INDUCTIVE COUPLED HIGH TEMPERATURE MONITOR

Abstract: An inductive coupled temperature monitor (10) is taught that is capable of operating in or nearby conductive objects (12) that produce unwanted reflected impedance. The temperature monitor (10) can be used to telemeter temperature information from a closed soup can (12) or other container, during an industrial cooking process.

Description: INDUCTIVE COUPLED HIGH TEMPERATURE MONITOR BACKGROUND OF THE INVENTION !• Field of the Invention The invention relates to an inductive coupled temperature monitor capable of operating through nearby conductive objects that produce unwanted reflected impedance. More particularly, the invention can operate within a metallic enclosure, such as a soup can, to monitor and telemeter temperature information via an inductive link. 2. Description of the Prior Art It has been known in the art that canned foods are often best cooked in the sealed can or jar. U.S. patent 4,092,111 issued to Gaignoux et al and entitled "Apparatus for the Heat Treatment of Products Contained in a Sealed Container" teaches an apparatus for processing food and pharmaceutical products that are sealed in cans or cups which are made of aluminum, plastic or glass. A problem in such industrial processes is that the heat supplied by the oven is not homogenous. It is therefore desirable to monitor the actual temperature in one or more of the sealed cans or jars within the oven. U.S. patent 3,875,854 issued to Wasenaar and entitled "Apparatus for Canning Food" teaches placing a thermocouple in a canning jar which is connected to a timer. The timer is activated once boiling of the fluid occurs in the jar. However, the reference teaches the use of thermocouples that must be connected to the timer circuit by a wire, making conveyor line production impossible. Therefore, there is the need, not taught in the prior art, to have a miniature transmitter placed within the cooking container (can, jar, etc.) to monitor the temperature and telemeter that information to a remote control system. SUMMARY OF THE INVENTION The present invention is a temperature responsive transmitter that is capable of operating near or within a metallic enclosure, such as a soup can, film processing can, or metal mixing enclosures used in industrial processes. To operate in this environment the miniature transmitter must: (1) be able to send an information signal through the shielding effect of the metallic enclosure; and (2) be minimally effected by unwanted reflected impedance from the metallic enclosure. To meet the first objective, an inductive signal is generated to telemeter temperature information from the metallic enclosure. Undesired shielding provided by the metallic enclosure is frequency dependent - the lower the frequency the less unwanted shielding provided by the can. The shielding can block the emission of the telemetry signal from the metallic enclosure. If a propagating field were used at the required lower frequencies, an extremely long antenna, inappropriate for a miniature transmitter, would be necessary. The inventor overcame this problem by using a non-propagating inductive field, which only requires a miniature coil to generate the magnetic field at the appropriate frequency. To meet the second objective, a unique circuit design isolates the tank circuit (which contains the inductive coil) from the temperature sensitive oscillator. In traditional circuit design the reflective impedance produced by the conductive enclosure would affect the impedance of the oscillator and thereby change its frequency of oscillation. Since the temperature information is conveyed through a change in the oscillator's frequency, any reflected impedance would modify the frequency and therefore would product an unknown error signal. To overcome this problem, the oscillator circuit is coupled to a buffer amplifier. The buffer amplifier then injects current pulses into the tank circuit to generate the inductive signal. The buffer amplifier isolates the oscillator from the reflected impedance coupled through the tank circuit. This isolation enables accurate temperature measurements to be telemetered in spite of unwanted reflected impedance caused by nearby conductive objects, including the metallic enclosure. The invented temperature monitor generally includes: a temperature sensitive crystal for producing an output frequency that is temperature dependent; an oscillator for producing an output at a frequency determined by the temperature sensitive crystal; a tank circuit containing a coil inductor to be emit the magnetic signal; and, a buffered output amplifier coupled to the oscillator for buffering the oscillator from reflective loads due to nearby conductive objects and for injecting current pulses into the tank circuit, wherein the coil inductor generates an inductive output signal having a frequency dependent on temperature sensed by the temperature sensitive crystal. A first novel feature of the apparatus is its ability to continually measure temperature inside a metallic enclosure, such as a soup can. A second novel feature of the invention is the use of a buffer amplifier that is coupled to a temperature sensitive crystal oscillator to isolate the oscillator from reflective impedance provided by nearby conductive objects. A third novel feature is the selection of a frequency of operation for the inductive link such that the undesirable shielding effect of a metallic enclosure is minimized. A fourth novel feature is the use of an inductive field to send a signal through a metallic enclosure thereby transmitting temperature information. A fifth novel feature reduces coupling with the metallic can thereby allowing a larger inductive signal to be emitted by orienting the inductor coil parallel to the ends of the can. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a drawing of the present invention telemetering temperature information through a metallic enclosure to a remote receiver. Figure 2 is a schematic diagram of the present invention in which a two-stage buffer amplifier isolates the temperature sensitive oscillator from the tank circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Figure 1 shows one of the intended environments for the present invention. The miniature temperature sensor 10 is placed within a metallic enclosure, such as a soup can, and measures the interior temperature. Induction coil 14 generates an inductive field which couples to and is detected by the remote receiver 16. The frequency of the inductive field is caused to vary as a function of sensed temperature, thereby telemetering through the soup can information on its interior temperature. The metallic enclosure 12 presents a difficult environment to telemeter information through. The metallic surface acts as a shield, rapidly impeding any transmitted radio signal. The soup can