Dispensing With Six-Axis Robots
If you've ever applied caulk around the edge of a bathtub, you know how difficult it is to produce a smooth, even bead. You need a steady hand, constant tube speed, and even pressure on the trigger. Inevitably, you need the help of a Popsicle stick, putty knife or the ever-practical finger.
Now imagine trying to lay a bead of flange sealant along the edge of a car door. The height, width and location of the bead must be accurate to within a millimeter. If not enough sealant is applied, or if it's applied in the wrong spot, you'll get an inadequate seal. Applying too much wastes material and results in extra work to clean up the overflow. If that's not challenging enough, you also have to finish one door every 40 seconds.
Popsicle sticks are not an option. This is a job for a six-axis robot. With their agility, speed and long reach, six-axis robots are ideal for dispensing adhesives on long, wide objects with undulating contours. A six-axis robot can apply a continuous bead of adhesive to a confined area, like the rear window frame of a car, while keeping the dispense nozzle at a consistent angle. Such a task would be difficult for a human and impossible for a four-axis Cartesian or SCARA robot.
The real advantage of articulated robots over SCARA robots is their ability to handle complex part geometries. The SCARA is fine if you're dispensing on a single plane, but if there's a contour to the part, which requires you to change the orientation of the dispense tip, you need a six-axis robot.
In the automotive industry, six-axis robots dispense a variety of one- and two-part materials, including adhesives, formed-in-place gaskets, lubricants, sealants, sound deadeners and structural foams. Six-axis robots can even be rigged to apply tape. Six-axis robots apply materials not just for the body and frame, but also for subassemblies, such as headlights, taillights, filters, batteries and ignition systems. One supplier uses a six-axis robot equipped with a spray valve to apply dots of color to leaf springs for identification purposes.
Automakers aren't the only assemblers that use six-axis robots for dispensing. For example, window and door manufacturers use six-axis robots to apply adhesives and gaskets. Appliance assemblers employ six-axis robots to dispense adhesives, gaskets and insulation. And, manufacturers of plumbing fixtures use six-axis robots to apply adhesives for assembling sinks.
Flexibility is another advantage of using six-axis robots for dispensing. The one or two additional axes of motion enable the robot to handle any part geometry that may be around the corner.
Outfitting the Robot
Assemblers have two options for dispensing with six-axis robots. Most often, the dispenser is mounted to the robot. The metering equipment is located on or near the robot, and the material is transferred through one or more hoses to a valve attached to the robot's end-effector.
Alternatively, the dispenser can be mounted in a fixed position, 5 to 12 feet off the ground. Instead of moving the valve above the part, the robot moves the part beneath the valve. This configuration enables the robot to move the part through a series of process steps. For example, the robot could pick up a part from a conveyor, pass it beneath the dispenser, present it to a resistance welder, and place it on a second conveyor.
"Having the robot hold the part can shorten cycle time in many cases," says David Mandeville, director of marketing for Sealant Equipment and Engineering Inc. "If the robot holds the dispenser, it usually can't do can additional operation. You can't do both welding and dispensing with one robot."
However, assemblers can mount two valves to the same robot, enabling one machine to dispense two materials or two bead patterns. Another possibility is to have two robots work in tandem: One holds the dispenser, while the second holds the part.
Responsibility for marrying robotic and dispensing technologies varies, but the task usually falls to the systems integrator or the manufacturer of the dispensing equipment. However, this may change in the near future, as robot suppliers introduce application-specific robots. For example, Motoman has developed a six-axis robot specifically for arc welding. The welding cables travel inside the upper arm of the robot, which improves accessibility to the parts and prolongs the life of the cables.
Quality Control Issues
A number of variables are important when dispensing with six-axis robots, including material viscosity, dispensing pressure, robot speed, the dispense path and pattern, the precision of the parts and fixtures, and the height of the nozzle above the workpiece.
Because the material must travel a considerable distance to get from the reservoir to the part, assemblers need to keep the viscosity of the material consistent, says Mandeville. For example, gasketing material inside a 55-gallon drum is often more viscous on a Monday morning than it is later that afternoon, after it has warmed up. As a result, the feed hoses and reservoir may need to be heated or insulated to isolate the system from swings in ambient temperature.
The speed of the robot along the dispense path is also critical, says Jim Victoria, application development manager with Nordson EFD Inc. To maintain a consistent bead, the robot controller and the dispenser controller must work together seamlessly. At the start of the dispense cycle, there's often a delay between when the dispenser is actuated and when the material actually starts moving. That delay can cause a gap at the start of the bead, if the robot begins moving before the material does. Similarly, if the robot pauses a split-second before starting to move, there could be a surge of material at the start of the bead. At the conclusion of the dispense cycle, a delay between when the dispenser stops and when the material stops can produce a blob at the end of the bead.
"When drawing a bead, you want to start the robot milliseconds before initiating the dispense cycle, to eliminate any surge of material," explains Victoria. "When you turn the dispense cycle off, you want to continue movement of the robot so that you don't end up with a ball at the end of the bead."
