Travel sensor having approximately constant measured signal temp

Details Travel sensor having approximately constant measured signal temp Travel sensor having approximately constant measured signal temp

1997-07-02

1999-04-27

Strecker, Gerard

Electricity: measuring and testing

Magnetic

Displacement

32420716, 32420724, 336 84M, 336130, G01D 3028, G01D 520, G01B 714

Patent

active

058983007

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a travel sensor and, more particularly, to a travel sensor for measuring a penetration depth of a body in a measuring coil through which an alternating current passes by measuring the damping of the alternating current.
2. Prior Art
From DE-OS 31 09 930.0 a travel sensor is known, in which the bar core with the exploring coil is inserted into a copper tubing attached inside a pipe. The frequency of the alternating current flowing through the coil is timed in such a way, that the eddy currents only develop in the copper surface. But the so-called eddy current principle used here has a low temperature dependency. However, at differing temperatures a temperature drift occurs which is a function of the depth of penetration of the insertion body into the exploring coil. In an electrical evaluation circuit this temperature drift can only be compensated with difficulty and requires a lot of time.
In the inductive travel sensor known from DE-OS 40 20 369.7 an insertion body is also moved in a measuring coil. In this case the insertion body consists of a layer of ferromagnetic and a layer of non-ferromagnetic material with good electrical conductivity properties. However, in practice it is very difficult to apply these two layers and to synchronize the two in order to avoid a temperature drift. For this reason the manufacture of this type of travel sensor is very time consuming, complicated and relatively expensive.


SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved travel sensor of the above-described type that avoids the above-mentioned disadvantages.
The travel sensor according to the invention includes at least one measuring coil through which an alternating current flows, a shielding sleeve enveloping the at least one measuring coil and consisting of a ferromagnetic material and an electrically conductive, non-ferromagnetic body arranged in the at least one measuring coil to extend into the at least one measuring coil (15) to a penetration depth which is moveable relative to the at least one measuring coil so that the penetration depth is variable. A measured signal characteristic of the penetration depth of the body in the at least one measuring coil is obtainable by measuring a damping of the alternating current. Respective volumes of the ferromagnetic material and the electrically conductive, but non-ferromagnetic, material are selected for a predetermined fixed eddy current frequency so that the temperature dependence of the measured signal is at least approximately constant across an entire measurement range for the measured signal.
In contrast, the travel sensor in accordance with the invention has the advantage, that the measuring curves determined at different temperatures run nearly parallel across the entire measuring range. Thus, the dependence of the temperature is nearly constant across the entire measuring range and can easily be considered in an electrical evaluation circuit. It is known that the offset variance of the measuring curve, i.e. the shift of the zero point, for example, can be relatively easily compensated with the aid of an electrical evaluation circuit. By using a shielding sleeve of ferromagnetic material, electromagnetic stray fields can be shielded, which for example are caused by electromagnets located close to the travel sensor. The ferromagnetic properties of the sleeve thus only have a minimal effect on the inductivity of the coil through which electricity flows, since on their way the lines of flux of the magnetic field of the coil are concentrated in the sleeve. The magnetic flow is thus independent of the other geometric properties of the travel sensor or of possible exterior interference fields.
The manufacture of the travel sensor is particularly easy, since only the thickness of the insertion body and of the exterior sleeve and the frequency must be synchronized.
In a preferred embodiment of the invention the measuring coil is arranged on a c

REFERENCES:
patent: 3891918 (1975-06-01), Ellis
patent: 4667158 (1987-05-01), Redlich
patent: 5068607 (1991-11-01), Redlich et al.

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