US2791697A - Particle counting apparatus - Google Patents

Description

May 7,1957 H, A, DELL 2,791,697

PARTICLE COUNTING APPARATUS- Filed March 25,. 1954 s sheets-sheet 1 fg,1 lNvENroR HUGH ALEXANDER DELL AGENT May 7, `1957 H. A. DELL v 2,791,697

PARTICLE COUNTING APPARATUS Filed March 2s, 1954 s sham-sheet 2 c L1 A @D Lg U l L V @HI1- gw?? s-8 Annu vvvvvvv AAAAAA +AAAAA INVENTOR HUGH ALEXANDER DELL BY www AGENT May 7, 1957 H. A. DELL 2,791,697

PARTICLE COUNTING APPARATUS Filed March 25,A 1954 3 Sheets-Sheet 3 JVr (b) V V e) INVEN TOR HUGH A LEXANDER DEL L BY www AGENT PARTICLE COUNTING APPARATUS Hugh Alexander Dell, Horiey, England, assignor, by

mesne assignments, to North American Philips Cornpany, Inc., New York, N. Y., a corporation of Delaware Application March 23, 1954, Serial No. 418,050 Claims priority, application Great Britain March 23, 1953 8 Claims. (Cl. Z50-217) The invention relates to apparatus for counting particles and is particularly but not exclusively concerned with the assessment of the dust content of an air sample.

There has been disclosed in the copending application Serial No. 295,586, iiled June 25, i952, apparatus for effecting such an assessment which includes means such as a flying spot scanner of the cathode ray tube type for scanning the sample, the guard spot technique being employed to distinguish between a terminal interception of the particle by the main beam i. e. a first or last interception of the particle by the main scanning beam and other interceptions of the particle by both main and guard beams or by the guard beam alone, the normal scanning motions of the scanning beam being arrested when a particle is rst (or last) seen, the scanning beam being then caused to explore the particle to determine its nature or size after which the scanning beam resumes its normal scanning motions. The individual particle sizes are deduced from the information obtained by the explorations and the total number of particles from the number of times the normal scanning is arrested.

Whilst such an apparatus enables memory devices to be dispensed with, the use of the guard spot technique may have disadvantages in certain cases. On the other hand, the arrested spot technique above referred to enables, when desired, much more information to be obtained of the nature or size of the particles of a sample than from the more interaction of the scanning raster with the particles since at the most, in this latter case, only the length of the line intercepts of a particle or the number of complete scanning lines it overlaps can be obtained. With certain shapes of particles even this information may not be conveniently presented.

The object of the present invention is to provide a particle counting apparatus having the desirable attri butes of the arrested spot scanning technique without the disadvantages of the guard spot technique for distinguishing between first (or last) and other interceptions of a particle.

According to the present invention a particle counting apparatus comprises particle sample scanning means including means for producing a scanning beam tracing out a rectilinear raster, scanning beam arresting means for arresting the normal scanning motions of the beam only when a terminal interception of the particle by the beam occurs, i. e. only when a particle is first (or last) intercepted, particle exploring means for causing the or a scanning beam (or beams) to explore a particle thus intercepted, line to line memory means for distinguishing betwen the iirst (or last) interception of the particle by the scanning beam and any other interceptions of the same particle and counting and sizing means giving' a count of the total number of particles scanned and/or individual counts of particles of diierent natures or sizes.

According to a further feature of the invention the line to line memory means is of the cyclic type and comprises for example, a magnetic recording disc or drum States arent' having a lirst channel bearing a single permanent recorded signal which, through the medium of a line synchronising reading head, is used to trigger or synchronise the line scan of the scanning means, a second recording channel on which an explore synchronising head writes a signal each time a particle is first (or last) intercepted and a third channel with which co-operate spaced writing and reading heads which respectively record the interception of particles in one scanning line and reproduce them during the succeeding scanning line. Signal erasing means are associated with the second and third channels for erasing, when required, the signals on these channels and switching means is also provided for energisingV or de-energising the reading and writing heads at appropriate times.

