Method and apparatus for measuring particle concentration in a fluid suspension of particles

Claim 2

Apparatus for measuring particle concentration in a fluid suspension of particles wherein each particle is sensed when it is in a sensing zone by an electrical system which produces a particle pulse having poorly defined leading and trailing edges and a duration related to the time the particle is in the sensing zone, said apparatus including A. means connected to the output of the electrical system for producing upon sensing each particle pulse above a given threshold level an output pulse having a given amplitude and a selected clearly defined duration established therebetween i. a clearly defined leading edge of said output pulse occurring at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse and ii. a clearly defined trailing edge of said output pulse occurring at substantially the same time that the instantaneous value of the particle pulse at its trailing edge falls below the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, and B. means for measuring and displaying the duty factor of said output pulses to obtain a measurement of the concentration of particles in the fluid traversing the sensing zone.

Claim 3

The apparatus as claimed in claim 1 wherein said means for producing said output pulse includes means for initiating said output pulse at a point in time substantially equal to the time when there is an inflection of the slope of the leading edge of the particle pulse.

Claim 4

The apparatus as claimed in claim 2 wherein said means for initiating said output pulse includes means for taking the second derivative of the particle pulse to provide a second derivative signal which has a first zero crossing at the point of inflection of the slope of the leading edge of the particle pulse.

Claim 5

The apparatus as claimed in claim 3 wherein said means for initiating said output pulse includes means for amplifying said second derivative signal and low pass filter means for reducing the signal to noise ratio of the amplified second derivative signal.

Claim 6

The apparatus as claimed in claim 4 including low threshold circuit means connected to said electrical system for detecting a threshold amplitude level of the particle pulse, said low threshold circuit means being adapted to prevent operation of said apparatus if the amplitude level of the particle pulse does not exceed a predetermined threshold level.

Claim 7

The apparatus as claimed in claim 5 including means for delaying the time of application of the particle pulse to said low threshold circuit means to compensate for the time delay in said means for taking the second derivative of the particle pulse occasioned by the amplification and filtering of said second derivative signal.

Claim 8

The apparatus as claimed in claim 3 including low threshold circuit means connected to said electrical system for preventing operation of said apparatus if the amplitude of the particle pulse does not exceed a predetermined threshold level.

Claim 9

The apparatus as claimed in claim 7 including means for clamping the particle pulse applied to said low threshold circuit to a common potential for said apparatus.

Claim 10

The apparatus as claimed in claim 7 wherein the quiescent output signal from said low threshold circuit means is a logic one output signal and said low threshold circuit means is adapted to change its logic one output to a logIc zero output when the instantaneous value of the particle pulse applied to the low threshold circuit means exceeds the predetermined threshold level and wherein said apparatus includes means for summing said output signal from said low threshold circuit with said second derivative signal whereby the leading edge of said second derivative signal is added to a logic one signal and said first zero crossing of said second derivative signal is added to a logic zero signal to produce a timing signal having a specific first zero crossing.

Claim 11

The apparatus as claimed in claim 9 wherein said means for initiating said output pulse includes comparator circuit means connected to said summing means for producing a trigger signal at the time when said timing signal makes its first zero crossing.

Claim 12

The apparatus as claimed in claim 1 wherein said means for producing said output pulse includes means for terminating said output pulse when the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse.

Claim 13

The apparatus as claimed in claim 11 wherein said means for terminating said output pulse includes a signal comparing circuit for sensing the particle pulse, for sensing and holding the instantaneous value of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse, for comparing the changing instantaneous values of the particle pulse with the held instantaneous value of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse and for producing a control signal at the time the instantaneous value of the trailing edge of the particle pulse falls below the held instantaneous value level, said control signal being utilized to terminate said output pulse.

Claim 14

The apparatus as claimed in claim 1 wherein said means for producing said well-defined output pulse includes first means for initiating said output pulse at a point in time substantially equal to the time when there is an inflection of the slope of the leading edge of the particle pulse and second means for terminating said output pulse when the instantaneous value of the trailing edge of said particle pulse falls below the level of the instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse.

