GB2185311A - Sensing a parameter by means of radiation attenuation - Google Patents

Abstract

A parameter of a medium e.g. sheet thickness, density, moisture content, fill level or presence/absence is measured by attenuation of radiation as a function of the parameter upon passage through the medium. Transmitted radiation is detected by a scintillation probe 20 for producing an electrical current output 24, a current integrator 28 for continuously integrating the current output and producing an output signal 43, a charge injector 30 for supplying to the input of the current integrator at 38 current pulses of predetermined total charge and of opposite polarity relative to the output of the probe 20 in response to the output signal 43 reaching a predetermined threshold set in comparator 29, and output circuitry 70, 80, 31 for generating count signals at the frequency at which the current pulses are supplied by the charge injector. Accordingly the current output of the probe 20 is measured even during the duration of the current pulses i.e. there is no dead line. <IMAGE>

Description

SPECIFICATION System detector and method for sensing a parameter of a medium by means of radiation attenuation This invention relates generallyto systems, detectors and methods for sensing a parameter of a medium by means of radiation attenuation and, more particularly, to such a system, detector and method characterised buy a high dynamic range.

Radiation attenuation techniques have been used to measure or sense a parameter of a medium, such as thickness, density, presence of a part, etc. Such techniques generally involve the passage or location ofthe medium between a source of radiation and a radiation detector located in the shadow of the medium. The radiation emitted by the source has to pass through the medium in orderto be detected by the detector. The radiation is selected to be of a type which will be attenuated as a function of the parame terto be measured or sensed, so that the amount of radiation reaching the detector will vary as a function of that parameter.

One common type of radiation detector or probe, used for sheet metal gauging applications, for instance, includes a scintillation crystal which is optically coupled to a photomultipl ier tube. The scintillation crystal converts impinging invisible radiation into bursts of visible light, which are converted by the photomultipliertu be to electrical charge pulses.

The charge pulses emitted by the photomultiplier tube are processed by electronic circuitry, such as a nuclear instrumentation module, which transmits pulse count data, for example, to an associated dis playorsystem controller.

In sheet gauging systems where the radiation is attenuated as a function of sheetthickness, the rate at which scintillations are produced in the detector by such radiation is also a function of sheet thick- ness. If only the sci nti I lations caused bytheattenua- ted radiation result in pulse count signals, the rate of such signals is likewise a function of sheet thickness; the higher the rate, the thinner the sheet.

The sensing speed of the gauging system or other parameter measuring system employing radiation attenuation techniques is related to the flux level or density ofthe radiation. In other words, the system sensing speed can be increased by increasing the flux level ofthe radiation which isto be attenuated as it passes through the medium for detection by the detector. Detectors or detector systems previously used in gauging applications cannot be used suc cessfullywith high radiation flux levels, however, be cause oftheir limited dynamic range. For example, detector systems employing pulse counting circuitry are limited by the problem of pulse pile-up, i.e., saturation ofthe electronic circuitry at high rates of detected scintillations.Consequently, this limits the system sensing speed, which in turn limits the overall system speed, such as the rate at which sheet materials can be processed, e.g. in a sheet manufacturing and/or processing line.

Detectors or detector systems are also known, which utilize current-to-frequency conversion circuitry. Such circuitry operates to produce count signals atafrequency proportionally representative ofthe current output of a photornultipliertube. Typically, this is accomplished by a current integrator, which produces an electrical voltage output proportionally representative of the integral of the current output of the photomultipliertube.When the integrator output reaches a predetermined level, indicating accumula tionofacertain amountofchargeinthefeedback capacitor of the current integrator, a field effect transistor switch is triggered to discharge the feedback capacitor, i.e., to reset the capacitor to a ground or base line reference potential, and thereby generate a pulse count signal. One problem with such cur rent-to-frequency conversion circuitry is that there is a certain amount of dead time when the circuitry is not responsive to the current output ofthe photo multipliertube, namely the time needed to discharge the capacitor to produce the pulse count signal.Consequently, there is a loss of stability at high current input rates, such as when the rate at which the feedback capacitor is being charged is of the same order as the discharge rate of the feedback capacitor. Accordingly, such circuitry has limited dynamic range.

The present invention provides an improved system and method for sensing a parameter of a medium by means of radiation attenuation. Such improved system and method enables the use of high flux level radiation to obtain high system sensing speeds.