The memory means therefore performs three functions; first, it ensures that the commencement of each line scan takes place with the recording medium (disc drum or the like) in a pre-determined (angular) position, second, it stores signals (hereinafter called particle signals) corresponding to all interceptions of particles in a scanning line and erases, if present, all previous particle signals and third, when, during the scanning of a line, a new particle is encountered, a signal (hereinafter called the explore sync signal) is written on the second channel and substantially simultaneously with the termination of this signal the aforesaid reading, writing and erase heads are de-energised, normal scanning of the sample is interrupted and exploration of the particle takes place. Resumption of normal scanning only takes place after a signal denoting the end of the exploration has appeared and when the emission of an explore sync signal from the explore sync head acting as a re-producer next takes place. By this means it is ensured that the recording medium is in the `same (angular) position at the resumption of normal scanning as it was when the scanning was interrupted so that the particle signals in the third channel are recordedon the medium in the same spatial relationship as the particles in that particular scanning line. Thus, however, many new particles are discovered in a scanning line and in spite of possible and probable differences in exploration time for these particles, the recording means preserves an accurate record of the presence and distribution of all particlesin that line until it has been used during the succeeding scanning line to distinguish between new interceptions and repeated interceptions of large particles extending from the preceding line. ln this way, a modified form of the guard spot technique is carried out without the disadvantages associated with other practical embodiments of this technique.

Other features of the invention will be apparentfrom the following description of a practical embodiment which is given by way of example only with reference to the drawings in which:

Figure l is a schematic diagram partly in block schematic form of a dust particle counter according to the invention;

Figure 2 is aview of a position of a particle sample;

Figure 3 is a circuit diagram of a pulse limiter and shaper unit;

Figure 4 is a circuit diagram of a new signal indicator;

Figure 5 shows signal waveforms; i Figure 6 is a circuit diagram of a switch control unit; and

Figure 7 is a circuit of a pulse producer.

In Figure l, the scanning means is represented as a iiying spot scanner incorporating a cathode ray tube 1, having deflecting means 2, to cause the cathode ray beam, when the t'ube is in operation, to trace out a rectilinear raster on the tube face. An optical system shown as lens 3 may be employed to-focus an imagerof theiiying spot to the same or a different scale on the sample 4,

and a further optical system, shown as lens 5, may be employed to cause the light passing through the sample to fall on a photo-electric pick-up device 6. It is assumed that the sample is a transparent plate on which dust particles or the like have been deposited and fixed or is a photographic reproduction of such a plate to the same or a different scale in either the positive or negative condition. Alternatively, the reproduction may be on an opaque surface in which case the pick-up device is suitably positioned so as to receive light reflected therefrom.

The output from the pick-up device is fed to a pulse limiter and shaper unit 7, which may be of the form shown in Figure 3, and which will be hereinafter more fully described.

The arrangement and functions of the remaining switching, control and recording units will be more clearly understood from the operation of the apparatus on a typical sample a portion of which is shown in Figure 2. The centres of three adjacent scanning lines are indicated at L1, L2 and L3, line L1 intersecting the four particles A, B, C and D, line L2 intercepting particles B and C only and line La intercepting particle C only.

The line scanning motion of the cathode ray beam is caused by the line time base 8 which is triggered by a pulse derived from a rst channel of the magnetic memory unit 9 through the medium of the reproducing head 10 which will be known hereafter as the line sync head. A frame scanning generator is denoted by 8.

The memory unit 9 may be of the known disc or drum type having a magnetisable surface of sufficient extent to accommodate three separate recording channels and is rotated at substantially constant speed by a suitable motor. As a matter of convenience the drum may be arranged to make one half of a revolution for one complete uninterrupted line scan.

When the ilying spot commences to scan line L1 under the action of the line time base unit S, no signal is emitted from the pick-up device 6 until particle A is encountered. Thereupon a signal is fed to the pulse limiter and Shaper unit 7 which delivers a suitably shaped, for example, a substantially rectangular, pulse of the desired polarity to switches S1 and S2. Switch S1 is normally in the position which passes the pulse to a writing head 11 associated with the third channel of the memory unit 9 to record thereon a particle signal representing particle A. Switch S2 is, in the starting condition, in the position to pass the pulse to the new signal indicator 12 the function of which is to ascertain whether the received pulse represents a first or subsequent interception of a particle. This is achieved as will be more clearly described later by comparing the incoming pulse from the limiter 7 with any signal substantially simultaneously received from the reading head 13 associated with the third channel and disposed at for example 180 from the writing head 11.

