chapter 3 sensor technology. a transducer is defined as any device that converts energy from one...

Chapter 3Chapter 3Sensor TechnologySensor Technology

A A transducertransducer is defined as any device that is defined as any device that converts energy from one form to another in such converts energy from one form to another in such a way that the output is proportional to the input.a way that the output is proportional to the input.

The output from most transducers is in the form The output from most transducers is in the form of electrical energy. of electrical energy.

For example a loudspeaker is a transducer that For example a loudspeaker is a transducer that transforms electrical signals into sound energy. transforms electrical signals into sound energy.

IntroductionIntroduction

A A sensorsensor is a transducer that is used to make is a transducer that is used to make measurement of a physical variable.measurement of a physical variable.

It is a device that responds to a physical stimulus It is a device that responds to a physical stimulus (as heat, light, sound, pressure, magnetism, or a (as heat, light, sound, pressure, magnetism, or a particular motion) and transmits a resulting particular motion) and transmits a resulting impulse (a signal relating to the quantity being impulse (a signal relating to the quantity being measured). measured).

For example, certain sensors convert temperature For example, certain sensors convert temperature into a change in resistance.into a change in resistance.



Sensors differ slightly from transducers in that, Sensors differ slightly from transducers in that, although changes occur, the changes do not although changes occur, the changes do not necessarily involve energy conversion. necessarily involve energy conversion.

It is more likely that a It is more likely that a change in some propertychange in some property of of the sensor occurs. the sensor occurs.

For example, many position-measuring For example, many position-measuring transducers utilize a component called a photocell.transducers utilize a component called a photocell.

A photocell is a device that changes its electrical A photocell is a device that changes its electrical resistance in proportion to the amount of light resistance in proportion to the amount of light falling on it.falling on it.


If a voltage is connected across the photocell, then If a voltage is connected across the photocell, then this voltage will change as the amount of light this voltage will change as the amount of light changes. changes.

The photocell can be caused to act like a The photocell can be caused to act like a transducer although strictly speaking no energy transducer although strictly speaking no energy conversion is taking placeconversion is taking place

Limit Switches

Limit Switches

A limit switch has the same ON/OFF characteristics. A limit switch has the same ON/OFF characteristics. The limit switch usually has a pressure-sensitive The limit switch usually has a pressure-sensitive

mechanical arm. mechanical arm. When an object applies pressure on the mechanical When an object applies pressure on the mechanical

arm, the switch circuit is energized. arm, the switch circuit is energized. An object might have a magnet attached that causes a An object might have a magnet attached that causes a

contact to rise and close when the object passes over contact to rise and close when the object passes over the arm. the arm.

Limit Switches

Limit switches can be either Limit switches can be either - Normally open (NO) or - Normally open (NO) or - Normally closed (NC) and may have - Normally closed (NC) and may have

multiple poles. multiple poles. A normally open switch has continuity when A normally open switch has continuity when

pressure applied and a contact is made. pressure applied and a contact is made. While a normally closed switch opensWhile a normally closed switch opens when when

pressure is applied.pressure is applied.

Limit Switches

Figure A.2: Normally Open-Normally Closed Limit Switches

Limit Switches

A single pole switch allows one circuit to A single pole switch allows one circuit to be opened or closed upon switch contact.be opened or closed upon switch contact.

Multiple-pole switch allows multiple Multiple-pole switch allows multiple circuits to be opened or closed.circuits to be opened or closed.

Limit Switches

Limit switches are mechanical devices. Limit switches are mechanical devices.

Limit Switches has following drawback:Limit Switches has following drawback: They are subject to mechanical failure.They are subject to mechanical failure. Their mean time between failures (MTBF) is Their mean time between failures (MTBF) is

low compared to non-contact sensors.low compared to non-contact sensors. Their speed of operation is relatively low; the Their speed of operation is relatively low; the

switching speed of photoelectric switching speed of photoelectric microsensors is up to 3000 times fastermicrosensors is up to 3000 times faster

Limit Switches -Advantages

Limit switches are mechanical position-sensing Limit switches are mechanical position-sensing devices that offer simplicity, robustness, and devices that offer simplicity, robustness, and repeatability to processes. repeatability to processes.

