Analog photosensor operating on the power from a standard 4-20 ma instrumentation current loop

• US 5,254,846 A
• Filed: 02/24/1992
• Issued: 10/19/1993
• Est. Priority Date: 06/01/1990
• Status: Expired due to Term

• Current Assignee: Banner Engineering Corporation
• Original Assignee: Banner Engineering Corporation
• Inventors: Fayfield, Robert W.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): Lee, John R.
• Application Number:
  US07/840,301
• Time in Patent Office: 603 Days
• Field of Search: 250/205, 250/221, 250/222.1, 250/201.4, 356/4, 315/149, 315/150, 315/151
• US Class Current: 250/205
• CPC Class Codes: G01S 17/32 using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated

Abstract

A photoelectric sensor interfaces and operates directly on the power available from a standard current loop without requiring an independent power supply. The sensor includes an emitter and receiver for measuring a parameter such as the distance between a target and the sensor. The intensity of light received by the receiver controls, as an inverse function, the amount of current drawn by the emitter. As the target becomes distant and the intensity of received light decreases, the sensor directs more current to the emitter, which also serves to enhance the sensor'"'"'s range. As the target becomes closer to the sensor, the intensity of received light increases and the sensor directs less current to the emitter because less current is required to produce a signal of sufficient strength for the receiver to detect. With less current directed to the emitter, sufficient current from the current loop is still available to operate the remaining circuitry in the sensor.

Claims

1. Photoelectric measuring and indicating apparatus, comprising:

light emitting means, coupled to a current source, for transmitting light having an intensity which varies with a current supplied thereto;

light responsive means for receiving light and producing a signal indicative of the intensity of the received light;

control means for controlling a current from said current source supplied to said light emitting means as a function of the intensity signal produced by said light responsive means, so that the current supplied to said light emitting means is at a maximum magnitude when the light received by said light responsive means is at a minimum magnitude, and the total current supplied to said light emitting means is at a minimum magnitude when the light received by said light responsive means is at a maximum magnitude, and the total current drawn by the apparatus is an inverse analog function of the intensity of the received light;

said control means comprising a timing circuit operable in response to the voltage of the intensity signal such that said timing circuit draws a DC current from said current source having a magnitude that varies inversely with the magnitude of the voltage of the intensity signal; and

said control means further comprising means, coupled to said timing circuit, for selecting a first resistance to draw a DC current through said apparatus having a maximum magnitude when the intensity signal has a minimum magnitude, and for selecting a second resistance to draw a DC current through said apparatus having a minimum magnitude when the intensity signal has a maximum magnitude.

2. A sensor operating on the power from a 4-20 milliamp instrumentation current loop, comprising:

first and second output terminals coupled to said instrumentation current loop;

emitter means for transmitting a signal toward a target to be sensed, said emitter means comprising means for varying the intensity of said transmitted signal as a function of the magnitude of an input current supplied to said emitter means;

receiver means for receiving at least a portion of said transmitted signal; and

control means, coupled to said first and second terminals, said emitter means, and said receiver means, for providing power to said emitter means, comprising;

first means for drawing a first current from said instrumentation current loop to provide power for the operation of said control means; and

second means for drawing a second current from said instrumentation current loop, for varying said second current as an inverse function of the intensity of said received signal, and for supplying said second current to said emitter means as said input current,

whereby the amount of current drawn by said sensor provides an indication on said instrumentation current loop of a measured parameter between said emitter and said receiver.

3. The sensor of claim 2 wherein said emitter means comprises means for transmitting an electromagnetic signal.

4. The sensor of claim 2 wherein said emitter means comprises means for transmitting a sonic signal.

5. The sensor of claim 2 wherein said control means comprises:

a first amplifier having an input coupled to said receiver means and an output;

a sample-and-hold circuit having an input coupled to said output of said first amplifier, a control terminal, and an output;

a second amplifier having an input coupled to said output of said sample-and-hold circuit and an output coupled to said emitter means; and

an oscillator coupled to said emitter means and said control terminal of said sample-and-hold circuit.