Measuring article thickness ultrasonically

• US 3,929,006 A
• Filed: 11/26/1973
• Issued: 12/30/1975
• Est. Priority Date: 11/26/1973
• Status: Expired due to Term

• Current Assignee: AT&T, Inc.
• Original Assignee: Howard John Flichman, Luther Miles Boggs
• Inventors: Flichman, Howard John, Boggs, Luther Miles
• Primary Examiner(s): Queisser, Richard C.
• Assistant Examiner(s): Beauchamp, John P.
• Application Number:
  US05/419,170
• Time in Patent Office: 764 Days
• Field of Search: 73/67.7,67.8R,67.8S,67.9
• US Class Current: 73/609
• CPC Class Codes: G01B 17/025 for measuring thickness of coating
• CPC Class Codes: G01S 15/101 Particularities of the measurement of distance G01S15/12, G01S15/14, and G01S15/18 take precedence
• CPC Class Codes: G01S 7/527 Extracting wanted echo signals Doppler systems G01S15/50
• CPC Class Codes: H01B 13/00 Apparatus or processes specially adapted for manufacturing conductors or cables

Abstract

In a system for measuring ultrasonically cable jacket thickness, an ultrasonic pulse is transmitted into engagement with the jacket as the cable is advanced through an upstream portion of a cooling trough adjacent an extruder. Signals developed as a result of a sequence of valid associated first and second echo pulses received from surfaces of the cable which define the jacket are processed. The system is provided with the capability of avoiding interference of the testing of a first echo pulse by a closely spaced second echo pulse which occurs in the measurement of thin-walled cable jackets. The system provides a first signal in response to a determination that the first echo pulse exceeds a predetermined minimum threshold amplitude. A second signal, distinguishable from the first signal, is provided in the event that the first echo pulse does not exceed a time varying threshold amplitude greater than the predetermined minimum threshold value beyond a preset time, and the receipt of a second echo pulse exceeding a predetermined minimum threshold amplitude. Should the second echo pulse then be validated with respect to a predetermined peak amplitude and a duration characteristic, the time interval between the first and second signals is converted into a measurement indicative of jacket thickness.

Claims

1. A method of measuring the thickness of an article having first and second surfaces bounding a material of known acoustical properties, which comprises the steps of:

establishing a medium having known acoustical characteristics contiguous with the first surface of the article;

generating a longitudinal wave in the medium directed toward the first surface, one part of the wave being reflected therefrom back through the medium, as a first echo, the other part of the wave passing through the first surface and the article to the second survace, and a portion of the other part being reflected back through the article and the medium as a second echo;

generating control signals to establish first and second predetermined minimum intensity levels and a time varying intensity associated with and greater than the first predetermined minimum intensity;

comparing the intensity of a first echo having the first predetermined minimum intensity level with the time varying intensity after a preset time and the intensity of a second echo with the second predetermined minimum intensity;

generating a quantity proportional to the time interval between a first echo having the first predetermined minimum intensity and of which the intensity is less than the time varying intensity after the preset time and a second echo having the second predetermined minimum intensity and a duration during which the second predetermined minimum intensity is exceeded to establish the thickness of the article; and

inhibiting the generation of any quantity when at least one of the predetermined minima or time varying intensity characteristics do not exist.

2. A method of measuring ultrasonically the thickness of an elongated member in which an on signal and an off signal are developed in response to the receipt of a predetermined sequence of first and second associated echo pulses of predetermined test characteristics from one surface and an opposite surface, respectively, of the member and in which relative motion is caused to occur between the successive sections of the elongated member and a source of ultrasonic pulses, which includes the steps of:

establishing a test period during which echo pulses are tested;

transmitting an ultrasonic pulse into the member and then receiving a first echo pulse reflected from the one surface of the member and then receiving an associated second echo pulse from an opposite surface of the member;

while excluding extraneous low level signals;

testing the first echo pulse and then an associated second echo pulse for a qualifying predetermined minimum amplitude;

initiating in response to receiving a qualifying first echo pulse the development of an on signal and the examination of a validating duration characteristic of the first echo pulse, a valid duration characteristic being a pulse having an amplitude which does not exceed a time varying amplitude greater than the predetermined minimum amplitude beyond a preset time;

conditioning partially in response to the first echo pulse being validated facilities for enabling the generation of a control pulse;

