LECTURE № 4

 

SENSORS AND SIGNAL PROCESSING. DISPLACEMENT, POSITION AND PROXIMITY SENSORS

 

4.1. EDDY CURRENT PROXIMITY SENSORS

 

Eddy current proximity sensors are used to detect non-magnetic but conductive materials. They comprise of a coil, an oscillator, a detector and a triggering circuit. Figure 4.1.1 shows the construction of eddy current proximity switch.

When an alternating current is passed thru this coil, an alternative magnetic field is generated. If a metal object comes in the close proximity of the coil, then eddy currents are induced in the object due to the magnetic field. These eddy currents create their own magnetic field which distorts the magnetic field responsible for their generation. As a result, impedance of the coil changes and so the amplitude of alternating current. This can be used to trigger a switch at some pre-determined level of change in current.

Eddy current sensors are relatively inexpensive, available in small in size, highly reliable and have high sensitivity for small displacements.

 

 

Fig. 4.1.1. Schematic of Inductive Proximity Sensor

 

Applications of eddy current proximity sensors:

·          automation requiring precise location;

·          machine tool monitoring;

·          final assembly of precision equipment such as disk drives;

·          measuring the dynamics of a continuously moving target, such as a vibrating element;

·          drive shaft monitoring;

·          vibration measurements.

 

Inductive proximity switch

Inductive proximity switches are basically used for detection of metallic objects. Figure 4.1.2 shows the construction of inductive proximity switch. An inductive proximity sensor has four components; the coil, oscillator, detection circuit and output circuit. An alternating current is supplied to the coil which generates a magnetic field. When,  a metal object  comes closer to the end of the coil, inductance of the coil changes. This is continuously monitored by a circuit which triggers a switch when a preset value of inductance change is occurred.

 

Fig. 4.1.2. Schematic of Inductive Proximity Switch

 

Applications of inductive proximity switches:

·          Industrial automation: counting of products during production or transfer;

·          Security: detection of metal objects, arms, land mines.

 

Optical encoders

Optical encoders provide digital output as a result of linear / angular displacement. These are widely used in the Servo motors to measure the rotation of shafts. Figure 4.1.3 shows the construction of an optical encoder.

 

 

Fig. 4.1.3. Construction and working of optical encoder

It comprises of a disc with three concentric tracks of equally spaced holes. Three light sensors are employed to detect the light passing thru the holes. These sensors produce electric pulses which give the angular displacement of the mechanical element e.g. shaft on which the Optical encoder is mounted. The inner track has just one hole which is used locate the «home» position of the disc. The holes on the middle track offset from the holes of the outer track by one-half of the width of the hole. This arrangement provides the direction of rotation to be determined. When the disc rotates in clockwise direction, the pulses in the outer track lead those in the inner; in counter clockwise direction they lag behind. The resolution can be determined by the number of holes on disc. With 100 holes in one revolution, the resolution would be:

360⁰/100 = 3.6⁰.

 

 

Pneumatic Sensors

 

 

Fig. 4.1.4. Working of Pneumatic Sensors

 

 

Pneumatic sensors are used to measure the displacement as well as to sense the proximity of an object close to it. The displacement and proximity are transformed into change in air pressure. Figure 4.1.4 shows a schematic of construction and working of such a sensor. It comprises of three ports. Low pressure air is allowed to escape through port A. In the absence of any obstacle / object, this low pressure air escapes and in doing so, reduces the pressure in the port B. However when an object obstructs the low pressure air (Port A), there is rise in pressure in output port B. This rise in pressure is calibrated to measure the displacement or to trigger a switch. These sensors are used in robotics, pneumatics and for tooling in CNC machine tools.

 

 

 

 

Proximity Switches

 

 

Fig. 4.1.5. Configurations of contact type proximity switch

 

 

Figure 4.1.5 shows a number of configurations of contact-type proximity switch being used in manufacturing automation. These are small electrical switches which require physical contact and a small operating force to close the contacts.They are basically employed on conveyor systems to detect the presence of an item on the conveyor belt.

