自动化专业外文文献

Development of Sensor New Technology

Sensor is one kind component which can transform the physical quantity,

chemistry quantity and the biomass into electrical signal. The output signal

has the different forms like the voltage, the electric current, the frequency, the

pulse and so on, which can satisfy the signal transmission, processing,

recording, and demonstration and control demands. So it is the automatic

detection system and in the automatic control industry .If automatic

Technology is used wider, then sensor is more important.

Several key words of the sensor：

1 Sensor Elements

Although there are exception ,most sensor consist of a sensing element and a

conversion or control element. For example, diaphragms,bellows,strain tubes and

rings, bourdon tubes, and cantilevers are sensing elements which respond to changes

in pressure or force and convert these physical quantities into a displacement. This

displacement may then be used to change an electrical parameter such as voltage,

resistance, capacitance, or inductance. Such combination of mechanical and electrical

elements form electromechanical transducing devices or sensor. Similar combination

can be made for other energy input such as thermal. Photo, magnetic and

chemical,giving thermoelectric, photoelectric,electromaanetic, and electrochemical

sensor respectively.

2 Sensor Sensitivity

The relationship between the measured and the sensor output signal is usually

obtained by calibration tests and is referred to as the sensor sensitivity K1=

output-signal increment / measured increment . In practice, the sensor sensitivity is

usually known, and, by measuring the output signal, the input quantity is determined

from input= output-signal increment / K1.

3 Characteristics of an Ideal Sensor

The high sensor should exhibit the following characteristics.

ahigh fidelity-the sensor output waveform shape be a faithful reproduction of the

measured; there should be minimum distortion.

bThere should be minimum interference with the quantity being measured; the

presence of the sensor should not alter the measured in any way.

cSize. The sensor must be capable of being placed exactly where it is needed. dThere should be a linear relationship between the measured and the sensor signal.

eThe sensor should have minimum sensitivity to external effects, pressure sensor,for

example,are often subjected to external effects such vibration and temperature.

fThe natural frequency of the sensor should be well separated from the frequency and

harmonics of the measurand.

Sensors can be divided into the following categories：

1 Electrical Sensor

Electrical sensor exhibit many of the ideal characteristics. In addition they offer

high sensitivity as well as promoting the possible of remote indication or

mesdurement.

Electrical sensor can be divided into two distinct groups:

avariable-control-parameter types,which include:

iresistance

iicapacitance

iiiinductance

ivmutual-inductance types

These sensor all rely on external excitation voltage for their operation.

bself-generating types,which include

ielectromagnetic

iithermoelectric

iiiphotoemissive

ivpiezo-electric types

These all themselves produce an output voltage in response to the measurand input

and their effects are reversible. For example, a piezo-electric sensor normally

produces an output voltage in response to the deformation of a crystalline material;

however, if an alternating voltage is applied across the material, the sensor exhibits

the reversible effect by deforming or vibrating at the frequency of the alternating

voltage.

2 Resistance Sensor

Resistance sensor may be divided into two groups, as follows:

iThose which experience a large resistance change, measured by using

potential-divider methods. Potentiometers are in this group.

iiThose which experience a small resistance change, measured by bridge-circuit

methods. Examples of this group include strain gauges and resistance thermometers. 3 Capacitive Sensor

The capacitance can thus made to vary by changing either the relative permittivity,

the effective area, or the distance separating the plates. The characteristic curves

indicate that variations of area and relative permittivity give a linear relationship only

over a small range of spacings. Thus the sensitivity is high for small values of

d. Unlike the potentionmeter, the variable-distance capacitive sensor has an infinite

resolution making it most suitable for measuring small increments of displacement or

quantities which may be changed to produce a displacement.

4 Inductive Sensor

The inductance can thus be made to vary by changing the reluctance of the inductive

circuit.

Measuring techniques used with capacitive and inductive sensor:

aA.C. excited bridges using differential capacitors inductors.

bA.C. potentiometer circuits for dynamic measurements.