In the case under review there is no signal from the reading head 13 since particle A is the first particle to be intercepted on the rst scanning line so that the new signal indicator 12 passes a pulse to the switch control unit 14 and to the pulse producer unit 15. Upon the receipt of this pulse the switch control unit 14 functions to open switches S1, S and S9 thereby isolating respectively the writing head 11, the reading head 13 and the erasing heads 16. It also opens switches S3 and S4 associated with the line and frame time bases which arrest the normal scanning motions. Further, it passes a signal to the signal routing switch 17 and to the explore switch 18. The routing switch 17 operates switch Sz to the position in which any further signal from the pick-up k6 is routed to the explore switch 18 and the explore switch 18 operates the switch Se of the Y-direction shift unit 19 to energise this unit so as to cause the light spot produced by the cathode ray beam to explore the par- 4 ticle in the Y-direction. At the moment that the particle is encountered the pulse producer unit 15 passes a signal via switch Se to the explore sync write-read head 20 to cause it to record a signal on the second channel. The unit 15 also passes a signal to the sync switch 21 which operates to alter the position of switch Ss so that any signals received from the head 20 acting as a reading head will be routed, as soon as exploration is completed, to both the switch control unit 14 and signal routing switch 17 via switch S1 which was operated to its open position by explore switch 18 when this was initially energised.

Since particle A is a small one a small deflection of the exploring light spot will cause it to traverse the particle until it is seen by the pick-up device 6 which thereupon passes a signal to the explore switch 18 to cause it to be re-set thus terminating the exploration and operating switch Si to its closed position.

It will be appreciated that the time delay between the interception of particle A and the end of its exploration depends on the size of the particle and during this period the magnetic memory drum will have rotated through an indeterminate angle which may be less or greater than one revolution. All that is necessary is that normal scanning should commence when the drum is in the same angular position as it was when particle A was first intercepted and this is ensured by the explore sync signal laid down in the second channel.l When the drum reaches this position the explore sync head 20 acting as a reproducing head delivers a pulse through switch S1 to the switch control unit 14 and to the signal routing switch 17. The unit 14 is thereupon re-set which causes closure of switches Si, S5 and Se and opening of switches Sa and S4 thus permitting normal scanning to be resumed. Routing switch 17 is also re-set which causes switch Sz `to be operated to pass any new signal from the limiter 7 to the new signal indicator 12. Further, the pulse from the explore sync head 20 is passed to the sync switch 21 to re-set it thus operating switch Sa to the position in which the head can receive a new recording signal from the pulse producer 15 when the next particle is encountered. Since switch S9 is closed the erase head 16 co-operating with the second channel erases the second channel explore sync signal thus clearing this channel for subsequent use. The erase signal is provided by an erase signal source 24.

The units 12, 14, 15, 17, 18 and 21 and the switches operated by them are thus once more in their starting condition and the iirst channel of the memory unit carries a record of the particle A at a position along the channel from its commencement corresponding to the position of the particle A in the rst scanning line.

The complete sequence of operations above described is repeated when particles B, C and D are tirst encountered in line Li, so that at the completion of this first line the third channel of the memory unit carries a record of these interceptions in their correct spatial relationship and the total length of this record is arranged to correspond to the distance between `the writing head 11 and the reading head 13 both distances being measured along the channel track. By this arrangement, at the beginning of the scan of line L2 (which is determined by the line sync signal from the head 10-co-operating with the first channel) the start of the record of line L1 co-operates with the reading head 13 so that as scanning of line L2 proceeds the reading head 13 delivers signals corresponding to interceptions of the particles in line L1. Thus a form of the guard spot technique is achieved so that in the case of particle A which, docs not overlap line L2 the new signal indicator 12 receives a signal from the head 13 but no signal from the limiter 7 when the scanning beam reaches, in line Lz, a position corresponding to the position of particle A in line L1. This signal from the head 13 does not operate the new Vsignal indicator 12 so that normal scanning proceeds until particle B is encountered.

r`At this moment the new signal indicator 12 receives a pletion of the scan of line L2 the third channel record of line L1 has been erased by the erase head 16 which cooperates with this channel as well as the second channel.

