MICROWAVE DETECTION SYSTEM

Abstract: A microwave detection system is constructed which is useful for detecting the presence or absence of a target with­ in a microwave transmission line.

Description: The present invention relates generally to a microwave detection system and, in particular, to a microwave detection system that is capable of determining the continuity or discontinuity of a tool of a machine tool at relatively close range. Most detection systems used in industry to automatically determine the presence or the absence of a tool or a continuous workpiece such as a finely drawn filament or a thin coating do not have the sensitivity to accurately and repeatedly detect the condition of such tool or workpiece when the element is relatively small. For example, many holes that are presently being drilled in printed circuit boards are 0.013" (0.33 mm) or less in diameter. The boards are usually manufactured in highly automated machines capable of carrying out a number of drilling operations at one time. A machine operator normally will be responsible for loading and unloading stacks of boards into and out of a number of machines. Accordingly, he cannot visually inspect the drilling tools with any degree of accuracy or consistency. Failure to detect a broken tool can thus lead to the production of a large number of defective pieces that must be either scrapped or reworked at added expense. "IEEE Transactions on microwave theory and techniques, vol. MTT-32, No.1, January 1984, pages 120-122, IEEE, New-York, US ; P.I. Somlo : Radar-echo location of Conducting spheres in waveguide" does not relate to an inspection or detection system for the tool of a machine tool. US-A-3 980 974 teaches a microwave transmitter and a receiver which cooperate with a mixer to generate a Doppler signal. Summary of the Invention The present invention provides a microwave detection system which is capable of determining the continuity or discontinuity of a tool of a machine tool within the microwave system. The microwave detection system of the present invention is particularly useful for detecting the presence or absence of a tool of a machine tool at relatively close range. This ability is particularly useful for detecting the presence of a particular tool or portion thereof in an automated machine, thereby providing the capability to detect breakage of such a tool during the course of an automated operation. The microwave detection system of the present invention can be readily constructed and retrofitted to an existing machine or added to the structure of a newly 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 those devices heretofore known in the art. The microwave detection system of the present invention is particularly useful because it is relatively insensitive to outside noise, debris and abrasion which frequently interfere with detection systems heretofore known in the art. Accordingly, the present invention relates to a detection system detecting the presence or absence of a tool of a machine tool, this tool being movable into and out of a detection zone, comprising (a) a microwave transmission line, (b) a transmitter operative for transmitting microwaves along the transmission line in one direction toward said zone, (c) a receiver operative for receiving a first return signal, having a first magnitude when the tool is located in the zone and reflects the transmitted microwaves incident thereon in the opposite direction, and also for receiving a second return signal having a different second magnitude when the tool is located out of the zone, and (d) detecting means operative for comparing the magnitude of either the first or the second returnsignal with a threshold, and for generating an output signal indicative of the presence or absence of the tool relative to the zone. According to one embodiment of the present invention, the transmission line is a rectangular waveguide having a top broadwall formed with a first opening and a bottom broadwall formed with a second opening that is aligned with the first opening and wherein the tool is an elongated drill bit which is positionable in the waveguide so as to extend through both openings. According to another embodiment, a shorting stub is positioned within the waveguide behind the bit as considered along said one direction. According to a further embodiment, the microwaves have a predetermined wavelength λ and the distance between the bit and the stub is N λ / 4 or N λ / 8 , where N is an integer, thereby optimizing the magnitude of either the first of the second return signal. According to a further embodiment of the present invention, the transmission line is a rectangular waveguide having an open end and the tool is positioned at this open end in close proximity with the end face of the waveguide. According to a further embodiment of the present invention, the microwaves have a predetermined wavelength λ and the distance between the receiver and the tool is N λ / 4 or N λ / 8 , where N is an integer, thereby optimizing the magnitude of either the first or the second return signal. According to a further embodiment of the present invention, the transmitter is a Gunn diode, the receiver is a Schottky diode and the detecting means includes a mixer, operative for heterodyning either the first or second return signal with a sample of the microwaves transmitted by the Gunn diode to generate a D C signal indicative of the amplitude of either the first or the second return signal, and comparator to which the D C signal is applied for comparing the magnitude thereof with