Robotic Systems ECE 401RB Fall 2007

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1 Robotic Systems ECE 401RB Fall 2007 Lecture 3: Sensors, Part 2 I. Touch Sensors The following notes are from: Chapter 6, James L. Fuller, Robotics: Introduction, Programming, and Projects, Second Edition, Prentice Hall, Chapter 29, G. McComb and M. Predko, Robot Builder s Bonanza, Third Edition, McGraw-Hill, Touch sensing is also called sensing How is touch sensing useful in the picking up and examining parts? Recognizing parts Determining part positions Determining part orientation Controlling force that is applied. Why does force need to be controlled? To avoid damaging parts. To avoid over-tightening and stripping threads. Lecture 3, Page 1 of 20

2 The sense of human touch is greatly underrated. Touch is many times more important than vision. Such as in screwing together a nut and a bolt. Teleoperators were created to handle dangerous materials. - But designers soon found this was difficult and exasperating without a sense of touch. Human touch involves a combination of sensing force and temperature. Touch can be sensed in three basic ways. Forces on joints as they seek to rotate to put pressure on an object. Forces on the objects themselves. Slippage between the robot and an external body. This is more a way of determining when force is too weak. Forces on joints Can be measured directly with strain gauges. Can be measured indirectly by noting the change in current of the electric joint motors. Or in air pressure of a pneumatic control, or in fluid pressure for a hydraulic control. Forces on objects themselves Simplest is determining contact versus no contact. A microswitch can be used. - For example, on the end of a robotic finger. A long whisker can be used to extend the reach of the microswitch. Bare wires held apart by a spring. - Sometimes called a membrane sensor. - This is common inside touchpads that must be pushed with pressure. Lecture 3, Page 2 of 20

3 Contact-sensitive touchpads (from wikipedia). - Like with touchpads on laptops, switches, etc. - Commonly operate by sensing the capacitance of a finger, or the capacitance between sensors. - Capacitive sensors are laid out along the horizontal and vertical axis of the touchpad. The location of the finger is determined from the pattern of capacitance from these sensors. Problem with switches bouncing Contacts do not immediately open or close. They may bounce back and forth until they reach their new state. The following example circuit could be used to eliminate this in hardware. - The forms a flip-flop that only changes state when the switch definitely changes state. - Both Schmidt triggers have to be properly biased for the output to change. Lecture 3, Page 3 of 20

4 Or one could write software that ignores changes in state until a time delay ends after the first change of state is detected. - A good rule of thumb is to wait 20 msec. Knowing the level of applied force may be necessary. Spring-loaded potentiometers - Use a spring to hold a sensor rod fully extended. - Upon contact, the rod is pushed back a distance proportional to the force. - This causes a change in the resistance of the potentiometer. Pressure-sensitive paint sensor - Using paint made from mixing piezo-resistant semiconductor powders with an organic material between to metal electrodes. - Pressure changes the resistance in proportion to the pressure. Lecture 3, Page 4 of 20

5 Slippage between the robot and an external body Installing a roller on the side of a gripper. Slippage causes the roller to rotate. Some resistance is needed to keep the roller from rotating too easily. Single versus multiple points of contact Why might it be helpful to be able to sense contact with an object at multiple points of contact? To determine size, shape, texture, weight. Human skin can sense contact at many, many points. - Allows us to even sense texture. Texture is very difficult to determine and would require many sensors. Multi-point contact is accomplished through a matrix of sensors. Matrix of microswitches - The matrix for this will be large and not suitable to be mounted on a gripper. Matrix of strain gauges - Can measure forces on various parts of a gripper as it grasps a part. Matrix of crossing bare wires. Lecture 3, Page 5 of 20

6 Matrix of conductive foam or rubber. - A sheet situated between sets of horizontal and vertical wires. - Pressure on the foam lowers the resistance to the nearest crossing of horizontal and vertical wires. - A rubber or foam sheet could also be placed top of a printed circuit board that has many contacts. Lecture 3, Page 6 of 20

