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Control valves
Published in Raymond F. Gardner, Introduction to Plant Automation and Controls, 2020
When a pneumatic signal is sent to a control-valve actuator, there is typically a small delay in response, until the signal grows large enough to produce an adjustment. Delays in valve response can be due to unbalanced pressure forces on the valve disk, especially when closed, the slip-and-stick action of friction in the moving parts, or looseness in linkages due to manufacturing tolerances. Three response imperfections are hysteresis, stiction, and deadband, and are shown in Figure 9.23. Hysteresis is caused by friction within the valve itself that results in a difference between the valve position on its upstroke compared its position on the downstroke for the same input signal. Hysteresis occurs from the reversal of the friction direction. Deadband, on the other hand, occurs when the valve reverses direction but there is no valve-stem movement until all tolerances in the actuator are taken up. Stiction is similar to deadband but does not depend on a direction change. With stiction, the signal demands a change, but the signal is not strong enough to reposition the valve mechanisms. Eventually, the process deviates sufficiently from its setpoint so that the corrective signal becomes strong enough to overcome static friction, and the actuator jumps. Stiction is a result of the stick-and-slip nature of friction.
Fundamentals of Controller Performance Diagnosis
Published in Raghunathan Rengaswamy, Babji Srinivasan, Nirav Pravinbhai Bhatt, Process Control Fundamentals, 2020
Raghunathan Rengaswamy, Babji Srinivasan, Nirav Pravinbhai Bhatt
Numerous surveys have indicated that nearly 20-30% of all process control loops oscillate due to stiction and lead to loss of productivity [8, 19, 64]. A process control loop where the process block is separated from a valve block (where stiction usually occurs) is depicted in Figure 10.1. The presence of stiction in control valve introduces nonlinear behaviour between the controller signal and manipulated variable. Stiction prevents proper valve movement and is one of the common causes for oscillations in industrial control loops, leading to poor performance, inferior quality products, and larger than normal rejection rates. Considerable attention has been paid to the development of techniques for detection and quantification of stiction in control valves [11, 40].
Low Current and High Frequency Miniature Switches: Microelectromechanical Systems (MEMS), Metal Contact Switches
Published in Paul G. Slade, Electrical Contacts, 2017
Benjamin F. Toler, Ronald A. Coutu, John W. McBride
As mentioned earlier, stiction or adhesion is a failure mode which is commonly caused by capillary, electrostatic, chemical, and van der Waals forces [88]. The surface of contacts in air can become hydrophilic due to oxidation and the formation of a liquid meniscus by water vapor causes stiction [81]. Many researchers have proposed reducing the surface adhesion force by novel switch design, contact materials, and sealing the micro-contacts in inert gases [3,81,95–98]. Adhesion can be described by Hertz, JKR, or DMT theories [99]. Hertz theory, mentioned in the contact resistance modeling section, is traditionally used for modeling elastic adhesion between non-deformable surfaces [99]. For deformable surfaces, JKR or DMT theory is utilized. The JKR theory takes into account the surface energy of the contacting interfaces. Comparatively, DMT theory emphasizes the cohesive forces at the contact periphery [99]. The JKR model is valid for “soft” elastic materials with higher surface energy while the DMT model is applicable for “hard” stiff solids with low surface energy [99].
Material-related and various dependences of adhesion force on piezo velocity revealed on an AFM at moderate humidity
Published in The Journal of Adhesion, 2023
Tianmao Lai, Siyuan Qiu, Runsheng Wang
Adhesion forces at solid-solid interfaces play a significant role in developing micro/nano devices, such as micro-electro-mechanical systems (MEMS). With the further miniaturization of small-scale systems, surface effects are dominant. The adhesion force between small-scale structures can cause a “stiction” problem. The adhesion force is the primary failure mechanism in MEMS.[1,2] Over the past few decades, studies have been extensively carried out to deepen the understanding of adhesion forces.