3 Sensing and Sensors.ppt
- 2. Introduction • Industrial robot requires sensory feedback to: 1. Locate randomly placed object; 2. Allow for variations in shape of objects; 3. Protect against dangerous and unexpected situations. Especially if the robot must work close to humans: 4. Allow “intelligent” recovery form error conditions; 5. Perform quality control. • The main objective of incorporating sensors in robotic system is to enable robots to work in nonstructural and random environments. • Sensors will make robots more intelligent. But the associated robotic software must have the ability to receive data from the sensors and to process the necessary real time information and commands needed for the decision making. 2 ROBOTICS
- 3. What is Sensing ? • Collect information about the world • Sensor - an electrical/mechanical/chemical device that maps an environmental attribute to a quantitative measurement • Each sensor is based on a transduction principle - conversion of energy from one form to another 3 ROBOTICS
- 4. Transduction to electronics • Thermistor: temperature-to-resistance • Electrochemical: chemistry-to-voltage • Photocurrent: light intensity-to-current • Pyroelectric: thermal radiation-to-voltage • Humidity: humidity-to-capacitance • Length (LVDT: Linear variable differential transformers) : position-to-inductance • Microphone: sound pressure-to-<anything> 4 ROBOTICS
- 5. Human sensing and organs • Vision: eyes (optics, light) • Hearing: ears (acoustics, sound) • Touch: skin (mechanics, heat) • Odor: nose (vapor-phase chemistry) • Taste: tongue (liquid-phase chemistry) 5 ROBOTICS
- 6. Extended ranges and modalities • Vision outside the RGB spectrum – Infrared Camera, see at night • Active vision – Radar and optical (laser) range measurement • Hearing outside the 20 Hz – 20 kHz range – Ultrasonic range measurement • Chemical analysis beyond taste and smell • Radiation: a, b, g-rays, neutrons, etc 6 ROBOTICS
- 7. Sensors Used in Robot 7 ROBOTICS
- 8. Solar Cell Digital Infrared Ranging Compass Touch Switch Pressure Switch Limit Switch Magnetic Reed Switch Magnetic Sensor Miniature Polaroid Sensor Polaroid Sensor Board Piezo Ultrasonic Transducers Pyroelectric Detector Thyristor Gas Sensor Gieger-Muller Radiation Sensor Piezo Bend Sensor Resistive Bend Sensors Mechanical Tilt Sensors Pendulum Resistive Tilt Sensors CDS Cell Resistive Light Sensor Hall Effect Magnetic Field Sensors Compass IRDA Transceiver IR Amplifier Sensor IR Modulator Receiver Lite-On IR Remote Receiver Radio Shack Remote Receiver IR Sensor w/lens Gyro Accelerometer IR Reflection Sensor IR Pin Diode UV Detector Metal Detector
- 9. Sensors used in robot navigation • Resistive sensors – bend sensors, potentiometer, resistive photocells, ... • Tactile sensors – contact switch, bumpers… • Infrared sensors – Reflective, proximity, distance sensors… • Ultrasonic Distance Sensor • Inertial Sensors (measure the second derivatives of position) – Accelerometer, Gyroscopes, • Orientation Sensors – Compass, Inclinometer • Laser range sensors • Vision • Global Positioning System 9 ROBOTICS
- 10. Classification of Sensors • Internal state (proprioception) v.s. external state (exteroceptive) – feedback of robot internal parameters, e.g. battery level, wheel position, joint angle, etc, – observation of environments, objects • Active v.s. non-active – emitting energy into the environment, e.g., radar, sonar – passively receive energy to make observation, e.g., camera • Contact v.s. non-contact • Visual v.s. non-visual – vision-based sensing, image processing, video camera 10 ROBOTICS
- 11. • In general, robotic sensors can be divided into two classes: i. Internal state sensors - device being used to measure the position, velocity and acceleration of the robot joint and/or end-effector. These devices are potentiometer, tachometers, synchros, resolvers, differential transformers, optical interrupters, optical encoders and accelerometer. ii. External state sensors – device being used to monitor the relationship between the robot kinematics and/or dynamics with its task, surrounding, or the object being manipulated. Robotic Sensor Classification 11 ROBOTICS