Briefly, a system, detector and method according to the invention are characterised bytheuseofarad- iation detector having a high dynamic range, obtained by means of cu rrent-to4requency conversion circuitry which uses a delta modulation technique to produce signals at a frequency proportional to the current output of a scintillation probe at the front end ofthe detector; More particularly, the invention provides a system and a related method for sensing a parameter of a a medium, which include a radiation sourceforemit- ting radiation, which is attenuated as a function of such parameter upon passage through the medium, and a radiation detector operatively positioned to detech such attenuated radiation, wherein the radiation detector includes:: a scintillator probe, for producing an electrical current output proportionally representative of detected radiation; a current integrator, for continuously integrating the electrical current output ofthe scintillator probe and having an inputforreceiving such electrical current output and an output for producing an electrical signal representative of the integral ofcurrent received at the input means; ; a current supply for supplying to the input current pulses of predetermined total charge and of opposite polarity relative to the electrical current output ofthe probe in response to the electrical output of the current integrator having a predetermined characteristic, and an outputfor generating count signals at a frequ- ency proportionally representative ofthefrequency at which such current pulses are supplied to the input bythe current supply.

Preferably, the predetermined characteristic of the electrical outputofthe integrator is such electrical output exceeding athreshold level.

In orderthatthe invention may be understood and appreciated, the following description, in conjunc tionwiththeannexed drawings, sets forth in detail byway of illustration, a preferred embodiment of the invention, as one of the various ways in which the principles ofthe invention may be employed. In the drawings: Figure lisa diagrammatic illustration of a sheet gauging system according to the invention; Figure2 is a diagrammatic illustration of a radiation detector employed in the sheet gauging system of Figure 1; and Figure 3 is a circuit diagram ofthe electronics illustrated in Figure 2.

Referring to Figure 1, the basic components of a sheet gauging system, in accordance with the invention, include a radiation source 10, a radiation detector 11 and a pulse count signal processor 12, which may be at a location remote from the detector 11.

Although the invention is described with reference to such a gauging system, itwill be readily appreciated that the principies of the invention may be employed for purposes otherthan measuring the thickness of sheet material, such as in other process control systems, e.g. for measuring the density of a medium, measuring the moisture content of a medium, det ecting the fill level of a medium, etc. The invention may also be employed in partsassemblysystems, such as for sensing the presence or absence of a part in an assembly.

As shown in Figure 1, sheet material 16to be gauged is passed between the radiation source 10 and the radiation detector 11, the detector 11 being located within the shadow ofthe sheet material 16. Ac- cordingly, radiation emitted by the radiation source 10 has to pass through the sheet material 16 in order to be detected by the radiation detector 11. The radiation emitted by the source 10 may be of any suitable type which will be attenuated, i.e., partially absorbed, bythe sheet material 16, as a function ofthe thickness of the sheet material through which it passes.

As diagrammatically illustrated in Figure 2, the radiation detector 11 generally includes a scintillator probe 20 at its front end and supporting current-tofrequency conversion electronic circuitry 21. The scintillator probe 20 includes a scintillation element 22 optically coupled to a photoelectrictransducer 23.

The scintillation element 22 may be a scintillation crystal, such as are sold under the trademark "POLY SCIN" by the Harshaw/Filtrol Partnership of Cleveland, Ohio, and the transducer 23 may be a photo mu Itipliertube ofsuitable type. As is well known, the scintillation crystal or element 22 converts impinging invisible radiation into bursts ofvisible light, which impinge on the photoemissive cathode ofthe photomultipliertube 23. The electrons emitted by the action of the scintillation light produce negative going charge pulses at the anode ofthe photomultiplier tube 23, so as to provide at an output 24 of the photomultipliertube 23 a negative current (current going to the anode) proportional to the flux level of detected radiation.As is preferred, the scintil lator probe 20 and supporting electronic circuitry 21 may be mounted in a compact housing.

The electronic circuitry 21, which uses a delta modulation technique, generally comprises a current in tegrator 28, a comparator 29, a charge injector 30 and a differential line driver 31 for transmitting pulse count signals, generated in the manner described below, at a frequency proportional to the current at the output 24 of the photomultipliertube 23. Accordingly, the detector 11 will emit count signals at a frequency proportional to the flux level of radiation det ectedatthefrontend of the detector 11 which, as above indicated, is a function of the thickness of the sheet material 16 in the gauging system shown. Lowvoltage electrical power may be provided to the cir cuitry21 from a conventional powersupplyviafiltered input regulators, which are not shown.