7 Artificial Skin This is a very dense touch matrix sensor. The goal is to sense shape, hardness, texture, smoothness, Artificial skin may also have some flexibility. This would allow a kind of three dimensional feeling. This is still very much in research labs and not readily available. Lecture 3, Page 7 of 20

8 Types of Detection Single-point detection Allows one to detect whether something is present or not. Also helps with collisions with obstacles. - The robot must also have a way of responding after that collision. Simple edge detection Can sense the length of an object in one dimension. Multi-point shape detection Can determine the shape of one surface of an object. Might also be able to detect size and location of holes. Surface detection Hardness, pliability, smoothness, roughness, patterns in the texture. Interpretation of touch information is very similar to interpretation of vision information. II. Range and Proximity Sensors Ranging and Proximity Proximity objects in the immediate vicinity of the robot. Proximity detectors can be contact or noncontact devices. - Contact devices touch objects to know where they are. Ranging farther away. Human sensing How do humans judge the distance to objects? Vision with two eyes to sense 3-D. Hearing Maybe smell. Temperature. Lecture 3, Page 8 of 20

9 Robots cannot use these senses very effectively for distance sensing. Vision would be hard because it would require two cameras for 3-D vision. Plus a way to distinguish objects from the background. Robotic distance sensing Includes laser, radar, sonar, vision, infrared, temperature, and magnetic detectors. Magnetic detectors Respond to the presence of magnetic materials like iron, cobalt, some ceramics, and some metals. - Only over short distances. Can work in conjunction with a relay that opens or closes in the presence or absence of the material. Inductive metal detectors Tuned radio circuits are used. Will change frequency of operation and current flow in the presence of certain materials. - Also only short range. Metal detectors are an example. Light ranging detectors Simple photoelectric eye - Something interrupts the beam between a light-beam source and beam detector. - Then the presence of an object can be known. Photoelectric reflection - Light bounces off different colors in different ways. - Can be used to detect differences in colors or differences between objects. Sonar range detectors Developed during World War II. - Only recently used in consumer products. - Fish finders use underwater sound waves to locate the bottom, fish, and other objects. Sony has a device to measure distance through the air using sound waves. - For distances up to 100 feet. Lecture 3, Page 9 of 20

10 Polaroid developed an ultrasonic range finder for robots. - Distances from 0.9 to 35 feet. - Overcame problem of particular frequencies not reflecting from some surfaces. - Used multiple frequencies 49.7, 53, 57, and 60 khz. Ultrasonic sound (above human hearing frequencies) is attenuated very rapidly in the air. - So they can only be relatively short range. Operates by measuring the length of time between emitting a signal and receipt of its echo. - If you know the speed of sound in that medium, you can calculate distance. - Some materials reflect sound better than others. - The angle of incidence of the sound wave upon an object may prevent the echo from returning to the detector. Some sonic proximity detectors can just listen for sounds. - Moving objects give off ultrasound. - Motion detectors and burglar alarms can use this. Capacitance detectors The capacitance of an electrical circuit changes as the dielectric constant of the circuit changes. Devices with a capacitance detector can sense a hand placed next to them. - As already discussed for mouse pads, touch switches (lamps, light switches, etc.). Can also sense nonmagnetic and nonconducting materials. - Like to find studs in a wall. Capacitance detectors are only good for a short range generally, 1 inch or less. Radar Radio detection and ranging. Developed during World War II. Can be used to detect objects hundreds of miles away. Active radar - Sends out radio waves to measure the length of time it takes for an echo to return. Inactive radar - Records existing radio waves. - Like for listening to stars. Lecture 3, Page 10 of 20

11 III. Temperature Sensors The following notes are from: Chapter 6, James L. Fuller, Robotics: Introduction, Programming, and Projects, Second Edition, Prentice Hall, Chapter 3, George A. Bekey, Autonomous Robots: From Biological Inspiration to Implementation and Control, The MIT Press, Uses Detection of very hot or very cold objects. Monitoring temperatures inside a robot. Detection of the presence of humans. Primary sensors Thermocouples Thermistors Bimetallic strips Thermocouples Made from two dissimilar materials. One end is welded together and the rest is open. A voltmeter is connected to the end of the thermocouple. A voltage proportional to the temperature of the tip of the thermocouple is measured. Lecture 3, Page 11 of 20