- 12. Sensor Selection/Sensing Taxonomy • There are many different types of robot sensors available and there are many different parameter measured by these sensors. • The application process, should be carried out in a top down manner, starting with task requirements, and going through several levels of analysis, eventually leading to the selection of a specific device. • A taxonomy for sensing to aid this process consists of five levels of refinement leading to sensor selection: 1. Specification of task requirements :eg localization, slippage detection, size confirmation, inspection, defect testing. 2. Choice of modality :eg,vision, force, tactile 3. Specification on sensor attributes :eg,output, complexity, discrete or continuous variable, imaging or non-imaging, local or global 4. Specification of operational parameters :eg size, accuracy, cost 5. Selection of mechanism :eg switching devices, inductive sensors, CCD vision imaging 12 ROBOTICS
- 13. •Some tasks requirements features: •Insertion Monitoring •Assembly Verification •Detection of Reject Parts •Recognition of Part Types •Assembly Test Operations •Check Gripper/Tool Operation •Location & Orientation of Parts •Workspace Intrusion Detection •Check Correct Manipulation of Parts •Analysis of Spatial Relations Between Parts 13 ROBOTICS
- 14. Some typical sensor operational data: • Ultrasonics • Resistive Effects • Capacitive Efects • Piezo-Electric Effects • Visible Light Imaging • Photo-Electric & Infrared • Mechanical Switching • Inductive Effects • Thermal Effects • Hall Effect Primary physical mechanisms employed in sensors: Cost Range Accuracy Repeatability Power Requirements Output Signal Specification Processing Reuirements Sensitivity Reliability Weight Seze 14 ROBOTICS
- 15. SENSORS FOR INDUSTRIAL ROBOTS Proximity and Range Sensors Tactile Sensors Vision Sensors Miscellaneous Sensors 15 ROBOTICS
- 17. I • It is a technique of detecting the presence or absence of an object with electronic noncontact sensors. • Typical application of proximity sensors includes: ש Object detection ש Collision avoidance ש Object verification & counting • Commonly available proximity sensors are: 1. Photoelectric/optical sensors 2. Inductive proximity sensors 3. Capacitive proximity sensors 4. Ultrasonic proximity sensors 17 ROBOTICS
- 18. Bend Sensors • Resistance = 10k to 35k • As the strip is bent, resistance increases Potentiometers • Can be used as position sensors for sliding mechanisms or rotating shafts • Easy to find, easy to mount Light Sensor (Photocell) • Good for detecting direction/presence of light • Non-linear resistance • Slow response to light changes Resistive Sensors Resistive Bend Sensor Photocell Potentiometer R is small when brightly illuminated 18
- 19. Sensor Measure bend of a joint Wall Following/Collision Detection Weight Sensor Sensors Sensor Applications 19 ROBOTICS
- 20. Inputs for Resistive Sensors Voltage divider: You have two resisters, one is fixed and the other varies, as well as a constant voltage V micro R1 R2 Vsense Comparator: If voltage at + is greater than at -, digital high out + - Binary Threshold V V R R R Vsense 2 1 2 A/D converter Digital I/O 20 ROBOTICS
- 21. Infrared Sensors • Intensity based infrared – Reflective sensors – Easy to implement – susceptible to ambient light • Modulated Infrared – Proximity sensors – Requires modulated IR signal – Insensitive to ambient light • Infrared Ranging – Distance sensors – Short range distance measurement – Impervious to ambient light, color and reflectivity of object 21 ROBOTICS
- 22. Intensity Based Infrared • Easy to implement (few components) • Works very well in controlled environments • Sensitive to ambient light time voltage time voltage Increase in ambient light raises DC bias Break-Beam sensor Reflective Sensor 22 ROBOTICS
