Stiction: Definition and Discussions
- PDF / 635,894 Bytes
- 9 Pages / 441 x 666 pts Page_size
- 99 Downloads / 201 Views
Stiction: Definition and Discussions
The presence of nonlinearities such as stiction, deadband and backlash in a control valve limits the control-loop performance. Stiction is the most common problem in spring-diaphragm-type valves, which are widely used in the process industry. There have been many attempts to understand, define and model the stiction phenomenon, and several of these will be reviewed in this chapter. The chapter also presents an approach to stiction modelling that can be related directly to the practical situation as observed in real valves in the process industry. It focusses on the definition of stiction and understanding of the mechanism that causes stiction.
11.1 Introduction The moving part in a process control loop is a control valve. If the control valve develops nonlinearities such as stiction, backlash and deadband, the valve output may be oscillatory, which in turn can cause oscillations in the process output. Among the many types of nonlinearities in control valves, stiction is the most common and one of the long-standing problems in the process industry. It hinders the achievement of good performance of control valves as well as control loops. Many studies (Horch, 2000; McMillan, 1995; Sharif and Grosvenor, 1998; Horch and Isaksson, 1998; Horch et al., 2000; Aubrun et al., 1995; Wall´en, 1997; Taha et al., 1996; Gerry and Ruel, 2001; Ruel, 2000; Armstrong-H´elouvry et al., 1994) have been conducted to define and detect static friction or stiction. However, a unique definition and description of the mechanism of stiction have not been available previously.
11.2 What Is Stiction? Terms such as deadband, backlash and hysteresis are often misused and wrongly used in describing valve stiction. For example, a deadband in a valve is commonly referred to as backlash or hysteresis. Therefore, before proceeding to the definition of stiction, these terms are defined for a better understanding of the stiction mechanism and a formal definition of stiction. M. A. A. S. Choudhury et al., Diagnosis of Process Nonlinearities and Valve Stiction, c Springer-Verlag Berlin Heidelberg 2008
143
144
11 Stiction: Definition and Discussions
11.2.1 Definition of Terms Relating to Valve Nonlinearity This section reviews the American National Standard Institution’s (ANSI) formal definition of terms related to stiction. The aim is to differentiate clearly between the key concepts that underlie the ensuing discussion of friction in control valves. These definitions can also be found in EnTech, (1998) and Fisher-Rosemount, (1999), which also refer to ANSI. ANSI (ISA-S51.1-1979, Process Instrumentation Terminology) defines the above terms as follows: • Backlash: “In process instrumentation, it is a relative movement between interacting mechanical parts, resulting from looseness, when the motion is reversed”. • Hysteresis: “Hysteresis is that property of the element evidenced by the dependence of the value of the output, for a given excursion of the input, upon the history of prior excursions and the directio
Data Loading...