initiating in response to the validation of the first each pulse and the receipt of a qualifying associated second echo pulse the examination of validating test characteristics of a qualifying associated second echo pulse;

while causing the development of an off signal distinguishable from the on signal upon the validation of the first echo pulse and the receipt of a second echo pulse of a qualifying predetermined minimum amplitude from the opposite surface of the jacket with the second pulse occurring a predetermined time interval followIng the beginning of the first pulse, the time duration between the on signal and the off signal being related to the elapsed time between the receipt of the first echo pulse from the one surface and the receipt of the associated second echo pulse from the opposite surface;

measuring the elapsed time between the on signal and the off signal;

completing the conditioning of the enabling facilities in response to validating the associated second echo pulse to generate the control pulse; and

converting the measured elapsed time in response to the generation of the control pulse into a signal, the magnitude of which is proportional to the thickness of the elongated member.

3. The method of claim 2, wherein the test characteristic for completing the validation of the associated second echo pulse includes a peak amplitude in excess of a preset value and an amplitude which exceeds the qualifying minimum amplitude during a preset time.

4. The method of claim 2, wherein the elongated member is a cable jacket and the surfaces thereof defining the thickness of the jacket.

5. The method of claim 4, wherein the transmitting of ultrasonic pulses includes the exciting of a transducer immersed in a coupling medium and spaced from the cable.

6. The method of claim 5, wherein a pair of transducers are positioned in alignment with diametrically opposed points on the cable jacket, wherein the transducers are pulsed cyclically sequentially and which also includes the step of:

determining the eccentricity of the jacket jacket by processing the thickness measurements of the jacket at diametrically opposed points.

7. A method of discriminating among pulses received from a sensor to indicate the receipt of a sequence of valid associated pulses, which includes the steps of:

permitting after a predetermined delay the testing of pulses within a predetermined time interval;

enabling the testing of a threshold amplitude of a first pulse upon the receipt thereof;

developing an on signal upon receipt of a first pulse having the threshold amplitude;

while further comparing the amplitude of the first pulse with respect to a time varying parameter subsequent to a first preset time;

the threshold amplitude and an amplitude less than the time varying amplitude after the preset time being indicative of a valid first pulse;

then developing an off signal in response to the validation of the first pulse and the receipt of an associated second pulse having the threshold amplitude; and

then measuring a relationship between the on signal and the off signal;

enabling the testing of the threshold amplitude of the second pulse after a second preset time;

while further testing the second pulse for the validity thereof with respect to the peak amplitude thereof;

the validation of the first pulse and the validity of the second pulse with respect to exceeding the threshold amplitude beyond the second preset time enabling a control pulse generator;

the validation of the second pulse with respect to the peak amplitude thereof further enabling the generator;

the enabled generator being operated at the end of the predetermined time interval for causing the measured relationship to be converted into the measurement of another parameter as an indication of the receipt of a sequence of valid associated first and second pulses.

8. A method of determining the validity of each successive sequence of associated first and second echo pulses reflected from surfaces subjected to ultrasonic waves, a valid first pulse having a predetermined minimum amplitude and an amplitude which does not exceed a variable amplitude beyond a preset duration and a valid second pulse having a predetermined minimum amplitude which is also exceeded for a duration greater than a preset time and of generating a control pulse for valid sequences of associated echo pulses, which includes the steps of:

applying the first echo pulse and then the associated second ecHo pulse as inputs to a logic circuit;

testing the minimum amplitude level of the first echo pulses and then the second echo pulse compared to the validity level;

developing an on signal for a counter upon determining that the first echo pulse has the required level to cause the counter to begin counting;

causing a first echo pulse having the predetermined minimum amplitude to generate a control pulse to delay the examination of the existence of the first echo pulse after the preset value;

determining whether a first echo pulse exceeds a time varying amplitude after the preset value, a valid first echo pulse not exceeding the variable amplitude beyond the present time;

developing an off signal for the counter to discontinue the count upon validating the first echo pulse and receiving a qualifying second echo pulse;

enabling a verification circuit for an associated second echo pulse subsequent to validating a first echo pulse;

causing the enabled second echo pulse verification circuit to generate a holdoff and a validation pulse during the receipt of a second echo pulse;

testing the peak amplitude of the second echo pulse;

while determining the presence of the second echo pulse at the predetermined minimum amplitude following the holdoff pulse of the preset time;

enabling the device should the first pulse be valid and the second echo pulse exceed the minimum amplitude during a time greater than the preset time; and

further enabling the device for generating the control pulse should the peak amplitude of the second pulse be validated.

