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Mechatronics: Ticket to Integrated Design
Increasingly, system integrators are being challenged to provide design solutions that feature a blend of engineering technologies
Lawrence D. Maloney, Contributing Editor -- Design News, June 1, 2008
When INVOTEC opened its doors 15 years ago, most customers wanted the engineering firm to provide specific “stand-alone” services.
“Some needed help with controls and others wanted mechanical design assistance or a solution to a manufacturing problem,” says Engineer John Hanna, company president and co-founder. “Nowadays, many customers want us to supply integrated systems, requiring us to marry several engineering and manufacturing disciplines to create one finished product.”
As a result, INVOTEC has built a reputation as a custom designer of automated, multiple-station manufacturing systems. Aimed at a whole range of tasks, including assembly, test, welding and laser-based materials removal, these systems serve applications ranging from automotive to materials handling to medical.
“The trend is clearly toward more sophisticated equipment with integrated inspection and error-proofing features,” says Hanna. “This often requires the marriage of vision systems, sensors and motion control — all integrated with a central PLC or PC control system.”
A Creative Combination
Case in point is an automated system INVOTEC recently designed to laser-weld a .015-inch-thick stainless-steel plate inside of a stainless-steel U-shaped channel. Positional accuracy: ±.004 inch. The welded components form a subassembly for a surgical device.
The machine, configured in a four-station dial index arrangement, incorporates a host of technologies: Pulsed Nd:YAG laser welder, laser power meter, fume extraction system, laser alignment and viewing optics, argon gas delivery system, servo-driven X-Y positioning stages, high-resolution vision system and lighting, programmable logic control, a variety of sensors, vacuum pick-ups, three-position pneumatic pick-and-place device, a vibratory feed system, cam-driven indexer, mechanical safety system and a laser safety system.
“One of the more significant design challenges we faced was in the overall layout of the system,” says Mechanical Engineer Daryl Greywitt, INVOTEC vice president and co-founder. “We needed to incorporate this wide array of mechanical and controls system components into a relatively small footprint.”
The machine's four assembly nests, enclosed by aluminum panels that include a laser-safe viewing window, are mounted to a four-position indexer unit. In the first position, a machine operator loads the stainless-steel channel component. Photo-electric sensors verify the component is properly seated in the nest prior to indexing. At station two, a vibratory bowl feeds the other component of this subassembly — the stainless-steel plate — onto a fixed nest position. A pneumatic pick-and-place unit with gripper and vacuum head then picks up the plate and places it into the channel. A spring-loaded clamp holds the plate in position as it is indexed to station three, where the actual welding operation takes place.
Featuring a Miyachi Unitek pulsed Nd:YAG laser, station three precisely integrates several mechanisms. First, a nest containing a channel and plate clamped together in proper alignment moves into the station beneath the weld head. A protective aluminum shroud is then lowered over the nest and engages in an interlocking slot in the dial plate. A magnetic coded safety switch assures the shroud is completely engaged before the laser is activated. When the shroud is in place, the argon cover gas flow is initiated, providing an inert atmosphere locally around the weld area to create a clean, contamination-free weld. The laser weld controller then initiates a preset weld schedule, creating the first spot weld. Then, the X-Y slide positions the weld head to the next location and repeats the operation until all five weld spots are completed. The argon flow is then stopped and the fume extraction system initiated. Finally, the protective shroud is raised. An Ophir LaserStar meter, mounted in this station, automatically monitors the power output of the laser to ensure proper output before welding.
Following the welding operation, the nest containing the subassembly indexes to station four for inspection and unloading. A high-resolution Cognex In-Sight vision sensor mounted above the nest acquires an image of the weld area and verifies weld spot location and weld quality. After an accepted vision check, a pick-and-place unit removes the completed assembly from the nest and places it in a tray.
Average cycle time for all these processes is just 25 sec per assembly, including load and unload.
Pursuit of Precision
A critical requirement of the overall system was to ensure accurate positioning of the plate relative to the inside of the channel, according to Project Engineer Mark Ruane, who adds that many components in the system were selected for their ability to contribute to this goal of positioning accuracy. The design team also needed a laser that could create a strong, cosmetically clean weld with minimal particle generation. They found the Miyachi Unitek LW series fiber laser offered easy integration, with full control of laser settings and functions in a low-maintenance package. In addition, the device's adjustable pulse-shaping feature delivers proper weld joint strength and quality.
From an electronics standpoint, Control Engineer Ed Baker says the project's toughest obstacle was designing a motion control system that could accurately position the laser heads over the weld locations, while still allowing simple adjustments, if necessary, by an operator. The weld spot size is approximately 0.045 inch, with some of the weld locations being only 0.055 inch wide.
To meet this control challenge, the team chose an Allen Bradley CompactLogix L43 PLC and two Allen Bradley SERCOS interface motion modules to position the laser welding head. The laser weld head is mounted to a Parker X-Y positioning slide controlled by the SERCOS interface drives. With this system, all the servo programming is done in the PLC, so a separate motion control software package is not required.
The design also incorporates an Allen Bradley Panelview Plus 600 operator interface. “With the Panelview, you can easily change weld positioning by simply entering the new position values, which are then written directly into the motion commands in the PLC,” says Baker.
Problem-Solving Tools
Engineering software also played a key role in the project. The INVOTEC engineers relied on SolidWorks 3D software for mechanical design and PDMWorks document management kept team members current on design revisions as the project unfolded.
“The interaction involving the component nest, the servo-positioned laser head and the laser guard sub-assemblies had to provide an interference-free, light-tight path for the laser welds,” says Ruane. “SolidWorks allowed us to create multiple configurations that showed the movement in each sub-assembly to complete the points on the weld path.”
In addition, each of these configurations had to be interrogated for interferences and isolation of light from the rest of the machine. The SolidWorks' 3D interference detection tool made the examination of each design configuration fast and accurate, adds Ruane.
Looking back on the project, the INVOTEC engineers emphasize the importance of close cooperation between mechanical and control engineers. “Selection of motion control hardware, for instance, needed to be a collaborative effort, because we had to optimize the solution based on accuracy, load bearing capability, adjustability and available space,” says Greywitt.
Overall time to complete this mechatronic design from initial concept through final system de-bug was approximately 22 weeks, including time for product design iterations. And the final design, according to INVOTEC engineers, provided the customer with a precise, highly productive automated alternative to a very complicated, labor-intensive manual operation.
