INDUSTRIAL ROBOTICS

39.1 INTRODUCTION · The nice definition (by the Robot Institute of America): "A robot is a reprogrammable

multifunctional manipulator designed to move material, parts, tools, or specialized devices through variable programmed motions for the performance of a variety of tasks"

 · The not-so-nice definition: Robots are one armed, blind, stupid, deaf, mute, and cannot

feel and understand what they are doing.

39.1.1 Basic Terms Links and Joints - Links are the solid structural members of a robot, and joints are the

movable couplings between them. Degree of Freedom (dof) - Each joint on the robot introduces a degree of freedom. Each

dof can be a slider, rotary, or other type of actuator. Robots typically have 5 or 6 degrees of freedom. 3 of the degrees of freedom allow positioning in 3D space, while the other 2or 3 are used for orientation of the end effector. 6 degrees of freedom are enough to allow the robot to reach all positions and orientations in 3D space. 5 dof requires a restriction to 2D space, or else it limits orientations. 5 dof robots are commonly used for handling tools such as arc welders.

 Orientation Axes - Basically, if the tool is held at a fixed position, the orientation

determines which direction it can be pointed in. Roll, pitch and yaw are the common orientation axes used. Looking at the figure below it will be obvious that the tool can be positioned at any orientation in space. (imagine sitting in a plane. If the plane rolls you will turn upside down. The pitch changes for takeoff and landing and when flying in a crosswind the plane will yaw.)

 

 

Position Axes - The tool, regardless of orientation, can be moved to a number of positions in space. Various robot geometries are suited to different work geometries. (more later)

 Tool Centre Point (TCP) - The tool centre point is located either on the robot, or the tool.

Typically the TCP is used when referring to the robots position, as well as the focal point of the tool. (e.g. the TCP could be at the tip of a welding torch) The TCP can be specified in cartesian, cylindrical, spherical, etc. coordinates depending on the robot. As tools are changed we will often reprogram the robot for the TCP.

 

 Work envelope/Workspace - The robot tends to have a fixed, and limited geometry. The

work envelope is the boundary of positions in space that the robot can reach. For a cartesian robot (like an overhead crane) the workspace might be a square, for more sophisticated robots the workspace might be a shape that looks like a `clump of intersecting bubbles'.

 

 Speed - refers either to the maximum velocity that is achievable by the TCP, or by

individual joints. This number is not accurate in most robots, and will vary over the workspace as the geometry of the robot changes (and hence the dynamic effects). The number will often reflect the maximum safest speed possible. Some robots allow the maximum rated speed (100%) to be passed, but it should be done with great care.

 Payload - The payload indicates the maximum mass the robot can lift before either failure

of the robots, or dramatic loss of accuracy. It is possible to exceed the maximum payload, and still have the robot operate, but this is not advised. When the robot is accelerating fast, the payload should be less than the maximum mass. This is affected by the ability to firmly grip the part, as well as the robot structure, and the actuators. The end of arm tooling should be considered part of the payload.

 Repeatability - The robot mechanism will have some natural variance in it. This means

that when the robot is repeatedly instructed to return to the same point, it will not always stop at the same position. Repeatability is considered to be +/-3 times the standard deviation of the position, or where 99.5% of all repeatability measurements fall. This figure will vary over the workspace, especially near the boundaries of the workspace, but manufacturers will give a single value in specifications.

 Accuracy - This is determined by the resolution of the workspace. If the robot is

commanded to travel to a point in space, it will often be off by some amount, the maximum distance should be considered the accuracy. This is an effect of a control system that is not necessarily continuous.

 Settling Time - During a movement, the robot moves fast, but as the robot approaches the

final position is slows down, and slowly approaches. The settling time is the time required for the robot to be within a given distance from the final position.

 Control Resolution - This is the smallest change that can be measured by the feedback

sensors, or caused by the actuators, whichever is larger. If a rotary joint has an encoder that measures every 0.01 degree of rotation, and a direct drive servo motor is used to drive the joint, with a resolution of 0.5 degrees, then the control resolution is about 0.5 degrees (the worst case can be 0.5+0.01).