Claim 15

The apparatus as claimed in claim 13 wherein said first means for initiating said output pulse includes means for producing a first control signal at substantially the same time when there is a point of inflection of the slope of the leading edge of the particle pulse and said second means for terminating said output pulse includes a comparator circuit for comparing the instantaneous value of the particle pulse with the instantaneous value of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse and for producing a second control signal at the time when the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value at the point of inflection of the slope of the leading edge of the particle pulse.

Claim 16

The apparatus as claimed in claim 14 wherein said second means includes a track and hold circuit for tracking the instantaneous value of the particle pulse, for holding the instantaneous value at the point of inflection of the leading edge of the particle pulse and for applying this held instantaneous value to said comparator circuit.

Claim 17

The apparatus as claimed in claim 15 wherein said track and hold circuit has a switch control input and said second means includes a switch control circuit connected between the output of said first means and said switch control input of said track and hold circuit, said switch control circuit being adapted, upon receiving said first control signal from saiD first means to apply a switch control signal to said track and hold circuit to cause same to hold the level of the instantaneous value of the particle pulse being tracked whereby said track and hold circuit holds an instantaneous value which is substantially equal to the instantaneous value at the point of inflection of the slope of the leading edge of the particle pulse.

Claim 18

The apparatus as claimed in claim 13 wherein said first means for initiating said output pulse includes means for taking the second derivative of the particle pulse and for producing a second derivative signal having a first zero crossing at a time substantially equal to the time there is an inflection of the slope of the leading edge of the particle pulse.

Claim 19

The apparatus as claimed in claim 17 including low threshold circuit means connected to said electrical system and adapted to sense the instantaneous value of the particle pulse and to prevent operation of said apparatus if the instantaneous value of the particle pulse does not exceed a predetermined threshold level.

Claim 20

The apparatus as claimed in claim 18 wherein said low threshold circuit means produces a quiescent logic one output signal and is adapted to provide a logic zero output signal when said low threshold circuit senses an instantaneous value of the particle pulse above said predetermined threshold level and wherein said apparatus includes summing circuit means for summing said logic output signal from said low threshold circuit with said second derivative signal whereby the leading edge of said second derivative signal is added to a logic one signal and said first zero crossing of said second derivative signal is added to a logic zero signal to produce a control signal having a specific first zero crossing.

Claim 21

The apparatus as claimed in claim 19 wherein said first means includes signal comparing means for sensing the first zero crossing of said control signal and for producing an output pulse initiating signal having a well-defined leading edge at a time substantially equal to the time of inflection of the slope of the leading edge of the particle pulse.

Claim 22

The apparatus as claimed in claim 13 wherein said first means includes means for producing a first control signal at substantially the time of inflection of the slope of the leading edge of the particle pulse and said second means includes means for producing a second control signal at the time the amplitude level of the trailing edge of the particle pulse falls below the amplitude level of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse, and wherein said apparatus includes control circuit means for producing said output pulse, said control circuit means being connected to said first and second means and being adapted to initiate said output pulse upon receiving said first control signal from said first means and being adapted to terminate said output pulse upon receiving said second control signal from said second means.

Claim 23

The apparatus as claimed in claim 21 wherein said second means includes an AND circuit, the quiescent output of which is a logic zero output signal, and means for producing an intermediate control signal which is applied to said AND circuit and which changes from logic one to logic zero at substantially the time of inflection of the slope of the leading edge of the particle pulse and which changes from logic zero to logic one at substantially the time the instantaneous value of the trailing edge of the particle pulse falls below the instantaneous value of the leading edge of the particle pulse at the point of inflection of the slope of the leading edge of the particle pulse, said first control signal from said first means being a logic one signal which is also applied to said AND circuit whereby, when said intermediate control signal changes from logic zero to logic one, the two inputs to said AND circuit are both logic one inputs resulting in a logic one oUtput from said AND circuit which constitutes said second control signal which is applied to said control circuit means for causing said control circuit means to terminate said output pulse.

