MAPUA INSTITUTE OF TECHNOLOGY AT LAGUNA Academic Year 2014 - 2015
EXPERIMENT NO. 2 SENSORS
Lea Fe A. SANGLITAN GRP. # 1 MARCH 25, 2015
ENGR. JIPCY MAURRIS NARVAEZ ECE135P/A21
I.
INTRODUCTION It is expected that some new types of sensors will be developed in the future. Passive sensors detect the reflected or emitted electro-magnetic radiation from natural sources, while active sensors detect reflected responses from objects which are irradiated from artificially generated energy sources, such as radar. A sensor is classified as a combination of passive, scanning, and imaging are classified further into image plane scanning sensors, such as TV cameras and solid state scanners, and object plane scanning sensors, such as multispectral scanners and scanning microwave radiometers. And for a sensor that is classified as an active, non-scanning and on-imaging sensors is a profile recorder such as a laser spectrometer and laser altimeter. The most popular sensors used in remote sensing are the camera, solid state scanner, such as the CCD (Charge Coupled Devices) images, the multi-spectral scanner and in the future the passive synthetic aperture radar. Laser sensors have recently begun to use more frequently for monitoring air pollution by laser spectrometer and for measurement of distance by laser altimeters. Those sensors which use lenses in the visible and reflective infrared region, are called optical sensors. Figure 1.1 summarizes the types of sensors now used or being developed in remote sensing.
Figure 1.1 Classification of Sensors
Types of sensors that are used to measure basic physical phenomena including: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Acceleration - Shock & Vibration. Angular / Linear Position Chemical/Gas Concentration Humidity Flow Rate Force Magnetic Fields Pressure Proximity - Spatial Presence Sound Temperature Velocity
Factors to consider when choosing a sensor. Accuracy - The statistical variance about the exact reading. Calibration - Required for most measuring systems since their readings will drift over time. Cost Environmental - Sensors typically have temperature and/or humidity limits. Range - Limits of measurement or the sensor. Repeatability - The variance in a sensor's reading when a single condition is repeatedly measured. Resolution - The smallest increment the sensor can detect. II.
METHODOLOGY Background Suppression Photoelectric Switch When using photoelectric sensors in simple diffused mode (a.k.a. proximity mode), the sensor uses the target to reflect light back to itself, eliminating the need for a secondary device such as a reflector. But the ability to precisely control the sensing range is not possible even if the sensor offers a sensitivity adjustment. In many applications this can cause significant problems because shiny objects well beyond the sensor¡¦s specified sensing range are erroneously detected. In fact it is not uncommon for a standard diffused mode sensor with a specified range of 15 inches to falsely detect a piece of metal, Plexiglas or other highly reflective object that is 6 feet or farther away from the sensor. Targets can vary greatly in color and this will directly affect the range of the sensor. This varying sensing distance is known as black-white differential. The black-white differential is simply the difference in distance between where a diffused sensor detects a 90% reflective white card vs. a 6% reflective black test card under the same conditions. Sensor manufacturers normally provide blackwhite differential data in the form of a graph so that customers have a guideline when applying them to the application.
Figure 2.1 black-white differential graph for a standard diffused mode sensor Color significantly affects sensing range for a diffused mode sensor, as the black-white differential for the sensor is approximately 190mm. The dramatic reduction in sensing range is due to the fact that standard diffused sensors recognize a target based on the light reflected back to the sensor¡¦s receiver. The received light must be strong enough to overcome any ambient light or any electrical noise at the sensor¡¦s receiver. If the target is black, it absorbs large amounts of energy and therefore must be close to the sensor in order to return enough of the emitted light to be detected. Polarized Retro-reflective Photoelectric Switch Retro-reflective mode is the second primary mode of photoelectric sensing. As with diffused mode sensing, the transmitter and receiver are in the same housing, but a reflector is used to reflect the light from the transmitter back to the receiver. The target is detected when it blocks the beam from the photoelectric sensor to the reflector. Retro-reflective mode typically allows longer sensing ranges than diffused mode due to the increased efficiency of the reflector compared with the reflectivity of most targets. The target color and finish do not affect the sensing range in retroreflective mode as they do with diffused mode. Retro-reflective mode photoelectric sensors are available with or without polarization filters. A polarization filter only allows light at a certain phase angle back to the receiver, which allows the sensor to see a shiny object as a target and not incorrectly as a reflector. This is because light reflected from the reflectors shifts the phase of the light, whereas light reflected from a shiny target does not. A polarized retro-reflective photoelectric sensor must be used with a corner-cube reflector, which is a type of reflector with the ability to accurately return the light energy, on a parallel axis, back to the receiver. Polarized retro-reflective sensors are recommended for any application with reflective targets.
