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Proximity sensors


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Proximity sensors

  1. 1. Proximity sensor A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. INFRARED
  2. 2.  A proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal.  The object being sensed is often referred to as the proximity sensor's target. Different proximity sensor targets demand different sensors. For example, a capacitive or photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor always requires a metal target. INDUCTIV E CAPACITIVE
  3. 3. Types of PROXIMITY SENSOR  Capacitive  Inductive  Passive optical  Ultrasonic sensor  MAGNETIC
  4. 4. Magnetic Proximity Sensors Magnetic proximity sensors are non-contact proximity devices that are used to detect magnetic objects (e.g. permanent magnets). They sense the presence of a magnetic object, commonly referred to as the target. The target, characterized by its magnetic field, triggers the switching process when it enters the detection range of the sensor. The switch may be a reed switch or the device could operate due to inductive, variable reluctance, magneto resistive or hall effect operating principles.
  5. 5. Sensors Specifications Performance Criteria • Rated operating distance is the critical distance at which switching takes place. It is important to choose a sensor that will operate in the required sensing range. This could be dictated by process requirements and mounting option. • Repeatability is the distance within which the sensor repeatably switches. It is a measure of precision. Depending on the application, precision could be an important design criterion when selecting a sensor.
  6. 6. OPERATING PRINCIPLES FOR MAGNETIC SENSORS Magnetic sensors are actuated by the presence of a permanent magnet. Their operating principle is based on the use of reed con- tacts, whose thin plates are hermetically sealed in a glass bulb with inert gas. The presences of a magnetic field makes the thin plates flex and touch each other causing an electrical contact. The plate’s surface has been treated with a special material par- ticularly suitable for low current or high inductive circuits. Magnetic sensors compared to traditional mechanical switches have the following advantage: • Contacts are well protected against dust, oxidization and corrosion due to the hermetic glass bulb and inert gas; contacts are activated bymeans of a magnetic field rather than mechanical parts • Special surface treatment of contacts assures long contact life • Maintenance free • Easy operation • Reduced size When using the NO (normally open) type the open reed contact closes as the magnet approaches. NO Magnetic sensors are two wires. When using the NO+NC type both NO (normally open) and NC (normally closed) functions are made available by means of a single glass bulb. NO+NC Magnetic sensors are supplied with three wires, one is in common, one is NO and one is NC
  7. 7. TYPICAL REED CONTACT PROTECTIONS The lifespan of a magnetic sensor at low values of voltage and current depends on the mechanical characteristics of the contact while for higher values the operating life depends upon the charectristics of load.
  8. 8. Magnetic Proximity Sensors Extremely small dimensions and high operating distances characterize these magnetic sensors in metallic case. To actuate sensor a magnetic is required. Features: • High operating distance • Threaded metallic case • Protection degree of IP 67 • Hermetically sealed • Compliant to the EMC directive Output VA V A MODEL NO 10 220 0.5 S3390 S3391 S3392 S3393 NO+NC 20 150 1 S3398 S3399 S3400 S3401 Dimensions: mm 1" = 25.4 mm 1 mm = .03937” External Dimensions ∅ 6 mm M8 x1 M10 x 1 M12 x 1 Operating Distance See Table 1 Switching Frequency NO output = 230 Hz max/ NO+NC output = 250 Hz max Case Nickel-Plated Brass Protection Degree IP 67 Operating Temperature -25 to +100°C (-13 to +212°F) Output Connection Cable: 2 x 0.14 mm², L=2m Dimensions:mm, 1" = 25.4 mm, 1 mm = .03937” Wiring NO Changeover, NO+NC Table 1. Operating distances as a function of the magnetic unit (mm) not to scale 4 Output NO NO/NC Magnet S3410 8 6 S3411 20 17 S3412 40 33
  9. 9. Rectangular Magnetic Proximity Sensors To actuate sensor a magnetic is required. Features: • • • • High operating distance Rectangular case Protection degree of IP 67 Hermetically sealed • Compliant to the EMC directive
  10. 10. CAPACITIVE proximity sensor
  11. 11. Introduction  Capacitance is an electrical property which is created by applying an electrical charge to two conductive objects with a gap between them. The capacitance of a parallel plate capacitor is given by: Where C is the capacitance, k is the permittivity of free space constant, K is the dielectric constant of the material in the gap, A is the area of the plates, and h is the distance between the plates.