- 23. IR Reflective Sensors • Reflective Sensor: – Emitter IR LED + detector photodiode/phototransistor – Phototransistor: the more light reaching the phototransistor, the more current passes through it – A beam of light is reflected off a surface and into a detector – Light usually in infrared spectrum, IR light is invisible • Applications: – Object detection, – Line following, Wall tracking – Optical encoder (Break-Beam sensor) • Drawbacks: – Susceptible to ambient lighting • Provide sheath to insulate the device from outside lighting – Susceptible to reflectivity of objects – Susceptible to the distance between sensor and the object 23
- 24. Modulated Infrared • Modulation and Demodulation – Flashing a light source at a particular frequency – Demodulator is tuned to the specific frequency of light flashes. (32kHz~45kHz) – Flashes of light can be detected even if they are very week – Less susceptible to ambient lighting and reflectivity of objects – Used in most IR remote control units, proximity sensors Negative true logic: Detect = 0v No detect = 5v
- 25. IR Proximity Sensors • Proximity Sensors: – Requires a modulated IR LED, a detector module with built-in modulation decoder – Current through the IR LED should be limited: adding a series resistor in LED driver circuit – Detection range: varies with different objects (shiny white card vs. dull black object) – Insensitive to ambient light • Applications: – Rough distance measurement – Obstacle avoidance – Wall following, line following limiter demodulator bandpass filter amplifier comparator integrator 25 ROBOTICS
- 26. IR Distance Sensors • Basic principle of operation: – IR emitter + focusing lens + position-sensitive detector Location of the spot on the detector corresponds to the distance to the target surface, Optics to covert horizontal distance to vertical distance Modulated IR light
- 27. IR Distance Sensors • Sharp GP2D02 IR Ranger – Distance range: 10cm (4") ~ 80cm (30"). – Moderately reliable for distance measurement – Immune to ambient light – Impervious to color and reflectivity of object – Applications: distance measurement, wall following, … 27 ROBOTICS
- 29. Range Finder • Time of Flight • The measured pulses typically come form ultrasonic, RF and optical energy sources. – D = v * t – D = round-trip distance – v = speed of wave propagation – t = elapsed time • Sound = 0.3 meters/msec • RF/light = 0.3 meters / ns (Very difficult to measure short distances 1-100 meters) 29 ROBOTICS
- 30. Ultrasonic Sensors • Basic principle of operation: – Emit a quick burst of ultrasound (50kHz), (human hearing: 20Hz to 20kHz) – Measure the elapsed time until the receiver indicates that an echo is detected. – Determine how far away the nearest object is from the sensor D = v * t D = round-trip distance v = speed of propagation(340 m/s) t = elapsed time Bat, dolphin, … 30 ROBOTICS
- 31. Ultrasonic Sensors • Ranging is accurate but bearing has a 30 degree uncertainty. The object can be located anywhere in the arc. • Typical ranges are of the order of several centimeters to 30 meters. • Another problem is the propagation time. The ultrasonic signal will take 200 msec to travel 60 meters. ( 30 meters roundtrip @ 340 m/s )
- 32. Ultrasonic Sensors • Polaroid ultrasonic ranging system – It was developed for auto-focus of cameras. – Range: 6 inches to 35 feet Ultrasonic transducer Electronic board Transducer Ringing: transmitter + receiver @ 50 KHz Residual vibrations or ringing may be interpreted as the echo signal Blanking signal to block any return signals for the first 2.38ms after transmission http://www.acroname.com/robotics/info/articles/sonar/sonar.html
- 33. Operation with Polaroid Ultrasonic • The Electronic board supplied has the following I/0 – INIT : trigger the sensor, ( 16 pulses are transmitted ) – BLANKING : goes high to avoid detection of own signal – ECHO : echo was detected. – BINH : goes high to end the blanking (reduce blanking time < 2.38 ms) – BLNK : to be generated if multiple echo is required t
