Electrical Engineering Technology EE306 Sensors, Measurement of Displacement and Mechanical Strain Lecture 9 Northrop : Chapter 6

Sensors and Transducers A sensor or transducer is a device which permits the conversion of energy from one form to another. It is the first element in an instrumentation or measurement system. (For example, it might convert temperature to voltage.) Its linearity, range, noise and dynamic response determine the resolution, sensitivity and bandwidth of the overall system.

Examples of transducers include :- A transducer is a device that obeys reciprocity (e.g. the physical input quantity is converted to an output voltage and if a voltage is applied to the output terminals, the input quantity is generated at the input interface of the transducer). Examples of transducers include :- Piezoelectric crystals used to sense force or pressure, Electrodynamic devices such as loudspeakers and D’Arsonval meter movements. Not all sensors are transducers.

There are two approaches to categorizing sensors. Based on applications Based on the mechanism by which they work, (e.g generation of an open circuit voltage due to the input quantity under measurement(QUM), or a change in resistance proportional to the QUM.

Resistive Sensors As the input quantity varies, the resistance of resistive sensors varies around a baseline or average value, Ro. The change in R due to the change in input is converted to an output voltage. Hence, the transducer is included as an arm of a Wheatstone bridge.

A four-active arm Wheatstone bridge

As long as the ratio R/Ro is  1, the bridge output voltage will be linear with R. As R/Ro  1, the bridge output voltage vs R/Ro tends to saturate, hence sensitivity and linearity are lost. In unbonded strain gauge force sensors, two resistors are available, whose values increase linearly (Ro + R) with the applied force, and two resistors whose values decrease with the input (Ro - R). These resistors can be assembled into a four-active arm Wheatstone bridge which has a linear output in R.

Types of Resistor Sensors Resistive Temperature Sensors (covered in Lecture 8) Resistive Strain Gauge.

Strain Gauge What is strain? When a material is stretched or compressed, the force generates a corresponding stress inside the material. The stress will generates a proportional tensile strain or compressive strain which deforms the material by (L + L) or (L - L). L = length of material Strain =  = L/L

What is strain gauge? When external force being applied to a ferritic material, it will generates physical deformation and change the electrical resistance of the material. In case that such material is sticked onto test specimen via electrical insulation, the material produces change of electrical resistance corresponding to the deformation. Strain gauges consist of electrical resistance material and measure proportional strains to the resistance changes.

Resistive Strain Gauge (6.3.2) Resistive strain gauges fall into two, broad categories—bonded and unbonded gauges. Bonded gauges consist of fine wires or conducting films which are cemented to some structural beam or machine part in which we wish to measure the strain. The minute/small elongation or compression of the beam, in response to a mechanical load, causes a length change in the bonded gauge conductors. This length change, in turn, causes a small change in the resistance of the conductors, which is usually sensed with a Wheatstone bridge circuit.

Consider a length of wire, L cm, with a circular area of cross-section A cm2, and a resistivity of  cm. The resistance of this wire (at a given temperature) is given by If the wire is stressed mechanically by a load W newtons (pounds), a stress of s = W/A Pa (psi) will occur. A is in m2. As a result of this stress, a strain, , will occur, given by: Y is the Young’s modulus for the material of the wire,

The outputs of strain gauge bridges generally need amplification. Gauge Factor, The outputs of strain gauge bridges generally need amplification. The frequency response of bonded strain gauges largely depends on the mechanical properties of the structure to which they are bonded. It generally ranges from dc through to the audio range. Unbonded strain gauges are used in several applications—the direct measurement of small forces and the measurement of pressure (pressure acts on a diaphragm or piston to produce a force).

Other Resistor Sensors Photoconductors :- Photoconductors (PCs) are materials whose resistance decreases upon illumination with light. (They should not be confused with photodiodes or solar cells - which produce an EMF or short circuit current in response to the absorption of light quanta.) Applications of photoconductors - meters for cameras, light sensors in spectrophotometers, light sensors in a variety of counting systems where an object interrupts a light beam hitting the photoconductor, systems which sense a decrease of overall ambient illumination and turn on outside lighting. PCs are also called light dependent resistors or photoresistors.

Conductive Relative Humidity Sensors :– Humidity sensors respond to the amount of water vapor in the air or other gas. Humidity sensors change their conductance in response to the partial pressure of water vapor to which they are exposed. Use of Resistance Change to Sense Position or Angle :- A single- or multi-turn potentiometer is used to measure the shaft angle in applications such as electromechanical, position feedback control systems or through gears. The potentiometer is one arm of a Wheatstone bridge.

