Getting a Grip on Small Parts
Actuators, grippers, vacuum cups and other end-of-arm tooling often take a back seat to sexier industrial robot components, such as controllers and vision systems. However, these devices are an essential part of any robotic application, especially when small parts are involved.
Today, small components are a fact of life for manufacturing engineers. Electronic subassemblies, medical devices and consumer products are tinier than ever. Ever since the first mass-produced transistors appeared 40 years ago, there’s been a continual push toward smaller parts and components.
For instance, consider the ubiquitous cell phone. When Motorola Inc. (Schaumburg, IL) unveiled the world’s first commercial, portable cellular telephone 20 years ago, it weighed 28 ounces and measured 13 by 1.75 by 3.5 inches. By comparison, the company’s smallest wireless phone today weighs less than 3 ounces and measures only 3.3 by 1.5 by 0.8 inches.
Many automotive components are also shrinking, especially as suppliers heed the call for lighter, less intrusive under-the-hood devices. For instance, anti-lock brake system controllers are now roughly half the size they were when they first appeared on cars 10 years ago. The 4-inch by 4-inch footprint has shrunk to a 2-inch by 2-inch format, while the size of the electronic stack inside has decreased from approximately 0.5 inch to 0.25 inch.
Smaller products usually prove popular with consumers, but they pose numerous challenges to manufacturing engineers. As parts get smaller, robots and other automated equipment become essential. Unfortunately, small parts can be difficult to manipulate.
“Most small part processes are [typically handled] with dedicated automation rather than flexible robotics,” says Milton Guerry, sales and marketing manager at Schunk Inc. (Morrisville, NC). As robots become smaller and more repeatable, they’re being used for smaller tasks. Guerry predicts they’ll be used for even more applications with small parts as the design of the robot arm becomes smaller and lighter, enabling it to move faster, with less power and interference.
To be effective, robotic grippers must apply enough pressure to hold the part without damaging it in the process. Also, small parts are often presented in arrays spaced closely together. The grippers must be small themselves to work in these fine-pitch matrices.
“Gripping and releasing small parts is typically more challenging because of the accuracy, repeatability and speed involved,” says Guerry. “The smaller the part, the more challenging the part is to grip and release. As a general rule, with smaller parts, more accuracy is involved in the positioning. And, higher quantities require lower cycle times.”
“The smaller the part, the tougher the application becomes,” adds Dennis Stuerzenberger, director of business development at PHD Inc. (Fort Wayne, IN). “Part handling complexity increases with the smaller items because the range of gripping options and available contact surfaces shrinks as part size decreases.
“Precision of the actuator is key as the assembly positional tolerances shrink with part size,” Stuerzenberger points out. “Consistent actuator output force is also of prime concern, as many of these components are very fragile.”
Indeed, a big challenge of handling small parts occurs before the grip. “Small parts must be orientated and isolated so that they are ready to be gripped,” says Marc Czaplicki, applications engineer at R&I Manufacturing Co. (Thomaston, CT). “To address this issue, engineers must experiment with escapements, nonrotating cylinders and shuttle devices.”
Hard to Handle
The term “small part” can mean many different things to different people. Most engineers define “small” as any component under 1 inch in length, width or height. To some observers, that would actually be a big part. For instance, Guerry considers any part less than 5 millimeters to be small.
“How small a part we can pick up depends on the finger design,” says Umesh Cooduvalli, product manager for robotic automation at De-Sta-Co Industries (Madison Heights, MI). “One of our smallest applications was a surface mount resistor that measured 0.08 by 0.06 by 0.05 inch.”
The size and shape of parts typically determine how easy they are to manipulate. According to Cooduvalli, conical parts are typically the most difficult to handle. “The steeper the angle on the cone, the tougher it gets,” he explains. “Parts that are not rigid, such as springs or flexible plastics, can pose problems with maintaining part position and orientation.”
