robotics-final.pdf

Introduction Robotics is an applied science that emerges from the utilization of knowledge in many disciplines together for analyzing and designing the robots. One of the fundamental sciences is mechanical engineering which will be useful for the design and analysis of the mechanisms that can produ...

Introduction Robotics is an applied science that emerges from the utilization of knowledge in many disciplines together for analyzing and designing the robots. One of the fundamental sciences is mechanical engineering which will be useful for the design and analysis of the mechanisms that can produce the desired motion of an industrial robot. Robotics Timeline 1922 Czech author Karel Capek wrote a story called Rossum’s Universal Robots and introduced the word “Rabota”(meaning worker) 1954 George Devol developed the first programmable Robot. 1955 Denavit and Hartenberg developed the homogenous transformation matrices 1962 Unimation was formed, first industrial Robots appeared. 1973 Cincinnati Milacron introduced the T3 model robot, which became very popular in industry. 1990 Cincinnati Milacron was acquired by ABB A robot is a reprogrammable, multifunctional manipulator designed to move materials, parts, tools, or specialized devices through variable programmed motion for the performance of a variety of tasks. A robot can also be defined as a computer controlled machine with some degrees of freedom. Degree of freedom - is the ability to move about in its environment. Types of Robots: 1. Mobile robots – robots that move freely in their environment a. We can subdivide these into indoor robots, outdoor robots, terrain robots, etc. based on the environment(s) they are programmed to handle 2. Robotic arms – stationary robots that have manipulators, usually used in construction (e.g., car manufacturing plants) a. These are usually not considered AI because they do not perform planning and often have little to no sensory input  3. Autonomous vehicles – like mobile robots, but in this case, they are a combination of vehicle and computer controller a. Autonomous cars, autonomous plane drones, autonomous helicopters, autonomous submarines, autonomous space probes b. There are different classes of autonomous vehicles based on the level of autonomy, some are only semi-autonomous 4. Soft robots – robots that use soft computing approaches (e.g., fuzzy logic, neural networks) 5. Mimicking robots – robots that learn by mimicking a. For instance, robots that learn facial gestures or those that learn to touch or walk or play with children 6. Softbots – software agents that have some degrees of freedom (the ability to move) or in some cases, software agents that can communicate over networks 7. Nanobots – theoretical at this point, but like mobile robots, they will wander in an environment to investigate or make changes a. But in this case, the environment will be microscopic worlds, e.g., the human body, inside of machines Current Uses of Robots: There are over 3.5 million robots in use in society of which, about 1 million are industrial robots – 50% in Asia, 32% in Europe, 16% in North America Factory robot uses: – Mechanical production, e.g., welding, painting – Packaging – often used in the production of packaged food, drinks, medication – Electronics – placing chips on circuit boards – Automated guided vehicles – robots that move along tracks, for instance as found in a hospital or production facility Other robot uses: – Bomb disabling – Exploration (volcanoes, underwater, other planets) – Cleaning – at home, lawn mowing, cleaning pipes in the field, etc – Fruit harvesting Robot Examples: The robot usually has a three phase sequence of operations: 1. sense (perception) 2. process (interpretation and planning) 3. action (movement of some kind) A robot typically has the following: a. sensors to sense its environment, particularly to make sure it does not hit any obstacles in its way b. goals (otherwise there is no need to have the robot) c. planning to determine how to accomplish those goals some robots are pre-programmed with the plan steps to carry out the given goals so planning is not needed  d. path planning to determine how to move about its environment using the available degrees of freedom this may be the motion of an arm to pick something up or it may be a series of movements to physically move it from location 1 to location 2. Sensor Interpretation Sensors are primarily used to – ensure the vehicle/robot is following an appropriate path (e.g., corridor, road) – and to seek out obstacles to avoid It used to be very common to equip robots with sonar or radar but not cameras because – cameras were costly – vision algorithms required too much computational power and were too slow to react in real time Today, outdoor vehicles/robots commonly use cameras and lasers (if they can be afforded) Additionally, a robot might use GPS, – so the robot needs to interpret input from multiple sensors Performing Sensor Interpretation There are many forms – Simple neural network recognition more common if we have a single source of input, e.g., camera, so that the NN can respond with “safe” or “obstacle” – Fuzzy logic controller can incorporate input from several sensors – Bayesian network and hidden Markov models for single or multiple sensors – Blackboard/KB approach post sensor input to a blackboard, let various agents work on the input to draw conclusions about the environment Since sensor interpretation needs to be real-time, we need to