9. An apparatus for measuring the thickness of an article having first and second surfaces bounding a material of known acoustical properties, which comprises:

means for establishing a medium having known acoustical characteristics contiguous with the first surface of the article;

means for generating a longitudinal wave in the medium directed toward the first surface, one part of the wave being reflected therefrom back through the medium, as a first echo, the other part of the wave passing through the first surface and the article to the second surface, and a portion of the other part being reflected back through the article and the medium as a second echo;

means for generating control signals to establish first and second predetermined minimum intensity levels and a time varying intensity associated with and greater than the first predetermined minimum intensity;

means for establishing a first preset time associated with the first minimum intensity level and the time varying intensity and a second preset time associated with the second minimum intensity level;

means responsive to the control signals for generating a quantity proportional to the time interval between the first echo having the first predetermined minimum intensity and of which the intensity is less than the time varying intensity after the first preset time and a second echo which exceeds the second predetermined minimum intensity during the second preset time to establish the thickness of the article; and

means for inhibiting the generation of any quantity when either of the predetermined minima or duration characteristics do not exist.

10. An apparatus for measuring ultrasonically the thickness of successive sections of an elongated member in which an on signal and an off signal are developed in response to the receipt of first and second associated echo pulses of predetermined test characteristics from one surface and an opposite surface, respectively, of the member and in which relative motion is caused to occur between the successive sections of the elongated member and a source of ultrasonic pulses, which includes:

means for establishing a test period during which echo pulses are tested;

means for transmitting an ultrasonic pulse into the member and then receiving a first echo pulse reflected from the one surface of the member and then receiving an associated second echo pulSe from an opposite surface of the member;

means for excluding extraneous low level signals;

means for establishing qualifying predetermined minimum amplitudes for first and second associated echo pulses;

means for comparing the first echo pulse and then an associated second echo pulse with the associated qualifying predetermined minimum amplitude;

means for establishing first and second preset times associated with first and second associated echo pulses, respectively;

means for establishing a time varying amplitude associated with the first echo pulse and greater than the associated predetermined minimum amplitude;

means actuated in response to receiving a qualifying first echo pulse for initiating the development of an on signal and the comparison of the amplitude of the first echo pulse with a time varying amplitude after the first preset time;

means responsive to the validation of the first echo pulse as one having the qualifying predetermined minimum amplitude and not exceeding the time varying amplitude after the first preset time and the receipt of a qualifying associated second echo pulse for initiating the examination of validating test characteristics of a qualifying associated second echo pulse, means for causing the development of an off signal distinguishable from the on signal with the time duration between the on signal and the off signal being related to the elapsed time between the receipt of the first echo pulse from the one surface and the receipt of the associated second echo pulse from the opposite surface;

means for measuring the elapsed time between the on signal and the off signal;

means for generating a pulse to control the disposition of the measured elapsed time;

means responsive to the first echo pulse being validated for enabling partially the means for generating a control pulse;

means responsive to validating the associated second echo pulse for completing the enabling of the pulse generating means to cause the generating means to generate the control pulse; and

means responsive to the generation of a control pulse for converting the measured elapsed time into a signal the amplitude of which is proportional to the thickness of the elongated member.

11. The apparatus of claim 10, wherein the means for transmitting of ultrasonic pulses includes a transducer immersed in a coupling medium and spaced from the elongated member and means for exciting the transducer to transmit the ultrasonic pulse.

12. The apparatus of claim 10, wherein:

the means for initiating the examination of the qualifying associated second echo pulse including the examination of peak amplitude and a duration pulse characteristic of a qualifying associated second echo pulse;

a valid second echo pulse having at least a predetermined peak amplitude and the minimum amplitude occurring beyond the second preset time.

13. The apparatus of claim 12, wherein the means for transmitting of ultrasonic pulses includes a transducer immersed in a coupling medium and spaced from the elongated member and means for exciting the transducer to transmit the ultrasonic pulse.

14. The apparatus of claim 10, wherein the elongated member is a cable jacket and the measured elapsed time is converted into a signal proportional to the thickness of the cable jacket.

