Robot Geometry

*Information Software and Technology*FUNCTIONS OF ROBOTSTechnical aspects of robotics: Axes of motionDegrees of freedomRobotic armsActuatorsEnd effectorsSensorsRobot Vision

Information Software and Technology

*Information Software and Technology*Robot GeometryRobot VisionSensorsEnd EffectorsRobotic ArmsActuators


*Information Software and Technology*Robot GeometryWhat is robot geometry?Types of robotic armsActuators


*Information Software and Technology*Robot GeometryWhat is Robot Geometry?

The degrees of freedom a robotic arm has and the axes of motion

How many shapes can be seen in a robot

The angles that a robot makes with the ground


*Information Software and Technology*What is Robot Geometry?The design of a robot arm needs to take into consideration whether it is to be able to cover a large area, perform intricate movements, lift heavy loads or move with great speed or a combination of these things. Robots can be programmed to do any job, however it is best if a robot is designed with some particular job in mind.


*Information Software and Technology*Axes of MotionThis diagram has the axes of motion marked with arrows there are 3A robot may in theory have any number of axes.The more axes there are the more manoeuvrable the robot is, however the more complicated it will be to programAxes of motions do not need to be rotations they can be motions along a straight lineAxes that allow rotations are known as revolute jointsAxes that allow movement in a straight line are known as prismatic joints


*Information Software and Technology*

Degrees of FreedomThe number of independent directions in which the end effector (tool or gripper) of the robot can moveAny solid object has a maximum of six degrees of freedomX, Y and Z represent movement along a lineRx, Ry and Rz represent rotationsRxRyRz


*Information Software and Technology*Types of Robotic ArmsAnthropomorphic SystemCartesian SystemCylindrical SystemPolar SystemSCARA System


*Information Software and Technology*The Anthropomorphic SystemThe articulated systemLike a human arm (sections are joined together)Shoulder and Elbow are used to refer to the two jointsMore sections can be added if needed.Most manoeuvrableUsed for paint sprayingCannot cover a large areaDifficult to move the end of the arm in a straight line


*Information Software and Technology*The Cartesian SystemThe X, Y Z system three independent directionsBest used when a large area needs to be coveredNot good when intricate movements are requiredX axis allows the arm to move along the work pieceY axis allows the arm to move towards and away from the work pieceZ axis allows movement upwards and downwardsThe three movements are at 90 degrees to the other twoThis robotic arm can move over the surface of an imaginary rectangle


*Information Software and Technology*The Cylindrical SystemSimilar to the cartesian except no X axisArm can rotate on a central supportAngle of rotation is referred to by the symbol Three axis of motion (Y, Z and )This robotic arm can move over the surface of an imaginary circleCan move much faster than the cartesian robot armCan be used for loading or unloading


*Information Software and Technology*The Polar System

Spherical systemSame Y and axes as cylindricalArm is pivoted so that it can rotate in the vertical plane instead of moving up and down along the z axisThis robotic arm can cover the surface of a sphereCan move faster in the vertical direction than a cylindrical armRange of movement is much more restricted


*Information Software and Technology*The SCARA SystemSelective Compliant Assembly Robot Arm All revolute joints in the arm rotate about the vertical axesThree degrees of freedomUsed for assembly operations


*Information Software and Technology*The Work Envelope The work envelope is the area that a robot can cover. The exact size and shape of the work envelope will be one of the main factors in deciding whether the robot is suitable for a particular job.The size and shape vary enormously


*Information Software and Technology*ActuatorsActuator is the term used for the mechanism that drives the robotic arm.There are 3 main types of ActuatorsElectric motorsHydraulicPneumatic cylinderHydraulic and pneumatic actuators are generally suited to driving prismatic joints since they produce linear motion directlyHydraulic and pneumatic actuators are also known as linear actuators. Electric motors are more suited to driving revolute joints as they produce rotation


*Information Software and Technology*Hydraulic ActuatorsA car makes use of a hydraulic system. If we look at the braking system of the car we see that only moderate force applied to the brake pedal is sufficient to produce force large enough to stop the car.The underlying principle of all hydraulic systems was first discovered by the French scientist Blaise Pascal in 1653. He stated that if external pressure is applied to a confined fluid, then the pressure is transferred without loss to all surfaces in contact with the fluid The word fluid can mean both a gas or a liquidWhere large forces are required we can expect to find hydraulic devices (mechanical diggers on building sites, pit props in coal mines and jacks for lifting cars all use the principle of hydraulics.


