Modular luminescence-based measuring system using fast digital signal processing

• US 5,107,445 A
• Filed: 12/04/1990
• Issued: 04/21/1992
• Est. Priority Date: 12/04/1990
• Status: Expired due to Term

• Current Assignee: Luxtron Corp. (Advanced Energy Industries, Inc. (China))
• Original Assignee: Luxtron Corp. (Advanced Energy Industries, Inc. (China))
• Inventors: Sun, Mei H., Jensen, Earl M., Melen, Robert E., Vecht, David L.
• Primary Examiner(s): CANGIALOSI, SALVATORE A
• Application Number:
  US07/621,900
• Time in Patent Office: 504 Days
• Field of Search: 364/525-527, 364/557, 364/578, 374/128, 374/129, 374/131, 374/159, 374/161, 250/227.18, 250/227.21, 372/41
• US Class Current: 374/161
• CPC Class Codes: G01K 11/3213 using changes in luminescence, e.g. at the distal end of the fibres

Abstract

A luminescence-based integrated optical and electronic system for measuring temperature or some other parameter from the decay time of a luminescent sensor is disclosed. A high bandwidth, low noise amplifier applies a detected decaying luminescent signal to a digital system that acquires that signal and processes it in order to determine its decay time characteristics that are related to temperature or another parameter being measured. The digital signal processing includes use of a digital curve-fitting technique. A preferred luminescent material for temperature measurement is a chromium-doped yttrium gallium garnet material. The entire optical and electronic portions of the measuring system can be accommodated on a small single circuit card.

Claims

1. A system for exciting a luminescent sensor of a parameter and measuring a time rate of decay of luminescence therefrom which is proportional to a magnitude of said parameter, comprising:

a diode having a light radiation output that excites the sensor to luminescence when directed thereagainst,

a photodetection system adapted to convert luminescence returning from the sensor into an electronic signal;

means for providing optical communication between said sensor and each of said diode and said photodetection system,

means connected with the diode for driving said diode to emit controlled pulses of light having an intensity proportional to a control signal,

means receiving said photodetection system signal for generating a digital representation thereof during at least a portion of an interval between said diode pulses of light, thereby to digitize a decaying intensity of the returning luminescence,

means receiving said photodetection system signal in the absence of substantially any returning luminescence for measuring a signal offset level,

means receiving said digital representation and said offset for calculating a decay rate of said photodetection system signal by fitting said digital representation, after adjustment by said offset, with a model decaying function,

means receiving said digital representation for determining a magnitude thereof at a fixed time with respect to an end of an exciting light pulse, and

means receiving said fixed time magnitude for providing the control signal to the diode driving means with a value that maintains said fixed time magnitude substantially constant.

3. The system according to either of claims 1 or 2 wherein said decay rate calculating means includes means for performing a least squares fit of a model function to the adjusted digital representation, thereby to determined the best value of the decay rate.

4. The system according to either of claims 1 or 2 wherein said decay rate calculating means includes means for taking a logarithm of the adjusted digital representation and then performing a linear least squares fit to said logarithm, whereby the resulting decay rate is a time constant when the luminescent material is characterized by an exponentially decaying luminescent emission after cessation of excitation.

5. The system according to either of claims 1 or 2 wherein said driving means includes means for pulsing the diode at least 100 times per second, wherein said calculating means includes means for forming a composite decaying signal digital representation as an average of a large number digital representations of individual decaying signals fitting said composite digital representation with the model decaying function, whereby an average rate of decay is obtained.

6. The system according to either of claims 1 or 2 wherein said driving means includes means for pulsing the diode at least 100 times per second, and said system additionally comprises means receiving the calculated decay rate for averaging a large number of individual decay rate calculations, whereby an average decay rate is obtained.

7. The system according to either of claims 1 or 2 which additionally comprises means receiving the calculated decay rate for determine the parameter being measured.

9. The system according to any one of claims 1 or 2 which includes said sensor, the luminescent material of said sensor being formed of a powder consisting essentially entirely of grains of chromium activated yttrium gallium garnet.

10. The system according to claim any one of claims 1 or 2 wherein said photodetection system includes either a photodiode or photomultiplier positioned to receive the luminescence of the sensor.

