Get a Grip
New part-handling tools, techniques power microassembly
For the task of assembling small products, manufacturers must find a suitable tool for handling components measured in millimeters and, sometimes, microns. In many cases, the choice is a diminutive gripper that grasps parts with little fingers. A number of different grippers are available for microassembly processes.
When handling tiny parts during assembly, however, grippers aren’t the whole story. Alternatives include suction, sticking and mating techniques, as well as novel nontouch methods developed specifically to solve problems in moving the world’s smallest components.
To assemble parts with dimensions in the single-digit-millimeter range, manufacturers have a variety of handling options. Perhaps the most common tools for the job are pneumatically actuated grippers with fingers that move parallel to each other. Among the commercially available products are grippers from PHD Inc., Fort Wayne, Ind. PHD’s smallest pneumatic parallel grippers are for electronics assembly, such as placing capacitors on computer chips. Typically, these grippers weigh a couple of ounces, with total jaw travel in the 4mm range and grip force of less than 2 lbs.
“Usually, grip force isn’t a major factor in these applications because what we’re picking up can be described as a flake of pepper,” said John Ross, PHD’s director of market segment management.
A Schunk MWPG 20 device gripping a 0.7mm-dia. graphite pen lead insert in a 0.75mm-dia. hole, a mounting process that can be checked with a guppy camera through the middle hole of the gripper. Photo courtesy Schunk.
PHD offerings include new Series GRA microgrippers, which measure less than 0.4" wide, allowing users to place them close together in assembly processes. The closer the grippers, the more parts that can be picked simultaneously, shortening assembly time, Ross explained.
Another manufacturer of tiny pneumatic parallel grippers is Schunk GmbH & Co. KG. Based in Germany, the company operates in the U.S. as Schunk Inc., Morrisville, N.C. The specifications for Schunk’s MPG 10 gripper include a 10mm finger length, a 1mm stroke and 7 N of gripping force. In assembly applications, this compact gripper can handle small mechanical parts, such as bolts and component housings from 1mm to 5mm, according to Samuel Härer, an engineer at Schunk GmbH who works on microassembly products.
A new Schunk offering for microassembly is the MWPG, which measures 20mm in diameter and can produce up to 5 N of gripping force. The stroke of the gripper is normally 1mm but can be adjusted down to 0.5mm. An optical sensor in the changing head checks the position of the fingers to see whether parts are gripped or not.
The MWPG also has a central gripper through-bore measuring 7mm in diameter, which can feed parts to the gripper or monitor the working area with an image-processing system.
Härer cited a couple of microassembly applications where the MWPG’s hole comes in handy. In the optoelectronics industry, a manufacturer uses the hole to see if lenses are in the right position for assembly. In another application, the hole allows a camera to check whether gears are oriented at the correct angle before being mounted in a gearbox.
Unlike parallel grippers, such as the MPG and MWPG, so-called “angular grippers,” like the Schunk SWG, have fingers that move in an angular manner, which allows them to open wider than the fingers of a similarly sized parallel gripper, according to Härer. As a result, he said, “you can grip a part with a larger diameter, but you don’t need a larger gripper to do it.” The smallest version of the SWG has a finger length of 10mm and can grip parts with diameters between 5mm and 10mm.
The electric advantage
Among gripper manufacturers, there is a widespread desire to replace standard pneumatic actuation systems with electromechanical technology, according to Ross. The three main reasons are:
Less contamination risk. Electrical technology is cleaner than pneumatic, which gives off particulates that find their way onto computer chips and other products during assembly. Using electrical technology, “it’s easier to make a clean-room version of a gripper,” Ross said.
Better control. Electromechanical actuation systems can control grip force more precisely than a pressure regulator.
Longer product life. Electromechanical systems last longer than pneumatic ones because they eliminate the need for dynamic seals, which would be among the first components to fail in a pneumatic system.
A Schunk MPG 10 device connected with a micro change system, gripping a pen housing with a maximum diameter of 10mm. The diameter at which the device is contacting the housing is 2.5mm. Photo courtesy Schunk.
