Plug & Play Control of Hydraulic Networks

Typically, control systems are designedwith little or no consideration for possible changes in the structure of the system process to be controlled. In classic control design, a monolithic approach is taken where structural changes in the system process require the development of a new mathematical model of the system and a subsequent redesign of the control system. This process can be expensive and time consuming. Therefore, an attractive alternative is to design the control system such that it automatically reconfigures whenever structural changes occur. This is the aim of the Plug & Play Process Control
research program, which the work presented here is a part of.
An industrial case study involving a large-scale hydraulic network with non-linear dynamics is studied. The hydraulic network underlies a district heating system, which provides heating water to a number of end-users in a city district. The case study considers a novel approach to the design of district heating systems in which the diameter of the pipes used in the system is reduced in order to reduce the heat losses in the system, thereby making it profitable to provide district heating to areas with low energy demands. The
new structure has the additional benefit that structural changes such as the addition or removal of end-users are easily implementable. In this work, the problem of controlling the pressure drop at the end-users to a constant reference value is considered. This is done by the use of pumps located both at the end-users and at designated places across the network.
The control architecture which is used consists of a set of decentralized linear control actions. The control actions use only the measurements obtained locally at each end-user. Both proportional and proportional-integral control actions are considered. Some of the work considers control actions which are constrained to non-negative values only. This is due to the fact that the actuators in this type of system typically consist of centrifugal pumps which are only able to deliver non-negative actuation. Other parts of the work consider
control actions which have been quantized. That is, they are restricted to piecewise constant signals taking value in a bounded set. This is done in order to facilitate sending the control signals across a finite bandwidth communication network. This is necessary since the actuators in the system are geographically separated from the logic circuitry implementing the control actions.
The results presented here consist of a series of global stability results of the closedloop system using the control actions described above. The stability analysis is complicated by the non-linearities present in the system process. Specifically, global practical output regulation can be shown when using proportional control actions, while global asymptotical output regulation can be shown when using proportional-integral control actions. Since the results are global in the state space, it is concluded that the closed-loop system maintains its stability properties when structural changes are implemented.