Apparatus, system and method for measuring stress

A system for measuring stress including a coilless sensor including at least one band of electrically conductive and magnetostrictive material, the band having a first end and a second end defining a gap therebetween, a measuring circuit electrically connected to the first and second ends of the coilless sensor, the measuring circuit being configured to pass a current through the coilless sensor and measure at least one of an inductance, a resistance and an impedance of the coilless sensor in response to the current, and a processor in electrical communication with the measuring circuit, the processor being configured to calculate an amount of stress being applied to the coilless sensor based upon the measured inductance, resistance and impedance.

BACKGROUND

The present application is directed to stress sensors and, more particularly, to apparatus, systems and methods for measuring stress.

Stress sensors may be used to measure an amount of force, torque or pressure applied to a material. Traditional stress sensors have employed a conductive wire coil that is wrapped around a separate core member. The core member is formed from a magnetostrictive material. An electrical current flowing through the wire coil establishes a magnetic field that surrounds the wire coil and propagates into the core member. A stress applied to the core member changes the magnetic permeability of the core member. The inductance of the wire coil is a function of the permeability of the material through which the coil member's magnetic field flows. Therefore, the stress applied to the core member changes the inductance of the wire coil and the change in inductance may be correlated into a measured force value.

U.S. Ser. No. 11/244,792, filed on Oct. 6, 2005, the entire contents of which are incorporated herein by reference, discloses a stress sensor in which the wire coil and core member have been functionally combined as a coilless sensor. The coilless sensor includes an electrically conductive member comprising a magnetostrictive material that is configured to receive an applied force, wherein the electrically conductive member has a change in impedance in response to the applied force. The coilless sensor further includes first and second covering members such that the electrically conductive member may be disposed between the first and second covering members. The second covering member has first and second apertures extending therethrough. The coilless sensor further includes first and second electrical terminals disposed through the first and second apertures, respectively, of the second covering member that are coupled to the electrically conductive member. The coilless sensor has increased sensitivity for measuring forces and provides more consistent force measurements since manufacturing tolerances associated with the air gap between various elements of the core member have been eliminated.

However, there remains a need for a coilless stress sensor and associated system having improved functionality and design flexibility. There is also a need for a coilless stress sensor capable of being used in an electric motor-operated brake caliper assembly.

SUMMARY

In one aspect, the disclosed system for measuring stress may include a coilless sensor including at least one band of conductive and magnetostrictive material, the band having a first end and a second end defining a gap therebetween, a measuring circuit electrically connected to the first and second ends of the coilless sensor, the measuring circuit being configured to pass a current through the coilless sensor and measure at least one of an inductance, a resistance and an impedance of the coilless sensor in response to the current, and a processor in electrical communication with the measuring circuit, the processor being configured to calculate an amount of stress being applied to the coilless sensor based upon the measured inductance, resistance and impedance.

In another aspect, the disclosed coilless sensor may include a band formed from an electrically conductive and magnetostrictive material, the band including a first end and a second end, wherein the band is shaped as an open ring and defines a gap between the first and second ends, a first terminal connected to the first end of the band, and a second terminal connected to the second end of the band.

In another aspect, the disclosed method for measuring stress may include the steps of providing a coilless sensor including a band formed from an electrically conductive and magnetostrictive material, the band including a first end and a second end, shaping the band such that a gap defined by the first and second ends has a length that is less than a length of the band, passing an electric current through the coilless sensor, measuring at least one of an inductance, a resistance and an impedance of the coilless sensor in response to the electric current, and correlating the measured inductance, resistance and impedance into an amount of stain being applied to the coilless sensor.

Other aspects of the disclosed apparatus, system and method for measuring stress will become apparent from the following description, the accompanying drawings and the appended claims.

DETAILED DESCRIPTION

Referring toFIG. 1, one aspect of the disclosed system for measuring stress, generally designated10, may include a coilless sensor12, a measuring circuit14and a processor16. When a force F is applied to the coilless sensor12, the change in electrical properties of the sensor12may be measured by the measuring circuit14and communicated to the processor16. The processor16may process the measured electrical properties to determine the amount of force F being applied to the sensor12.

In one aspect, as shown inFIG. 2A, a coilless sensor12′ may include an elongated band102having a first end104and a second end106. A first terminal108may be connected to the first end104of the band102and a second terminal110may be connected to the second end106of the band102to connect the sensor12′ to the measuring circuit14. The terminals108,110may be connected to the band102by, for example, soldering or any other available techniques.