make sure that the approach is not overly elaborate Obstacle Avoidance What happens when an obstacle is detected by sensors? It depends on the type of robot and the situation – in a mobile robot, it can stop, re-plan, and resume – in an autonomous ground vehicle, it may slow down and change directions to avoid the obstacle (e.g., steer right or left) while making sure it does not drive off the road – notice that it does not have to re-plan because it was in motion and the avoidance allowed it to go past the obstacle or it might stop, back up, re-plan and resume – an underwater vehicle or an air-based vehicle may change depth/altitude While obstacle avoidance is a low-level process, it may impact higher level processes (e.g., goals) so planning may take place at higher levels Mission Planning As the name implies, this is largely a planning process – Given goals, how to accomplish them? this may be through rule-based planning, plan decomposition, or plans may be provided by human controllers – In many cases, the mission goal is simple: go from point A to point B so that no planning is required – For a mobile robot (not an autonomous vehicle), the goals may be more diverse reconnaissance and monitoring search (e.g., find enemy locations, find buried land mines, find trapped or injured people) go from point A to point B but stealthily monitor internal states to ensure mission is carried out Path Planning How does the vehicle/robot get from point A to point B? – Are there obstacles to avoid? Can obstacles move in the environment? – Is the terrain going to present a problem? – Are there other factors such as dealing with water current (autonomous sub), air current (autonomous aircraft), blocked trails (indoor or outdoor robot)?  Path planning is largely geometric and includes – Straight lines – Following curves – Tracing walls Additional issues are – How much of the path can be viewed ahead? – Is the robot going to generate the entire path at once, or generate portions of it until it gets to the next point in the path, or just generate on the fly? – If the robot gets stuck, can it backtrack? The robot must balance the desire for the safest path, the shortest distance path, and the path that has fewer changes of orientation – Variations of the A* algorithm (best-first search) might be used – Heuristics might be used to evaluate safety versus simplicity versus distance Once a path is generated, the robot must follow that path, but the technique will differ based on the type of robot – For an indoor robot, path planning is often one of following the floor using a camera, find the lines that make up the intersection of floor and wall, and use these as boundaries to move down – For an autonomous car, path planning is similar but follows the road instead of a floor using a camera, find the sides of the road and select a path down the middle – For an all-terrain vehicle, GPS must be used although this may not be 100% accurate Robot Anatomy It is the physical construction of the body, arm, and wrist of the machine. The body, arm, and wrist assembly is sometimes called the manipulator, and attached to the robot’s wrist is a hand or a tool called the end effector. End effector is not considered as part of the robot’s anatomy but the arm and body joints of the manipulator are used to position the end effector, and the wrist joints of the manipulator are used to orient the end effector. Example of End Effectors 1. Grippers 2. Welding Torches 3. Collision sensors 4. Tool changers Four Common Robot Configurations 1. Polar configuration It uses a telescoping arm that can be raised or lowered about a horizontal pivot. The pivot is mounted on a rotating base. The various joints provide the robot with the capability to move its arm within a spherical space. Spherical/Polar Robots A robot with 1 prismatic joint and 2 rotary joints – the axes consistent with a polar coordinate system. Commonly used for: handling at die casting or fettling machines handling machine tools arc/spot welding  Spherical/Polar Robots Advantages: large working envelope. two rotary drives are easily sealed against liquids/dust. Disadvantages: complex coordinates more difficult to visualize, control, and program. exposed linear drive. low accuracy. 2. Cylindrical configuration These configuration uses a vertical column and a slide that can be move up and down along the column. The robot arm is attached to the slide so that it can be moved radially with respect to the column. By rotating the column, the robot can move like a cylinder. Cylindrical Robots In this, the robot body is a vertical column that swivels about vertical axis The arm consists of several orthogonal slides which allow the arm to be moved up and down and in or out w.r.t. to body Commonly used for: handling at die-casting machines assembly operations handling machine tools spot welding Cylindrical Robots Advantages: can reach all around itself rotational axis easy to seal relatively easy programming rigid enough to handle heavy loads through large working space good access into cavities and machine openings  Disadvantages: linear axes are hard to seal won’t reach around obstacles exposed drives are difficult to cover from dust and liquids 3. Cartesian coordinate configuration The Cartesian coordinate robot illustrated above uses three perpendicular slides to construct the x, y, and z axes. By moving the three slides relative to one another, the robot is moving within a rectangular work envelope. Cartesian Robots It consists of three orthogonal slides. Three slides are parallel to x, y and z axes of the Cartesian coordinate system Commonly used for: pick and place work assembly operations handling machine tools arc welding Cartesian Robots Advantages: ability to do straight line insertions into furnaces. easy computation and programming. most rigid structure for given length.  