Proximity sensor having a bridge circuit with a voltage controlled resistance

• US 5,079,502 A
• Filed: 03/02/1990
• Issued: 01/07/1992
• Est. Priority Date: 05/16/1988
• Status: Expired due to Term

• Current Assignee: Syron Engineering and Manufacturing LL C (IMI PLC)
• Original Assignee: Syron Engineering and Manufacturing LL C (IMI PLC)
• Inventors: Rogacki, Steven A., Herbermann, Alfred F.
• Primary Examiner(s): Snow, Walter E.
• Application Number:
  US07/487,691
• Time in Patent Office: 676 Days
• Field of Search: 324/226, 324/207.11, 324/207.13, 324/207.14, 324/207.15, 324/207.16, 324/207.19, 324/207.22, 324/207.26, 324/236, 324/207.12, 340/686, 340/870.31
• US Class Current: 324/207.19
• CPC Class Codes: G01V 3/101 by measuring the impedance of the search coil; by measuring features of a resonant circuit comprising the search coil measuring impedance or characteristics derived therefrom G01R27/00, e.g. quality factor G01R27/26
• CPC Class Codes: H03K 17/9547 with variable amplitude

Abstract

Proximity sensors for detecting the presence of metal objects such as sheet metal workpieces within workpiece engaging devices such as grippers and shovels. The proximity sensors are jacketed in a stainless steel casing, making them rugged and resistant to impact damage. Monitor circuits are used with the sensor which enable the devices to sense objects through their metal casing, these objects being disposed upon the sensor casing. A thresholding circuit and a delay circuit are further disclosed and which reduce erroneous indications of the presence of the metal objects upon the sensor. Other embodiments are disclosed which allow for the proper alignment of metallic workpieces within an area and for the determination of the height of a stack of workpieces.

Claims

1. A sensor for sensing the presence of a metal workpiece in contact with a surface of said sensor and removably fixed thereon, said sensor comprising:

emission means for generating a magnetic field therefrom and causing said generated magnetic field to be directed to said metal workpiece and for generating a signal indicating said presence of said metal workpiece upon said surface of said sensor;

metal casing means, surrounding said emission means, for defining said surface of said sensor in contact with said metal workpiece;

means, coupled to said emission means, for causing said signal to be generated, for a specified time, in the absence of said metal workpiece from said surface of said sensor;

said emission means comprises a coil having an impedance value associated therewith;

circuit means, coupled to said coil, for exciting said coil at a frequency less than 20 KHz and for measuring an amount of change of said impedance value caused by the presence of said metal workpiece upon said surface of said sensor and for generating said signal in response to said measured change in said impedance value;

said circuit means comprises threshold means, coupled to said coil and responsive to said change of said impedance value of said coil, for defining a threshold value and for comparing said measured amount of change of said impedance value of said coil with said threshold value and for allowing said generation of said signal to occur only if said measured amount of change of said impedance value is different than said threshold value; and

said circuit means comprises a resistance bridge having four legs in which said coil and a capacitor comprise one of said legs and two nodes of said bridge form inputs to an amplifier and the amplifier output is connected to another mode of said bridge and controls a voltage controlled resistor which forms another leg of said bridge.

2. The sensor of claim 1 wherein said metal casing means comprises:

stainless steel.

3. The sensor of claim 1 wherein said contact of said workpiece with said surface of said sensor causes an impaction force to be applied to said sensor, said metal casing means comprises:

a top plate;

an end plate; and

beam means, vertically disposed between and coupled to said top plate and to said end plate for transferring said impaction force from said top plate to said end plate.

4. A sensor for sensing the presence of a metal workpiece in contact with a surface of said sensor and removably fixed thereon, said sensor comprising:

impedance means, having an impedance value associated therewith, for generating a magnetic field therefrom and for causing said generated magnetic field to be received by said metal workpiece;

oscillator means, coupled to said emission means, for exciting said impedance means at a frequency less than 20 KHz and for measuring a change of said impedance value caused by the presence of said metal workpiece upon said surface of said sensor and for generating a signal, having an amplitude associated therewith, in response to said measured change in said impedance;

delay means, coupled to said oscillator means, for causing said signal to be generated for a predetermined amount of time in absence of said metal workpiece from said surface of said sensor; and

metal casing means encapsulating said impedance means, said oscillator means, and said delay means for defining said surface of said sensor in contact with said metal workpiece;

threshold means, coupled to said signal from said oscillator means, for defining a threshold value and for comparing said amplitude of said signal with said threshold value and for generating a second signal therefrom in response to said comparison; and

a resistance bridge having four legs in which said induction means and a capacitor comprise one of said legs and two nodes of said bridge from inputs to an amplifier and the amplifier output is connected to another node of said bridge and controls a voltage controlled resistor which forms another leg of said bridge.

