Inductive proximity sensor with periodic switching for sensing the presence of objects

• US 5,498,958 A
• Filed: 11/03/1993
• Issued: 03/12/1996
• Est. Priority Date: 12/21/1990
• Status: Expired due to Term

• Current Assignee: Detra SA
• Original Assignee: Detra SA
• Inventors: Schwab, Michel, Tu, Mai X.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US08/145,180
• Time in Patent Office: 860 Days
• Field of Search: 324/207.15, 324/207.16, 324/207.17, 324/207.26, 324/233, 324/234, 324/234-240, 324/329, 324/336, 340/551, 340/686, 340/941, 307/116, 361/179, 361/180
• US Class Current: 324/207.16
• CPC Class Codes: H03K 17/9525 controlled by an oscillatory signal H03K17/9537 takes precedence

Abstract

An inductive proximity sensor determines if a metallic object is in close proximity. An inductor is placed near the object. Several switches control the direction of the voltage passing through the inductor. During a first time period, the voltage flows through the inductor to induce a magnetic field in the object. During the second time period, the voltage is reversed in the inductor to quickly eliminate any current in the inductor. The decaying magnetic field in the object induces a voltage in the inductor which is measured during a third time period and compared to a reference value to determine the proximity of the object to the sensor.

Claims

1. An inductive proximity sensor for sensing the presence of objects in close proximity to the sensor, comprising:

an inductor;

a current directing means for supplying a first voltage to the inductor to produce a current in the inductor to induce a magnetic field in the object during a first period, and for supplying a second voltage to the inductor in a direction opposite to the first voltage during a second period which begins upon conclusion of the first period and terminates upon establishing a substantially zero current in the inductor;

determining means connectable to the inductor for determining when the object is within a predetermined proximity to the inductor, the determining means measuring an induced voltage in the inductor generated by a decaying magnetic field in the object during a third period that begins shortly after the termination of the second period, and comparing the measured induced voltage with a predetermined value.

2. The inductive proximity sensor of claim 1, wherein the third period is composed of a first partial duration and a second partial duration, the measured voltage is the difference between the integral values of the induced voltage at a terminal of the inductor during the first and second partial durations.

3. The inductive proximity sensor of claim 1, wherein the third period has a first partial duration and a second partial duration, the first partial duration measuring the induced voltage that is amplified, the second partial duration measuring the induced voltage that is inverted and amplified by an inverter amplifier.

4. The inductive proximity sensor of claim 1, wherein the current directing means comprises:

a first current control means connecting a power source and a first terminal of the inductor;

a second current control means connecting the power source and a second terminal of the inductor, an output terminal being connected to the second terminal of the inductor;

a third current control means connecting said first terminal of the inductor to ground; and

a fourth current control means connecting the second terminal of the inductor to ground.

5. The inductive proximity sensor of claim 4, wherein the first through fourth current control means are switches.

6. The inductive proximity sensor of claim 4, wherein the first through fourth current control means are transistors.

7. The inductive proximity sensor of claim 1, further including a resistor connected in parallel with the inductor, the resistor having a value substantially larger than the resistance of the inductor.

8. The inductive proximity sensor of claim 1, wherein the determining means comprises:

a first switching means for sampling an output of the inductor during the third period;

a first amplifier for amplifying the output of the inductor;

a second amplifier for inverting the output of the first amplifier;

a second switching means for supplying an output of the first amplifier to a comparator;

a third switching means for supplying an output of the second amplifier to the comparator; and

a comparator for generating an output signal based on the output of the first and second amplifiers and a reference signal.

9. The inductive proximity sensor of claim 8, further including a low-pass filter to remove a DC offset voltage introduced by the first amplifier before comparing the output of the first and second amplifiers with the reference signal.

10. The inductive proximity sensor of claim 9, wherein the low-pass filter comprises a resistor connected between the output of the first and second amplifier and an input of the comparator, a capacitor connected between the input of the comparator to ground.

11. The inductive proximity sensor of claim 8, further including a fourth switching means for supplying a ground signal to the comparator.

12. The inductive proximity sensor of claim 11, further including a fifth switching means for supplying a ground signal to the first amplifier.

13. The inductive proximity sensor of claim 1, wherein the inductor has an intermediate output separating the inductor into two parts, a direction of inductor windings of each of the two parts being arranged to yield a resultant induced voltage equal to the sum of two partial induced voltages in phase opposition.

20. The inductive proximity sensor of claim 1, wherein the current directing means is disconnected from the inductor between the second and third period.

14. A method for sensing the presence of objects in close proximity to an inductive proximity sensor, comprising the steps of:

generating a first current in an inductor during a first period to induce a magnetic field in the object by applying a first voltage to the inductor;

applying a second voltage to the inductor in a direction opposite to the first voltage during a second period which begins upon conclusion of the first period and terminates upon establishing a substantially zero current in the inductor;

measuring an induced voltage in the inductor generated by the decaying magnetic field in the object during a third period which begins shortly after the termination of the second period to produce an intermediate signal; and

comparing the intermediate signal with a reference signal to determine whether an object is in close proximity with the inductive proximity sensor.

15. The method of claim 14, wherein a plurality of switches control the direction of the current in the inductor, and an operation of the plurality of switches is controlled by a plurality of control signals.

16. The method of claim 15, wherein the switches are transistors.

17. The method of claim 14, wherein the third period is composed of a first partial duration and a second partial duration, the measured voltage is the difference between the integral values of the induced voltage at the terminal of the inductor during the first and second partial durations.

18. The method of claim 14, wherein the third period has a first partial duration and a second partial duration, the measured induced voltage being amplified by an inverter amplifier during the first partial duration and the measured induced voltage being inverted and amplified during the second partial duration.

19. The method of claim 14, further including the step of removing a DC offset voltage using a low-pass filter before said comparing step.