Measuring Transducer

a measuring device that converts a physical quantity being measured into a signal for subsequent transmission, processing, or recording. In contrast to a measuring instrument, the signal at the output of a measuring transducer (the output quantity) cannot be perceived directly by an observer. A mandatory condition for measuring conversion is the storage in the output quantity of the measuring transducer of information on the quantitative value of the quantity being measured. Measuring conversion is the only method of construction for all measuring devices.

Measuring transducers differ from other types of converters in their ability to perform transformations with established accuracy. Measuring conversion of a given type (such as the conversion of temperature into mechanical displacement) may be accomplished by various measuring transducers, such as a mercury thermometer, a bimetallic strip, or a thermocouple with a millivoltmeter.

The concept of representing measuring instruments as devices that perform a number of successive conversions from the perception of the quantity measured to the reception of the result of the measurement was first advanced in the USSR by M.L. Tsukerman and was finally formulated as applicable to the measurement of nonelectrical quantities by F.E. Temnikov and R.R. Kharchenko in 1948. In the 1960’s this concept became generally recognized in all fields of measurement technology, instrument-making, and metrology.

The principle of operation of measuring transducers may be based on the use of virtually any physical phenomenon. The conversion of any measured quantities into an electrical signal became the predominant trend between the 1940’s and the 1970’s. Depending on the type of quantities converted, a distinction is made among measuring transducers that convert electrical quantities into electrical quantities, electrical into nonelectrical, nonelectrical into electrical, and nonelectrical into nonelectrical. Voltage and current dividers, instrument transformers, and measuring current and voltage amplifiers are examples of the first type; sensors of ultrasonic flowmeters and mechanisms of electrical measuring devices that convert a change in current intensity or voltage into a deflection of a pointer or beam of light are examples of the second type; thermocouples, thermoresistors, tensoresistors, photocells, and rheo-static, volumetric, and inductance displacement pickups are examples of the third type; and pneumatic measuring transducers, levers, gear drives, diaphragms, bellows, and optical systems are examples of the fourth type.

The design combination of several measuring transducers is also a measuring transducer. Examples of such a combination include a sensor that is a set of measuring transducers applied to the measured object and the so-called intermediate measuring transducer, which is a set of measuring transducers that convert the output signals of sensors into other signals that are more convenient for transmission, processing, or recording. In terms of structure, the constituent measuring transducers are subdivided into direct and equalizing types. The first type is characterized by the fact that all transformations of quantities are performed in only one direction (directly from an input quantity to an output quantity). In this case the resultant error is determined as the sum of the errors of all constituent measuring transducers, taking into consideration the correlation links of the errors. The use of inverse transformation of the output quantity into a quantity that is like the input quantity and that equalizes it is characteristic of the latter. In this case the resultant error is determined only by the error of the inverse transformation and by the degree of imbalance. Equalizing measuring transducers are subdivided into slave transducers with feedback and with static or astatic equalization and program-equalized transducers. Slave measuring transducers with feedback ensure temporal continuity of conversion; their shortcoming is the danger of the loss of stability, which is manifested in the appearance of natural oscillations as the depth of feedback is increased. Program-equalized measuring transducers do not have this shortcoming, but they are characterized by discontinuity of the output quantity—that is, the appearance of the output quantity only at certain discrete instants.

In the 1960’s a trend toward the conversion of measured quantities into the frequency of electrical impulses by means of so-called frequency measuring transducers took shape. Such measuring transducers have been developed for almost all known physical quantities. The main advantages of frequency measuring transducers are the simplicity and high accuracy of transmission of their output quantity (frequency) through communications channels, as well as the relative simplicity of the digital readout of the result of measurement by means of numerical frequency meters. Measuring transducers of analog quantities into digital code and of digital quantities into analog code are widely used in digital measuring devices. The principles of both frequency measuring transducers (integrating analog-digital units) and program-equalized measuring transducers (time-impulse and digital-coding analog-digital transducers) are used.