The band102may be formed into a generally open ring114and the first and second ends104,106of the band102may be separated by a gap112. The ring114may be generally round or oval-shaped in top view and may have a diameter D. In one aspect, the band102may form about 270 to about 355 degrees of the ring114and the gap112may form the rest of the ring114. In another aspect, the band102may form about 90 to about 359 degrees of the ring114and the gap112may form the rest of the ring114.

Referring toFIG. 2B, the band102may have a generally rectangular cross-section having a height H and a width W, wherein the height H is greater than the width W. In one aspect, the cross-section of the band102may have an aspect ratio (i.e., the ratio of the height H to the width W) of about 1 to about 10. Those skilled in the art will appreciate that the height H, width W, diameter D and gap112of the ring114may be selected based upon design considerations. For example, a sensor12′ may have an average diameter D of about 50 mm, a height H of about 5 mm, a width W of about 2 mm and a gap112of about 2 mm.

The band102may be formed from an electrically conductive and magnetostrictive material. For example, the band102may be formed from a magnetostrictive material such as cobalt, iron, nickel, rare-earth elements having magnetostrictive properties and alloys and combinations thereof. In one aspect, the band102of the sensor12′ is formed from a nickel/iron alloy.

The force F being measured may be applied to a portion of the band102, or to the entire surface of band102. Alternatively, the force F may be applied partially through the band102and partially through a parallel, non-sensing member (not shown inFIG. 2). All three cases are contemplated by the present disclosure. However, applying the force F to a portion of the band102results in the sensor being larger than necessary. Also, having the force F applied partially through the band102and partially through a parallel, non-sensing member may be attractive in cases where the force F is large, so as to keep the size of the sensor small. However, in that situation, the sensor reading will be an accurate image of the entire force only if there is a consistent and repeatable relation between the portion of the force F being sensed and the entire force F. Overall, therefore, with the exception of designs for large forces, the most desirable approach may be the one where the force F is exerted over approximately the entire surface of the band. To achieve this, the practitioner will design the shape of the band102(generally round, oval, or for that matter any other shape) to match the shape and cross-sectional dimensions of the force-transmitting member, so that the band surface corresponds to the footprint of the force F.

In another aspect, as shown inFIG. 3A, the coilless sensor12″ may include an elongated band202having a first end204and a second end206. As discussed above, terminals208,210may be connected to the first and second ends204,206, respectively, of the band202. The band202may be formed into a generally open washer-shaped ring112having a gap214.

Referring toFIG. 3B, the band202may have a generally rectangular cross-section having a height H and a width W, wherein the width W is greater than the height H. In one aspect, the cross-section of the band202may have an aspect ratio (i.e., the ratio of the height H to the width W) of about 0.1 to about 1. For example, a sensor12″ may have an average diameter D of about 50 mm, a height H of about 2 mm, a width W of about 5 mm and a gap214of about 2 mm.

In another aspect, multiple bands102,202may be stacked together, either in series or parallel, to provide an overall sensor12, as discussed below.

For example, as shown inFIGS. 4A,4B, a coilless sensor12′″ may include two bands302A,302B connected to each other by a bridge portion304. The bands302A,302B may be formed into a concentric open ring310having a first end306and a second end308separated by a gap312. A first terminal314may be connected to the first end306and a second terminal316may be connected to the second end308to connect the sensor12′″ to the measuring circuit14. Therefore, sensor12′″ effectively includes two bands302A,302B connected in parallel.

Alternatively, as shown inFIGS. 5A,5B, a coilless sensor12″″ may include two bands402A,402B formed into a concentric open ring404having a first end406and a second end408separated by a gap410. A bridge portion412(FIG. 5A) may be positioned between the two bands402A,402B at the second end408of the ring404. A first terminal414may be connected to the first end406of the first band402B and a second terminal416may be connected to the first end406of the second band402A to connect the sensor12″″ to the measuring circuit14. Therefore, sensor12″″ ofFIGS. 5A,5B effectively includes two bands402A,402B connected in series.

At this point, those skilled in the art will appreciate that the coilless sensors12of the disclosed stress measuring system10may have various shapes, dimensions and sizes, preferably but not necessarily matching the footprint of the stress being measured, and may be formed from various electrically conductive and magnetostrictive materials having various rectilinear or curvilinear shapes, the selection of which may be driven by cost, design and manufacturing considerations.

