Industrial robots and positioning equipment traditionally perform their tasks by following programmed, fixed paths along x-y-z coordinates. Some of them augment and modify these programs with inputs from contact, proximity and vision sensors to accommodate changes and variations in the nature and position of the work. But what about operations in which proximity doesn't provide enough precision, or vision can't see what needs to be done?
"The classic example is putting a peg into a hole," says Dave Gavel, technical expert, robotics, at Ford Motor, Livonia, Mich. "Using position control, any misalignment will cause the robot to jam. If you're assembling the sun gear into a planetary set and you don't know the positions of everything, or if the positional uncertainty due to stacking tolerances exceeds the clearances, you have to do a blind search."
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Much like we humans grope for a light switch in the dark, robots can use today's touch technologies to do that blind search and gently align heavy components into precise assemblies such as gearboxes. "Touch sensing for us entails the ability to sense surfaces and sometimes textures on manufacturing work pieces, allowing for smarter and more-precise material handling," says Henry Loos, Jr., application engineer, Applied Robotics, Glenville, N.Y.
Touch also can enable machines to handle delicate items without crushing them, reliably pick up soft or varying objects, and work in close proximity to people with little risk of injury. As part of the never-ending quest for the perfect android, emerging technologies promise to give machines human-like ability to assess and accommodate variations in the hardness, texture and temperature of touched materials.
Ford Relies on Force Control
Today's state-of-the-art touch application is "typically applied with a six-degrees-of-freedom, force-torque sensor that's mounted on the tool flange or part of a tool changer and connected to the robot controller through an analog/digital control box," says Nick Hunt, manager of technology and support, ABB Robotics North America. In the U.S., ABB standardized on force-torque converters from ATI, Hunt says. "There are other ways to get the six inputs, but that's the typical way."
ABB recently introduced Integrated Force Control, a consolidation of discrete software features that previously were available only in the company's machining or assembly Force Control offerings. The module makes it possible to automate complex tasks that previously required skilled personnel and advanced fixed automation, such as machining and small parts assembly that require dexterous handling of work pieces and tools. Hunt says it can dramatically reduce the stress robot programmers are under when faced with processing parts of complex and varying geometries.
"The essence of force control, whether you buy it or roll your own using our core, is the ability for the robot to feel its way around," Hunt says. "It enables the robot to adapt to the application, not the other way around."
For example, in a casting deburring operation (Figure 1), the robot typically grinds or mills a rough casting to a specific contour. "If it's hard-wired, when the robot gets to a big burr, it grinds too hard," Hunt says. "It's hard on the robot, the tool, the grinder motor and the part."
Force control allows the user to easily program the contour. You can walk the robot through the operation by hand — set a point, move the robot, set a point — to teach the robot the contour of the part. Then you can put it in auto, test and run. "Now when the robot encounters a burr, it slows down —you specify the force," Hunt adds. "If it's a big burr, you can set it to grind in several passes. Imagine trying to program that as a straight robot program. That's a mighty complex program."
On a torque converter assembly line at Ford, "We had ergonometric issues with people doing three vertical installs — a hub splined into the turbine, a one-way clutch into the torque converter and the impeller into the pump — because they are blind searches, the parts are heavy, and the parts have high inertia," Gravel says. "The impeller was the hardest because the mating features are 180º apart." Using force control, Gravel adds, an assembly robot can apply a small force and oscillating motion. When the part goes down, the robot knows it's in position, and it can move on.