Transducers and Sensors

Transcription

1 Transducers and Sensors Dr. Ibrahim Al-Naimi Chapter THREE Transducers and Sensors 1

2 Proximity Sensors Sensors which detect whether or not an object is located at a certain position, i.e. discrete sensors that sense when an object has come near to the sensor face. These sensors are known as proximity sensors. Sensors of this type provide a "Yes" or "No" statement depending on whether or not the position, to be defined, has been taken up by the object. These sensors, which only signal two status, are also known as binary sensors or in rare cases as initiators. Proximity Sensors With many production systems, mechanical position switches are used to acknowledge movements which have been executed. Additional terms used are microswitches, limit switches or limit valves. Because movements are detected by means of contact sensing, relevant constructive requirements must be fulfilled. Also, these components are subject to wear. In contrast, proximity sensors operate electronically and without contact. 2

3 Proximity Sensors The advantages of contactless proximity sensors are: Precise and automatic sensing of geometric positions Contactless sensing of objects and processes; no contact between sensor and workpiece is required with electronic proximity sensors Fast switching characteristics; because the output signals are generated electronically, the sensors are bounce-free and do not create error pulses. Wear-resistant function; electronic sensors do not include moving parts which can wear out Unlimited number of switching cycles Suitable versions are also available for use in hazardous conditions (e.g. Areas with explosion hazard). Proximity Sensors Today, proximity sensors are used in many areas of industry for the reasons mentioned above. They are used for sequence control in technical installations and as such for monitoring and safeguarding processes. In this context sensors are used for early, quick and safe detection of faults in the production process. The prevention of damage to man and machine is another important factor to be considered. A reduction in downtime of machinery can also be achieved by means of sensors, because failure is quickly detected and signalled. 3

4 Fields of Application for Proximity Sensors Typical fields of application for proximity sensors are in the areas of: Automotive industry Mechanical engineering Packaging industry Timber industry Printing and paper industry Drinks and beverages industry Ceramics and brick industry 4

5 Proximity Sensors Applications Detecting Objects Positioning Counting Proximity Sensors Applications Measuring rotational speed Detecting materials Defining direction 5

6 Proximity Sensors Applications Tools monitoring Level monitoring Proximity Sensors Applications Accident protection Shape recognition 6

7 Inductive Proximity Sensor As with all proximity sensors, inductive proximity sensors are available in various sizes and shapes as shown in the following figure. As the name implies, inductive proximity sensors operate on the principle that the inductance of a coil and the power losses in the coil vary as a metallic (or conductive) object is passed near to it. Because of this operating principle, inductive proximity sensors are only used for sensing metal objects. They will not work with non-metallic materials. Inductive Proximity Sensor 7

8 Inductive Proximity Sensor To understand how inductive proximity sensors operate, consider the cutaway block diagram shown in the following figure. Mounted just inside the face of the sensor (on the left end) is a coil which is part of the tuned circuit of an oscillator. When the oscillator operates, there is an alternating magnetic field (called a sensing field) produced by the coil. This magnetic field radiates through the face of the sensor (which is non-metallic). The oscillator circuit is tuned such that as long as the sensing field senses non-metallic material (such as air) it will continue to oscillate, it will trigger the trigger circuit, and the output switching device (which inverts the output of the trigger circuit) will be off. The sensor will therefore send an off signal through the cable extending from the right side of the sensor in the figure. Inductive Proximity Sensor 8

9 Inductive Proximity Sensor Inductive Proximity Sensor When a metallic object (steel, iron, aluminium, tin, copper, etc.) comes near to the face of the sensor, as shown in the following figure, the alternating magnetic field in the target produces circulating eddy currents inside the material. To the oscillator, these eddy currents are a power loss. As the target moves nearer, the eddy current loss increases which loads the output of oscillator. This loading effect causes the output amplitude of the oscillator to decrease. 9

10 Inductive Proximity Sensor Inductive Proximity Sensor As long as the oscillator amplitude does not drop below the threshold level of the trigger circuit, the output of the sensor will remain off. However, as shown in the following figure, if the target object moves closer to the face of the sensor, the eddy current loading will cause the oscillator to stall (cease to oscillate). When this happens, the trigger circuit senses the loss of oscillator output and causes the output switching device to switch on. 10

