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Abstract Model based control strategies are not well established in the heating
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ventilating and air conditioning industry yet. However, these techniques offer
considerable advantages over traditional approaches based on expert knowledge
and therefore may overcome deficiencies of existing control concepts. This paper
outlines a model based control policy for an industrial test plant. The method uses
describing functions to avoid undesired limit cycles. The proposed technique proves
to be a systematic tool to improve the performance of heating ventilating and air
conditioning systems.5573
1 Introduction
The main objectives of heating ventilating and air conditioning (HVAC) systems are
temperature and humidity control. In [4] the problem of limit cycles in temperature
control systems is analyzed. For an air conditioning system of an office room, the
describing function method is used to eliminate periodic oscillations of the room air
temperature. Simulations reveal that the proposed approach is suitable for improving
an existing PI-controller. In this article an industrial HVAC pilot plant is introduced
in order to verify these simulation results in practice.
The paper is organized as follows: In “Test Plant” a mathematical descrip-
tion of the test plant and of its components is outlined. “Describing Function
Method” is dedicated to the describing function method, “Conclusion” concludes
the manuscript.2Testplant
In the following subsections, a functional description as well as a mathematical
model of the pilot plant shown in Fig. 1 are given. The pilot plant was erected by the
company Fischer&Co.
1
It serves for research purposes exclusively and offers the
opportunities of
• Heating / cooling
• Humidifying / dehumidifying
air which can be transported to a factory building and/or to a test room.2.1 Description of the test plant
A photo and a schematic representation of the test plant are shown in Fig. 1.Solely
standard industrial HVAC components are integrated into the plant in order to
provide realistic testing conditions. A CPU from the company Bernecker&Rainer
2
is used as a programmable control unit.
The major components are labeled in Fig. 1. The air passes the plant from right
to the left. Via air dampers, the mixing ratio of outer air and air from the test room
(150m3
) or the factory building (5000m3
) can be adjusted. A fan is installed
to transport the air. The heating and cooling coils are installed to increase and to
decrease the air temperature. The air humidity can be adjusted with the help of a
steam humidifier. The air dampers on the left hand side of the test plant are used
to select whether the test room or the factory building is supplied with conditioned
air. In the present application, the room air temperature #a (see Fig. 1) is controlled
using the heating coil (see “Describing Function Method”).
In order to preserve transparency, only the utilized components, i.e. heating coil
and room, will be explained in detail.
The used heating coil is a finned tube type heat exchanger. A fluid with a higher
temperature than the air passing the heating coil increases the air temperature.
With the help of a hydraulic circuit, the fluid temperature of the heating coil and
consequently the heating power can be adjusted.
In the following experiments, the test room is supplied with conditioned air. The
temperature #a of the air leaving the test room is the controlled variable.
2.2 Model of the test plant
In this section mathematical models of the HVAC components required for the ap-
plication of the describing function method and for controller design are presented.
2.2.1 Heating coil