing solution of this description will operate on magnetic principles as they are not substantially affected by light, sound, air-pressure, dust, and/or humidity. The sensor system may be described as consisting three main parts: (1) the sensing module (or Magnetic Field Sensor Array), the sensor electronics, and the target-point. The sensing module and the sensing electronics are connected with each other by a number of insulated electrical wires (for example four wires can be utilized). The sensing module can be placed at the one end of the air-spring and can be referred to as the base-point. The sensor electronics can be powered by a low DC (direct current) voltage. The target- point or reference device may be a small and high strength permanent magnet. The physical dimension and the absolute surface-magnetic-field-strengths of the permanent magnet are subject to a number of application-dictated parameters, including the measurement distance to be covered, available space, and environmental factors, including ferro-magnetic objects that may be situated near to the measurement path. For purposes hereof the "measurement path" is a vertical straight line between the target- point and the base point. In general, larger and more powerful permanent magnets are needed with larger measurement distances with stronger surface-magnetic-field- strengths being required. In any case, the area around the measurement should be free of moving ferro-magnetic objects as they can interfere negatively with the distance measurement to be taken. However, within limits, static (not moving) ferro-magnetic objects can be tolerated with appropriate correction factors. This description is focusing on non-contact proximity measurement using a magnetic principle based differential mode measurement solution.

The reference device referred to in this description may in particular be one or more permanent magnets. Thus, the reference device does not need any energy supply or any other external connections. However, due to aging of the permanent magnet, the distance measuring device may have to meet the following requirements: ability to deal with changes in the magnetic field strength (aging, low cost product); limited accuracy in the actual orientation of the North-South magnetic pole axis (when having to deal with low cost magnets, for example); ability of compensating the effects of interfering magnetic stray fields (like the Earth Magnetic Field); maximum freedom in the final placement of the actual sensing device and the reference device and not having to rely on strict geometrical (rectangular shaped) design requirements; simple measurement data processing (fast signal processing using low current, slow operating, and low cost processors).

Fig. 1 schematically shows a distance measuring device 100 with a first sensing module 110 having a first sensing direction 111, a second sensing module 120 having a second sensing direction 121, and a reference device 130 that is a permanent magnet 132 with a north pole and a south pole.

The reference device 50 emanates a magnetic field 50. The intensity of the magnetic field 50 is detected by the first and second sensing modules 110, 120. However, the intensity of the magnetic field 50 is measured along the first and second sensing directions 111, 121, i.e., perpendicular to the movement trajectory 160 of the reference device. As can be seen from this schematic representation in Fig. 1, the intensity of the magnetic field 50 will change even along the directions 111, 121 if the reference device 130 moves along the movement trajectory 160.

In a coordinate system, the sensing directions correspond to the Y-direction and the movement direction of the reference device corresponds to the Z-direction, both of which are shown in Fig. 1. Y and Z-directions are perpendicular with respect to each other.

The two magnetic field sensing modules 110, 120 (Sensor 1 and Sensor 2) are measuring in the Y-axis in opposite direction to each other. The first and second sensing modules 110, 120 are used as a single sensing unit by determining a differential signal (signal of first sensing module 110 - signal of second sensing module 120). The differential signal indicates the intensity of the magnetic field. The magnetic field intensity measured by this sensing unit is the difference between the two individual Y-axis measurements. Thus, the unwanted effects caused by the Earth Magnetic Field 10 (or any other symmetrical magnetic stray field) will be cancelled.

The reference device 130 is a passive permanent magnet that is passively providing a symmetrically shaped cloud of magnetic flux lines, circling the magnet from one to the other pole. For example, the permanent magnet may be a symmetrically shaped bar magnet.

Fig. 2 shows a side view of the distance measuring device 100. Basically, the view corresponds to that of Fig. 1.

A base line 150 is shown interconnecting the first and second sensing modules 110, 120. The movement trajectory 160 is perpendicular to the base line 150. The distance 170 to be measured corresponds to the distance between the base line 150 and the reference device along the Z-axis.

The lateral distance 112A between the first sensing module 110 and the center point (intersection of the base line 150 with the movement trajectory 160) and the lateral distance 112A between the second sensing module 120 and the center point are equal.

The distance measuring device 100 further comprises a calibration sensor 470 and a calibration unit 148. The calibration sensor 470 may be a three-axes sensor and the calibration unit 148 may be configured to determine the distance between the base line and the reference device using a more complex algorithm compared to how the first and second sensing modules are operated. The calibration sensor 470 and the calibration unit 148 may be used to calibrate the distance measuring device on a regular basis, for example to deal with aging effects of the permanent magnet.