At the completion of the scan line L2 the third channel Aof the memory'unit carries signals representing the second interceptions of particle B and C for comparison with such signals as may be received in line L3.

In the example shown only particle C overlaps line VLaso that scanning of this line will proceed from end to end without the flying, spot being arrested and at the end of the line the signals in the third channel of the memory unit, representing the interceptions in line L2, will have been erased and only the interception of particle C in line La will remain for usek during the succeeding scanz ning line.

It will be clear from the above description that each time the ying spot is arrested to explore a particle the Y-direction shift unit is operated and the measured size of the particle'being explored will be for example, a function of the timethe unit is in operation 0r proportional to a potential which varies with Y-direction deflection. The latter caseis diagrammatically illustrated by the resistor 22 across which a potential appears when a capacitor whose charge is related to the extent of theV exploration scan, is discharged at the termination of the exploration. At the termination ofthe exploration the switch Se is returned to the position in which the Y- direction shift unit is de-energised or blocked (the lower position in Figure 1) and the transientV potential generated across the resistor 22 is passed to a pulse `height analyser and counter 23 of known form. Thus, as scanning of the sample proceeds, a train of pulses having a random distribution in time and having varying Vamplitudes willbe presented to the analyser 23 which may present an analysis of the pulse train in any convenient manner for example the pulses may be sorted into any arbitrary number of groups so that individual counts of particles in each group may be presented as well as a total count of rall the particles. This latter count, may, if desired, be simply derived from the number oftimes the switch control unit 14 is operated.

Suitable circuit arrangements given by way of example only for the limiter unit 7, the various switch units and the pulse producer 15 will now be described with reference to Figures 3, 4, 5, 6 and 7.

The [imiter unit One suitable circuit for the limiter unit is shown in Figure 3 and may comprise a cathode coupled double triode thermionic valve 24a, 241:. rl`he signals from the pick-up device 6 are impressed on the control grid of the ltriode 24a and the anode is connected -through resistor Z5 to the control grid of triode 24h, this grid being connected through resistor 26 to a negative potential source. The

:tially rectangular -form and having a length representa- .tivetof the length ofithe particle in the direction of line scan.

New vsignal indicator `The positive goingpulses from the limiter 7 are, as above described, passed to the new signal indicator 12 which also receives signals from the read head 13 associated with'the third channel of the memorymeans. The new signal indicator 12 Amay be of the form shown in the circuit diagram of AFigure 4. In this circuit the signal from the manner, applied to Itheanode of diode V1 the cathode of which is connected to the control grid of triode valve V2 which acts as a pulseinvertor. Signals appearing at the lanode of valve V2 are applied to the control-grid of valve V3 which lwith valve V4 forms a bi-stable multivibrator, the anode of valve V3 being connected to the control grid of valve V4 and the anode of valve V4 being connected to the control grid of valve V3 in the usual manner. The differentiated incoming signal is also applied to the suppressor grid 0i the valve V4 and the control grid of this valve is connected to a negative H. T. line through a resistor 29 across which is shunted triode valve V5. The anode of valve V5 is therefore coupled to the control grid of valve V4 and has its cathode connected going signalof substantiallyV rectangular waveform, Ais

applied tothe control grid of valve V5.

The method of operation of this circuit is as follows: initially the v alve V3 is conducting and the valve V4 is cut off lon its control grid and'valve V5 is nonconducting idue tothe connection ofits control grid to `a negative bia-s line designated -Hfl2 in EFigure 4.