Mechanical limit switches are simplest in which Mechanical limit switches are simplest in which contact is made and a switch is engaged. contact is made and a switch is engaged.

Limit switches are easy to maintain because the Limit switches are easy to maintain because the operator can hear the operation of the switch and operator can hear the operation of the switch and can align it easily to fit the application.can align it easily to fit the application.

Limit Switches -Advantages

They are also robust. They can handle an inrush current They are also robust. They can handle an inrush current 10 times that of their steady state rating.10 times that of their steady state rating.

Reliability is another benefit. Published claims for repeat Reliability is another benefit. Published claims for repeat accuracy for standard limit switches vary from within accuracy for standard limit switches vary from within 0.03mm to within 0.001mm over temperature range of -4 0.03mm to within 0.001mm over temperature range of -4 to +200F. to +200F.

Limit switch dissipate energy spikes and rarely break Limit switch dissipate energy spikes and rarely break down under normal mode surges. They will not be down under normal mode surges. They will not be affected by electromagnetic interferences (EMI).affected by electromagnetic interferences (EMI).

Proximity Sensors

Proximity Sensors

Proximity sensing is the technique of detecting the Proximity sensing is the technique of detecting the presence or absence of an object with an electronic non-presence or absence of an object with an electronic non-contact sensor. There are contact sensor. There are three typesthree types of proximity sensors: of proximity sensors:

1. Inductive,2. Capacitive,3. Magnetic.

Proximity Sensors• Mechanical limit switches are the first devices

to detect objects in industrial applications.

• Inductive proximity sensors are used in place of limit switches for non-contact sensing of metallic objects.

• Capacitive proximity switches can also detect non-metallic objects.

• Both inductive and capacitive sensors are limit switches with ranges up to 100mm.

Inductive Proximity SensorsInductive Proximity Sensors

Inductive sensorsInductive sensors are used to detect the presence of are used to detect the presence of metallic objects.metallic objects.

These sensors require DC or AC voltage for the These sensors require DC or AC voltage for the power to drive circuitry to generate the fields and to power to drive circuitry to generate the fields and to produce output signal.produce output signal.

Inductive Proximity SensorsInductive Proximity Sensors

• An inductive proximity sensor consists of four basic elements:

1. Sensor coil and ferrite core

2. Oscillator circuit3. Trigger/Detector circuit4. Solid-state output circuit

Inductive Proximity Sensor Inductive Proximity Sensor working principleworking principle

The oscillator circuit generates a radio-The oscillator circuit generates a radio-frequency electromagnetic field that frequency electromagnetic field that radiates from the ferrite core and coil radiates from the ferrite core and coil assembly.assembly.

The field is centered around the axis of the The field is centered around the axis of the ferrite core, which shapes the field and ferrite core, which shapes the field and directs it at the sensor face.directs it at the sensor face.

When a metal target approaches and When a metal target approaches and enters the field, eddy current are induced enters the field, eddy current are induced into the surfaces of the targetinto the surfaces of the target..


This results in a loading effect, or This results in a loading effect, or “damping” that causes a reduction in “damping” that causes a reduction in amplitude of the oscillator signal.amplitude of the oscillator signal.

The detector circuit detects the change The detector circuit detects the change in oscillator amplitude. The detector will in oscillator amplitude. The detector will switch ON at specific operate amplitude.switch ON at specific operate amplitude.

This ON signal generates a signal to This ON signal generates a signal to turn ON the solid state output. This is turn ON the solid state output. This is often referred to as the often referred to as the dampeddamped condition.condition.


As the target leaves the sensing field, the As the target leaves the sensing field, the oscillator responds with an increase in oscillator responds with an increase in amplitude. amplitude.