“Smooth, featureless parts are a challenge, especially if they are fragile,” notes Scott Ames, technical support engineer at Robohand Inc. (Monroe, CT). “If there is a small lip or other feature on the part, you can encapsulate the part and require little grip force. If the part is smooth, you rely solely on the coefficient of friction and you must apply much more force to hold the part.”
“Any shape that does not provide a flat, clean contact surface is difficult to manipulate consistently,” adds Stuerzenberger. He says an excellent example would be the small springs used for assembling the pumps found in hand soaps, shampoos and other consumer products.
In addition, odd-shaped, molded parts make it difficult to identify distinct features to allow repeatable gripping and handling. “Very thin, flat parts can be difficult to grip with traditional grippers,” notes Costas Charalambous, gripper component sales manager at Techno-Sommer Automatic (New Hyde Park, NY). “Nonuniform shapes, such as soft, flexible components, can also be difficult to pick up. In many cases, vacuum cups cannot be used because the material is porous. We have developed off-the-shelf needle grippers for this type of problem.”
According to Cooduvalli, the biggest challenge isn’t in handling the small parts themselves. It’s shrinking the grippers to fit in the close quarters that often come with small-part applications. “Small parts are often picked from trays, which limits how much of the part is accessible,” he points out.
Selecting the type of gripper to use depends on the application. “There is no ‘magic formula’ to picking the right gripper,” explains Charalambous. “Usually, it is dictated by the shape of the part and what will be done to it after it is gripped.” He says adding any sort of articulation at the tooling fingers can allow a robot to be more flexible and agile.
Grippers are available to meet the high repeatability demands of smaller parts, but the finger design must complement the grippers’ repeatability, correctly locating the work-piece in the fingers. “With extremely small parts, conventional grippers are sometimes too large,” warns Guerry. “Here, we try to take a fresh look at the gripping requirements and often come up with a solution that is more customized than conventional off-the-shelf grippers.”
Guerry says parallel grippers typically offer more repeatable positioning than angular grippers. However, angular grippers are often simpler in design and therefore more economical. Concentric grippers are mostly used for parts with round ends. “Due to the enormity of part shapes and sizes, it is very difficult to recommend one design over another as a rule, but parallel grippers should probably be a first consideration,” explains Guerry.
A wide range of specialized grippers are available to handle small parts. For example, mini-finger grippers feature jaws that can be machined down to very tiny shapes to fit the part, whether it be an interior or exterior grip.
Czaplicki claims that traditional mini grippers are very versatile and can be adapted to small parts, depending on the tooling mounted to the gripper fingers. “To address the demand for smaller and smaller parts, we offer a line of mini angular grip heads, mini parallel grip heads, and a mini angular stack-pack gripper with a very narrow profile,” he points out. “They can be stacked to allow for close center distance mounting, which is typically needed in racking-deracking, palletizing-depalletizing and other similar applications.”
Three-point grippers feature jaws that swing into the center rather than slide in. “This gives the jaws a much longer stroke range in a small package, allowing the gripper to be used for a variety of part sizes without having to change tooling fingers,” notes Charalambous. “The swing design lends itself well to applications where the part is not always centered.”
For delicate parts, Charalambous suggests choosing compliant finger grippers that are actuated by bladders instead of traditional mechanical linkages. “We have also developed a series of two- and three-jaw grippers that are electrically actuated,” he explains. “They have adjustments that allow changing of the grip force to [accommodate] the pickup of small, fragile parts.”
In some cases, Charalambous says electric grippers can be used effectively with small parts, because they allow a greater adjustment of stroke and grip force to protect smaller, more delicate parts.
No matter what type of gripper is used, experts believe the biggest pitfall in handling small parts is not completely accounting for manipulator speed and repeatability. They point out that many components must be “de-rated” for high-speed applications.
“A common mistake is to use virtually all the stroke of a gripper,” says Ames. “As part size gets smaller, so does the gripper and with it, the stroke gets shorter. If there isn’t adequate stroke for clearance as well as over travel, there can be issues when there are slight variations in part size. This usually isn’t a concern with large parts because large parts require large grippers which usually have long strokes.”
