Parallel system interconnection using control.interconnect()

[sdahdah]

I'm trying to use control.interconnect() to connect one system input to two subsystem inputs, but I can't figure out a clean way to do it. The best way I've found so far is to create a dummy summing junction for this purpose, as shown below:

import control

# Systems to connect in parallel
a = control.TransferFunction([1], [1], name='a')
b = control.TransferFunction([1], [2], name='b')
sum = control.summing_junction(2, name='sum')
split = control.summing_junction(1, name='split')

# Interconnection using ``control.interconnect``
P_interconnect = control.interconnect(
    syslist=[a, b, sum, split],
    connections=[
        ['sum.u[0]', 'a'],
        ['sum.u[1]', 'b'],
        ['a', 'split'],
        ['b', 'split'],
    ],
    inplist=['split'],
    outlist=['sum'],
)
print(P_interconnect)

# Interconnection using ``control.parallel``
P_parallel = control.parallel(a, b)
print(control.tf2ss(P_parallel))

Am I missing something, or is this the recommended way to feed two subsystem blocks the same system-level input?

[slivingston]

Did you try to use the inputs and outputs parameters? These are shown in the "auto-splitting" section of examples/interconnect_tutorial.ipynb.

[sdahdah]

That makes sense, thanks! Does that only work when connecting signals by name (i.e. when connections is left unspecified)?

[slivingston]

connections specifies internal connections (i.e., between subsystems), whereas auto-splitting behavior only affects how system inputs map to subsystem inputs. Therefore, connections can be given when doing this.

Note that automatically (also known as "implicitly") making internal connections will not work with an explicit connections argument. For reference, the relevant part of the code is

python-control/control/nlsys.py

Lines 2260 to 2273 in 1bb0a46

	# If connections was not specified, assume implicit interconnection.
	# set up default connection list
	if connections is None:
	connection_type = 'implicit'
	# For each system input, look for outputs with the same name
	connections = []
	for input_sys in syslist:
	for input_name in input_sys.input_labels:
	connect = [input_sys.name + "." + input_name]
	for output_sys in syslist:
	if input_name in output_sys.output_labels:
	connect.append(output_sys.name + "." + input_name)
	if len(connect) > 1:
	connections.append(connect)

[sdahdah]

Got it. Thanks for the help!