Control of an Electronic Expansion Valve Using an Adaptive Pid Controller

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Control of an Electronic Expansion Valve Using an Adaptive PID Controller Antônio A. T. Maia*, Marconi A. Silva, Ricardo N. N. Koury, Luiz Machado, Alexandre C. Eduardo Federal University of Minas Gerais, Mechanical Engineering Department, Belo Horizonte, Minas Gerais, Brazil

Phone: + (55 31) 3409 6667, Fax: + (55 31) 3443 3783, e-mail: aamaia@ufmg.br

ABSTRACT
In many refrigeration systems, electronic expansion devices have been used to replace the conventional expansion devices like capillary tubes and thermostatic expansion valves. The electronic expansion devices are usually provided with an automatic controller that is responsible for determining the valve opening that keeps the superheat at the outlet of the evaporator within the desired limits. Most of these controllers permit only the adjustment of the desired superheat, the proportional, the integral and the derivative gains. After being adjusted for one operating point, these parameters do not suffer any automatic correction even when the operating conditions changes. This could penalize the system efficiency because the controller parameters defined initially may not be the most suitable for the system when operating in this new condition. Within this context, in this work it was developed an adaptive PID-controller to regulate the superheat degree at the outlet of the evaporator. A dynamic model obtained from experimental tests was used in the controller design. The controller effectiveness was evaluated by means of computer simulation and through experimental tests. The results obtained showed that the employed technique is effective in regulating the superheat degree at the outlet of the evaporator with an acceptable performance.

1. INTRODUCTION
The increase in the energy prices in the last decades has motivated many research works to identify great energy consumers and ways to improve the efficiency of these systems. In this context, refrigerating machines have a representative participation in the daily energy consumption. Most of the domestic refrigerating systems are equipped with a capillary tube or a thermostatic valve as expansion device. These expansion devices are not able to deal with wide range of operation conditions and they also present some response lag. Bearing in mind the same installation, energy savings can be obtained by replacing the conventional expansion device by an electronic expansion valve (EEV). The employment of this valve can be advantageous when compared with the conventional expansion devices because it has shorter response time and the controller used in most of these systems is generally able to keep the superheat close to the optimal value under every condition, which contributes to improve the refrigerating capacity (Lazzarin & Noro, 2008; Fallahsohi, 2010). The PID controller that comes with the electronic expansion valve normally permits adjustments in the proportional, integral and differential gains. If it is not properly adjusted the system can display a less efficient response or even an unstable response. On the other hand, when the controller gains are correctly defined it is generally done for one operating point and these settings do not suffer any automatic change even when the machine is working in a different operation point, which also may reduce the system efficiency. A strategy that could be used to overcome these problems is to use a controller with an auto tuning algorithm. To improve its efficiency, this algorithm should be executed continuously and the controller gains updated at every change in the operating point. Several control methods are available for controlling the superheat at the outlet of the evaporator using an electronic expansion valve. Outtagarts et al. (1997) proposed two control algorithms (PD and Qualitative Optimal Regulation), based on the system characteristics obtained from experimental data, to control an EEV. Ekren and Küçüka (2009) proposed a fuzzy logic...
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