METHOD AND CIRCUIT FOR DETECTING A CHANGE IN INDUCTANCE
A method and circuit for detecting a change in inductance of a variable inductance element. An oscillating signal has a frequency that varies with inductance of the element. An intermediate voltage is produced at a level that varies according to frequency of the oscillating signal. The intermediate voltage is scaled to produce an output voltage.
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The present invention relates generally to inductive sensors, and more particularly to a method for detecting a change in the inductance of an inductive sensor.
BACKGROUND OF THE INVENTIONConventional inductive sensors may use an inductive coil positioned relative to a magnetostrictive object such that magnetic flux lines induced by an alternating electric current in the coil pass through the object in a direction substantially parallel to the strain direction. The inductance of the coil is measured over time. A change in permeability of the object due to a change in strain of the object is detected or determined from a change in the measured inductance over time.
What is needed is an improved method for accurately detecting a change in the inductance of such inductive sensors as well as other variable inductance elements.
SUMMARY OF THE INVENTIONIn a first aspect, a method for detecting changes in inductance of a variable inductance element involves the steps of: a) producing an oscillating signal having a frequency that varies in proportion to variations in inductance of the variable inductance element; b) producing an intermediate analog voltage that varies in proportion to variations in frequency of the oscillating signal of step a); c) scaling the intermediate analog voltage of step b) to produce an output analog voltage; and d) detecting changes in inductance of the variable inductance element based upon changes in the output analog voltage of step c).
In another aspect, a method is provided to convert a known range of inductance change of a variable inductance element between a first inductance and a second inductance into a desired range of analog voltage change between a first voltage level and a second voltage level. The method involves the steps of: a) establishing an oscillator circuit incorporating the variable inductance element so as to produce an oscillating signal having a frequency that varies with inductance of the variable inductance element, the oscillating signal produced with a first frequency when the variable inductance element has the first inductance and produced with a second frequency when the variable inductance element has the second inductance; b) establishing a circuit to convert the frequency of the oscillating signal to an intermediate analog voltage, the intermediate analog voltage produced at a first intermediate level when the oscillating signal has the first frequency and produced at a second intermediate level when the oscillating signal has the second frequency; and c) establishing a circuit to scale the intermediate analog voltage so as to produce an output voltage within the desired range, the output voltage produced at the first voltage level when the intermediate analog voltage is at the first intermediate level and produced at the second voltage level when the intermediate analog voltage is at the second intermediate level.
In a further aspect, a circuit for producing a voltage level substantially proportional to inductance of a variable inductance element includes an oscillator stage having the variable inductance element connected therein and producing an oscillating signal having a frequency that varies with inductance of the variable inductance element. A conversion stage is operatively connected to receive the oscillating signal and produces an intermediate analog voltage that varies in proportion to variations in the frequency of the oscillating signal. An amplification stage is operatively connected to receive the intermediate analog voltage and operates to offset and amplify the analog voltage to produce an output analog voltage with a voltage level proportional to inductance of the variable inductance element.
The foregoing methods and circuit provide a practical, effective and relatively inexpensive way to detect changes in inductance of a variable inductive element.
SUMMARY OF THE DRAWINGS
A flow chart 10 illustrating one embodiment of a method is shown in
At step 16 an oscillator circuit 102 (
Accordingly, the basic method of detecting changes in inductance of a variable inductance element involves producing an oscillating signal 104 having a frequency that varies in proportion to variations in inductance of the variable inductance element; producing an intermediate analog voltage VI that varies in proportion to variations in frequency of the oscillating signal 104; scaling the intermediate analog voltage VI to produce an output analog voltage VO; and detecting changes in inductance of the variable inductance element based upon changes in the output analog voltage VO. In one implementation the scaling step involves both amplifying and offsetting the intermediate analog voltage.
Referring now to
Referring to
The foregoing description has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise forms or procedures disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, while specific embodiments of oscillator circuit or stage 102, conversion circuit or stage 106, amplification or scaling circuit or stage 108 and detection unit 110 are shown and described with reference to
Claims
1. A method for detecting changes in inductance of a variable inductance element comprising the steps of:
- a) producing an oscillating square wave signal having a frequency that varies in proportion to variations in inductance of the variable inductance element;
- b) inputting the oscillating square wave signal to a phase-locked loop circuit and producing an intermediate analog voltage that varies in proportion to variations in frequency of the oscillating square wave signal of step a);
- c) scaling the intermediate analog voltage of step b) to produce an output analog voltage; and
- d) detecting changes in inductance of the variable inductance element based upon changes in the output analog voltage of step c).
2. The method of claim 1, wherein the variable inductance element comprises an inductive sensor.
3. The method of claim 2 where the inductive sensor includes at least one coil located adjacent a magnetostrictive object.
4. (canceled)
5. The method of claim 1, wherein step d) includes inputting the output analog voltage to an analog to digital converter.
6. (canceled)
7. The method of claim 1, wherein step a) involves connecting the variable inductance element in a feedback path of an oscillator circuit.
8. The method of claim 1 wherein the scaling of step c) involves offsetting and amplifying the intermediate analog voltage.
9. A circuit for producing a voltage level substantially proportional to inductance of a variable inductance element, the circuit comprising:
- an oscillator stage having a feedback path and the variable inductance element connected therein and producing an oscillating signal having a frequency that varies with inductance of the variable inductance element;
- a conversion stage comprising a phase-locked loop circuit operatively connected to receive the oscillating signal and producing an intermediate analog voltage that varies in proportion to variations in the frequency of the oscillating signal; and
- an amplification stage operatively connected to receive the intermediate analog voltage and operating to offset and amplify the intermediate analog voltage to produce an output analog voltage with a voltage level proportional to inductance of the variable inductance element.
10. (canceled)
11. (canceled)
12. The circuit of claim 9 wherein the amplification stage includes an adjustable offset control component.
13. The circuit of claim 12 wherein the adjustable offset control component comprises a potentiometer.
14. The circuit of claim 12 wherein the adjustable offset control component comprises an automated control component.
Type: Application
Filed: Oct 8, 2004
Publication Date: Apr 21, 2005
Applicant:
Inventors: Curtis Cyran (Dayton, OH), Robert Disser (Dayton, OH)
Application Number: 10/961,450