12 Thermistors Made from a material whose electrical resistance changes at various temperatures. Usually made from various metal oxide semiconductor materials. Bimetallic strips Used in many traditional household thermostats. Consist of two strips of metal fused together. The two metals expand and contract at different rates with changes in temperature. Causes the strips to bend one way or the other. Can be configured so the metal will touch electrical contacts if the temperature has become too low or too high. IV. Navigation Industrial robots have not needed navigation capabilities. Since they did not move! Robots with navigation capabilities can follow lines or guide themselves. Line following robots A line of some kind is placed on the floor. Hopefully with high contrast to the color of the rest of the floor. Then at least two sensors are used. Differences might exist in the amount of light reflected back into the sensors. - Which can tell the robot if it is centered, too far to the right, or too far to the left. - And can tell the presence of curves or intersections. Lecture 3, Page 12 of 20

13 What are keys to making such a system work properly and efficiently? Proper spacing of sensors in comparison to the width of the line. Having the right number of sensors. Responding properly (not too quickly or too slowly) to make adjustments. The robot may have its own light source. To heighten the contrast to be able to see better. Instead of a visible line, a magnetic cable could be used. Buried below the surface. Sensors detect their orientation (centered, left, or right) from the cable. Would also need to take into account noise and inaccuracies in the magnetic measurements. Automated Guided Vehicles (AGV s) Could be taught to follow lines in environments were lines can be installed. - Like in a warehouse or factory. Or follow guides that are already normally present. - Line lines on streets. - Or walls or shelving in indoor environments. Lecture 3, Page 13 of 20

14 Beacon followers Robots could sense ultrasonic beacons. Sent out by devices that the robot is supposed to go to. Or sent out by landmarks for the robot to be aware of. It would be complicated if more than one ultrasonic beacon could be heard by a robot. - Maybe the beacons could cycle on and off at different schedules. - Or maybe have an encoded format. - Like different on and off patterns like streams of bits. With one sensor, the robot could detect changes in strength of the signal as it moves. If there were two sensors, like two human ears, then what could the robot do? Sense differences in strength between the two ears. To give two dimensional orientation. But these sensors must have high enough precision to show differences between each other. Would even more sensors be useful? Navigation using predetermined routes. Robots could direct themselves. What capability that we have already talked about would be very important for this to be successful? Internal sensors to sense that the robot has gone the distance that was expected. Lecture 3, Page 14 of 20

15 May also wish to have inertial guidance systems to make sure the robot stays oriented in the right direction. - Even have an internal compass or gyroscope. And also have sensors to make sure the robot has not fallen over. Outdoor, long distance navigation systems. Like with robotic automobiles. Could use a combination of maps and GPS. And collision avoidance sensors. Navigation by touch Simple applications could use simple approaches to navigation. If navigating a maze, the robot can just follow the walls. The Roomba robotic vacuum moves in circles, follows walls when it hits them, and reacts to obstacles. V. Hearing (Audition) Provided by microphones. Transducers convert acoustic energy to electrical signals. In general, robotic sensors are not as effective as animal hearing. Animals are highly sensitive to stereo, time, and frequency. Primarily would be used for detection of alarm signals. But increasingly one finds robots that can respond to voice commands. Good for helping physically disabled people. Will have a small vocabulary. If designed to be used by many people. Or can be trained to understand the voices of particular people. Speech recognition software and extra processing power might be required. Lecture 3, Page 15 of 20

