Method and Apparatus for Providing Primary Coincidence Correction During Particle Analysis Utilizing Time Generation Techniques

This invention is directed to particle counting methods and apparatuses which provide a time correction to the time period particle pulses are counted so that coincidence phenomena resulting in fewer particle pulses being counted in a predetermined time period does not induce an error in the particle pulse count ultimately obtained. The particle counting methods and apparatuses concerned employ particle sensing zones wherein the presence of one particle within the zone masks or hides the presence of another particle which also may be within the sensing zone. This invention particularly is directed to, but not limited to, the correct determination of non-electrical properties, such as size and count of microscopic particles by measuring electrical properties (Patent Office Class 324-71NE).

Now well known in the art of electronic particle counting and analyzing is apparatus marketed primarily under the trademark "Coulter Counter," and owned by the assignee of this application. Such apparatus and portions thereof are disclosed in several U.S. Pat., for example: Nos. 2,656,508; 2,985,830; and 3,259,842 (Class 324-71). A significantly important portion of such Counter type of apparatus is the minute scanning aperture or scanning ambit or sensing zone relative to or through which pass and are detected and counted, single particles at a rate often well in excess of one thousand per second. Because of the physical parameters of the scanning aperture, and particle concentration, coincidence of two particles in the scanning ambit occurs quite often.

It has been found that once a particle pulse produced by the presence of a particle in the scanning ambit, exceeds the threshold setting of the apparatus and is detected, no other pulse will be produced in response to other particles in the scanning ambit until such time as the particles have cleared the sensing zone. As the presence of a first particle masks a second particle causing a coincidence error, the coincidence error is a function of the time that detected particles are in the scanning ambit, and the total coincidence error is a function of the total time that particles are in the scanning aperture during the detecting and counting process. Since the coincidence error is a function of the time that each particle is in the scanning aperture, it can also be said that the coincidence error is a function of the duration of each particle pulse produced and detected in response to passage of a particle through the scanning ambit. It has also been found that the coincidence error is functionally related to the rate at which particles pass through the scanning ambit and, therefore, the particle pulse repetition rate, if this rate is determined only by particle concentration.

In practicing this invention an apparatus is provided for automatically correcting a particle pulse count subject to coincidence error wherein particle pulses are developed in response to passage of particles in a particulate system through a particle sensing device. The apparatus includes a timer which is operative to count particle pulses received for a predetermined period of time. A control device is coupled to the timer and is operative to increase the predetermined period of time by a time increment in response to each particle pulse. The duration of the time increment is a function of particular characteristics of each particle pulse, for example the pulse duration or amplitude.

In one embodiment of this invention, a second time increment is added to the predetermined period of time in response to each particle pulse. This second time increment may be either a fixed duration or a duration which is proportional to the repetition rate of the particle pulses received.

The method for correcting a particle pulse count subject to a coincidence error is also envisioned as being within the scope of this invention. This method includes the steps of

a. counting the particle pulses received for a predetermined period of time, and

b. increasing the predetermined period of time by a time increment that is a function of particular characteristics of each pulse counted such as, for example the pulse width.

FIG. 1 is a block diagram of an apparatus embodying the features of this invention;

FIG. 2 is a block diagram of a threshold circuit which may be used in the apparatus shown in FIG. 1;

FIG. 3 is a graph of curves representative of signals at various points of the apparatus shown in FIGS. 1 and 2; and

FIG. 4 is a block diagram of another embodiment of the apparatus of this invention.

Referring to FIG. 1 there is shown an apparatus embodying the invention and identified generally by the reference number 10. A pulse source 12 can be the aperture tube and related circuitry of a Coulter type particle detector such as is shown and described in U.S. Pat. Nos. 2,656,508 and 3,259,842, wherein particle pulses are developed in response to the passage of particles in a particulate system through the sensing zone of the aperture tube. The particle pulses developed by the pulse source 12 are coupled via conductor 14 to a threshold circuit 16. Pulses such as appear on conductor 14 are shown in Graph A of FIG. 3, and are identified as pulses 44, 46, 48 and 50.

Threshold circuit 16 has a low threshold level. This low threshold level is represented by the dashed line 52 in Graph A of FIG. 3. When the low threshold level 52 is exceeded by a particle pulse from pulse source 12, threshold circuit 16 develops a threshold pulse having a fixed amplitude and duration which is equal to the period that the particular pulse exceeds the threshold level. Graph B of FIG. 3 shows the threshold pulses developed in response to particle pulses 44, 46, 48 and 50. The periods of these pulses are T.sub.44, T.sub.46, T.sub.48 and T.sub.50, respectively. The threshold pulses shown in Graph B are developed at conductors 17 and 19 in FIG. 1. Conductor 19 couples the threshold pulses to one input of an AND gate 18.

