US 3836828 A
The apparatus comprises an electronic system including an antenna positioned about the equipment to be protected and coupled to a resistor-capacitor bridge, to which is coupled an oscillator. The output of the bridge is coupled through a differential amplifier to a demodulator, and the output of the oscillator is also coupled directly to the demodulator, The demodulator output is coupled through an amplifying system both to a display device, which is used to indicate the state of balance of the system, and to a control circuit for generating a signal which is coupled to the equipment to be protected to turn it off or to otherwise exert a protective control action when a certain level of unbalance in the bridge occurs.
Description (OCR text may contain errors)
United States Patent 11 .Siegel ELECTRONIC PROTECTION AND SENSING APPARATUS  Inventor: Leon J. Siege], Livingston, NJ.
 Assignee: Weldotron Corporation, Piscataway,
22 Filed: ,Iunel8, 1973  Appl. No.: 370,783
Related US. Application Data  Continuation-in-part of Ser. No. 273,692, June 21,
 US. Cl. 317/146, 3l7/DIG. 2, 340/258 C  Int. Cl. .1 H0111 47/12  Field of Search.....-340/248 C; 317/146, DIG. 2
3,493,954 2/1970 Bartlett et al. 340/258 c 3,497,995 3/1970 Forsberg 340/258 c 3,551,919 1/1971 Forbes 340/258 c Primary ExaminerJames D. Trammell [5 7] ABSTRACT The apparatus comprises an electronic system including an antenna positioned about the equipment to be protected and coupled to a resistor-capacitor bridge, to which is coupled an oscillator. The output of the bridge is coupled through a differential amplifier to a demodulator, and the output of the oscillator is also coupled directly to the demodulator, The demodulator output is coupled through an amplifying system both to a display device, which is used to indicate the state of balance of the system, and to a control circuit for  References Cited generating a signal which is coupled to the equipment UNITED STATES PATENTS to be protected to turn it off or to otherwise exert a 3 109 893 11/1963 B 340/258 C protective control action when a certain level of unurns 3,235,857 2/1966 Bagno 340/258 0 balance m the budge occurs 3,409,842 11/1968 Embling et al 317/D1G. 2 11 Claims, 8 Drawing Figures 40 O 50,- so, 70, Diff. I Osc. Bridge Amp. Dem d Filter 130 Di ff. D11 f LO ic Machine 80 Amp 9 Control D i f f. Ladder Meter -1OO L' M I '9 D isploy ELECTRONIC PROTECTION AND SENSING APPARATUS This application is a continuation-in-part of application Ser. No. 273,692, filed July 21, 1972, now abandoned.
BACKGROUND OF THE INVENTION SUMMARY OF THE INVENTION Briefly, apparatus embodying the invention includes a stable sine wave oscillator coupled to a sensitive, stable, resistance-capacitance bridge to .which a sensing antenna is coupled. The output of the bridge is coupled to a series of differential circuits and to visual indicators and to apparatus for turningoff the operating machinery.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the system of the invention;
FIG. 2 is a partly schematic, partly block, representation of the system of FIG. 1;
FIG. 3 is a side elevational representation of apparatus protected by the invention showing the mechanical relationship therebetween;
FIG. 4 is a front elevational view of a portion of the apparatus of FIG. 3;
FIG. 5 is a perspective view of a box for containing the electronic system of the invention,
FIG. 6 is a schematic representation of a portion of a system of the invention showing a modification thereof;
FIG. 7 is a schematic representation of a front elevational view of a portion of the system, of FIG. 6; and
FIG. 8 illustrates another modification of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus of the invention can be used to protect automatic operating machinery from the standpoint of operator safety and from the standpoint of intrusion of unauthorized personnel. Other uses will occur to those skilled in the art.
