Angular sensor with piezoresistive material

Abstract

Angular sensor (1) comprising: a support (2); a torsion spring (10) having a first end (11) mechanically connected to the support (2) and a second end (12) connected to a drive element (9) movable for rotation relative to the support (2); the spring (10) comprising an electrically conductive piezoresistive material; a resistance measurement circuit having terminals electrically connected to the spring (10). Figure 2

Description

Title of the invention: Angular sensor with piezoresistive material

[0001] The present invention relates to the field of angular sensors.

[0002] BACKGROUND OF THE INVENTION

[0003] In a manner known per se, an angular sensor is associated with a pivoting element to determine the angular position of the pivoting element relative to a reference frame. For example, in aeronautics, angular sensors make it possible to determine the position of a rotary actuator.

[0004] In industry, several angular sensor technologies can be cited.

[0005] A first angular sensor technology concerns rotary potentiometers. However, such use is not recommended in environments subject to strong vibrations and / or the presence of particles that may penetrate the angular sensor, which may impair the operation of the angular sensor.

[0006] A second technology concerns optical sensors, otherwise known as "encoders". However, it is known from the prior art that optical sensors are sensitive to external disturbances such as dirt and extreme temperatures.

[0007] Other technologies relate to capacitive and magnetic sensors. Capacitive sensors are known to be sensitive to temperature and humidity, while magnetic sensors are sensitive to external magnetic fields, which can lead to measurement errors.

[0008] A possible alternative involves inductive sensors, and more specifically RVDT sensors (Rotary Variable Differential Transformer). These sensors are known for their robustness and can be used in harsh environments subject to extreme temperatures, foreign objects, etc. However, due to their structure, RVDT sensors are relatively bulky, heavy, and expensive. Furthermore, this type of sensor has a limited rotary operating range of ±35°.

[0009] SUBJECT OF THE INVENTION

[0010] The invention aims in particular at an angular sensor which remedies at least in part the aforementioned drawbacks. Summary of the invention

[0011] For this purpose, according to the invention, an angular sensor is provided comprising: - a support; - a torsion spring having a first end mechanically linked to the support and a second end linked to a drive element movable in rotation relative to the support, the spring comprising an electrically conductive piezoresistive material; - a resistance measurement circuit comprising a first terminal and a second terminal which are electrically connected to two parts of the spring separated from each other.

[0012] Thus, the torsion spring is fixed to the support via its first end. The second end of the spring is connected to the drive element, which rotates relative to the support. It is therefore understood that the shape of the torsion spring is determined by the drive element and depends on the force exerted by the drive element on the spring, and thus on the position of said drive element relative to the support. Consequently, the piezoresistivity of the spring material causes a variation in the electrical resistance of the spring, proportional to and in response to a variation in the position of the drive element relative to the support. The measuring circuit makes it possible to evaluate the variation in electrical resistance across the spring. It is therefore understood that when the drive element is mechanically connected to a system, the invention makes it possible to determine the angular position of said system.

[0013] According to optional features, used individually or in whole or in combination: - the spring is a spiral spring; - the material is a metallic alloy containing copper and nickel; - the material is a metallic alloy containing approximately 55% copper and approximately 45% nickel; - the material is constantan; - the spring is wrapped in insulating material; - the support is a housing containing the spring; - the measurement circuit includes an ohmmeter; - The measurement circuit includes a Wheatstone bridge.

[0014] The invention also relates to an actuator comprising a frame, a shaft mounted pivotally on the frame and a sensor, the support being fixed relative to the frame and the drive member being rotationally linked to the shaft.

[0015] Other features and advantages of the invention will become apparent from the following description of particular, non-limiting embodiments of the invention. Brief description of the drawings

[0016] Reference will be made to the attached drawings, among which:

[0017] [Fig-1] [Fig.1] is a perspective view of an angular sensor according to the invention;

[0018] [Fig.2] [Fig.2] is a perspective view of an internal structure of the sensor angular illustration in [Fig.1];

[0019] [Fig.3] [Fig.3] is an electrical diagram of a sensor measurement circuit angular illustrated in [Fig.2] according to a first embodiment;

[0020] [Fig.4] [Fig.4] is an electrical diagram of a sensor measurement circuit angular illustrated in [Fig.2] according to a second embodiment;

[0021] [Fig.5] [Fig.5] is a graph representing an angular position as a function of an electrical resistance measured by one of the measuring circuits illustrated in figures 3 or 4;

[0022] [Fig. 6] [Fig. 6] is a partial schematic view of a vehicle equipped with a actuator according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] With reference to [Fig. 6], the invention is described herein in application to an actuator A mounted on a vehicle structure V. Here, the actuator comprises a motor Mot having a stator forming a frame and a rotor pivotally received within the stator and carried by a shaft S passing through the stator. The shaft S has one end intended to be rotationally linked to an element to be moved in rotation and an opposite end associated with an angular sensor 1.