Timer 22 is of the type commonly known in the art which when enabled via a push-button or a start signal associated with the start of the operation of the pulse source 12 will develop an output signal at conductor 23 for a fixed period of time. That is, timer 22 once initiated, will measure a predetermined time interval of, for example 5 minutes and, during that predetermined time interval, it will develop an output or timer signal at conductor 23. The timer signal developed by timer 22 is coupled via conductor 23 to the second input of AND gate 18 for the predetermined time interval or period of operation of timer 22, thus partially enabling AND gate 18. AND gate 18 will be fully enabled during the predetermined time period by receipt of a threshold pulse on conductor 19 such as, for example those pulses shown in Graph B of FIG. 3. When AND gate 18 is fully enabled it will develop an AND gate output pulse which is coupled to pulse counter 24. Pulse counter 24 will count the pulse coupled from AND gate 18. As AND gate 18 develops its AND gate signal during the predetermined time period in response to the threshold pulses, which are developed in response to the particle pulses in excess of the threshold level, pulse counter 24 will be counting each particle pulse in excess of the threshold level during the predetermined time period.

Conductor 17 couples the threshold pulses developed by threshold circuit 16 to one input of an OR gate 20, and to the input of a trailing edge detector circuit (TED) 26. Trailing edge detector circuit 26 is operative in response to the trailing edge of each one of the threshold pulses received to develop a pulse. Pulses developed in response to threshold pulses T.sub.44, T.sub.46, T.sub.48 and T.sub.50 are shown in Graph C of FIG. 3. The pulses developed by trailing edge detector circuit 26 are coupled to the input of a monostable multivibrator 28, commonly known in the art as a one shot. Monostable multivibrator 28 will develop an output pulse of fixed amplitude and predetermined duration in response to each pulse coupled to its input from trailing edge detector circuit 26. The duration of the pulse developed by monostable multivibrator 28 may be varied by adjusting the Potentiometer 29 shown as being coupled to monostable multivibrator 28. The pulses of fixed amplitude and predetermined duration, developed by monostable multivibrator 28 in response to threshold pulses T.sub.44, T.sub.46, T.sub.48 and T.sub.50 are shown in Graph D of FIG. 3. They are shown as having a fixed duration identified as T.sub.2. These pulses are coupled from monostable multivibrator 28 to a second input of OR gate 20.

OR gate 20 develops an output pulse in response to an input pulse at either one of the two inputs. The output or OR gate pulse will be present for the full duration of the input pulse at either one of the two inputs. Accordingly, OR gate 20 will develop an OR gate pulse for the full duration of each threshold pulse. Because the threshold pulse is coupled to one input of OR gate 20 via conductor 17, and the monostable pulse is coupled from monostable vibrator 28 to the second input of OR gate 20, immediately upon the termination of a threshold pulse, OR gate 20 will maintain the OR gate pulse for a fixed increment or period T.sub.2. Graph E of figure represents the output of OR gate 20 in response to particle pulses 44, 46, 48 and 50. It can be seen that the pulse identified as having a period T.sub.64 is equal to the period of threshold pulse T.sub.44 and T.sub.2. Similarly, the pulses identified as T.sub.66, T.sub.68, and T.sub.70 correspond in duration to the period of the corresponding threshold pulse plus a fixed period T.sub.2.

Timer 22 also includes an inhibit circuit such that when a signal is coupled to inhibit input 21 of timer circuit 22 measurement or counting of the predetermined time period will be inhibited. The measurement or counting of the predetermined time period will be inhibited for the duration of the signal coupled to inhibit input 21. The OR gate signals developed at output of OR gate 20 are coupled to the inhibit input 21 of timer 22 inhibiting the measuring or counting of the predetermined time period for the duration of the OR gate signals.

The circuitry coupling threshold circuit 16 to the inhibit input 21 of timer 22 acts as previously noted to inhibit the operation of timer 22 for particular time increments in response to each particle pulse received and detected. A portion of each particular time increment is a function of the duration of each particle pulse received in excess of the threshold level. Another portion of the time increment is of fixed duration in response to each particle pulse received and detected. These time increments extend the total time period counted or measured by timer 22 so that particle pulses may be counted for a time period greater than the predetermined time period originally selected. This additional time period counted, and resulting from these time increments, and the particle pulses detected and counted during this additional time period, provide an error correction to the particle pulse count obtained during the predetermined time period such that the total particle pulse count will be an error corrected particle pulse count for the predetermined time period.

Although the threshold circuit 16 is shown and described in FIG. 1 as a single, low level threshold circuit, it is to be understood that other types of threshold circuits may be employed in the apparatus shown in FIG. 1. For example, a "window" type threshold circuit may be employed wherein pulses only falling within a particular amplitude range are detected and produce threshold pulses at conductor 19. The operation of a "window" type threshold circuit is shown and described in U.S. Pat. No. 3,259,842. FIG. 2 shows an embodiment of a "window" type threshold circuit for use in the apparatus of FIG. 1.