Referring to FIG. 1-, the system of the invention 10 includes a highly stable oscillator connected to a bridge 30 to which the protective sensing antenna 40 is coupled. The output ofthe bridge can be a positive or negative" signal, depending on the nature of the disturbance sensed by the antenna. Accordingly, two output lines run from the bridge to a differential amplifier 50 and from the differential amplifier to a demodulator 60 to which the'output of the oscillator is also coupled. Y
The output of the demodulator 60 is coupled to a filter arrangement 70 and then to two differential amplifiers 80 and 90 connected in cascade. The output of the second differential amplifier iscoupled (l) to a differential meter which indicates the magnitude and direction of the error signal, (2) to a ladder network and thence to a visual display for providing a visual representation of the error signal, and (3) to a control signal generating circuit 130. The circuit responds to an error signal representative of the alarm condition for which the operating machinery is to be turned offand (1) turns on an alarm indicator in display 110 and (2) operates a machine control device for turning off the operating machinery.
Portions of the system 100 are shown in greater detail in FIG. 2. All routine elements such as ground connections, bias voltage and bias resistor elements and the like may not be shown since such elements can readily be provided by those skilled in the art. The oscillator 20 is a high precision circuit which, in one mode of operation of the system, is set to generate an undistorted sine wave of I25 kilocycles (kc) at about 1 volt rms. The oscillator includes an integrated circuit operational amplifier 150, andthe desired stability is achieved by means of a twin-T feedback network from the output to the input of the amplifier 150. The feedback network includes a pair of series resistors and 174 in parallel with a pair of series capacitors I80 and 184, with the junction points of each pair interconnected by a series resistor and capacitor 194. The junction of resistor I70 and capacitor 180 is connected by lead 200 to the input of the operational amplifier I50, and the junction of resistor 174 and capacitor 184 is connected by lead 210 to the output of the operational amplifier. A network 218, including a pair of parallel, oppositely-oriented diodes 220 in series with a resistor 230, is connected between leads 200 and 210.
The oscillator 30 is highly regulated and produces a stable sine wave at the desired frequency. The twin-T feedback network 160 and the network 218 affect the desired stability. The output of amplifier 150 is connected by lead 240 to the junction point 242 of bridge 30.
The bridge 30 is preferably a resistance-capacitance bridge including resistors 250 in two legs and variable capacitors in the other two legs. In one specific embodiment of the invention, the capacitance legs each include one parallel, variable, air-capacitors 260. The system antenna 40 is coupled to the junction point 270 and a trimmer capacitor 280, which isused to set the bridge to a null point, is connected across a capacitor leg.
The antenna 40, typically, comprises metal tubing suitably shaped about the apparatus to be protected. The output of the bridge 30, when the antenna is disturbed, is the oscillator sine wave changed in phase and amplitude, with the direction of the change and the amplitude being determined by the nature of the disturbance to the antenna. If an individual approaches the antenna, one phase change, by definition a positive change, occurs; and if the antenna is physically altered, the opposite phase change, by definition a negative phase change, occurs. The amplitude of the signal out of the bridge depends on the magnitude of the disturbance or the nearness of a human body. Thus, two resistive output paths 290 and 300, one positive and one negative, are coupled from junction points 270 and 273 of the bridge to the inputs of the differential amplifier 50. The resistance of the paths and the stray capacitance of the amplifier 50 combine to nullify any transient signals, such as static electricity, which might be picked up by the antenna.
The differential amplifier 50, in one embodiment of the invention, is a fairchild 730 module. The outputs of the differential amplifier 50, which comprise positive or negative error voltages, are coupled by capacitors 310 and 320 to demodulator 60 which also is coupled to the output of the oscillator by lead 330 and receives the oscillator sine wave output. The demodulator, in one embodiment of the invention, is a Fairchild 796 module.
The error voltage outputs of the demodulator 60 are coupled by leads 340 and 350 through the low-pass filter network 70 to remove any vestiges of the oscillator signal and to provide a clean DC error voltage. The outputs of the filter are coupled through resistive paths 360 and 370 to the inputs of the first differential amplifier 80 which is a low gain amplifier and serves to decouple the error signals from the remainder of the system. The output of differential amplifier 80 is coupled through a resistive path 374 to differential amplifier 90 connected as a variable gain amplifier. In one embodiment of the invention, the amplifiers 80 and 90 are Fairchild 741 amplifiers.
