A digitally - programmable analogue Fuzzy Logic Controller (FLC) is presented. Input and output signals are processed in the analog domain whereas the parameters of the controller are stored in a built-in digital memory. Some new functional blocks have been designed whereas others were improved towards the optimization of the power consumption, the speed and the modularity while keeping a reasonable accuracy, as it is needed in several analogue signal processing applications. A nine-rules, two-inputs and one-output prototype was fabricated and successfully tested using a standard CMOS 2.4? Technology, showing good agreement with the expected performances, namely: from 2.22 to 5.26 Mflips (Mega fuzzy logic inferences per second) at the pin terminals (@CL=13pF), 933-µW power consumption per rule (@Vdd=5V) and 5 bits of resolution. Since the circuit is intended for a subsystem embedded in an application chip (@CL ≤5pF) up to 8 Mflips may be expected.
In the last years the application of Fuzzy Logic has been extended beyond the classical Process Control area where it has been employed from the beginning. Signal Processing, Image Processing, Power Electronics, seem to be others niches where this soft-computing technique can meet a broad range of applications. As real time processing mode need ever faster, more autonomous and less power consuming circuits the choice of on-chip controllers become an interesting option. Digital Fuzzy Logic chips provide enough performance for general applications but their speed is limited, if compared with their analogue counterparts. Furthermore, in real-time applications digital fuzzy processors needs A/D and D/A converters to interface sensors and actuators, respectively.
On the other hand, pure analog processors suffer the lack of suppleness since full analog programmability is only feasible in special technologies allowing analog storage devices (i.e.: floating gate transistors). However, in the frame of standard CMOS technologies, a trade-off between accuracy and flexibility is achieved when a finite discrete set of analogue parameters is provided. For instance, a voltage parameter can be settled by using a binary-scaled set of currents sources yielding a discrete set of voltage drops through a linear resistor. In such a case, it is possible to use a digital memory to store a given binary combination of the set of currents. This technique gives rise to the so-called Mixed-Signal analogue computation circuits . It has been shown that analogue current-mode FLCs lend themselves to simple rules-evaluation and aggregation circuits that can work at a reasonable speed. If some of the unwanted current-to-voltage and/or voltage to- current intermediate converters can be avoided, the delay through cascaded operators may be even shortened and higher speeds achieved. This is interesting when Fuzzifiers and defuzzifiers circuits are being designed for these circuits interact normally with a voltage-mode controlled environment. On the other hand, to reduce die silicon area and power consumption some building blocks can be shared without altering functionality. As a result a relatively low-complexity layout can be obtained which leads to an additional gain of speed. In this work, a low-power digitally programmable analogue Fuzzy Logic Controller (Mixed-Signal FLC) is introduced intended for embedded subsystems, as it is required for analogue signal processing applications of Medium-accuracy (i.e.: non-linear filtering , power electronics , etc). Keeping in mind the above exposed issues, new operators were designed while others were optimized achieving a flexible and high performance controller notwithstanding the limits imposed by the technology that was used for the demonstrator.
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