Sensors. Definition and features

Sensors are a very important part of the instrumentation and control of industrial processes, are used to determine the status of the process where they are installed and transform the variations of the measured variable into an electrical signal conditioned so that it can be received at their destination.

The output signal of a sensor usually goes to an indicator, to a recorder or a controller. There are a variety of sensors according to the variable to be measured, eg pressure, temperature, level, flow, position (proximity), speed, weight, voltage, current, frequency, viscosity, resistivity, radiation, pH, electrical conductivity, humidity and others.

When the sensors have digital outputs (only two possible values) are called switches and when they have analog outputs (more than two possible values) are called transmitters, in this sense they have pressure switches and pressure transmitters, for example.

Sensors. Block diagram.

Sensors features

Many of the features of the sensors depend on the variable to be measured, but others are common to all sensors. Some aspects to consider when selecting a sensor are:

• Accuracy specifies the difference between the measured value and the actual value of the variable being measured.
• Repeatability, the degree to which successive measurements differ from each other.
• Resolution is the smallest change that can be measured.
• Precision consists of the characteristics of repeatability and resolution.
• Sensitivity is given by the minimum value of the measured variable that causes a change in output.
• Error is the deviation between the measured value and true value.
• Linearity indicates that so close is the correlation between the input and output to a straight line.
• Range is the difference between the highest and the lowest value that can be measured.
• Response time is the ability of the instrument to follow the input variations.

There are other aspects to be taken into consideration, eg the sensor supply, the output type, output connection type (two-wire or three-wire). Proximity sensors, which are among the most used, are of several types: photoelectric, inductive, capacitive, ultrasonic and limit switch. These aspects will be discussed later in another post.