The output of amplifier 90 is connected to several circuit modules. One connection is by lead 380 to the differential meter 100 which, as is well known, can read a central null point and positive and negative deviations therefrom. In a preferred arrangement, which may be auxiliary to the differential meter, lead 380 is also connected to a ladder network 110, the outputs of which are coupled to display 120 which includes a plurality of lamps 400, preferably solid state lamps known as LEDS.
The details of the ladder circuit 110 are not shown, but, briefly, the ladder includes a series of PNP transistors and a series of NPN transistors to accommodate both positive and negative error signals. At null, when the bridge is balanced, the center lamp 400C is on, and as an error signal appears and increases in amplitude, lamps on either side of center turn on, with end lamps 400E turning on when an emergency signal is present.
The output ofthe amplifier 90 is also coupled by lead 410 to the control circuit 130 which includes two Schmitt trigger chains, one responsive to positive error signals and the other responsive to negative error signals. Lead 410 is connected to one channel by resistive path 420 and to the other channel by resistive path 430.
The control circuitry includes a positive channel which comprises a differential amplifier including a pair of NPN transistors 440 and 442 connected emitter-to-emitter with the base of transistor 442 coupled to a Zener diode 450 oriented as shown. The collector of transistor 440 is connected to the base of a PNP transistor amplifier 460 which has its collector connected to the last lamp 400E on the positive side of the display 120. The collector of transistor 460 is also connected through a diode 464 to an inverter transistor 470A and a relay driven transistor 4708 which drives the machine control apparatus 140 which may be a relay or the like, and is used to turn off the operating machine protected by the system 10. Transistor 470A is normally OFF, and transistor 4708 is normally ON.
The negative channel of the logic circuit is similarly constructed but with opposite-type transistors and with transistor 460' coupled to the last lamp 400E on the negative side of the display 120. The negative channel is coupled to and operates the transistor chain 470 which operates the control relay 140. Transistors 440 and 442 and diode 450 and their counterparts comprise trigger circuits, and the diodes 464 and 464 comprise an OR gate through which the inverter transistor and relay driver transistor are operated.
in operation of the system 10, with the oscillator 30 operating and the protected machine operating and with the bridge balanced and all indicators set at a null reading by the trimmer capacitor 280, if an individual approaches the operating machine, the antenna senses this fact. The system is sensitive enough to detect and respond to an individual who comes within several feet of the antenna. As the individual approaches, the bridge becomes unbalanced and a positive error signal appears on lead 290 into the differential amplifier 50. The differential amplifier produces an error sine wave which is out of phase with the oscillator sine wave. The error wave is fed into the demodulator 60 on lead 310, and the oscillator signal is fed into the demodulator on lead 330, and the output on lead 340 is a positive DC error voltage. This error voltage passes through the filter 70, and it emerges as a clean, positive DC voltage free of any residual oscillator sine wave.
This DC error voltage is fed through the amplifier and 90, and the resultant signal provides a positive reading on the meter, and it turns on one of the lamps 400 in the display depending on its amplitude. The output of amplifier 90 is also fed into the positive channel of the control circuit 130. The output of the differential amplifier 90 is generally proportional to the magnitude of the original error signal, and, if this signal is below a predetermined alarm level, nothing happens in circuit and the protected machine is not turned off. However, if the alarm level is exceeded, the positive trigger circuit fires and Zener diode 450 conducts and turns on transistor 460. This causes the alarm lamp 400E on the positive side of the display to light, and it turns transistor 470 ON and transistor 470B OFF to drop out relay and to turn off the operating machinery.
Similarly, if the antenna is somehow damaged or physically disturbed, a negative phase change occurs at the output of the bridge 30, and negative error signals pass through the system and operate the system as described above.
Once the alarm condition is corrected, the system resets, the relay 140 locks up again, and the machine can be operated.
The system 10, and particularly bridge 30, can be designed to operate with an antenna up to about 50 feet in length with the bridge arranged as shown. If desired, the system can be operated with longer antennas, up to about feet in length, by moving one or more of the capacitors 260 of one leg into parallel relationship with the capacitors 260 in the other leg.
