CFD Analysis, Sensing and Control of a Rotary Pulse Width Modulating Valve to enable a Virtually Variable Displacement Pump
2017-08
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CFD Analysis, Sensing and Control of a Rotary Pulse Width Modulating Valve to enable a Virtually Variable Displacement Pump
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2017-08
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Abstract
Hydraulic systems have been widely utilized for heavy duty industries for their competitive advantages of high power density, low cost, and flexible circuit design. However, the efficiency of hydraulic systems typically is not very competitive due to high throttling losses, which limits their applications. On/off valve based control of a hydraulic system is an approach that can potentially increase the hydraulic system's efficiency significantly. This approach combines the strengths of throttling valve control and variable displacement unit control. The former has the advantage of high control bandwidth and precision, but the disadvantage of low efficiency due to throttling loss; the latter has the advantage of high efficiency, but the disadvantage of being bulky, heavy, costly, and the control bandwidth is low when compared to valve control. To create a potentially high efficiency with relatively low cost solution, a fixed displacement pump, an accumulator, and a high speed on/off valve are combined to create a virtually variable displacement pump (VVDP). By pulse width modulating (PWM) the flow from the supply to the load via the on/off valve, the average output flow can be varied by adjusting the PWM duty ratio. The key technology to this hydraulic configuration is the pulse width modulated on/off valve. A novel rotary high speed on/off valve concept has been proposed. This concept can enable different digital hydraulic configurations, such as VVDP, VVDPM (pump/motor), and virtually variable displacement transformer. Research conducted in this dissertation supports the design, modeling, and control of the rotary on/off valve. A 3-way, high-speed, rotary, self-spinning on/off valve was developed for the VVDP configuration. The valve has two degrees of freedom. The spool's rotary motion realizes the high-speed switching required for the PWM function. This motion can be self-driven by capturing the fluid's angular momentum via a unique valve spool turbine design. The spool's axial motion determines the valve PWM duty ratio, and this motion is driven externally. Firstly, to understand the flow inside the valve, and to quantify the valve pressure drop with the key valve parameters, a computational fluid dynamics (CFD) analysis is conducted in chapter 2. Analytical and semi-empirical formulas to model the pressure drop across the valve spool as a function of flow rate and key valve geometrical parameters are developed. The torque generated by the valve turbines are also analyzed using CFD to validate the analytical models which calculate the torque as a function of flow rate and key valve geometrical parameters. These equations are utilized in an optimization analysis to optimize the valve geometry, targeted at reducing the valve's power loss. CFD is also utilized to optimize the valve's interior flow path to reduce the fluid volume inside the valve while maintaining a low pressure drop, so that both the compressible loss and the throttling loss of the valve are reduced. The CFD analysis enabled reducing the throttling loss pf a prototype valve design by 62.5% and reducing the compressible loss by 66%. Secondly, the sensing and estimation of the valve spool's rotary position and velocity are addressed in chapter 5. Given the limitation on sensing distance and the requirement of a simple sealing structure, a coarse, non-contacting, optical sensor is proposed to measure the spool's angular position. Measurement events in the form of encoder count changes are obtained at irregular times and infrequently. An event-based Kalman filter is developed to improve the resolution and to provide continuous estimates of the spool's angular position and velocity. Thirdly, the spool's axial motion actuation, sensing, and control development are addressed. The on/off valve's duty ratio is regulated by controlling the valve spool's axial position. In chapter 4, a driving mechanism to work with the self-spinning valve's feature and the corresponding sensing and control methods are developed to manipulate the spool's axial position. In the first generation's driving system, a geroter pump is hydro-statically connected to both ends of the spool chamber to move the spool axially. This design simplifies the sealing structure in order to achieve self-spinning. An optical sensor is utilized as a non-contact approach to measuring the spool's axial displacement. The measurement is corrupted by a structured noise caused by the spool's rotary motion. A periodic time varying model is proposed to model the structured noise, which can capture the main dynamics with a low order system. An analysis of the observability of the augmented system (plant plus structured noise) is conducted. A state observer can be built to distinguish between the axial spool position and the structured noise, and the estimated position can then be used in the control law. The sleeve chamber pressure dynamics are ignored, and a linear feed-forward with a Proportional-Integral controller is developed for spool axial positioning. The self-spinning function ties the spool rotary speed with the valve flow. The controller was experimentally implemented, and achieved good spool regulation results. In order to investigate the PWM frequency and the flow rate properties independently, an external driving mechanism is developed in chapter 5. A new passivity based nonlinear controller has been proposed which considers the pressure dynamics inside the sleeve chamber. This controller can provide more robust axial position control. From theoretical analysis' point of view, a passivity framework for hydraulic actuators is developed by considering the compressibility energy function for a fluid with a pressure dependent bulk modulus. It is shown that the typical actuator's mechanical and pressure dynamics model can be obtained from the Euler-Lagrange equations for this energy function and that the actuator is passive with respect to a hydraulic supply rate. The hydraulic supply rate contains the flow work $(PQ)$ and the compressibility energy, whereas the latter one has typically been ignored. A storage function for the pressure error is then proposed and the pressure error dynamics are shown to be a passive two port subsystem. Trajectory tracking control laws are then derived using the storage function. Since some of the states utilized in the passive controllers are from an estimator instead of being directly measured, the chapter also provides the analysis on the convergence of both tracking errors and the estimation errors to zero. This passivity-based nonlinear controller implemented with a high gain observer is applied experimentally on the valve. Experimental results validate the effectiveness of this new control system. Lastly, the VVDP is implemented as the variable displacement pump in a direct displacement control open circuit, as presented in chapter 6. A variable flow source (VVDP), a directional valve, and a proportional valve are coordinated to manipulate the motion of the hydraulic actuator in an energy efficient way. The passivity-based nonlinear controller as discussed in chapter 5 is proposed to realize accurate actuator trajectory tracking. A nominal method to optimally distribute the control efforts between the control valve and the variable flow pump is proposed. This method can accommodate different control bandwidths from the valve and the pump, so that the valve has a large nominal opening to reduce the throttling loss. Experimental results validate the effectiveness of the control strategy.
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University of Minnesota Ph.D. dissertation. August 2017. Major: Mechanical Engineering. Advisors: Perry Li, Thomas Chase. 1 computer file (PDF); xi, 174 pages.
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Wang, Meng. (2017). CFD Analysis, Sensing and Control of a Rotary Pulse Width Modulating Valve to enable a Virtually Variable Displacement Pump. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/193428.
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