15. A logic circuit for validating a sequence of associated first and second pulses, which includes:

comparator means for testing the amplitude characteristics of the pulses;

means for establishing a time varying amplitude test parameter;

means for testing the duration of a first pulse having a required amplitude characteristic as compared to the time varying amplitude test parameter;

means for testing the duration of an associated second pulse, the testing of the duration of the second pulse initiating the comparator to test the peak amplitude of the second pulse;

means for developing an on signal upon receipt of a first puLse having the required amplitude and an off signal upon validation of the first pulse and during the receipt of a second pulse, a predetermined relationship of the on signal and the off signal being related to a parameter to be measured;

means for measuring the relationship between the on signal and the off signal;

means for causing the measurement to be further processed;

first logic means responsive to receipt of a first pulse having the required amplitude for initiating the developing means and the first pulse duration testing means;

second logic means responsive to a valid first pulse, a valid first pulse exceeding a required minimum threshold amplitude characteristic and not exceeding the time varying amplitude characteristic beyond a preset duration, and a valid associated second pulse, a valid second pulse having a required amplitude characteristic occurring at least within a preset duration, for enabling the causing means;

third logic means responsive to the validation of a first pulse and to the receipt of a second pulse of required minimum amplitude for enabling the second logic means, for developing the off signal and for operating the second pulse peak amplitude verification means;

the validation of the second pulse with respect to peak amplitude further enabling the causing means; and

means beginning a test cycle for operating the causing means when the causing means is enabled at the end of the test cycle and for conditioning the circuit for another cycle of operation.

16. An apparatus for measuring ultrasonically the thickness of a cable jacket, which includes:

means for cyclically transmitting ultrasonic pulses into engagement with the outwardly and the inwardly facing surfaces of the cable jacket;

means for receiving a first echo pulse reflected from the outwardly facing surface and an associated second echo pulse from the inwardly facing surface;

first comparator means capable of testing pulses of either polarity for testing an amplitude characteristic of the first echo pulse and responsive to a valid first echo pulse for testing an amplitude characteristic of the second echo pulse, a valid first echo pulse having a predetermined minimum amplitude at threshold, and subsequently exceeding a time varying threshold greater than the predetermined minimum only within a preset duration, a valid second echo pulse having a minimum amplitude at threshold, the minimum amplitude also occurring within a duration in excess of a preset time, and a peak amplitude greater than a predetermined value;

logic means when operated for developing an on signal and responsive to being disabled for developing an off signal with the time duration between the on signal and the off signal being related to the elapsed time between the transmission of a pulse into the outwardly facing surface of the jacket and receipt of a corresponding echo pulse at the outwardly facing surface after reflection from the inwardly facing surface;

means for measuring the time between the on signal and the off signal;

a monostable device which when operated causes the time between the on signal and the off signal to be recorded;

means delayed a predetermined time for enabling the comparator means and for enabling the monostable device;

means connected to the first comparator means and rendered effective by the receipt of a first echo pulse having a required predetermined minimum amplitude at threshold for causing the generation of an amplitude of time varying intensity and for causing the first comparator means to compare the amplitude of the first echo pulse with the time varying amplitude to validate a duration characteristic of the first echo pulse by determining that the varying intensity is not exceeded beyond a preset time;

means interposed between the first comparator means and the first echo pulse validating means and responsive to the receipt of a first echo pulse having the required minimum amplitude for operating The first echo pulse echo validating means and for causing the developing means to develop the on signal;

means for validating a duration characteristic of the second echo pulse;

second comparator means for testing the peak amplitude of the second echo pulse during the testing of a duration characteristic thereof;

switching means connected to the first pulse duration test means and the second pulse duration test means and rendered effective by the validation of the first echo pulse and by the receipt of the second echo pulse spaced from the first for operating the second pulse validation means while causing the logic means to develop the off signal and responsive to an invalid first pulse for inhibiting the operation of the second pulse duration test;

the means for validating the duration of the second echo pulse causing the second comparator means to be operated;

switching means rendered effective by the validation of the first echo pulse and the second echo pulse having at least the predetermined duration test characteristic for further enabling the monostable device;

the validation of the peak amplitude of the second pulse still further enabling the monostable device to operate the monostable device and cause the count to be stored; and

sequence control means for causing the transmitting means to operate and for conditioning the apparatus for each successive cycle of operation.