Disadvantages: requires large operating volume. exposed guiding surfaces require covering in corrosive or dusty environments. can only reach front of itself 4. Articulated/Jointed-arm configuration The Jointed-arm robot is made up of rotating joints moving similar to the human arm. This chain of revolute joints provides greater freedom and dexterity in movement of the articulated robotic arm. SCARA and PUMA are the most popularly used articulated robots in assembly lines and packaging processes. Robot Motions Industrial robots are designed to perform productive work such as pick and place, welding, assembly and more. The work is accomplished by enabling the robot to move its body, arm, and wrist through series of motions and positions. The individual joint motions associated with the performance of a task are referred to by the term degrees of freedom (DOF) and a typical robot is equipped with four to six DOF. The robot’s motions are accomplished by means of powered joints. Three joints are normally associated with the action of the arm, body, and the two or three joints are generally used to actuate the wrist. Connecting the various manipulator joints together are rigid members that are called links. The links can be connected to form a serial chain or a parallel chain. The joints used in the design of industrial robots typically involve a relative motion of the adjoining links that is either linear or rotational. Example design of a robot: Linear joints involve a sliding or translational motion of the connecting links. Rotating or revolute joints involve a revolving motion of the connecting links The arm and body joints are designed to enable the robot to move its end effector to a desired position within the limit of the robot’s size and joint movements. For robots of polar, cylindrical, or jointed-arm configuration, the three degrees of freedom associated with the arm and body motions are: 1. Vertical traverse: This is the capability to move the wrist up or down to provide the desired vertical attitude. 2. Radial traverse: This involves the extension or retraction (in or out movement) of the arm from vertical center of the robot. 3. Rotational traverse: This is the rotation of the arm about the vertical axis (right or left swivel of the robot arm). The degrees of freedom associated with the arm and body of the robot are shown below: The wrist movement is designed to enable the robot to orient the end effector properly with respect to the task being performed. Three degrees of freedom for orientation of the wrist: 1. Wrist roll: also called as wrist swivel, this involves rotation of the wrist mechanism about the arm axis. 2. Wrist pitch: also called wrist bend, given that the wrist roll is in its center position, the pitch would involve the up and down rotation of the wrist. 3. Wrist yaw: involve the right or left rotation of the wrist. These degrees of freedom for the wrist are shown below.: Robot Basic Motion System Examples of Industrial Robots are PUMA and SCARA PUMA Robot (Programmable Universal machine for assembly) PUMA is the most commonly used industrial robot in assembly, welding operations and university laboratories. PUMA resembles more closely to the human arm than SCARA. PUMA has greater flexibility than SCARA but with the increased compliance comes the reduced precision. Thus, PUMA is preferably used in assembly applications which do not require high precision, such as, welding and stocking operations. PUMA robot having six degrees of freedom. SCARA (Selective compliance assembly robot arm) It is a simple articulated robot which can perform assembly tasks precisely and fast. SCARA is most adept in pick and place operations in any assembly line in industries with speed as well as precision. SCARA is more or less like a human arm the motion is restricted horizontal sweeping and vertical movement, it cannot rotate along an axis other than vertical. SCARA robot design for assembly  Articulated robots used in machining, spray painting, welding Work volume It is the term that refers to the space within which the robot can manipulate its wrist. The convention of using the wrist end to define the robot’s work volume is adopted to avoid complication of different sizes of the end effectors. The work volume is determined by the following physical characteristics of the robot: a. The robot’s physical configuration (type of joints, structure of links) b. The size of the body, arm, and wrist components c. The limits of the robot’s joint movements Polar robot has a work volume of a partial sphere. Cylindrical robot has a work volume that is cylindrical. Cartesian robot has a work volume of a rectangular shape. Joint Notation Scheme The physical configuration of the robot manipulator can be described by means of a joint notation scheme using the join types defined earlier. Uses the joint symbols (L, R, T, V) to designate joint types used to construct robot manipulator Separates body-and-arm assembly from wrist assembly using a colon (:) Example: TLR : TR Manipulator Joints Linear joint (type L) Rotational joint (type R) Twisting joint (type T) Revolving joint (type V) Notation