  12. 12. Capacitive sensor  The capacitive sensor, consists of a target plate and a second plate known as the sensor head. These two plates are separated by an air gap of thickness h and form the two terminals of a capacitor.
  13. 13. Capacitive sensor  The guard ring essentially moves the distorted edges of the electric field to the outer edge of the guard, significantly improving the uniformity of the electric field over the sensor area and extending its linearity.
  14. 14. Capacitive sensor
  15. 15. Sensitivity  It is clear that the capacitance impedance Zc is linear in h and that methods of measuring ΔZc will permit extremely simple plates to act as a sensor to measure the displacement Δh.  Cylindrical sensor heads are linear and is valid provided that 0<h<D/4 where D is the diameter of the sensor head. Fringing in the electric field produces nonlinearities if h >D/4. The linear range can be extended to h=D/2 if a guard ring surrounds the sensor.  The sensitivity of the probe is given by :
  16. 16. Sensitivity  Sensitivity can be improved by reducing the area of the probe, however the range of the probe is limited by linearity to about D/2.  Low frequency improves sensitivity but limits frequency response of the instrument.  It is also important to note that the frequency of the ac power supply must remain constant to maintain a stable calibration constant.
  17. 17. Advantages  It is non-contacting and can be used with any target material.  The sensor is extremely rugged and can be subjected to high shock loads and intense vibratory environments.  Can be used at high temperature.  Sensitivity remains constant over a wide range of temperature.
  18. 18. Industrial application  Typical capacitive sensor construction shows two plates: one connects to the oscillator (sensor electrodes), and the other is the object being sensed, which is detected within the electrical field.
  19. 19. Industrial application  Capacitive proximity sensors can detect objects composed of a wide variety of materials. Here, a capacitive sensor detects the contents of a box.
  20. 20. Industrial application  A capacitive sensor functions like a typical capacitor. The metal plate in the end of the sensor electrically connects to the oscillator, and the object to be sensed acts as the second plate. When this sensor receives power, the oscillator detects the external capacitance between the target and the internal sensor plate. This arrangement completes the circuit and provides the necessary feedback path for the output circuit to evaluate.  Capacitive sensors can detect many different kinds of objects. For example, solids, liquids, or granular targets are all detectable (including metals, water, wood, and plastic).
  21. 21. Inductance proximity sensors  Inductive proximity sensors operate under the electrical principle of inductance. Inductance is the phenomenon where a fluctuating current, which by definition has a magnetic component, induces an electromotive force (emf) in a target object.  these are best used when your application calls for metallic target sensing with a range that is within an inch of the sensing surface.
  22. 22.  An inductive proximity sensor has four elements: coil, oscillator, trigger circuit, and an output. The oscillator is an inductive capacitive tuned circuit that creates a radio frequency. The electromagnetic field produced by the oscillator is emitted from the coil away from the face of the sensor. The circuit has just enough feedback from the field to keep the oscillator going. When a metal target enters the field, eddy currents circulate within the target. This causes a load on the sensor, decreasing the amplitude of the electromagnetic field. As the target approaches the sensor, the eddy currents increases, increasing the load on the oscillator and further decreasing the amplitude of the field.  The trigger circuit monitors the oscillator’s amplitude and at a predetermined level switches the output state of the sensor from its normal condition (on or off). As the target moves away from the sensor, the oscillator’s amplitude increases. At a predetermined level the trigger switches the output state of the sensor back to its normal condition (on or off).
  23. 23. Eddy current sensor  An eddy current sensor measures distance between the sensor head and an electrically conducting surface.sensor operation is based on eddy currents that are induced at the conducting surface as magnetic flux lines from the sensor intersect with the surface of the conducting material.  The magnetic flux lines are generated by the active coil in the sensor,which is driven at a very high frequency(1 MHz).
  24. 24.  The magnitude of the eddy current produced at the surface of the conducting material is a function of the distance between the active coil and the surface.the eddy currents increase as the distance decreases.