While it may be easy to appreciate the benefits of electromechanical microgrippers, Ross pointed out that creating such products is far from easy. “When you think of the size of these grippers, trying to put components like motors into them is very difficult,” he said. “But that’s where everybody’s trying to go.” Users want an electrical gripper that offers the same grip force, jaw travel and moment capacity as a pneumatic gripper and is the same in terms of size and weight.
Despite these challenges, electrically actuated microgrippers are on the market. For example, grippers sold by the German firm piezosystem jena feature a flexure design that’s actuated by a piezoelectric effect. This effect is “completely electric but nonmagnetic, so you don’t have any issues with magnetic fields,” which can cause problems for electronic components, said Jim Litynski, who runs the company’s U.S. subsidiary, piezosystems jena Inc. in Hopedale, Mass.
One piezoelectric gripper sold by the company has a V-groove for handling fibers. This device can grip a fiber 125µm to 600µm in diameter and place it into a fiber-optic switch, for example. The firm also sells a general-purpose piezoelectric gripper for handling small parts. It can be used for electronic assembly applications, such as aligning tiny semiconductor leads on a board.
The piezoelectric actuation system “gives you fine control over the gripping force so you’re not crushing the wires when you’re handling them,” Litynski said.
A PHD gripper for semiconductor manufacturing. Photo courtesy PHD.
Piezo fibergripper holding a bare fiber. Photo courtesy Piezosystem Jena.
A general-purpose, piezoelectrically actuated gripper. Photo courtesy Piezosystem Jena.
Electrically actuated grippers aren’t the best option for handling components in some microassembly applications. Advanced Micro Robotics LLC, which develops robotic arms for pick-and-place processes, often uses suction cups rather than grippers to move small parts. Measuring just a few millimeters in diameter, these cups have air-hose connections through which the suction effect is created, noted Ken Loewenthal, president of AMR, Dulles, Va.
Loewenthal maintains that configuring a suction cup assembly is much faster and cheaper than developing a gripper system to do the same job. For one, he said, the actuation mechanism for a suction cup system is simpler than that needed for a gripper. A standard vacuum pump will do—and not even a high-quality one is required. “All you need is a little negative vacuum pressure, and for $200 you can buy a low-cost vacuum pump from any scientific Web site to do the job,” he said.
Or you can dispense with vacuum pumps entirely, Loewenthal added, replacing them with in-house compressed air, which is available in most facilities where assembly operations take place. For about $30, he noted, a manufacturer can purchase a Venturi vacuum generator and connect it to an in-house air supply. When air traverses the Venturi tube, it creates a vacuum that produces the suction needed to pick up small parts.
“The cost of (using suction cups) is negligible, but the equipment is all industrial-strength,” he said. By contrast, he noted, the costs involved in purchasing a gripper mechanism, machining the fingers and attaching the device to a robot arm can be “thousands and thousands” of dollars.
Suction cups are among the many part-handling devices sold by De-Sta-Co. The Auburn Hills, Mich., company offers off-the-shelf devices that manufacturers can use to construct so-called “end effectors” for robotic arms that meet their specific assembly needs. For microassembly applications, flexible suction cups can be an attractive alternative to grippers, which can damage small, fragile parts, said Jason Kniss, De-Sta-Co’s global product manager for end effectors.
Kniss also pointed out that, unlike grippers, which grab parts by the sides, suction cups hold parts from the top. This is an advantage when parts are being placed into tight spaces with other components nearby. In such cases, gripper fingers might make unwanted contact with the surrounding components while trying to position a part, but suction cups “let you come in from above and drop the part into place.”
Another important pick-and-place consideration is whether a particular end effector can even pick up a part. According to Kniss, common suction cups can pick up many types of parts, while gripper jaws may have to be changed from job to job to accommodate different components.
On the other hand, not everything can be picked up with a suction cup. Small, irregularly shaped parts, for example, “are very hard to pick up properly with suction cups,” Loewenthal said. In addition, suction cup systems are dependent on proper orientation of parts before they’re picked up. With grippers, that may not be the case.
Grippers can also apply more force than a flexible suction cup, which allows them to perform more tasks. In microassembly, “the forces aren’t really high—maybe 10 or 20 lbs.—but you’re not going to get that with a suction cup,” Loewenthal said. “So, if you’re trying to push a pin into a press-fit hole, a suction cup isn’t going to work.”
