By Mohieddine Jelali, Biao Huang
The presence of non-linearities, akin to stiction and deadband, locations limits at the functionality of keep an eye on valves. certainly, within the method industries, stiction is the most typical valve challenge, and over the past decade a variety of various suggestions for overcoming it were proposed.
Detection and analysis of Stiction on top of things Loops represents a finished presentation of those equipment, together with their rules, assumptions, strengths and downsides. instructions and dealing techniques are supplied for the implementation of every procedure and MATLAB®-based software program may be downloaded from www.ualberta.ca/~bhuang/stiction-book allowing readers to use the the right way to their very own info. equipment for the obstacle of stiction results are proposed in the basic context of:
• oscillation detection up to speed loops;
• stiction detection and prognosis; and
• stiction quantification and prognosis of a number of faults.
The cutting-edge algorithms offered during this publication are proven and in comparison in business case reviews of numerous foundation – chemical substances, construction, mining, pulp and paper, mineral and steel processing. Industry-based engineers will locate the ebook to be precious suggestions in expanding the functionality in their regulate loops whereas educational researchers and graduate scholars attracted to keep an eye on functionality and fault detection will find a wealth of static-friction-related examine and invaluable algorithms.
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Extra resources for Detection and Diagnosis of Stiction in Control Loops: State of the Art and Advanced Methods
F. Thornhill, M. L. Shah – If the slope of the input signal is positive, the sign(slope) returns +1. – If the slope of the input signal is negative, the sign(slope) returns –1. – If the slope of the input signal is zero, the sign(slope) returns 0. op(k) Look-up table (converts mA to valve %) x( k ) no xss = xss y (k ) = 0 x(k ) > 0? yes xss = xss y(k ) = 100 no yes x(k ) < 100? v− new = [ x(k ) − x(k − 1)] / Δt no no sign(v− new) = 0? sign(v− new) = sign(v− old)? yes I =0 yes I =1 yes | x(k ) − xss |> J ?
Moreover, it seems that the valve shows some asymmetrical behaviour since it sometimes sticks in one direction but not in the other. Controller Output, OP 90 85 80 75 70 65 8000 8050 8100 8150 8200 8250 8300 8350 8400 8450 8500 (a) Valve Position, MV 75 actual predicted 70 65 60 8000 8050 8100 8150 8200 8250 Samples 8300 8350 8400 8450 8500 (b) Fig. 5 Summary and Conclusions This chapter has described valve-stiction models. Both physics-based models and data-driven models are discussed. Although a physics-based model gives insights into the stiction phenomena that are observed in practice, it is difficult to use for simulation purposes because it is hard to determine the physical parameters, such as spring constant and static and dynamic friction forces.
21]. 2 Two-parameter Stiction Model For a dynamic stiction model, the challenges are (i) to model the tendency of the valve to stay moving once it has started until the input changes direction or the velocity goes to zero, and (ii) to include the effects of deadband and the slip jump. This section describes two-parameter stiction models that can meet these challenges. The two-parameter stiction model first appeared in Choudhury et al. . Parameters of 2 Stiction Modelling 25 this valve-stiction model can be related directly to plant data and, furthermore, such a model produces the same open-loop and closed-loop behaviour as the physical model.
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