11 Inductive Proximity Sensor Inductive Proximity Sensor The sensing range (switching distance) of a proximity sensor is the maximum distance the target object may be from the face of the sensor in order for the sensor to detect it. One parameter affecting the sensing range is the size (diameter) of the sensing coil in the sensor. Small diameter sensors (approximately ¼ in diameter) have typical sensing ranges in the area of 1mm, while large diameter sensors (approximately 3" in diameter) have sensing ranges in the order of 50mm or more. Additionally, since different metals have different values of resistivity (which limits the eddy currents) and permeability (which channels the magnetic field through the target), the type of metal being sensed will affect the sensing range. 11

12 Inductive Proximity Sensor Inductive proximity sensor manufacturers derate their sensors based on different metals, with steel being the reference (i.e., having a derating factor of 1.0). Some other approximate derating (reduction) factors are stainless steel: 0.85, aluminium: 0.40, brass: 0.40, and copper As an example of how to apply the derating (reduction) factors, assume you are constructing a machine to automatically count copper pennies as they travel down a chute, and the sensing distance will be 5mm. In order to detect copper (derating factor 0.30), you would need to purchase a sensor with a sensing range of 5mm / 0.30 = 16.7mm. Let s say you found a sensor in stock that has a sensing range of 10mm. If you use this to sense the copper pennies, you would need to mount it near the chute so that the pennies will pass within (10mm)(0.30) = 3mm of the face of the sensor. Inductive Proximity Sensor Inductive proximity sensors are available in both DC and AC powered models. Most require 3 electrical connections: ground, power, and output. However, there are other variations that require 2 wires and 4 wires. Most sensors are available with a built-in LED that indicates when the sensor is on. One of the first steps a designer should take when using any proximity sensor is to acquire a manufacturer s catalogue and investigate the various types, shapes, and output configurations to determine the best choice for the application. 12

13 Inductive Proximity Sensor Since the parts of machines are generally constructed of some type of metal, there are an enormous number of possible applications for inductive proximity sensors. They are relatively inexpensive (~$30 and up), extremely reliable (not sensitive to dust), operate from a wide range of power supply voltages, are rugged, and since they are totally self contained, they connect directly to the discrete inputs on a PLC with no additional external components. In many cases, inductive proximity sensors make excellent replacements for mechanical limit switches. Applications: Inductive Proximity Sensor By placing an inductive proximity next to a gear, the proximity can sense the passing gear teeth to give rotating speed information. This application is currently used as a speed feedback device in automotive cruise control systems where the proximity is mounted in the transmission. All helicopters have an inductive proximity mounted in the bottom of the rotor gearbox. Should the gears in the gearbox shed any metal chips (indicating an impending catastrophic failure of the gearbox), the inductive proximity senses these chips and lights a warning light on the cockpit instrument panel. Inductive proximities can be mounted on access doors and panels of machines. The PLC can be programmed to shut down the machine anytime any of these doors and access panels are opened. Very large inductive proximities can be mounted in roadbeds to sense passing automobiles. This technique is currently used to operate traffic lights. 13

14 Example: Technical Characteristics Inductive Proximity Sensor 14

15 Inductive Proximity Sensor Inductive Proximity Sensor 15

16 Inductive Proximity Sensor Inductive Proximity Sensor 16

17 Capacitive Proximity Sensor Capacitive proximity sensors are available in shapes and sizes similar to the inductive proximity sensor. However, because of the principle upon which the capacitive proximity sensor operates, applications for the capacitive sensor are somewhat different. Capacitive Proximity Sensor To understand how capacitive proximity sensors operate, consider the cutaway block diagram shown in the figure. The principle of operation of the sensor is that an internal oscillator will not oscillate until a target material is moved close to the sensor face. The target material varies the capacitance of a capacitor in the face of the sensor that is part of the oscillator circuit. As is shown in the waveform diagram in the figure, as the target approaches the face of the sensor, the oscillator amplitude increases, which causes the sensor output to switch on. 17

18 Capacitive Proximity Sensor Capacitive Proximity Sensor If an object (metal, plastic, glass, wood, water) is introduced into the active switching zone, then the capacitance of the resonant circuit is altered. This change in capacitance essentially depends on the following parameters: The distance of the object from the active surface, The dimensions of the object and The dielectric constant of the object. 18