As the robot moves down the bead path, it can accelerate or decelerate just as a race car driver slows down in the curves and speeds up on the straightaways. If the robot slows to take a sharp corner, the dispense system must restrict the flow rate to maintain a consistent bead. When the robot accelerates, the system boosts flow. A servo-controlled metering system is the key. "The robot controller takes control of the metering system motor as if it were a seventh axis," says Mandeville.
Another important factor when dispensing with six-axis robots is the height of the nozzle above the workpiece, says Brad Trees, sales representative for welding and process monitoring products with Precitec Inc. If the standoff height is too large, the bead can swirl and stray from the desired path. If the standoff height is too small, the material oozes out from the nozzle, accumulating ahead of the tip and producing a flat, misshapen bead. Worse, the nozzle could crash into the workpiece, damaging the part or the dispenser.
There are several ways to avoid this problem. One option is to tighten the tolerances for locating the parts in their fixtures. However, this can increase cycle time and raise the cost of both the parts and the fixtures.
Another way to ensure consistent standoff height is with an inductive distance sensor. Attached directly to the dispense nozzle, the sensor relies on changes in electric current to measure its height above any metallic object. This positional information is relayed to the robot controller, enabling the robot to maintain the correct standoff height to within ±0.2 millimeter, explains Trees.
A third solution to the standoff height problem is the streamed bead process developed by Sealant Equipment & Engineering. In most dispensing applications, the nozzle is positioned a few millimeters above the workpiece. However, with the streamed bead process, the nozzle is located 1 to 2 inches above the workpiece. Under high pressure, the material is ejected from the nozzle in a coherent stream.
"That allows you to loosen up your tooling requirements and make up for variances in the parts," says Mandeville. "In most dispensing applications, if the part moves a few thousandths of an inch in the fixture, the bead will get smashed or become a squiggle. But, if you're off the deck an inch and shooting material across that gap, the location of the parts is not as critical."
Depending on the material rheology and fluid pressure, the streaming process can produce a smooth, flat ribbon of material or an oval-shaped bead.
Machine vision is another way to ensure that material is dispensed accurately. The camera can be mounted in a fixed position above the dispensing station, or for greater flexibility, to the robot arm itself. When a part arrives at the dispensing station, the vision system determines the exact position of various reference points, and adjusts the dispense path accordingly.
Quality control doesn't end once the material is on the part. Many assemblers inspect the workpiece after dispensing to ensure that the right amount of material has been deposited in the right locations.
The Glue Bead Monitor from Precitec does just that-as the material is dispensed, says Trees. The heart of the system is a small vision sensor mounted on the robot arm behind the dispense valve. The system measures the position, width, height and cross-sectional area of the bead using laser triangulation. A laser diode onboard the camera projects a laser beam horizontally across the bead. The reflected light is then analyzed to get the bead measurements.
The system is taught the quality control parameters for the bead by scanning a known good part. The engineer then sets the upper and lower limits for deviation from the standard.
Robots Assemble Headlights
Because of its ability to create a hermetic seal, adhesive bonding is the method of choice for assembling automotive headlights, which must be impervious to the elements.
However, for bonding to work, two things have to happen. First, the adhesive must be able to stick to the plastic. Adhesives don't bond well to some plastics with low surface energy. Second, the right amount of adhesive must be applied in the right areas. If too little adhesive is applied, the headlight will not have an adequate seal. If too much is applied, the excess will ooze over the sides, causing cosmetic defects and wasting material.
These were the challenges facing Reinhardt-Technik GmbH & Co., a supplier of meter, mix and dispense equipment in Kierspe, Germany. Reinhardt was hired to build a robotic cell to apply a thick two-part silicone adhesive to headlight housings.
The cell's workhorses are two six-axis robots from Kuka Robotics Corp. One robot, a KR-6, applies a plasma treatment to the housings to improve their bondability. A second, larger robot, a KR-30, dispenses the adhesive. The KR-6 can carry a payload of 6 kilograms and has a maximum reach of 1.6 meters. The KR-30 can carry a 30-kilogram payload and has a maximum reach of 2 meters.
An operator loads the housings onto pallets, which are transported via conveyor into the cell. Before a pallet reaches the first station, a sensor determines whether a housing is present, and if so, whether it is a left or right headlight. This data is then sent to the robot controller.
At the first station, the KR-6 uses a torch nozzle to spray ionized air into a groove that runs around each housing. This increases the surface tension of the plastic and enables the adhesive to bond better.
At the next station, the KR-30 dispenses the silicone into the groove. Because the groove is slanted in most places, this application could only be accomplished by a six-axis robot. While moving around the component, the robot must hold the nozzle perpendicular to the bottom of the groove at all times. In any other position, the downward motion of the nozzle would ruin the bead.
Reinhardt installed the dispenser's metering unit on the fourth axis of the robot, while the mixer and nozzle were located on the sixth axis. This shortened the supply lines and ensured that the adhesive could be delivered at a consistent velocity. This is critical for precise dispensing.
After the adhesive is applied, the pallet exits the cell, where an operator fits a polycarbonate lens to the housing and unloads the assembly.
The cell outputs five headlights per minute. In addition, the cell accommodates multiple headlight models. Changeover is accomplished by swapping pallets and changing the robot program.
For more information on six-axis robots, call Kuka at 586-569-2082, visit www.kukarobotics.com.