In using the system 10 to protect a piece of machinery, for example a press 500 illustrated schematically in FIG. 3, the antenna 40 is suitably shaped so that it properly encloses the critical portions of the machine. The machine 500 usually includes a switch box 510 including a push-button switch 520 which is used by an operator to start and stop the machine. The antenna usually includes a portion 40 which is positioned adjacent to the switch box. In order to permit the system to be used with such apparatus without'having the machine turned off every time the operator attempts to approach the switch box, a shield 530 comprising a generally rectangular curved metal plate is secured along one edge to the switch box. The shield plate is shaped so that it curves downwardly in front of the antenna portion 40', with the curvature being such that it is spaced further and further from the antenna as it passes from box 510 toward and in front of antenna portion 40'. The lower margin of the shield plate terminates parallel to the antenna portion and completely shielding it from an oncoming person.
The circuit elements of system described above are preferably mounted in a metal box 550 (FIG. 3) which is located near the operating machinery and has a'lead running toit from the antenna. In addition to providing protection for the operating machine, it is necessary for the system to be protected from tampering by unauthorized personnel. Accordingly, the circuit is suitably mounted within the box which contains a first lockable cover 560 hinged at the side and having a glass portion through which the meter 100 and/or display 120 can be seen. The first cover carries a keyoperated lock 570 which permits the cover to be raised to provide access to a second cover 570 hinged at the side and provided with a key-operated lock 580 and concealing the circuit elements disposed within the box itself. Cover 570 carries the lamps 400 and meter 100,
if desired, (not shown). A knob 280', which operates trimmer capacitor 280, is disposed adjacent to the second cover 570. With this arrangement, only authorized personnel, having the required keys can open the first cover 560 to provide access to the adjustment knob 280 and can open the second cover 560 to provide access to the circuit itself within the box.
The system 10 has high sensitivity and excellent signal-to-noise ratio because of the extensive use of differ? ential circuitry throughout. Other important features include the stable, precise oscillator which provides a stable, regulated sine wave; and the series differential amplifiers 80 and 90, with amplifier 80 having low gain and amplifier 90 having variable gain whereby optimum signalcontrol is achieved. In addition, the use of air capacitors in the bridge provides stability and, particularly, immunity to thermal effects.
It is to be noted that the system of the invention can be modified for uses other than that specifically described above. For example, by suitably modifying the antenna sensing element, the system could be used to measure movement of any type, and it could function as a pressure gauge by detecting movement of a diaphragm; it could be used as a liquid level gauge, etc. Thus. in effect, the system could be used to measure any linear displacement motion.
In a modification of the invention illustrated schematically in FIG. 6, a lead 600 is connected from the Output of the oscillator to an amplifier 602 which produces a signal of very low impedance and of the same phase and frequency as the oscillator output signal. The output of the amplifier circuit is coupled to and drives a conductive shield plate 602 in the form of a frame placed in front of the system antenna 40 represented schematically. The shield frame 604 has the same general shape as the antenna and is positioned perhaps inches away from the antenna. The shield is 6 shaped and positionedfsothat it covers the antenna, as illustrated in FIG 7fand, when considered from the viewpoint of an individual approaching the antenna, prevents the antenna from seeing" an individual approaching the apparatus being protected.
This arrangement permits an individual to approach the apparatus as close as the shield and to work in the area without tripping the alarm circuit. However, if the individual puts his hand through the shield frame, which represents a danger condition, he affects the antenna and operates the system, as described above, to turn off the protected apparatus.
Instill another modification of the invention illustrated in FIG. 8, the oscillator 20 and bridge 30 are disposed within a temperaturecontrolled chamber represented by block 610. Means 612 is provided for generating heat in the chamber, and a thermistor 614 is also provided in the chamber and coupled to a control circuit 616 which is coupled to and operates the heating means 612 for maintaining a constant temperature, of the order of in the chamber 610.
Maintaining the oscillator and bridge in a temperaturecontrolled atmosphere reduces the drift which the system would ordinarily experience due to temperature changes.
