Thermal measuring and testing – Temperature measurement – By electrical or magnetic heat sensor
Reexamination Certificate
2001-12-12
2003-10-07
Fulton, Christopher W. (Department: 2859)
Thermal measuring and testing
Temperature measurement
By electrical or magnetic heat sensor
C374S183000, C340S870170, C340S870190, C340S870050, C340S539230, C600S300000
Reexamination Certificate
active
06629776
ABSTRACT:
BACKGROUND OF THE INVENTION
In a first aspect, the present invention relates to an electronic thermometer and more particularly, but not exclusively, to a thermometer useful for measuring body temperature of human or animal subjects or for measuring ambient temperature. In a second aspect, the present invention relates to a telemetry system consisting of a transmitting measurement device (preferably a body temperature thermometer) and a portable receiving unit which displays the measured value of a sensed parameter.
Several prior art inventions utilize a temperature-dependent resistive element together with an oscillator circuit to form a digital thermometer. In the prior art, a thermistor is sometimes used as a temperature-dependent, variable-resistance device in series with a charging capacitor to form the frequency-controlling elements of the oscillator network. The equation
f
=
1
(
2
π
RC
)
determines the frequency of oscillation, where R is the resistance of the resistive element (thermistor) and C is the capacitance of the series charging capacitor. As the temperature varies, the resistance of the thermistor varies, and the frequency varies as a result. By measuring the frequency, and knowing the value of the capacitance, the value of R can be determined. Because R is uniquely related to temperature, the temperature can be determined as well. For a thermistor, the resistance is related to the temperature via the Steinhart-Hart equation. The use of a multivibrator as the oscillator circuit is disclosed in U.S. Pat. No. 4,359,285 by Washburn for low-power oceanographic applications. U.S. Pat. Nos. 4,602,871 and 4,464,067 issued to Hanaoka disclose thermometers based on thermistor-controlled oscillators whose properties emphasize miniaturization, light weight, and improved accuracy using correcting circuits. These latter two patents refer to applications wherein the sensor may be used with low-power wristwatch devices.
One disadvantage of measuring the frequency of the oscillator is that one must know the value of the capacitor extremely accurately in order to derive the value of the resistance accurately. Generally, it is difficult to do capacitance measurements accurately, and in addition, the capacitance value is known to be a temperature-dependent parameter. The capacitance can increase or decrease with changing temperature and the degree of change is related to the exact type of material used in the capacitor (Y5V, X7R, NPO, etc.). A further disadvantage of this approach is that the active circuit elements in the oscillator circuit can themselves have temperature-dependencies. These dependencies are nearly impossible to predict and may vary from circuit to circuit.
Some prior arts attempt to reduce the undesirable temperature dependencies by way of calibration techniques. As an example of prior art, U.S. Pat. No. 4,150,573 discloses the use of a thermistor to control a pulse oscillator circuit. In that patent, the pulse oscillator input is switched between the thermistor and a fixed resistor. A ratio is formed between the frequency produced by the thermistor and the frequency produced by the fixed resistor. This ratio divides out uncertainties associated with circuit component values and power supply variations. This provides the advantage of reducing the need for high accuracy parts and reduces the effects of power supply variations. However, this concept is unnecessarily complicated and it does not accurately measure the non-ideal behavior of the oscillator circuit nor does it null out temperature dependencies in the active components of the oscillator circuit. This concept may also introduce errors due to the temperature variations in the switching device.
For a medical thermometer, or other applications where extreme accuracy is required (less than 0.05 degrees C. uncertainty), the errors introduced by capacitance variation and by active circuit element variation cannot be tolerated. A method is needed that reduces these effects to a level of less than 0.01 degrees C. In addition, for a low-power application such as a miniature ingestible temperature sensor, it is not possible to use sophisticated, computer-controlled correction techniques, because the thermometer must be miniature, and is expected to be powered from a 1.5 volt battery source or a 3.0 volt battery source.
At present there are ingestible temperature responsive transmitters or ingestible temperature monitoring pills available. U.S. Pat. No. 4,689,621 issued to Kleinberg, and U.S. Pat. No. 4,844,076 issued to Lesho et al describe temperature responsive transmitters for use in ingestible capsules. Both devices disclosed employ crystal-controlled oscillators which transmit continuously on a single frequency determined by the temperature of the device. Lesho et al. also discloses a receiver employing a frequency counter to determine the frequency of the transmitter, and perform the calculation to determine the temperature sensed by the pill.
However, both of these devices have severe application limitations as they are purely analog devices, continuously transmitting on a single frequency. This prevents the use of multiple devices on a single subject, or on subjects in close proximity to each other because the signals from individual devices interfere with each other and cannot be distinguished. In addition, the prior art uses the temperature characteristics of a crystal to vary the oscillation frequency of the transmitter, requiring a frequency counter or other coherent detector in the receiver to determine the absolute frequency, and hence, the temperature. Use of a crystal to determine the oscillation frequency also requires an extensive calibration procedure, and requires the user of the device to input those calibration values into the receiver prior to use.
To prevent the batteries used in the ingestible capsule from being drained during storage, the prior art places a magnetic reed switch between the battery and the circuitry. Consequently, the device must be stored with a magnet in close proximity to keep the device de-activated, or it must use a rechargeable battery, and a recharger as disclosed in Lesho et al.
SUMMARY OF THE INVENTION
The basis of the invention in a first aspect is a circuit containing a temperature-dependent resistive element that controls the charge and discharge times of a multivibrator. By measuring the charge and discharge times, and converting those time elements with a formula, the resistance value of the resistive element can be determined. Because the resistance of the resistive element is uniquely related to temperature, the temperature can be uniquely determined.
As in the prior art, our invention also utilizes a thermistor-controlled multivibrator whose oscillation frequency is determined by the RC combination of the thermistor resistance and the value of the charging capacitor. However, the preferred embodiment contains several unique designs not introduced in the prior art. These designs provide novel means to (1) null out errors introduced by the non-ideal behavior of the multivibrator circuit and (2) vastly improve accuracy by nulling out undesirable temperature-induced effects within the passive and active circuit elements.
Our invention utilizes a CMOS 555 timer as the multivibrator circuit in the preferred embodiment. However, other oscillator designs might be used in other applications as well. For example, our method could be used with a bipolar, 5-volt 555 timer, when higher voltage power supplies would be available.
A first novel feature of the preferred embodiment of the first aspect of this invention is the determination of temperature through the measurement of the charge and discharge times of the sensor digital waveform. By measuring the ratio of the discharge time to the charge time, a sensor response may be obtained that is uniquely determined by the temperature that the sensor is in equilibrium with, e.g. body temperature, skin temperature, ambient temperature, etc.
A second novel feature of the preferred embodiment of t
Barton Donna K.
Bell Florian G.
Jones Christopher T.
Laird Jesse S.
Fulton Christopher W.
Mini-Mitter Company, Inc.
Smith-Hill John
Smith-Hill and Bedell
Verbitsky Gail
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