 

 

 

 

Fig. 4.1.6. Reed Switch

 

 

Magnet based Reed switches are used as proximity switches. When a magnet attached to an object brought close to the switch, the magnetic reeds attract to each other and close the switch contacts. A schematic is shown in Figure 4.1.6.

 

 

 

Fig. 4.1.7. LED based proximity sensors

 

Photo emitting devices such as Light emitting diodes (LEDs) and photosensitive devices such as photo diodes and photo transistors are used in combination to work as proximity sensing devices. Figure 4.1.7 shows two typical arrangements of LEDs and photo diodes to detect the objects breaking the beam and reflecting light.

 

Hall effect sensor

Figure 4.1.8 shows the principle of working of Hall effect sensor. Hall effect sensors work on the principle that when a beam of charge particles passes through a magnetic field, forces act on the particles and the current beam is deflected from its straight line path. Thus one side of the disc will become negatively charged and the other side will be of positive charge. This charge separation generates a potential difference which is the measure of distance of magnetic field from the disc carrying current.

 

 

Fig. 4.1.8. Principle of working of Hall effect sensor

 

The typical application of Hall effect sensor is the measurement of fluid level in a container. The container comprises of a float with a permanent magnet attached at its top. An electric circuit with a current carrying disc is mounted in the casing.

When the fluid level increases, the magnet will come close to the disc and a potential difference generates. This voltage triggers a switch to stop the fluid to come inside the container.

These sensors are used for the measurement of displacement and the detection of position of an object. Hall effect sensors need  necessary signal conditioning circuitry. They can be operated at 100 kHz. Their non-contact nature of operation, good immunity to environment contaminants  and ability to sustain in severe conditions make them quite popular in industrial automation.

 

1.1.         VELOCITY, MOTION, FORCE AND PRESSURE SENSORS

 

Tachogenerator

Tachogenerator works on the principle of variable reluctance. It consists of an assembly of a toothed wheel and a magnetic circuit as shown in figure 4.2.1. Toothed wheel is mounted on the shaft or the element of which angular motion is to be measured. Magnetic circuit comprising of a coil wound on a ferromagnetic material core. As the wheel rotates, the air gap between wheel tooth and magnetic core changes which results in cyclic change in flux linked with the coil. The alternating emf generated is the measure of angular motion. A pulse shaping signal conditioner is used to transform the output into a number of pulses which can be counted by a counter.

 

Fig. 4.2.1. Principle of working of Techogenerator

 

 

Fig. 4.2.2. Construction and working of AC generator

 

An alternating current (AC) generator can also be  used as a techognerator. It comprises of rotor coil which rotates with the shaft. Figure 2.4.2 shows the schematic of AC generator. The rotor rotates in the magnetic field produced by a stationary permanent magnet or electromagnet. During this process, an alternating emf is produced which is the measure of the angular velocity of the rotor. In general, these sensors exhibit nonlinearity error of about ± 0.15% and are employed for the rotationsup to about 10000 rev/min.

 

Pyroelectric sensors

 

 

Fig. 4.2.3. Principle of pyroelectricity

 

These sensors work on the principle of pyroelectricity, which states that a crystal material such as Lithium tantalite generates  charge in response to heat flow.  In presence of an electric field, when such a crystal material heats up, its electrical dipoles line up as shown in figure 4.2.3. This is called as polarization. On cooling, the material retains its polarization. In absence of electric field, when this polarized material is subjected to infra red irradiation, its polarization reduces. This phenomenon is the measure of detection of movement of an object.

 

Fig. 4.2.4. Construction and working a Pyroelectric sensor

 

Pyroelectric sensor comprises of a thick element of polarized material coated with thin film electrodes on opposite faces as shown in figure 4.2.4. Initially the electrodes are in electrical equilibrium with the polarized material. On incident of infra red, the material heats up and reduces its polarization. This leads to charge imbalance at the interface of crystal and electrodes. To balance this  disequilibrium, measurement circuit supplies the charge, which is calibrated against the detection of an object or its movement.

