High-sensitivity, Long-time-constant Thermistor Bolometer

Abstract

A high-sensitivity, long-time-constant thermistor bolometer having a heat sink and two thermistor elements in the same plane thermally connected to the heat sink through a path of high thermal impedance including a cylinder of material of low heat conductivity.

Parent Case Text

RELATED APPLICATIONS

This application is a division of application Ser. No. 864,842 filed Oct. 9, 1969, now U.S. Pat. No. 3,631,434 which is a continuation-in-part of application Ser. No. 564,391, filed July 11, 1966, and now abandoned.

Description

BACKGROUND OF THE INVENTION

The problem of moving objects which are at a higher temperature than their surroundings, or at least at a different temperature, is one of great importance in warfare, and is also useful in peacetime. The moving objects may be human beings, animals, or vehicles which have relatively hot areas, such as the exhaust from a motor vehicle. The problem may be considered as the detection of a moving intrusion into or across a particular background. It is also important, particularly when an instrument is to be carried, to have an instrument that is light and does not consume any large amounts of power and so can be operated for extended periods of time with self-contained power sources, such as batteries, either primary or secondary.

SUMMARY OF THE INVENTION

The present invention is directed to a high-sensitivity, long-time-constant thermistor bolometer, in which the conventional two thermistor flakes in opposition are mounted on a substrate on a cylinder of material of low thermal conductivity, which in turn is mounted on a conventional heat sink, the bolometer being evacuated. It has very high sensitivity with a very long time constant, typically of about 200 msecs.

The bolometer which can be operated with batteries and has very low power requirements is particularly useful in passive intrusion detectors, such as one pointed down a jungle path and which responds to infrared radiations from intruders, such as human beings, crossing the field of view of the intrusion detector. A typical diagrammatic representation of such a detection system with an illustrative form of electronic processing circuits is also described, although the detector is claimed as such, regardless of its use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of an intrusion detection system, and

FIG. 2 is an enlarged section through the detector of the present invention .

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows in diagrammatic form an intrusion detector system using a detector of the present invention, and illustrative electronic processing circuits. No mechanically movable parts are required during operation. The instrument is provided with imaging optics which is shown diagrammatically as a lens 10. The optics images a particular field of view sharply in the plane of the detector, with two sensitive thermistor flakes 4 and 5. The detector flakes, the size of which is shown exaggerated, are sufficiently far apart so that at ordinary distances an intruder would be imaged as a sufficiently small image so that the image would not cover both flakes at the same time.

The more detailed description of the new type of detector can be understood best in connection with FIG. 2. This detector, which has a conventional metal base 1 is equipped with a window 2 of suitable infrared transmitting material, such as KRS5 or germanium, which will transmit infrared in the wavelengths around 10.mu.. The two thermistor flakes 4 and 5 are mounted on a thin insulating layer 9 of polyglycol terephthalate, which is cemented across the top of a hollow cylinder or ring 3 of nylon resting on the base 1 of the thermistor bolometer. The three pins of the bolometer 6, 7, and 8 are connected to one end of flake 4, the junction of flakes 4 and 5, and the opposite end of flake 5 respectively. The bolometer is evacuated, and shows a high sensitivity with a fairly long time constant of about 200 msecs.

As will be seen from FIG. 1, pins 6 and 8 are connected to the positive and negative sides of a D.C. potential of about 13 volts. Pin 7 is connected to the input of an amplifier 24 which shows good low-frequency response. The input circuit of the amplifier is shown outside the amplifier symbol in schematic, and is a differentiating circuit of 1 .mu.f capacitor with 10M.OMEGA. resistor. It has a time constant of approximately 10 seconds. The amplifier 24 should preferably respond down to about 0.2 cycle.

The amplified output from 24 is coupled through a 150 .mu.f tantalum capacitor into an integrating circuit with a 10k resistor and two 15.mu.f tantalum capacitors connected back to back. This leads to the input of an amplifier 11 which should have good low-frequency characteristics. The output of amplifier 11 is connected in parallel to two monostable multivibrators 12 and 15. The outputs of the multivibrators are connected to an AND gate 13 and AND gate 16 respectively. The other connections to the AND gates is the common input to the multivibrators. Each AND gate actuates its own alarm 14 and 17 respectively.

