Implementation of a PID Controller for Building Automation

Abstract: This master thesis project has been performed in cooperation with TAC, a building automation company in Malmö and the Department of Automatic Control at LTH. TAC has experienced problems for some time with their PID controllers. This master thesis has therefore investigated the use of PID control at TAC, the problems with the existing controllers. The thesis also describes a new PID controller that has been implemented at TAC. The issues treated in the thesis are not only technical but also deal with the processes within a company when changing a control algorithm. The non-technical issues are for instance how the force of habit affects the development and that backward compatibility must be ensured. The new algorithm that was decided on and implemented is of a parallel-positional form. This basic form of the PID algorithm is proven to be the best choice. From a control point of view the controller successfully controls the processes in TAC's area of business. The PID controllers at TAC are used to control HVAC (Heating Ventilation and Air Condition) systems. The conclusion that the new PID controller is successful is based on simulations as well as tests at a commercial system. The PID controller is described in detail in the thesis. Features of the controller like setpoint weighting is described, as well as a tracking signal that allows the controller to be forced to externally specified values. Other features that are described include an offset added during control without the integral part, bumpless transfer, a rate of change limiting mechanism and other necessary features to make the controller practically usable. An investigation concerning possible concepts that may be useful in a new application programming tool has been performed as a part of the master thesis. The concepts determined to suit TAC are incorporated in IEC 61131. The concepts discussed in detail are amongst others SFC, the use of processes-tasks, instantiation and inheritance.