said threshold. According to a further embodiment of the present invention, the openings are located on the waveguide relative to the transmitter and the receiver at positions selected to optimize the amplitude of the return signal. According to a further embodiment of the present invention, the waveguide opening is 0.5" (12,7 mm) or less. The present invention also includes a method of detecting the presence or absence of a tool of a machine tool, this tool being movable into and out of the detection zone of the system of claim 1, comprising the steps of : (a) providing a microwave transmission line, (b) transmitting microwaves along the transmission line in one direction toward said zone, (c) receiving a first return signal having a first magnitude when the tool is located in the zone and reflects the transmitted microwaves incident thereon in the opposite direction and also receiving a second return signal having a different second magnitude when the tool is located out of the zone and (d) comparing the magnitude of either the first or the second return signal with a threshold and generating an output signal indicative of the presence or absence of the tool relative to the zone. The method of the present invention may include the further step of forming the transmission line as a rectangular waveguide having a top broadwall with a first opening and a bottom broadwall of the waveguide with a second opening that is aligned with the first opening and wherein the tool is an alongated drill bit which is positionable in the waveguide so as to extend through both openings. According to a further embodiment of the method of the present invention, it includes the step of positioning a shorting stub within the waveguide behind the bit as considered along said one direction. According to a further embodiment of the method of the present invention, it includes the step of generating the microwaves with a predetermined wavelength λ and wherein the distance between the bit and the stub is N λ / 4 or N λ / 8 , where N is an integer, thereby optimizing the magnitude of either the first or the second return signal. According to a further embodiment of the method of the present invention, there may be a further step of forming the transmission line as a rectangular waveguide with an open end and wherein the tool is positioned at this open end in close proximity with the end face of the waveguide. According to a further embodiment of the method of the present invention, it includes the step of generating the microwaves with a predetermined wavelength λ and wherein the distance between the receiver and the tool is N λ / 4 or N λ / 8 , where N is an integer thereby optimizing the magnitude of either the first or the second return signal. According to a further embodiment of the method of the present invention, it includes the step of heterodyning either the first or the second return signal with a sample of the transmitted microwaves to generate a D C signal indicative of the amplitude of either the first or the second return signal and conducting the D C signal to a comparator for comparing the magnitude thereof with said threshold. According to a further embodiment of the method of the present invention, it includes the step of locating the openings on the waveguide at positions selected to optimize the amplitude of the return signal. Description of the Drawings Fig. 1 is a partial side view of a machine tool 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 drill; Fig. 3 is a section through the waveguide of the broken took detecting system shown in Fig. 2 illustrating the E-field intensity in the waveguide when the target is removed therefrom; Fig. 4 is also a section through the waveguide illustrating the E-field 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 suitable for detecting a wide variety of targets, but is particularly useful for detecting relatively small targets such as drill bits at relatively close range. Thus the microwave detection system according to the present invention is particularly useful in combination with an automatic drilling machine wherein a number of drilling operations are being conducted simultaneously as the microwave detection system according to the present invention is capable of detecting the presence or absence of a plurality of targets. With reference to the Figures, automated drilling machine 9 is utilized to drill extremely small holes in a stack of printed circuit boards 10. The machine includes at least one pressure foot assembly 11 in each drilling station that houses a retractable drill 12. While the present invention is particularly useful for drill targets having a diameter that is at or below 0.013" (0,33 mm) it is also useful for larger size targets and planar surfaces as well. The microwave detection system of the present invention is shown as 13 retrofitted to the pressure foot assembly 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 adjustable shorting stub 18 of the type that is well known and used in the art. The waveguide shown is rectangular in cross-sectional form and contains two small coaligned holes 19-19 centered in the top broadwall 20 and the bottom broadwall 21 of the waveguide. Normally, the elongated shank of the drill 12 is contained in the holes and sufficient clearance is provided therebetween to enable the drill to move vertically towards and away from the work. The drill in Fig. 1 is shown in a broken condition although in operation the drill would be within the waveguide for detection. With reference to Figure 2, transceiver module 17 