| http://www.w3.org/ns/prov#value | - The system of claim 6 wherein:said first voltage signal ranges from a first voltage level V1 to a second voltage level V2 over a period TP1 in voltage increments (V1 -V2)/n, where each increment has a duration TD1, n is a positive integer, and n(TD1)=TP1 ; and said second voltage signal ranges from a third voltage level V3 to a fourth voltage level V4 over said period TP2 in voltage increments (V4
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