A　semi-analytical　model　for　a　double-plate　capacitive　proximity　sensor　is　presented　according　to　the　effective　theory.　Three　physical　models　are　established　to　derive　the　final　equation　of　the　sensor.　Measured　data　are　used　to　determine　the　coefficients.　The　final　equation　is　verified　by　using　measured　data.　The　average　relative　error　of　the　calculated　and　the　measured　sensor　capacitance　is　less　than　7.5%.　The　equation　can　be　used　to　provide　guidance　to　engineering　design　of　the　proximity　sensors.　(C)　2014　Author(s).　All　article　content,　except　where　otherwise　noted,　is　licensed　under　a　Creative　Commons　Attribution　3.0　Unported　License.