- 34. Ultrasonic Sensors • Applications: – Distance Measurement – Mapping: Rotating proximity scans (maps the proximity of objects surrounding the robot) chair Robot chair Doorway Scan moving from left to right Length of Echo Scanning at an angle of 15º apart can achieve best results
- 36. Laser Ranger Finder • Range 2-500 meters • Resolution : 10 mm • Field of view : 100 - 180 degrees • Angular resolution : 0.25 degrees • Scan time : 13 - 40 msec. • These lasers are more immune to Dust and Fog http://www.sick.de/de/products/categories/safety/ 36 ROBOTICS
- 38. • Tactile sensing includes any form of sensing which requires physical touching between the sensor and the object to be sense. • The need for touch or tactile sensors occurs in many robotic applications, from picking oranges to loading machines. Probably the most important application currently is the general problem of locating, identifying, and organizing parts that need to be assembled. • Tactile sensor system includes the capability to detect such things as: 1. Presence 2. Part shape, location, orientation, contour examination 3. Contact are pressure and pressure distribution 4. Force magnitude, location, and direction 5. Surface inspection : texture monitoring, joint checking, damage detection 6. Object classification : recognition, discrimination 7. Grasping : verification, error compensation (slip, position ,orientation) 8. Assembly monitoring 38 ROBOTICS
- 39. • The major components of a tactile/touch sensor system are: 1. A touch surface 2. A transduction medium, which convert local forces or moments into electrical signals. 3. Structure 4. Control/interface 39 ROBOTICS
- 40. • It is the transduction method in tactile sensor design which has received the most attention. It is concerned with the change in resistance of a conductive material under applied pressure. • This technique involves measuring the resistance either through or across the thickness of a conductive elastomer. Most elastomers are made from carbon- or silicon-doped rubber. Resistive Resistive Tactile Element – Resistance Measured Through The rubber 40 ROBOTICS
- 41. • Advantages: 1. Wide dynamic range 2. Durability 3. Good overload tolerance 4. Compatibility with integrated circuitry, particularly VLSI. • Disadvantages: 1. Hysteresis in some designs. 2. Elastromer needs to be optimized for both mechanical and electrical properties. 3. Limited spatial resolution compared with vision sensors. 4. Larger numbers of wires may have to be brought away from the sensor. 5. Monotonic response but often not linear. 41 Resistive Tactile Element – Resistance Measured Across the rubber
- 42. Piezoelectric & Pyroelectric Effects • Piezoelectric effect is the generation of a voltage across a sensing element when pressure applied to it. The voltage generated is proportionally related to the applied pressure. No external voltage is required, and a continuous analogue output is available from such sensor. • A pyroelectric effect is the generation of a voltage when the sensing element is heated or cooled. • Polymeric materials with piezoelectric and pyroelectric properties are appropriate for use with sensors. Piezoelectric/Pyroelectric Effects Tactile element 42 ROBOTICS
- 43. • Advantages: 1. Wide dynamic range 2. Durability 3. Good mechanical properties of piezoelectric from pyroelectric materials 4. Temperature as well as force sensing capabilities • Disadvantages: 1. Difficult of separating piezoelectric from pyroelectric effects 2. Inherently dynamic - output decay to zero for constant load 3. Difficult of scanning elements 4. Good solution are complex 43 ROBOTICS
- 44. CAPACITIVE TECHNIQUE • Tactile sensors within this category are concerned with measuring capacitance, which made to vary under applied load. • The capacitance of a parallel plate capacitor depends upon the separation of the plates and their area, so that a sensor using an elastomeric separator between the plates provides compliance such that the capacitance will vary according to applied load.