As the amplitude increases above a specific As the amplitude increases above a specific value, it is detected by the detector circuit, value, it is detected by the detector circuit, which switches OFF, causing the output which switches OFF, causing the output signal to return to the normal or signal to return to the normal or

OFF(OFF(undampedundamped) state.) state.

Inductive Proximity SensorInductive Proximity Sensor

Typical applicationsTypical applications of inductive proximity sensors in of inductive proximity sensors in control systems: control systems:

• Motion position detectionMotion position detection• Motion controlMotion control• Conveyor system controlConveyor system control• Process controlProcess control• Machine controlMachine control• Verification and countingVerification and counting

Capacitive Proximity Sensors

Capacitive Proximity Sensors Capacitive sensing is based on dielectric capacitance.Capacitive sensing is based on dielectric capacitance. Capacitance is the property of insulators to store an Capacitance is the property of insulators to store an

electric charge. electric charge. A capacitor consists of two plates separated by an A capacitor consists of two plates separated by an

insulator, usually called a dielectric. insulator, usually called a dielectric. When the switch is closed a charge is stored on the When the switch is closed a charge is stored on the

two plates.two plates. The distance between the plates determine the ability The distance between the plates determine the ability

of a capacitor to store a charge and can be calibrated of a capacitor to store a charge and can be calibrated as a function of stored charge to determine discrete as a function of stored charge to determine discrete ON and OFF switching status.ON and OFF switching status.

Capacitive Proximity Sensors

The capacitive proximity sensor has the same four basic elements as an inductive sensor:

1.Sensor (the dielectric plate)

2.Oscillator circuit3.Detector circuit4.Solid-state output circuit

Capacitive Proximity Sensors The oscillator circuit includes feedback capacitance The oscillator circuit includes feedback capacitance

from the external target plate and the internal plate.from the external target plate and the internal plate. In a capacitive switch, the oscillator starts oscillating In a capacitive switch, the oscillator starts oscillating

when sufficient feedback capacitance is detected. when sufficient feedback capacitance is detected. The oscillation begin with an approaching target until The oscillation begin with an approaching target until

the value of capacitance reaches a threshold.the value of capacitance reaches a threshold. At threshold point the trigger circuit will turn on the At threshold point the trigger circuit will turn on the

output switching device. output switching device. Thus the output modules function as normally open, Thus the output modules function as normally open,

normally closed, or changeover switches.normally closed, or changeover switches.

Capacitive Proximity SensorsFeatures of capacitive sensors:

• They can detect non-metallic targets• They can detect lightweight or small objects that cannot be detected by mechanical limit switches• They provide a high switching rate for rapid response in object counting applications.• They can detect liquid targets through non- metallic barriers, (glass, plastic, etc)• They have long operational life with a virtually unlimited number of operating cycles.• The solid-state output provides a bounce-free contact signal

Capacitive Proximity SensorsTypical applications of capacitive proximity sensorsin control systems:

•Liquid level detection•Bulk material level control•Process control

Magnetic Proximity Sensors

Magnetic Proximity SensorMagnetic Proximity Sensor

As with inductive proximity sensors, magnetic As with inductive proximity sensors, magnetic proximity sensor hasproximity sensor has

1.1. LC oscillating circuit, LC oscillating circuit, 2.2. A signal strength A signal strength

indicator and indicator and 3.3. A switching amplifier.A switching amplifier.4.4. Strip of magnetically Strip of magnetically

soft-glass metal.soft-glass metal.

Magnetic Proximity SensorMagnetic Proximity Sensor This strip attenuates the oscillating circuit . This strip attenuates the oscillating circuit . If a magnet is brought closer, the oscillating de-attenuates.If a magnet is brought closer, the oscillating de-attenuates. The power consumption of a magnetic proximity sensor The power consumption of a magnetic proximity sensor

therefore increases as the magnet is brought closertherefore increases as the magnet is brought closer

(in inductive proximity sensor the power consumption (in inductive proximity sensor the power consumption reduces as the switching target is brought closer.)reduces as the switching target is brought closer.)

