Documentation 
Summing junction for namebased interconnections
S = sumblk(formula)
S = sumblk(formula,signalsize)
S = sumblk(formula,signames1,signames2,...)
S = sumblk(formula) creates the transfer function, S, of the summing junction described by the string formula. The string formula specifies an equation that relates the scalar input and output signals of S.
S = sumblk(formula,signalsize) returns a vectorvalued summing junction. The input and output signals are vectors with signalsize elements.
S = sumblk(formula,signames1,signames2,...) replaces aliases (signal names beginning with %) in formula by the signal names signames. The number of signames arguments must match the number of aliases in formula. The first alias in formula is replaced by signames1, the second by signames2, and so on.
formula 
String specifying the equation that relates the input and output signals of the summing junction transfer function S. For example, the following command: S = sumblk('e = r  y + d') creates a summing junction with input names 'r', 'y', and 'd', output name 'e' and equation e = ry+d. If you specify a signalsize greater than 1, the inputs and outputs of S are vectorvalued signals. sumblk automatically performs vector expansion of the signal names of S. For example, the following command: S = sumblk('v = u + d',2) specifies a summing junction with input names {'u(1)';'u(2)';'d(1)';'d(2)'} and output names {'v(1)';'v(2)'}. The formulas of this summing junction are v(1) = u(1) + d(1); v(2) = u(2) + d(2). You can use one or more aliases in formula to refer to signal names defined in a variable. An alias is a signal name that begins with %. When formula contains aliases, sumblk replaces each alias with the corresponding signames argument. Aliases are useful when you want to name individual entries in a vectorvalued signal. Aliases also allow you to use input or output names of existing models. For example, if C and G are dynamic system models with nonempty InputName and OutputName properties, respectively, you can create a summing junction using the following expression. S = sumblk('%e = r  %y',C.InputName,G.OutputName) sumblk uses the values of C.InputName and G.OutputName in place of %e and %y, respectively. The vector dimension of C.InputName and G.OutputName must match. sumblk assigns the signal r the same dimension. 
signalsize 
Number of elements in each input and output signal of S. Setting signalsize greater than 1 lets you specify a summing junction that operates on vectorvalued signals. Default: 1 
signames 
Signal names to replace one alias (signal name beginning with %) in the formula string. You must provide one signames argument for each alias in formula. Specify signames as:

S 
Transfer function for the summing junction, represented as a MIMO tf model object. 
Summing Junction with ScalarValued Signals
Create the summing junction of the following illustration. All signals are scalarvalued.
This summing junction has the formula u = u1 + u2 + u3.
S = sumblk('u = u1+u2+u3');
S is the transfer function (tf) representation of the sum u = u1 + u2 + u3. The transfer function S gets its input and output names from the formula string.
S.OutputName,S.Inputname
ans = 'u' ans = 'u1' 'u2' 'u3'
Summing Junction with VectorValued Signals
Create the summing junction v = u  d where u,d,v are vectorvalued signals of length 2.
S = sumblk('v = ud',2);
sumblk automatically performs vector expansion of the signal names of S.
S.OutputName,S.Inputname
ans = 'v(1)' 'v(2)' ans = 'u(1)' 'u(2)' 'd(1)' 'd(2)'
Summing Junction with VectorValued Signals That Have Specified Signal Names
Create the summing junction
$$\begin{array}{l}\text{e}\left(1\right)=\text{setpoint}\left(1\right)\text{alpha}+\text{d}\left(1\right)\\ \text{e}\left(2\right)=\text{setpoint}\left(2\right)\text{q}+\text{d}\left(2\right)\end{array}$$
The signals alpha and q have custom names that are not merely the vector expansion of a single signal name. Therefore, use an alias in the formula specifying the summing junction.
S = sumblk('e = setpoint  %y + d', {'alpha';'q'});
sumblk replaces the alias %y with the cell array {'alpha';'q'}.
S.OutputName,S.Inputname
ans = 'e(1)' 'e(2)' ans = 'setpoint(1)' 'setpoint(2)' 'alpha' 'q' 'd(1)' 'd(2)'