*Information Software and Technology*Hydraulic ActuatorsEach hydraulic actuator contains the following parts:PistonsSpring return pistonDouble acting cylinderHydraulic transfer valueAnd in some cases a hydraulic accumulatorAdvantages of the hydraulic mechanismA hydraulic device can produce an enormous range of forces without the need for gears, simply by controlling the flow of fluidMovement of the piston can be smooth and fastPosition of the piston can be controlled precisely by a low-current electrically operated valueThere are no sparks to worry about as there are with electrical motor, so the system is safe to use in explosive atmospheres such as in paint spraying or near inflammable materials


*Information Software and Technology*Pneumatic ActuatorsA pneumatic actuator uses air instead of fluidThe relationship between force and area is the same in a pneumatic system compared to a hydraulic systemWe know that air is compressible, so in order to build up the pressure required to operate the piston, extra work has to be done by the pump to compress the air. This means that pneumatic devices are less efficientIf you have ever used a bicycle pump you may have noticed that it becomes hot as it is used. The heat produced by the mechanical work done in compressing the air. Heat represents wasted energy.


*Information Software and Technology*Pneumatic ActuatorsAdvantages of the Pneumatic system:Generally less expensive than an equivalent hydraulic system. Many factories have compresses air available and one large compressor pump can serve several robotsSmall amount of air leakage is ok, but in a hydraulic system it will require prompt attentionThe compressibility of air can also be an advantage in some applications. Think about a set of automatic doors which are operated pneumatically. If a person is caught in the doors they will not be crushed. A pressure relief valve can be incorporated to release pressure when a force is exceeded, for example the gripper of a robot will incorporate a relief value to ensure it does not damage itself or what it is grippingPneumatic devices are faster to respond compared to a hydraulic system as air is lighter than fluid.A pneumatic system has its downfalls and the main one is that it can produce the enormous forces a hydraulic system can. Another is concerned with the location of the pistons. As air is compressible heavy loads on the robot arm may cause the pistons to move even when all the valves on the cylinder are closed. It is for this reason that pneumatic robots are best suited for pick and place robots.


*Information Software and Technology*Electric MotorsNot all electric motors are suited for use as actuators in robotsThere are three basic characteristics of a motor, when combined will determine the suitability of a motor for a particular job. The 3 characteristics are power, torque and speed. Each of these characteristics are interdependent, that means that you can not alter one without affecting the others.


*Information Software and Technology*Electric MotorsTwo types of power: electrical and mechanical, both are measured in watts.Torque is how strong a motor is or how much turning force it is able to produce and is measured in newton-metres.The speed is measured in revolutions per minute and is rotation of the motorThere are 3 different types of motorsAC motor which operates by alternating current electricityDC motor which operates by direct current electricityStepper motors which operates by pulses of electricityAny type of electric motor could be used for a robot as long as it is possible to electronically control the speed and power so that it behaves the way we want.DC motors and Stepper motors are commonly used in robotics


*Information Software and Technology*Hardware to control robotsEnd EffectorsSensorsRobot Vision


*Information Software and Technology*End EffectorsWhat is an end effector?Types of end effectorsGrippersTools


*Information Software and Technology*What is an end effector?An end effector is the device that is at the end of a robotic arm.There are two main types of end effectors: Grippers and tools.We can think of an end effector like a human hand. Even though a human hand is very versatile, an end effector has one great advantage that humans do not have and that is the interchangablility of end effectors. If the end effector is not suitable than it can be changed unlike the human hand.