- 46. • Advantages: 1. Wide dynamic range 2. Linear response 3. Robust • Disadvantages: 1. Susceptible to noise 2. Some dielectrics are temperature sensitive 3. Capacitance decreases with physical size ultimately limiting spatial resolution. 46 ROBOTICS
- 47. Mechanical Transduction • A Linear Potentiometer • Advantages: 1. Well known Technology 2. Good for probe application • Disadvantages: 1. Limited spatial resolution 2. Complex for array construction Mechanical Transducer A linear Potentiometer 47 ROBOTICS
- 48. Magnetic Transduction Methods • Sensors using magnetic transduction are divided into two basic categories: Groups together sensors which use mechanical movement to produce change in magnetic flux. • Advantages: 1. Wide dynamic range 2. Large displacements possible 3. Simple • Disadvantages: 1. Poor spatial resolution 2. Mechanical problems when sensing on slopes. Magnetic tactile Element 48 ROBOTICS
- 49. 2. Concerns magnetoelastic materials which show a change in magnetic field when subjected to mechanical stress. • Advantages: 1. Wide dynamic range 2. Linear response 3. Low hysteresis 4. Robust • Disadvantages: 1. Susceptible to stray field and noise. 2. A.C. circuit required Magneto resistive tactile Element 49 ROBOTICS
- 50. Optical Transduction Methods • Advantages: 1. Very high resolution 2. Compatible with vision sensing technology 3. No electrical interference problems 4. Processing electronics can be remote from sensor 5. Low cabling requirements • Disadvantages: 1. Dependence on elastomer in some designs – affects robustness 2. Some hysteresis Optical Tactile Element Pressure to light Transduction 50 ROBOTICS
- 52. • Vision is the most powerful robot sensory capabilities. Enables a robot to have a sophisticated sensing mechanism that allows it to respond to its environment in intelligent and flexible manner. Therefore machine vision is the most complex sensor type. • Robot vision may be defined as the process of extracting, characterizing, and interpreting information from images of a three-dimensional world. This process, also known as machine or computer vision may be subdivided into six principle areas. These are: 1. Sensing : the process that yields visual image 2. Preprocessing : deals with techniques such as noise reduction and enhancement of details 3. Segmentation : the process that partitions an image into objects of interest 4. Description: deals with that computation of features for example size or shape, suitable for differentiating one type of objects from another. 5. Recognition: the process that identifies these objects (for example wrench, bolt, engine block, etc.) 6. Interpretation: assigns meaning to an ensemble of recognized objects. 52
- 53. • The imaging component, the “eye” or sensor, is the first link in the vision chain. Numerous sensors may be used to observe the world. There are four type of vision sensors or imaging components: • 1. Point sensors capable of measuring light only at a single point in space. These sensor using coupled with a light source (such as LED) and used as a noncontact ‘feeler’ It also may be used to create a higher – dimensions set of vision Information by scanning across a field of view by using mechanisms such as orthogonal set of scanning mirrors IMAGING COMPONENTS 53 ROBOTICS
- 55. 55 Image scanning using a point sensor and oscillating deflecting mirrors
- 56. 2. Line Sensor • Line sensors are one-dimensional devices used to collect vision information from a real scene in the real world. • The sensor most frequently used is a “line array” of photodiodes or charger-couple-device components. • It operates in a similar manner to analog shift register, producing sequential, synchronized output of electrical signals, corresponding to the light intensity falling on an integrated light-collecting cell. Circular and cross configurations of light sensors 56 ROBOTICS
- 57. • Line array may be used to image scene. E.g. by fixing the position of a straight-line sensor and moving an object orthogonally to the orientation of the array, one may scan the entire object of interest. 57 An automated robot sorting system using a line scan camera to generate two-dimensional images.
- 58. 3. Planar Sensor • A two dimensional configuration of the line-scan concept. Two generic types of these sensors generally in use today are scanning photomultipliers and solid-state sensors. • Photomultipliers are represented by television cameras, the most common of which is the vidicon tube, which essentially an optical-to- electrical signal converter. • In addition to vidicon tubes, several types of solid-state cameras are available. Many applications require the solid-state sensors because of weight and noise factor (solid-state arrays are less noisy but more expensive). This is important when mounting a camera near or on the end- effector of a robot. 58 ROBOTICS