With reference to the circuit diagram of Figure3, the current integrator 28 includes an operational amplifier 36, which has an inverting input terminal 37, connected to a current summing junction 38, and a feedbackwapacitor39. The currentsumming jun- ction 38 is connected via a circuit input terminal 40to the output 24 of the photomultipliertube 23 and to the output of the charge injector 30. If desired, a test input 41 may be provided and also connected to the terminal 40 by a high value resistor 42.

The voltage output of the operational amplifier 36 ramps positive in response to negative current at the inverting input terminal 37, i.e., current going to the anode ofthe photomultipliertube 23 (ora negative voltage atthetest input 41), and negative in response to a positive current supplied by the charge injector 30, in the manner hereinafter described. The resulting voltage output of the operational amplifier 36 on a line 42 is proportional to the net charge received at the summing junction 38.

The operational amplifier 36 may have a limited current sinking capacity, in which case such amplifier36 may be provided with a dynamic buffer 44.

The dynamic buffer 44 includes a transistor 45 and a resistor 46, which serve as a base drive starved em it ter follower, which provides emitter-to-collector current, but at a level insufficient to override the output of the operational amplifier 36. A capacitor47 couples the base drive of the operational amplifier 36 to the base of a transistor 45, whereupon a negative operational amplifier swing,which turns off its NPN emitter-follower output stage, turns on the transistor 45 to provide short duration high negative output current capacity. The duration of such high negative output current capacity is the product of the capacitance ofthe capacitor 47 and the resistance of theresistor 46.

The voltage output of the operational amplifier36 ofl the line 43 is fed to an input 50 of the comparator 29. The comparator 29 compares the voltage atthe input 50 with a threshold voltage and, when such threshold voltage is exceeded (thereby indicating a predetermined characteristic of the voltage output of the operational amplifier 36, the comparator 29 causes a logictrue signal to be applied via a line 51 to the charge injector 30 which, in response to such signal, supplies a positive current pulse of controlled total charge to the summing junction 38 in the manner described below. Such a current pulse results in a short negative ramping step at the output of the operational amplifier 36.

The charge injector 30 controls the injection current pulse by control ling both the current and time of such current pulse. The current is controlled by a constant current source 55, which includes an operational amplifier 56, a current sensing resistor 57 and a high current gain transistor 58. Due to the high current gain of the transistor 58, the collector current is very nearly equal to the emitter current. The operational amplifier 56 forces the voltage across the current sensing resistor 57 to be equal to the voltage supplied by regulated reference voltage circuitry 59 to the wipervoltage of a potentiometer 60, which sets the emitter cu rrent of the transistor 58.

The time or period of the injection current pulse is controlled by a system clock 70. The system clock 70 includes an oscillator 71 controlled buy a SMHzcrystal 72, for example. The output signal of the oscillator71 is divided by jumpered connection of a flip-flop 73to provide a 2.5MHz system clock signal on a line 74. If desired, theflip-flop 73 may be bypassed to provide a5.OMHzclocksignal on the line 74, although this may result in some loss of stabiiity.

The line 74 is connected to the clock input ofaJ-K flip-flop 80 which has its J input connected to the output line 51 ofthe comparator29. When thecom- paratoroutputon the line 51 becomes true (logic high), the next negative clock edge on the line 74 causes the flip-flop 80 to change state, whereupon the Q and Qoutputs of the fl ip-flop 80 drive PNP transistors 84 and 85 to switch on a current pulse, which is applied to the summing junction 38 via a PNP common base switch 86. The transistors 84 and 85 serve as a differential switch, providing a nearly constant load for the current source 55.The current pulse applied at the summing junction 38 will continue until the next clock edge at the clock input ofthe flip-flop 80 resetsthe flip-flop 80to its original state, thereby terminating the charge pulse. Accordingly, the time of the current pulse is one system clock period and, even at high input currents where the output ofthe comparator 29 may still or again be true after one clock period, the next negative clock edge will still reset the flip-flop 80 to terminate the current pulse. For photomultipliertube anode currents of 10.0nanoampsto 1.25 milliamps,thecharge percur- rent pulse has an optimal range of 0.75to 2.0 nanocoulombs, but can be set, for example, as low as 0.20 nanocoulombs at some sacrifice of stability.The charge per pulse is determined bythe system clock speed and the setting of the potentiometer 60.