 Coordinates - The robot can move, therefore it is necessary to define positions. Note that

coordinates are a combination of both the position of the origin and orientation of the axes.

 

 

 

  

39.1.2.1 - Accuracy and Repeatability · The accuracy and repeatability are functions of,

1. - resolution- the use of digital systems, and other factors mean that only a limited number of positions are available. Thus user input coordinates are often adjusted to the nearest discrete position.

2. - kinematic modeling error - the kinematic model of the robot does not exactly match the robot. As a result the calculations of required joint angles contain a small error.

3. - calibration errors - The position determined during calibration may be off slightly, resulting in an error in calculated position.

4. - random errors - problems arise as the robot operates. For example, friction, structural bending, thermal expansion, backlash/slip in transmissions, etc. can cause variations in position.

 · Accuracy,

1. · "How close does the robot get to the desired point" 2. · This measures the distance between the specified position, and

the actual position of the robot end effector. 3. · Accuracy is more important when performing off-line

programming, because absolute coordinates are used.  · Repeatability

1. · "How close will the robot be to the same position as the same move made before"

2. · A measure of the error or variability when repeatedly reaching for a single position.

3. · This is the result of random errors only 4. · repeatability is often smaller than accuracy.

 · Resolution is based on a limited number of points that the robot can be commanded to

reach for, these are shown here as black dots. These points are typically separated by a millimeter or less, depending on the type of robot. This is further complicated by the fact that the user might ask for a position such as 456.4mm, and the system can only move to the nearest millimeter, 456mm, this is the accuracy error of 0.4mm.

 

 · In a perfect mechanical situation the accuracy and control resolution would be

determined as below, 

 · Kinematic and calibration errors basically shift the points in the workspace resulting in

an error `e'. Typically vendor specifications assume that calibration and modeling errors are zero.

 

 · Random errors will prevent the robot from returning to the exact same location each

time, and this can be shown with a probability distribution about each point.  

 

 · The fundamental calculations are, 

  

39.2.1 Basic Robotic Systems · The basic components of a robot are,

1. Structure - the mechanical structure (links, base, etc). This requires a great deal of mass to provide enough structural rigidity to ensure minimum accuracy under varied payloads.

2. Actuators - The motors, cylinders, etc. that drive the robot joints. This might also include mechanisms for a transmission, locking, etc.

3. Control Computer - This computer interfaces with the user, and in turn controls the robot joints.

4. End of Arm Tooling (EOAT) - The tooling is provided be the user, and is designed for specific tasks.

5. Teach pendant - One popular method for programming the robot. This is a small hand held device that can direct motion of the robot, record points in motion sequences, and begin replay of sequences. More advance pendants include more functionality.

 

  

39.2.2 Types of Robots  · Robots come in a wide variety of shapes, and configurations. · The major classes of robots include,

1. arms - fixed in place, but can reach and manipulate parts and tools 2. mobile - these robots are free to move

39.2.2.1 - Robotic Arms · Typical joint types are,

1. Revolute - rotary joints often driven by electric motors and chain/belt/gear transmissions, or by hydraulic cylinders and levers.

2. Prismatic - slider joints in which the link is supported on a linear slider bearing, and linearly actuated by ball screws and motors or cylinders.

 · Basic configurations are,

1. Cartesian/Rectilinear/Gantry - Positioning is done in the workspace with prismatic joints. This configuration is well used when a large workspace must be covered, or when consistent accuracy is expected from the robot.

2. Cylindrical - The robot has a revolute motion about a base, a prismatic joint for height, and a prismatic joint for radius. This robot is well suited to round workspaces.

3. Spherical - Two revolute joints and one prismatic joint allow the robot to point in many directions, and then reach out some radial distance.

4. Articulated/Jointed Spherical/Revolute - The robot uses 3 revolute joints to position the robot. Generally the work volume is spherical. This robot most resembles the human arm, with a waist, shoulder, elbow, wrist.