Claim 24

The apparatus as claimed in claim 22 wherein said first means includes means for producing an intermediate control signal at substantially the same time as the point of inflection of the slope of the leading edge of the particle pulse, and a signal generating circuit having a trigger input, a reset input and an output and being adapted to produce said first control signal when said intermediate control signal is applied to said trigger input, said first control signal being applied to said AND circuit and to said control circuit means, and wherein said control circuit means has a second output which is connected to said reset input of said signal generating circuit of said first means, the quiescent output signal from said second output of said control circuit means being a logic one output signal and changing from logic one to logic zero when said first control signal from said signal generating circuit of said first means is applied to said control circuit means and changing from logic zero to logic one when said second control signal from said AND circuit is applied to said control circuit means, the trailing edge of said output signal from said second output of said control circuit means being applied to said reset input of said signal generating circuit to terminate said first control signal, the termination of which terminates said second control signal from said AND circuit to reset said apparatus for sensing another particle pulse.

Claim 25

The apparatus as claimed in claim 23 wherein said second means includes signal tracking means for tracking the particle pulse, signal value holding means for holding a given instantaneous value of a signal at a given point in time and switch control means, connected between said signal value holding means and said output of said signal generating circuit, for controlling said signal value holding means to hold a given instantaneous value of the particle pulse upon receiving said first control signal from said signal generating circuit.

Claim 26

The apparatus as claimed in claim 1 wherein said means includes first and second input channels for receiving the particle pulse, said first input channel including means for clamping each particle pulse to a common potential and means for delaying the clamped particle pulse, said second input channel including means for producing a signal equal to the second derivative of said particle pulse, said second derivative signal having at least two zero potential crossings corresponding respectively to the points of inflection of the respective slopes of the leading and trailing edges of the particle pulse, and filter means for increasing the signal to noise ratio of the second derivative signal, a low threshold circuit connected to the output of said first input channel and including means for changing a logic one output signal from said low threshold circuit to a logic zero output signal when the instantaneous value of the clamped and delayed particle pulse from said first input channel reaches a predetermined threshold value, summing circuit means connected to said output of said low threshold circuit and to the output of said second input channel for combining the filtered second derivative signal with the logic output signal from said low threshold circuit whereby the leading edge of said second derivative signal is added to a logic one signal and the first zero crossing of said second derivative signal is added to a logic zero signal to produce a combined second derivative signal having a specific first zero crossing, first circuit means connected to said summing circuit means for receiving the combined second derivative signal and for producing a first intermediate control signal having a clearly defined leading edge at a time substantially equal to the first zero crossing of said second derIvative signal, second circuit means connected to said first circuit means for producing, at an output thereof, a first logic one control signal upon sensing the leading edge of said first intermediate control signal, said second circuit means also having a reset input for receiving a reset signal for terminating said first control signal, a triggered, signal comparing circuit including a track and hold circuit connected to said output of said first input channel, a switch control circuit connected between said output of said second circuit means and said track and hold circuit, and a comparator having a first input terminal connected to said output of said first input channel and a second input terminal connected to the output of said track and hold circuit, said track and hold circuit being adapted to track said clamped and delayed particle pulse from said input channel and to hold the instantaneous value of said clamped and delayed particle pulse at the time when said track and hold circuit is placed in its hold mode by a switch control signal from said switch control circuit, said switch control circuit producing said switch control signal upon receiving said first control signal from said second circuit means whereby said switch control signal commences at a point in time substantially equal to the time said second derivative signal of said leading edge of said particle pulse makes said first zero crossing and the instantaneous value held by said track and hold circuit is the instantaneous value at the point of inflection of the leading edge of the particle pulse, the quiescent output signal from said comparator being a logic one signal and said comparator being adapted to produce a logic zero output signal during the time the instantaneous value of said clamped and delayed particle pulse received at said first input terminal of said comparator is greater than the held instantaneous value received at said second input terminal of said comparator from said track and hold circuit, an AND circuit having a first input terminal connected to the output of said comparator and a second input terminal connected to said output of said second circuit means and adapted to produce a second control signal when the output signal from said comparator changes from logic zero to logic one and said first control signal is applied to said second input terminal of said comparator, a third circuit means for producing said well-defined output pulse, said third circuit means having first and second inputs and first and second outputs, said first input of said third circuit means being connected to said output of said second circuit means, said second input of said third circuit means being connected to the output of said AND circuit, said second output of said third circuit means being connected to said reset input of said second circuit means, and said means for measuring and displaying the duty factor of said output pulses includes an integrator connected to said first output of said third circuit means for integrating each output pulse received from said third circuit means, means for timing said integrator, means for resetting said integrator, and means for displaying a readout of the output from said integrator, said third circuit means being adapted, upon receiving at its first input said first control signal from said second circuit means, to change the signal at said first output thereof from logic zero to logic one to start one of said output pulses, the leading edge of said output pulse occurring at substantially the same time as the inflection of the slope of the leading edge of the particle pulse, and to change the signal at said second output thereof, which is applied to said reset input of said third circuit means, from logic one to logic zero, and being adapted upon receiving at its second input said second control signal from said AND circuit to change the logic output signal at said first output thereof from logic one to logic zero to terminate said output pulse, the traIling edge of said output pulse from said third circuit means occurring at a time substantially equal to the time the amplitude of the trailing edge of the particle pulse falls below the held instantaneous value at the point of inflection of the slope of the leading edge of the particle pulse and to change the output signal at said second output thereof from logic zero to logic one, the trailing edge of this output signal being applied to said reset input of said second circuit means to cause said second circuit means to terminate said first control signal at the output thereof which in turn terminates said second control signal from said AND circuit to reset said apparatus for receiving another particle pulse, said timing means and said resetting means being operable to said integrator to integrate for a predetermined measuring interval, and said displaying means providing a display of the final charge accumulated and held by said integrator.