Scheme for designating robot configurations Example: TRL Consists of a sliding arm (L joint) actuated relative to the body which can rotate about both vertical axis (T joint) and horizontal axis (R joint) TRT: R TVR:TR RR:T Practice Exercises Sketch the manipulator configurations of the following: a. LVR:R b. RLR:RT c. LRR:T Robot Control System BY: JOHNALYN FIGUERAS Robot Drive Systems It provides the robot’s capacity to move its body, arm, and wrist. It also determines its speed of operations, load capacity, and its dynamic performance. Types of Drive Systems Hydraulic Drive Electric Drive Pneumatic Drive Hydraulic Drive Generally associated with larger robots. It advantages are that it provides with greater speed and strength. It disadvantages are that it typically adds to floor space required by the robot and it is inclined to leak oil which is a trouble. Rotary vane actuator can be utilized to provide rotary motions, and hydraulic pistons can be used to accomplish linear motions. Electric Drive This drive has a better accuracy and repeatability It is smaller robot that required less space and the applications tend towards more precise work such as assembly. Electric drive robots are actuated by dc stepping motors or dc servomotors; motors that are suited to the actuation of rotational joints through appropriate drive train and gear systems and linear joints (telescoping arms) by means of pulley systems. Pneumatic Drive Generally reserved for smaller robots that possess fewer degrees (two-four joint motions). These robots are often limited to simple pick and place operations with fast cycles. This drive has an advantage in compliance or ability to absorb some shock contact with the environment. Can be readily adapted to the actuation of piston devices to provide translation movement of sliding joints. It can also be used to operate rotary actuators for rotational joints Speed of Motion The speed capabilities of current industrial robots range up to a maximum of about 500 degrees per second for certain joints. Current day industrial robots can have cycle times as low as 0.8 sec (e.g., Adept Viper) Speed determines how quickly the robot can accomplish a given work cycle. It is generally desirable in production to minimize the cycle time of a given task. Factors for Speed Determinations 1. The accuracy with which the wrist (end effectors) must be positioned 2. The weight of the object being manipulated 3. The distance to be moved  Load Carrying Capacity The size, configuration, construction, and drive system determine the load carrying capacity of the robot. The load capacity is specified under the condition that the robot’s arm is in its weakest position. In the case of a polar, cylindrical, or jointed-arm configurations, the robot is at maximum extensions. The rated weight-carrying capacities of industrial robot ranges from less than a kilogram for some small robots up to several hundred kilograms for very large robots which has a rated load of 2000lb. To use this specification, the user must consider the weights of the end effector. Example Load capacity of a give robot: 5 kgs Weight of end effector: 2 kgs Net weight-carrying capacity of the robot would be only 3 kg. ROBOT CONTROL SYSTEMS In order to operate, a robot must have a means of controlling its drive system to properly regulate its motions. Types of Robot Controls Limited- sequence robots Playback robots with point-point control Playback robots with continuous path control Intelligent robots Limited- sequence robots Do not use servo control to indicate relative positions of the joints They are controlled by setting limit switches and/or mechanical stops to establish the endpoints of travel for each joints Establishing the positions and sequence of these stops involves a mechanical set-up of the manipulator rather than robot programming in the usual sense of the term Generally, no feedback association Any of the three-drive system can be used however pneumatic drive is commonly used.  Playback Robots Use a more sophisticated control unit in which a series of positions or motions are taught to the robot, recorded into memory, and then repeated by the robot under its own control. It usually has some form of servo-controlled to ensure that the positions are achieved by the robot. Playback robots with point-point control Are capable of performing motion cycles that consist of a series of desired point locations and related actions The robot is taught each point, and the points are recorded in the robot’s control unit. Playback robots with continuous path control capable of performing motions cycles in which the path followed is controlled. the robot moves through a series of closely spaced points which describe as desired path Straight lines are a common path for industrial robots. Intelligent robots constitute a growing class of industrial robot that possesses the capability not only to play back a programmed motion cycle but also to interact with its environment in an intelligent way. The controller unit consists of a digital computer or similar device (e.g, programmable controller). This robot has the capacity to communicate during the work cycle with human or computer-based systems PRECISION MOVEMENT A robot is expected to perform repeated task in the real world and its performance is ultimately measured by its ability to position and orient the end effectors at the desired locations on a large number of times. Features to define precision function: Spatial Resolution