Studying electrical measurements

2. RESULTS

A measure is a measuring instrument designed to reproduce and (or) store PQ of one or several sizes, the values ​​of which are expressed in established units and are known with the required accuracy. [3]

The operation of reproducing a quantity of a given size can be formally represented as a conversion of a digital code N into a given physical quantity X_M based on the unit of a given physical quantity [Q]. Therefore, the transformation equation of the measure will be written in the form X_M = N [Q].

The output of the measure is a quantized analog value X_M of a given size, and the input should be the numerical value of the value N (Fig. 2).

Measure
(Q)

Х_М


N₂[Q]

N₁[Q]






N X_M = N[Q]

а)






N





N₁ N₂ N_n


b)

Fig. 2. The notation of a measure in structural diagrams (a) and the transformation function of a multi-valued measure (b)

Measures are classified into the following types:

- unambiguous, reproducing a physical quantity of one size, for example: a weight of 1 kg, a plane-parallel terminal block 100 mm, a capacitor of constant capacity, a normal element;

- multi-valued, reproducing PQ of different sizes, for example: variable capacitor, dashed measure of length. In addition, there are sets of measures, measure stores, installation, built-in and imported measures.

A measuring transducer (MT) is a technical device that performs the operation of converting an input signal X into an output X, the informative parameter of which, with a given degree of accuracy, is functionally connected with the informative parameter of the input signal and can be measured with a sufficient degree of accuracy. An informative parameter of the MD input signal is the parameter of the input signal, functionally related to the measured value and used to transmit its value, or being the measured value itself. [4]

The measuring transducer is designed to perform one measuring conversion. Its operation takes place under conditions when, in addition to the main signal X associated with the measured value, it is affected by many other signals Z, considered in this case as interference (Fig. 3, a).

Y

Real

MT
F(x)

Ideal




X Y=f(X, Z_i)

X

Z_i
a) b)

Fig. 3. Block diagram of the measuring transducer (a) and its conversion function (b)

The most important characteristic of the MT is the transformation function (equation) (Fig. 3, b), which describes the static properties of the converter and is generally written in the form Y = F (X, Z_i).

A comparison device (comparator) is a measuring device that makes it possible to compare measures of homogeneous quantities or readings of measuring devices.

The generalized block diagram of MD is shown in Fig. 1. The input signal is a measuring signal, one of the parameters of which is uniquely associated with the measured PQ:

_{ },

where: - informative parameter of the input signal; _{ } - measured PQ; _{ }- uninformative parameters of the input signal.

An uninformative parameter of the MD input signal is its parameter, which is not used to transmit the value of the measured value.

Comparators are especially widespread in modern electronic technology, where they are used to compare voltages and currents. For this purpose, a special type of integrated circuit has been developed.

The comparison performed by the comparator can be one and different in time. The first one is used much more often.

In electronic comparators, it is realized (Fig. 4) by serial connection of a subtractor (S), which forms the difference of input signals (X₁-X₂), and an AC voltage amplifier with a large gain (amplifier – limiter AL), which serves as an indicator of the sign of the difference.

The output signal of the AL is equal to its positive supply voltage (taken by a logical unit) if the difference (X₁-X₂)>0 and negative supply voltage (taken by a logical zero) if (X₁-X₂)<0.

Comparator

Х₁ – Х₂
Y 1

+U

Х

S


AL
₁




Y X₂ X₁

Х₂ 0

а) -U b)
Figure: 4. Block diagram of the comparator (a) and its transformation function (b)

The ideal comparator conversion function shown in Fig. 4b, is described by the equation

The degree of perfection of the comparator is determined by the minimum possible threshold of sensitivity, as well as by its speed - the time: switching from one state to another. For an ideal comparator, the threshold and the switching time are equal to zero. In a real comparator, the presence of a threshold will lead to an additive error.

A measuring device is a measuring device designed to generate a signal of measuring information in a form accessible for direct perception by an observer, i.e. in the form of direct reading, automatic registration, in the form of data for a computer or for automatic regulation and control devices. In electrical measurements, a large number of types of devices are used, which differ in design and structural diagram, in the method of comparing the measured value with a known, according to the nature of the measured value, counting and other signs. Each measuring device can be characterized by a structural diagram, which shows what transformation the measured value X undergoes in the device before we receive information about it at the output. Therefore, the main components of the device are measuring transducers. In addition, to ensure the normal operation of the measuring device, there are auxiliary units: power supplies, lighting devices, etc. [5]

A measuring device whose readings are a continuous function of changes in the measured value is called an analog measuring device. This is a device with a reading device in the form of an arrow and a scale. A measuring device that automatically generates discrete (coded). The signal measuring information and giving readings in digital form is called a digital measuring device. Measuring devices that can be read or read and recorded or only recorded are called recording measuring devices. If a recording measuring device records readings in the form of a diagram, then such devices are called self-recording devices. [6]

The registering measuring device, which provides for printing readings in digital form, is called a printing device. If the readings of the measuring instruments are functionally related to the sum of two or more values ​​supplied to them through different channels, they are called summing.

An example is a wattmeter that measures the total power of several generators. There are also integrating instruments in which the measured value is integrated over time. For example, an electricity meter. Devices, called null indicators, are used in comparison devices and some others to establish the absence of current in any section of the circuit or the equality of voltages in any sections of the circuit. These are galvanometers and electronic devices that produce a current pulse (or stop issuing pulses) when two voltages are equal, supplied to their outputs.

Depending on the means (units) used, electromechanical and electronic devices are distinguished.