17. An electrical logic circuit for developing an on signal and an off signal with the measured time duration between the on signal and the off signal being related to the elapsed time between the receipt of a valid first echo pulse reflected from a first surface and defined as exceeding a predetermined threshold value and having an amplitude not exceeding a variable threshold beyond a preset time, and the receipt of a valid second echo pulse reflected from a second surface, spaced from the first surface, and defined as having at least a predetermined peak amplitude and a preset threshold value which is exceeded within another preset time duration, and for causing the measured time duration associated with a valid sequence of valid first and second pulses to be processed, which includes:

a first comparator for amplitude detection of pulses of either polarity exceeding a predetermined level;

a second comparator for determining if the peak amplitude of the second echo pulse is greater than a predetermined value;

the comparators having an input signal applied thereto in response to the receipt of the echo pulses;

means operated after a predetermined delay for enabling the comparators;

a first bistable element normally in a first state;

first echo pulse verification means connected to the first bistable element for determining the validity of the first pulse with respect to a duration characteristic and that the end of the first echo pulse is spaced from the beginning of the second echo pulse;

first logic means connected to the first bistable element and responsive to like signals being impressed on the inputs thereof for developing the on signal and responsive to the disabling thereof for developing the off signal;

means for enabling the first logic means after the delay;

second logic means interposed between the first comparator and the first bistable element and responsive to an echo pulse having the required threshold amplitude impressed upon the input of the first comparator for causing the first bistable element to assume a second state to operate the enabled first logic means and cause the first echo pulse verification means to be operated;

means connected to the first echo pulse verification means and to the first comparator and responsive to the operation of the first echo pulse verification means for causing the comparator to compare the amplitude of the first echo pulse with a time varying amplitude level after a preset time;

third logic means connected to the output of the second logic means associaTed with the first comparator and connected to the first echo pulse verification means;

second echo pulse verification means for validating the associated second echo pulse with respect to the threshold amplitude thereof being exceeded during the other preset time, the second echo pulse verification means being effective during the other preset time for enabling the second comparator to test the peak amplitude of the second echo pulse;

means for causing the time interval to be processed;

means connected to the third logic means and the second echo pulse verification means and operated upon the inputs thereof being impressed simultaneously with like signals caused by a valid first pulse and a second pulse exceeding the predetermined threshold level within a preset duration and connected to the causing means for enabling the causing means, a first echo pulse which exceeds the time varying amplitude level after the preset time causing the second logic means to apply a signal to the third logic means such that the enabling means connected to the third logic means does not enable the causing means, further, a second echo pulse which does not exceed the predetermined level after another preset time causing the second pulse verification means to apply a signal to the enabling means connected thereto such that the causing means is not enabled;

a second bistable element connected to the developing means interposed between the third logic means and the second echo pulse verification means and when in a second state for causing the developing means to develop the off signal and for operating the second echo pulse verification means; and

a third bistable element interposed between the second comparator and the causing means and responsive to being set to a second state for further enabling the initially enabled causing means to cause the time interval to be converted into a measurement of the distance between the two surfaces, the third bistable element being set to a second state by the second comparator receiving a second echo pulse having a peak amplitude greater than the preset value;

the causing means being operated by the comparator enabling means after a predetermined time greater than the combined duration of the first and second echo pulses plus the separation therebetween.

18. A circuit for measuring ultrasonically the thickness of a jacket of a cable being advanced along a predetermined path with a transducer being spaced from the cable and at least partially immersed in a coupling medium, which includes:

means initiating a cycle of operation for causing the transducer to emit ultrasonic waves into engagement with the outwardly facing surface of the cable jacket whereupon successive first echo pulses are reflected, portions of the ultrasonic waves being propagated through the jacket and upon engagement with the interface of the inwardly facing surface causing successive associated second echo pulses to be reflected;

a first comparator for determining if threshold amplitude of successive ones of the associated pairs of first and second echo pulses exceeds a predetermined minimum level;

a second comparator for determining if the peak amplitude of the successive ones of the second echo pulses is at least a predetermined value;

a first monostable multivibrator which when operated generates a pulse having a predetermined time interval during which a valid sequence of first and second echo pulses may occur, the first multivibrator being connected to the first comparator such that the generation of the pulse having the predetermined time interval enables the first comparator;

a second monostable multivibrator interposed between the cycle initiation means and the first multivibrator for delaying the generation of the pulse having the predetermined time interval;

a third monostable multivibrator responsive to the validation of a sequence of associated first and second echo pulses for generating a control pulse;