Giant Magnetoresistive Effect Based Sensors :- The giant magnetoresistive (GMR) effect is a physical phenomenon in which a magnetic field changes the resistance of a layered, thin film sensor. GMR sensors have a resistance that is modulated by an imposed magnetic flux density and thus, a Wheatstone bridge can be used to sense the R/RM and consequently, B.

Voltage Generating Sensors Thermocouple and thermopile – in lecture 8. Photovoltaic Cells and photodiodes :- Photovoltaics (PV) is the name of a method/process of converting solar energy into direct current electricity using semiconducting materials that exhibit the photovoltaic effect. A photodiode is a semiconductor device that converts light into current. The current is generated when photons are absorbed in the photodiode Photovoltaic cells are used to measure light intensity, as well as to generate dc electric power, when used as solar cells.

Piezoelectric Transducers :- Piezoelectric effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress, hence will generate an open circuit EMF. Piezoelectric materials include a number of natural crystals and manmade ceramic materials. Quartz, Rochelle Salt, and ammonium dihydrogen phosphate (ADP) are examples of naturally occurring piezomaterials.

Pyroelectric sensors – Pyroelectric materials (PyMs) are crystalline or polymer substances that generate internal, electrical charge transfer, in response to internal heating, caused by the absorption of long wave, infrared radiation (LIR). The charge transfer can be sensed as a current or voltage change.

Sensors Whose Voltage Output is Proportional to d/dt. An EMF will be induced in a coil of N turns surrounding a magnetic flux, , when the magnetic flux changes in time and many input transducers make use of this principle. The Variable Reluctance Phonograph Pickup Electrodynamic Accelerometer Linear Velocity Sensors

Sensors Whose Output EMF Depends on the Interaction of a Magnetic Field with Moving Charges The fluid version of this class of generating sensor is generally used with a constant magnetic (B) field, to measure the average velocity of a fluid flowing in an insulating pipe or conduit, including blood vessels. The solid state version of this class of sensor is the well-known Hall-effect device, used to measure magnetic fields.

Sensors Based on Variable Magnetic Coupling This class of sensors operates with ac excitation. Their outputs vary according to the degree of magnetic coupling between the excitation coil(s) and the output winding(s). Most sensors which operate on the principle of variable magnetic coupling between coils are mechanotransducers (i.e. they can be used to measure or generate linear or rotational displacements). LVDT - linear variable differential transformer Synchros and Resolvers - These are angular position sensors that work on the principle of variable mutual inductance.

Variable Capacitance Sensors Capacitive sensors have been designed to measure force by means of displacement of one or two capacitive electrodes causing a change in capacitance. The mechanical input quantity causes a change in the separation between two (or more) capacitor plates. They can also be used to measure acceleration, thickness, depth of a dielectric liquid and pressure.

Wheastone Bridge Two known resistances P and Q, a known variable resistance R and the unknown resistance X. R is adjusted until there is no deflection on G even with the resistance of A reduced to zero. Junctions D and E are then at the same potential, so that the p.d. between C and D is the same as that between C and E, and the p.d. between D and F is the same as that between E and F – NULL DEFLECTION p.d across P = I1P p.d across R =I2R p.d across Q = I1Q p.d across X =I2X Q/P = X/R I1P = I2R X = R x Q/P I1Q = I2X

Wheastone Bridge for Strain Gauge Measurement The strain gage is placed into one of the bridge arms and other three arms are completed with fixed resistors. R4 is taken as the strain gage. R3 is made variable to balance (null) the bridge when there is no force applied. Let R1 = R2 = R3 = R and R4 = Rx = R +  R = R(1 +  R/R), and let x = R/R. The open circuit voltage E0 = 0 at balance ( R = 0). At slight unbalance ( R  0) Since x << 1.

Bridge with Two Active Elements (Half Bridge) Let R2 = R3 = R; R1 = R - R; R4 = R + R, the open circuit voltage E0 = 0 at balance (R = 0). At slight unbalance (R  0)

Bridge with Four Active Elements (Full Bridge) The strain gauges that are working together are placed into opposite (non-neighbouring) arms of the bridge. The strain gauge resistors are manufactured for a perfect match to have the open circuit voltage E0 = 0 at balance (R = 0). At slight unbalance (R  0) with R1 = R3 = R - R; R2 = R4 = R + R.