Accuracy Repeatability Compliance Spatial Resolution Spatial Resolution The spatial resolution of a robot is the smallest increment of movement into which the robot can divide its work volume. Spatial resolution depends on two factors: the system’s control resolution and the robot’s mechanical in accuracies The control resolution is determined by the robot’s position control system and its feedback measurement system. It is the controller’s ability to divide the total range of movement for the particular joint into individual increments that can be addressed in the controller. Spatial Resolution The increments are sometimes referred to as “addressable points.” The ability to divide the joint range into increments depends on the bit storage capacity in the control memory. The number of separate, identifiable increments (addressable points) for a particular axis is given by: Number of increments = 2n where n = the number of bits in the control memory For example, Robot with 8 bits of storage can divide the range into 256 discrete points. The control resolution would be defined as the total motion range divided by the number of increments. Example Using our robot with one degree of freedom as an illustration, we will assume it has one sliding joint with a full range of 1.0m. The robot’s control memory has 12-bit storage capacity. Determine the control resolution for this axis of motion. Note: This is an example of one joint robot. For robots with several degrees of freedom, each joint of motion will have a control resolution. To obtain the control resolution of the entire robot, components resolutions for each joint would have to be summed vectorially. Accuracy It refers to a robot’s ability to position its wrist end at a desired target point within the work volume. The accuracy of a robot can be defined in terms of spatial resolution because the ability to achieve a given target points depends on how closely the robot can define the control increments for each of its joint motions. accuracy and control resolution when mechanical inaccuracies are assumed to be zero Accuracy and Control resolution when mechanical inaccuracies are assumed to be zero Factors affecting Robots accuracy a. Accuracy varies within the work volume b. Accuracy is improved if the, motion cycle is restricted to a limited work range c. Accuracy is the load carried by the robot. The mechanical errors will be reduced when the robot is exercised through a restricted range of motions. Local Accuracy – the robot’s ability to reach a particular reference point within the limited workspace. Global Accuracy - when accuracy is assessed within the robot’s full work volume. Repeatability Is a concerned with the robot’s ability to position its wrist or an end effector attached to its wrist at a point in space that have previously been taught to the robot. Illustration of repeatability and accuracy Compliance The compliance of the robot manipulator refers to the displacement of the wrist end in response to a force or torque exerted against it. High compliance means that the wrist is displaced a large amount by a relatively small force. The term spring is sometimes used to describe a robot with high compliance. Low compliance means that the manipulator is relatively stiff and is not displaced by a significant amount. Robot manipulator compliance is a directional feature. The compliance of the robot arm will be greater in certain directions that in other directions because of the mechanical construction of the arm BASIC CONTROL SYSTEMS CONCEPTS AND MODELS Five Major Components of a Control Systems 1. Input/s 2. Controller and actuating devices 3. Plant (mechanism or process being controlled) 4. Output (controlled variable) 5. Feedback elements (sensors) Seatwork 1. The telescoping arm of a certain industrial robot obtains its vertical motion by rotating (type R joint) about a horizontal axis. The total range of the rotation is 90 o. The robot possesses a 10-bit storage capacity for this axis. When fully extended, the robot’s telescoping arm measures 50 in. from the pivot point. When fully retracted, the arms measure 30 in. from the pivot point. a. Determine the robot’s control resolution for this axis in degrees of rotation. b. Determine the robot’s control resolution on a linear scale in both fully extended and fully retracted position. c. Sketch the side view of the robot’s work volume as determined by thus pivoting axis. Your best quote that reflects your approach… “It’s one small step for man, one giant leap for mankind.” - NEIL ARMSTRONG Robot End Effectors END EFFECTORS o It is commonly known as robot hand. o It is mounted on the wrist, enables the robot to perform specified tasks. o Various types of end-effectors are designed for the same robot to make it more flexible and versatile. End-effectors are categorized into two major types: 1. Grippers 2. Tools GRIPPERS o Grippers grasp and manipulate objects during the work cycle. o Typically, the objects grasped are work parts that need to be loaded or unloaded from one station to another. o It may be custom-designed to suit the physical specifications of the work parts they have to grasp. Grippers are described in detail in table below: Mechanism of gripping Generally, the gripping mechanism is done by the grippers or mechanical fingers. Though in the industrial robotics due to less complications, two finger grippers are used. The fingers are also replaceable. Due to gradual wearing, the fingers can be replaced without actually replacing the grippers. Shape of the gripping surface The shape of the gripping surface on the fingers can be chosen according to the shape of the objects that are lifted by the grippers. For example, if the robot is designated a task to lift a round object, the gripper surface shape can be a negative impression of the object to make the grip efficient, or for a square shape the surface can be plane. Types: 1. Impactive – jaws or claws which physically grasp by direct impact upon the object. 