An advance seen increasingly in robots used for aerospace manufacturing is the application of force control. This is the ability of the machine to monitor and react to forces exerted on the robot end effector. Many aerospace manufacturing processes require a "soft touch"—as in polishing or grinding turbine blades. More sensitive force control helps the robot create more uniform finishes.
"It's not uncommon to get a flat spot or a high spot in grinding," said Nick Hunt, Manager of Product Support for ABB Robotics-North America. "With force control, we can direct the grinder to maintain a certain [level of] force across the entire surface of the blade."
The speed of the system has always been the primary limitation of force control.
"The robot has to respond quickly to changes in force," said Hunt. "When we see the smallest amount of change in force, we have to respond very quickly so the process doesn't recognize the change in force. You don't want the process to know that a change has occurred because if the process did recognize the change, then it would result in a flat or high spot on the turbine blade. With force control, we can be more sensitive than the process we're dealing with."
Force control can be especially important when drilling holes in a wing structure or fuselage, particularly if a combination of composite and other materials is involved.
"Drilling has to be done in an exact way, and when you drill a hole in a composite wing you would initially encounter composite material and then maybe aluminum," said Hunt. "You have to make sure that when you puncture through the wing that the hole is normal [perpendicular] to the surface. Even though robots are rigid, they tend to flex during the drilling process, and force control keeps the drill properly positioned."
Manufacturers strive to build stiffness into their robotic systems to minimize flex when the robot applies a tool to a workpiece. If they don't, then they have to compensate with expensive features such as vision that guide the tool.
A concept similar to force control is also being developed for motion control, where a welding robot, for example, can sense changes in material thickness and compensate by altering the current used in the welding process.
"Combining motion control with welding control opens up a new realm of possibility, such as joining thick and thin materials," said Panasonic's Kevin Pagano, Engineering Manager for Panasonic North America's Robotics division, which specializes in arc-welding robots, particularly gas tungsten arc-welding robots for metal joining in aerospace manufacturing. "The robot can move from side to side between the two materials and apply a low current to the thin material and a higher current to the thicker one. The synchronization of the welding current with motion control hasn't been possible before because of the communications lag between the two."
Such a capability is now being used for the manufacture of instrument panel beams that run across a car dashboard, for example, where the beam is built from aluminum and a variety of metallic brackets have to be attached.
"Companies that do TIG [tungsten inert gas] welding in aerospace like the idea of an integrated, automatic voltage control that adjusts the height of the electrode and controls the path of the robot as the material distorts due to the heat of the welding process," Pagano said.