What is claimed is:
l. A sensing system comprising a sine wave oscillator,
a bridge network coupled to the output of said oscillator and having a positive output channel and a negative output channel and including means for balancing the bridge when said system is operated,
an antenna coupled to said bridge and said oscillator,
said antenna causing said bridge to produce an output signal when its electrical field is disturbed, a first two-channel differential amplifier coupled to said output channels of said bridge, a demodulator coupled to the outputs of said first differential amplifier and to the output of said oscillator for comparing the output of said oscillator with an output signal from said bridge and producing an errorsignal therefrom, a filter coupled to the output of said demodulator for filtering the output error signal therefrom and producing a DC output signal, a plurality of differential amplifiers in cascade coupled to the output of said filter and receiving said DC output signal therefrom, the output of said plurality of differential amplifiers being coupled (l) to display means for providing a visual representation of the magnitude of said DC output signal, and (2) to control circuit means for producing a control signal from said DC output voltage, said control signal being usable to carry out a con'trol function, said display means including a ladder circuit connected to the output of said plurality of differential amplifiers and having one portion for generating signals responsive to positive error signals and a second portion for generating signals responsive to negative error signals, and
a series of lamps coupled to said ladder circuit and including a center lamp which represents a null condition, a first group of lamps on one side of said center lamp for glowing when a positive error signal is present and a second group of lamps on the other side of said center lamp for glowing when a negative error signal is present, a first auxiliary lamp associated with said first group of lamps and a second auxiliary lamp associated with said second group oflamps, both auxiliary lamps representing an emergency condition,
said control circuit including a first differential amplifier having its input connected to the output of said plurality of differential amplifiers and having its output coupled both to said first auxiliary lamp and through an OR gate to an amplifier chain coupled to, and operating, means for turning off apparatus protected by said system, and
a second differential amplifier having its input connected to the output of said plurality of differential amplifiers and having its output coupled both to said auxiliary lamp and to said OR gate and to said amplifier chain for operating said means for turning off said apparatus protected by said system.
2. The apparatus defined in claim 1 wherein said bridge network is a resistor-capacitor bridge.
3. The apparatus defined in claim 2 wherein said capacitors are variable air capacitors.
4. The apparatus defined in claim 2 wherein two legs of said bridge include two variable air capacitors in parallel.
5. The apparatus defined in claim 1 wherein said plurality of differential amplifiers includes a first differential amplifier of low gain and a second differential amplifier of variable gain.
6. The apparatus defined in claim 1 wherein said light-producing elements are light-emitting semiconductor diodes.
7. The system defined in claim 1 and including a box in which said system is mounted, said box including a first hinged cover which can be locked in place concealing said system within said box with a shaft from said means for balancing said bridge extending through a hole in said first cover and carrying an operating knob on its end, i
said display means carried by said first cover, and a second hinged'cover which can be locked in place over said first cover and including a transparent portion which permits viewing of said display means,
said first and second covers having different locking mechanisms.
8. The system defined in claim 1 and including a shield disposed in front of said antenna and shielding said antenna from an individual approaching said antenna and standing close to said antenna, with'said shield interposed between said individual and said an tenna.
9. The system defined in claim 8 and including a connection from said oscillator to said shield whereby said shield carries a signal of the same phase and frequency as said antenna.
10. The system defined in claim 1 wherein said oscillator and bridge network are disposed in a temperature-controlled chamber.
11. Control apparatus for use in conjunction with a machine which includes an operating mechanism and a switch box, said apparatus comprising an antenna shaped to enclose said machine and including an antenna portion positioned adjacent to said switch box, and
an arcuate shield plate disposed adjacent to said switch box and between said switch box and said antenna portion and thus shielding said antenna portion from an operator who approaches and touches said switch box,
said shield plate having one edge secured to said machine adjacent to said switch box and positioned relatively close to said antenna portion, said shield plate extending away from said switch box and said antenna portion, the curvature of said shield plate being such that, as said plate extends from its area of attachment to said machine, it is spaced farther and farther from said antenna portion.