19 Capacitive Proximity Sensor There are two types of capacitive sensor, each with a different way that this sensing capacitor is formed. In the dielectric type of capacitive sensor, there are two side-by-side capacitor plates in the sensor face. For this type of sensor, the external target acts as the dielectric. As the target is moved closer to the sensor face, the change in dielectric increases the capacitance of the internal capacitor, making the oscillator amplitude increase, which in turn causes the sensor to output an on signal. The conductive type of sensor operates similarly; however, there is only one capacitor plate in the sensor face. The target becomes the other plate. Therefore, for this type of sensor, it is best if the target is an electrically conductive material (usually metal or water-based). Capacitive Proximity Sensor Dielectric type capacitive proximity sensors will sense both metallic and non-metallic objects. However, in order for the sensor to work properly, it is best if the material being sensed has a high density. Low density materials (foam, bubble wrap, paper, etc.) do not cause a detectable change in the dielectric and consequently will not trigger the sensor. Conductive type capacitive proximity sensors require that the material being sensed be an electrical conductor. These are ideally suited for sensing metals and conductive liquids. For example, since most disposable liquid containers are made of plastic or cardboard, these sensors have the unique capability to look through the container and sense the liquid inside. Therefore, they are ideal for liquid level sensors. 19

20 Capacitive Proximity Sensor Capacitive proximity sensors will sense metal objects just as inductive sensors will. However, capacitive sensors are much more expensive than the inductive types. Therefore, if the material to be sensed is metal, the inductive sensor is the more economical choice. As with the inductive proximity sensors, capacitive proximity sensors are available with a built-in LED indicator to indicate the output logical state. Also, because capacitive proximity sensors are used to sense materials with a wide range of densities, manufacturers usually provide a sensitivity adjusting screw on the back of the sensor. Then when the sensor is installed, the sensitivity can be adjusted for best performance in the particular application. Capacitive Proximity Sensor Some of the potential applications for capacitive proximity sensors include: They can be used as a non-contact liquid level sensor. They can be place outside a container to sense the liquid level inside. This is ideal for milk, juice, or soda bottling operations. Capacitive proximity sensors can be used as replacements for pushbuttons. They will sense the hand and, since they have no moving parts, they are more reliable than mechanical switches. Since they are hermetically sealed, they can be mounted inside liquid tanks to sense the tank fill level. 20

21 Example: Technical Characteristics Capacitive Proximity Sensor 21

22 Capacitive Proximity Sensor Capacitive Proximity Sensor 22

23 Capacitive Proximity Sensor Ultrasonic Proximity Sensor Sound frequency which is above the limit of human hearing is described as ultrasound. The lower limit is at approximately 20 khz. The particular characteristics of ultrasound applied to proximity sensors are the result of the high frequency and the correspondingly short wavelength. 23

24 Ultrasonic Proximity Sensor The propagation of sound is the result of propagation of mechanical long waves, which manifests itself in a periodic density variation in the carrier medium, leading to alternating compressions and dilutions. The propagation of sound waves is dependent on a transmitting medium, it is not possible in a vacuum. The following formula applies for the speed of sound in dry air at temperature T: Ultrasonic Proximity Sensor There are three different methods of generating ultrasound: mechanical, magnetic and electrical. Electrical generation: With electrorestriction (inverse piezoelectrical effect) an alternating voltage of high frequency is connected to a crystal plate. This plate then carries out the mechanical oscillations of the corresponding frequency, which become particularly strong with resonance. Frequencies of up to approximately khz are achieved. 24

25 Ultrasonic Proximity Sensor Ultrasonic Proximity Sensor Nowadays, instead of crystals, piezoelectrical materials, which are widely distributed under the trade name Piezoxide (e.g. by Valvo), are used to generate ultrasound. These materials are made of lead-zirconate-titanate. 25

26 Ultrasonic Proximity Sensor Ultrasonic Proximity Sensor 26

27 Ultrasonic Proximity Sensor The operational principle of an ultrasonic proximity sensor is based on the emission and reflection of acoustic waves between an object and a receiver. Normally, the carrier of these sound waves is air. The travelling time of the sound is measured and evaluated (time of flight). The ultrasonic transmitter emits sound waves in the nonaudible range at any frequency usually between khz then receives an echo if the object is present. Filters inside the ultrasonic proximity sensor check whether the sound received is actually the echo of the emitted ultrasonic waves. Ultrasonic Proximity Sensor 27

28 Ultrasonic Proximity Sensor Ultrasonic Proximity Sensor An object must not be present in the sound field of the proximity sensor within the socalled near field, as this can lead to error pulses at the proximity sensor output. For an ultrasonic proximity sensor with a transducer diameter of 15 mm and an emitting frequency of 200 khz, the range of the near field is approximately 130 mm. 28

