16. Robot Applications
The nice definition of a robot by the Robot Institute of America is “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 is "Robots are one armed, blind, stupid, deaf, mute, and cannot feel and understand what they are doing".
• 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.
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.
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.
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.
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.
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.
Services - mail delivery, vacuuming, etc.
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,
- 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.
- spot welding - spot welding electrodes are clamped in place, and the weld is made. The robot allows many welds to be done.
- arc welding- continuous path robots are used to slowly track a path with a continuous rate, and with control of welding parameters.
- investment casting - robots can be used in the pick and place operations involved in making the molds.
- 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.
- press work- the robot handles loading parts into the press, and removing the resulting work pieces.
- 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.
- 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.
- foundry process- robots can be used for ladling materials, and preparation of molds.
- machine tools- robots can be used for loading and unloading machine tools, and material transfer systems.
- heat treatment process - parts can be loaded into the ovens, unloaded from the ovens, quenched and dried by robots.
- metal deburring - continuous path robots can be used to track rough edges with a compliant tool design.
- palletizing process - parts can be placed in boxes, or on skids in preparation for shipping. Most robots have program commands to support this.
- brick manufacture - a robot can be used for loading and unloading a kiln, and stacking bricks for shipping.
- 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.
16.0.1 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.
• These tasks are characterized by the need for,
- conformity to specified paths
• each robot may perform a variety of sub-assemblies
• requires a conveyor and inspection station
• A host computer must synchronize robot actions
• A bad part rejection function should be available
• 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.
16.0.4 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.
16.1 End Of Arm Tooling (EOAT)
• The best known universal gripper - the human hand
• End of arm tooling is typically purchased separately, or custom built.
16.1.1 EOAT Design
• Typical factors to be considered are,
pre- and post- processing geometry
coefficient of friction between gripper and part
maximum accelerations during motion
robot accuracy and repeatability
use of separate wear components
use of mounting plate on robot
gripper flexible enough to accommodate product design change
• The typical design criteria are,
- low weight to allow larger payload, increase accelerations, decrease cycle time
- minimum dimensions set by size of workpiece, and work area clearances
- widest range of parts accommodated using inserts, and adjustable motions
- rigidity to maintain robot accuracy and reduce vibrations
- maximum force applied for safety, and to prevent damage to the work
- power source should be readily available from the robot, or nearby
- maintenance should be easy and fast
- safety dictates that the work shouldn’t drop when the power fails
• Other advanced design points,
- 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.
- holding pressures/forces/etc are hard to control, try to hold parts with features or shapes
- compliance can help guide work into out-of-alignment conditions.
- sensors in the EOAT can check for parts not in the gripper, etc.
- the gripper should tolerate variance in work position with part alignment features
- gripper changers can be used to make a robot multifunctional
- multiple EOAT heads allow one robot to perform many different tasks without an EOAT change.
- *** Don’t try to mimic human behavior.
- design for quick removal or interchange of tooling by requiring a small number of tools (wrenches, screwdrivers, etc).
- provide dowels, slots, and other features to lead to fast alignment when changing grippers.
- use the same fasteners when possible.
- eliminate sharp corners/edges to reduce wear on hoses, wires, etc.
- allow enough slack and flexibility in cables for full range of motion.
- use lightweight materials, and drill out frames when possible.
- use hard coatings, or hardened inserts to protect soft gripper materials.
- examine alternatives when designing EOAT.
- the EOAT should be recognized as a potential bottleneck, and given extra design effort.
- use shear pins, and other devices to protect the more expensive components.
- consider dirt, and use sealed bearings where possible.
- move as much weight away from the tip of the gripper towards the robot.
16.1.2 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.
1. Physically mate with the part for a good grip
2. Apply enough force to the part to prevent slipping
- pivoting (often uses pivotal linkages)
- linear or translational movement (often uses linear bearings and actuators)
- miscellaneous - eg. bladder, diaphragm
184.108.40.206 - Vacuum grippers
• Suction cups can be used to grip large flat surfaces. The cups are,
- typically made of soft rubber or plastic
- 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.
- requires only one surface of a part to grasp
- a uniform pressure can be distributed over some area, instead of concentrated on a point
- many different types of materials can be used
- the maximum force is limited by the size of the suction cups
- positioning may be somewhat inaccurate
- time may be needed for the vacuum in the cup to build up
16.1.3 Magnetic Grippers
• Can be used with ferrous materials
- easy to control, requires a power supply, and a controller
- polarity can be reversed on the magnet when it is put down to reverse residual magnetism
- external power is not required
- a mechanism is required to separate parts from the magnet when releasing
- good for environments that are sensitive to sparks
- variation in part size can be tolerated
- ability to handle metal parts with holes
- requires only one surface for gripping
- can pick up the top sheet from a stack
- residual magnetism that remains in the workpiece
16.1.4 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.