[0024] With reference to [Fig. 1], the angular sensor 1 comprises a support, here a housing 2, here made of an electrically insulating material. The housing 2 has a substantially cylindrical shape. The housing 2 is formed in at least two parts, namely a first part forming a base 3 of the housing and a second part forming a cover 4 of the housing. The base 3 and the cover 4 have a circular cross-section.

[0025] The base 3 comprises a circular peripheral rim 5. The peripheral rim 5 carries a conduit 6 extending radially outwards from the peripheral rim 5. The conduit 6 is formed in at least two portions. A first portion 7 has a substantially cubic shape. The first portion 7 is connected to the peripheral rim 5. A second portion 8 has a substantially cylindrical shape. The second portion 8 includes an external thread and is mounted on top of the first portion 7. In this case, the peripheral rim 5, the first portion 7, and the second portion 8 form a single piece.

[0026] The first portion 7 and the second portion 8 delimit a central channel opening into an internal cavity of the housing 2, via an opening in the peripheral border 5.

[0027] The base 3 and the operculum 4 are assembled to delimit the internal cavity of the housing 2. The internal cavity communicates with the outside of the housing 2 via an orifice made here in the center of the operculum 4.

[0028] The angular sensor 1 also includes a drive member 9 received by pivoting in the orifice provided in the center of the operculum 4 to extend along a axis of rotation X, the axis of rotation X being perpendicular to a principal surface of the operculum 4.

[0029] The drive member 9 includes a first end which extends outside the housing 2 and which has a fork shape forming a mechanical coupling element in rotation of the drive member 9 with the shaft S.

[0030] The drive member 9 also includes a second end which extends inside the housing 2.

[0031] With reference to [Fig. 2], the angular sensor 1 also includes a spring 10 housed in the internal cavity of the housing 2. The spring 10 is a torsion spring and preferably a spiral spring. The spring 10 comprises a plurality of non-contiguous and equidistant coils and extends in a winding plane perpendicular to the axis of rotation X.

[0032] The spring 10 comprises a first end 11 and a second end 12. The first end 11 is mechanically linked to the housing 2. The second end 12 is mechanically linked to the second end of the drive member 9.

[0033] Furthermore, the spring 10 comprises a piezoresistive electrically conductive material. By "piezoresistive," it is understood that the electrical resistance of the spring 10 varies following the application of a mechanical stress on said spring 10. The electrical resistance of the spring 10 varies because the electrical resistivity of the material varies with the mechanical stress. Preferably, the material is a metallic alloy comprising copper and nickel. More precisely, the metallic alloy comprises between 45% and 65% copper and between 35% and 55% nickel. In the present case, the spring 10 comprises constantan and, more precisely, is made of constantan. Constantan is characterized by a set of physical properties, as is known per se. Constantan has a tensile strength of between 300 and 860 megapascals (MPa). Constantan has an elastic modulus of approximately 160,000 MPa, and more precisely, approximately 162,000 MPa.Furthermore, the constantan has an electrical resistivity of approximately 50 ohm-meters (Ωm). Electrical resistivity is the ability of a material to oppose the flow of electric current. The electrical resistivity of the constantan is practically independent of the surrounding temperature, which is very advantageous when the angular sensor is intended to be subjected to temperature variations. The thermal resistivity coefficient of the constantan is approximately 0.00002 K⁻¹.

[0034] The spring 10 is optionally covered with a protective layer of electrically insulating material. The protective layer is arranged to prevent potential short circuits between the coils, constantan being an electrically conductive material as described above. For example, the protective layer may be made of polyimide, such as that produced under the Kapton brand by DuPont, or polytetrafluoroethylene, such as that produced under the Teflon brand by the same DuPont company.

[0035] The sensor 1 also includes a measuring circuit 13, disposed outside the housing 2, which is arranged to measure an electrical resistance of the spring 10.

