Engineering / 12. Automated control systems in
production
Muhtarova M.N., Ermaganbetova S.D., Alibayeva A., Khozhabayeva Zh.
Al-Farabi Kazakh National University, Kazakhstan
Information on sensors from the composition of
measuring systems, their classification and their metrological characteristics
Annotation. A measuring system is a collection of measuring,
connecting, computational components that form measuring channels and auxiliary
devices functioning as a single unit intended to obtain information about the
state of an object by means of measuring transformations in the general case of
a set of time-varying and spatially distributed quantities characterizing This
state; Machine processing of measurement results; Registration and indication
of measurement results and the results of their machining; Transforming these
data into system output signals for different purposes. "
Keywords: sensors, measuring transducer, parameter, signal.
Sensors are information devices that convert a
controlled physical parameter into a signal suitable for further processing by
a person or a machine; Sensors are called devices whose signals affect,
bypassing a person, the subsequent links in the processing of information. There
are a huge number of sensors that can be used in various measuring systems.
When classifying the sensor, it is first of all
indicated by the sensor of what physical quantity it is (for example, a
pressure sensor).
By types of input and output values, the sensors can
be divided into converters:
- Electrical quantities in electrical;
- non-electrical quantities in non-electrical
quantities.
The sensors are also divided into generator and
parametric. If the output is an "active" electrical quantity -
voltage, charge, current, electromotive force (EMF), then we have a generator
sensor. For example, a thermocouple is a generator sensor, it has an EMF
output. If the output is a "passive" electric quantity, often
referred to as a parameter, that is, resistance, inductance, mutual inductance,
capacitance, dielectric or magnetic permeability, etc., then this sensor is
parametric. For example, an inductive displacement sensor is a parametric
sensor. Here the inductance of the sensor is a function of displacement.
Therefore, when classifying a sensor, it is necessary to indicate which active
value or parameter is output.
Sensors can be classified according to the following
characteristics: by designation, by output signal, by the way information is
presented, by remoteness of action.
The functions performed by instruments and sensors can
be understood from the consideration of information schemes of two typical
tasks - the task of determining the characteristics of an object and the task
of controlling an object.
The task of determining characteristics, which is
aimed at evaluating the physical properties of an object, is solved by
analyzing the accumulated information about the object's response to
deterministic or random external influences. The accumulation of information is
carried out by an open circuit (Fig. 1, a): the output coordinate of the object
x(f),
depending on the external action u(t)t
is perceived by the device or sensor and converted into a signal y(t).
The task of management, aimed at organizing the
necessary behavior of the object, is solved by comparing and analyzing
information about the actual and specified parameters of the controlled process
and the corresponding impact on the management bodies of the facility. The
control is performed in a closed circuit (Figure 1, b), in which the output
coordinate of the object x(t),
converted by the instrument or sensor into the signal y(t), is compared with the desired (required) output coordinate xo(f)
using the master to the signal yo(t).
The control can be manual or automatic. With manual
control, a person compares the reading of the instrument y(t) with the set value of the monitored parameter yo(t)
and acts in the necessary manner on the
control to ensure that
y(t)=yo(t).
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The operation of the operator is facilitated if the
instrument shows directly the amount of the error
Δy(t)=y(t)-yo(t),
while the
operator's task is to continuously note the instrument reading to zero, such a
device is called a director.
There is a set of metrological characteristics that
are available for all sensors regardless of their type, the principle of
operation and the type of physical quantity being converted. Metrological
support of the measuring system includes standardization and calculation of
metrological characteristics, tests for the purpose of type approval and
testing for compliance with the approved type, certification, verification and
calibration.
Speaking about the metrological support of the
measuring system, we must not forget that the modern measuring system consists
of two large parts: hardware and software. It is difficult to distinguish which
of these parts is more important, and both of them directly affect the
metrological characteristics of the system. In some cases, the impact of
software on the measurement results is significantly higher than the hardware.
Therefore, the software must also undergo metrological certification.
The conversion function f(x), the calibration characteristic and the range of measurements
(transformations) indicate, respectively, the theoretical and experimental
nature of the relationship between the input and output quantities, and also
the limits in which the sensor or any measuring transducer transforms the input
quantity with an allowable error. The values of the input
quantity that limit the range of measurements (transformations) from below and
from above are called respectively the lower and upper limits of the
measurements (transformations). The transformation function and the calibration
characteristic of the sensor, like any measuring transducer, are tried to be as
linear as possible.
Parameters of the sensor are the sensitivity S and the conversion factor Kc of the input quantity to
the output value. The sensitivity of a chain of sensors connected in series and
secondary measuring converters with linear transformation functions is equal to
the product of their sensitivities:
S = ∏ Si ïðè i = 1,
2, …. N
The above formula determines the resulting sensitivity
and determines the requirement of linearity of conversion of each of the
measuring transducers.
The settling time characterizing the decay rate of the
transient (unsteady) process in the sensor plays a decisive role when choosing
the latter as an input measuring transducer when it acts on a measuring
instrument operating in a dynamic mode. In this case, we sometimes use the
analogous term - the time constant, which also characterizes the inertial
properties of the converter. The requirement for the speed of the sensor is hence.
Since the sensors are measuring instruments, the most
important metrological characteristic is the error in the conversion. To the
previously mentioned requirements, the requirements of stability of time
characteristics, noise immunity, reliability should also be added. In practice,
the parameters and characteristics of the sensors are improved by applying
circuit and algorithmic solutions for processing.
As an example, consider the temperature sensor
HEL-700. This is a platinum thermistor (RTD) with a linear temperature
dependence of the resistance. It features high accuracy, speed,
interchangeability and a wide operating temperature range. The sensor
parameters are as follows:
1) Operating temperature range (degrees C) - (-200 ..
+ 260) ° C.
2) Resistance at 0 ° C - 1000 ± 2 Ohm.
3) Time constant - less than 1 s.
4) Absolute error - no more than ± 0,5 ° Ñ.
6) Recommended measuring current is 1.0 mA.
7) Sensitivity in the operating temperature range -
not worse than 3.0 Ohm/deg.
8) Overall dimensions - 2,8x4,75 mm.
9) Measured medium - gas, water.
Many modern sensors, which are often called
intelligent, are combined with electronic circuits for signal normalization, as
well as control circuits for operating modes and preprocessing circuits.
1. Tsapenko M.P. Measuring information systems: structure and
algorithms, circuit design: Proc. Manual for universities. - Moscow:
Energoatomizdat, 1985.
2. Yevdokimov Yu.K., Lindval V.R., Shcherbakov G.I. LabVIEW for the
radio engineer: from the virtual model to the real device. A practical guide
for working in the LabVIEW software environment. - Moscow: DMK Press, 2007.
3. Lebedev A.V., Petrov A.B. Computer-aided design of measuring
information systems / Methodological instructions for use in graduate engineering
/ MIREA. - M., 2004.
4. V.I. Kalashnikov, S.V. Nefedov, A.B.Putilin and others; Ed.
G.G.Ranneva.Information and measuring technology and technology: Textbook for
universities - Moscow: Higher School, 2002.
5. N.N. Evtikhiev, J.A. Kupershmidt, V.F. Papulovsky, V.N. Skugorov. Measurement
of electric and non-electrical quantities: Textbook. Manual for schools. - M.:
Energy Atomizdat, 1990.