Referring to FIG. 2, each particle pulse from pulse source 12 in FIG. 1 is coupled via conductor 14 to an upper threshold 30 and a lower threshold 36. For purposes of the description of the operation of this "window" threshold circuit the particle pulses shown in Graph A of FIG. 3 will be assumed to be coupled via conductor 14 to upper and lower threshold 30 and 36 respectively. Referring to Graph A, the lower threshold level is represented by the dashed line 52 and the upper threshold level is represented by the dashed line 54. If a particle pulse exceeds the lower threshold level 52 of lower threshold 36, a threshold pulse will be developed by lower threshold 36 and coupled to conductor 17. Pulse T.sub.44 in Graph B represents the output threshold pulse at conductor 17 for particle pulse 44 when threshold level 52 of lower threshold 36 is exceeded. If the particle pulse coupled to upper threshold 30 exceeds the upper threshold level 54, upper threshold 30 will develop an output pulse which is coupled to a bistable multivibrator 32. Graph F of FIG. 3 shows the output of upper threshold 30 during the period that particle pulses 44, 48, and 50 in Graph A exceed upper threshold level 54.

The pulse developed by upper threshold 30 and coupled to bistable multivibrator 32 causes bistable multivibrator 32 to change states and develop an output pulse. This output pulse is coupled via an inverted input to AND gate 34. The inverted input acts to transform AND gate 34 to a veto type AND gate. As a consequence, when bistable multivibrator 32 develops an output signal, that is, when upper threshold 30 is activated in response to a pulse which exceeds the upper threshold level 54, AND gate 34 will be inhibited from developing an output signal at conductor 19. Because of this inhibit when the upper threshold is exceeded, indicating that the particle pulse amplitude is outside of the window, a pulse will not be coupled to AND gate 18 for counting by pulse counter 24.

Each pulse developed by lower threshold 36 such as for example, pulse T.sub.44 shown in Graph B is also coupled to a trailing edge detector (TED) 38. Trailing edge detector 38 will develop a pulse in response to the trailing or terminating edge of the threshold pulse. Graph C shows the pulse developed by trailing edge detector circuit 38 in response to T.sub.44. The pulse developed by trailing edge detector circuit 38 is coupled to a monostable multivibrator 40 which, in the embodiment shown, can be identical to monostable multivibrator 28 and have the same time period, so that the pulses shown in Graph D of FIG. 3 could equally as well represent the pulses developed by monostable multivibrator 40 as the pulses represented by the monostable multivibrator 28. Monostable multivibrator 40 develops a pulse of fixed amplitude and predetermined duration in response to the pulse from trailing edge detector 38. This pulse is represented in FIG. 3, Graph D. The pulse developed by monostable multivibrator 40 is coupled to a second trailing edge detector 42 and to a second input of AND gate 34. When AND gate 34 is not inhibited, that is, when the upper threshold level 54 is not exceeded, the pulse coupled from monostable multivibrator 40 will cause AND gate 34 to develop an AND gate signal at output 19. When AND gate 34 is inhibited via an output signal from bistable multivibrator 32, it will not develop an AND gate signal at conductor 19 in response to a pulse from monostable multivibrator 40. In Graph A of FIG. 3 it can be seen that particle pulses 44, 48 and 50 exceed the low threshold level 52 of the low threshold 36 and the upper threshold level 54 of upper threshold 30. Particle pulse 46 exceeds the lower threshold level 52 of lower threshold 36 and does not exceed upper threshold level 54 of upper threshold 30. Consequently only particle pulse 46 is within the window. Referring now to Graph G of FIG. 3, it can be seen that the output of veto AND gate 34, represented in Graph G only develops an output signal after particle pulse 46.

Trailing edge detector 42, coupled to the output of monostable multivibrator 40 acts in response to the trailing edge of the monostable multivibrator pulses as represented in Graph D of FIG. 3 to develop pulses which are coupled to the reset input of bistable multivibrator 32. These pulses act to reset bistable multivibrator 32 and terminate its output signal such that AND gate 34 is no longer inhibited from responding to a pulse at its second input. This resetting of bistable multivibrator 32 is necessary in order to allow threshold circuit 16 to respond to the next particle pulse in the particle pulse train.