5. Scara (Selective Compliance Arm for Robotic Assembly) - This robot conforms to cylindrical coordinates, but the radius and rotation is obtained by a two planar links with revolute joints.

 

 

 

 

 

  

39.3.1 Overview · Unlike many machines, robots are easy to imagine performing tasks, because of their

similarity to the human form. This has caused many companies to adopt robots without properly assessing what their strengths and weaknesses are.

 · The early days of experimentation lead to many failed applications, as well as some

notable successes. · A useful dichotomy is,

1. Point-to-Point - A robot that typically only has 2 (or very few) possible positions. These are good for pick and place type operations, and they are often constructed with pneumatic cylinders.

2. Manipulation - A robot that assembles, or moves parts requires good end of path motion, but does not require as much accuracy in the middle of the path. A higher speed between path endpoints is often desired.

3. Path Tracking - When arc welding, gluing, etc. the robot must follow a path with high accuracy, and constant speed. This often results in slower motion, and more sophisticated control software.

4. Operating - The robot will be expected to apply forces to perform work at the end of the tool, such as doing press fits. While the demands for these robots is essentially the same, they must be capable of handling the higher forces required when in working contact with the work.

5. Telerobotics - Acts as a remote extension of human control, often for safety or miniaturization purposes. In these cases the robots often mimic the human form, and provide some forms of physical feedback.

6. Services - mail delivery, vacuuming, etc. 7. Biomedical - prosthetic and orthotic devices.

 · The number of degrees of freedom of the robot should be matched to the tasks. · Note: 5 d.o.f. robots will allow the tool to reach all points in space if the tool has an axis

of symmetry. For example, a welding torch flame has a symmetrical axis. · Some commercial applications that have been done with robots are,

1. - die casting - used for unloading parts from dies, quenching parts, and trimming them with a trim press. The robot may also be used to put inserts into the die.

2. - spot welding - spot welding electrodes are clamped in place, and the weld is made. The robot allows many welds to be done.

3. - arc welding- continuous path robots are used to slowly track a path with a continuous rate, and with control of welding parameters.

4. - investment casting - robots can be used in the pick and place operations involved in making the molds.

5. - forging- a robot can be used to precisely position the work under the impact hammer, freeing a worker from the handling hot heavy work pieces.

6. - press work- the robot handles loading parts into the press, and removing the resulting work pieces.

7. - spray painting- a very popular application in which the robot sweeps the paint head across the surface to deposit a spray. This process has been coupled with electrostatics to improve efficiency and distribution.

8. - plastic molding - they can be used for loading the hoppers, and unloading the parts. This is most effective when the parts are hard to handle.

9. - foundry process- robots can be used for ladling materials, and preparation of molds.

10. - machine tools- robots can be used for loading and unloading machine tools, and material transfer systems.

11. - heat treatment process - parts can be loaded into the ovens, unloaded from the ovens, quenched and dried by robots.

12. - metal deburring - continuous path robots can be used to track rough edges with a compliant tool design.

13. - palletizing process - parts can be placed in boxes, or on skids in preparation for shipping. Most robots have program commands to support this.

14. - brick manufacture - a robot can be used for loading and unloading a kiln, and stacking bricks for shipping.

15. - glass manufacture - a robot can handle the breakable glass with a wide EOAT that prevents sagging, etc. The robot can also be used for grinding edges.

  

39.3.2 Spray Painting and Finishing  · Air spraying - air under pressure causes the paint to atomize and be propelled to the

article to be painted · Airless spraying - finishing materials, such as paint, are sprayed under considerable

hydraulic pressure through a fixed orifice, which causes the paint to be atomized directly without the need for air.

 · Electrostatic spraying - Atomized particles (paint or powder droplets) are

electrostatically charged. These are attracted to the object being sprayed by the applied electrostatic field. Considerable material savings are achieved since very little of the sprayed material bypasses the object and is lost. Objects being sprayed are kept at a ground potential to achieve a large electrostatic field.