A major advantage of this technology is that large sensing A major advantage of this technology is that large sensing ranges are possible even with small sensor types.ranges are possible even with small sensor types.

Magnetic Proximity SensorMagnetic Proximity Sensor

Permanent magnets are usually used to trigger Permanent magnets are usually used to trigger magnetic proximity sensors. magnetic proximity sensors.

EgEg: magnetically hard-substances, such as steel alloyed : magnetically hard-substances, such as steel alloyed with other metals such as aluminum, cobalt and with other metals such as aluminum, cobalt and nickel.nickel.

Photoelectric Sensors

• A photoelectric sensor is a semiconductor component that reacts to light or emits light. The light may be either in visible range or the invisible infrared range.


• Infrared sensors may be active or passive. The active sensors send out an infrared beam and respond to the reflection of the beam against a target.


• The distinct advantage of photoelectric sensors over inductive or capacitive sensors is their increased range.

• Dirt, oil mist and other environmental factors will hinder operation of photoelectric sensors during manufacturing process.


There are three modes of detection There are three modes of detection used by photoelectric sensors:used by photoelectric sensors:

• Through-beam detection method• Reflex/retro-reflective detection

method• Proximity/Diffuse reflective detection

method

Photoelectric Sensors The light source used for each of the three The light source used for each of the three

modes comes from a LED.modes comes from a LED. LEDs emit a visible colored light (red, green, LEDs emit a visible colored light (red, green,

yellow) or invisible (infrared) light.yellow) or invisible (infrared) light.


Visible LEDs are used for in retro-Visible LEDs are used for in retro-reflective applications, they provide reflective applications, they provide easy reflector alignment to the sensor.easy reflector alignment to the sensor.

Light intensity of infrared LEDs is Light intensity of infrared LEDs is greater than the visible ones. They are greater than the visible ones. They are better suited for through-beam and better suited for through-beam and diffused style sensors.diffused style sensors.


Switching the LED off and on at a Switching the LED off and on at a predetermined frequency (modulating), predetermined frequency (modulating), increases the light intensity and lifetime of increases the light intensity and lifetime of the LED while reducing the average power the LED while reducing the average power consumed.consumed.

The pulsed LED provides a stronger signal The pulsed LED provides a stronger signal when compared to a continuously illuminated when compared to a continuously illuminated LED, therefore, a larger sensing range can be LED, therefore, a larger sensing range can be obtained.obtained.


Another key advantage to modulating Another key advantage to modulating the sensor is to provide protection the sensor is to provide protection against external light interference. against external light interference.

The receiving circuit, typically The receiving circuit, typically phototransistor based, is modulated at phototransistor based, is modulated at the same frequency as the emitter’s.the same frequency as the emitter’s.


Photoelectric sensors are comprised of the following components :

1. Light Source (LED)2. Receiver

(phototransistor)3. Signal Converter4. Amplifier


The generated light pulses that are The generated light pulses that are received by the phototransistor are received by the phototransistor are converted into electrical signals. converted into electrical signals.

These signals are analyzed in order to These signals are analyzed in order to determine if they are the result of the determine if they are the result of the actual transmitted light.actual transmitted light.

Upon verification, the output of the Upon verification, the output of the sensor is switched accordingly.sensor is switched accordingly.

With the appropriate conditioning, light With the appropriate conditioning, light or dark sensing is achieved.or dark sensing is achieved.

Photoelectric SensorsThrough-beam detection method

• Sensor have separate source and detector elements aligned opposite each other, with the beam of light crossing the path that an object must cross.

• When an object passes between the source and detector, the beam is broken, signaling detection of the object.


The effective beam area is that of column of The effective beam area is that of column of light travels straight between the lenses.light travels straight between the lenses.


