The Gripping Point

Electric motors can provide a touch for delicate applications

Pneumatic actuators are often touted for their holding strength, energy density, and ability to handle applications with high force ratings. Frequently found in large machinery, they can even be applied to precision devices, if of the smaller cylinder variety. In the case of more delicate applications, electric motors can be the better choice, Leah Scully of Machine Design writes.

“However, designers often opt to use electric motors in sensitive applications that require a wide variation in force because they are easier to maintain and fine tune,” Scully describes, demonstrating the versatility of electric motors in this case. The specific example used is a replacement of pneumatic actuators on a robotic gripper with 12 electric motors, performed by designers at Keller HCW, the German machinery and service company with primary divisions of automation and robotics. The demonstration took place at the Automatica show in Munich last year. Although videos aren’t available, here is a similar application, albeit on a more primitive scale:


The switch-out yielded a wider range of force distribution for the robotic grippers, as they were able to pick up objects with a wide variety of weight and density – from bricks to tissue packs. Pneumatic cylinders were replaced with direct-drive linear motors. Other benefits included heightened functionality, repeatability and precision, as well as better outlooks on maintenance and energy efficiency. The upgraded gripper was still suitable for heavy lifting (hence the brick portion of the demonstration) as well.

A specific advantage was breaking the previous limitations of the pneumatics’ stroke adjustment range, which was confined to 10 mm. This ruled out the capability of grabbing different sizes of bricks. This forced builders to continually switch out the grip ends between tasks, resulting in more work time and “ultimately wasted worker and mechanical energy” Scully notes in her article, published last Friday, January 29th. “The cost of four or five additional gripper types also contributed to the expenses of construction. It was proposed that a rotary servo drive be used to adjust the stroke length for improved variability in grasping, but this solution does not eliminate the problem of adjustment repeatability.”

The reason for these limitations in pneumatics is due to their sensitivity to humidity and ambient temperature, which can cause unpredictable reactions when adjusted. Motors are less prone to the effects of such factors, can be controlled more precisely, and can cover a longer stroke in a shorter time. The electric motors used in this case, from LinMot, an Elkhorn, Wisc., based company, also took up less space.

Keller is headquartered in Ibbenburen, Germany, with outlets in Europe, Africa, Asia, and Australia.



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