 · Heating of materials - paint decreases in viscosity when heated and can be sprayed with

lower pressures. Less solvent is required and there is less overspray of paint. Heating may be used with any of the preceding systems

 · Air spraying and electrostatic spraying are the most common methods of application for

paints, enamels, powders, and sound absorbing coatings.  

39.3.3 Welding  · These tasks are characterized by the need for,

1. - smooth motion 2. - conformity to specified paths 3. - consistent tool speed

39.3.4 Assembly  · General concepts are,

1. · one or more robots 2. · each robot may perform a variety of sub-assemblies 3. · requires a conveyor and inspection station 4. · A host computer must synchronize robot actions 5. · A bad part rejection function should be available 6. · An organized output should be used, e.g. pallets, or shipping

crates.  · These tasks are common, but face stiff competition from fixed automation and manual

labor.  

39.3.5 Belt Based Material Transfer · When a robot is used in a workcell, the raw part is delivered in, worked on, and then

moved out. This can be done using moving belts, etc. · Parts are placed directly on the belt, or placed on pallets first. · Belts can travel in straight paths, or in curved paths if flexible belt link designs are used. · If straight belts are used, transfer points can be used at the end to change part/pallet

direction · When pallets are used, there is a fixture on top designed to hold the part in an accurate

position so that robots and other equipment will be able to locate the part within some tolerance.

 · Vision systems may be necessary if part orientation cannot be fixed.  

39.4 END OF ARM TOOLING (EOAT) · The best known universal gripper - the human hand · Useful classifications are,

1. - Grippers 1. - multiple/single 2. - internal/external

2. - Tools 1. - compliant 2. - contact 3. - non-contact

 · End of arm tooling is typically purchased separately, or custom built. 

39.4.1 EOAT Design · Typical factors to be considered are,

1. Workpiece to be handled 1. part dimensions 2. mass 3. pre- and post- processing geometry 4. geometrical tolerances 5. potential for part damage

2. Actuators 1. mechanical 2. vacuum 3. magnet 4. etc.

3. Power source of EOAT 1. electrical 2. pneumatic 3. hydraulic 4. mechanical

4. Range of gripping force 1. object mass 2. friction or nested grip 3. coefficient of friction between gripper and

part 4. maximum accelerations during motion

5. Positioning 1. gripper length 2. robot accuracy and repeatability 3. part tolerances

6. Maintenance 1. number of cycles required 2. use of separate wear components 3. design for maintainability

7. Environment 1. temperature 2. humidity 3. dirt, corrosives, etc.

8. Temperature protection 1. heat shields 2. longer fingers 3. separate cooling system 4. heat resistant materials

9. Materials 1. strong, rigid, durable 2. fatigue strength 3. cost and ease of fabrication 4. coefficient of friction 5. suitable for environment

10. Other points 1. interchangeable fingers 2. design standards 3. use of mounting plate on robot 4. gripper flexible enough to accommodate

product design change  

· The typical design criteria are,1. - low weight to allow larger payload, increase accelerations,

decrease cycle time 2. - minimum dimensions set by size of workpiece, and work area

clearances 3. - widest range of parts accommodated using inserts, and adjustable

motions 4. - rigidity to maintain robot accuracy and reduce vibrations 5. - maximum force applied for safety, and to prevent damage to the

work 6. - power source should be readily available from the robot, or

nearby 7. - maintenance should be easy and fast 8. - safety dictates that the work shouldn't drop when the power fails

  · Other advanced design points,

1. - ensure that part centroid is centered close to the robot to reduce inertial effects. Worst case make sure that it is between the points of contact.

2.  

1. - holding pressures/forces/etc are hard to control, try to hold parts with features or shapes

2.  

1. - compliance can help guide work into out-of-alignment conditions.

2. - sensors in the EOAT can check for parts not in the gripper, etc.

