Development of Control Strategies for the SvDP-Concept

Abstract

Hastigheds variabel differentialpumpe (SvDP)-konceptet, designet af Bertelsen og Madsen i samarbejde med Bosch Rexroth A/S, har til formål at være et alternativ til den klassiske hydraulikopsætning, hvor en ventil bruges til at styre olieflowet. Ventilen er erstattet af to pumper, som er monteret på samme aksel, hvilket resulterer en højere energieffektivitet af det hydrauliske system. Indledningsvist er en ulineær simulerings model af systemet etableret. Simuleringsmodellen er verificeret ved hjælp af eksperimentelle målinger fra laboratoriet. For at belyse egenskaberne af SvDP-konceptet, er en grundig stationær analyse af systemet udført. Analysen af systemet afslører at trykket i systemet stiger når omløbsretningen på pumperne er negativ, hvorimod trykket altid falder når omløbsretningen er positiv. Det betyder at trykket kun kan kontrolleres ved negativ omløbsretning af pumperne. To typer af trajektorier er designet til at evaluere ydelsen af de designede regulatorer. To trajektorier, baseret på femtegrads polynomier, er designet for at sikre at kravene til acceleration og hastighed er realiserbare. Ud over disse trajektorier, er to rampe baserede trajektorier designet, som kræver spring i hastighed. Modellen er lineariseret og de kritiske operationspunkter er identificeret på baggrund af den indledende system analyse, samt de designede trajektorier. Det lineære system, evalueret i de kritiske operationspunkter, resulterer i to lineære systemer, som afhænger af omløbsretningen. En decentraliseret kontrolstrategi er designet for de to lineære systemer for at SISO kontrol kan anvendes. Grundet begrænsninger i laboratoriet, hvor kvaliteten af hastigheds signalet er lav, designes to sæt regulatorer. Et sæt regulatorer er designet konservativt, sådan at disse kan testes, på trods af den lave kvalitet af hastigheds signalet. Derudover er et sæt regulatorer designet til at vise den potentielle ydeevne af systemet. Regulatorerne, designet for høj ydeevne, anvender tilbagekobling af det virtuelle lasttryk for at øge dæmpningen i systemet. Proportionalforstrækningen i hastighedsregulatorerne umuliggør implementering, da det støjfyldte hastigheds signal resulterer i et uhensigtsmæssigt kontrol signal. De konservativt designede regulatorer er implementeret og testet i laboratoriet og viser at konceptet virker. Rms hastighedsfejlen, målt i laboratoriet, afviger fra simuleringerne med 23-55 \%. Hvis det antages at denne afvigelse også er repræsentativ for simuleringerne med høj-ydeevne-regulatorerne, kan det forventes at opnå en rms hastigheds fejl på 2-3 mm/s. Det er derfor konkluderet at SvDP-konceptet kan anvendes som et alternativ til den traditionelle ventil-cylinder løsning.

The Speed-variable Differential Pump (SvDP)-concept, designed by Bertelsen and Madsen (2013) in collaboration with Bosch Rexroth A/S, is intended to be an alternative to the classic hydraulic setup, which utilizes a valve to direct the oil flow. The valve is replaced with two pumps mounted on the same servo drive, resulting in a higher energy efficiency of the hydraulic system. Initially, a nonlinear simulation model of the system is established. The simulation model is verified using experimental measurements, obtained in the test facility provided by Bosch Rexroth A/S. To highlight the properties of the SvDP-concept, a thorough steady state analysis of the system is performed. The analysis of the system reveals that the system pressure levels rise when the direction of rotation in the pumps is negative, while the pressure levels always decrease when the direction of rotation is positive. This means that the pressures, in the current configuration of the SvDP-concept, can only be controlled during negative rotation of the pumps. Two types of trajectories are designed to evaluate the performance of the designed controllers. Two trajectories based on quintic functions are designed, ensuring realizable demands to acceleration and velocity of the piston. In addition to the quintic trajectories, two ramp based trajectories are designed, which require step in velocity. The model is linearized, and the critical operating points are identified, based on the results of the steady state analysis and the designed trajectories. The linear system, in the critical operating points, results in two linear systems depending on the direction of rotation. A decentralized control strategy is designed for the two linear systems, such that SISO control can be applied. Two sets of controllers are designed, due to limitations in the test facility, where a poor velocity estimate is available. A set of conservative controllers are designed, which can be implemented and tested despite the limitations, and a set of high performance controllers are designed to prove the potential of the SvDP-concept. The high performance controllers utilize pressure feedback to improve the damping of the system, but the proportional gain of the PI-controllers regulating the velocity, prevents implementation, as a noisy position signal results in chattering of the control signal. The conservative controllers are implemented and tested, resulting in a proof of concept. The rms velocity error, obtained from the experimental results, deviate from the simulations by 23-55 %. Assuming this deviation applies to the simulation results, for the high performance controllers, an rms velocity error of 2-3 mm/s can be expected, when a decent velocity feedback is available. It is therefore concluded that the SvDP-concept can be an alternative to the traditional valve-cylinder configuration.

A master thesis from Aalborg University

Development of Control Strategies for the SvDP-Concept