Data processing: measuring – calibrating – or testing – Measurement system – Temperature measuring system
Reexamination Certificate
1999-07-30
2002-02-05
Wilson, Allan R. (Department: 2815)
Data processing: measuring, calibrating, or testing
Measurement system
Temperature measuring system
C257S470000, C327S512000, C327S513000
Reexamination Certificate
active
06345238
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of temperature sensors, and more particularly to temperature sensors based on the intrinsic characteristics of semiconductor devices.
BACKGROUND OF INVENTION
The sensing of temperature is one of the fundamental requirements for environmental control, as well as certain chemical, electrical and mechanical controls. Often, by controlling a process based on a measured temperature, the efficiency or other parameter may be altered, typically to an optimum. However, where the temperature sensor itself is expensive, cumbersome, or requires substantial signal conditioning and processing, these factors weigh heavily against the sensing of every relevant temperature, and rather to employ the minimum number of temperature sensors possible.
Thus, the logistical cost of providing and operating the sensor to optimize the operation of the system or process is weighed against the potential gains in system efficiency. Therefore, as the sensor systems become less expensive, more reliable, more robust, and easier to deploy, the reasonableness of widespread deployment is enhanced.
There are two ways of looking at the efficiency (or gains in efficiency) of a process: first, one considers a particular apparatus, operating under particular conditions. Second, one looks at all examples of the class of process as a whole. In the first case, in order to justify reconfiguration of the process to gain efficiency, the efficiency gains must not only exceed the incremental costs of the reconfiguration, but also the administrative costs involved as well. Therefore, it is often difficult to justify any changes to a system or process unless the gains are substantial, or the value of improved performance is high. On the other hand, when one looks at multiple installations of the process, or the design of new processes for widespread distribution, even small changes in the cost, availability, robustness, or other characteristics of a component may result in change in engineering policy.
In particular, the known electrical temperature sensor technologies have shortcomings which limit widespread deployment. Thus, many common processes operate at suboptimally, resulting in inefficient energy utilization, increased greenhouse gas production, and possibly increases in other pollutants. Thus, by providing temperature sensors and control systems responsive thereto, many processes may be improved. These improvements result in reduced energy utilization and environmental benefits.
While the control systems are central to the improvements discussed above, the present availability of advanced microprocessors and embedded control systems typically allow any reasonable degree of complex control to be provided, and the cost of the control itself is not prohibitive. On the other hand, linearization of sensors poses certain problems even for these available control systems. The most critical problem is the fact that the linearization must often be calibrated to the sensor, therefore, a significant individual step must be taken for each sensor to set up the system properly, and further, replacement of sensors becomes difficult, requiring a recalibration. Thus, where a temperature sensor has an uncalibrated nonlinear output, a substantial additional cost is incurred.
Often, in order to control a real process, an actuator is required. These actuators are indeed typically expensive and have other significant costs as well; however, these same actuators may already be provided within the process or system, or may be justified based on improvements in efficiency or performance.
Of course, other kinds of sensors may provide gains; however, as a class, temperature sensors tend to be one of the most common and critical sensor classes employed, especially in systems which employ large amounts of energy, such as combustion processes or heavy electrical systems.
There are a number of known temperature sensing technologies which produce an electrical output. These include thermocouples, resistive temperature sensors, thermistors, and semiconductor sensors. Each of these technologies has different applicable physics and other characteristics, and therefore may have different applications.
In many applications, it is desired to have a passive, two terminal, linear temperature sensor. Ideally, of course, the sensor is linear, accurate and of low cost. However, most thermal sensing technologies have an exponential feature in their sensitivity curves. Therefore, these sensors are limited to a particular linear approximate operating range and environment of operation and possibly require linearization as well.
Further, it is also desirable to provide temperature sensors which require little or no calibration. Thus, for example, the thermocouple type sensors have well defined characteristics, and require little calibration. Likewise, resistive temperature devices (RTDs) require only simple calibration, with very well defined variations with temperature. Thermistors, on the other hand, are more difficult to calibrate and maintain in calibration.
Semiconductor temperature sensors come in a variety of types. Normally, under constant current, the voltage across a bipolar semiconductor junction (or indeed virtually any type of semiconductor diode) will vary exponentially. Likewise, under constant voltage across the junction (within the limits of the device), the current will vary exponentially. Therefore, typical semiconductor sensor systems seek to linearize this output, and indeed may produce a high quality output. However, this leads to four problems. First, the linearization circuitry is active, and therefore requires a power supply; second, the active electronics typically have a relatively constrained temperature operating range; third, these sensors can be expensive; and fourth, the linearization circuitry may require calibration. Thus, while there are well understood methods for compensating performance of nonlinear temperature sensors to produce a linear output, these are not ideal for all purposes.
The so-called military range of temperatures, −50 to 125° C., is typically considered the full range of terrestrial environmental conditions. In an electrical system, the maximum temperature may exceed 125° C., for example 150° C., and in automotive environments, for example, sensors are sought which may reliably operate up to 200° C. In combustion systems, or other environments, the temperatures may extend substantially above this level. In terms of thermodynamics, this automotive range represents a significant range, i.e., 223 to 423° K, an almost 2:1 range. Therefore, sensors which have a temperature sensitivity which depends strongly on an exponential (or logarithmic) function of temperature are expected to exhibit substantial non-linearity over this range of temperatures.
Thermistors are typically formed of a semiconductor material, e.g., an n-type doped semiconductor material, without any significant bipolar junction. The semiconductor has an intrinsic and non-linear change in conductivity with temperature. While it is possible to use silicon semiconductor alloys as thermistors, the materials in common use have a significantly higher sensitivity to temperature, and thus produce a higher level output. Thermistors, may have a positive temperature coefficient (PTC) or a negative temperature coefficient (NTC), depending on the base material and doping. Compensation networks may be used to linearize the thermistor, but these may require individual calibration. Thermistors and their associated linearization components are difficult to calibrate to obtain high accuracy (and do not maintain calibration), and the devices may be subject to multivariate environmental influences, making stability poor. Thermistors, however, are inexpensive and typical devices offer a logarithmic temperature response over a range of about −65° C. to 150° C.
Another type of temperature sensor is an Resistive Temperature Device (RTD), which has a wide operating temperature range and high accuracy
Airpax Corporation, LLC
Milde Hoffberg & Macklin, LLP
Wilson Allan R.
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