CA1239685A

CA1239685A - Microwave detection system

Info

Publication number: CA1239685A
Application number: CA000472513A
Authority: CA
Inventors: Norbert E. Ii Gelston
Original assignee: U S Technology Corp
Current assignee: U S Technology Corp
Priority date: 1984-02-13
Filing date: 1985-01-21
Publication date: 1988-07-26
Also published as: JPH0619265B2; JPS60201205A

Abstract

ABSTRACT OF THE DISCLOSURE

A microwave detection system is con-structed which is useful for detecting the presence or absence of a target within a micro-wave transmission line.

Description

, The present invention relates genera]]y to a microwave detection system and, in particu]ar, to a microwave detection system th~t is capab]e of determining the continuity or discontinuity of a target at re]ative]y c]ose range.
Most detection systems used in industry to ; automatica]]y determine the presence or the absence of a too] or a continuous workpiece such as a fine]y drawn fi]ament or a thin coating do not have the sensitivity to 10 accurate]y and repeated]y detect the condition of such too]
or workpiece when the e]ement is re]ative]y small. For examp]e, many ho]es that are present]y being dri]]ed in printed circuit boards are 0 0]3" or less in diameter. The boards are usua]]y manufactured in high]y automated -15 machines capab]e of carrying out a number of dri]]ing operations at one time. A machine operator norma]ly wi]l be responsib]e for ]oading and un]oading stacks of boards into and out of a number of machines. According]y, he cannot visua~]y inspect the dri]]ing too]s with any degree of -~

., .

12396~5 accuracy or consistency. Fai~ure to detect a broken too]
can thus ]ead to the production of a ]arge number of defective pieces that must be either scrapped or reworked - at added expense ~ 5 Summar of the Invention :~ Y
3 The present invention provides a microwave detection system which is capab~e of determining the continuity or discontinuity of a target within the microwave sys~tem. Such targ-~ts include too]s, wires, - 10 surface discontinuities. sizes, and vo]ume capacities. The microwave detection system of the present invention is particu]ar]y usefu] for detecting the presence or absence of a target at re]ative~y c~ose range. This abi]ity is particu]ar]y usefu] for detecting the presence of a 15 particu~ar too] or portion thereof in an automated machine, thereby providing the capabi]ity to detect breakage of such a too] during the course of an automated operation.
The microwave detection system of the present invention can be readi]y constructed and retrofitted to an 20 existing machine or added to the structure of a new]y produced machine.
The detection system of the present invention provides a broad sensitivity range which constitutes an improvement both as to sensitivity and breadth compared to lZ39685 those devices heretofore known in the art.
The microwave detection system of the present invention is particu]ar~y usefu] because it is re]ative]y insensitive to outside noise, debris and abrasion which frequently interfere with detection systems heretofore known in the art.
According to one embodiment of the present invention, the microwave detection system comprises a microwave transmission ]ine having an opening within which the target is situated, means for transmitting a microwave signa] down the transmission ]ine toward the target and for receiving a ref]ected return wave signa] trave]ing the opposite direction, positioned at one end of the transmission ]ine, a mixer circuit connected to said system for heterodyning the tran~mitted and ref]ected signa]s to provide an output signa] having a component which indicates the presence or absence of the target within the 3, transmission ]ine opening and detecting means coup]ed direct]y to the output of the mixer circuit ~or comparing the component to a predetermined ]eve] to determine the presence or absence of the target. The position of the transmission ]ine opening re]ative to the transmitting and receiving means is se~ected to optimize the signa] strength from the mixer circuit.
According to another embodiment of the present 12396~5 invention, the microwave detection system comprises a waveguide as the microwave transmission ]ine having an opening therein in which the target such as a dri~l bit, wire, surface area, or the ]ike is positioned qo that it is within the waveguide opening. The means for transmitting the microwave signa] down the waveguide toward the target and for receiving reflected return wave ~ignals may be a transceiver modu]e connected to the waveguide at one end thereof. The mixer circuit heterodynes the transmitted and ref]ected sig,na]s so that the''''~mp]itude and phase shift , changes produced by the presence of the target are readi]y discernib]e.
According to another embodiment, the mixer circuit heterodynes the transmitted and reflected signa]s down to the baseline and the amp~itude and phase shift-changes produced by the presence of the target are readi]y discernible as a D,C, output signa~ component. ~ comparator .~ may be direct]y coup]ed to the mixer circuit in order to sense the D.C. component and produce an output whi'ch indicates either the presence or absence of the target within the transmission ]ine or waveguide.
According to a further embodiment of the present invention, a shorting stub is disposed within the microwave transmission ]ine behind the target in order to tune the 25 system for optimum detection of the presence or absence of --- lZ39685 the target. Preferab]y, the system is tuned for each target.
The microwave detection system of the present invention is particu]ar]y usefu~ for detecting the breaking of a portion of the target, such as, for examp]e, the breaking of a portion of a dri]l bit.
According to a further embodiment of the present invention, a shorting stub is positioned a distance N ~/4 or N ~/8 from the waveguide opening wherein N is any integer and A is the wave]ength of the transmittad signa].
According to a further embodiment of the present invention, the transceiver comprises a Gunn diode and a Schottky diode.
According to a further embodiment of the present invention, an amp]ifier is operative~y positioned between the mixer and comparator for amp]ifying the output signa].
According to a further embodiment of the present invention, the transmission ]ine opening is ~ocated a distance N ~4 or N ~8 from the transmitting and receiving means where N is any integer and Ais the wave]ength of the transmitted signa].
According to a further embodiment of the present invention, the transmission ]ine is a waveguide having an opening centered in the waveguide and a shorting stub is positioned in the waveguide behind the target.