Applications of Pyroelectric sensors:

·          Intrusion detector;

·          Optothermal detector;

·          Pollution detector;

·          Position sensor;

·          Solar cell studies;

·          Engine analysis.

 

Strain Gauge as force Sensor

 

 

Fig. 4.2.5. Strain gauge based Load cell

 

Strain gauge based sensors work on the principle of change in electrical resistance. When, a mechanical element subjects to a tension or a compression the electric resistance of the material changes. This is used to measure the force acted upon the element.

Figure 4.2.5 shows a strain gauge load cell. It comprises of cylindrical tube to which strain gauges are attached. A load applied on the top collar of the cylinder compress the strain gauge element which changes its electrical resistance. Generally strain gauges are used to measure forces up to 10 MN. The non-linearity and repeatability errors of this transducer are ±0.03% and ±0.02% respectively.

 

Fluid pressure

Chemical, petroleum, power industry often need to monitor fluid pressure. Various types of instruments such as diaphragms, capsules, and bellows are used to monitor the fluid pressure. Specially designed strain gauges doped in diaphragms are generally used to measure the inlet manifold pressure in applications such as automobiles. A typical arrangement of strain gauges on a diaphragm is shown in figure 4.2.6. Application of pressurized fluid displaces the diaphragm. This displacement is measured by the stain gauges in terms of radial and/or lateral strains. These strain gauges are connected to form the arms of a Wheatstone bridge.

 

 

 

Fig. 4.2.6. Schematic of Capsule and Bellow

 

Capsule is formed by combining two corrugated diaphragms. It has enhanced sensitivity in comparison with that of diaphragms. Figure 2.4.7 shows a schematic of a Capsule and a Bellow. A stack of capsules is called as «Bellows». Bellows with a LVDT sensor measures the fluid pressure in terms of change in resultant voltage across the secondary coils of LVDT.

 

Fig. 4.2.7. Bellow with a LVDT

 

Figure 2.4.8 shows a typical arrangement of the same.

 

Tactile sensors

 

 

Fig. 4.2.8. Schematic of a tactile sensor

 

In general, tactile sensors are used to sense the contact of fingertips of a robot with an object. They are also used in manufacturing of ‘touch display’ screens of visual display units (VDUs) of CNC machine tools. Figure 2.4.9 shows the construction of piezo-electric polyvinylidene fluoride (PVDF) based tactile sensor. It has two PVDF layers separated by a soft film which transmits the vibrations. An alternating current is applied to lower PVDF layer which generates vibrations due to reverse piezoelectric effect. These vibrations are transmitted to the upper PVDF layer via soft film. These vibrations cause alternating voltage across the upper PVDF layer. When some pressure is applied on the upper PVDF layer the vibrations gets affected and the output voltage changes. This triggers a switch or an action in robots or touch displays.

 

Piezoelectric sensor

Piezoelectric sensor is used for the measurement of pressure, acceleration and dynamic-forces such as oscillation, impact, or high speed compression or tension. It contains piezoelectric ionic crystal materials such as Quartz (Figure 4.2.9). On application of force or pressure these materials get stretched or compressed. During this process, the charge over the material changes and redistributes. One face of the material becomes positively charged and the other negatively charged. The net charge q on the surface is proportional to the amount x by which the charges have been displaced. The displacement is proportion to force. Therefore we can write:

 

q = kx = SF                                                     (2.2.1)

 

where k is constant and S is a constant termed the charge sensitivity.

 

 

Fig. 4.2.9. Principle of working of Piezoelectric sensor

 

 

Liquid flow

Liquid flow is generally measured by applying the Bernoulli’s principle of fluid flow through a constriction. The quantity of fluid flow is computed by using the pressure drop measured. The fluid flow volume is proportional to square root of pressure difference at the two ends of the constriction. There are various types of fluid flow measurement devices being used in manufacturing automation such as Orifice plate, Turbine meter etc.

