Interface of transducers with a measuring apparatus. Capacitance-resistance transducers
The chart of connection of transducers has a major value not only because there is a large number of their different types. Fortunately, the charts used for this purpose are identical for the devices of different type and them easily to generalize. At the same time an engineer must realize distinctions between transducers, neatly to adapt the chart of interface for a concrete device. At creation of the measuring systems it is necessary to work out also the problems of noises and cross-coupling of transducers and other blocks of the systems on each other.
Most principles of providing of compatibility of measuring transducers can be understood, considering as an example capacitance-resistance transducers. For them resistance changes at varying of measurand. Usually an interface chart is used in an order to cause the change of resistance the change of tension. This tension after forms an entrance signal for other part of the measuring system.
The great number of methods of transformation of change of resistance is known in the change of tension. The simplest chart for this purpose is a divizor of tension (fig. 1, а) in which resistance of transformer R1 is included consistently with other resistance of Rt and tension of excitation of Vexc. Output tension of Vout changes at varying of resistance of capacitance-resistance transformer in accordance with the known formula of divizor of tension
Vout = Vexc (R1/ R1 + Rt)
When changing resistance of transformer causes the change of loading of source of excitant tension, for providing of excitation preferably to use the source of direct-current.
Really, if the source of direct-current is used, then in a chart it is not needed to include successive resistance is tension, generated on resistance of transformer, it is possible to measure directly (fig. 1, б).
Certainly, the most widespread method of connection of capacitance-resistance transducers with the measuring system is application of the unbalanced bridge (fig. 1, в) at which resistance of device forms one of shoulders of bridge of Uytstone. If a capacitance-resistance transformer has one more pickoff, then in ideal case them it is necessary also to connect in a bridge chart. Usually consistently with a transformer the tuning resistor of Rtrim is included, that a bridge can it was be balanced in any point (say, in the point of the least resistance) of turn-down of measurand.