16 VI. Smell (Olfaction) This is essentially the detection of certain chemicals in the air. Can be quite sophisticated and able to detect chemicals that animals cannot detect. But will generally not be nearly as versatile or sensitive as a animal s, like a dog s, nose. So, olfaction sensors are usually built for specialized applications. Examples VII. Taste Nitrates and explosives Smoke Car exhaust emissions Military sniffers for unauthorized people. Natural gas leaks Freon leaks Robots do not usually need this kind of capability. Sensors can certainly exist for detecting chemicals within fluids or solids. VIII. Other Sensors But not along typical taste characterizations of sour, sweet, or bitter. Radiation sensors For detection of X rays and other forms of radiation. Humans cannot detect these forms of radiation. Although some animals do have that capability. Particularly useful in power plants and radiation contaminated areas. Vibration Can detect vibrations a certain frequencies that can cause harm to humans and animals. Or detect vibrations that might signal impending harm to a bridge or building. And more generally for earthquakes. Lecture 3, Page 16 of 20

17 Chemical concentration gradients Follow chemical trails that might get stronger. To try to follow back to the source. IX. Multiplicity of Sensors Obviously, robots can use a wide variety of sensors to assist in a particular application. Sensors with higher precision have significantly higher cost. How does one determine that level of precision that is needed for a particular sensor for a particular application? Consider the impact of failure. Other ideas Other ideas Other ideas Other ideas In some cases, two sensors of lower precision would be preferred over one sensor of high precision. Why? Lower cost. Two sensors give more than one perspective on an environment. Lecture 3, Page 17 of 20

18 And it is always important to remember that it is the of the sensor information that is most difficult, not just the collection of that sensor information. X. Electric Noise and Robot Sensor Information Noise can cause many unexpected problems for the sensors in a robot. Industrial environments have a high level of electrical noise. Starting and stopping of electric motors causes noise. Generates pulses that last several milliseconds. Can have amplitudes of thousands of volts for larger motors. This can be seen in momentary blinking lights, radio buzz, or television static. But impulse noise like this is much more of a problem with a robotic controller. - Not just if it might damage a component. - But also if the controller responds by thinking this is a valid input. - Then it might take action. Ways to reduce electrical noise problems Separate the power supply for the motors from the power supply for the controller and sensors. Also route the wiring separately. Or use shielded cables. - Which have grounded shields surrounding the wires. Be aware of long distance wiring The farther apart devices are physically, the more the wiring acts as an antenna to pick up noise. - This is worst with point-to-point wiring, which uses individual wires from on point on the circuit to another point on the circuit. - Noise is decreased by using twisted pairs of wires. - And noise is lowest if using shielded wires. Distance also causes voltages to decrease along the length of the wire. - Due to the voltage drop along the wire from the resistance. - And voltages may not be high enough at the end of the wire for proper detection. Lecture 3, Page 18 of 20

19 - For example, if 5 volts were expected. - So, boosting the voltage at the sending end may be required. - Also, it might be useful to convert 0 and 5 volt signals to + and signals. - Like RS-232 which uses +2.5 volts and 2.5 volts. Differential signals can also be used over long distances. - The signal is sent over two wires. - And it is the difference in voltage between the two wires that is the received information. Far-away communication More general communication approaches should be used for longer distances between objects. Wired approaches - Modems - Sending tones in coded messages over telephone lines. - Fiber optics - Not susceptible at all to electrical noise. Wireless approaches - Bluetooth - Wireless local area networks - Many other approaches (see our CSEE wireless courses) Error detection and correction If errors in control signals could cause serious problems, use methods to identify errors in those messages. Send parity check bits that will detect if bit errors have occurred to a signal. - How does parity work? Parity Even parity make sure there are an even number of ones. Will detect if 1, 3, 5, etc. bits are in error. Lecture 3, Page 19 of 20

20 Or send duplicate messages. - If both copies of the messages do not agree, then tell the source to retransmit. - Or send three messages and use the two that agree. - Or use other approaches. XI. Summary of sensors Types of sensors Internal sensors Primarily testing movement of motors and wheels. And the movement of the whole robot in comparison with intended goals. External sensors Vision Touch Range and Proximity Navigation Hearing Smell Radiation Vibration Chemical Highly customized to the application And the budget Lecture 3, Page 20 of 20

Intelligent Robotics Sensors and Actuators

Intelligent Robotics Sensors and Actuators Luís Paulo Reis (University of Porto) Nuno Lau (University of Aveiro) The Perception Problem Do we need perception? Complexity Uncertainty Dynamic World Detection/Correction