The reference device 130 is moving up-and-down on a Z-axis path. This path is placed exactly halve -way-distance between the two sensing modules 110, 120. Thus, the distance from the first sensing unit 110 and from the second sensing unit 120 (on the Y- axis) towards the movement trajectory 160 where the reference device is moving along is the same. Fig. 3 shows a top view of the distance measuring device 100 with the first and second sensing modules 110, 120 and the reference device 130 in between. The sensing modules 110, 120 and the reference device 130 are arranged along the common base line that extends along the Y-axis from the first sensing module 110 to the second sensing module 120. In the top view, the Z-axis (not shown) extends out of and into the drawing plane. The X-axis is perpendicular to the Z-Axis and to the Y-axis. The reference device 130 is not offset from the base line along the X-axis. Furthermore, the reference device 130 does not change its position along the Y-axis when moving up and down (towards and away from the base line). Fig. 4 shows an embodiment with four sensing modules 110, 120, 410, 420. The first and second sensing modules 110, 120 are interconnected by the first base line 150. The third and fourth sensing modules 410, 420 are interconnected by the second base line 460. The first base line 150 intersects the second base line 460 at an angle of 90°. In an initial state, the reference device 130 is located below the intersection point of the first and second base lines, i.e., there is a common intersection point of the first and second base lines and the movement trajectory 160.

Based on magnetic principles, four independent sensing modules 110, 120, 410, 420 are provided. The four sensing modules are monitor the magnetic field pattern that is emanated from the reference device 130. Each pair of magnetic field sensors that are placed opposite to each other form one differential mode linear position sensor. The four sensing units represent two separate differential mode linear position sensors, thus providing redundancy and also enabling to detect if the reference device leaves the ideal movement trajectory. In that case, the distance values measured by the two pairs of sensing units are different. The four sensing units 110, 120, 410, 420 are arranged in the same plane and also are the sensing directions. The sensing directions are oriented towards a common center point. The movement trajectory 150 is orthogonal to said plane.

Fig. 4 shows one dual differential linear position sensor system that is built of two single differential linear position sensors that are placed to each other by a 90° angle.

Fig. 5 shows two design options of the distance measuring device 100. The design option on the left is already shown in Fig. 3 and the design option on the right is isometrically shown in Fig. 4.

Looking from the top-down onto the linear position sensor one can recognize the first differential mode linear position sensor (namely the group of the first and second sensing modules 110, 120, see left side of the drawing). A second and basically identical differential mode linear position sensor (namely the group of the third and fourth sensing modules 410, 420) is placed with a 90° angle in relation to the first sensor.

It is one aspect of a passive magnetic principle based sensing technology that is has to master compensating the potential interferences caused by magnetic stray field sources, like the earth magnetic field, accidental magnetizations of the application itself (static magnetic stray fields), and temporarily magnetic stray fields caused by moving objects that are coming close to the sensor. Fig. 6 shows one possible implementation of a distance measuring device 100 within a wheel suspension 320. The first and second sensing modules 110, 120 and the evaluating unit 140 are arranged at a static part of the wheel suspension while the reference device is arranged at the moving part of the wheel suspension. However, the elements might also be positioned and assigned conversely. In such an application as shown in Fig. 6, the reference device will not only move along the Z-axis (while nevertheless moving up-and-down), but also in the X-axis direction with respect to the sensing modules. This is an air- spring application specific characteristic and is shown in a top view in Fig. 7.

Fig. 7 shows the relative movement of the reference device 130 of Fig. 6 with respect to the first and second sensing modules from a top view.

The arc-like movement of Fig. 6 causes the reference device changing its position along the X-axis, as shown in Fig. 7. However, there is no change of the position of the reference device along the Y-axis.

In an air spring application, the kinematics may result in that the reference device is not only moving up-and-down along a single axis (Z). Under certain circumstances, the reference device will show also movements in the X-axis direction as shown in Fig. 7. As long as the distance from the reference device to the first sensing unit and the second sensing unit remains identical, the here described linear position sensor solution may, with certain limitations, compensate for the movements in the X-axis. Fig. 8 schematically shows the absolute value of the differential measurement signal (signal of first sensing module - signal of second sensing module). The vertical axis shows the measured magnetic field in Gauss and the horizontal axis shows the distance between the reference device and the base line. As can be seen, the curve has an exponential character.