2. A system for measuring a parameter, comprising:

an optical block, comprising;

means for terminating one end of a length of optical fiber transmission medium within said block, another end thereof being adapted to be positioned in optical communication with a luminescent material sensor characterized by emitting radiation that decays in intensity, after being pulsed with excitation radiation, with a rate proportional to the parameter being measured,

a diode light source positioned to direct an output into the optical fiber through its said one end, and

a photodetection system positioned to receive returning luminescence from the sensor through said one end of the optical fiber, thereby to generate an electrical signal proportional thereto,

means connected with the diode for driving said diode to emit controlled pulses of light having an intensity proportional to a control signal,

means receiving said photodetection system electrical signal for generating a digital representation thereof during at least a portion of an interval between said diode pulses of light, thereby to digitize the decaying intensity of said returning luminescence,

means receiving said photodetection system electrical signal in the absence of any returning luminescence for measuring a signal offset level,

means receiving said digital representation and said offset for calculating a decay rate of said photodetection system electrical signal by fitting said digital representation, after adjustment by said offset, with a model decaying function,

means receiving said digital representation for determining an intensity level thereof at a fixed time with respect to an end of an exciting light pulse, and

means receiving said fixed time intensity level for providing the control signal to the diode driving means with a value that maintains said fixed time intensity level substantially constant,

whereby said decay rate is proportional to the parameter being measured.

8. The system according to claim 2 wherein said optical block, said driving means, said digital representation generating means, said offset measuring means, said decay rate calculating means, said fixed time determining means and said control signal providing means are all mounted on a single circuit card.

11. An opto-electronic module for measuring a parameter, comprising:

a length of an optical fiber medium optically communicating at one end with a luminescent material sensor characterized by emitting light radiation that exponentially decays in intensity, after being pulsed with excitation light radiation, with a time constant that is proportional to the parameter being measured and is less than substantially one millisecond,

an optical block including;

means for retaining said optical fiber with another end thereof terminating in said block,

a diode light source positioned to direct a light output thereof into the optical fiber through its said another end in response to a driving electrical signal,

a photodetector positioned to receive the luminescent emissions from the optical fiber through its said another end and produce an electrical signal output that follows intensity variations of the luminescent emissions, and

a dichroic beamsplitter positioned to reflect a majority of the diode light output to said fiber another end and to transmit a majority of the sensor luminescent radiation at fiber another end to said photodetector,

means electrically connected with the diode for driving said diode to emit controlled pulses of light having an intensity proportional to a value of a control signal and a repetition rate in excess of 100 Hertz, whereby the electrical signal output of said detector follows the decaying light signal between successive excitation pulses,

means connected with said photodetector for amplifying its electrical signal output,

means receiving the amplifier detector output signal for digitizing said amplifier signal,

means receiving the digitized signal for determining a value thereof when the luminescent light emission is substantially zero, thereby to provide an offset signal level,

means receiving the digitized signal for subtracting said offset therefrom, thereby producing an offset corrected digitized signal,

means accumulating a plurality of said offset corrected digitized signals for producing a composite offset corrected digitized signal,

means receiving said composite signal for calculating a logarithm thereof, whereby a straight line function results,

means receiving said straight line function for calculating a time constant of said exponentially decaying light signal and a magnitude proportional to a beginning value thereof, and

means receiving said beginning value proportional magnitude for monitoring any changes thereof, whereby a system malfunction can be detected,

whereby a time constant proportional to the parameter being measured is accurately determined.

12. The module according to claim 11 which additionally comprises means receiving said time constant for calculating a magnitude of the parameter being measured.

13. The module according to claim 12 wherein said parameter is temperature.

14. The module according to any one of claims 11-13 wherein all of said optical block, said diode driving means, said amplifying means, said digitizing means, said offset determining means, said composite signal means, said logarithm calculating means, said linear least square calculating means, and said monitoring means are mounted on a single circuit card.

15. The module according to claim 11 wherein said luminescent material includes chromium activated yttrium gallium garnet.

16. The module according to claim 13 wherein said luminescent material includes a specific composition

17. The module according to any one of claims 11-13 wherein said sensor includes said luminescent material in a powder form and a binder, said powder consisting essentially entirely of grains of chromium activated yttrium gallium garnet.

18. The module according to claim 11 which additionally comprises means receiving said digitized signal for providing the control signal to the diode driving means with a value that maintains the digitized signal substantially constant at a predefined time after termination of said diode light pulses.

19. The module according to claim 11 wherein said time constant calculating means includes means receiving said straight line function for calculating said time constant and beginning value by a least squares algorithm.

20. The module according to claim 11 which additionally comprises means responsive to said diode driving means for significantly reducing a gain of said amplifying means during the emitted pulses of light.

21. The module according to claim 11 wherein said composite offset corrected digitized signal producing means includes means dividing the composite offset corrected digitized signal by an impulse response of the amplifying means for correcting for a limited bandwidth of said amplifying means, thereby to allow curve fitting beginning at an earlier time in the luminescence decay.

22. The module according to claim 11 wherein said photodetector includes a photodiode, and said amplifying means includes:

means connected to said photodiode for positively biasing said photodiode,

a first limited gain operational amplifier with an input connected directly to said photodiode and a voltage output, and

means including at least a second operational amplifier having an input connected to receive said first operational amplifier voltage output for providing an amplified voltage output that is connected to said digitizing means.