What is a robot without its arm or means to grip and move objects? Not much. Technically, robots have existed for at least 100 years. Their rougher mechanical movement is relatively easy to generate, particularly with electric motors or pneumatics. However, it’s only been over the last 40 years that mechanical motion precision, controls, sensing, and feedback have advanced enough to refine robotic handling to the realm of indispensible.
A PDF on best gripper design can be downloaded here.
A lineup of gripper products can also be found here.
You see, unlike more basic kinematic linkages, robotic arms by definition have multiple degrees of freedom that must all be synchronously directed in a kind of electromechanical (or servofluidic) ballet. In fact, the feedback required to actuate sensitive robotic grippers — ones responsive enough to the environment to be of practical use — is something of a new possibility: Therefore, it’s only been 25 or so years since the first practical grippers went into production for real industrial applications.
It all began in 1969: After much toil in the school’s machine and computer labs, Stanford University mechanical-engineering student Victor Scheinman developed his Stanford arm, an early robot that would come to be known as the first readily controllable gripper. Predecessors such as the Hydraulic Stanford arm were effective and fast, but known for being uncontrollable and even dangerous.
In contrast, Scheinman’s better-behaved Stanford arm was steerable by Stanford-lab computers in six full degrees of freedom; dc electric motors with gear reducers and harmonic drives generated its motion.
The grippers and robotic hands detailed in this article are manufactured by these companies: PBC Linear • Schmalz Inc. • Techno-Sommer Automatic • SCHUNK Inc. • Festo Corp. • Altra Industrial Motion • ABB Inc. • Barrett Technology Inc. • Shadow Robot Co. Ltd. • Motoman Inc.
By the early 1980s, rougher gripper designs inspired by the Stanford arm (and made possible with increasingly powerful microchips) were in mass production, and used in heavy industry. Though many Stanford-arm feedback and control elements were copied (feedback tachometers and potentiometers sent speed and position to controllers) most early industrial arms were powered by air, and used for automotive manufacturing. Coincidentally, that legacy survives: Many of the latest gripper advancements come from the field of fluid power, and the majority of grippers are still pneumatic.
Another rotating-parallel gripper is detailed at machinedesign.com.
A variation — the two fingered angle gripper — came in the late 1970s. Each of the two fingers in this design swings on a pivot point, closing like a gate or lobster claw on target objects. What’s the difference between parallel and angle action? Parallel jaw action simplifies finger design, and force remains the same throughout the stroke — unlike some two-finger angular grippers. The parallel design also offers distinct design options: Grippers with a direct-acting piston and wedge allow for shorter stroke and high grip force, to 10,000 lbf. By comparison, straight direct-piston grippers generate slightly smaller force, but offer longer strokes, to 24 in. in some cases. “I would estimate that 60 to 70% of applications end up using parallel grippers,” adds Hayes. Many of these units leverage fluid power as well, though some grippers are offered in both pneumatic and electromechanical versions.
The next gripper innovation came in the late 1980s: A three-finger grasper, developed at the Massachusetts Institute of Technology was licensed to spinoff Barrett Technology Inc., Cambridge, Mass., in 1990. The design, now called the Barrett hand, embeds servocontrollers, software, communication, and four brushless motors.
Also check out the full story of the HIT-DLR robotic hand at MicroMo Electronics Inc. - Faulhaber Group.
Two fingers have an extra degree of freedom, with 180° synchronous lateral mobility for lots of ways to grasp.
Remember that two-finger grippers perform best when dedicated to a single specific task — and must be swapped out for other grippers on a turret if parts or target shapes change. In contrast, the latest BH8-series Barrett hand performs the functions of several custom parallel grippers. Here, Windows-based BHControl user interface plus C-Function Library and firmware help the gripper adapt to new situations.
As it happens, late last year, Barrett Technology also licensed the polymer-based SDM Hand, a grasper developed by Robert D. Howe of Harvard University’s School of Engineering and Applied Sciences, and Aaron Dollar. This robotic hand has fingers with flexible joints, so it conforms to and grips objects ranging in shape and mass — without excessive force or requiring perfect positioning. Barrett expects production SDMs availability by 2011.