WO2012168757A1

WO2012168757A1 - Bearing device including a deformable housing and a sensor

Info

Publication number: WO2012168757A1
Application number: PCT/IB2011/002058
Authority: WO
Inventors: Mathieu Hubert; Benoit Locher; Ulrich Schroeder
Original assignee: Aktiebolaget Skf (Publ)
Priority date: 2011-06-10
Filing date: 2011-06-10
Publication date: 2012-12-13

Abstract

The invention relates to a bearing device for supporting a shaft (12) having a rotation axis (10), the bearing device including a deformable housing (22) and at least one sensor (34, 36) for measuring a deformation of the housing (22) due to load applied to the shaft (12) in a load direction (10) perpendicular to the rotation axis (10). It is proposed that the said sensor (34, 36) is arranged in a plane spanned by the rotation axis (10) and the load direction (D).

Description

BEARING DEVICE INCLUDING A DEFORMABLE HOUSING AND A SENSOR

The invention relates to a bearing device for supporting a shaft having a rotation axis, the device including the deformable housing and at least one sensor for measuring the deformation of the housing, and to a device such as a washing machine equipped with a bearing device of the above-mentioned type.

In many machines, in particular in washing machines, it is necessary to measure the load acting on a shaft in a direction perpendicular to the rotation axis of the shaft. It is therefore known to provide bearing devices for supporting the shaft with suitable load sensors. Many solutions exist e. g. for measuring the load in the drum of a washing machine. Most solutions measure dynamics of the drum movement and/or inertia and deduce the load inside the drum from the data thus obtained. However, these solutions are not accurate for low loads, e. g. for loads of less than 1 kg. In particular, it is known to use mechanical load sensors with a moveable piston for this purpose. However, the friction of the piston and/or of other movable sensor parts generates a large hysteresis which is the predominant factor in the low load regime such that this type of sensors is very imprecise for low loads. The inertia of the drum depends strongly on the position of the load inside the drum which changes during the operation of the machine and therefore perturbs inertia-based measurements of the load. Finally, the known systems are strongly dependent on temperature and are subject to wear.

The above-mentioned problem is partially solved with a device as proposed in the document WO2007/077307A1 , which relates to a bearing device with a system for detecting the load applied to an element arranged in the housing. The housing is elastically deformable and two lateral sensors arranged in a plane perpendicular to the load direction are used to detect the deformation of the housing. The sensors employed are of capacitive type and are sensitive for humidity in a gap between the sensor electrodes. This is problematic for washing machines, which are often operated in a humid environment.

Further, there is room to improve the thermal stability of the device proposed in WO2007/077307A1. In order to solve the above problems, the invention proposes a bearing device according to claim 1. Further favourable embodiments of the invention are defined in the dependent claims.

The invention starts from a bearing device for supporting the shaft having a rotation axis, the bearing device including a deformable housing and at least one sensor for measuring the deformation of the housing due to a load applied to the shaft in a load direction. The load direction is at least essentially perpendicular to the rotation axis.

It is proposed that the sensor is arranged in a plane spanned by the rotation axis and the load direction. The sensor is arranged in the plane where the maximum

deformation of the housing is to be expected due to the action of the load because the point of attack is arranged in the same plane. As a consequence, a very direct deformation measurement can be achieved and a ratio between thermal deformations and load induced deformations is as low as possible. As a result thereof, the arrangement of the sensor according to the invention may improve the thermal stability of the system. According to a further embodiment of the invention, it is proposed that the sensor is arranged in an elongate opening of the housing. A longitudinal axis of the elongate opening is then preferably arranged perpendicular to the load direction. This elongate opening preferably induces a high elasticity of the housing in the load direction, whereas a maximum rigidity is kept in the direction perpendicular thereto. This may help to reduce the risk of resonant vibrations in the system.

Further, is proposed that the sensor is an inductive sensor. This sensor may in particular comprise a coil wound around a core material with high magnetic

permeability for guiding the flux across an air gap. The air gap may in a preferred embodiment extend perpendicular to the load direction such that the width of the air gap changes immediately and directly in response to the load acting onto shaft.

Preferably, a target of a material with high magnetic permeability is arranged on a side of the air gap opposite to the coil. This may reduce the loss due to leakage of the magnetic field and increase the precision of the device. This object may be achieved e.g. by making the target of a ferromagnetic or permanent magnetic material, preferably of ferrite. In a particularly advantageous embodiment of the invention, the housing itself is made of a ferromagnetic material. In this case, the housing may be used as a yoke for closing a magnetic circuit the flux inside of which is measured by the magnetic sensor.

In an alternative embodiment, the housing includes a core made of a ferromagnetic material. The rest of the housing may then be constructed in a light-weight manner with a particular focus on optimizing the elasticity and/or the manufacturing costs.

In a preferred embodiment of the invention, two sensors are arranged on diametrically opposite sides of the rotation axis. The deformations perceived by the sensors are therefore opposite to each other. This enables a differential measurement.

The differential measurement may be realized in the device itself and no external signal processing is needed if the bearing device includes a control device configured to measure the difference between the signals of the two sensors.

