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In scientific research, industrial and agricultural production, national defense construction, or even daily life, the vast majority of information that people get is non-electric quantity information,which is often difficult to measure accurately. And even if this information is detectable, it is so hard to enlarge, process and transmit. Hence, a special device is often used to detect the relevant information sensitively and accurately and then transform the information into physical quantities which are convenient for processing. It is well known that electrical signals are easy to be amplified, processed, stored and transmitted over a long distance. Furthermore, computers can only process electrical signals. Consequently, most measured non-electric quantities need to be converted into electric quantities for measurement, which forms the electrical measurement technology of non-electrical quantity. This non-electrical quantity measurement is also a kind of conversion measuring method.
The electrical measuring system of non-electric quantity generally includes sensors, measuring circuits, amplifiers, indicators, recorder, and other components. Among all of them, the most critical component is the realization device of variable conversion-sensor. The sensor can convert the non-electric quantity into electrical signals which can be measured by electronic amplification, calculation, display and other devices at a certain level of accuracy. At present, various kinds of sensors, such as resistance type, inductance type, magneto electric type, piezoelectric type, thermoelectric type, photoelectric type, etc. , have been extensively used in related fields.
Due to the advantages of high measuring accuracy, fast speed of response, automatic continuous measurements, and convenient long-distance measurements, the application fields of electrical measuring technology of non-electric quantity are becoming wider and wider. Several commonly used electrical measurement methods of non-electric quantity are described as follows.
Thermoelectric conversion is the measurement of converting thermal quantity into electrical quantity. For example, based on the principle of thermo electromotive force, the temperature measurement is converted into the thermo electromotive force measurement of thermocouple. According to the law of resistance changes with temperature, the measurement of temperature variation is con verted into the measurement of resistance change of thermistor.
It is a conversion between pressure and voltage. Both piezoelectric ceramic chips and pressure sensors belong to this kind of converter. The common electronic scale is one of the applications with piezoelectric converter. In addition, microphones and speakers that we are familiar with also belong to this kind of converter. A microphone converts the pressure change of a sound wave into the corresponding voltage change, while a speaker performs the opposite conversion. That is the way to convert the changed electrical signal into the corresponding sound wave. Based on this kind of conversion method and some electrical instruments, we can also measure some physical quantities like sound pressure, sound speed, and sound frequency, etc.
It is a measuring method of converting luminous flux into electrical quantity based on the photoelectric effect. Photoelectric converters include photocell, photomultiplier, photosensitive diode and many others. And they all have been widely used in controlling and measuring systems. In recent years, photoelectric converters are also widely used in optical communication and optical input equipment of computers. Optical measurement can be transformed into electrical measurement by using photoelectric elements.
This is a conversion measurement to convert the measured physical quantity (such as displace ment, speed, pressure, magnetic field, etc. ) into electromotive force based on the electromagnetic induction principle or Hall effect. According to the differences based on magneto electric conversion principle, there are two kinds of common sensors: magneto electric induction sensor and Hall sensor. Due to the limitation of measurement principle, magneto electric induction sensor is only suitable for dynamic measurement which can directly measure the velocity of vibrating object and the angular velocity of rotating object. If the integral circuit or differential circuit is adopted in the measuring circuit, it can be used to measure the displacement or acceleration directly.
At present, many kinds of conversion measuring methods have been widely used in different application fields. Therefore, in the design and application of experimental scheme, one should pay attention to the following points:
(1) The correctness of the conversion principle and parameter relation should be determined.
(2) The sensor should have stable and large-scale output to effectively amplify or transmit the detected signals.
(3) Determining whether there are other effects accompanying in the conversion process is re quired. If so, extra steps must be taken to compensate or eliminate these effects.
(4) The feasibility and economic benefits of the conversion system and measuring process should be considered with full hearts.