WO2000057147A1

WO2000057147A1 - Stress-wire antenna

Info

Publication number: WO2000057147A1
Application number: PCT/SE2000/000476
Authority: WO
Inventors: Carl Tyrén; Manuel VÁZQUEZ VILLALABERITIA; Antonio Hernando Grande; Christian Quinones
Original assignee: Tyren Carl; Vazquez Antonio Hernando Grand; Christian Quinones
Priority date: 1999-03-21
Filing date: 2000-03-15
Publication date: 2000-09-28
Also published as: EP1181513A1; SE9901045D0; AU4154700A

Abstract

A simple piece of magnetic wire is a wire-less mechanical stress sensor by combining magneto-elastic and magneto-impedance effects allowing the wire stress level to be remotely measured via radio detection.

Description

STRESS-WIRE ANTENNA

Background :

The world market is since many years still waiting for low-cost, wire-less sensors to measure mechanical stress. A classical need is torque which often is referred to as the most demanded measurement (after temperature) in the world. Maybe also not so surprising as the majority of electrical-to- mechanical energy converters are rotating machines.

The ever growing computer control of our, in the last instance, mechanical world indefinately relies on the availability of mechanical sensors as well as actuators.

A useful sensor for mechanical signals must be compatible with the often rough, dirty and temperature exposed environment of the mechanical world. Simplicity and wire-less signal transfer are here not preference but necessity.

Invention :

What could be simpler and more mechanically compatible than a metallic wire A piece of metallic wire is, in general, a dipole antenna and as such will respond to a radio signal. Given that this response will depend on the mechanical stress the wire is exposed to and an incredibly simple and wireless mechanical sensor is a reality

This invention will define such a wire and radio detection system.

The basic underlying principle controlling the radio signal response of the stress wire sensor is the so called (Giant) Magneto impedance effect. This effect relates to a well-known phenomena of high frequency electrical signals in electrical conductors namely the so-called skin depth. High-frequency conduction takes place in a skin layer of the conductor. The penetration depth of the skin is related to the AC signal frequency as well as the electrical resistivity and magnetic permeability of the conductor itself.

The effectiv resistance of the conductor is therefore determined by the skin layer cross-section in product with the resistivity of the conductor material. (Note that this effect is different from the magnetoresistance effect for which the actual resistivity of the conductor changes). The effective resistance in turn controls the radio signal amplitude response of the metallic stress-wire.

The magneto-impedance effect depends on the permeability of the metallic wire conductor. Therefore, using a magneto-elastic coupling, the radio response of the wire can be coupled to the mechanical stress situation of the wire.

The combination of magneto-elastic coupling and magneto-impedance in one metallic wire gives the stress wire antenna sensor.

Note that the stress wire antenna sensor is not a resonance sensor nor does it require any mechanical freedom for its intrinsic functioning. Radio detection of the stress wire antenna can be realized in several manners. In most practical cases it should be desirable to create a modulation of the stress-wire signal in order to identify its signal from other returns of the detection transmission signal.

1. Direct

Under certain conditions, eg differential or non-echoic, the amplitude of the return signal from the stress-wire can be directly mesured and monitored.

2. Magnetically driven

By exposing the stress-wire also to a dynamic, eg sinewave, magnetic bias field signal its return radio signal will be amplitude-modulated at the frequency of the AC bias field. This modulation is created by the magneto- impedance effect as the AC bias field modulates the magnetic permeability of the stress-wire. The average permeability value of the stress-wire will be determined by the stress level applied to the wire and the AC bias signal will generate an identifying oscillation of the wire permeability around the level set by the stress.

3. Vibrational driven

If the mechanical signal applied to the stress-wire contains a vibration component, i.e. a DC and an AC stress component, the AC part of the stress signal will generate a characteristic amplitude modulation of the return radio signal that can be utilized to identify the stress-wire radio signal from other competing radio signals coming in to the radio detection antenna.

4. Antenna aspect driven When the antenna aspect angles oscillates, eg as in the case where a stress wire is applied to a rotating shaft for torque mesurements, the return radio signal from the stress-wire will contain a characteristic amplitude modulation generated by the antenna aspect oscillation.

For a certain amplitude of AC bias or AC stress signal the resulting radio signal amplitude modulation will depend on the DC bias field and DC stress level in the stress-wire.