Light from the source is transmitted Light from the source is transmitted directly to the photo-detector , through-directly to the photo-detector , through-beam sensors offer the following beam sensors offer the following benefitsbenefits::

• Longest sensing range• Highest possible signal strength• Greatest light/dark contrast ratio• Best trip point repeatability


Through-beam detection generally provides the longest Through-beam detection generally provides the longest range of the three operating modes and provides high power range of the three operating modes and provides high power at a shorter range to penetrate steam, dirt, or other at a shorter range to penetrate steam, dirt, or other contaminants between the source and detector. contaminants between the source and detector.

The limitation of through-beam sensors are as follows:The limitation of through-beam sensors are as follows:• They require wiring of the two components across the detection zone.• It may be difficult to align the source and the detector.• If the object to be detected is smaller than the effective beam diameter, an aperture over the lens may be required.• Alignment of the source and detector must be accurate.

Reflex/Retro-reflective detection method

Reflex photoelectric controls position the source and Reflex photoelectric controls position the source and detection parallel to each other on the same side of the target.detection parallel to each other on the same side of the target.

The light is directed to a retro-reflector and returns to the The light is directed to a retro-reflector and returns to the detector. The switching and output occur when an object detector. The switching and output occur when an object breaks the beam.breaks the beam.

Figure A.15: Reflex Photoelectric Controls

Reflex/Retro-reflective detection method

Since the light travels in two directions (hence twice the Since the light travels in two directions (hence twice the distance), reflex controls will not sense as far as through-distance), reflex controls will not sense as far as through-beam sensors. However, reflex controls offer a powerful beam sensors. However, reflex controls offer a powerful sensing system that is easy to mount and does not require that sensing system that is easy to mount and does not require that electrical wire to be run on both sides of the sensing areas.electrical wire to be run on both sides of the sensing areas.

The main limitation of these sensors is that a shiny surface on The main limitation of these sensors is that a shiny surface on the target object can trigger false detection. Hence the object the target object can trigger false detection. Hence the object to be detected must be less reflective than the retro-reflector.to be detected must be less reflective than the retro-reflector.

The reflex method is widely used because it is flexible and The reflex method is widely used because it is flexible and easy to install and provides the best cost-performance ratio of easy to install and provides the best cost-performance ratio of the three methods.the three methods.

Proximity/Diffuse reflective detection method

The proximity detection method requires that the source The proximity detection method requires that the source and detector are installed on the same side of the object to and detector are installed on the same side of the object to be detected and aimed at a point in front of the sensor. be detected and aimed at a point in front of the sensor.

Figure A.17: Proximity Detection


When an object passes in front of the source and When an object passes in front of the source and detector, light from the source is reflected from the detector, light from the source is reflected from the object’s surface back to the detector, and the object object’s surface back to the detector, and the object is detected.is detected.

Each sensor type has a specific operating range. In Each sensor type has a specific operating range. In general, through-beam sensors offer the greatest general, through-beam sensors offer the greatest range, followed by reflex sensors, then by range, followed by reflex sensors, then by proximity sensors.proximity sensors.

The maximum range for through-beam sensors is The maximum range for through-beam sensors is of primary importance. At any distance less than of primary importance. At any distance less than the maximum range, the sensor has more than the maximum range, the sensor has more than enough power to detect an object.enough power to detect an object.


The optimum range for the proximity and reflex The optimum range for the proximity and reflex sensors is more than significant than the maximum sensors is more than significant than the maximum range. The optimum range is the range at which the range. The optimum range is the range at which the sensor has the most power available to detect sensor has the most power available to detect objects. objects.

Figure A.18: Proximity detection

Ultrasonic Sensors

Ultrasonic sensorsUltrasonic sensors are used in non-contact material are used in non-contact material monitoring applications including web loop control, level monitoring applications including web loop control, level control, positioning, flow monitoring and conveyor control, positioning, flow monitoring and conveyor transfer.transfer.