Considering tirstjthe Voperation at the iirst intercepltion of -a particle, thepositive-going pulse produced by the ditterentiator representingt-he leading edge of thc particle `traverses valve V1 an-d causes valve V2 to con- 'duct so.that a negative pulse from the `anode yof this u valve is passed to the `control grid of valve V3. Valve V3 .-is 'thereby 'cut-off and -the multivibrator ilips lto its Y other stable `state ,in which ,valve V4 is conducting being openboth on `its ycontrol grid and its suppressor grid. When the negative pulse produced by the ydiferentiator representing the trailing yedge yof the particle appears it vcannot eect .the control grid of valve V2 due to rectifier valve V1 but it appears on thesuppressor grid of valve V4 limiting the kcurrent to the anode, which causes -the multivibrator to tllop to its initial stable state. Substantially Ithe Whole of the emission current 'of the valve momentarily goes `to the screen grid, the potential of which exhibits Ia negative pulse which is passed to the switch control unit 14 and :the pulse producer .115.

Considering now the :operation of the indicator, for `example under the condi-tion of 'the second interception of particle B (-lineLz) :and assuming that .the signal from the vread head 13 appears slightly before .the signal from the limiter 7 then -the control grid of valve V5 will go positive so that V5 fully Lconducts and reduces the potential of the control lgri-d of V4. When the positive pulse of the differentiated signal from the limiter 7 is Iapplied the Valve V2 and the corresponding negativego-ing pulse is :applied `to valve V3 the multivibrator cannot flop since the control grid of` valve V4 is being forceably held yat a very low potential. Thus, when the negative-going pulse of the differentiated lsignal is 'applied .to the suppressor grid of valve V4 no signal aplimiter 7 is, after differentiation in the normal overlapping two or more scanning lines. The waveforms at (b) are produced when a long substantially parallel sided particle sloping in the direction of line scan, is encountered and at (c) when such a particle slopes in the opposite direction at (d) and (e) are shown the waveforms produced when a particle having tapering sides is encountered in the case of (d) 4the sides are divergent in the direction of frame scan and in (e) convergent in the same direction. VAt -(f) the read head signal alone is present (for example when the memory unit provides in line L2 a Isignal corresponding to the 'first interception of particle A in line L1).

In each ot these cases (b) to (f) no signal -is passed on .to the switch control unit 14 since the read head signal has either prevented the multivibrator being set or has returned it to the condition in ywhich it is non-responsive to the trailing negative-going pulse of 4the differentiated signal from the limiter 7.

Switch control unit As above described the new signal indicator only emits a pulse when a particle is rst intercepted and this pulse is passed to the switch control unit 14 now to be described and to the pulse producer 15 which will be later described. A suitable circuit for the switch control unit is .shown in Figure 6. This unit may conveniently comprise a cathode coupled double: triode -thermionic valve Ve, Vr forming a bi-stable multi-vibrator. The anode of valve Vs is connected to the control grid of valve V9 and the anode of valve Vv is cross-connected to the control grid `of valve Vs. Both control grids are connected through grid leaks to a negative H. il". line and the anodes are each connected to a positive H. T. line through loa-d resist-ors. Negative-going signals from the new signal indicator 12 are applied to the control grid of valve Vs which is initially in the conducting state the valve V7 being cut oil. The multivibrator may be tripped -to its original stable condition by negative going signals applied to the control grid of valve Vr these signals being those derived'from the explore sync head 20.

Upon the application of a negative pulse from the new signal indicator 12 a negative-going pulse Iappears at the anode of valve Vr and this may -be utilised to open switches Si, S3, S4 and S5.

Since these switches have been shown diagrammatically as mechanical switches there is included in the anode circuit of valve V7 a multi-contact relay 30 of which the contacts lconstitute the aforesaid switches. 'It will be clear, however, that such mechanical switches may be replaced by electronic switches in which case of course the relay is not employed.