3. - the gripper should tolerate variance in work position with part alignment features

4. - gripper changers can be used to make a robot multifunctional 5. - multiple EOAT heads allow one robot to perform many different

tasks without an EOAT change. 6. - *** Don't try to mimic human behavior. 7. - design for quick removal or interchange of tooling by requiring a

small number of tools (wrenches, screwdrivers, etc). 8. - provide dowels, slots, and other features to lead to fast alignment

when changing grippers. 9. - use the same fasteners when possible. 10. - eliminate sharp corners/edges to reduce wear on hoses, wires, etc. 11. - allow enough slack and flexibility in cables for full range of

motion. 12. - use lightweight materials, and drill out frames when possible. 13. - use hard coatings, or hardened inserts to protect soft gripper

materials. 14. - examine alternatives when designing EOAT. 15. - the EOAT should be recognized as a potential bottleneck, and

given extra design effort. 16. - use shear pins, and other devices to protect the more expensive

components. 17. - consider dirt, and use sealed bearings where possible. 18. - move as much weight away from the tip of the gripper towards

the robot.   

39.4.2 Gripper Mechanisms  · A gripper is specifically EOAT that uses a mechanical mechanism and actuator to grasp

a part with gripping surfaces (aka fingers) · Quite often gripper mechanisms can be purchases, and customized fingers attached. · Fingers are designed to,

1. 1. Physically mate with the part for a good grip

2. 2. Apply enough force to the part to prevent slipping  

 · Movements of the fingers

1. - pivoting (often uses pivotal linkages) 2. - linear or translational movement (often uses linear bearings and

actuators)  · Typical mechanisms

1. - linkage actuation 2. - gear and rack 3. - cam 4. - screw 5. - rope and pulley 6. - miscellaneous - eg. bladder, diaphragm

 

 

 

 

 

39.4.2.1 - Vacuum grippers · Suction cups can be used to grip large flat surfaces. The cups are,

1. - typically made of soft rubber or plastic 2. - typically round, or oval shapes

 · A piston operated vacuum pump (can give a high vacuum), or a venturi valve (simpler)

can be used to generate the vacuum. · The surfaces should be large, smooth, clean. · The force of a suction cup depends on the effective area of the vacuum and the

difference in the vacuum, and air pressures. 

 

· e.g. 

 · Advantages,

1. - requires only one surface of a part to grasp 2. - a uniform pressure can be distributed over some area, instead of

concentrated on a point 3. - the gripper is light weight 4. - many different types of materials can be used

 · Disadvantages,

1. - the maximum force is limited by the size of the suction cups 2. - positioning may be somewhat inaccurate 3. - time may be needed for the vacuum in the cup to build up

  

39.4.3 Magnetic Grippers  · Can be used with ferrous materials · Electromagnets,

1. - easy to control, requires a power supply, and a controller 2. - polarity can be reversed on the magnet when it is put down to

reverse residual magnetism  

· Permanent magnets,1. - external power is not required 2. - a mechanism is required to separate parts from the magnet when

releasing 3. - good for environments that are sensitive to sparks

 · Advantages,

1. - variation in part size can be tolerated 2. - ability to handle metal parts with holes 3. - pickup times fast 4. - requires only one surface for gripping 5. - can pick up the top sheet from a stack

 · Disadvantages,

1. - residual magnetism that remains in the workpiece 2. - possible side slippage

 

 

39.4.3.1 - Adhesive Grippers

39.4.3.1 - Adhesive Grippers  · Can handle fabrics and other lightweight materials · These grippers are basically a sticky surface on the end of the robot 

· As the adhesive gripper is repeatedly used, it loses stickiness, but a tape roll can be used to refresh the sticky surface.

  

39.4.4 Expanding Grippers · Some parts have hollow cavities that can be used to advantage when grasping. · A bladder can be inserted into a part, and then inflated. This forms a friction seal

between the two, and allows manipulation. When done the pressure is released, and the part freed.

 

 · Expanding grippers can also be used when gripping externally. 

  

39.4.5 Other Types Of Grippers  · Most grippers for manipulation are sold with mounts so that fingers may be removed,

and replaced. · Gripper fingers can be designed to reduce problems when grasping.