The absolute values of the differential measurement signal shown in Fig. 8 may be linearized to simplify distance calculation. The linearized distance measurement curve is shown in Fig. 9. The linearization is done between two calibration points, one for a position of the reference device that is close or closest to the base line and one for a position of the reference device that is far or farthest from the base line. The relevant measurement range is between the two calibration points.

The linearized distance measurement applies a linearization algorithm that is described below. The equation constants of the linearization algorithm are calculated to operate best between the two defined calibration locations, here called "near" (at 90 mm) and "far" (at 220 mm). The dashed line represents the ideal measurement curve while the continuous line shows the actual linearized measurement curve.

As an example, the equation for calculating the distance between the reference device and the base line can be determined as follows: b

Distance = a +—

Distance: measurement result in a unit that is defined by the other variables used, a: Offset variable

b: Signal gain variable

Y: Measured differential magnetic field strength in the Y-axis direction n: Exponent used to linearize the exponential measurement curve

The units to be used in this very simple and straight forward equation depend on the users desire what format the distance result should be (metric units, imperial units, others ...) and in what format the magnetic field strength will be measured (Tesla, Gauss, or simple counts).

^Differential ¾emsorl ¾ensor2 To be independent and immune to environmental magnetic stray fields, the differential value will be used (the difference between the Y-axis measurement from the two magnetic field modules).

Depending on the placement of the reference device into the application (north pole at the top or south pole at the top towards the base line) and depending on the orientation of the sensor system in relation to the EMF (Earth Magnetic Field), the calculated Y value might be negative. As we are only interested in a positive value for the distance between the reference device and the base line, YDifferentiai will be converted into an absolute value (and therefore will be always positive).

Y = absolute (Υ ΜΙ , βΓΜίΜ )

To assure that the final measurement is always positive, the above conversion is used.

In the following, it is described how the equation constants: a, b, and n are calculated. A two-point calibration procedure is performed to calibrate and linearize the measured differential signal Y into a reliable distance value. The two calibration locations (here called "near" and "far" or distance^ and distancef ar ) can be chosen freely but have to be placed within the usable measurement range. The values of the constants a, b, n will change depending on design specific aspects, like: Fixed distance A (distance from Sensor to the center line); Mmgnetic field strength of the used reference device; physical dimension and shape of the reference device (wide, short, long,); the exact locations of the two-calibration used for the system linearization; the usable magnetic field measurement range (when does saturation occur).

The exponent n may be experimentally chosen such that the measurement curve shown in Fig. 9 is obtained. Having chosen where the exact locations will be for the two point calibration process, the equation (to calculate the signal gain constant b) is based on only two Y-field measurement values: Y at the location Distance ne ar (e.g., 90 mm) and Y at the location Distancefar (e.g., 220 mm).

near 'far /

The equation to calculate the signal gain constant b is shown above. After the signal gain has been calculated, the signal offset constant a can be calculated with the following equation:

b

a = Distance far

Yfar

In this example, the value for Distancefar is 220 mm. Fig. 10 shows a measured value of the sensing modules over the distance between the reference device and the base line. With a small distance between the reference device and the base line, the sensing module may run into saturation so that the measured value is not correct. However, with increasing distance, the measured value gets exponentially smaller. Thus, the usable measurement range of the sensing modules needs to be defined somewhere between the point of saturation (on the left) and a point where the signal is not almost zero (on the right). For distances that are smaller than the most-left distance in Fig. 10 (left of the indicated usable measurement range), the fifth sensing module can be used, see Fig. 13. Each of the magnetic field sensing units measure the intensity of the magnetic field in the Y-axis direction in relation to the distance between the reference device and the base line. For the largest part, this measurement curve has an exponential characteristic. However, when the distance between reference device and base line becomes very short, the magnetic field direction will become exponentially large. It is very possible that some magnetic field sensors may not be able to manage such large magnetic field strength (risk of magnetic field internal saturation). Therefore, the usable measurement range has its lowest distance value at a distance that is greater than zero for the distance value between the reference device and the base line.

To nevertheless deal with distances that are smaller than the smallest value of the usable measurement range, a fifth sensing module 430 is placed in the center between the first, second, third, and fourth sensing modules at the intersection of the first and second base lines 150, 460, as shown in Fig. 13. Placing a fifth magnetic field sensing module in the center of the sensor system arrangement allows measuring in the direction towards the reference device (along the direction of movement of the reference device).