The Q output of the flip-flop 80 also is sent via line 89to the differential line driver 31, which preferably is an RS-422 balanced differential line driver which provides differential output pulses on respective lines 90 and 91 for each injection current pulse.Accordingly, the frequency at which the differential output pulses or count signals are provided will be equal to the frequency at which the current pulses are produced by the charge injector 30. Since the fiip- flop 80 is reset to terminate each current pulse as above indicated, the maximum pulse rate at the det ector output 92 is limited to one-half the speed ofthe system clock signal on the line74. The line driver31 transmits the frequency output of the detector to the processing unit 12 (including a receiver operated in balanced configuration with the driver 31), for conventional monitoring and/or overall system control purposes.

Summarizing, the output of the operational amplifier 36 is proportional to the net charge received at the summing junction 38 and such output is compared to a predetermined threshold level by the comparator 29. When the output voltage of the operational amplifier 36 exceeds the comparator threshold, the charge injector 30 is caused to supply a current pulse of controlled total charge to the sum- ming junction 38. As above noted, this results in a short negative ramping step at the output of the op erational amplifier 36.If the output of the operational amplifier 36 continues to equal or exceed the threshold level,the charge injector30 will supply an- other cu rrent pu Ise of equal charge to the su mming junction 38 and this will continue periodically until the output of the operational amplifier 36 has been reducedtoa level belowthethreshold level.

It is important to note that when a current pulse is being supplied to the summing junction 38 bythe charge injector 30, the current output of the photomultipliertube 23 will be summed with such current pulse for continuous processing of such current output by the current integrator 28, although then in summed combination with the injection current pulse. Accordingly, there is no dead time when the current output of the photomultipliertube 23 is not being processed. The result is circuitry having a high dynamic range, e.g., a O.Oto 2.5 milliamp input current range with a scale factor which may be adjusted by the potentiometer 60 (Figure 3) from 500 Hzto 5 KHz per microamp, for example.

Claims (12)

1. Asystemforsensing a parameter of a medium, which includes a radiation source for emitting radiation, which is attenuated as a function of such parameter upon passage through the medium, and a radiation detector operatively positioned to detect such attenuated radiation, wherein the radiation detector includes:: a scintillator probe, for producing an electrical current output proportionally representative of detected radiation; a current integrator, for continuously integrating the electrical current output of the scintillator probe and having an input for receiving such electrical currentoutputand an outputforproducing an electrical signal representative of the integral of current received at the input; a current supply for supplying to the input current pulses of predetermined total charge and of opposite polarity relative to the electrical current output of the probe in response to the electrical output of the current integrator having a predetermined characteristic, and an output for generating count signals at a frequency proportionally representative of the frequency at which such current pulses are supplied to the input bythe currentsupply.

2. A system according to claim 1, wherein the predetermined characteristic of the electrical output of the integrator is such electrical output exceeding a threshold level.

3. A system according to claim 1 or 2, wherein the electrical current output causes the integrator electrical output to ramp positive and the current pulses cause the electrical output to ramp negative.

4. A system according to any of claims 1 to 3, wherein the current supply includes a comparator, for producing a comparator output signal in responsetothe integrator electrical output having such predetermined characteristic, and a charge injectorfor generating current pulses of like total charge in responseto the comparator output signal.

5. Asystem according to claim 4, wherein the charge injector includes a current source for supplying constant current and means for electrically connecting the current source to the integrator input for a period of controlled duration.

6. Asystem according to claim 5,whereintheel- ectrically connecting means include a clockfor producing clock pulses for controlling the duration of such period.

7. A system according to claim 6, wherein the el ectricallyconnecting means include a switch, operative in an on state to connect the currentsource el ectricallyto the integrator input and in an "off" state to disconnectthe current source electricallyfrom such input, and means responsive to the comparator output signal forturning on the switch upon receipt of a first clock pulse from the clock and then turning offthe switch upon receipt of a next clock pulse.

8. A system according to claim 7, wherein the comparator produces a logic true signal when the in tegratorelectrical output has such predetermined characteristic and a logic false signal when it does not have such predetermined characteristic, and the switch operating means serve to turn the switch on and off periodically in response to clock pulses as long as the comparator produces a logic true signal.

9. A system according to claim 7 or 8, wherein the output includes a line driverfortransmitting a count signal in response to each operation of the switch.

10. A system according to claim 1, substantially as described with reference to the accompanying drawings.

11. Amethodforsensing a parameterofa medium which comprises the step of using a system as defined in any preceding claim.

12. A detectorfor sensing a parameterofa medium, substantially as hereinbefore described with reference to the accompanying drawings.