Electrical Instrumentations and Measurements Lecture 4 PDF

Electrical Instrumentations and Measurements ELC 243 Lecture 4  Sensors and Transducers A sensible distinction is to use 'sensor' for the sensing element itself, and 'transducer' for the sensing element plus any associated circ...

Electrical Instrumentations and Measurements ELC 243 Lecture 4  Sensors and Transducers A sensible distinction is to use 'sensor' for the sensing element itself, and 'transducer' for the sensing element plus any associated circuitry. For example, a thermistor is a sensor, since it responds to a stimulus (changes its resistance with temperature), but only becomes a transducer when connected in a bridge circuit to convert change in resistance to change in voltage, since the complete circuit then transduces from the thermal to the electrical domain. Sensor Circuits Transducer  Types of transducer Since the conversion of energy from one form to another is an essential characteristic of the sensing process, it is useful to consider the various forms of energy.  ❑ The signal is fed to an input transducer, which changes the form of energy, usually into electrical. The block labelled 'Modifier' represents an amplifier or other device that operates on the transduced signal, and an actuator (output transducer) then converts the energy into a form suitable for display or recording. ❑ For instance, the temperature of a hot body (thermal energy) could be measured by a thermocouple (input transducer) feeding an operational amplifier (modifier) followed by an LED display (actuator) producing radiant energy.  Transducer parameters The response of a transducer to an input signal is known as its responsivity r, defined by For a displacement transducer the units would be V/m. The definition may be applied between any chosen terminals of the device, so r may be in Ω/ OC for a thermistor or V/°C at the output of a bridge circuit connected to the thermistor. The noise produced by a transducer, which limits its ability to detect a given signal, is known as its detectivity d, defined by  Transducers for Temperature Resistive temperature transducers These transducers are rather like wire-wound resistors, being in the form of a non-inductively wound coil of a suitable metal wire, usually platinum, copper or nickel. The relationship between resistance and temperature is nearly linear, but is more accurately described by equation. α is typically about 0.4 x 10-2 for platinum, but β and ɣ are very small (of the order 10-6 and 10-12 respectively).  Thermistors ❑ Thermistors are small semiconducting devices, usually in the shape of beads, discs or rods. They are made by mixtures of oxides of various materials, such as cobalt, nickel and manganese, and are often encapsulated in glass. ❑ where 𝑅𝑇0 and 𝑅𝑇1 are the resistances at temperatures 𝑇0 and 𝑇1. β is usually of value about 3000. The resistance at room temperature may be between a few hundred ohms and several megohms, depending on the type; a typical value is 10 kΩ at 300K with a slope of 1 kΩ/oC. Most thermistors are known as NTC types (negative temperature coefficient) but it is possible to produce PTC types (positive temperature coefficient) over a limited range by suitable doping. ❑ Thermistors have the advantages of high responsivity and small mass. However, besides being non-linear their characteristics may vary appreciably from sample to sample, and they have a limited temperature range of about -l00°C to 200°C. Thermoelectric transducers Thermoelectric transducers are known as thermocouples, and are self- generating devices comprising junctions between two dissimilar wires, one junction is maintained at 0oc (in melting ice) and the other attached to the object to be measured. ❑ A more convenient practical arrangement is shown in the figure. The two wires are laid out side by side and are connected directly to the voltmeter. The two junctions between the wires and the terminals of the voltmeter do not produce any error if they are both at the same temperature, but there is no proper reference junction.  Optical transducers Optical transducers have become very popular in recent years, partly because they are mostly inherently digital and partly because they are fairly immune from electrical interference. The main types are encoders, carrying some form of code representing position, and gratings. Encoders may be absolute or incremental, and are usually used for angular measurements, whereas gratings are purely incremental and are used for both translation and rotation. Absolute angular encoders It consists of a number of concentric tracks containing a pattern of opaque and transparent sections such that a unique binary code can be read from the patterns for any angle. The patterns may be read by means of photocells.  Incremental angular encoders Incremental encoders comprise a disc with a single binary track; this enables them to be made in 'slotted' form rather than transparent/opaque, as for coded discs. The slots are detected by a photocell arrangement feeding a counting system, so the resolution is essentially equal to the angle between the slots ❑ Typical discs usually have numbers of slots between about 48 and 96