Fig. 11 shows the distance measuring device 100 installed in an air spring 200. The air spring 200 comprises a top plate 210 and a bottom plate 220. The first and second sensing modules 110, 120 are mounted to the top plate 210 and the reference device 130 is mounted to the bottom plate 220.

The top plate 210, the bottom plate 220 and a bellows that forms the outer surface of the cylindrically shaped air spring enclose a pressurizable chamber 240. The distance measuring device is arranged within the pressurizable chamber 240 to measure the working stroke of the air spring.

When installed inside an air spring, the two magnetic field sensing modules 110, 120 may be placed at the static (relative to the truck's chassis) top-end of the air spring while the reference device will be mounted at the moving bottom end of the air spring. The installation process may be facilitated when the supply lines for the sensing modules not need to be brought to the moving bottom plate of the air spring. When the air spring compresses or expands, the reference device is moving along the center axis (dashed perpendicular line). The moving path of the reference device may coincide with the center axis of the air spring. However, this is not necessary. The moving path of the reference device may be laterally offset with respect to the center axis of the air spring.

Fig. 12 shows an evaluating module 140. The evaluating module 140 comprises two sensor array interfaces 144, 145 for interconnecting the first and the second sensing module. Further, the evaluating module 140 comprises a controller 143, for example a processor or any other kind of automated calculation unit, and a power supply unit 141. The power supply unit 141 may provide the evaluating module with electrical energy received from an energy source (not shown) via power interface 146. The controller 143 may send or receive data via an input/output (I/O) interface 142 which may comprise a serial digital I/O, an analogue I/O, and/or an interface to be connected to a data bus, for example a digital bus like CAN or CAN-Open.

The sensing modules 110, 120, 410, 420, 430 are configured to provide the absolute magnetic field strength measured in direction of the Y-axis. This information is transmitted to the evaluating unit 140 via the interfaces 144, 145.

The controller 143 may determine the distance value by using the equation described above. However, the controller 143 may alternatively or additionally request distance values from a lookup table by entering the measured signal values of the sensing modules.

Fig. 14 shows an air spring 200 with a first mounting element 210, a second mounting element 220, and a flexible member 230, for example a bellow. The first mounting element in form of a top plate, the second mounting element in form of a bottom plate, and the bellow contain or include a volume which is the pressurizable chamber 240. The first and second sensing modules 110, 120 are arranged at the first mounting element 210 and the reference device 130 is arranged at the second mounting element 220 opposite to the first mounting element. There is a relative movement of the mounting elements 210, 220 along the movement trajectory 160. In an operating mode of the air spring, the top plate and the bottom plate may move towards each other along the direction arrow 160 by movements of the bottom plate and/or by movements of the top plate.

Fig. 15 illustrates a wheel suspension 320 and a vehicle's chassis 310, which are mechanically linked to each other and have an air spring 100 for dampening vibrations of the wheel 325 due to uneven road conditions, wherein one of the mounting elements of the air spring is mounted to the wheel suspension 320 and the other one of the mounting elements of the air spring is mounted to the vehicle's chassis 310.

The wheel suspension 320 may move along the arrow 322 when the wheel rolls over an uneven street and, as a result of the vibrations of the wheel 325 and of the wheel suspension 320, the mounting elements of the air spring are moving frequently towards and away from each other as indicated by arrow 160. The air spring and in particular the pressurizable chamber within the air spring is adapted to dampen the vibrations of both the wheel suspension and the vehicle's chassis as to not transfer or transmit these vibrations from one of these parts to the other one, respectively. It should be understood that the features described in individual exemplary embodiments may also be combined with each other in order to obtain a more fail safe air spring height sensor or air spring as well as to enable error detection and correction of the measured height signal. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. List of Reference signs

10 magnetic stray field

50 magnetic field generated by the reference device

100 distance measuring device

110 first sensing module

111 first sensing direction

112A lateral distance between sensing module and center

120 second sensing module

121 second sensing direction

130 reference device

132 permanent magnet

140 evaluating module

141 power supply unit

142 I/O interface

143 controller

144 sensor array interface

145 sensor array interface

146 power interface

148 calibration unit

150 base line

160 movement trajectory

170 measured distance

200 air spring

210 first mounting plate

220 second mounting plate

230 flexible member

240 pressurizable chamber

310 chassis 320 wheel suspension

322 suspension movement

325 wheel

410 third sensing module

420 fourth sensing module

430 fifth sensing module

460 second base line

470 calibration sensor