[0036] With reference to [Fig. 3] and in a first embodiment, the measuring circuit 13 comprises an ohmmeter 14. Thus, the measuring circuit 13 is a simple circuit in which a first terminal of the ohmmeter 14 is electrically connected to the first end 11 of the spring (symbolized by the resistance R in the diagram) and a second terminal of the ohmmeter 14 is electrically connected to the second end 12 of the spring 10. The ohmmeter 14 is connected to the ends 11, 12 by cables passing through the channel delimited by the conduit 6. An electrical resistance Rw associated with said cables is taken into account in the resistance measurement performed by the measuring circuit 13.

[0037] With reference to [Fig. 4] and in a second embodiment, the measuring circuit 13 includes a Wheatstone bridge 15. The Wheatstone bridge 15 is a well-known measuring instrument capable of evaluating a minimal change in electrical resistance. The Wheatstone bridge 15 is therefore a voltage divider circuit, arranged here to measure the electrical resistance of the spring 10, symbolized by the resistance RI in the diagram, connected to the measuring circuit by cables passing through the channel delimited by the conduit 6. As shown in [Fig. 4], the measuring circuit 13 includes a voltage source and five resistors (RI, R2, R3, R4, and R5). The measuring circuit 13 includes a first branch comprising the two resistors RI and R2, and a second branch, parallel to the first, in which the two resistors R3 and R4 are located. The resistor R5 has a first terminal on the first branch and a second terminal on the second branch.

[0038] The operating principle of the two embodiments of the invention will now be described.

[0039] The sensor 1 is here screwed onto the structure of the vehicle V to be immobile relative to the frame of the actuator A and the second end of the shaft S is engaged and coupled in rotation with the fork of the drive member 9.

[0040] Regardless of the embodiment chosen for the measuring circuit, the angular sensor 1 makes it possible to determine the angular position of the actuator shaft S. It is therefore understood that when the actuator shaft S is in motion, this causes the drive member 9 to rotate. Consequently, the rotation of the drive member 9 exerts a force, a mechanical stress, on the spring 10, which then winds or unwinds depending on the direction of rotation of the drive member 9. The change in the shape of the spring 10 alters the resistivity of the material and therefore the electrical resistance across the terminals of the spring 10.

[0041] With reference to [Fig. 5], a graph is shown with the measured electrical resistance value on the x-axis and the associated angular position on the y-axis. This graph is constructed empirically to allow the relationship between the value of the rotation angle and the measured resistance value. Alternatively, the graph could be obtained through modeling.

[0042] In the first embodiment described above, the ohmmeter 14 directly measures the electrical resistance across the spring 10, and the angular position of the shaft S of the actuator A can be deduced from the graph in [Fig. 5]. The reading and processing of the electrical resistance measurement can be performed by any processing unit, such as a microcontroller or other computing device, connected to the measuring circuit 13.

[0043] In the second embodiment of the invention, the measuring instrument is the Wheatstone bridge 15. As a reminder, the resistance RI corresponds to the electrical resistance of the spring 10 that we seek to measure. The values ​​of the resistances R2, R3, R4, and R5 are known. A notion of equilibrium is established when the voltage measured across the terminals of the resistance R5 is zero: AL - Ai R2 ~ R4

[0044] Indeed, if the ratio of the resistances in the two branches is equal, then the Wheatstone bridge 15 is balanced and the voltage across the resistor R5 is zero.

[0045] Therefore, if the voltage across resistor R5 is non-zero, it is understood that the resistance RI across spring 10 has changed. The change in resistance RI is related to a change in the resistivity of the spring 10 material. This implies a change in the shape of spring 10 and thus a change in the angular position of the actuator. Referring to the graph in [Fig. 5], the angular position of the actuator can be determined. Similarly, the reading and processing of the electrical resistance measurement can be performed by any processing unit.

[0046] Knowledge of the angular position of the actuator also allows us to deduce the rotational speed and acceleration.

[0047] By way of non-limiting example, consider an actuator forming a cylinder whose total stroke represents 90 revolutions of the shaft S. For this application, a gearbox with a reduction ratio of 3 is integrated between the drive element 9 and the shaft S. Therefore, the sensor 1 will measure a maximum of 30 revolutions. To achieve this, a spring 10 is chosen whose length and number of coils are adapted to achieve 30 revolutions.

