Digitally programmable analog membership-function circuits have been invented for use in fuzzy-logic systems. Heretofore, fuzzy membership functions have been implemented, variously, by use of purely analog or purely digital circuits. Purely digital circuits afford flexibility at the cost of less speed and greater circuit area. Purely analog circuits offer greater speed and occupy less area but tend to be limited in flexibility and programmability. The present invention takes advantage of both the programmability of digital circuitry and the speed and compactness of analog circuitry. The analog aspect of the invention creates the potential for high-speed parallel processing with relatively low power consumption.

A Combination of Analog and Digital Circuits implements a trapezoidal membership function of an analog input signal. The digital inputs establish the parameters of the trapezoid.
In designing a circuit according to the invention, one uses current-mode circuitry to implement fuzzification in a fully parallel architecture while using a digital interface for programmability of attributes of membership functions. In a typical case, a membership-function circuit accepts an analog input potential and implements a trapezoidal membership function (see figure). First, the input potential signal (X) is converted to a current-mode signal (Iin) for further processing. Two current-mode digital-to-analog converters (DACs) provide currents IL and IR that represent zero-crossing locations of the left and right legs, respectively, of the trapezoidal membership function.

Current subtractors generate IinIL and IRIin, and feed these difference currents as analog inputs to dividing digital-to-analog converters (DIV-DACs). [Alternatively, one could use multiplying digital-to-analog converters (M-DACs); the advantage of DIV-DACs is that they provide more evenly spaced increments, which makes the range of programmable slopes more useful.] The digital input to each DIV-DAC is a number inversely proportional to the slope of the corresponding leg of the trapezoidal membership function. In each DIV-DAC, the analog difference-current input is divided by the digital inverse-slope input to obtain the ordinate on the affected leg of the trapezoid. A comparator circuit selects the output of either the left- or the right-leg DIV-DAC, and a limiting circuit clips the output at a level corresponding to the top of the trapezoid. The output current of the limiting circuit is converted to an output voltage.

This work was done by Tyson Thomas and David Weldon of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at under the Electronics & Computers category.