2. Ingressive – pins, needles or hackles which physically penetrate the surface of the object (used in textile, carbon and glass fiber handling). 3. Astrictive – suction forces applied to the objects surface (whether by vacuum, magneto– or electro adhesion). 4. Contigutive – requiring direct contact for adhesion to take place (such as glue, surface tension or freezing). Now, let’s discuss some of the grippers. Mechanical Grippers A mechanical gripper is used as an end effector in a robot for grasping the objects with its mechanically operated fingers. In industries, two fingers are enough for holding purposes. As most of the fingers are of replaceable type, it can be easily removed and replaced. A robot requires either hydraulic, electric, or pneumatic drive system to create the input power. The power produced is sent to the gripper for making the fingers react. It also allows the fingers to perform open and close actions. Most importantly, a sufficient force must be given to hold the object. In a mechanical gripper, the holding of an object can be done by two different methods such as: 1. Using the finger pads as like the shape of the work part. 2. Using soft material finger pads. In the first method, the contact surfaces of the fingers are designed according to the work part for achieving the estimated shape. It will help the fingers to hold the work part for some extent. In the second method, the fingers must be capable of supplying sufficient force to hold the work part. To avoid scratches on the work part, soft type pads are fabricated on the fingers. As a result, the contact surface of the finger and co – efficient of friction are improved. This method is very simple and as well as less expensive. It may cause slippage if the force applied against the work part is in the parallel direction. The slippage can be avoided by designing the gripper based on the force exerted. During rapid grasping operation, the work part will get twice the weight. To get rid out of it, the modified equation 1 is put forward by Engelberger. The g factor in the equation 2 is used to calculate the acceleration and gravity. a. The values of g factor for several operations are given below: b. g = 1 – acceleration supplied in the opposite direction. c. g = 2 – acceleration supplied in the horizontal direction. d. g = 3 – acceleration and gravity supplied in the same direction. Example: Suppose a stiff cardboard carton weighing 10lb is held in a gripper using friction against two opposing fingers. The coefficient of friction between the fingers contacting surfaces and the carton surface is 0.25. The orientation of the carton is such that the weight of the carton is directed parallel to finger surfaces. A fast work cycle is anticipated so that a g factor of 3.0 should be applied to calculate the required gripper force. Determine the required gripper force for the conditions given. Solutions: 0.25 * 2 * Fg = 10 * 3.0 Fg = 30/0.5 = 60lb, the force the gripper must cause by the fingers against the surface of the carton There are various ways of classifying mechanical grippers and their actuating mechanisms. One method is according to the type of finger movement used by the gripper. In this classification, the grippers can actuate the opening and closing of the fingers by one of the following: 1. pivoting movement 2. linear or translational movement Mechanical Gripper can also be classed according to the type of kinematics device used to actuate the finger movement. In this classification, we have the following types: 1. Linkage actuation - covers a wide range of design possibilities to actuate the opening and closing of the Gripper. The design of the linkage determines how the input force Fa to the gripper is converted the gripping force Fg applied by the fingers. It also determines how wide the gripper fingers will open and how quickly the gripper actuate. 2. Gear and rack actuation - the rack gear would be attached to a piston or some other mechanism that provide linear motion. Movement of the rack would drive two partial pinion gears, and these would in turn open and close the fingers. 3. Cam actuation – a cam-and follower arrangement often using a spring-loaded follower can provide the opening and closing of the gripper. 4. Screw actuation - the screw is turned by a motor, usually accompanied by a speed reduction mechanism. When the screw is rotated in one direction, this causes a threaded block to be translated in one direction. When the screw is rotated in the opposite direction, the threaded block moves in the opposite direction. The threaded block is, in turn, connected to the gripper fingers to cause the opening and closing action. 5. Rope-and –pulley actuation- this mechanism can be designed to open and close a mechanical gripper because of the nature of these mechanisms, some form of tension device must be used to oppose the motion of the rope or cord in the pulley system. 