Chapter 8- End Effectors

1. Chapter 8- End Effectors Landstown High School Governor’s STEM & Technology Academy Advanced Robotics Advanced Robotics

2. Robot Geometry • In order to understand how end effectors work on a robot, you must understand Robot Geometry. Advanced Robotics

3. 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. Advanced Robotics

4. Axes of Motion • This diagram has the axes of motion marked with arrows – there are 3 • A 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 program • Axes of motions do not need to be rotations – they can be motions along a straight line • Axes that allow rotations are known as revolute joints • Axes that allow movement in a straight line are known as prismatic joints Advanced Robotics

5. x y z Degrees of Freedom • The number of independent directions in which the end effector (tool or gripper) of the robot can move • Any solid object has a maximum of six degrees of freedom • X, Y and Z represent movement along a line • Rx, Ry and Rz represent rotations Rx Ry Rz Advanced Robotics

6. Types of Robotic Arms Advanced Robotics

7. The Anthropomorphic System • The articulated system • Like a human arm (sections are joined together) • “Shoulder” and “Elbow” are used to refer to the two joints • More sections can be added if needed. • Most manoeuvrable • Used for paint spraying • Cannot cover a large area • Difficult to move the end of the arm in a straight line Advanced Robotics

8. The Cartesian System • The X, Y Z system – three independent directions • Best used when a large area needs to be covered • Not good when intricate movements are required • X axis allows the arm to move along the work piece • Y axis allows the arm to move towards and away from the work piece • Z axis allows movement upwards and downwards • The three movements are at 90 degrees to the other two • This robotic arm can move over the surface of an imaginary rectangle Advanced Robotics

9. The Cylindrical System • Similar to the cartesian except no X axis • Arm can rotate on a central support • Angle of rotation is referred to by the symbol ø • Three axis of motion (X, Y, and Z ) • This robotic arm can move over the surface of an imaginary circle • Can move much faster than the cartesian robot arm • Can be used for loading or unloading Advanced Robotics

10. The Polar System • Spherical system • Same Y and Z axes as cylindrical • Arm is pivoted so that it can rotate in the vertical plane instead of moving up and down along the z axis • This robotic arm can cover the surface of a sphere • Can move faster in the vertical direction than a cylindrical arm • Range of movement is much more restricted Advanced Robotics

11. The SCARA System • Selective Compliant Assembly Robot Arm • All revolute joints in the arm rotate about the vertical axes • Three degrees of freedom • Used for assembly operations Advanced Robotics

12. 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 Advanced Robotics

13. Actuators • Actuator is the term used for the mechanism that drives the robotic arm. • There are 3 main types of Actuators • Electric motors • Hydraulic • Pneumatic cylinder • Hydraulicand pneumatic actuators are generally suited to driving prismatic joints since they produce linear motion directly • Hydraulic and pneumatic actuators are also known as linear actuators. • Electric motors are more suited to driving revolute joints as they produce rotation Advanced Robotics

14. Hydraulic Actuators • A 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 liquid • Where 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. Advanced Robotics

15. Hydraulic Actuators • Each hydraulic actuator contains the following parts: • Pistons • Spring return piston • Double acting cylinder • Hydraulic transfer value • And in some cases a hydraulic accumulator • Advantages of the hydraulic mechanism • A hydraulic device can produce an enormous range of forces without the need for gears, simply by controlling the flow of fluid • Movement of the piston can be smooth and fast • Position of the piston can be controlled precisely by a low-current electrically operated value • There 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 Advanced Robotics

16. Pneumatic Actuators • A pneumatic actuator uses air instead of fluid • The relationship between force and area is the same in a pneumatic system compared to a hydraulic system • We 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 efficient • If 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. Advanced Robotics

17. Pneumatic Actuators • Advantages 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 robots • Small amount of air leakage is ok, but in a hydraulic system it will require prompt attention • The 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 gripping • Pneumatic 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. Advanced Robotics

18. Electric Motors • Not all electric motors are suited for use as actuators in robots • There 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. Advanced Robotics

19. Electric Motors • Two 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 motor • There are 3 different types of motors • AC motor which operates by alternating current electricity • DC motor which operates by direct current electricity • Stepper motors which operates by pulses of electricity • Any 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 Advanced Robotics

20. End Effectors Advanced Robotics

21. 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. Advanced Robotics

22. Types of Grippers Advanced Robotics

23. Grippers • Grippers 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 Advanced Robotics

24. Types of Grippers • There are four main categories which makes use of a gripper • No 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 drilling • Coarse 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 sacks • Precise gripping – A robot holds the workpiece which requires accurate positioning for example unloading and loading machine tools • Assembly – 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. Advanced Robotics

25. Mechanical Grippers • We can think of a mechanical gripper as a robot hand. A basic robot hand will have only two or three fingers • A 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 closed • Mechanical 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. Advanced Robotics

26. Suction Grippers • There are two types of suction grippers: • Devices operated by a vacuum – the vacuum may be provided by a vacuum pump or by compressed air • Devices 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 object • The 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 Advanced Robotics

27. Magnetic Grippers • Magnetic 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 object • A 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 Advanced Robotics

28. 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 mould • A 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. Advanced Robotics

29. Tools • Tools 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 Advanced Robotics

30. Types of Tools • A 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 tool • Other common tools are paints praying, deburring tools, pneumatic tools such as a nut runner to tighten nuts. Advanced Robotics

31. Citations • Robotics: Therory and Industrial Applications, • mrjob.mathsclass.net‎ Advanced Robotics