It should be noted that coilless sensors12disclosed herein may be used to measure an amount of stress applied to a material and/or to measure the amount of force, torque and/or pressure that affects the stress level in a material. However, those skilled in the art will appreciate that coilless sensors12may be used to measure any physical quantity that can be determined from an applied stress to the sensor.

As shown inFIGS. 2A and 3A, the force F applied to the sensors12′,12″ is in a direction substantially perpendicular to a direction of the electrical current flowing through the sensors. However, it should be noted that depending on the design and application of the sensor12, including the overall shape of the sensor12and the materials used to form the sensor12, the stress may be in-line, perpendicular or at any angle with respect to a direction of the electrical current flowing through the sensor12.

Accordingly, referring again toFIG. 1, when an electric current is supplied to the coilless sensor12by the measuring circuit14, the current establishes a magnetic field (not shown) around the sensor12in a path transverse to the current flow. As a force F is applied to the sensor12, its magnetic permeability changes due to imposed stress, thereby affecting the measured inductance. Furthermore, at higher frequencies and because of skin effects, the effective resistance of the sensor12is also a function of magnetic permeability. Therefore, the stress applied to the sensor12may be a function of, or otherwise correlated to, the amount of inductance, resistance and/or impedance of the sensor12.

Accordingly, by measuring and processing the electrical signals (e.g., inductance, resistance and/or impedance) received from the coilless sensor12in response to the introduced current, the system10may determine the amount of force F being applied to the coilless sensor12.

Referring toFIG. 6, an electric brake caliper assembly, generally designated500, may be provided with a coilless sensor12of the present disclosure to measure the amount of braking force being applied to a rotor (not shown) by the brake caliper assembly500. In one aspect, the assembly500may include a caliper housing502, an electric motor assembly504having a drive shaft506extending therefrom, a caliper508and a ballnut/ball screw assembly510. The caliper housing502may include a central bore512extending therethrough and the caliper508may be slidably received within the bore512. Rotational power from the motor504may be communicated to the ball screw assembly510by the shaft506(possibly via a gear, not shown), and the ballnut/ball screw assembly510may convert the rotational power of the motor504into distal advancement (arrow A) of the caliper508through the bore512. A coilless sensor12(e.g., a washer-shaped coilless sensor12″ ofFIG. 3A) having terminals208,210connected to a measuring circuit14(FIG. 1) may be positioned between the caliper508and the ballnut/ball screw assembly510, on the one hand, and the caliper housing502, on the other hand, to sense the reaction force (arrow F′) generated when the caliper508is being driven into engagement with the brake pads (not shown) and rotor (not shown).

Those skilled in the art will appreciate that the ring shape of the coilless sensor12″ corresponds with the generally cylindrical shape of the caliper508, thereby occupying less space within the caliper housing502, while sensing the full reaction force (except for the small gap214) being applied to the caliper housing502by the caliper508. In other words, the ring shape of the coilless sensor12″ generally matches the footprint of the reaction force F′. This is an advantage in brake systems, because although the reaction force F′ may be uniformly distributed at low braking levels, it may not be so at higher braking levels when the caliper assembly may become distorted. A sensor that would sense only a portion of the force may thus provide an inaccurate measure of the force.

While suggestion has been made to use the disclosed coilless sensors12in an electric brake caliper assembly500(FIG. 6), those skilled in the art will appreciate that the disclosed coilless sensor12may be used in a wide variety of applications and the overall size and shape of the sensor12may be dictated by the application, intended use and/or desired result.

Referring again toFIG. 1, the measuring circuit14may supply an electric current to the coilless sensor12and may measure the inductance, resistance and/or impedance in the coilless sensor12in response to the electric current. The measured inductance, resistance and/or impedance in the coilless sensor12may in turn be communicated to the processor16.

The coilless structure of coilless sensor12may result in low inductance levels, which may be difficult to measure by the measuring circuit14. For example, a single ring coilless sensor12formed from a nickel-iron alloy and having diameter D on the order of 50 mm, a width W of about 2 mm and a height H of about 2 mm may have an inductance of about 1.5 micro-Henries (μH) at 10 kilohertz (kHz). If two such rings are placed in series, as shown inFIG. 5B, the inductance may be about 3 μH.