 

 

Fig. 4.2.10. Orifice Plate

 

Figure 4.2.10 shows a schematic of Orifice plate device. It has a disc with a hole at its center, through which the fluid flows. The pressure difference is measured between a point equal to the diameter of the tube upstream and a point equal to the half the diameter downstream. Orifice plate is inexpensive and simple in construction with no moving parts. It exhibits nonlinear behavior and does not work with slurries. It has accuracy of ± 1.5%

 

 

Fig. 4.2.11. Schematic of turbine meter

 

Turbine flow meter has an accuracy of ±0.3%. It has a multi blade rotor mounted centrally in the pipe along which the flow is to be measured. Figure 4.2.11 shows the typical arrangement of the rotor and a magnetic pick up coil. The fluid flow rotates the rotor. Accordingly the magnetic pick up coil counts the number of magnetic pulses generated due to the distortion of magnetic field by the rotor blades. The angular velocity is proportional to the number of pulses and fluid flow is proportional to angular velocity.

 

Fluid level

The level of liquid in a vessel or container can be measured:

a)            directly by monitoring the position of liquid surface;

b)           indirectly by measuring some variable related to the height.

Direct measurements involve the use of floats however the indirect methods employ load cells. Potentiometers or LVDT sensors can be used along with the floats to measure the height of fluid column. Force sensed by the load cells is proportional to the height of fluid column.

 

1.2.         TEMPERATURE AND LIGHT SENSORS

 

Temperature conveys the state of a mechanical system in terms of expansion or contraction of solids, liquids or gases, change in electrical resistance of conductors, semiconductors and thermoelectric emfs. Temperature sensors such as bimetallic strips, thermocouples, thermistors are widely used in monitoring of manufacturing processes such as casting, molding, metal cutting etc. The construction details and principle of working of some of the temperature sensors are discussed in following sections.

 

Bimetallic strips

 

 

Fig. 4.3.1. Construction and working of Bi-metallic strip

 

Bimetallic strips are used as thermal switch in controlling the temperature or heat in a manufacturing process or system. It contains two different metal strips bonded together. The metals have different coefficients of expansion. On heating the strips bend into curved strips with the metal with higher coefficient of expansion on the outside of the curve. Figure 4.3.1 shows a typical arrangement of a bimetallic strip used with a setting-up magnet. As the strips bend, the soft iron comes in closer proximity of the small magnet and further touches. Then the electric circuit completes and generates an alarm. In this way bimetallic strips help to protect the desired application from heating above the pre-set value of temperature.

Figure 4.3.1 shows the construction of a RTD. It has a resistor element connected to a Wheatstone bridge. The element and the connection leads are insulated and protected by a sheath. A small amount of current is continuously passing though the coil. As the temperature changes the resistance of the coil changes which is detected at the Wheatstone bridge.

RTDs are used in the form of thin films, wire wound or coil. They are generally made of metals such as platinum, nickel or nickel-copper alloys. Platinum wire held by a high-temperature glass adhesive in a ceramic tube is used to measure the temperature in a metal furnace. Other applications are:

·          air conditioning and refrigeration servicing;

·          food processing;

·          stoves and grills;

·          textile production;

·          plastics processing;

·          petrochemical processing;

·          micro electronics;

·          air, gas and liquid temperature measurement in pipes and tanks;

·          exhaust gas temperature measurement.

 

Thermistors

Thermistors follow the principle of decrease in resistance with increasing temperature. The material used in thermistor is generally a semiconductor material such as a sintered metal oxide (mixtures of metal oxides, chromium, cobalt, iron, manganese and nickel) or doped polycrystalline ceramic containing barium titanate (BaTiO3) and other compounds. As the temperature of semiconductor  material increases the number of electrons able to move about increases which results in more current in the material and reduced resistance. Thermistors are rugged and small in dimensions. They exhibit nonlinear response characteristics. Thermistors are available in the form of a bead (pressed disc), probe or chip. Figure 4.3.2 shows the construction of a bead type thermistor. It has  a small bead of dimension from 0.5 mm to 5 mm coated with ceramic or glass material. The bead is connected to an electric circuit through two leads. To protect from the environment, the leads are contained in a stainless steel tube.