*Information Software and Technology*GrippersGrippers are devices which can be used for holding or gripping an object. They include what you might call mechanical hands and also anything like hooks, magnets and suction devices which can be used for holding or gripping.Grippers take advantage of point-to-point control (exact path that the robot takes between what it is picking up and where it is placing it.Grippers should be designed so that it requires the minimum amount of manoeuvring in order to grip the work piece


*Information Software and Technology*Types of GrippersThere are four main categories which makes use of a griperNo gripping in this situation the workpiece is held in a jig (a specially designed purpose built holder) and the robot performs an activity on it. Jobs which use no gripping can include spot welding, flame cutting and drillingCoarse gripping in this case the robot holds the workpiece but the gripping does not have to be precise. Jobs which use coarse gripping include handling and dipping castings, unloading furnaces, stacking boxes or sacksPrecise gripping A robot holds the workpiece which requires accurate positioning for example unloading and loading machine toolsAssembly the robot is required to assemble parts which requires accurate positioning and some form of sensory feedback to enable the robot to monitor and correct its movements.


*Information Software and Technology*Types of Grippers MechanicalSuctionMagneticHooks and Scoops


*Information Software and Technology*Mechanical GrippersWe can think of a mechanical gripper as a robot hand. A basic robot hand will have only two or three fingersA mechanical hand that wraps around an object will rely on friction in order to secure the object it is holding.Friction between the gripper and the object will depend on two things, First is the type of surface whether it be metal on metal, rubber on metal, smooth surfaces or rough surfaces and the second is the force which is pressing the surfaces together. Mechanical grippers are often fitted with some type of pad usually made from polyurethane as this provides greater friction. Pads are less likely to damage the workpiece. Pads are also used so to have a better grip as the polyurethane will make contact with all parts of the surface when the gripper is closedMechanical grippers can be designed and made for specific purposes and adjusted according to the size of the object. They can also have dual grippers. We are all familiar with the saying two hands are better than one and robots benefit from having dual grippers as they can increase productivity, be used with machines that have two work stations where one robot can load two parts in a single operation, operations in which the size of objects or part change due to the machining processes and where the cycle time of the robot is too slow to keep up with the production of other machines.


*Information Software and Technology*Suction GrippersThere are two types of suction grippers:Devices operated by a vacuum the vacuum may be provided by a vacuum pump or by compressed airDevices with a flexible suction cup this cup presses on the workpiece. Compressed air is blown into the suction cup to release the workpiece. The advantage of the suction cup is that if there is a power failure it will still work as the workpiece will not fall down. The disadvantage of the suction cup is that they only work on clean, smooth surfaces.There are many more advantages for using a suction cup rather than a mechanical grip including: there is no danger of crushing fragile objects, the exact shape and size does not matter and the suction cup does not have to be precisely positioned on the objectThe downfalls of suction cups as an end effector include: the robot system must include a form of pump for air and the level of noise can cause annoyance in some circumstances


*Information Software and Technology*Magnetic GrippersMagnetic grippers obviously only work on magnetic objects and therefore are limited in working with certain metals. For maximum effect the magnet needs to have complete contact with the surface of the metal to be gripped. Any air gaps will reduce the strength of the magnetic force, therefore flat sheets of metal are best suited to magnetic grippers. If the magnet is strong enough, a magnetic gripper can pick up an irregular shaped object. In some cases the shape of the magnet matches the shape of the objectA disadvantage of using magnetic grippers is the temperature. Permanent magnets tend to become demagnetized when heated and so there is the danger that prolonged contact with a hot workpiece will weaken them to the point where they can no longer be used. The effect of heat will depend on the time the magnet spends in contact with the hot part. Most magnetic materials are relatively unaffected by temperatures up to around 100 degrees.Electromagnets can be used instead and are operated by a DC electric current and lose nearly all of their magnetism when the power is turned off.Permanent magnets are also used in situations where there is an explosive atmosphere and sparks from electrical equipment would cause a hazard


*Information Software and Technology*Hooks and Scoops Hooks and scoops are the simplest type of end effectors that can be classes as grippers.A scoop or ladle is commonly used to scoop up molten metal and transfer it to the mouldA hook may be all that is needed to lift a part especially if precise positioning in not required and if it is only to be dipped into a liquid.


