Vision systems are excellent for guiding robots. However, for some applications, such as tightening bolts, a vision system is of little help, because the end-effector and tool are so close to the fastener they block the camera’s view.

Vision systems are excellent for guiding robots-but only as long as there’s a direct line of sight between the camera and the part. However, for some applications, such as tightening bolts, a vision system is of little help, because the end-effector and tool are so close to the fastener that they block the camera’s view.

Engineers at NASA’s Goddard Space Flight Center (Greenbelt, MD) have found a way around that problem. Led by John M. Vranish, the engineers developed advanced capacitive proximity sensors, called capaciflectors, that can help a robot align a wrench socket or other tool with the head of a bolt or other electrically conductive object.

A capaciflector includes at least one sensing electrode, excited with an alternating voltage, that puts out a signal indicative of the capacitance between that electrode and a nearby object. In a bolt-tightening application, the robot’s end-effector is equipped with a wrench socket that is instrumented with a capaciflector and an array of sensing electrodes. As the position of the socket changes with respect to the bolt head, the capaciflector senses differences in capacitance. The capacitance increases as the socket strays from the bolt head, and decreases as the socket centers on the bolt head.

The outputs of the electrodes are converted to DC, digitized, and fed to a microprocessor. The microprocessor then uses the variations in capacitance to generate data equivalent to a fictitious force field that is a source of signals to control the motion of the robotic manipulator in the vicinity of the object.

Machine vision is still used to get the robot close to the bolt head, while the capaciflector is used to ensure the socket mates with the head.

For more information, call 301-286-2000 or visit www.nasa.gov/centers/goddard