In a preferred embodiment of the invention, the two sensors are variable reluctance sensors and the control device includes one reference oscillator for supplying oscillating measurement currents to the sensors and an operational amplifier for amplifying the difference between the signals of the sensors.

A bearing supporting the shaft is preferably held by being press-fitted into a central opening of the deformable hosing with an interference fit.

The invention has multiple fields of application including applications in the field of home appliances such as washing machines as well as in the filed of automotive technology and process engineering.

The invention can be well understood and other advantages thereof appear more clearly in the light of the following description of a preferred bearing device and sensor device in accordance with the principle of the invention, given purely by way of example and made with reference to the accompanying drawings in which:

Fig. 1 shows a bearing device with a deformable housing and two sensors according to the invention in an axial view.

Fig. 2 is a schematical representation of the electronic concept of the

invention. Fig. 3 is a example of a simple analog circuit realising the electronic system of

Fig. 2.

Fig. 4 is a illustration of the general principle of the inductive sensor according to the invention.

Fig. 1 is an axial view of a bearing device for supporting a shaft having a rotation axis 10. The shaft 12 is supported in a roller bearing 14 and is fit into the inner ring 16 of the bearing 14. The outer ring 18 is press-fitted into an opening 20 of a deformable housing 22, the outer circumference of which is coaxial with the opening 20 and the rotation axis 10.

The deformable housing 22 is generally ring-shaped with for elongate openings 24, 26, 28, 30, 32. The deformable housing 22 is mounted into a washing machine in such a way that the load direction D, which is parallel to the direction of the gravitational force of the drum acting on the shaft is aligned with the vertical direction in the representation in Fig. 1. The upper opening 30 and the lower opening 32 are essentially shaped like section of the ring, wherein the inner boundaries of the openings 30, 32 are slightly convex surfaces, whereas the outer boundaries are cylinder sections concentric with the rotation axis 10. The lateral openings 24, 26 are slightly shorter than the upper opening 30 and the lower opening 32 and the shape thereof is basically chosen such that the material walls between the neighbouring openings 24-32 and/or between one of the openings 24-32 and the central opening 20 have a basically constant thickness. Thicker portions of the material, which is cast ferromagnetic iron in the preferred embodiment, result from rounded corners of the openings 24, 26, 30 and 32. The upper opening 30 and the lower opening 32 are equipped with sensors 34, 36 respectively. The sensors 34, 36 are arranged in the centre of the respective openings 30, 32 and in a plane P spanned by the rotation axis 10 and the load direction D. The plane P is perpendicular to the drawing plane of Fig. 1. Each sensor 34, 36 is composed of two parts connected to the inner and outer boundary of the respective openings 30, 32. In the embodiment illustrated in Fig. 1 , the sensors 34, 36 are composed of coils 38, 40 wound around a core material with high permeability such as iron or ferrite and of a target 42, 44. The targets 42, 44 are made of a ferromagnetic permanent magnetic material such as ferrite and fixed to the inner boundaries of the openings 30, 32.

Since the entire housing 22 is made of a ferromagnetic material (cast iron), the magnetic flux generated by the coils 38, 40 is guided around the respective openings 30, 32 from the core of the coils 38, 40 to the targets 42, 44 opposing the coils 38, 40.

Between the vertical end faces of the coils 38, 40 and the corresponding target 42, 44, a small vertical air gap 46 is formed, which extends perpendicular to the load direction D.

When a load is applied in the load direction D, the load is transferred via the bearing 14 through the central opening 20 of the housing 22, which is elastically deformed in such a way that the opening 20 moves downward, i.e. the rotation axis 10 and the centre of the inner opening 20 are displaced in the load direction D in relation to the outer circumference of the housing 22. As a consequence of this displacement, the upper air gap 46 is widened, whereas the lower air gap 48 is narrowed.

As a result of this change in the widths of the air gaps 46, 48, the magnetic reluctance of the magnetic circuit generated by the upper coil 38 is increased and the reluctance of the magnetic circuit generated by the lower coil 40 is decreased. This change in the reluctance will immediately affect the effective inductions of the coils 38, 40 (i.e. the inductions of the system comprising the coil and the housing 22). Fig. 2 is a schematic representation of the electronic concept of the invention. A reference oscillator 50 is connected to the coils 38, 40, the other ends of which are connected with each other so as to form a half-bridge topology. The latter connected outputs of the coils 38, 40 are connected via a capacitance to a first amplifier 52, the output of which is connected to a synchronous demodulator 54 receiving the reference oscillation from the reference oscillator 50 as an input. The reference oscillator 50 generates an oscillating measurement current with a frequency between e.g. 10 to 40 kHz. Fig. 3 is an example of a simple analog circuit realising the electronic system of Fig. 2. The first amplifier 52 of Fig. 2 is realized by means of an operational amplifier 56 being part of an inversion- and amplification circuit. The output thereof is connected to an inversion circuit 58 and with a parallel bypass line 60. Both the inversion circuit 58 and the bypass line 60 are connected to a further gain amplifier 62 via switches and via a damping capacitance. The switches are operated by a synchronous switching device 64 triggered by the reference oscillator 50.

