Sensors

Electric sensors

Also in this series:
  1. Temperature Sensors: Thermocouples
  2. Temperature sensors: RTD, an accurate alternative
  3. Thermistor, a champion of sensitivity

The transducers or sensors are essential devices in a control system or data acquisition. Think, for example, to control the temperature of the water in a radiator, brightness in a damp environment or humidity in a greenhouse.

In all cases, there is a physical quantity to be controlled and it should be read by devices that are capable of providing an electric measure (resistance, capacitance, voltage, current, etc.) proportional to the value of the physical quantity to be measured.

The output signal from the transducer cannot always be used directly, but often must be adapted to the characteristics of the circuit to which it is to be applied. If it is a microprocessor control, transducer signal (if analog) should be converted into a digital signal before it can be acquired.

DAQ

A signal conditioning circuit, interposed between the transducer and A/D converter, is designed to adapt the output of the transducer to the characteristics of the analog-to-digital converter and it generally performs the following functions:

  • convert the transducer output signal into a voltage signal, making thus a conversion that, depending on the type of transducer, will be temperature/voltage, pressure/voltage, humidity/voltage, etc.
  • modify the slope of the transducer output characteristic, this is necessary, for example, if the output is decreasing in corrispondence with an input physical increasing quantity;
  • linearize the input/output characteristics of the transducer, or it makes the output signal proportional with respect to the measuring input;
  • adjust the offset, or, ideally, obtain a zero output (0 Volts) when the transducer has minimal amplitude output;
  • adjust the scale factor, i.e. it makes the transducer output compatible with the maximum allowable input to the A/D converter;
  • limit the bandwidth of the output signal of the transducer;
  • galvanically separate the transducer from the acquisition system, as often requested by security specifications;
Sensors and transducers

Transducers are classified in many ways but for the purposes of data acquisition a basic division is between:

  1. Analog. The output signal is a continuous signal i.e. it varies continuously within a range of values delimited by a maximum and minimum. Within that range, the output will then assume all infinites real values. This type of transducers must interface with digital acquisition system through an A/D converter;
  2. Digital. The output signal takes on two levels, compatible with digital systems, and can be directly interfaced with digital acquisition systems.
  3. In future articles we will look at the main types of transducers belonging to these two broad categories, with some reference to commercially available sensors.

    [ssba]