The operation of the instrument is as follows:

An ordinary background in which there is no motion of the image of an intruder from flake 4 to flake 5 within the time constant of the input to amplifier 24 will not result in a signal to amplifier 24 unless the image on one or other flake moves on and off the flake. For example, if a tree in the background having different temperature than the average of the background sways in the wind, there may be a signal, if the image sways on and off one flake. This would result in amplifier pulses from the amplifiers 24 and 11, but the pulses would have the same polarity. Thus if the image of the moving object only went on and off of flake 4, there would be only positive pulses reaching the multivibrators. Let us assume that multivibrator 12 responds only to positive pulses, and places a negative pulse on the AND gate 13. The multivibrator 15 would not be acted on by a positive pulse, and therefore it would not put any signal on the gate 16. In a similar manner, if the image of an object moved on and off flake 5, the resulting negative pulses would cause the multivibrator 15 to operate, but not the multivibrator 12. In either case there would be no alarm signal given, because neither AND gate 13 nor 16 would have received signals of proper polarities in both of their inputs.

Now let us assume that a man walks across the field of view, his image first striking flake 4 and then a second or so later leaving flake 4 and striking flake 5. The moving signal on flake 4 would put out a positive pulse from amplifier 11 which would cause multivibrator 12 to put a signal on gate 13 but would not trigger off alarm 14 as there would be no negative signal in the other input of gate 13. However, as soon as the man's image comes onto flake 5, a negative signal will be generated in the output of amplifier 11. This will reach AND gate 13 and since the gate now has signals of the proper polarity on both of its inputs, its output will set off the alarm 14. The negative pulse from flake 5 through the amplifiers will, of course, cause the multivibrator 15 to operate, but it will not cause the AND gate 16 to pass on the signal because the other input to this gate will not have a signal of the right polarity. Therefore, alarm 17 will not be actuated, and the alarm signals will show that a moving target moved from the left (looking at the flakes 4 and 5 on FIG. 1). If a man moved across the field of view from the opposite direction, the pulses would be reversed, multivibrator 15 would be activated by the first negative pulse, and then the following positive pulse would cause AND gate 16 to pass its signal on to the alarm 17, whereas gate 13 would not have received the signals of the proper polarity in both its inputs and so alarm 14 would not be actuated.

Monostable multivibrators and AND gates are conventional electronic devices, and therefore they have been shown purely diagrammatically in block diagram form. Of course the multivibrators must have the proper time constants so that there will be an alarm if a man moves across from one flake to another in a reasonable time. Also, of course, the gates must have the proper circuitry for the functions which they are performing and which have been described above. Similarly, the amplifier preceding the logic circuits may incorporate automatic gain control features and clamp circuits of conventional form in order to function optimally in detecting targets at any distance within the limit of the instrument and varying in intensity with the background, depending on particular background conditions.

After an alarm is given, the operator of the instrument, if it is being monitored by a human operator, can reset the alarm. As this is a conventional electronic operation, its circuit is not shown. It is possible to have the alarm unattended, or record at a remote location, and in some such cases it is desirable to have the multivibrators clear themselves after the expiration of a predetermined delay, their preset time constant. This also is a conventional type of electronic circuitry, and is not specifically shown in schematic form.

The new detector, which is extremely sensitive (though slow, which does no harm and actually is an advantage in the present use), permits operation with fairly low voltages and very moderate power inputs. It is possible to use primary batteries and run an instrument for a week or more. Also, because there are no moving parts and no need for the expense of immersion optics, the new bolometers can be built very economically in comparison to their high sensitivity. Other types of detectors, such as thermocouples and thermopiles, may also be used, but they do not lend themselves as well to the compact construction with good sensitivity which is made possible with the slow, highly sensitive, unimmersed bolometers shown particularly in FIG. 2.

Claims

I claim

1. A high sensitivity, long time constant thermistor bolometer comprising, in combination,

a. a heat sink,

b. a hollow thermally insulating support mounted on said heat sink with one edge of the support in contact with the heat sink,

c. a thin electrically insulating membrane mounted on the other edge of the hollow support and therefore out of direct contact with the heat sink,

d. a pair of thermistor flakes mounted on said thin membrane, the flakes being electrically connected together,

e. externally extending electrical leads, the first of said leads being connected to the electrical connection between the extremities of the two flakes, second and third leads connected respectively to the extremities of each flake opposite the extremity connected to the other flake, whereby when the second and third leads are connected to different polarities of a DC current source, the thermistor flakes are in opposition, and thermal connection of the thermistors to the heat sink is through a path consisting of the membrane and the hollow support.

2. A thermistor bolometer according to claim 1 in which the membrane is of a thin plastic and the hollow support of thermally insulating material is a hollow cylinder.

3. A bolometer according to claim 1 in which the bolometer is evacuated.

4. The thermistor bolometer according to claim 1 in which the thin membrane is polyglycol terephthalate, and the hollow support is of nylon.

5. A thermistor bolometer according to claim 4 in which the bolometer is evacuated.