Solid state temperature measuring device and method

Details Solid state temperature measuring device and method Solid state temperature measuring device and method

2001-08-14

2004-01-20

Fulton, Christopher W. (Department: 2859)

Thermal measuring and testing

Temperature measurement

By electrical or magnetic heat sensor

C257S467000

Reexamination Certificate

active

06679628

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a device and a method for measuring temperature, and more particularly to a device and method for digitally measuring an analog voltage, proportional to temperature.
BACKGROUND OF THE INVENTION
Numerous methods exist to measure temperature by using electronic devices such as semiconductors and resistors. Semiconductor devices used vary from a simple, low cost diode to a pair of transistors, specially manufactured for high accuracy of temperature measurement. Other devices use resistors of varying stability to support a highly accurate and easily calibrated temperature measuring device. The resistors are placed in a Wheatstone bridge configuration. A Wheatstone bridge is a common technique wherein a probe, typically a thermistor, causes the bridge to become unbalanced as the temperature changes. The temperature changes sensed by the probe cause the bridge to transmit an analog signal.
Temperature measurement is critical to many applications. Typically, a thermocouple device is insert into a liquid, gas, or in contact with a surface. As the thermocouple senses a temperature change in its environment, the thermocouple circuit creates an analog signal. As opposed to a digital signal, or a series of zeros and ones that represent the state of an object, the analog signal is continuously variable with temperature. However, every analog signal must be converted to a digital signal for use by a computer system, such as a programmable logic controller (PLC) system.
A PLC control system has numerous inputs acting as conditions that a PLC acts upon. Based upon the input temperature, the control system may cause an alarm to sound, or slow the reaction rate through a reduction in a catalyst. It is the critical nature of temperature measurement across numerous applications that result in a wide variety of measuring devices.
A widely used temperature measurement device
100
in
FIG. 1A
uses a simple, low cost diode in which a constant current I
A
is applied from a current source. Every diode
104
has a junction voltage with varies with temperature. This voltage shift with temperature is called the slope of the diode, (S). The voltage
106
across the diode is proportional to the temperature of the junction represented by the equation T~=K−S/degree Celsius. The slope S and the constant K are measured in millivolts. In addition, each diode junction voltage
108
or slope has an error term as the diode heats up because of the constant current source
102
.
Other devices use a resistor circuit to adjust the slope of the diodes in the circuit. These resistor circuits are used to compensate for the cold junction temperature of the thermocouple. The cold junction or reference temperature is the analog output of the thermocouple circuit at zero degrees Celsius. A thermocouple device produces an analog output proportional to the measured temperature. Compensating for the cold junction temperature improves the accuracy of the thermocouple device. The analog-to-digital converter reads the voltage difference between the measured temperature and the cold junction temperature. A fixed cold junction signal produces an accurate and repeatable base signal to the analog-to-digital converter. A similar system is described in U.S. Pat. No. 4,441,071, which is incorporated herein by reference. In another system, amplifiers and a power supply are added to the resistor network to compensate for the cold junction temperature. A similar system is described in U.S. Pat. No. 4,126,042, which is incorporated herein by reference.
Other devices use a combination of resistors and amplifiers to measure a voltage change, hence, a temperature change over a predefined range. Typically, these systems are used for measuring high temperatures requiring a highly accurate result. The temperature represented by the output signal is low to moderate requiring the use of amplifiers. A similar system is described in U.S. Pat. No. 5,611,624, which is incorporated herein by reference.
Yet in other devices, the resistor not the diode provides the analog signal based upon the sensed temperature change. The resistors are configured in a Wheatstone bridge circuit with a power source applied across the circuit. A first circuit called a reference circuit provides a first signal that is constant to the analog-to-digital converter. A second circuit called a bridge circuit provides a second signal, the second signal's output is proportional to the temperature change, to the analog-to-digital converter. The comparison of the two separate signals allows the use of inexpensive resistors in the circuit to achieve a highly accurate temperature measurement. A similar system is described in U.S. Pat. No. 5,655,305, which is incorporated herein by reference.
FIG. 1B
shows a specially manufactured transistor circuit
150
for measuring temperature. This circuit
150
improves measuring accuracy at a much greater cost than the single diode device
100
of
FIG. 1A. A
pair of transistors
158
and
160
, having a known base-emitter junction area ratio are inputs to a differential amplifier
166
. A voltage
152
is applied across the transistors
158
,
160
and a pair of resistors
154
,
156
to draw currents
162
and
164
through the transistors. The ratio of the base-emitter current density of the transistor pair
158
and
160
yields a known slope. the error term of the junction voltage at the base emitter cancels out when the current density or surface area of each transistor is controlled at the time of manufacture.
The voltage change due to the measured temperature causes the currents
162
and
164
to flow through the transistor pair
158
,
160
, which amplify the small voltage change. The amplified voltage across the transistor pair
158
,
160
is the input to a differential amplifier
166
, having an output proportional to the absolute temperature measured (PTAT). One of the transistors
158
,
160
in the transistor pair is the cold junction or reference temperature circuit. A similar system is described in the National Semiconductor LM34, LM82, LM83, and LM84 specifications, which are incorporated herein by reference. A similar system is described in U.S. Pat. No. 4,475,103, which is incorporated herein by reference.
Although some devices use a diode or a group of diodes, each temperature device design trades cost for accuracy and temperature range. The temperature device of
FIG. 1A
has the advantage of low cost and an output signal capable of being measured without additional circuitry. Additional circuitry increases manufacturing cost, thus, an increased cost to the end product. Moreover, additional circuitry increases cost because of engineering and design considerations in the product using the more complicated circuit.
The diode circuit of
FIG. 1A
has many disadvantages shared by other low cost circuits. The temperature range and accuracy depends upon the diode type and details of its manufacture. The design engineer using the device of
FIG. 1A
must have engineering information on the diode itself. Regardless of the diode, the circuit of
FIG. 1A
has an inherently low accuracy and requires calibration before use. Calibration fixes the use of the
FIG. 1A
circuit, thus, limiting its reuse in other applications. Last, the higher the temperature measured the lower the voltage output from the circuit. This would require additional circuitry thus leading to an increased cost.
The cold junction or reference temperature circuit used in thermocouple based devices of other devices leads to additional cost and engineering complexity. Without a cold junction temperature, the actual measurement has no baseline. Additional circuitry provides a cold junction analog signal as an input to the circuitry of the temperature measuring device. The additional circuitry narrows the use of the temperature measuring devices. The narrowed use results from a need to improve accuracy and reduce the calibration time. Electronic circuits lose accuracy because of the heat gener

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