Figure 2.2 Retro-reflective mode photoelectric sensors Non-polarized retro-reflective photoelectric sensors usually allow longer sensing ranges than polarized versions, but can falsely identify a shiny target as a reflector.
Capacitive Proximity Switch Capacitive sensing is a noncontact technology suitable for detecting metals, nonmetals, solids, and liquids, although it is best suited for nonmetallic targets because of its characteristics and cost relative to inductive proximity sensors. In most applications with metallic targets, inductive sensing is preferred because it is both a reliable and a more affordable technology. Capacitive proximity sensors are similar in size, shape, and concept to inductive proximity sensors. However, unlike inductive sensors which use induced magnetic fields to sense objects, capacitive proximity generate an electrostatic field and reacts to changes in capacitance caused when a target enters the electrostatic field. When the target is outside the electrostatic field, the oscillator is inactive. As the target approaches, a capacitive coupling develops between the target and the capacitive probe. When the capacitance reaches a specified threshold, the oscillator is activated, triggering the output circuit to switch states between ON and OFF.
Figure 2.3 Capacitive Proximity Operation The ability of the sensor to detect the target is determined by the target’s size, dielectric constant and distance from the sensor. The larger the target’s size, the stronger the capacitive coupling between the probe and the target. Materials with higher dielectric constants are easier to detect than those with lower values. The shorter the distance between target and probe, the stronger the capacitive coupling between the probe and the target.
Inductive Proximity Switch Inductive proximity sensors are designed to operate by generating an electromagnetic field and detecting the eddy current losses generated when ferrous and nonferrous metal target objects enter the field. The sensor consists of a coil on a ferrite core, an oscillator, a trigger‐signal level detector and an output circuit. As a metal object advances into the field, eddy currents are induced in the target. The result is a loss of energy and a smaller amplitude of oscillation. The detector circuit then recognizes a specific change in amplitude and generates a signal which will turn the solid‐state output “ON" or “OFF."
Figure 2.3 Principles of Operation for Inductive Proximity Sensors A metal target approaching an inductive proximity sensor (above) absorbs energy generated by the oscillator. When the target is in close range, the energy drain stops the oscillator and changes the output state.
Limit Switch A limit switch is an electromechanical device that consists of an actuator mechanically linked to a set of contacts. When an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection. Limit switches are used in a variety of applications and environments because of their ruggedness, ease of installation, and reliability of operation. They can determine the presence or absence, passing, positioning, and end of travel of an object. They were first used to define the limit of travel of an object; hence the name “Limit Switch.” The electrical advantages of limit switches are:
Suitable for switching higher power loads than other sensor technologies (5 A at 24V DC or 10A at 120V AC typical vs. less than 1 A for proximities or photo-electrics) Immunity to electrical noise interference Immunity to radio frequency interference (walkie-talkies) No leakage current Minimal voltage drops Simple normally open and/or normally closed operation
III.
DATA AND RESULTS
EXERCISE NO. 1: INTRODUCTION TO SENSORS ANSWERS TO PROCEDURE QUESTIONS: 1. TABLE. 1-1 Visible Red and Infrared Light Beams PHOTOELECTRIC SENSORS BACKGROUND SUPPRESSION PHOTOELECTRIC SWITH, MODEL 6373 POLARIZED RETROFLECTIVE PHOTOELECTRIC SWITCH, MODEL 6374
VISIBLE RED
INFRARED
3. Development view of the Reflective Block 1 – UNDERSIDE 2 – DEPOLARIZING RETROFLECTIVE SURFACE 3 - MATTE BLACK METTALIC SURFACE
4 – SHINY METTALIC SURFACE 5 – BLACK plastic SURFACE 6 – WHITE WOODEN SURFACE
6. YES 7. Yes. The red pilot light turns on when an object is placed against the sensing face of the Capacitive Proximity Switch. When the sensor detects the presence of an object, current flows through the relay coil, causing the relay contacts to switch to the activated mode, causing the red pilot light to turn on 8. When lamp L1 turns on, lamp L2 turns off. When the sensor detects the presence of an object, current flows through the relay coil. This switches the NC relay contacts to the activated mode, causing lamp L2 to turn off.