6. Miscellaneous – for gripper-actuating mechanisms that do not logically fall into the above categories. Gripper Force Analysis the purpose of the gripper mechanism is to convert input power into the required motion and force to grasp and hold an object. Let us illustrate the analysis that might be used to determine the magnitude of the required input power in order to obtain a given gripping force in the example below. Suppose the gripper is a simple pivot-type device used for holding the cardboard carton. The gripper force, calculated in the previous example is 60lb. The gripper is to be actuated by a piston device to apply an actuating force Fa. The corresponding lever arms for the two forces are shown in the diagram: The analysis would require that the moments about the pivot arms are summed and made equal to zero. The piston device would have to provide an actuating force of 240lb to close the gripper with a force against the cartoon of 60lb. Another example, The above figure shows the linkage mechanism and dimensions of a gripper used to handle a workpart for a machining operation. Suppose it has been determined that the gripper force is to be 25 lb. What is required is to compute the actuating force to deliver this force of 25 lb? Solution: Figure a shows how the symmetry of the gripper can be use so that only one half of the mechanism needs to be considered Figure b shows how the moments might be summed about the pivot point for the finger link againt which the 25-lb gripper force is applied. TOOLS o The robot end effecter may also use tools. o Tools are used to perform processing operations on the work part. o Typically, the robot uses the tool relative to a stationary or slowly moving object. o In this way the process is carried out. Examples of the tools used as end effectors by roots to perform processing applications include: a. Spot welding gun b. Arc welding tool c. Spray painting gun d. Rotating spindle for drilling, routing, grinding, etc. e. Assembly tool (e.g. automatic screwdriver) f. Heating torch g. Water-jet cutting tool THE ROBOT/END EFFECTOR INTERFACE An important aspect of the end effector applications engineering involves the interfacing of the end effector with the robot. This interface must accomplish at least some of the following functions: 1. Physical support of the end effector during the work cycle must be provided. Power to actuate the end effector must be supplied through the interface. Control signals to actuate the end effector must be provided. This is often accomplished by controlling the actuating power. 2. Feedback signals must sometimes be transmitted back through the interface to the robot controller. In addition, certain other general-design objectives should be met. These include high reliability of the interface, protection against the environment, and overload protection in case of disturbances and unexpected events during the work cycle. Robotics Sensors What makes a machine a robot? Sensing Planning Acting action information on the about the environment environment  What is Sensing ? Collect information about the world Sensor - an electrical/mechanical/chemical device that maps an environmental attribute to a quantitative measurement attribute mixtures - often no one to one map hidden state in environment Each sensor is based on a transduction principle - conversion of energy from one form to another Also known as transducers Uses of Sensors Where am I? ? localization Uses of Sensors What is the angle of my arm?  internal information Uses of Sensors Will I hit anything? obstacle detection Uses of Sensors Where is the cropline? Autonomous harvesting Uses of Sensors Where are the forkholes? Autonomous material handling Uses of Sensors Where is the face? Face detection & tracking Sensors with various properties that describe their capabilities a. Sensitivity (change of output and change of input) b. Linearity (constancy of output and input) c. Response Time (time required for a change in input to force a change in the output) d. Measurement/Dynamic range (difference between minimum and maximum) e. Accuracy (difference between measured & actual) f. Repeatability (difference between repeated measures) g. Resolution (smallest observable increment) h. Bandwidth (result of high resolution or cycle time) Types of Sensors Active send signal into environment and measure interaction of signal w/ environment e.g. radar, sonar Passive record signals already present in environment e.g. video cameras Types of Sensors Classification by medium used based on electromagnetic radiation of various wavelengths vibrations in a medium concentration of chemicals in environment by physical contact Types of Sensors Exteroceptive: deal w/ external world where is something ? how does is look ? (camera, laser rangefinder) Proprioceptive: deal w/ self where are my hands ? (encoders, stretch receptors) am I balanced ? (gyroscopes, INS) Types of Sensors Interoceptive what is my thirst level ? (biochemical) what is my battery charge ? (voltmeter) For the most part we’ll ignore these in this class Available sensors in the market Touch sensors Bend sensors Tilt sensors Light sensors Encoders Temperature sensors Laser rangefinders Potentiometers Cameras Touch sensors electrical flow a simple switch force voltage measurement Tilt sensors another simple switch gravity Encoders Encoders measure rotational motion. They can be used to measure the rotation of a wheel. Servo motors: Used in conjunction with an electric motor to measure the motor’s position and, in turn, control its position. Encoders Voltage square wave on on off off off 1 2 3 4... Important spec: Number of counts per revolution Examples Sensor Analysis 10 cm 16 counts per rev. 10 cm wheel diameter How far does the wheel travel for 1 encoder count? What happens if we change the wheel diameter? How many counts are there per meter of travel? Examples C = D Diameter C = 10 cm Circumference 10 cm 1 rev 1.96 cm x = 1 rev 16 counts count  Examples Suppose I want 1.0 cm / count. What should my wheel diameter be? 1.0 cm 16 counts 16 cm x = count 1 rev rev C = 16 cm C 16 D =  =  = 5.09 cm  Examples For my 10 cm wheel, how many encoder counts will there be for 1 meter of travel? 