According to a further embodiment of the present invention, the shorting stub is positioned re]ative to the waveguide opening to optimize the signa] strength from the mixer circuit.
According to a further embodiment of the present ; invention, the waveguide opening is 0 5" or less.

The present invention a]so inc]udes à method of detecting the presence or absence of a target within the microwave transmission ]ine of the microwave detection system of the present invention which comprises positioning the target within the opening in the transmission ]ine, transmitting a microwave signal down the transmission ]ine toward tne target, receiving a return wave signa] ref]ected - back along the transmission ]ine in the opposite direction from the target, heterodyning the transmitted signal and the return signal to provide an output signa~ having a component indicative of the presence or absence of the target within the opening, and comparing the component of the signal to a given va]ue to determine the presence or absence of the target~ The position of the transmission ]ine opening re]ative to the transmitting and receiving means is se]ected to optimize the signa] strength from the ~Z3968~

mixer circuit The method of the present invention may inc]ude the further step of positioning a shorting stub with the transmission ]ine behind the target.
According to a further embodiment of the method of the present invention, the stub is positioned a distance of N A/4 or N ~8 from the target where N is a positive integer and ~ lS the wavelength of the transmitted signa].
Accordïng to a further embodiment of the method of the present invention, the target is positioned immediate]y adjacent to the end of the transmission ]ine and is ~ocated a distance of N ~/4 or N ~8 from the point of microwave propagation where N is a positive integer and ~ is the wave]ength of the transmitted signal.

According to a further embodiment of the method of the present invention, there may be a further step of amp]ifying the heterodyned signa].
According to a further embodiment of the method of the present invention, the transmission ]ine is a waveguide having an opening centered in the waveguide and a shorting stub is positioned in the waveguide behind the target.
According to a further embodiment of the method of the present invention, the shorting stub is positioned re]ative to the waveguide opening to optimize the signa]
streng~h from the mixer circuit.

"

-~Z39685 Description of the Drawings Fig 1 is a partia~ side view of a machine too]fitted with the present invention;
Fig 2 is a schematic view showing one embodiment of the invention for detecting a target in the form of a dri]];
Fig 3 is a section through the waveguide of the broken took detecting system shown in Fig. 2 i~ustrating the E-fie~d intensity in the waveguide when the target is ,~ . ~ . r removed therefrom;
Fig~ 4 is a~so a section through the waveguide i~]ustrating the E-fie]d intensity in the waveguide when the target is present; and Fig 5 is another embodiment of the invention showing the target positioned at the end of the waveguide The microwave detection system according to the present invention is suitab]e for detecting a wide variety of targets but is particu]ar]y usefu] for detecting re]ative]y smal~ targets such as dri~ bits at re~ative~y - 20 c]ose range Thus the microwave detection system according to the present invention is particu]ar]y usefu] in combination with an automatic dri]]ing machine wherein a number of dri]]ing operations are being conducted simu]taneous]y as the microwave detection system according to the present invention is capab]e of detecting the . ~