REVIEW QUESTIONS: 1.
HOW DO PHOTOELECTRIC SENSORS DETECT THE PRESENCE OF OBJECT? They use a light beam to sense the presence of objects.
2.
WHAT IS THE DIFFERENCE BETWEEN LIGHT SENSING AND DARK SENSING? Light sensing means the receiver detects the presence of the light beam, while dark sensing means the receiver detects the absence of the light beam.
3.
WHAT ARE THE THREE TYPES OF PHOTOELECTRIC SENSING MODES? Diffuse-reflective, through-beam, and retro-reflective
4.
WHAT IS MEANT BY EXCESS GAIN RATIO WHEN DESCRIBING PHOTOELECTRIC SWITCHES? The ratio of light intensity available at a given distance of a sensor to the light intensity needed to trigger the sensor
5.
WHAT IS MEANT BY HYSTERISIS WHEN DESCRIBING PROXIMITY SWITCHES? Hysteresis is the difference between the "operating point" and "releasing point".
EXERCISE NO. 2: BACKGROUND SUPPRESION PHOTOELECTRIC SWITCH 3. Table 2-1 SURFACE BLACK WOODEN SURFACE WHITE WOODEN SURFACE MATTE BLACK METTALLIC SURFACE SHINY METTALIC SURFACE DEPOLARIZING RETROREFLECTIVE
DETECTED
NOT DETECTED
4. The Background Suppression Photoelectric Switch is capable of detecting objects placed in front of a very reflective background. 5. The power indicator (green LED) turns off when the sensor output is activated.
REVIEW QUESTIONS: 1.
WHAT ARE THE BACGROUND SUPPRESSION SWITCHES DESIGN FOR? Background suppression photoelectric switches are designed for short range applications where the background behind the target is very close and very reflective.
2.
HOW DO THE BACKGROUND SUPPRESSION PHOTOELECTRIC SWITCHES IGNORE THE BACKGROUND BEHIND THE TARGET? Background suppression photoelectric switches do not ignore the background. Rather, they use sophisticated electronics to detect its presence actively
3.
NAME TWO ADVANTAGES OF BACKGROUND SUPPRESSION PHOTOELECTRIC SWITCHES. Their simplicity and their ability to detect targets placed in front of shiny surfaces
4.
EXPLAIN HOW BACKGROUND SUPPRESION PHOTOELECTRIC SWITCHES CAN DETECT OBJECTS IN FRONT AND BEHIND THE NOMINAL SENSING DISTANCE OF THE SWITCH. They use two sensing elements. One element detects reflections from in front of the nominal sensing distance, and the other detects reflections from behind the nominal sensing distance.
5.
AT WHICH DISTANCE SHOULD THE TARGET BE DISTANCED FROM THE BACKGROUND TO OBTAIN A RELIABLE OPERATION? For reliable operation, a minimum separation of 10% the maximum sensing distance is recommended between the target object and the background.
EXERCISE NO. 3: POLARIZED RETROREFLECTIVE PHOTOELECTRIC SWITCH 4. table 3-1 SURFACE BLACK WOODEN SURFACE WHITE WOODEN SURFACE MATTE BLACK METTALLIC SURFACE SHINY METTALIC SURFACE DEPOLARIZING RETROREFLECTIVE
DETECTED
NOT DETECTED
5. The Polarized Retroflective Photoelectric Switch detects only the retroreflective surface. The other surfaces of the Reflective Block do not depolarize the light beam. 6. Yes. The Polarized Retroflective Photoelectric Switch detects the presence of fingers because they are large enough to break the entire light beam emitted by the photoelectric switch. 7. No. Lamp 1 turns off when the photoelectric switch detects an object. This sensor operates in the LO mode and lamp L1 is supplied through the NO contact of the photoelectric switch. 8. YES, IT STILL CAN DETECT. 9. YES, IT STILL CAN DETECT. 10. THE SENSOR IS EXPERIENCING HYSTERISIS AND ONLY THE ORANGE LIGHT IS ON.
