Object of measurement and its model. Informative parameters of measuring signals

Researcher's, engineer’s attention may be concentrated on all the objects of the material world. Level of their cognition may be determined as the information on the inherent properties, first of all, on their quantitative characteristics obtained by means of a measurement.

In the process, as a rule, one can be interested not in all of the object properties and their conventional numerical displays, so-called parameters, but only in those, which are necessary for implementation of certain indispensable functions.

That’s the reason, why during researches, connected with a measurement, a real object of the material world, that may be characterized with a vast number of properties with inherent numerical values of parameters, is substituted for a simplified model, displaying only one parameter, which may be interesting for the given research. It’s known as the informative parameter.

For example, a resistor is used, as a rule, for electric current limitation or its transformation into a voltage drop. In this case the only property we are interested in is the property to make a resistance to an electric current flow and this one is measured and expressed by numerical values (e.g. R=100 Ohm). On electrical schemes this simplified model is represented as a rectangle.

But a real resistor is a rather complicated object of the material world that may possesses by the vast number of various characteristics and parameters, which in this case can be treated as non-informative. E.g., its geometrical dimensions, form, mass, colour, etc.

Such simplification, substitution of the real objects for the models is inherent to all fields of science, the simplest of which, by the way, is mathematics, that deals with not the unlimitedly complicated objects of the material world, but with the simplest, “ideal ”, non-existing in nature concepts, models and symbols.

A competent selection of an object model accelerates achievement of practical results. But for this purpose the deep knowledge and the researcher’s experience are necessary, as he, when creating a model, makes the considerations and foresees what properties of the object may be neglected and which ones have to be considered, because they will influence significantly the processes being researched.

For example, the above – mentioned resistor when used in direct current circuitry is well simulated by the pattern in Fig.7a.

However for alternating current, especially on the higher frequencies this simplified model describes the property of the resistor to influence the current inadequately. Since apart from the active resistance of the resistor R its inductance L and capacity C must be taken into consideration.

The later two parameters depend upon the resistor`s construction, its geometrical dimensions, materials it is made of, etc.

 Mostly they reveal itself for the resistors designed as a coil of the high resistivity wires (manganin, nichrome, constantan, etc).

Any coil has some inductance and capacitance, that consists of the capacitance of the neighbouring wire turns.

Fig7. Models of a resistor: a) is for direct current, b) is for high frequencies current, c) is for superhigh frequencies current

Model of such resistor for a high frequency current is represented in Fig.7b; hereby the parameters R, L and C depend upon the frequency.

If a resistor is used under superhigh frequencies, for example, in some units of radars, converters, especially for satellite TV equipment, where frequencies are measured in GHz and wave-length in cm, the resistor is considered as a distributed-parameter line (Fig. 7c).

In principle, the universal models of objects, so-called metamodels, may be developed, but for the majority of applications it can hardly be profitable, because it complicates the measurements and calculations of the metamodel parameters.

The largest part of modern measurement instruments are highly selective and measure only one parameter, while paying no attention to the other parameters of the object being researched.

Some MIs can convey information about two parameters of the object researched. For example, alternating current bridges yield an active and reactive conductivity, quality factor of a coil, capacitance and loss-angle tangent of a capacitor.

These MIs present information about an object in accordance with their measurement or metrological model, even simplified, unlike the above mentioned mathematical model.

Some alternating current bridges present information in terms of active and reactive conductivities connected in parallel, others – in terms of active and reactive resistances, connected in series. In these cases the MI properties dictate the structure of the model used.

Recently, the measurement instruments (for example, the bridges for biological research) presenting at once three parameters of the object investigated, have appeared.

The most modern computer-controlled measurement complexes can measure many parameters of an object at once, and, after performance of all necessary calculations, can display a complex parameter that characterize the object condition.

Questions for self-verification

1. What is a measurement instrument? What requirements must it satisfy?

2. What are the functions of a material measure? Give examples of material measures of electrical and non-electrical quantities.

3. What are an indicating (measurement) instrument and a measurement transducer? What is the difference between them? What examples of those MIs can you give?

4. What is an auxiliary MI? What examples of those MIs can you give?

5.  What is a measurement installation? What are the examples of those MIs?

6. What is a measurement system? What examples of those MIs can you give?

7. What is the difference between the informative and non- informative parameters? What examples of electrical and non-electrical measurements can you give?

8.   Why do we substitute a real object of the material world by the simplified models? What advantages does this substitution offer?

9. What selectivity do the different MIs have and how is it connected with the model of the object examined?