It should be noted for purposes of this application that when a "window" type threshold circuit 16 is employed, the output coupled to conductor 19 is not the same as the output coupled to conductor 17. The output coupled to conductor 19 is a count output which is ultimately coupled to pulse counter 24 in FIG. 1 in order to count the number of pulses falling within the "window" of threshold circuit 16. However, all pulses in excess of the lower threshold level 52 of low threshold 36 in threshold circuit 16 must be considered in determining the amount of error correction time to be added to the time period measured by timer 22. For this reason, the output of threshold circuit 16 is coupled to conductor 17 which couples any and all pulses in excess of the lower threshold level 52 to the error correction circuitry in FIG. 1 consisting of OR gate 20, negative trailing edge detector 26 and monostable multivibrator 28. All of these pulses are employed, therefore, for developing the OR gate signal coupled from OR gate 20 to the inhibit input 21 of timer 22.

As previously noted, negative trailing edge detector 26 and monostable multivibrator 28 in the embodiment of FIG. 1 form a circuit which inhibits operation of timer 22 for a fixed or predetermined period of time in response to each particle pulse detected. It has been found that the error correction can be enhanced if timer 22 is inhibited for a second time increment whose duration is a function of the rate of detection of the particle pulses rather than a fixed time increment. FIG. 4 is a block diagram of an apparatus including circuitry for inhibiting operation of timer 22 by a period which is proportional to the rate of detection of particle pulses. As all other portions of the apparatus shown in FIG. 4 are identical to the apparatus shown in FIG. 1, these identical portions, which are identically numbered, will not be described. The operation of these portions as they are identical to that described with respect to FIG. 1 also will not be repeated. Only the operation of the circuitry which inhibits timer 22 for a time period which is a function of the repetition rate of the detected particle pulses will be described.

The threshold pulses developed at conductor 17 by threshold circuit 16 in the embodiment of FIG. 4 are coupled to an inverted input of AND gate 78, one input of an OR gate 80, one input of an AND gate 76, and rate meter 82. Rate meter 82 will develop a DC voltage whose amplitude is a function of the repetition rate of the threshold pulses coupled thereto. The DC voltage developed by rate meter 82 is coupled to one input of analog switch 84.

Inverting one input of AND gate 78 makes AND gate 78 a veto AND gate. Consequently, whenever a threshold pulse is coupled to the inverted input via conductor 17, AND gate 78 will be inhibited from developing an AND gate signal at its output. The threshold pulses coupled via conductor 17 to the output of OR gate 80 causes OR gate 80 to develop an OR gate signal at its output which is coupled to a second input of AND gates 76 and 78. As AND gate 78 is inhibited, it will not develop an AND gate signal. As the first input to AND gate 76 is coupled to threshold circuit 16 via conductor 17, the presence of a threshold pulse on conductor 17 and the OR gate signal at its second input will cause AND gate 76 to change states and develop an AND gate signal at its output. The AND gate signal is coupled from AND gate 76 to the control input of analog switch 84, opening analog switch 84 and allowing the DC voltage developed by rate meter 82 to be coupled to integrator 86. Integrator 86 will integrate this DC voltage for the period that the threshold pulse is present at conductor 17 and develop an integration voltage. The integration voltage developed by integrator 86 is coupled to one input of comparator 88. The second input of comparator 88 in this embodiment is coupled to a ground or reference potential. Consequently, when the integration voltage developed by integrator 86 exceeds zero or reference potential, comparator 88 will develop an output signal which is coupled to a second input of OR gate 80 and to the second input of OR gate 20. This comparator output signal will cause OR gate 20 to develop an OR gate signal at its output which is coupled to the inhibit terminal 21 of timer 22 inhibiting the measurement or counting of timer 22.

The comparator output signal developed by comparator 88 and coupled to the input of OR gate 80 causes OR gate 80 to maintain its OR gate signal at the second input of AND gates 76 and 78 after the termination of the threshold pulse developed at conductor 17. Upon the termination of the threshold pulse developed at conductor 17, the inverted input of AND gate 78 will no longer inhibit the operation of veto AND gate 78. With an OR gate signal coupled to the second input of AND gate 78, AND gate 78 will change states and develop an AND gate signal at its output which is coupled to the control input of analog swith 90. Analog switch 90 is coupled via a potentiometer 92 to negative supply potential. Upon receipt of the AND gate signal from veto AND gate 78 analog switch 90 will couple the negative potential from potentiometer 92 to integrator 86. Integrator 86 will begin integrating this negative voltage causing the integration voltage developed at its output to decrease towards zero or reference potential. When zero or reference potential is reached, comparator 88 will change states and terminate its comparator output signal. When the comparator output signal is terminated, OR gates 20 and 80 will no longer develop an OR gate signal at their outputs, so that veto AND gate 78 will no longer develop an AND gate signal at its output and timer 22 will no longer be inhibited from counting or measuring the predetermined time.

It is to be understood that the method for providing the above noted particle pulse count corrected is also contemplated as being within the scope of this invention. Although a number of embodiments of apparatus for providing error correction of a particle pulse count have been shown and described, it is to be understood that other modifications of the above described circuitry are capable of being made without departing from the spirit or scope of the invention as defined in the appended claims.