  25. 25.  Changes in the eddy currents are sensed with an impedance(inductance) bridge.two coils in the sensor are used for two arms of the bridge. The other two arms are housed in the associated electronic package.  The first coil in the sensor is the active coil and the second coil is inactive or balance coil changes inductance with target movement which is wired into the active arm of the bridge.thw second coil is wired into an opposing arm of the same bridge,where it serves as a compensating coil and cancel the effects of temperature change.
  26. 26.  The output from the impedance bridge is demodulated and becomes the analog signal,which is linearly proportional to distance between the sensor and the target.  The sensitivity of the sensor is dependent on the target material,with higher sensitivity associated with higher conductivity materials.  Thus eddy current sensors are high output devices if the specimen is non magnetic and from the graph it says that the sensitivity decreases significantly if the specimen material is magnetic.
  27. 27.  For aluminium the sensitivity is typically 100mV/mil(4mV/mm).  For non conducting,poorly conduting or magnetic materials,it is possible to bond a thin film of aluminium foil to the surface of the target at the location of the sensor to improve the sensitivity.the thickness of the foil can be little as 0.7mm.
  28. 28.  The effect of temperature on the output of the eddy current sensor is small. The sensing head with with dual coils is temperature compensated,however a small error can be produced by temperature changes in the target material,since resistivity of the target materil is a function of temperature.  So while measuring output we should even take care of sensitivity.
  29. 29.  The range of eddy current sensor is controlled by the diameters of the coils,with the larger sensors exhibiting the larger ranges.the range to diameter is usually about 0.25.  linearity is typically better than 0.05 percent
  30. 30.  Eddy-Current Sensor Advantages  Compared to other noncontact sensing technologies such as optical, laser, and capacitive, high-performance eddy-current sensors have some distinct advantages.  Tolerance of dirty environments  Not sensitive to material in the gap between the probe and target  Less expensive and much smaller than laser interferometers  Less expensive than capacitive sensors  Eddy-Current sensors are not a good choice in these conditions:  Extremely high resolution (capacitive sensors are ideal)  Large gap between sensor and target is required (optical and laser are better)
  31. 31.  The Eddy Current Sensor Precision eddy current noncontact measuring systems have been used for more than 30 years for displacement, vibration, thickness, alignment, dimensioning, and parts sorting applications. All these can be classified as variations on displacement because in each case the parameter being measured is the distance from the target to the sensor. The differences lie in the interpretation and implementation of the displacement data.
  32. 32.  The fact that eddy current sensors do not require contact for measuring displacement is quiet a result of this feature,they are often used in transducer systems for automatic control of dimensions in fabrication process.  They are also applied extensively to determine thickness of organic coatings that are non-conducting.
  33. 33. Ultrasonic Sensors
  34. 34. Ultrasonic sensors are based on measuring the properties of sound waves with frequency above the human audible range. Systems typically use a transducer which generates sound waves in the ultrasonic range, above 18 kHz, by turning electrical energy into sound, then upon receiving the echo turn the sound waves into electrical energy which can be measured. Ultrasonic sensors are non-intrusive in that they do not require physical contact with their target, and can detect certain clear or shiny targets otherwise obscured to some vision-based sensors.
  35. 35. Active Sensors Active ultrasound sensors emit sound waves from quartz-crystal transducers. The waves strike objects within the field of detection and as long as there are no movement the waves are not disrupted. However, when movement occurs the sound wave is disrupted and is reflected back to the system’s receiver.
  36. 36. Passive Ultrasonic Motion Sensors Passive sensors operate on the principle of sounds such as breaking glass or metal striking metal to trigger alarms. These sounds produce waves detected by the sensors that, like the active sensors, relay them to electronic control units to determine if the sound wave pattern falls within established normal parameters.