a valiD first pulse exceeding the predetermined minimum amplitude and an amplitude not exceeding a time varying amplitude greater than the minimum amplitude beyond one preset time;

a valid second pulse having at least a predetermined peak amplitude and exceeding the minimum threshold amplitude within another preset time;

a first logic element connected to the first comparator and operated by a signal impressed thereon by the first comparator in response to receiving a first echo pulse exceeding the required minimum amplitude level;

a first echo pulse verification circuit connected to the output of the first logic element for generating a holdoff pulse of the one preset time;

means connected to the first echo pulse verification circuit and to the first comparator and responsive to the trailing edge of the holdoff pulse for generating a time varying amplitude level, a first echo pulse which exceeds the time varying level beyond the one preset time causing the first logic element to be operated;

a second logic element connected to the first logic element for developing an on signal and an off signal with the time duration between the on signal and the off signal being related to the elapsed time between the receipt of a first echo from the outwardly facing surface of the cable jacket and the receipt of an associated second echo pulse from an opposite surface of the material;

means for measuring the time between the on signal and the off signal, the second logic element being operated when the first logic element is operated for developing an on signal to control the measuring means to begin counting;

a third logic element having inputs connected to the first logic element and to the first echo pulse verification circuit and being enabled by the first echo pulse verification means, the third logic element caused to be operated when the first logic element is being operated subsequent to the holdoff pulse;

a fourth logic element having inputs connected to the first logic element and to the first echo pulse verification circuit;

a second echo pulse verification circuit connected to the fourth logic element and connected to the second comparator for impressing a signal thereon to enable the second comparator;

a first bistable device switched in response to the third logic element being operated for discontinuing the operation of the circuit because of the extended duration of the first echo pulse, and switched responsive to the third logic element being in an unoperated state for conditioning the fourth logic element;

a fifth logic element interposed between the bistable means and the third monostable multivibrator and connected to the second echo pulse verification circuit, the fifth logic element being conditioned by the bistable device when switched responsive to the third logic element being unoperated;

the fourth logic element responsive to the first logic element being operated for impressing a signal on the second verification circuit and on the developing means to cause the developing means to develop an off signal to discontinue the count, the count corresponding to the time between the pulses and related to the jacket thickness;

the impression of the signal on the input of the second verification circuit causing the generation of a holdoff pulse of the other preset time;

a second bistable device normally in a first state interposed between and connected to the second comparator and to the third monostable multivibrator;

the enabled second comparator operated by a second echo pulse of the required peak amplitude causing the second bistable device to assume a second state, the switching of the second bistable device to the second state causing a signal to be impressed on the third multivibrator to enable the third multivibrator;

a sixth logic element connected to the second verification circuit and to the first logic element, the sixth logic element operated by the simultaneous impression of like signals from the second verification circuit and from the output of the first logic element to establish that the second pulse exceeds the minimum amplitude during a time interval in excess of the other preset time, the operation of the sixth logic element causing the enabled fifth logic element to be operated, the operation of the fifth logic element further enabling the third multivibrator;

the trailing edge of the pulse having the predetermined time interval operating the enabled third monostable multivibrator to cause the third monostable multivibrator to generate the control pulse which causes the count to be converted to a measurement of thickness; and

a pulse repetition sequencer for cyclically operating the initiating means and for resetting the elements of the circuit to condition the circuit for further cycles of operation.

19. An apparatus for measuring the thickness of an article having first and second surfaces bounding a material of known acoustical properties, which comprises:

means for generating a longitudinal wave in a medium having known acoustical chracteristics contiguous with the first surface of the article, the wave being directed toward the first surface, one part of the wave being reflected therefrom back through the medium as a first echo, the other part of the wave passing through the first surface and the article to the second surface, and a portion of the other part being reflected back through the article and the medium as a second echo;

means for generating control signals to establish first and second predetermined minimum intensity levels and a time varying intensity associated with a greater than the first predetermined minimum intensity;

means for generating a quantity proportional to the time interval between a first echo having the first predetermined minimum intensity and a duration beyond which the intensity is less than the time varying intensity greater than the first predetermined intensity and a second echo having the second predetermined minimum intensity and a duration during which the second predetermined minimum intensity is exceeded to establish the thickness of the article; and

means for inhibiting the generation of any quantity when either of the predetermined minima or duration characteristics do not exist.