5. The sensor of claim 4 wherein said metal casing means comprises:

stainless steel.

6. A sensor for sensing the presence of a metal workpiece in contact with a surface of said sensor and removably fixed thereon, said sensor comprising:

(a) impedance means, having an impedance value associated therewith, for generating a magnetic field therefrom for causing said generated magnetic field to be received by said metal workpiece;

(b) oscillator means, coupled to said impedance means, for exciting said impedance means at a frequency less than 20 KHz and for measuring a change of said impedance value caused by the presence of said metal workpiece upon said surface of said sensor and for generating a signal, having an amplitude associated therewith, in response to said measured change therefrom wherein said oscillator means comprises a resistance bridge having four legs in which said impedance means and a capacitor comprise one of said legs and two nodes of said bridge form inputs to an amplifier and the amplifier output is connected to another node of said bridge and controls a voltage controlled resistor which forms another leg of said bridge;

(c) delay means, coupled to said oscillator means, for causing said signal to be generated for a predetermined time in absence of said metal workpiece from said surface of said sensor;

(d) metal casing means, encapsulating said impedance means, said oscillator means, and said delay means for defining said surface of said sensor in contact with said metal workpiece; and

(e) threshold means, coupled to said signal from said oscillator means, for defining a threshold value and for comparing said amplitude of said signal with said threshold value and for generating a second signal therefrom in response to said comparison.

7. The sensor of claim 6 further comprising:

display means, coupled to said delay means, for causing a visual indication of said presence of said metal workpiece upon said surface of said sensor.

8. The sensor of claim 6 wherein said metal casing means comprises:

stainless steel.

9. A metallic work piece alignment apparatus comprising:

area means for defining a work piece placement area;

sensor means, in close proximity to said area means, for determining the presence of said workpiece within said workpiece placement area;

said sensor means comprises emission means for generating a magnetic field therefrom and causing said generated magnetic field to be directed to said metal workpiece and for generating a signal indicating said presence of said metal workpiece within said placement area;

means coupled to said emission means for causing said signal to be generated, for a specified period of time, in absence of said metal workpiece from said placement area;

said emission means comprises a coil having an impedance value associated therewith;

circuit means, coupled to said coil, for exciting said coil at a frequency less than 20 KHz and for measuring a change of said impedance value caused by the presence of said metal workpiece with said workpiece placement area and for generating said signal in response to said measured change in said impedance value;

said circuit means comprises threshold means, coupled to said coil and responsive to said measured change in said impedance value of said coil, for defining a threshold value and for comparing said measured change in said impedance value of said coil with said threshold value and for allowing said generating of said signal to occur only if said measured change of said impedance value is different then said threshold value; and

said circuit means comprises a resistance bridge having four legs in which said coil and a capacitor comprises one of said legs and two nodes of said bridge form inputs to an amplifier and the amplifier output is connected to another node of said bridge and controls a voltage controlled resistor which forms another leg of said bridge.

10. A height determining apparatus for determining the height of a stack of metallic workpiece comprising:

first sensor means for generating a first magnetic field therefrom and causing said first generated magnetic field to be received by said stack of metallic workpieces and for generating a first signal in response to said reception of said first generated magnetic field by said stack of metallic workpieces;

second sensor means positioned upon said first emission means for generating a second magnetic field therefrom and causing said second generated magnetic field to be received by said stack of metallic workpieces and for generating a second signal in response to said reception of said second generated magnetic field by said stack of metallic workpieces; and

height determination means, coupled to said first and second signals, for determining said height of said stack of metallic workpieces by use of said received first and second signals;

said first and said second sensor means comprises coil means having an impedance value associated therewith;

circuit means, coupled to said coil means, for exciting said coil means at a frequency less than 20 KHz and for measuring a change of said impedance value caused by the presence of said metal workpiece upon said surface of said sensor and for generating said first and said second signals in response to said measured change in said impedance value;

said height determining apparatus further comprises

threshold means, coupled to said coil means and responsive to said measured change in said impedance value of said coil means, for defining a threshold value and for comparing said measured change in said impedance value of said coil means with said threshold value and for allowing said generating of said signal to occur only if said measured change of said impedance value is different than said threshold value; and

said circuit comprises

a resistance bridge having four legs in which said coil means and a capacitor comprises one of said legs and two nodes of said bridge form inputs to an amplifier and the amplifier output is connected to another node of said bridge and controls a voltage controlled resistor which forms another leg of said bridge.