Several embodiments of such capacitive position sensors are known. Some of them, mostly intended for two dimensional position locating systems, also called digitizers and used as computer peripherals, need as a scale a matrix of wires embedded in a board and excited by electrical signals, said signals being picked up by a receiving electrode in the cursor, which may be a stylus or pointing device, as disclosed in U.S. Pat. No. 3,342,935. Such digitizers, due mostly to the complexity of the matrix of wires, are too costly to compete in applications where a simpler pointing device, such as a mouse, will be sufficient.
Other embodiments of such capacitive position sensors, mainly intended for linear or angular measuring instruments, such as jaw calipers, micrometers, dial indicators, rotary encoders, use a scale with a linear array of floating electrodes, i.e. completely insulated electrically from their surroundings, facing a cursor having transmitting and receiving electrodes. A capacitive position sensor of this type is described in U.S. Pat. No. 3,961,318. These floating electrodes eliminate the need for electrical connections to the scale, but there are still drawbacks. The scale may not be homogeneous, as an insulating substrate is needed under the conducting electrodes. Coupling between adjacent floating electrodes, as well as humidity induced surface conductivity of the scale, degrade performance. Moreover, the floating electrodes extend across the measuring path, as they need to provide two clearly separated functions, i.e. coupling to the transmitting electrodes and retransmitting the received signal to the receiving electrodes. Said extension across the measuring path, needed to implement both functions, prevents the extension of such an embodiment to a second dimension.
USPTO Backgrounds
USPTO Backgrounds