lZ396E~5 g - presence or absence of a plura]ity of targets With reference to the Figures automated dri]]ing machine 9 is uti]ized to dri]l extremely sma]l holes in a stack of printed circuit boards 10 The machine includes at least one pressure foot assembly 11 in each dril]ing station that houses a retractable drill 12 While the present invention is particu]arly usefu~ for drill targets having a diameter that is at or be]ow 0.013 it is also usefu] for larger size targets and planar surfaces as well.
10The microwave detection system of the present invention is shown as 13 retrofitted to the pressure foot assemb~y of the machine. Since the system does not require physical contact it can be mounted in any convenient location to detect the continuity or the discontinuity of the target. the detection system includes a waveguide 15 and a transceiver module 17. The waveguide passes through the work zone of the drilling station and is terminated by an adjustab]e shorting stub 18 of the type that is we~l known and used in the art. The waveguide shown is rectangular in cross-sectional form and contains two smal~
coa]igned ho]es 19-19 centered in the top broadwal] 20 and the bottom broadwa]l 21 of the waveguide. Normally the elongated shank of the drill 12 is contained in the holes and sufficient c]earance is provided therebetween to enab]e the dri]] to move vertical]y towards and away from the - lZ39S85 work. The dril] in Fig. 1 is shown in a broken condition a~though in operation the dri]~ wou]d be within the waveguide for detection.
With reference to Figure 2 transceiver modu]e 17 contains a ]oca~ oscil~ator 23 which may be a Gunn diode that is positioned at one end of the waveguide to propagate a microwave signal down the waveguide toward the target which for illustration is a dri~]. The adjustab]e shortening stub is positioned in the waveguide behind the _.; r_ .~ r -10 target and is arranged to reflect a we]] defined return signal back toward the module. The receiver section of the modu]e inc]udes a detector such as a Schottky diode 25 and a mixer circuit 26. The return wave signa] is picked up by the diode 25 and is applied to the mixer a]ong with a samp]e of the transmitted signa]. The two input signals are heterodyned down to baseband ln the mixer to provide an ieasily detectab]e D.C. output signa] indicative of the amp]itude rather than the frequency of the return signal.
the mixer output is coupled direct]y via ]ine 29 to a comparator 30 through amp]ifier 31 so that on~y the D C.
component of the output signa] is app]ied to the comparator.
The microwave system operates on the princip]e that the presence of a continuous target such as a fine dri~]
in or near the end of the waveguide at a predetermined ~Z39685 location produces a change in the standing wave ratio (SWR) of the transmitted signal. This change in the SWR is capab]e of being detected by the recelver to provide a signal that is indicative of the target condition. The comparator threshold can be set to detect either the presence or the absence of a target. Although the pre~sent system resemb]es a dopp]er radar, it shou]d be apparent that it does not operate on the doppler effect in that the frequency component of the output signa] is not uti]ized in the comparator circuit. As will be explained in greater detai] below, the present system is adjusted to provide a , clear indication of changes in the amp]itude of the D.C.
output signal component to determine the presence or absence of a specific target rather than its movement toward or away from the transceiver.
The waveguide dimensions are specifica]ly adjusted for the particular system application to provide for maximum detection sensitivity so long as the width of the waveguide permits transmission. Variations in the height of the waveguide wi~l affect the power ]evel of the transmissions and thus affect the overa]] sensitivity of the system. The ]ocation of the shorting stub is a]so important. Preferab]y the stub is ]ocated at the N ~4 or N
A/8 distance from the center]ine of the target where A is the wavelength of the transmitted signal and N is some ~Z39G85 positive integer. By so positioning the shorting stub the waveguide is tuned to the transmitted signal to allow detection to be carried out on the linear part of the return signa].
As noted the local osci]lator propagates an electromagnetic wave down the hollow rectangular waveguide which is terminated at the shorting stub. The openings 19 for the tool which are contained in the broadwalls of the w~veguide are minimized to provide sufficient clearance to allow the tool to move axially therein without touching the waveguide. Insertion of a tool into the waveguide causes a change in the magnitude and phase of the reflected wave incident at the transceiver 17. The transceiver heterodynes the signal down to baseband so that the amplitude of the signal D.C. component translates to an easi~y detectable offset change. The cross-sectional dimensions of the waveguide and the operating frequency of the local oscillator are scaled to any values that are compatible with dominant mode on]y wave propagation. The system is 20 tolerant in regard to the exact positioning of the tool opening 19 and the stub positioning; however it is preferred that these locations be optimized for specific app]ications The E-field intensity within the waveguide is shown in Figs 3 and 4 for dominant mode propagation. The norma~