16.1.5 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.
16.2 Advanced Topics
16.2.1 Simulation/Off-line Programming
• How a robot interacts with the environment makes it difficult to program off-line. To do this successfully, a complete simulation of the robot workspace is required.
• One excellent example of a simulation package is CIMStation by Silma. It allows full construction of the robots workspace, and subsequent testing.
• Examples of simulated operation in CIMStation are,
- kinematic and dynamic simulation
• The simulators available for the robots in the lab allow off-line programming and simulations.
16.4 Practice Problems
7. Suggest a type of robot suitable for the following tasks. Briefly explain your suggestion.
a) placing pallets on rack shelving
ans. cartesian - well suited to cartesian layout of shelves.
ans. scara - will work on a flat table well.
c) loading and unloading parts from an NC mill
ans. articulated - can easily move around obstructions.
8. Suggest a type of robot suitable for the following tasks. Briefly explain your suggestion.
a) a gas pump robot for placing the gas nozzle into the fuel tank.
b) for drilling holes in a printed circuit board.
3. We plan to use a pneumatic gripper to pick up a 4 by 8 sheet of glass weighing 40 lbs. Suggest a gripper layout and dimensions of the cups. State any assumptions.
4. A vacuum pump to be used in a robot vacuum gripper application is capable of drawing a negative pressure of 4.0 psi compared to atmospheric. The gripper is to be used for lifting stainless steel plates, each plate having dimensions of 15” by 35”, and weighing 52 lbs. Determine the diameter of the suction cups to be used for the gripper if it is decided to use two cups for greater stability. A factor of safety of 1.5 should be used in the computations.
5. Consider the following gripper design problems.
a) We plan to use a friction gripper to pick up a 50 lb iron plate. Suggest a gripper design and specify the force required.
b) Design an end effector, and describe the path planning approach for a robot unloading satellites from the space shuttle.
16.5 Laboratory - Robot Interfacing
Purpose: Basic robot interfacing
1. Develop a program that will put down and pick up balls at two different point for both robots.
2. Test both programs on-line.
3. Plan for a mode of robot operation either one robot can deposit a ball in a center pickup point, or at their own private pickup points. (Only a single ball will be used) Each robot will have a button connected to it. If the robot button and the other robot has the button, the robot will request the ball from other robot (using an I/O line). A list of the inputs and outputs is given below. Develop a simple diagram showing outputs and inputs to connect two robots and sensors. Develop a state diagram for the operation of both robots. Rewrite the robot programs from step 1 so that they will use the inputs.
input #1 ball in my private point
input #2 other robot wants ball
output #1 request ball from other robot
1. The instructor will discuss interfacing issues.
2. Each group should do each of the three parts below in turn.
1a. Test programs on the RV-M1, and add grippers/fixtures as required
1b. Test programs on the RT-3000, and add grippers/fixtures as required
1c. Connect and test the wiring for each of the robots and the interface
2. Integrate all of the equipment for the final task.
1. Individually developed programs for the robots.
2. Group programs for the robots and PLC.
16.6 Laboratory - Robot Workcell Integration
Purpose: Interfacing robots to workcells using PLCs.
1. Develop a program that will draw a square on both robots, and test them on-line.
2. Develop a program that will draw a circle on both robots, and test them on-line.
3. Develop ladder logic for a micrologix that will watch for inputs from both robots. The two inputs will indicate when either of the robot is using the drawing paper. The PLC will also have outputs connected to both robots. Each robot will have two inputs. One input will cause a circle to be drawn, the other will cause the square to be drawn. The PLC will use inputs from push-button switches to indicate when a circle or square is to be drawn. The ladder logic should share the work between the robots.
4. Combine and modify the robot programs so that they will interface to the PLC program.
1. Each group should do one of the tasks below,
1a. Enter and test the program on the RT-3000 robot. Use a voltage source to test the program.
1b. Enter and test the program on the RV-M1 robot. Use a voltage source to test the program.
1c. Enter and test the program on the micrologix.
2. Rotate between the stations until you have done all three modules.
3. Connect the PLC and two robots electrically, and enter and test the programs.
1. Individually developed programs for the robots and PLC.
2. Group programs for the robots and PLC.