The negative going pulse appearing Iat the anode of valve Vr when an input sign-al is received from the new signal indicator 12 is, as above stated, passed to the signal routing switch 17 to cause it to actu'ate vswitch Sz -to the position in which signals from the limiter 7 are passed to the explore switch 18 which in turn operates switch Ss and opens switch S7. l

`Both the signal routing switch and the explore switch may be of exactly the same .type as the switch control unit above described so that Ifurther description of these items is not necessary. The sync switch 21 the position of which has to beset by a pulse from the pulse pro- -ducer 15 and re-set by a signal from the explo-re sync head 20 can .also employ the same circuit as that shown in Figure 6.

Pulse producer A suitable circuit for the pulse producer is shown in Figure 7. This may conveniently `com-prise a cathode coupled `double triode thermionic valve Va, V9 having the anode of valve Vs connected to the control grid of valve' V9 through a capacitor C the lcontrol grid being connected to the earth line through a resistancev R the time constant of CR being suitably proportioned. Negative-going input pulses from the new signal indicator 12r a-re applied to .the control grid of valve Va which may be suitably biassed. In the initial or starting condition valve Va is conducting and valve V9 is cut olf. Upon the receipt of a negative going pulse at the control grid of valve Va yt-his valve ceases .to conduct and valve V9 becomes conducting so that the potential at the anode of valve V9 drops. After a time delay, determined by the time constant of CR, the valves Vs, V9 which form a mono-stable multi-vibrator revert to their initial condition so that a substantially rectangular pulse, having the desired duration in time, is available at the anode of valve V9. This pulse is fed to the explore sync head 20 whenever a particle .is first intercepted so as .to record a signal in the second channel which is available, as above described, for synchronising the action of the scanning means with the rotation of the memory drum.

It wil-l be understood that .the invention is not limited to -the arrangements described with reference to the block schematic diagram of Figure 1 since the magnetic memory device may be replaced by any other suitable device performing the same functions. Further, the invent-ion is not limited to the simple exploration of the particles as described herein. Such exploration may be of any other convenient or desired form. Such other methods of exploration may involve the use of more than one scanning flying spot. Such arrangements may involve alternati-on of the pick-up means 6 since more than one such means may be necessary but such changes will be obvious to those slcilled in the art.

lt will be understood that the invention is not limited to the particular circuit arrangements herein descii-bed since changes may be made to suit particular circumstances las they arise in practice.

What is claimed is:

1. Particle 'counting apparatusl comprising particle sample scanning means including means for producing a scanning beam tracing 4out a rectilinear raster, scanning `beam yarresting means for arresting the norm-al scanning motions of the beam only when a terminal interception of la particle Iby the beam occurs, particle exploring means for causing the scanning beam -to explore la par ticle thus intercepted, line Ito line memory means for dis tinguishing between the said terminal interception of the particle by the scanning beam and any other interceptions of the ysaine particle and counting and sizing means giving a count of the number of particles scanned and distinguishing between particles of different sizes.

2. Particle counting apparatus as claimed in claim l wherein the line to line memory means is of -the cyclic type having a plurality yof independent *signal channels the iirst of which carries a permanent signal which is utilised to trigger the line scan of the scanning beams.

3. Particle counting lapparatus -as claimed in claim 2 wherein the second of said signal channels carries particle signals corresponding -to all interceptions by the scanning beam of particles in a scanning line.

4. Particle counting apparatus as claimed in claim 3 including means for Whiting said particle signals .in said second channel during lscanning of a line and means for reading said signals dur-ing scanning of the succeeding line.

5. Particle counting apparatus as claimed in claim 2 wherein a third of said `signal channels is ladapted to store an explore sync signal each time a new par-ticle is encountered by the scanning beam.

6. Particle counting apparatus as claimed in claim 5 including means functioning as 4both writing land reading means for writing and reading said explore sync signal in said third `channel and means for erasing said signal `after it has been read.

7. Particle counting apparatus -as claimed in claim 6 comprising means 4for rie-energizing -said Ascanning beam arresting means afterY exploration is completed and when 9 v10 the associated explore sync signal is emitted by said References Citedinthe le of this patent fefdllg 15123118- t tu l ed l 2 UNITED STATES PATENTS a ce coun lng appara s as calm m caam l wherein the line to dine memory means comprises a 'I vlvithers ept'l magnetic recording element. 5 er an' 1

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