~23~;~35 E-fie]d profile across the cross section of the waveguide : is shown in Fig 3 with the maximum strength of the fie~d occurring at centra] axis 22 of the waveguide and fa]]ing off symmetrical]y to either side thereof. When a tool 12 is positioned in the waveguide it shorts the E-fie~d at the axis and produces a pronounced change in the profi~e of the E-fie~d as graphically disp~ayed by the arrows in Fig 4.
By se~ecting the operating frequency and the waveguide dimensions for dominant mode operation on~y the too~ acts ]ike a shorting stub to an incident wave and produces a discernible change in the phase and amplitude of the return wave.
Let the tota~ E-field (incident p~us reflected waves) at the transceiver be denoted by VE which can be written VE = ~O [~expax)+ (rexp -ax~ ;
where:
VO is the incident wave at the lo~d;
~ is the complex propagation constant for the waveguide r is the complex reflection coefficient of the load x is the distance from VE to the load A short section of waveguide can be regarded as being without ~oss if the too] aperature is sufficient~y small Therefore o becomes:

-14 - ~2~96 o= j~ / [_] - [--] o where: ~ is the operating frequency in rad/sec;
c is the speed of light in free space; and a-is the width of the waveguide broadwall.
If the transceiver unit generates a simple sinusoidal signal A exp j~t at the central axis, equations ~l) and (2) yield:

VE = ~ exp j~t) (exp - jBx) ~ (exp jBx)+(rexp -~Bx)~ (3) The load reflection coefficient is now equal to -1 since either the shorting stub or the tool will present a short circuit to the incident E-field. Equation (3) thus becomes:
VE = 2A(sin ~x) Cexp j (~ t - ~x + ~/2) (4) The transceiver, because it is directly coupled to the comparator, will only detect the D.C. related component of equation (4) which takes the form: ~
VEmax = ~cos(~t + ~) (5) where: ~ = 2A sin x; and = ~/2 - ~x ~- l~en the tool is removed from the waveguide:
x ~ lt + ls (6?
and when the tool is present in the waveguide:

x = ~t l7) Thus, equations (5) - (7) define the amp~itude and the phase at the transceiver for either the presence or the absence of the too], With reference to Fig~ 2, the present invention ~Z39685 wi]] be exp]ained in regard to a simp~e homodyne , transceiver. The baseband mixer D.C. component output of the homodyne transceiver is given by:

K
D.C. cos(~ - 0) (8) where: ~ is an arbitrary phase constant; and K is the mixer conversion constant.
Using equations (5) through (8), the following relationship is o~tained for the condition where the tool is absent from the waveguide:

