Hi @m
To describe the equation 
(a flat plane surface), use the syntax 'linear', [1 1 2] in the addMF() command for the output z.
(a flat plane surface), use the syntax 'linear', [1 1 2] in the addMF() command for the output z.%% Sugeno fuzzy system
fis = sugfis;
%% Fuzzy Inputs, x and y
fis = addInput(fis, [0+eps 1], 'Name', 'x');
fis = addMF(fis, 'x', 'linsmf', [0 1], 'Name', 'large'); % Large fuzzy set
fis = addInput(fis, [0+eps 1], 'Name', 'y');
fis = addMF(fis, 'y', 'linzmf', [0 1], 'Name', 'small'); % Small fuzzy set
%% Fuzzy Output, z = 1·x + 1·y + 2
fis = addOutput(fis, [2 4], 'Name', 'z');
fis = addMF(fis, 'z', 'linear', [1 1 2], 'Name', 'Eq1');
%% Rule: If (x is large) and (y is small), Then (Z is Eq1 = x + y + 2)
rule = "x==large & y==small => z=Eq1";
fis = addRule(fis, rule);
%% The surface
opt = gensurfOptions('NumGridPoints', 101);
gensurf(fis, opt), grid on, grid minor, zlim([2 4])
