The input to the finite state automata associated with controlled junctions are the power variables, effort and flow, and their output are control signals that determine the on/off state of the associated junction. Each controlled junction has an associated control specification function, called CSPEC, which switches the junction on and off. The state transition diagram from which the CSPEC is derived may have several internal states that map onto the on and off signals but in every transition sequence on/off signals have to alternate. Furthermore, CSPEC conditions have to result in at least one continuous mode of operation for all reachable energy distributions.
The set of local control mechanisms associated with controlled junctions constitute the signal flow model of the system. The signal flow model performs no energy transfer, therefore, it is distinct from the bond graph model that deals with the dynamic behavior of the physical system variables. Signal flow models describe the transitional, i.e., mode-switching behavior of the system. A mode of a system is determined by the combination of the on/off states of all the controlled junctions in its bond graph model. Note that the system modes and transitions are dynamically generated, they do not have to be pre-enumerated.
When on, a controlled junction behaves like a normal junction, but
when off it forces either the effort or flow value at all
connected bonds to become 0, thus inhibiting energy transfer
across the junction.
Therefore, controlled junctions exhibit ideal switch behavior,
and modeling discontinuous behavior in this way is consistent
with bond graph theory[12]. Deactivating controlled
junctions can affect the behaviors at adjoining junctions, and,
therefore, the causal relations among system variables.
Controlled junctions are marked by subscripts (e.g., 
, 
).
They define the
interactions between the signal flow and energy flow models of the
system.
The use of controlled junctions is illustrated for the
bullet-wood system whose hybrid model is shown in Fig. 1.
and 
are the
inertias (masses) of the bullet and wood block,
respectively. When the bullet attaches
itself to the wood block, the model switch occurs because the
0-junction turns on when 
. Once the
bullet is lodged in the wood there is no mechanism to dislodge it,
therefore, once turned on this junction cannot be turned off.
This is indicated by the FALSE condition on the on/off transition
for the junction. This example illustrates a seamless integration of
multi-mode behaviors in one model. Other examples of hybrid bond
graph models are discussed in [9, 10].