29 Ultrasonic Proximity Sensor an ultrasonic ping is sent from the face of the sensor. If a target is located in front of the sensor and is within range, the ping will be reflected by the target and returned to the sensor. When an echo is returned, the sensor detects that a target is present, and by measuring the time delay between the transmitted ping and the returned echo, the sensor can calculate the distance between the sensor and the target. Ultrasonic Proximity Sensor The sensor is only capable of sensing a target that is within the sensing range (sensor Field of View FOV). The sensing range is a funnel shaped area directly in front of the sensor as shown in the figure. Sound waves travel from the face of the sensor in a cone shaped dispersion pattern bounded by the sensor s beam angle. However, because the sending and receiving transducers are both located in the face of the sensor, the receiving transducer is blinded for a short period of time immediately after the ping is transmitted, similar to the way our eyes are blinded by a flashbulb. This means that any echo that occurs during this blind time period will go undetected. These echoes will be from targets that are very close to the sensor, within what is called the sensor s deadband. In addition, because of the finite sensitivity of the receiving transducer, there is a distance beyond which the returning echo cannot be detected. This is the maximum range of the sensor. These constraints define the sensor s useable sensing area. 29

30 Ultrasonic Proximity Sensor Ultrasonic proximity sensors that have a discrete output generally have a switch point adjustment provided on the sensor that allows the user to set the target distance at which the sensor output switches on. Note that ultrasonic sensors are also available with analog outputs that will provide an analog signal proportional to the target distance. Ultrasonic Proximity Sensor Ultrasonic proximity sensors are useful for sensing targets that are beyond the very short operating ranges of inductive and capacitive proximity sensors. Off the shelf ultrasonic proximities are available with sensing ranges of 6 meters or more. They sense dense target materials best such as metals and liquids. They do not work well with soft materials such as cloth, foam rubber, or any material that is a good absorber of sound waves, and they operate poorly with liquids that have surface ripple or waves. Also, for obvious reasons, these sensors will not operate in a vacuum. 30

31 Ultrasonic Proximity Sensor Since the sound waves must pass through the air, the accuracy of these sensors is subject to the sound propagation time of the air. The most detrimental impact of this is that the sound propagation time of air decreases by 1.7%/degree Celsius. This means that as the air temperature increases, a stationary target will appear to move closer to the sensor. The humidity content of air at a temperature range below 40 C effects a maximum change in the speed of sound by 1.4 % between a relative air humidity of 0 % and 100 %. Natural changes in atmospheric air pressure do not cause any significant changes in the speed of sound. Only at high altitudes does the speed of sound decrease slightly. They are not affected by ambient audio noise, nor by wind. However, because of their relatively long useful range, the system designer must take care when using more than one ultrasonic sensor in a system because of the potential for crosstalk between sensors. Ultrasonic Proximity Sensor Similarly as with light, ultrasound is deflected on flat surfaces. In this case, an ultrasonic sensor does not receive an echo signal. Objects with smooth, even surfaces, can no longer be detected if the deviation is for instance more than ±3 ±5 of the vertical alignment to the proximity sensor. With objects of a rough or irregular surface a wider angle is possible, whereby the ultrasonic wave length, the surface finish and distance are also relevant. 31

32 Ultrasonic Proximity Sensor Ultrasonic Proximity Sensor Preferred areas of application for ultrasonic proximity sensors are: Storage facilities Transport systems Food industry Metal, glass and plastics processing Monitoring of bulk material 32

33 Ultrasonic Proximity Sensor Ultrasonic proximity sensors have the following advantages: Relatively large range (up to several meters) Object detection irrespective of colour and material Safe detection of transparent objects (e.g. glass bottles) Relatively dust and dirt insensitive Fading out of background possible Outdoor application possible Ultrasonic Proximity Sensor Ultrasonic proximity sensors have the following disadvantages: If ultrasonic proximity sensors are used for slanting object surfaces, the sound is deflected. It is therefore important that the object surface to be reflected is at a right angle to the axis of the sound propagation or to use ultrasonic barriers instead. Ultrasonic proximity sensors react relatively slowly. Maximum switching frequency is between 1 Hz and 125 Hz. Ultrasonic proximity sensors are generally more expensive than optical proximity sensors (e.g. factor 2). 33