Grippers also have the edge when it comes to placement precision. “When a gripper grips a part—provided that it grips the part properly—it tends to grip it very precisely,” Loewenthal said. As a result, “you can easily get precision and repeatability of 0.001" or less. With a suction cup, you’re not as likely to get that kind of precision.”
De-Sta-Co’s ARV (auto-release Venturi) is a single-line vacuum generator. Instead of having two air lines (one for vacuum and one for blow-off), the ARV uses a single air line for both functions. The ARV uses 60 percent less compressed air in operation than a standard vacuum generator. Photo courtesy De-Sta-Co.
To improve the placement accuracy of suction cups, systems developed by De-Sta-Co normally include locating features such as pins, according to Kniss. In electronics assembly, for example, locating pins on the miniature end effector would mate with receptacles on the board or board fixture to ensure accurate placement.
Moving the smallest parts
When part dimensions are measured in millimeters, gravity is the dominant force in pick-and-place operations. But when dimensions get into the neighborhood of 100µm, other forces take precedence, according to Dan Popa, associate professor of electrical engineering at the University of Texas at Arlington’s Automation & Robotics Research Institute.
These so-called “surface forces” might cause a tiny part to stick to a gripper finger rather than drop into place when the jaws open, according to Svetan Ratchev, professor of manufacturing engineering and head of the manufacturing division at the University of Nottingham (England).
“Quite often, the problem is not grasping small parts. Instead, it’s releasing the parts after they’ve been grasped,” he said.
To deal with the “sticky situations” caused by surface forces, some manufacturers apply antistiction coatings to grippers. These include zirconium-oxide and various polymer coatings, Popa said. Another strategy is to “precharge” grippers to combat electrostatic forces that can cause tiny parts to stick to gripper fingers, according to Ratchev.
Like grippers, vacuum-based systems can be used to pick and place parts with micron dimensions. But also like grippers, these systems start to experience adhesion-related problems when part dimensions shrink to around 100µm, Popa noted.
Ironically, Popa pointed out, manufacturers can actually use adhesion to their advantage when assembling microparts. An adhesion-based assembly technique requires preparing the surfaces involved in a pick-and-place operation so that parts will stick to the end effector when picked up and then stick to the substrate when dropped off.
Another microassembly technique features a protrusion, or “jammer” (a name coined by Popa’s group), inserted and locked into a mating feature in the part so that it can be picked up. Obviously, this jammer technique can only be used to move microparts that can incorporate features that will lock the jammer in place.
No contact required
All of the methods of moving micro-parts discussed so far require some kind of device to make contact with the parts. At the smallest scales, however, these methods are plagued by problems caused by gravity-defying surface forces. In addition, contact methods can damage and contaminate tiny parts.
“In the micro domain, we often deal with parts that we cannot physically touch,” Ratchev noted. For this reason, a number of researchers are working on noncontact methods of moving and positioning small parts. One of Ratchev’s students has developed such a method, based on airflow between tiny objects arranged in an array. Altering the gaps between the objects changes the airflow between them, resulting in rotational and translational motion that can accurately position the objects, Ratchev explained.
Other noncontact positioning options include vibrating tiny parts and even pulling on them with laser beams, the latter of which works something like the tractor beams in “Star Trek.”
On the downside, Popa noted that such noncontact methods generally require more-complicated control software because the methods are more complex than conventional pick-and-place operations. The forces that can be applied by noncontact methods are also smaller than those applied by contact methods and may be insufficient for some assembly applications.
Today, noncontact assembly methods—and others aimed at microparts—appear to be confined mainly to research laboratories. To make them commercially viable, the techniques need to be industrialized, but that’s not happening yet, according to Ratchev. At the smallest scale, he said, “microassembly is a growing domain but not yet big enough to justify significant investment.”
However, technological advances in microgrippers as well as suction, sticking and mating techniques are providing new ways of moving the world’s smallest components. As micromanufacturers develop a broader range of products, they will need creative ways to move and assemble them as well. µ -----------------------------------------------------------------------------
Advanced Micro Robotics LLC
piezosystem jena Inc.
Automation & Robotics Research Institute University of Texas at Arlington
Nottingham Innovative Manufacturing Research Center University of Nottingham