 

Fig. 4.3.2. Schematic of a thermistor

Applications of Thermistors:

·          To monitor the coolant temperature and/or oil temperature inside the engine;

·          To monitor the temperature of an incubator;

·          Thermistors are used in modern digital thermostats;

·          To monitor the temperature of battery packs while charging;

·          To monitor temperature of hot ends of 3D printers;

·          To maintain correct temperature in the food handling and processing industry;

·          Equipments;

·          To control the operations of consumer appliances such as toasters, coffee makers, refrigerators, freezers, hair dryers, etc.

 

Light sensors

A light sensor is a device that is used to detect light. There are different types of light sensors such as photocell/photoresistor and photo diodes being used in manufacturing and other industrial applications. Photoresistor is also called as light dependent resistor (LDR). It has a resistor whose resistance decreases with increasing incident light intensity. It is made of a high resistance semiconductor material, cadmium sulfide (CdS). The resistance of a CdS photoresistor varies inversely to the amount of light incident upon it. Photoresistor follows the principle of photoconductivity which results from the generation of mobile carriers when photons are absorbed by the semiconductor material.

Figure 4.3.3 shows the construction of a photo resistor. The CdS resistor coil is mounted on a ceramic substrate. This assembly is encapsulated by a resin material. The sensitive coil electrodes are connected to the control system though lead wires.On incidence of high intensity light on the electrodes, the resistance of resistor coil decreases which will be used further to generate the appropriate signal by the microprocessor via lead wires.

 

 

Fig. 4.3.3. Construction of a photo resistor

 

Photoresistors are used in science and in almost any branch of industry for control, safety, amusement, sound reproduction, inspection and measurement.

Applications of photo resistor:

·          computers, wireless phones, and televisions, use ambient light sensors to automatically control the brightness of a screen;

·          barcode scanners used in retailer locations work using light sensor technology;

·          in space and robotics: for controlled and guided motions of vehicles and robots. the light sensor enables a robot to detect light. robots can be programmed to have a specific reaction if a certain amount of light is detected;

·          auto flash for camera;

·          industrial process control.

 

Photo diodes

Photodiode is a solid-state device which converts incident light into an electric current. It is made of Silicon. It consists of a shallow diffused p-n junction, normally a p-on-n configuration. When photons of energy greater than 1.1eV (the bandgap of silicon) fall on the device, they are absorbed and electron-hole pairs are created. The depth at which the photons are absorbed depends upon their energy. The lower the energy of the photons, the deeper they are absorbed. Then the electron-hole pairs drift apart. When the minority carriers reach the junction, they are swept across by the electric field and an electric current establishes.

Photodiodes are one of the types of photodetector, which convert light into either current or voltage. These are regular semiconductor diodes except that they may be either exposed to detect vacuum UV or X-rays or packaged with a opening or optical fiber connection to allow light to reach the sensitive part of the device.

 

 

Fig. 4.3.4. Construction of photo diode detector

 

Figure 4.3.4 shows the construction of Photo diode detector. It is constructed from single crystal silicon wafers. It is a p-n junction device. The upper layer is p layer. It is very thin and formed by thermal diffusion or ion implantation of doping material such as boron. Depletion region is narrow and is sandwiched between p layer and bulk n type layer of silicon. Light irradiates at front surface, anode, while the back surface is cathode. The incidence of light on anode generates a flow of electron across the p-n junction which is the measure of light intensity.

 

Applications of photo diodes

Camera: Light Meters, Automatic Shutter Control, Auto-focus, Photographic Flash Control.

Medical: CAT Scanners – X ray Detection, Pulse Oximeters, Blood Particle Analyzers.

Industry:

·          Bar Code Scanners;

·          Light Pens;

·          Brightness Controls;

·          Encoders;

·          Position Sensors;

·          Surveying Instruments;

·          Copiers – Density of Toner.