In equilibrium, i.e. when the air gaps 46, 48 have identical widths. The induced voltages in the coils 38, 40 are identical with opposite signs and cancel each other in the centre of the half-bridge. As a consequence, the output voltages of the first amplifying circuit 56 as well as the output of the gain amplifier 62 are zero.

If the inner opening 20 of the housing is biased, an oscillating voltage with an amplitude roughly proportional to the amount of deformation of the housing 22 is observed at the input of the amplifier 56, which is basically rectified by the combination of the inversion circuit 58 and the synchronous switching device 64 constituting the demodulator 54 and then amplified by the amplifier 62.

The resulting output voltage may be approximated by a linear function of the vertical displacement x of the ferrite targets 42, 44 in relation to the cores of the coils 38, 40. This can be derived as follows.

Fig. 4 is an illustration of the sensor circuit showing some of the quantities used in the following. Let the total inductance of the coils be , respectively wherein

is a constant leakage inductance and is a modulable contribution depending on

the displacement x.

For the total inductance of the upper coil 38 and for the induced voltage of

the lower coil 40, we obtain:

The dependence of the variable contribution may be well approximated by

where L₀ is a constant and

is the width of the air gaps 46, 48 at

equilibrium.

The resulting voltage at the centre of the half-bridge topology given the input

voltage amplitude U from the reference oscillator 50 an therefore be approximated

For we may use such that the signal may be approximated as

In this equation, the output voltage is a simple linear function of the displacement x, which is, in turn, proportional to the load acting on the axis in the load direction and thus to the weight of the drum with its contents.

The above described system is susceptible even for small displacements and small loads. The differential nature leads to a high temperature stability. Since the load is measured at the bearing, the position and distribution of the mass around the shaft, e.g. the position and distribution of the mass in the drum of a washing machine, are immaterial for the result. In alternative embodiments of the invention, the targets 42, 44 could be formed in one piece with the deformable housing 20. Further, the housing 20 could be made of resin or plastic materials with a ferromagnetic core. In order to enable load measurements in more than one load direction, more than two sensors could be distributed in pertinent openings around the rotation axis 10. For applications where the differential measurement for temperature compensation is less important, the skilled person could use only one sensor arranged in one of the openings and still obtain advantages over the prior art.

The above embodiments of the invention as well as the appended claims and figures show multiple characterizing features of the invention in specific combinations. The skilled person will easily be able to consider further combinations or sub-combinations of these features in order to adapt the invention as defined in the in the claims to his specific needs.

Claims

Claims:

1. Bearing device for supporting a shaft (12) having a rotation axis(10), the

bearing device including a deformable housing (22) and at least one sensor (34, 36) for measuring a deformation of the housing (22) due to load applied to the shaft (12) in a load direction (D) perpendicular to the rotation axis (10),

characterized in that said sensor (34, 36) is arranged in a plane (P) spanned by the rotation axis (10) and the load direction (D).

2. Bearing device according claim 1 , wherein said sensor (34, 36) is arranged in an elongate opening (30, 32) of the housing (22), a longitudinal axis of which is arranged perpendicular to the load direction (D).

3. Bearing device according to one of the preceding claims, wherein said sensor (34, 36) comprises a coil (38, 40) wound on a core material with high magnetic permeability for guiding a magnetic flux across an air gap (46, 48), wherein said air gap (46, 48) extends perpendicular to the load direction (D) such that the width of the air gap (46, 48) changes depending on the load applied to the shaft (12).

4. Bearing device according to claim 3, wherein a target (42, 44) of a material with high magnetic permeability is arranged on a side of the air gap (46, 48) opposite to the coil (38, 40).

5. Bearing device according to claim 4, wherein the target (42, 44) is made of a ferromagnetic or permanent magnetic material.

6. Bearing device according to claim 5, wherein the target (42, 44) is made of ferrite.

7. Bearing device according to one of claims 3 - 6, wherein the housing (22) is made of a ferromagnetic material.

8. Bearing device according to one of claims 3 - 6, wherein the housing (22) includes a core made of a ferromagnetic material.

9. Bearing device according to one of the preceding claims, wherein two sensors (34, 36) are arranged on diametrically opposite sides of the rotation axis (10).

10. Bearing device according to claim 9, further including a control device (50, 52, 54) configured to measure the difference between the signals of the two sensors (34, 36).

11. Bearing device according to claim 9, wherein the two sensors (34, 36) are

variable reluctance sensors and wherein the control device (50, 52, 54) includes one reference oscillator (50) for supplying an oscillating measurement current to the sensors (34, 36) and an operational amplifier (52) for amplifying the difference between the signals of the sensors (34, 36).

12. Bearing device according to one of the preceding claims, wherein the housing (22) includes a central opening (20) for receiving a bearing (14).

13. Bearing device according to claim 11 , further including a bearing (14) being press-fitted into the central opening (20) with an interference fit.

14. Washing machine including a drum supported on a shaft (12) and a bearing device according to one of the preceding claims for measuring the load in the drum.