New robotic arm promises to mind Newton’s third law

On Earth you may barely notice Newton’s third law, which famously describes how every force generates an equal and opposite reaction. But in space, with little gravity or friction to resist reaction forces, giving anything a nudge is likely to propel you backwards.

The same problem afflicts robotic arms. Simply moving such an arm produces reaction forces that can shift a spacecraft from its correct orientation.

A new approach that uses gyroscopes to move mechanical joints offers a new way around the problem that uses less energy and can move faster than existing space arms designed to avoid the problem.

Researchers at the Space System Design Studio at Cornell University, New York, have successfully tested a prototype on a microgravity “vomit comet” flight that simulates a space environment.

Gyro power

The established way to control reaction forces on a robotic arm is to use flywheels instead of motors. The speed of a constantly spinning flywheel is altered to generate a force that moves the arm.

But Cornell aerospace engineer Mason Peck and colleagues say they can accomplish the same thing, but expend much less power, using a device called a control-moment gyroscope (CMG).

Each consists of a gyroscope mounted on a powered gimbal that can tilt the spinning device. When this happens, changing the gyroscope’s axis of rotation, the gimbal receives a boost of torque, which twists the joint.

Such gyroscopes are used on some satellites, such as the International Space Station, to control their orientation in space. For a down-to-earth demonstration, see how this person on a moving platform can control themselves by holding a spinning bicycle wheel.

Triple-jointed

The Cornell arm has three joints. Each is controlled by a pair of gyroscopes that between them can exert forces in two directions to move the joint back and forth.

Each of the joints controls movement along only one 3D axis, so they do not work directly against one another.

“You can get an exchange of momentum and cause motion to occur without much power,” Peck told New Scientist, “it takes a little power, but nowhere near as much as if you were spinning up or down the disc,” Peck says.

Because the system will allow quick movement with little power, it could be especially useful for controlling a camera or telescope that had to quickly track a moving object. But it is equally useful as a basic robotic arm, Peck says.

“It’s a unique idea,” says Vaios Lappas, an aerospace engineer at the University of Surrey. “You can develop a very attractive, advantageous technology for space robotics using CMGs. They could have superior capability with the same or less physical resources, and pretty much the same complexity.”

Journal reference: IEEE Transactions on Robotics (DOI: 10.1109/TRO.2008.924264)