Dispensing Profits with Dispensing Robots
by Bennett Brumson
, Contributing Editor
Robotic Industries Association Posted 07/19/2002
Dispensing is one of the more demanding and varied robotic applications due to the range of tasks demanded of it and its need for precision. From dispensing fluids to wire to tape, this application can be especially rewarding compared to traditional systems. A big key to success is knowing what is possible and how to properly justify the technology. It also helps to understand the subtle but often significant difference between coating and dispensing.
'Just about any fluid that can be pumped, including chocolate, can be dispensed robotically,' asserts John Burg, president of Meritage Manufacturing Solutions (MMS). Meritage is a robotic systems integrator from Warrenville, IL.
Precision and Placement
Robotic dispensing is similar yet quite different from coating applications. Coating applications, such as painting, are less exacting in both robot accuracy and part positioning. This is balanced by the fact that, unlike most painting workcells, dispensing applications rarely need to be explosion proof.
'There is a need to be more precise in dispensing over coating applications. For example, if a robot cuts a corner a small amount in a coating application, it won't be noticed too much,' said Steven Krotzer, an applications engineering manager at Staubli Unimation, Inc. of Duncan, SC. 'In dispensing, where a recess is involved, the robot must follow a very precise path. This can't vary too much, even by half a millimeter, since the sealant or adhesive might need to go into that groove or recess. In coating, the thickness of a coat of paint can vary, but this is difficult to measure and much less important than in dispensing adhesives or sealers.' Krotzer went on to explain that CAD data or machine vision systems are deployed to meet this requirement of precision.
'There is a need for higher accuracy in robotic paths in dispensing applications. In painting applications, the robot applicator spray pattern usually covers an area 20 to 25 cm wide,' said George Wilson, a proposal engineer at Behr Systems, Inc. 'For instance, if a paint robot's path varies by one or two millimeters, it is not seen and generally does not become an issue for quality. If a dispensing (seam sealing) robot's path varys by the same one or two millimeters, then the welded seam is left uncovered and susceptible to water leakage. For this reason, the robot controllers used for dispensing (seam sealing) operations use a more sophisticated algorithm enabling the robot to 'track' its path more accurately improving the quality of the applied media. Generally, seam sealing requirements include applying a bead of material 25 mm wide, 5 mm thick with +/- 0.5 mm deviation.' Behr Systems, Inc. is a division of the Durr Group and has its U.S. base in Auburn Hills, MI.
In addition to the need for the robot to be relatively accurate, dispensing workcells need to have precise part placement.
'Sealant applications aren't as forgiving as coating in placement of a part. The location of the part within the workcell needs to be much more precise,' declared Cory Thomas of CTA, Inc. 'Updating of the reference frame, whether this is done by vision, a touch probe, or another part indicating device, is needed to place the sealant bead.' Thomas is a technical sales engineer at CTA, which is headquartered in Madison, AL.
Thomas' opinion is echoed by Dan Crowe, vice president of consulting engineering at iRobotics. Crowe stated, 'Part positioning is very important. It is much like welding applications in that respect.' iRobotics is located in Indianapolis, IN, and is a robot manufacturer, a systems integrator and a provider of robotic software. Crowe also pointed out, 'If manufacturers have been doing something that they want automated, there is a tendency to have a robot do the process the same way as people did it. That isn't necessarily the best way to have a robot do it, since a different sequence might make more sense.'
As important as the need is for robot accuracy and precise part positioning, there are other challenges to a successful dispensing workcell. These entail maintaining a consistent speed of the robot, dispensing a steady amount of liquid material around corners, and properly dispensing high viscosity liquids.
'The most challenging variable in dispensing applications is maintaining a constant speed of the robot, especially around corners. If the speed of the robot changes, the flow rate of the material being dispensed must change accordingly to match it,' contends Staubli's Krotzer. The bead will vary in size with changes in speed of the dispensing head. Staubli's system has an automatic correction feature. When the robot is cutting a corner, the system's software compares deviation from where the bead was placed to where it should have been put. The software 'learns' from this and makes a correction the next time by measuring deviation from an ideal speed and trajectory.
'Bead size and flow rate of the material being applied will be disrupted if the speed of the robot changes. The ability to control the output of the material being dispensed relative to the velocity of the robot is crucial,' said Greg Tarrant, president Flow Robotic Systems. Flow Robotics, of Wixon, MI, is a system integrator.
To synchronize flow rates and robot speed, there are two choices for robot users: open- or closed-loop systems. Open loop workcells have an operator controlling the size of the bead by varying the speed of the robot. Here, there is a continual output of the material being dispensed. A closed loop system changes the flow rate relative to the speed of the robot. The advantage of a closed loop setup is the robot can stop or change the flow of the dispensed liquid when slowing for corners. Otherwise there will be an undesirable accumulation of material on corners. Open loop systems have the advantage being less expensive than closed loop structures, while they require more programming time. Closed loops generally have more consistency in flow rates than do systems with open loops.