34 Example: Technical Characteristics Ultrasonic Proximity Sensor One popular use for the ultrasonic proximity sensor is in sensing liquid level. The following figure shows such an application. Note that since ultrasonic sensors do not perform well with liquids with surface turbulence, a stilling tube is used to reduce the potential turbulence on the surface of the liquid. 34

35 Ultrasonic Proximity Sensor Optical proximity sensors are devices which convert signals generated by light emission into electrical signals. The response of optical receivers varies according to different ranges of wavelength. The following figure indicates the spectral ranges of electromagnetic emission. 35

36 Optical proximity sensors employ optical and electronic means for the detection of objects. Red or infrared light is used for this purpose. Semiconductor light emitting diodes (LEDs) are a particularly reliable source of red and infrared light. They are small and robust, have a long service life and can be easily modulated. Photodiodes or phototransistors are used as receiver elements. When adjusting optical proximity sensors, red light has the advantage that it is visible in contrast to infrared light. Besides, polymer optic cables can easily be used in the red wavelength range because of their reduced light attenuation. 36

37 Infrared (non visible) light is used in instances, where increased light performance is required in order to span greater distances for example. Furthermore, infrared light is less susceptible to interference (ambient light). With both types of optical proximity sensor, additional suppression of external light influences is achieved by means of modulating the optical signal. I.e. the light output is pulsed at a high frequency, and a receiver is tuned to the frequency of the source. So, these types have a high degree of immunity to other potentially interfering light sources. Therefore, red LED and IR LED sensor types function better in areas where there is a high level of ambient light (such as sunlight), or light noise (such as welding). 37

38 Optical proximity sensors basically consist of two main units: the emitter and the receiver. Depending on type and application, reflectors and fibre-optic cables are required in addition. Emitter and receiver are either installed in a common housing (diffuse sensors and retro-reflective sensors), or housed separately (through-beam sensors). The emitter houses the source of red or infrared light emission, which according to the laws of optics extends in a straight line and can be diverted, focussed, interrupted, reflected and directed. It is accepted by the receiver, separated from external light and electronically evaluated. Optical sensors are an extremely popular method of providing discrete-output sensing of objects. Since the sensing method uses light, it is capable of sensing any objects that are opaque, regardless of the colour or reflectivity of the surface. They operate over long distances (as opposed to inductive or capacitive proximity sensors), will sense in a vacuum (as opposed to ultrasonic sensors), and can sense any type of material no matter whether it is metallic, conductive, or porous. Since the optical transmitters and receivers use focused beams (using lenses), they can be operated in close proximity of other optical sensors without crosstalk or interference. 38

39 Through-beam sensors Through-beam sensors consist of separately assembled emitter and receiver components whereby wide sensing ranges can be achieved. For the interruption of the light beam to be evaluated, the cross-section of the active beam must be covered. The object should permit only minimum penetration of light, but may reflect any amount of light. Failure of the emitter is evaluated as "object present". 39

40 Through-beam sensors Through-beam sensors 40

41 Through-beam sensors Advantages Enhanced reliability because of permanent light during non-operation. Wide range. Small objects can be detected even at large distances. Suitable for aggressive environment. Objects can be diffuse reflecting, mirroring or low transparent. Good positioning accuracy. Through-beam sensors Disadvantages Two separate proximity sensor modules (emitter and receiver) and separate electrical connections are required. Cannot be used for completely transparent objects. Applications 41

42 Diffuse sensors The emitter and receiver are fitted in the same housing. The object diffusely reflects a percentage of the emitted light thereby activating the receiver. Depending on the design of the receiver, the output is then switched through (normally open function) or switched off (normally closed function). Diffuse sensors 42

43 Diffuse sensors The switching distance largely depends on the reflectivity of the object. The size, surface, shape, density and colour of the object as well as the angle of impact determine the intensity of the diffused light so that as a rule only small distances within a range of a few decimetres can be scanned. The background must absorb or deflect the light emission, i.e. when an object is not present, the reflected light beam must be clearly below the response threshold of the receiving circuit. Diffuse sensors 43