- 59. 4. Volume Sensor • A sensor that provide three- dimensional information. The sensor may obtain the information by using the directional laser or acoustic range finders. Schematic representation of a triangulation range finder 59 ROBOTICS
- 60. IMAGE REPRESENTATION • From the diagram below. F(x,y) is used to denote the two-dimensional image out of a television camera or other imaging device. • “x” and “y” denote the spatial coordinates (image plane) • “f” at any point (x,y) is proportional to the brightness (intensity) of the image at that point. • In form suitable for computer processing, an image function f(x,y) must be digitized both spatially and in amplitude (intensity). Digitization of the spatial coordinates (x,y) will be known as image sampling, while amplitude digitization is known as intensity or grey-level quantization. • The array of (N, M) rows and columns, where each sample is sampled uniformly, and also quantized in intensity is known as a digital image. Each element in the array is called image element, picture element (or pixel). 60 ROBOTICS
- 61. ILLUMINATION TECHNIQUES • Illumination of a scene is an important factor that often affects the complexity of vision algorithms. • A well designed lighting system illuminates a scene so that the complexity of the resulting image is minimised, while the information required for object detection and extraction is enhanced. • Arbitrary lighting of the environment is often not acceptable because it can result in low contras images, specular reflections, shadows and extraneous details. • There are 4 main illumination techniques for a robot work space : 61 ROBOTICS
- 62. ILLUMINATION TECHNIQUES 1. DIFFUSE-LIGHTING • This technique is for smooth, regular surface object. It is used where surface characteristic are important. • Example: Diffuse-lighting technique 62 ROBOTICS
- 63. ILLUMINATION TECHNIQUES 2. BACKLIGHTING • Produce black and white image. This technique suited for applications in which silhouettes of object are sufficient for recognition or other measurement. • Example: Backlighting technique 63 ROBOTICS
- 64. ILLUMINATION TECHNIQUES 3. STRUCTURED LIGHTING • Consist of projecting points, stripes, grids onto work surface. • This lighting technique has 2 important advantages: 1. It establishes a known light pattern on the work space and disturbances of this indicate the presence of an object, thus simplifying the object detection problems. 2. By analysing the way which the light pattern distorted, it is possible to gain insight into three-dimensional characteristics of the object. 64 Structured lighting technique
- 65. • The following figure illustrates the structured lighting technique using two light planes projected from different directions, but converging on a single stripe on the surface. The two light sources guarantee that the object will break the light stripe only when it is directly below the camera. • This technique is suitable for moving object. • Note: “The line scan camera sees only the line on which the two light planes converge, but two-dimension information can be accumulated as the object move past the camera” 65 3. STRUCTURED LIGHTING (cont.) (a) Top view of two light planes intersecting in a line sight (b) Object will be seen by the camera only When it interrupts both light planes
- 66. ILLUMINATION TECHNIQUES 4. DIRECTIONAL LIGHTING • This method is used to inspection of object surfaces. • Defects on the surface such as scratches, can be detected by using a highly directed light beam (such as laser beam) and measuring the amount of scatter Directional lighting technique 66 ROBOTICS
- 67. ROBOT VISION SYSTEM • There are several commercial packages that can be bought for vision processing work. A typical hardware configuration is shown below. • Based on the technique used, the robotic vision systems can be grouped into the following major types: 1. Binary vision systems 4.Structured light vision systems 2. Gray-level vision systems 5.Character recognition vision systems 3. Ad hoc special-purpose vision systems 67 Vision system hardware
- 68. • A typical system will have facilities for controlling the camera remotely and perhaps interfaces for remote lighting control. • The main problem with commercial vision packages is that they have to be general purpose in order to be applicable in many situations. This very requirement sometimes means that they are not suitable or are over complicated for a particular robot task in hand. • In industrial robot world, vision is not used in an exploratory sense but is used to confirm or measure or refine existing known data. • Whichever commercial vision system one purchases, one is likely to use it for applications such as those listed in the next section. 68 ROBOTICS
- 69. Vision Dev. Tools: Survey • Commercial products – Matrox: MIL, Inspector – Coreco Imaging: Sapera, MVTools, WiT – MVTec: Halcon – Euresys: eVision, EasyAccess – AAI: Aphlion • Free tools – Intel: Open Source Computer Vision – Microsoft: Vision SDK – XMegaWave: XMegaWave – UTHSCSA: ImageTool 69