Therefore, one potential challenge presented by the coilless sensor12is the low voltage signals generated by such small inductance values. For example, a 3 μH inductance at 10 kHz is a 0.19 Ohm (Ω) impedance. If the coilless sensor12is excited with a 1 milliamp (mA) current, the resulting voltage is about 0.19 millivolts (mV). Therefore, even a 20 percent change in inductance as a result of stress measures in the 40 microvolt (μV) range.

Another challenge results from the possibility that the impedance of the connection between the sensor12and the measuring circuit14may not be small compared with the impedance of the sensor12. While it may be possible to minimize the length of the connection between the sensor12and the circuit14, the signal may remain smaller since only the inductance of the sensor12is affected by stress. Similarly, temperature compensation schemes which are based on sensing the sensor resistance will be complicated by the resistance of the leads and connections, even if the sensor12is in close proximity to the measuring circuit14.

The circuit discussed offers a solution to the challenges discussed above by using a 4-wire connection between the sensor12and the measuring circuit in order to circumvent the issues stemming from the sensor12to circuit14connection.

One aspect of a measuring circuit14, generally designated600inFIG. 7, may include a sinewave oscillator602having a 90 degree reference, a DC voltage input604, a voltage-to-current converter606, instrumental amplifier608, a high pass filter610, low pass filters612,614,616and synchronous detectors618,620. The coilless sensor12may be connected to the circuit600by four lead wires622A,622B,622C,622D (ground). The circuit600may have analog outputs624A,624B,624C.

The output of the sinewave oscillator602may be offset with DC voltage input604and, at converter606, the voltage may be converted into a sinusoidal current signal. For example, the current signal may have a 40 mA peak to peak of amplitude. The current may flow through two of the lead wires622A,622D to the coilless sensor12, which is modeled as a series ac resistance Rac and inductive reactance XI.

The instrumentation amplifier608may reject the voltage generated by current flowing through the lead resistance and may only measure voltage due to current flow through Rac and XI which combine in quadrature (i.e., the voltage from Rac is in phase with the current sinewave while the voltage from XI is leading the current sinewave by 90 degrees). This quadrature relationship allows the use of two synchronous detectors618,620to separate and detect the two voltages by using in-phase and quadrature reference signals, thus producing two analog output signals624A,624B. One output is proportional to Rac and the other is proportional to XI.

The high pass filter610may be used ahead of the detectors to reject the DC component of the amplifier output. Conversely, the low pass filters612,614on the output of the amplifier608reject the higher frequency AC components due to the fundamental of the current signal, and its harmonics, so as to produce outputs proportional to, respectively, Xl, and Rac. Also, the low pass filter616on the output of the amplifier608rejects most or all AC components and produces an output proportional to the DC resistance of the sensor12. The DC resistance of the sensor12may be useful for temperature measurement in general. It can also be used for temperature compensation of the output of sensor12, in ways known in the art.

Thus, circuit600uses a current source and amplifiers of sufficient sensitivity and amplification to result in sufficiently high output voltages for easy signal pick-up.

Those skilled in the art will appreciate that the circuit14as described inFIG. 7with a 4-wire connection is useful to measure low values of inductance, resistance and impedance. Further, the proposed circuit may be modified to measure only the resistance, only the inductance, or both the resistance and inductance. One may opt to use, or not to use, the DC resistance measuring means. One may adapt and adjust various aspects of the circuit to fit a specific application, in ways which may be driven by cost, design and overall electronic integration.

The processor16is provided for calculating an amount of force F applied to the coilless sensor12based on the output signal from the measuring circuit14. The processor16may be electrically coupled to the measuring circuit14and may receive an output signal from the measuring circuit14indicative of at least one of an amount of inductance, an amount of resistance and an amount of impedance of the coilless sensor12. In one aspect, the processor16may include a computer that receives the output signal from the measuring circuit14and calculates a numeric force value (e.g., in Newtons). In another aspect, the processor16may be part of a brake controller (not shown), and the measuring circuit14then provides an analog or digital value useful for the feedback control of the brake system.

Accordingly, the disclosed coilless sensor12and associated system, including the measuring circuit14and processor16, provide a low cost and robust method for measuring applied stress. Furthermore, the disclosed coilless sensor12may be adapted for use in various applications and designs because of its overall simplicity and relatively small size.

Although various aspects of the disclosed apparatus and system for measuring stress have been shown and described, modifications may occur to those skilled in the art upon reading the specification. The present application includes such modifications and is limited only by the scope of the claims.