44 Diffuse sensors Advantages Because the reflection on the object activates the receiver, an additional reflector is not required. Whereas with through-beam sensors objects can only be detected laterally to the light beam, diffuse sensors allow frontal detection, i.e. in the direction of the light beam. This type is more convenient than through-beam type because the emitter and receiver are located in the same housing, which simplifies wiring. Diffuse sensors Disadvantages This type of sensor does not work well with transparent targets or targets that have a low reflectivity (dull finish, black surface, etc.). Care must also be taken with glossy target objects that have multifaceted surfaces (e.g., automobile wheel covers, corrugated roofing material), or objects that have gaps through which light can pass (e.g. toy cars with windows, compact disks). These types of target objects can cause optical sensors to output multiple pulses for each object. 44

45 Diffuse sensors Applications Retro-reflective sensors Light emitter and light receiver are installed in one single housing. An additional reflector is required. The retroreflective optical sensors is the most sophisticated of all of the sensors. The sensor works similarly to the through beam sensor in that a target object passing in front of the sensor blocks the light being received. However, in this case, the light being blocked is light that is returning from a reflector. Therefore, this sensor does not require the additional wiring for the remotely located receiver unit. Compared to a diffuse sensor, the retro-reflective sensor has a greater range. 45

46 Retro-reflective sensors Retro-reflective sensors 46

47 Retro-reflective sensors Advantages Enhanced reliability because of permanent light during non-operation. Simple installation and adjustment. Object can be diffuse reflecting, mirroring or transparent as long as a sufficiently high percentage of the light is definitely absorbed. In most cases, a greater range in comparison with diffuse sensors. Retro-reflective sensors Disadvantages Transparent, very bright or shiny objects may remain undetected. Applications 47

48 Retro-reflective sensors Generally, this type of sensor would not work well with glossy target objects because they would reflect light back to the receiver just as the remote reflector would. However, this problem is avoided using polarizing filters. This polarizing filter scheme is illustrated in the following figure. Notice in the illustration that there is an added polarizing filter that polarizes the exiting light beam. In the illustration, this is a horizontal polarization. In the upper figure, notice that with no target object present, the specially designed reflector twists the polarization angle by 90 degrees, sending the light back in vertical polarization. Retro-reflective sensors At the receiver, there is another polarizing filter; however, this filter is installed with a vertical polarization to allow the light returning from the reflector to pass through and be detected by the receiver. In the lower illustration of the figure, notice that when a target object passes between the sensor and the reflector, not only is the light beam disrupted, but if the object has a glossy surface and reflects the light beam, the reflected beam returns with the same horizontal polarization as the emitted beam. Since the receiver filter has a vertical polarization, the receiver does not receive the light, so it activates its output. 48

49 Retro-reflective sensors Optical proximity sensors with fibre-optic cables Optical proximity sensors with fibre-optic cable adaptors are used if conventional devices take up too much room. Another application, where the use of fibre-optic cable adaptors is of advantage, is in areas with explosion hazard. With the use of fibre-optic cables the position of small objects can be detected with high accuracy. By using two separate fibre-optic cables it is possible to construct a through-beam sensor. Because of their handling flexibility, these can be used universally. 49

50 Optical proximity sensors with fibre-optic cables Optical proximity sensors with fibre-optic cables 50

51 Optical proximity sensors with fibre-optic cables Advantages Detection of objects in areas of restricted access, e.g. through holes. Possibility of remote installation of proximity sensor housing (e.g. Hazardous environment: heat, water, radiation, explosion risk). Accurate detection of small objects. Sensing elements can be moved. Glass fibre optics is suitable for higher temperatures Magnetic Proximity Sensor Hall Effect Sensor: The Hall effect was discovered in the last century by E. Hall. He discovered that a voltage difference is created on the opposite sides of a small thin gold plate, through which a current passes, if a magnetic field operates vertically to this. Subsequently, it was discovered that this effect also occurs with many semiconductors. Certain physical characteristics are required for this. The thickness of the small plate must be less than the dimensions of length and width. Voltages of up to 1.5 V can be created. 51

52 Magnetic Proximity Sensor Hall Effect Sensor: Magnetic Proximity Sensor Hall Effect Sensor: The formula for Hall voltage is: Hall sensor elements are used for the measurement of current and magnetic field or in combination with moving magnets for angle and position. 52