- 70. VISION APPLICATIONS • 1. OBJECT LOCATION Used in object handling and processing: -Position -Orientation • 2. OBJECT PROPERTIES Used in inspection, identification, measurement: -Size -Area -Shape -Periphery length / area ratio -Texture -Repetition of pattern -Properties of internal features • 3. SPATIAL RELATIONS Used in measurement and task verification -Relative positions -Relative orientations -Occlusions -Alignments -Connectivity • 4. ACTION MONITORING Used in actuator control and verification: -Direct feedback -Error measurement -Action confirmation -Inspection -Collision avoidance planning. 70
- 72. MISCELLANEOUS SENSORS • There are several type of sensor that can be used to determine the position of robot joints like potentiometer, optical encoder, Linear Variable Differential Transformer (LVDT) Force & Torque Sensors. POSITION, VELOCITY& ACCELERATION SENSORS 72 ROBOTICS
- 73. Potentiometer • Potentiometer transducers can be used to measure both linear and angular displacement 73 (a) Potentiometer (b) (b) Schematic diagram of the potentiometer
- 74. Linear Variable Differential Transformer (LVDT) • LDVT is a robust and precise device which produce a voltage output proportional to the displacement of a ferrous armature for measurement of robot joints or end-effectors. It is much expensive but outperforms the potentiometer transducer. Linear Variable Differential Transformer (LVDT) 74 ROBOTICS
- 75. Force & Torque Sensors • Force transducers are often based on displacement principles. There various type force and torque transducer available commercially A force-measuring device based on a compression spring and LDVT. 75 ROBOTICS
- 76. This figure illustrate a tension load cell. It can be used to measure the force required to pick up heavy load in industry 76 ROBOTICS
- 77. Force & Torque Sensors • Force can be measured using piezoelectric principle. • Figure shows a load washer type piezoelectric force transducer. It is designed to measure axial forces. It is preloaded when manufactured and can measure both tensile and compressive forces. 77 ROBOTICS
- 78. Force & Torque Sensors • Measured using piezoelectric principle. • Figure shows a three- component dynamometer type piezoelectric force transducer that measures three orthogonal components of force. 78 ROBOTICS
- 80. Incremental Optical Encoders - direction - resolution grating light emitter light sensor decode circuitry A B A leads B • Incremental Encoder: • It generates pulses proportional to the rotation speed of the shaft. • Direction can also be indicated with a two phase encoder: 80
- 81. Absolute Optical Encoders Gray Code • Used when loss of reference is not possible. • Gray codes: only one bit changes at a time ( less uncertainty). • The information is transferred in parallel form (many wires are necessary). 000 001 011 010 110 111 101 100 000 001 010 011 100 101 110 111 Binary 81
- 82. Other Odometry Sensors • Resolver • Potentiometer = varying resistance It has two stator windings positioned at 90 degrees. The output voltage is proportional to the sine or cosine function of the rotor's angle. The rotor is made up of a third winding, winding C 82
- 83. Inertial Sensors • Gyroscopes – Measure the rate of rotation independent of the coordinate frame – Common applications: • Heading sensors, Full Inertial Navigation systems (INS) • Accelerometers – Measure accelerations with respect to an inertial frame – Common applications: • Tilt sensor in static applications, Vibration Analysis, Full INS Systems 83 ROBOTICS
- 84. Accelerometers • They measure the inertia force generated when a mass is affected by a change in velocity. • This force may change – The tension of a string – The deflection of a beam – The vibrating frequency of a mass 84 ROBOTICS
- 85. Accelerometer • Main elements of an accelerometer: 1. Mass 2. Suspension mechanism 3. Sensing element High quality accelerometers include a servo loop to improve the linearity of the sensor. kx dt dx c t d x d m F 2 2 85 ROBOTICS
- 86. Gyroscopes • These devices return a signal proportional to the rotational velocity. • There is a large variety of gyroscopes that are based on different principles 86 ROBOTICS
- 87. Global Positioning System (GPS) Space Segment http://www.cnde.iastate.edu/staff/swormley/gps/gps.html 24 satellites (+several spares) broadcast time, identity, orbital parameters (latitude, longitude, altitude) 87
- 88. Noise Issues • Real sensors are noisy • Origins: natural phenomena + less-than-ideal engineering • Consequences: limited accuracy and precision of measurements • Filtering: – software: averaging, signal processing algorithm – hardware tricky: capacitor 88 ROBOTICS
- 89. 89 • Sensors for mobile robots: theory and applications, H. R. Everett, A. K. Peters Ltd, C1995, ISBN: 1-56881- 048-2 • Handbook of Modern Sensors: Physics, Designs and Applications, 2nd edition, Jacob Fraden, AIP Press/Springer, 1996. ISBN 1-56396-538-0. Literature: 89 ROBOTICS