REVIEW QUESTIONS: 1.
FOR WHICH APPLICATIONS ARE THE RETROREFLECTIVE PHOTOELECTRIC SENSORS DESIGNED? Retro-reflective photoelectric sensors are intended primarily for use in applications where an opaque target will completely block the light beam between the sensor and the reflective surface. They can detect most objects including shiny objects.
2.
NAME TWO REASONS WHY POLARIZED RETROREFLECTIVE SENSORS OFFER A SHORTER DETECTION DISTANCE THAN STANDARD RETROREFLECTIVE SENSORS. Polarized retro-reflective sensors offer a shorter distance than standard retro-reflective sensors because they use a less efficient visible red LED, and because of the additional light losses caused by the polarizing filters.
3.
WHAT ARE THE PURPOSES OF THE FILTERS IN A POLARIZED RETROREFLECTIVE SENSOR? The filters polarize the light.
4.
NAME THE TYPE OF RETROREFLECTOR THAT PROVIDES THE HIGHEST SIGNAL RETURN. The corner cube retro-reflectors provide the highest signal return.
5.
EXPLAIN WHY RETROREFLECTIVE SENSORS ARE NOT WELL SUITED TO DETECT SMALL OBJECTS. Because the light beam must be completely blocked for the sensor to be activated.
EXERCISE NO. 4: CAPACITIVE PROXIMITY switch 5. Table 4-1 SURFACE BLACK WOODEN SURFACE WHITE WOODEN SURFACE MATTE BLACK METTALLIC SURFACE SHINY METTALIC SURFACE DEPOLARIZING RETROREFLECTIVE 6. YES 7. YES
DETECTED
NOT DETECTED
8. Table 4-2 MATERIAL PAPER PLASTIC CARDBOARD GLASS
DETECTED
NOT DETECTED
REVIEW QUESTIONS: 1.
WHAT TYPES OF MATERIAL DO CAPACITIVE PROXIMITY SWITCHES DETECT? Capacitive proximity switches detect the presence of both metallic and nonmetallic objects.
2.
WHAT ARE THE FOUR MAIN SECTIONS OF A CAPACITIVE PROXIMITY SWITCH? The four main parts of a capacitive proximity switch are the capacitive probe, oscillator, rectifier (detector circuit) and transistor (output circuit).
3.
NAME TWO PARAMETERS THAT AFFECT THE SENSING DISTANCE OF A CAPACITIVE PROXIMITY SWITCH. The sensing distance of capacitive proximity switches depends on the size of both the probe and dielectric constant of the target object.
4.
EXPLAIN WHY THE CAPACITIVE PROXIMITY SWITCHES MUST BE SPACED FROM SURROUNDING SURFACES AND/OR OTHER SENSORS. Because surrounding surfaces and/or other sensors may falsely trigger the sensor.
5.
EXPLAIN WHY THE MOST CAPACITIVE PROXIMITY SWITCHES ARE EQUIPPED WITH A SENSITIVITY ADJUSTMENT. Because they measure a dielectric gap, being able to compensate for target and application conditions is important.
EXERCISE NO. 5: INDUCTIVE PROXIMITY SWITCH 4. Table 5-1 SURFACE BLACK WOODEN SURFACE WHITE WOODEN SURFACE MATTE BLACK METTALLIC SURFACE SHINY METTALIC SURFACE DEPOLARIZING RETROREFLECTIVE
DETECTED
NOT DETECTED
5. NO 6. YES 7. 0125 in.
REVIEW QUESTIONS: 1.
WHAT TYPE OF MATERIAL DO INDUCTIVE PROXIMITY SWITCHES DETECT? Inductive proximity switches detect the presence of metallic objects only.
2.
WHAT ARE THE FOUR MAIN PARTS OF AN INDUCTIVE PROXIMITY SWITCH? The four main parts of an inductive proximity switch are the wire coil, oscillator, rectifier (detector circuit) and transistor (output circuit)
3.