contains a local oscillator 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 drill. The adjustable shortening stub is positioned in the waveguide behind the target and is arranged to reflect a well defined return signal back toward the module. The receiver section of the module includes a detector such as a Schottky diode 25 and a mixer circuit 26. The return wave signal is picked up by the diode 25 and is applied to the mixer along with a sample of the transmitted signal. The two input signals are heterodyned down to baseband in the mixer to provide an easily detectable D.C. output signal indicative of the amplitude rather than the frequency of the return signal, the mixer output is coupled directly via line 29 to a comparator 30 through amplifier 31 so that only the D.C. component of the output signal is applied to the comparator. The microwave system operates on the principle that the presence of a continuous target, such as a fine drill, in or near the end of the waveguide at a predetermined location produces a change in the standing wave ratio (SWR) of the transmitted signal. This change in the SWR is capable of being detected by the receiver 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 present system resembles a doppler radar, it should be apparent that it does not operate on the doppler effect in that the frequency component of the output signal is not utilized in the comparator circuit. As will be explained in greater detail below, the present system is adjusted to provide a clear indication of changes in the amplitude 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 specifically 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 will affect the power level of the transmissions and thus affect the overall sensitivity of the system. The location of the shorting stub is also important. Preferably the stub is located at the N λ/4 or N λ/8 distance from the centerline of the target where λ is the wavelength of the transmitted signal and N is some 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 signal. As noted, the local oscillator 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 waveguide 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 easily 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 only wave propagation The system is 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 applications. The E-field intensity within the waveguide is shown in Figs. 3 and 4 for dominant mode propagation. The normal E-field profile across the cross section of the waveguide is shown in Fig. 3 with the maximum strength of the field occurring at central axis 22 of the waveguide and falling off symmetrically to either side thereof. When a tool 12 is positioned in the waveguide, it shorts the E-field at the axis and produces a pronounced change in the profile of the E-field as graphically displayed by the arrows in Fig. 4. By selecting the operating frequency and the waveguide dimensions for dominant mode operation only, the tool acts like a shorting stub to an incident wave and produces a discernible change in the phase and amplitude of the return wave. Let the total E-field (incident plus reflected waves) at the transceiver be denoted by V E , which can be written: V E = V o [(expσx) + (Γexp -σx)] where: V o is the incident wave at the load; σ is the complex propagation constant for the waveguide Γ is the complex reflection coefficient of the load x is the distance from V E to the load A short section of waveguide can be regarded as being without loss if the tool aperature is sufficiently small. Therefore σ becomes: where: ω 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 jωt at the central axis, equations (1) and (2) yield: V E = A( exp jωt) (exp - jβx) [(exp jβx)+(Γexp - jβx)] (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: V E = 2A(sin βx) [exp j (ω t - βx + π/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: V Emax = δcos(ωt + ) (5) where: δ = 2A sin x; and  = π/2 - βx When the tool is removed from the waveguide: x = 1 t + 1 s (6) and when the tool is present in the waveguide: x = 1 t (7) Thus, equations (5) - (7) define the amplitude and the phase at the transceiver for either the presence or the absence of the tool. With reference to Fig. 2, the present invention will be explained in regard to a simple homodyne transceiver. The baseband mixer D.C. component output of the homodyne transceiver is given by: SO D.C. = δK 2 cos( - ϑ) (8) where: ϑ is an arbitrary phase constant; and K is the mixer conversion constant. Using equations (5) through (8), the following relationship is obtained for the condition where the tool is absent from the waveguide: SO D.C. = KAsin [β(1 t +1 s )] cos [π/2 -β(1 t + 1 s ) - ϑ] (9) and when the tool is present: SO D.C. = KAsin(β1 t ) [cos(π/2 - β 1 t - ϑ)] (10) As can be seen, by using either equation (9) or (10) the physical parameters of the target detection system can be easily optimized for any given application so that the output of the comparator can be used to either automatically shut down the machine and/or sound an alarm 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 distal end of the guide is opened to atmosphere. As illustrated in Fig.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 located a distance λ/4 or λ/8 times some positive integer from the output of the transceiver. As explained above in greater detail, the parameters of the system can be easily optimized for the x = ] t mode of operation. In this application, the signal sent out by transceiver 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.

