Investigation of Separate Meter-In Separate Meter-Out Control Strategies: How does the separate meter-in separate meter-out control system work and what is the limits for separate control of the hydraulic cylinder? What are the consideration associated with chosen the most suitable control strategies for the hydraulic cylinder? What methods are sufficient for decoupling the separate meter-in separate meter-out hydraulic system?
Translated title
Investigation of Separate Meter-In Separate Meter-Out Control Strategies
Author
Berthing, Anders
Term
4. term
Education
Publication year
2019
Submitted on
2019-10-14
Pages
109
Abstract
Denne afhandling undersøger og designer styringer til en hydraulisk cylinder med separate meter-in/separate meter-out (uafhængig regulering af ind- og udløbsflow med to proportionalventiler). Formålet er at forstå, hvornår konceptet fungerer godt, og hvordan man undgår kritiske driftsforhold. Analysen dækker både positive og negative cylindrehastigheder samt to lasttyper: en overkørende last (som hjælper cylinderen) og en resistiv last (som modarbejder bevægelsen). Resultaterne viser to centrale begrænsninger: risiko for kavitation (dannelse af dampbobler ved for lavt tryk) og for højt tryk. Kavitation kan opstå, når hastigheden er positiv, lasten er overkørende, og der bruges meter-in-regulering, som begrænser tilstrømningen til stempelkammeret. Et tilsvarende problem kan opstå ved negativ hastighed, resistiv last og meter-in-regulering, hvor stangkammeret får for lidt olie. For højt tryk kan opstå, når der bruges meter-out-regulering ved positiv hastighed med overkørende last eller ved negativ hastighed med resistiv last. For at håndtere disse kritiske situationer sammenlignes to styringsstrategier: en slavefunktion, hvor det ene ventilinput afhænger af det andet (så kun én tilstand kan kontrolleres), og uafhængig styring af de to proportionalventiler. På grund af bedre fleksibilitet og ydeevne vælges uafhængig ventilstyring. Med uafhængig styring vælges cylindrens hastighed som primær styrevariabel og tryk som sekundær (enten på stempel- eller stangsiden). En koblingsanalyse med relative gain array (RGA) og singulærværdinedbrydning (SVD) viser, at parringen mellem hastighed og stangtryk giver mindst indbyrdes påvirkning. Derfor styres hastighed af inputsignalet up, mens stangtrykket styres af inputsignalet ur. For at afkoble systemet designes en prækompensator, så det kan betragtes som to uafhængige én-indgang/én-udgang-sløjfer. Der udvikles to PI-regulatorer (proportional–integral), hvor trykregulatoren er 10 gange hurtigere end hastighedsregulatoren for at undgå indbyrdes forstyrrelser. På en lineær model følger både hastighed og tryk tilfredsstillende. På en ikke-lineær model forringes hastighedssporing ved negative referencesignaler, fordi modellens parametre ændrer sig. Problemet løses ved at designe et ekstra sæt PI-regulatorer baseret på en lineær model for negative hastigheder og skifte mellem de to sæt afhængigt af referencesignalet. Denne løsning forbedrer hastighedssporingen og viser, at en velfungerende styring for separate meter-in/separate meter-out kan opnås.
This thesis investigates and designs controllers for separate meter-in/separate meter-out control of a hydraulic cylinder, where inlet and outlet flows are regulated independently by two proportional valves. The goal is to identify when the concept works well and how to avoid critical operating conditions. The study covers both positive and negative cylinder velocities and two load types: an overrunning load (assisting motion) and a resistive load (opposing motion). The analysis identifies two key limitations: the risk of cavitation (vapor bubbles due to very low pressure) and excessive pressure. Cavitation can occur with positive velocity, an overrunning load, and meter-in flow control that restricts supply to the piston chamber. A similar risk exists with negative velocity, a resistive load, and meter-in control that starves the rod chamber. Excessive pressure can occur when using meter-out control with positive velocity and an overrunning load, or with negative velocity and a resistive load. To handle these critical cases, two control strategies are compared: a slave-function approach, where one valve input depends on the other (so only one state can be controlled), and fully independent control of the two proportional valves. Independent valve control is chosen for greater flexibility and performance. With independent control, cylinder velocity is selected as the primary controlled variable and pressure as the secondary (either piston-side or rod-side). A coupling analysis using the relative gain array (RGA) and singular value decomposition (SVD) indicates the weakest interaction when pairing velocity with rod-side pressure. Accordingly, velocity is controlled by input up, and rod pressure by input ur. A pre-compensator is designed to decouple the system so it behaves like two single-input single-output loops. Two proportional–integral (PI) controllers are then designed, with the pressure loop 10 times faster than the velocity loop to reduce interference. On a linear model, both velocity and pressure tracking are acceptable. On a nonlinear model, velocity tracking degrades for negative velocity references due to changing model parameters. This is addressed by designing an additional set of PI controllers based on a linear model for negative velocities and switching between controller sets depending on the reference. This improves velocity tracking and demonstrates that an effective control method for separate meter-in/separate meter-out can be achieved.
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