WHAT ARE THE MAXIMUM SENSING DISTANCE OF AN INDUCTIVE PROXIMITY SWITCH TO BE REALATIVELY SHORT? Because the magnetic field associated with the induced eddy currents is quite small, the maximum sensing distance of an inductive proximity switch is also quite small.
4.
EXPLAIN WHY INDUCTIVE PROXIMITY SWITCHES MUST BE SPACED FROM SURROUNDING METALLIC SURFACES AND/OR OTHER SENSORS. Because surrounding metallic surfaces and/or other sensors may falsely trigger the sensor.
5.
NAME TWO PARAMETERS THAT AFFECT THE SENSING DISTANCE OF AN INDUCTIVE PROXIMITY SWITCH. The sensing distance of an inductive proximity switch depends on the size of the coil and the target composition.
EXERCISE NO. 6: LIMIT SWITCH 1. NORMALLY OPEN, NORMALLY CLOSE AND ANY COMBINATION OF THE TWO. 2. ROTARY LEVER ACTUATED. 3. RED LIGHT IS OFF. 6. WHEN THE SWITCH IS PRESS, THE RED LIGHT TURNS ON. 7. YES 8. YES
REVIEW QUESTIONS: 1.
WHAT PRECAUTION MUST BE TAKEN TO PREVENT ARCING OR WELDING OF THE CONTRACTS OF A LIMIT SWITCH? Do not connect a Single Limit Switch to two power supplies that are different in polarity or type. Do not design a circuit where a difference of voltage exists between contacts, otherwise contact welding may result?
2.
NAME THE TWO TYPES OF LIMIT SWITCHES. Inductive, capacitive, photoelectric, and ultrasonic.
3.
WHAT SHOULD BE DONE IF THE LOAD CURRENT EXCEEDS THE CONTACT RATING OF THE LIMIT SWITCH? Switch off or limit certain electrical currents.
4.
WHAT IS THE MAIN DIFFERENCE BETWEEN A LIMIT SWITCH AND A PHOTOELECTRIC SWITCH? Proximity switches open or close an electrical circuit when they make contact with or come within a certain distance of an object. Limit switches are most commonly used in manufacturing equipment, robotics, and security systems.
5.
IV.
ON WHICH PRINCIPLE DOES THE ROTARY LEVER-ACTUATED LIMIT SWITCH WORK? A limit switch is a switch that is actuated by a moving part of some machine. Its operation is much like any other switch in that there are contacts that move when a plunger or lever on the outside of the switch is pushed. Internally there is an over center spring mechanism that snaps the switch open or shut in response to a gradual motion of the plunger or lever.
DISCUSSION/CONCLUSION History has shown that advancements in materials science and engineering have been important drivers in the development of sensor technologies. For instance, the temperature sensitivity of electrical resistance in a variety of materials was noted in the early 1800s and was applied by Wilhelm von Siemens in 1860 to develop a temperature sensor based on a copper resistor. The high resonance stability of single-crystal quartz, as well as its piezoelectric properties, have made possible an extraordinarily wide range of high performance, affordable sensors that have played an important role in everyday life and national defense. More recently, a new era in sensor technology was ushered in by the development of large-scale silicon processing, permitting the exploitation of silicon to create new methods for transducing physical phenomena into electrical output that can be readily processed by a computer. Ongoing developments in materials technology will permit better control of material properties and behavior, thereby offering possibilities for new sensors with advanced features, such as greater fidelity, lower cost, and increased reliability. A sensor is a device that produces a measurable response to a change in a physical condition, such as temperature or thermal conductivity, or to a change in chemical concentration. Sensors are particularly useful for making in-situ measurements such as in industrial process control. Sensors are an important part to any measurement and automation application. The sensor is responsible for converting some type of physical phenomenon into a quantity measurable by a data acquisition (DAQ) system.
V.
REFERENCE
http://www.engineershandbook.com/Components/sensors.htm http://www.automation.com/library/articles-white-papers/sensors-sensingtechnologies/background-suppression-with-photoelectric-sensors http://www.ab.com/en/epub/catalogs/12772/6543185/12041221/12041231/Capac itive-Proximity-Sensing.html https://www.labvolt.com/downloads/cwa8036_40.pdf