[0048] The elastic limit, also called the shear stress (r) of the spring 10, is approximately 630 MPa and the elastic modulus E (Young's modulus) is approximately 162 gigapascals (GPa). The Poisson's ratio v is approximately 0.35. The internal diameter The spring's length is 10 millimeters (mm) and its outer diameter is 100 mm. This gives us a mean diameter (dm) of 55 mm. Furthermore, the spring's thickness (e) is 0.5 mm. The total twist angle is calculated:

[0050] 0 = 30*2* 7T = 60æ rad

[0051] The maximum torsional moment is calculated:

[0052] m _ oq 15 ^ewton _ meter}

[0053] The shear modulus corresponds to:

[0054] G= =60*109pfl 2^1+v)

[0055] The polar moment of inertia is calculated:

[0056] j — = 6.13*10 1

[0057] From this, we can deduce the length of the spring:

[0058] L

[0059] The number of turns can also be deduced from this:

[0060] turns = 25.0 turns

[0061] In this application, the spring 10 has a length of 4.49 meters and 25 turns. The measured electrical resistance is, in this case, 21.5 ohms (Q) at rest. After 30 turns, the measured electrical resistance is approximately 23.6 Q. It should be noted that the length of the spring influences the accuracy of the measurement: the longer the spring, the more accurate the measurement.

[0062] Of course, the invention is not limited to the embodiments described but encompasses any variant falling within the scope of the invention as defined by the claims.

[0063] In particular, the sensor support is here a housing, but it may be envisaged to use a cage or not to use a support.

[0064] Although here an insulating protection surrounds the spring, it may be envisaged to omit this insulating protection in the case where no risk of contact between the coils can take place.

[0065] Although here the drive member includes a fork, it may be envisaged to use other types of mechanical coupling such as keying, clamp couplers, bellows couplings or cardan couplings.

[0066] Although here the spring includes constantan, the spring can include any material whose resistivity varies according to the mechanical stress exerted on the spring.

[0067] Although here the cross-section of the wire forming the spring is rectangular, it could be circular, square or oval...

[0068] Although a gearbox is integrated in the example provided, it is possible to connect the actuator directly to the angular sensor. In any case, the operating range of the angular sensor according to the invention is greater than that of a standard RVDT sensor.

[0069] The conduit 6 can be used to fix the sensor to the frame and / or can house a connector for connecting the ends of the spring 10 to the measuring circuit.

[0070] Although the measuring circuit is connected to the ends of the spring, the measuring circuit can be connected simply to two parts of the spring which are separated from each other by a distance such that the section of spring extending between said parts has a length corresponding to the desired amplitude of rotation.

[0071] Although here the motor is integrated into the structure of the actuator, the motor can be offset from the actuator and linked to the shaft S by means of a flexible shaft, or any other mechanical linkage device.

Claims

Demands

1. Angular sensor (1) comprising: - a support (2); - a torsion spring (10) having a first end (11) mechanically linked to the support (2) and a second end (12) linked to a drive member (9) movable in rotation relative to the support (2), the spring (10) comprising an electrically conductive piezoresistive material; - a resistance measuring circuit (13) comprising a first terminal and a second terminal which are electrically connected to two parts of the spring (10) separated from each other.

2. Sensor (1) according to claim 1, wherein the spring (10) is a spiral spring.

3. Sensor (1) according to claim 1 or 2, wherein the material is a metallic alloy comprising copper and nickel.

4. Sensor (1) according to any one of the preceding claims, wherein the material is a metallic alloy comprising plus or minus 55% copper and plus or minus 45% nickel.

5. Sensor (1) according to any one of the preceding claims, wherein the material is constantan.

6. Sensor (1) according to any one of the preceding claims, wherein the spring (10) is wrapped in an insulating cover.

7. Sensor (1) according to any one of the preceding claims, wherein the support (2) is a housing enclosing the spring (10).

8. Sensor (1) according to any one of the preceding claims, wherein the measuring circuit (13) includes an ohmmeter (14).

9. Sensor (1) according to any one of claims 1 to 7, wherein the measuring circuit (13) comprises a Wheatstone bridge (15).

10. Actuator comprising a frame, a shaft pivotally mounted on the frame and a sensor (1) according to any one of the preceding claims, the support (2) being fixed relative to the frame and the drive member (9) being rotationally linked to the shaft.

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