Ultrasonic sensors use the propagation time of sound Ultrasonic sensors use the propagation time of sound pulse to calculate the distance of a target. Sound pulses pulse to calculate the distance of a target. Sound pulses are emitted and received by a diaphragm in the face of are emitted and received by a diaphragm in the face of the transducer as illustrated in the diagram below.the transducer as illustrated in the diagram below.

Ultrasonic Sensors

Figure A.19: Ultrasonic Sensing

Terminology

•Beam Angle: The beam angle is the angle formed by sound waves as they emanate from an ultrasonic sensor. The beam angle defines the usable area in which target detection is possible.

Terminology of Ultrasonic Sensor

•Deadband: The deadband is the unusable region that defines the minimum distance for target detection. The unusable region occurs because a transducer must be pulsed in order to produce a sound wave, and the oscillations from the shocked must stop before the transducer can register its echo pulse.


Linearity:Linearity: If the “perfect” If the “perfect” analog ultrasonic sensor could analog ultrasonic sensor could be produced, its output, from be produced, its output, from beginning-to-end of the span beginning-to-end of the span limits, would appear in limits, would appear in graphical form as a perfect graphical form as a perfect straight line. Linearity defines straight line. Linearity defines the tolerances within which the the tolerances within which the sensor’s output may vary from sensor’s output may vary from the “perfect” line during “real the “perfect” line during “real life” target monitoring. life” target monitoring. Linearity specifications are Linearity specifications are always given as a percentage.always given as a percentage. Figure A.21: Linearity of Ultrasonic Sensor


Resolution:Resolution: Resolution is the smallest target Resolution is the smallest target movement an ultrasonic sensor can identify and movement an ultrasonic sensor can identify and evaluate. For example, if an ultrasonic sensor has a evaluate. For example, if an ultrasonic sensor has a resolution of 10mm, the sensor output remains resolution of 10mm, the sensor output remains unchanged until the target moves more than 10mm.unchanged until the target moves more than 10mm.

Repeatability:Repeatability: Repeatability is the ability of a Repeatability is the ability of a sensor to consistently detect a target at the same sensor to consistently detect a target at the same point. Repeatability is expressed as a percentage of point. Repeatability is expressed as a percentage of sensing range and is frequently affected by sensing range and is frequently affected by environmental conditions. environmental conditions.

Terminology of Ultrasonic Sensor Target

Good Targets:Good Targets: Ultrasonic sensors function Ultrasonic sensors function best in the detection and monitoring of best in the detection and monitoring of objects with a relatively high density. Solid, objects with a relatively high density. Solid, liquid or granular media make ideal targets liquid or granular media make ideal targets due to their high acoustic reflectivity. due to their high acoustic reflectivity. Unlike photoelectric sensors, target color Unlike photoelectric sensors, target color and dusty atmospheric conditions do not and dusty atmospheric conditions do not affect ultrasonic sensors.affect ultrasonic sensors.

Target

Poor Targets: Poor Targets: Porous targets such as felt, cloth or Porous targets such as felt, cloth or foam rubber have very high sound absorption foam rubber have very high sound absorption properties, and subsequently make poor candidates properties, and subsequently make poor candidates for ultrasonic detection. In addition, liquid targets, for ultrasonic detection. In addition, liquid targets, typically excellent for ultrasonic detection, may typically excellent for ultrasonic detection, may become undetectable if bubbles or foam cover the become undetectable if bubbles or foam cover the surface. surface.

Unstable Targets:Unstable Targets: Standard ultrasonic sensors can Standard ultrasonic sensors can generate erroneous output signal when monitoring generate erroneous output signal when monitoring turbulent or unstable targets. turbulent or unstable targets.

chapter 3 sensor technology. a transducer is defined as any device that converts energy from one...

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sound energy

energy conversion

light changes

form of electrical energy

introduction slide

electrical resistance

place slide

limit switches slide