will act as a shorted turn of a secondary coil which can couple onto the inductive coil 14 and absorb a large portion of its magnetic field. Unwanted reflected impedance from the conductive surfaces can couple into the inductor coil 14 changing its impedance and altering the frequency of oscillation — thus placing in error the information contained in the telemetry signal. The present invention can uniquely overcome each of these problems. First, a magnetic coupled, non-propagating magnetic telemetry signal is used. This allows the transmitter to operate at a lower frequency where the shielding effect of the soup can is minimal, without requiring an extremely long antenna. In the preferred embodiment, the transmitter operates at 262.144 kH - a frequency low enough to minimize the shielding effect of a soup can. If a propagating field were used, the antenna would have to be too long to make a miniature transmitter practical. However, for a non-propagating inductive field only a miniature coil 14 is necessary to emit a magnetic flux of sufficient strength to penetrate through the soup can and be detected remotely. Second, the Applicant recognized that coil alignment is important when the transmitter is used within a shielded container, such as soup can 12. When used inside a typical soup can, the coil 14 axis should pass through the sides of the can (as shown in Figure 1) and not the ends. With this orientation, magnetic flux flows through the side perpendicular rather than being surrounded by a large shorted turn (i.e., the cylindrical surface of the soup can acts as a shorted secondary) . The Applicant has found that this orientation improves the external field transmitted through the soup can by about 20 db. Third, the apparatus contains a unique circuit design which minimizes the effect of unwanted reflected impedance caused by nearby conductive surfaces on the operation of the transmitter. As discussed previously, the reflected impedance can couple through the tank circuit and modify the oscillator frequency — thereby generating an error in the telemetry signal. Figure 2 is a schematic of the apparatus, showing the use of a buffer amplifier 18 between the temperature sensitive crystal oscillator 20 and the tank circuit 22 which contains inductive coil 24. The buffered amplifier provides the necessary isolation and allows the apparatus to operate in or near conductive objects that would generate reflective impedance. The oscillator 20 is a variant of the "Pierce" type oscillator, which uses an inverting amplifier and runs the crystal 26 in the inductive region. The frequency of oscillation is set by crystal 26. Crystal 26 is a Statex TS-2 tuning fork and produces an output frequency that changes with temperature. Therefore, the oscillator will produce an essentially square wave output at pin 11, the frequency of which will vary with sensed temperature. The oscillator output (pin 11) is coupled through a capacitor 32 to the input of the buffered amplifier 18. Two transistors are used in this class C buffer amplifier to obtain a high input impedance. The first transistor 28 forms a common collector stage which both isolates the following amplifier and has no Miller feedback. The second transistor 30 is a common emitter stage which provides amplification. The output from the buffer amplifier (pin 1) inputs to tank circuit 24 and injects current pulses into the tank circuit. Timing is well maintained since the coupling capacitor 32 is charging towards +3.6 volts until the pin 12 transistor conducts at which time the output transistor is again turned off. The 30mA current from pin 1 sustains about 200mA rms circulating current in the tuned circuit 22. This circulating current establishes the desired local inductive field at coil 24 necessary to telemeter temperature information. The inductor coil 24 is formed on a 5/8" diameter mandel using #18 copper wire to maintain a high Q. The coil is wound in two layers of 12 turns each. The inductance of the coil is about 8 Hy. The inductor coil 24 emits a magnetic field, the frequency of which is directly related to the sensed temperature. Due to the isolation between the tank circuit and the oscillator, taught above, the frequency of the magnetic flux accurately indicates the sensed temperature. In operation, the apparatus is sealed in an enclosed metallic container such as a soup can, and telemeters temperature information remotely. Although the apparatus has been described as operating in a metallic enclosure, such as a soup can, it must be understood that it would be equally applicable in various industrial applications where temperatures to be measured in an area surrounded totally, or in part, by conductive surfaces which will produce unwanted reflected impedance. Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: What is claimed is: 1. An apparatus for sensing a physical parameter and telemetering an inductive signal, wherein the apparatus is designed to operate nearby conductive objects that produce unwanted reflected impedance, said apparatus comprising: a sensor for producing an output signal that is dependent on the magnitude of the sensed physical parameter; an oscillator means, coupled to said sensor, for producing an output at a frequency determined by said sensor output signal; a tank circuit containing a coil inductor; and, a buffered output amplifier means, coupled between said oscillator and said tank circuit for buffering said oscillator from reflected loads affecting said tank circuit that are caused by nearby conductive objects and for injecting current pulses into said tank circuit, wherein the coil inductor in said tank circuit generates an inductive output signal having a frequency dependent on the magnitude of the sensed physical parameter. 2. The apparatus of claim 1, wherein said sensor is a temperature sensitive crystal for producing an output frequency that is temperature dependent. 3. The apparatus of claim 2, wherein the tank circuit is tuned to essentially the same frequency produced by the buffered output amplifier. 4. The apparatus of claim 2, wherein said buffered output amplifier contains a first transistor common collector stage for isolation and a second transistor common emitter stage for amplification. 5. The apparatus of claim 2, wherein said coil inductor is orientated with its axis pointing through the sides of a metal soup type can, when the apparatus is operating inside a soup type can, thereby reducing the inductive flux coupled to the soup type can. 6. The apparatus of claim 2, wherein the frequency band of operation is selected to minimize the shielding effect of the nearby conductive objects.

Inventors: HOGREFE ARTHUR F, HOGREFE, ARTHUR, F.
Applicants: UNIV JOHNS HOPKINS, THE JOHNS HOPKINS UNIVERSITY
Application year: 1990
Main classification: G08C19
IPC codes: G01K1, G01K7, G06K7, G08C17, G08C19