*Information Software and Technology*ToolsTools are devices which robots use to perform operations on an object, for example, drills, paint sprays, grinders, welding torches and any other tool which get a specific job done.Tools take advantage of continuous path control (the path the end effector takes needs to careful, steady and continuously controlled at every moment)If we think of a spray gun and if it moves to quickly then the paint will be too thin on the other hand if it moves to slowly the paint will be too thick or in blobs. Any tool required can be fitted to the end of the robotic arm and can be programmed to select and change tools without human intervention


*Information Software and Technology*Types of ToolsA common tool used as an end effector is the welding tool. Welding is the process of joining two pieces of metal by melting them at the join and there are 3 main welding tools: a welding torch, spot welding gun and a stud welding toolOther common tools are paints praying, deburring tools, pneumatic tools such as a nut runner to tighten nuts.


*Information Software and Technology*SensorsWhat is a sensor?Types of sensorsOpticalHeatAcousticProximity


*Information Software and Technology*What is a sensor?


*Information Software and Technology*Types of Sensors


*Information Software and Technology*Optical Sensors


*Information Software and Technology*Heat Sensors


*Information Software and Technology*Acoustic SensorWe are all familiar with the microphone a microphone is a device for converting sound waves into electrical signals.An acoustic sensor interprets the sound that it senses and requires a computerBasic acoustic sensors are used in robots to responds to a human shout or the noise of an explosion. A more sophisticated sensor would be able to recognise words and take appropriate action and makes use of the concept of speech recognition


*Information Software and Technology*Proximity SensorA proximity sensor is a device that can sense the presence of an object, just by being close to it. Sensors of this type have been made by using the magnetic effect induced by the presence of metals, the effect on electrical capacitance induced by the presence of most other materials or by detecting reflected light or reflected ultrasonic sound.


*Information Software and Technology*Robot VisionWhat is robot vision?


*Information Software and Technology*Robot Vision


*Information Software and Technology*CONGRATULATIONS


*Information Software and Technology*TRY AGAIN


*Information Software and Technology*ACTIVITY 1: Using five of the robots in your portfolio (from the artificial intelligence booklet), identify the job it is designed to do. For example spray painting a car. Then classify each robot into a size category; the spray painting robot can cover a large area.


*Information Software and Technology*ACTIVITY TWONow using the five robots you used in the previous activity, identify the axes of motion for each, drawing arrows on your robot diagram. If in doubt discuss with the person next to you


*Information Software and Technology*ACTIVITY THREE:

Once again use the five robots you used in the previous activity, identify the degrees of freedom for each, drawing arrows on your robot diagram. If in doubt discuss with the person next to you. Remember degrees of freedom is the number of movements the end effector can make.


*Information Software and Technology*ACTIVITY FOUR Think for a moment about the work envelop of your own arm.What movements can your arm make? What shape does it make?What size is the envelope?Investigate the TALON V polar geometry robot and the ASEA industrial robot and compare their work envelops.


*Information Software and Technology*The Work EnvelopStanding in one place, you can extend you arm upwards, downwards and to either side. Obviously it doesnt reach that far behind you because the joints only move through limited angles and some areas cannot be reached because the rest of your body gets in the way. You can visualise an imaginary chamber or envelop which surrounds your arm; the end of your arm can reach anything placed within this envelop but anything outside the envelope is inaccessible. The shape of this work envelop is hard to draw, but it is roughly the shape of a sphere, almost like an apple with a large bite taken out of it.


*Information Software and Technology*QuestionsName the types of robotic armsWhat is robot geometry? What is an end effector?What is an actuator?What do we mean by degrees of freedom?What do we mean by the term axes of motion? Describe the term work envelopName four types of grippers, giving a brief descriptionWhat is the difference between a hydraulic actuator and a pneumatic actuator?Describe the Anthropomorphic SystemDescribe the Cartesian systemDescribe the polar systemDescribe the SCARA systemDescribe the cylindrical system


Documents

Robot Geometry