  37. 37. Benefits of High Frequency  Uninterrupted coverage  Electronically adjustable reach  Detection through glass, wood, walls etc.  “Invisible” sensor that can be integrated in lights  Can be concealed behind trim panels  Uninterrupted signal propagation  Good quality of detection, even in long rooms, stairwells etc.  Extremely fast detection of the smallest of movements  Operates irrespective of ambient temperature and temperature of objects  Reach, twilight threshold and light ‘ON’ duration can be set to suit individual needs
  38. 38. Applications a)Bottle Counting on Drink Filling Machines
  39. 39. Thru-beam sensors Individual detection of conveyed bottles is normally too fast for sensing by ultrasonic sensors. The bottles pass the sensor too quickly and the gaps between the bottles are often too small. For this reason, ultrasonic thru-beam sensors are particularly suitable for bottle counting. The use of hot steam and chemicals for machine cleaning in these applications requires ultrasonic thru-beam sensors with a high degree of chemical resistance. Even in areas with strong steam generation, reliable detection of bottles is guaranteed with ultrasonic thru-beam sensors.
  40. 40. b) Vehicle Detection in Barrier Systems
  41. 41. In car parking lots and parking garages, entry is controlled using barrier systems. The barrier must not be lowered when there is a vehicle underneath. Ultrasonic sensors are particularly suitable for controlling this process. They detect objects regardless of vehicle type or color and monitor the entire area below the barrier. When mounting and aligning the sensors, ensure that the devices are installed at a sufficient distance from the ground (if necessary, angled slightly upwards)
  42. 42. Optical Proximity Sensors
  43. 43. Working Principle An optical proximity sensor offers non-contact sensing of almost any object up to a range of 10 meters. It includes a light source, (usually an LED in either infrared or visible light spectrum) and a detector (photodiode). The light source generates light of a frequency that the light sensor is best able to detect, and that is not likely to be generated by other nearby sources. Infra-red light is used in most optical sensors. To make the light sensing system more foolproof, most optical proximity sensor light sources pulse the infra-red light on and off at a fixed frequency. Due to the high intensity infra-red energy beam, these sensors have major advantages over other opto-electronic systems when employed in dusty enviroments.
  44. 44. There are two main types of Optical Proximity Sensors : 1) Beam Type
  45. 45. 2) Retro Reflective Type
  46. 46. Name Advantages Disadvantages Beam Type •Most accurate •Longest sensing range •Very reliable •Must install at two points on system: emitter and receiver •Costly - must purchase both emitter and receiver Reflective Type • Slightly less accurate than through-beam •Very reliable •Must install at two points on system: sensor and reflector •Sensing range less than beam type
  47. 47. Advantages  Effective in Dusty/ noisy enviroments  Uses focused beam  Long range  Higher sensing distance compared to Inductive and capacitive type sensors  Immune to visible light interference Drawbacks  Interference  Cost  Pb in fog/smoke/nontransparent materials
  48. 48. General Applications  Lift door mechanisms  Pipeline monitoring, wind turbine blade monitoring, fuel tank and ship hull monitoring, power line monitoring etc.  Component positioning sensing in Electronic industry  Security and safety applications in presses  Colour sensing applications  Counting of bottles/containers in factories etc.
  49. 49. proximity sensor for neurovascular bundle detection during dental implant surgery The basic implant procedure involves using a drill to create an osteotomy in the bone where a titanium screw is placed. A dental prosthesis is then placed onto the frame of the titanium screw. The success of these procedures is dependent on the anchorage by the formation of bony tissue around the implant, such that implant shows no mobility when loaded.
  50. 50. Dental implant surgery is done by using combined near infrared absorption (NIR) and optical coherence tomography (OCT) techniques. These have different sensitivity to the proximity of optical contrast from neurovascular bundles.
  51. 51. Implant depth is determined by the surgeon when drilling the channel in the mandible. The depth when drilling a dental implant channel within the mandible is limited by the risk of breaching the mandibular canal that contains a neurovascular bundle including the inferior alveolar nerve (IAN), which is the mental nerve providing sensory enervation to the lower lips and chin. Loss of sensation in the anterior mandible, such as numbness to the lower lip and chin, can occur due to the disruption of the IAN. The reported incidence of nerve injury from implant placement is as high as 44%, with 73% of dentists encountering neurosensory impairment within their practice.
  52. 52. The medical complications that could be avoided by using this device are (i) Vertical bone grafting (ii) Nerve lateralization (moving the nerve out of the jaw) which causes a high risk of nerve injury and sometimes permanent damage.
  53. 53. Thank you !