Claim 29

The method as claimed in claim 26 wherein said output pulse is initiated by taking the second derivative of the particle pulse to obtain a second derivative signal the first zero crossing of which is at substantially the same time as the inflection of the slope of the leading edge of the particle pulse, and said method includes the step of using this zero crossing of the second derivative signal to produce a control signal which initiates said output pulse.

Claim 30

The method as claimed in claim 27 including the steps of adding a logic signal to said second derivative signal, said logic signal consisting of a logic one signal up to a point in time when the instantaneous value of the particle pulse exceeds a predetermined threshold level and being a logic zero signal from this point in time until the trailing edge of the particle pulse falls below the predetermined threshold level whereby the leading and trailing edges of the second derivative signal are added to a logic one signal and the zero crossings of the second derivative signal are added to a logic zero signal to obtain a control signal having a specific first zero crossing at the time of inflection of the slope of the leading edge of the particle pulse.

Claim 31

The method as claimed in claim 26 wherein said output pulse is terminated by a control signal, said control signal being obtained by holding the instantaneous value of the leading edge of the particle pulse at the point of inflection of the leading edge of the particle pulse, by comparing this held instantaneous value with the changing instantaneous value of the particle pulse and by initiating said control signal when the instantaneous value of the trailing edge of the particle pulse falls below the held instantaneous value of the leading edge of the particle pulse at the time of inflection of the slope of the leading edge of the particle pulse.

Claim 34

The method as defined in claim 29 wherein said second control signal is only produced when said first control signal is in existence and said second control signal is used to terminate said first control signal with the termination of said first control signal serving to terminate said second control signal to enable said method to be quickly and accurately repeated for a subsequent particle pulse.

Claim 35

The method as claimed in claim 29 including the step of adding a logic signal to said second derivative signal to produce a trigger signal which is used to generate said first control signal, said logic signal consisting of a logic one signal up to a point in time when the instantaneous value of the particle pulse exceeds a predetermined threshold level and being a logic zero signal from this point in time until the trailing edge of the particle pulse falls below the predetermined threshold level whereby the leading and trailing edges of the second derivative signal are added to a logic one signal and the zero crossings of the second derivative signal are added to a logic zero signal to obtain a trigger signal having a specific first zero crossing at the time of the inflection of the slope of the leading edge of the particle pulse.