A. Amplitude detection

The skin depth level is set by the stress-related permeability and the HF frequency. The HF AM modulation amplitude depends on the modulation of the HF conduction ring area by the AC magnetic bias field and/or the AC mechanical vibrational amplitude as well as the HF signal level. The goal of the mesurement is to determine the permeability level as this value then can be tied to the mechanical stress level in the wire. When the AC bias field amplitude and /or the HF amplitude along the stress wire antenna are not known, as would be the many times the more common case as the stress wire antenna will be at some location and angle in the detection space for which these values are usually not known, this permeability level cannot readily be calculated from the radio response signal. However, assuming that stress and AC and HF levels are maintained constant, a known change of the HF frequency will change the skin depth with a known factor. Therefore, noting the corresponding change in the radio response signal AM amplitude - and also knowing the diameter of the stress wire-antenna - the actual skin depth can be calculated as the absolute change in the conductive ring area depends on the actual skin depth level ( 2 x pi x R x dR ). Knowing then the skin depth and the HF frequency ( and the wire electrical conductivity ) its momentarily magnetic permeability can be calculated and thereby also the mechanical stress level in the wire !

B. Skin depth saturation detection

One possibility to eliminate the influence of the radio signal (HF) amplitude is offered by the skin-depth saturation technique.

This technique represents another aspect of the above described skin layer penetration depth related to the AC signal frequency as well as the electrical resistivity and magnetic permeability of the conductor itself.

The effective resistance of the conductor is therefore related to the skin layer cross section and is basically an inverse square root relation UNTIL the skin depth reaches and equals the conductor radius.

At this point the AC resistance of the conductor cannot decrease further as the full cross-section of the wire now carries the high frequency current. . This point is therefore a discontinuity point in the AC resistance to skin-depth relationship (see fig. 2).

The stress-level in the stress-wire will set a particular skin depth level at a given HF frequency. An applied AC magnetic bias field will add a skin depth level oscillation. Increasing the amplitude of the AC bias will increase the skin level oscillation amplitude and finally the skin depth will reach the radius and center of the wire causing a response signal discontinuity. This point is independant of the HF signal amplitude and thus measurement procedure eliminates this influence on the stress level measurement.

To reduce the actual length of a stress wire antenna for a certain HF frenquency, while still maintaining a good antenna gain, a dielectric coating or layer around the stress wire antenna can be added to increase its appearant antenna length. An example of such a coating is a Barium-Titanate coating.

The stress wire antenna can also be conceived as a loop antenna and, as such, could also respond the lower frequency non-radiating magnetic interrogation field signals.

Claims

Claims :

1. Method for detection of mechanical stress or strain characterized b y the combination of magneto-elastic and (giant) magneto-impedance effects in a magnetic wire antenna element whereby the mechanical stress will control the radio response of the wire element.

2. Method according to claim 1 characterized by the addition of a dynamic magnetic bias field influence to the stress sensing wire in order to generate a corresponding modulation of the wire radio response.

3. Method according to claim 1 characterized b y using a dynamic stress applied to the stress sensing wire and the corresponding modulation of the wire radio response to identify the stress sensing wire radio signal.

4. Method according to claim 1 charactarized by using a dynamic antenna aspect relation to the stress sensing wire and the corresponding modulation of the wire radio response to identify the stress sensing wire radio signal.

5. Method according to claim 1 and 2 characterized by making a known change in the HF frequency and calculating the wire mechanical stress level from the resulting change in the AM level of the stress wire radio response signal.

6. Method according to claim 1 and 2 characterized by detection of a discontinuity in the AC impedance-to-AC bias amplitude relation at the point of skin depth saturation.

7. Stress wire sensor according to claim 1 characterized by an added dielectric coating or layer.

8. Stress wire sensor according to claim 1 characterized by being a

Co based alloy containing Fe or Ni or Mn or combinations of them so that the ratio of Co content to the rest of metallic elements be in the range (93- 96)/(7-4)

(e.g. Co95Mn5, Co96Fe3Ni1, etc) and as metalloids Si, B, C etc amounting between 15% to 25 % of the total alloy (e.g. (CoMn)78(SiBC)22).

9. Stress wire sensor according to claim 1 characterized by having been annealed by electrical current or transverse magnetic field techniques.