Claims: A detection system (13) detecting the presence or absence of a tool of a machine tool (12 ; 50), this tool being movable into and out of a detection zone, comprising (a) a microwave transmission line (15 ; 51), (b) a transmitter (23) operative for transmitting microwaves along the transmission line (15 ; 51) in one direction toward said zone, (c) a receiver (25) operative for receiving a first return signal, having a first magnitude when the tool (12 ; 50) is located in the zone and reflects the transmitted microwaves incident thereon in the opposite direction, and also for receiving a second return signal having a different second magnitude when the tool (12 ; 50) is located out of the zone, and (d) detecting means (26,29,30,31) operative for comparing the magnitude of either the first or the second returnsignal with a threshold, and for generating an output signal indicative of the presence or absence of the tool (12,50) relative to the zone. The system (13) according to claim 1, wherein the transmission line is a rectangular waveguide (15) having a top broadwall (20) formed with a first opening (19) and a bottom broadwall (21) formed with a second opening (19) that is aligned with the first opening (19) and wherein the tool (12) is an elongated drill bit (12) which is positionable in the waveguide (15) so as to extend through both openings (19,19). The system (13) according to claim 2, wherein a shorting stub (18) is positioned within the waveguide (15) behind the bit (12) as considered along said one direction. The system (13) according to claim 3, wherein the microwaves have a predetermined wavelength λ and the distance between the bit (12) and the stub (18) is N λ / 4 or N λ / 8 , where N is an integer, thereby optimizing the magnitude of either the first of the second return signal. The signal (13) according to claim 1, wherein the transmission line is a rectangular waveguide (51) having an open end and the tool is positioned at this open end in close proximity with the end face of the waveguide (51). The system (13) according to claim 5, wherein the microwaves have a predetermined wavelength λ and the distance between the receiver (25) and the tool (50) is N λ / 4 or N λ / 8 , where N is an integer, thereby optimizing the magnitude of either the first or the second return signal. The system (13) according to claim 1, wherein the transmitter (23) is a Gunn diode, the receiver (25) is a Schottky diode and the detecting means (26,29,30,31) includes a mixer (26), operative for heterodyning either the first or second return signal with a sample of the microwaves transmitted by the Gunn diode (23) to generate a D C signal indicative of the amplitude of either the first or the second return signal, and comparator (30) to which the D C signal is applied for comparing the magnitude thereof with said threshold. The system (13) according to claim 2, wherein the openings (19,19) are located on the waveguide (15) relative to the transmitter (23) and the receiver (25) at positions selected to optimize the amplitude of the return signal. A method of detecting the presence or absence of a tool of a machine tool (12 ; 50), this tool being movable into and out of the detection zone of the system of claim 1, comprising the steps of : (a) providing a microwave transmission line (15 ; 51), (b) transmitting microwaves along the transmission line (15 ; 51) in one direction toward said zone, (c) receiving a first return signal having a first magnitude when the tool (12 ;50) is located in the zone and reflects the transmitted microwaves incident thereon in the opposite direction and also receiving a second return signal having a different second magnitude when the tool (12 ; 50) is located out of the zone and (d) comparing the magnitude of either the first or the second return signal with a threshold and generating an output signal indicative of the presence or absence of the tool (12 ; 50) relative to the zone. The method according to claim 9, wherein it comprises the step of forming the transmission line as a rectangular waveguide (15) having a top broadwall (20) with a first opening (19) and a bottom broadwall (21) of the waveguide (15) with a second opening (19) that is aligned with the first opening (19) and wherein the tool (12) is an alongated drill bit (12) which is positionable in the waveguide (15) so as to extend through both openings (19,19). The method according to claim 10, wherein it comprises the step of positioning a shorting stub (18) within the waveguide (15) behind the bit (12) as considered along said one direction. The method according to claim 11, wherein it comprises the step of generating the microwaves with a predetermined wavelength λ and wherein the distance between the bit (12) and the stub (18) is N λ / 4 or N λ / 8 , where N is an integer, thereby optimizing the magnitude of either the first or the second return signal. The method according to claim 9, wherein it comprises the step of forming the transmission line as a rectangular waveguide (51) with an open end and wherein the tool (50) is positioned at this open end in close proximity with the end face of the waveguide (51). The method according to claim 13, wherein it comprises the step of generating the microwaves with a predetermined wavelength λ and wherein the distance between the receiver (25) and the tool (50) is N λ / 4 or N λ / 8 , where N is an integer thereby optimizing the magnitude of either the first or the second return signal. The method according to claim 9, wherein it comprises the step of heterodyning either the first or the second return signal with a sample of the transmitted microwaves to generate a D C signal indicative of the amplitude of either the first or the second return signal and conducting the D C signal to a comparator (30) for comparing the magnitude thereof with said threshold. The method according to claim 10, wherein it comprises the step of locating the openings (19,19) on the waveguide (15) at positions selected to optimize the amplitude of the return signal.

Inventors: GELSTON II, NORBERT E.
Applicants: U.S. TECH CORPORATION
Application year: 1985
Main classification: G01S13
IPC codes: G01D21, G01V3, G01D3, G01S13, B23B49, H05K3, B23Q17, G01B15, G01S1, H03F99