Friction Compensation in Pneumatic Control Valves Through Feedback Linearization
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Friction Compensation in Pneumatic Control Valves Through Feedback Linearization João R. Baeza1
· Claudio Garcia1
Received: 22 October 2017 / Revised: 21 January 2018 / Accepted: 28 March 2018 © Brazilian Society for Automatics–SBA 2018
Abstract Control valves are very important actuators in the process industry. Due to friction in the stem, they present a nonlinear behavior, which can generate errors in the stem position and oscillations in control loops. This paper presents five control algorithms based on feedback linearization technique, to deal with pneumatic control valves affected by high friction in their stems. These control algorithms measure the valve position and manipulate the actuator pressure, acting as a valve positioner. The paper also shows practical results with a control valve operating in a real flow control loop. Keywords Control valves · Friction · Nonlinear control · Feedback linearization · Sliding mode control · Internal model control
1 Introduction Although all the industrial processes are nonlinear, the majority of the applied control algorithms use linear control techniques, tuned at a defined operating point, mainly based on the PID controller, due to its well-known theory and simple structure, which allows to use it in different industrial applications (Mishra et al. 2014, 2015). However, several of these control loops present low performance, such as variability or errors relative to the set point, due to poor tuning, disturbances or nonlinearities (Arifin et al. 2014). Variability in control loops impacts the quality of the final product, accelerate equipment wear and increase the energy and raw material waste (Bacci et al. 2016). Control valves are one of the most used final control elements in the process industry. They are mainly responsible to control the flow, a fundamental task in the chemical and petrochemical industries, paper and cellulose, water and sewage treatment, etc. According to Kayihan and Doyle (2000), a typical plant can have up to 6000 valves.
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João R. Baeza [email protected] Claudio Garcia [email protected]
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As mechanical systems they are subject to friction, which produces phenomena such as backlash and stick-slip motion that are nonlinearities which may cause variability and set point errors in the control loops (Hägglund 2007). According to Srinivasan and Rengaswamy (2008), almost one-third of the low performance control loops are caused by friction in control valves. Therefore, an upgrade in their performance would generate an important impact in the process industry. High friction in control valves increases variability in control loops (Srinivasan and Rengaswamy 2005), which reduces the quality of the final product and causes energy waste. The first studies about friction started in the sixteenth century with Leonardo da Vinci, but the formal idea of a force which opposes the motion was consolidated by Coulomb in the eighteenth century. However, due to its importance and complexity, the researches in this area have never stopped. Several
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