Thick and Rich
Highly viscous fluids can pose a difficulty when dispensed robotically.
'The viscosity of sealants and adhesives change with temperature. Nylon-based materials require a heating system to make them flow more easily,' explained Thomas of CTA. 'A hot water jacket or heat tape is sometimes used to help high viscosity polyurethane sealants to flow, which require very high pressure equipment to dispense them. The challenge is more from a pumping standpoint. Some of the new polyurethane sealants have fillers for chip resistance or have granite in them to protect a car's rocker panel. These are very aggressive against pumps, causing more clogging and faster wear on equipment. This robotic application uses closed loop systems to compensate for changes in viscosity. The workcell either spreads the material fast enough or puts the material through a purge loop.'
Most robotic dispensing systems are in the automotive sector performing various tasks. These include gluing taillight lenses into place, sealing weld seams, applying sound dampening material to underbodies, and filling break fluid and wiper fluid reservoirs at the end of the assembly line.
Not Just for Liquids Anymore
Most robotic dispensing workcells disburse liquid materials, but there are a few that apply other manufacturing ingredients. Meritage Manufacturing Solutions has filled a market niche with a robotic tape dispensing system. John Burg recalled a project that had a six-axis pedestal robot laying tape.
'We wrapped tape on a solid bar that was 25 centimeters thick and 75 centimeters wide. It was similar to wrapping electrical tape on the ends of wires.'
Burg went on to describe another project still on the drawing board.
'We are currently looking at the robotic taping of composite materials for aerospace applications. A layer of tape is dispensed then another layer is put on at 90 degrees for increased strength. This plan is technically feasible but not yet cost effective enough to justify the use of robotics, although it will be soon. In robotic taping systems, it is critical to have an accurate path, the proper tension of the tape, the tape laid smoothly and without gaps, and applied with the proper pressure to assure adhesion.'
There are other robotic tape dispensing applications.
'CTA is now automating a system that will dispense pressure sensitive tape for bumper strips on cars. The tape is on a reel, with a pressure roller. A predetermined amount of tape is laid out and cut,' said CTA's Cory Thomas. 'This dispensing is quite a bit different from fluid dispensing, but the challenges are similar. Part placement is still key but this can be done with laser topography or with reference vision,' he said.
George Wilson of Behr also spoke of a tape dispensing application.
'A Mercedes-Benz plant in Tuscaloosa, AL, uses a system that robotically dispenses tape to fasten windshields to cars. That factory also uses robots to tape windows for the car's painting phase, which is removed afterwards.'
Flow Robotic Systems created a workcell that applied pinstriping. 'Flow has an application that dispenses pinstriping detailing on recreational vehicles exteriors. These pinstripes can be two-tone, with either thick or thin lines, and with varying colors. This is a very precise application and uses an open loop system,' claimed Bill Signorelli, a Flow Robotics general manager.
Wire is another item that can be dispensed with the use of robots.
'Staubli did a project that dispensed wire. The robot routed the wire, labeled it, stripped and cut it. Next, the robot inserted the wire into the proper place, then moved on to the next wire. This was for electrical components. A computer determined the best route for the wire to go. The system has a very specialized end effecter,' said Krotzer.
Krotzer recollected another wire dispensing project Staubli undertook.
'Staubli built a workcell using a similar process for dispensing wire into fluorescent light bulbs. In that application, the correct speed is critical. If the speed is to slow, too much wire will be dispensed. If the robot speed is too fast, the wire will be stretched.'
As new composite materials are introduced into manufacturing, it will be more cost effective to use robots for gluing and taping them together.
Taping applications have promise of being an area of growth in the robotics industry. MMS's John Burg spoke to this. 'Currently, no one knows the size of the 'universe' of robotic taping workcells. A hundred systems? A thousand systems? As of yet, there few market studies into the demand for robotic taping applications. Only if there is market demand will there be cost effective ideas. Taping has the potential to be in the top ten areas of robotic growth, but not in the top five.'
Other applications such as bathroom fixture production and biotechnology research have benefited from the use of robotic dispensing. Robots regularly provide the uptime and quality justifications that make it feasible to incorporate their use on a variety of dispensing tasks. Leading experts for robotic dispensing can be found within the ranks of the Robotic Industries Association, whose members are top companies in the industry, and serves as the only North American trade association dedicated solely to the development and knowledge transfer of best practices for robot applications such as dispensing and a host of other applications.