Capaciflector Camera

Figure 1 shows a multiple-element capacitive proximity sensor of the "capaciflector" type, with sensing electrodes formed as thin strips and arranged in rows and columns. Each row or column sensing electrode provides a measurement of the capacitance between itself and a proximate object. From the measurements provided by all the row and column electrodes, the known dependence of each capacitance on the positions and orientations of the electrodes and the object, and the known positions of the electrodes, associated processing circuitry can construct a three-dimensional capacitance image of the object. Because of this imaging capability, the sensor is called a "capaciflector camera." Sensors of this type could be installed on robot arms, and the threedimensional capacitance images that they generate could be used by robotcontrol circuits to prevent collisions with the capacitively sensed objects, to image and identify these objects, and to navigate amongst them.

Each electrode is covered by a thin layer of electrically insulating material. The sensing electrodes are driven by a fixedfrequency source (e.g., a crystal controlled oscillator), all at the same voltage, through current-measuring voltage followers. Like the capaciflectors described in several recent articles in NASA Tech Briefs, this sensor also includes a shielding electrode driven through a voltage follower from the same source. The driven shielding electrode in a capaciflector minimizes the capacitance between the sensing electrodes and electrical ground (electrical ground being the surface of the robot arm on which the sensor is mounted) and concentrates more of the sensing electric field outward from the sensing electrodes, increasing the sensitivity and range of the sensor. An optional floating shield can be driven in a similar way to serve as an additional sensing electrode that provides a single capacitance reading that serves as a nonimaging, gross indication of proximity. Because all of the electrodes are driven at the same voltage, the sensing electrodes do not sense the driven shielding electrode, and they do not sense each other. That is to say, there is no crosstalk between a sensing electrode and any of the other electrodes: each sensing electrode gives an independent measurement of the capacitance between itself and the sensed object.

The capacitance between each sensing electrode and the sensed object gives rise to a current in the sensing electrode. The current-measuring voltage follower that drives the sensing electrode puts out a voltage that varies with this current by virtue of the voltage drop of this current across the voltage-follower resistor R, as indicated in simplified form in Figure 2. Each such output voltage (one for each row and column sensing electrode and one for the floating shield if used) is fed to external processing circuitry, where it is compared with a reference voltage derived from the source voltage to obtain a signal proportional to the sensed capacitance. The sensed-capacitance signals from all the rows and columns are then processed further to obtain the three dimensional image of the sensed object.

This work was done by John M. Vranish of Goddard Space Flight Center. For further information, write in 32 on the TSP Request Card.

Point of Contact:
John Vranish
Mail Stop 723.4
NASA Goddard Space Flight Center
Greenbelt Road
Greenbelt, MD 20771
301-286-4031
John.M.Vranish.1@gsfc.nasa.gov

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