53 Selection Criteria for Proximity Sensors In the first instance, proximity sensors can be selected according to the material which they are to detect. Metals of any kind can be detected easily and economically with inductive proximity sensors if short switching distances only are required (e.g mm). For greater distances, optical proximity sensors of varying designs are available. The greatest distances can be spanned by means of through-beam sensors. Selection Criteria for Proximity Sensors Capacitive proximity sensors are suitable for the detection of a wide range of materials, but again only for relatively small distances, similar to inductive proximity sensors. Objects to be detected by a capacitive proximity sensor must be of a certain minimum volume. Ultrasonic and optical diffuse reflective proximity sensors are able to detect a wide range of different materials over greater distances. However, the detection of reflecting objects with tilted surfaces may create problems. 53

54 Selection Criteria for Proximity Sensors Further criteria for the selection of proximity sensors are the conditions under which the object is to be detected, what the installation requirements for the proximity sensor are and the environmental factors to be taken into account. Once all these requirements have been established, a suitable proximity sensor can be selected from the various alternative products on offer. A systematic listing of the above mentioned criteria is set out overleaf. Selection Criteria for Proximity Sensors Object material Electrically conductive material Steel Stainless steel Brass Copper Aluminium Nickel Chromium Metal-coated, electrically non-conductive materials, depending on specific coating thickness (mention one example) Graphite 54

55 Selection Criteria for Proximity Sensors Object material Electrically non-conductive material Plastics Cardboard, paper Wood Textiles Glass Size and shape Dimension of structure to be detected and possibly classification to standard shapes, e.g. block, cylinder, sphere, cone inter alia. Selection Criteria for Proximity Sensors Object material Nature of non-conductive materials Optically transparent or non-transparent Optical reflex ability of surface (absorbent to reflecting) Homogenous, non-homogenous (e.g. composite material) Porous, fibrous Solid, liquid, loose material Dielectric constant 55

56 Selection Criteria for Proximity Sensors Conditions for the detection of objects Contacting or non-contacting Required distance between proximity sensor and object, possibly taking into account any tolerances which may occur in respect of distance, e.g. in the case of moving objects. Speed of a moving object or time during which the object is present or down time. Constant or changing sensing requirements, e.g. different position of object. Distance to adjacent objects, required resolution of interrogation. Type of background or area below Selection Criteria for Proximity Sensors Installation conditions Free space available (distance/volume) around sensing area. The need to use miniature designs or remotely positioned proximity sensors when using fibre optic attachments or pneumatic sensor heads. The necessity for detecting "around the corner", in crevasses or through holes. Necessity of flush mounted installation. Required minimum distance between several adjacent proximity sensors. 56

57 Selection Criteria for Proximity Sensors Environmental considerations Ambient temperature Effect of dust, dirt, particles, humidity, splashing water, water jets inter alia. Influence of magnetic or electric fields, e.g. in a welding environment. Influence of external light emissions (peculiarities of ambient lighting). Area with explosion hazard Clean room environment Requirements for hygiene or sterilisation for use with food packaging or in a medical environment. Application in high pressure or vacuum conditions. Selection Criteria for Proximity Sensors Safety applications Application in areas with explosion hazard. Application for the purpose of accident prevention. Application where increased safety measures are required against breakdown. Options/features of proximity sensors Design/type with specification of dimensions Voltage supply (direct current, alternative current) 57

58 Selection Criteria for Proximity Sensors Options/features of proximity sensors Type of switch output and type of protective circuits: Positive switching (PNP output) Negative switching (NPN output) Short circuit protection Reverse polarity protection Connection: Cable or plug Protection class to IEC 529, DIN Permissible ambient temperature during operation Available special designs e.g. to DIN (NAMUR) or intrinsically safe design ("explosion protection"), or accident protection design Selection Criteria for Proximity Sensors Options/features of proximity sensors Extent of switching distance or range, fixed value or adjustable value Nominal switching distance or nominal range Switching hysteresis Maximum operating frequency (switching frequency) Maximum load current Flush mounted or non-flush mounted option Minimum required distance between adjacent proximity sensors of the same type Operating reserve factor for optical proximity sensors 58

59 Selection Criteria for Proximity Sensors Options/features of proximity sensors Fibre optic design available for optical proximity sensors. The following technical data apply in respect of fibre optic designs, e.g.: Range Dimensions of fibre optic head Fibre optic cable length Detection angle, response ranges Permissible ambient temperature Available accessories for retro-reflective sensors (reflectors, dimensions) Prices or price categories of proximity sensors Selection Criteria for Proximity Sensors Example: Metal Objects are to be detected on processing equipment in a highly inaccessible place where ambient temperature may increase up to 100 o C. Which type of sensor is particularly suitable in this case? 59