When there is risk of damage to a part due to the force applied by a gripper, vacuum cups should be considered. However, vacuum cups tend to lend themselves more to larger parts and applications were precision is not a concern. “In small parts handling, it is tough to compete with a gripper, and likewise it’s virtually impossible to compete with vacuum for large parts,” says Ames.
According to John Westbeld, design manager at SAS Automation Ltd. (Xenia, OH), “vacuum cups are a good alternative to grippers when they can be used.” He claims they are usually a cheaper solution to gripping and can be very accurate. “Vacuum cups should be considered when the parts are not porous or textured to the point of losing vacuum,” notes Westbeld.
Vacuum cups can be an economical and reliable way to pick up small parts that have sufficient flat space to place the cup. But, Guerry warns that “cups are often more challenging in respect to accurate positioning, due to the pick flexibility and the cups’ elasticity.”
Choosing a Robot
Many different types of industrial robots are available for manipulating small parts. The best configuration to use depends on the application and the work envelope. Cycle times, accuracy and flexibility can dictate a choice between one type of robot or another. Other considerations include the process parameters, part weight, repeatability and the working area to complete the tasks.
“With the advances made in software and hardware, nearly any style of robot [can be used] in small parts handling,” says Ames. “For high-speed, pick-and-place work with small parts, the SCARA robot really shines. The articulated robot lends itself to assembly of small parts and tends to be more flexible to accommodate job changes.”
As parts continue to get smaller and smaller, evolving technology such as machine vision systems, will be applied to more gripping applications. Cooduvalli says vision allows the robot to deal with differences in part size and location, while working in tightly confined areas.
Sensors can also help end effectors manipulate small parts. For instance, they allow for precise part placement and help ensure that parts are gripped correctly. They provide positive feedback of the manipulator’s condition before the next action.
“Position sensing and force sensing devices play an important part in small part handling,” claims Charalambous. “They help in eliminating wasted motion by the robot by detecting when a part is or isn’t present. They can also assist in preventing damage to smaller parts by monitoring the force applied to the part.”
Small parts are often fragile. With a pneumatic gripper, it is possible to produce enough force to damage the part. “Force sensors can be used to signal the robot that it has applied enough force to hold the part,” says Cooduvalli. “Stroke position sensors are helpful in providing feedback to the robot so it knows whether the gripper is open, closed or in transit. They can also be used to signal that a part is present. For the low cost of sensors, it is worthwhile to prevent costly damage due to [problems such as] the robot taking off before it is fully closed.”
The Next Frontier
How much smaller can we go? It’s inevitable that products such as hearing aids and pacemakers will continue to shrink.
“Everything we see and use in our daily life is getting smaller and smaller,” says Cooduvalli. “Look at the size of cell phones today. This trend will continue in the medical and electronics fields, as well as spread to other industries.
“There is definitely a limit to how small current gripper technology will let you go,” adds Cooduvalli. “I don’t think we’ve reached that limit yet, but it’s not too far off. The trend in the future may be to go with small, piezo motor-driven, electric grippers, or perhaps pneumatic grippers where the drive mechanism is remote from the jaws and fingers.”
As new developments in nanotechnology evolve, manufacturing engineers will be faced with more difficult part-gripping challenges. “Parts will continue to become smaller and smaller, and as this happens, parallel and angular grippers [may] become less of an option,” warns Charalambous. “More unique grippers that utilize magnets or other pick up methods will have to be used.”
According to Thierry Dumont, business unit manager-robotics at Bosch Rexroth Corp. (Buchanan, MI), one new gripping technique that shows promise in handling sensitive parts is noncontact technology. “Lifting power is combined with an air cushioning function to prevent physical contact with the part,” he explains. “Wafer handling is an excellent candidate for noncontact technology.”
Noncontact technology operates by the Bernoulli principle. Airflow under the device creates a vacuum and a lifting force between the center and the circumference. Because of the dynamic vacuum and the continuous flow, the lifted object will not attach to any surfaces.