1.96 cm 1 meter 0.0196 m x = count 100 cm count 1 = 51 counts/m 0.0196 m/ct  Physics 101 R (1000 Ohms) Ohm’s Law I (0.009 Amps) V=IxR voltage current resistance 9 = 0.009 x 1000 V (9 Volts)  Electrical analogy Voltage Current Res ista nce a larger pipe is a smaller pipe is less resistance more resistance so more water so less water Bend sensor a variable resistor resistance changes as it bends V=IxR assuming constant current, the measured voltage changes with resistance Light sensor photo-resistor resistance changes with light intensity Temperature sensor thermal resistor “thermistor” resistance changes with temperature Potentiometer another rotational sensor R resistance changes with position of dial  Examples Given a 5 V source, what is the min. and max. Bend sensor specs: current that is drawn? 100  when straight min = 5 = 5 mA 1000  when bent 1000 V=IxR 5 max = = 50 mA V 100 I= R Sensors Based on EM Spectrum Basically used for ranging Light sensitive eyes, cameras, photocells etc. Operating principle CCD - charge coupled devices photoelectric effect IR sensitive - FLIR sense heat differences and construct images night vision application EM Spectrum Radio and Microwave RADAR: Radio Detection and Ranging Microwave radar: insensitive to clouds Coherent light all photons have same phase and wavelength LASER: Light Amplification by Stimulated Emission of Radiation LASER RADAR: LADAR - accurate ranging The SICK Laser Rangefinder EM Spectrum Nuclear Magnetic Resonance (NMR) heavy duty magnetic field lines up lines up atoms in a body now expose body to radio signals different nuclei resonate at different frequencies which can be measured leading to an image Local Proximity Sensing in EM Infrared LEDs cheap, active sensing usually low resolution - normally used for presence/absence of obstacles rather than ranging operate over small range Sensors Based on Sound SONAR: Sound Navigation and Ranging bounce sound off of something measure time for reflection to be heard - gives a range measurement measure change in frequency - gives the relative speed of the object (Doppler effect) bats and dolphins use it with amazing results robots use it w/ less than amazing results Sonar and IR Proxmity Odor Sensors Detection of chemical compounds and their density in an area spectroscopy - mostly lab restricted fiber-optic techniques - recently developed chemical detection - sniffers aand electronic noses via “wet chemistry on a chip” No major penetration in robotics yet applications are vast (e.g. mine detection) Touch Sensors Whiskers, bumpers etc. mechanical contact leads to closing/opening of a switch change in resistance of some element change in capacitance of some element change in spring tension Proprioceptive Sensors Encoders, Potentiometers measure angle of turn via change in resistance or by counting optical pulses Gyroscopes measure rate of change of angles fiber-optic (newer, better), magnetic (older) Compass measure which way is north GPS: measure location relative to globe Propriceptive Sensors Sensor Choice What sensors to employ ? E.g. mapping ranging - laser, sonar, IR, stereo camera pair salient feature detection - doors using color Factors accuracy, cost, information needed Sensor Placement Where do you put them ? On/off board (e.g. localization using odometry vs. localization using beacons) If onboard - where ? Reasonable arrangements - heuristic Optimal arrangements - mathematically rigorous Sensor Placement Where do you put them ? On/off board (e.g. localization using odometry vs. localization using beacons) If onboard - where ? Reasonable arrangements - heuristic Optimal arrangements - mathematically rigorous Graded Exercises 1. It is a desired to design a safety monitoring system for a robot cell in which a robot loads an automatic production machine with parts arriving on a conveyor. The robot is large and it is considered dangerous for workers to wander into the work volume of the robot. A side from the other safety precautions that might be taken to ensure the safety of the works, the safety monitoring system will use one or more sensors to detect the presence of humans in the cell. With the above scenario, do the following below: a. Write the detailed “functional specifications” for the sensor system. That is, make up a list of the things the sensor system must do, and the ways in which it will have to operate. b. From the different types of sensors, select several alternative sensors that will satisfy the functional specifications , and compare its features against the specifications. For each sensor, explain how it will be configured in the safety monitoring system. Use sketches to illustrate the configuration. c. Select the best sensor alternative, and justify your selection. 2. How would you apply Tilt Sensors in a walking robot? 3. Describe your idea of using a GPS sensor in an industrial robot.