SOD C = KAsin ~(lt+15~ cos ~/2 -B(lt + ls3 - ~ (9) and when the tool is present:
SOD.C. KAsin(Blt) Ccos(7r/2 ~ glt ~ ~3)] (10) i As can be seen, by using either equation (9) or (10) the physica] parameters of the target detection system can be easi]y optimized for any given app]ication so that the output of the comparator can be used to either automatica]]y shut down the machine and/or sound an a]arm indicating a discontinuation in the target.
Because of certain size constraints, there may be times when the target cannot be passed through the waveguide. In this case, the shorting stub is removed from the waveguide and the dista~ end of the guide is opened to ~Z396~5 atmosphere As illustrated in ~ig. 5 the target which in this case is illustrated by fine wire 50 is positioned at the opening in close proximity with the end face of the waveguide. The target is ]ocated a distance .J4 or ./8 times some positive integer from the output of the transceiver. As explained above in greater detai] the parameters of the system can be easi~y optimized for the x = ~t mode of operation. In this application the signal sent out by t!ransceiver 52 along the waveguide 51 is somewhat attenuated because of the absence of the shorting stub however the return is detectable so that the condition of the target can be ascertained accurately.
Other and further uses and modifications of the - microwave detection system of the present invention will be more fully appreciated by those skilled in the art.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH
AN EXCLUSIVE PROPERTY OR PRIVILEGE IS
CLAIMED ARE DEFINED AS FOLLOWS

1. A microwave direction system useful for detecting the presence or absence of a target within a microwave transmission line which comprises a microwave transmission line having an opening within which the target is situated and means for transmitting a microwave signal down the transmission line toward the target and for receiving a reflected return wave signal traveling the opposite direction positioned at one end of the transmission line and forming a transmitter/receiver means connected to said system for transmitting and receiving signals to provide an output signal having a component which indicates the presence or absence of the target within the transmission line opening and processing means coupled directly to the output of said transmitter/receiver means for comparing the component to a predetermined level to determine the presence or absence of the target, the position of the transmission line opening relative to the transmitter/receiver means is selected to optimize the signal strength from the transmitter/receiver means and is less than the length of the transmission line.

2. The system as defined in claim 1 wherein the microwave transmission line opening is centered in the line and a shorting stub is positioned in the line behind the target.

3. The system as defined in claim 1 wherein the transmitter/receiver means is a transceiver.

4. The system according to claim 2 wherein the shorting stub is positioned a distance N .lambda./4 or N .lambda./8 from the transmission line opening wherein N is any integer and is the wavelength of the transmitted signal.

5. The system as defined in claim 3 wherein the transceiver comprises a Gunn diode and a Schottky diode.

6. The system as defined in claim 1 which further comprises an amplifier operatively positioned between the transmitter/receiver means and the processing means for amplifying the output signal.

7. The system as defined in claim 1 wherein the transmission line opening is located a distance N .lambda./4 or N/.lambda.8 from the transmitter/receiver means where N is any integer and .lambda.is the wavelength of the transmitted signal.

8. The system as defined in claim 1 wherein the transmission line is a waveguide having an opening centered in the transmission line and a shorting stub is positioned in the waveguide behind the target.

9. The system as defined in claim 8 wherein the shorting stub is positioned relative to the waveguide opening to optimize the signal strength from the transmitter/receiver means.

10. The system as defined in claim 9 wherein the waveguide opening is 0.5" or less.

11. A method of detecting the presence or absence of a target within the microwave transmission line of the system of claim 1 which comprises positioning the target within the opening in the transmission line, transmitting a microwave signal down the transmission line in the opposite direction from the target to the transmitter/receiver means to provide an output signal having a component indicative of the presence or absence of the target within the opening, and comparing the component of the signal to a given value to determine the presence or absence of the target, the position of the transmission line opening relative to the transmitter/receiver means is selected to optimize the signal strength from the transmitter/receiver means and is less than the length of the transmission line.

12. The method as defined in claim 11 which includes the further step of positioning a shorting stub in the line behind the target.

13. The method as defined in claim 11 wherein the stub is positioned a distance of N .lambda./4 or N .lambda./8 from the target where N
is a positive integer and .lambda. is the wavelength of the transmitted signal.

14. The method as defined in claim 11 wherein the transmission line opening is located a distance N .lambda./4 or N .lambda./8 from the transmitter/receiver means where N is a positive integer and is the wavelength of the transmitted signal.

15. A method as defined in claim 11 which includes the further step of amplifying the transmitter/receiver means signal.

16. A method as defined in claim 11 wherein the transmission line is a waveguide having an opening centered in the waveguide and a shorting stub is positioned in the waveguide behind the target.

17. The method as defined in claim 11 wherein the shorting stub is positioned relative to the waveguide opening to optimize the signal strength from the transmitter/receiver means.