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The Maha Fluid Power Research Center at Purdue University was formed in 2004 from a $4 million endowment from fluid power pioneer Otto Maha. The research center is a member of the Global Fluid Power Society (GFPS), a world-wide commmunity of institutes involved in promoting fluid power networking and awareness.
Fluid power is the discipline that involves the use of fluids to perform mechanical actuations. It is a well-established and independent discipline that has had a defined research area and scientific activities for at least seven decades. Many examples from our daily lives show how fluid power has enabled us to avoid long mechanical systems without affecting the energy density. Examples include electrohydraulic actuation systems in airplanes that move the wings, hydraulic actuation systems in mining that lift tons of ore, and hydro-mechanical power split transmissions in tractors. Several components inside the hydraulic system, such as pumps, valves and actuator sensors, use fluid power to multiply human strength and push, pull, lift, rotate or hold very heavy loads.
A recent study conducted by the U.S. Department of Energy shows that about five percent of the U.S. energy consumption is transmitted by fluid power equipment. Nevertheless, this study also shows that the efficiency of fluid power averages 21 percent. “This offers a huge opportunity to improve the current state of the art of fluid power machines, in particular to improve the energy consumption of current applications,” explains Dr. Andrea Vacca, associate professor at Purdue University and head of a research team of 17 people at the Maha Fluid Power Research Center.
Along with energy efficiency and study of lubrication principles, operator safety is a central concern of fluid power machine builders. This implies improving systems noise and vibration emission. Regarding these challenges, Vacca’s main research subjects at Maha are analyzing the source of noise and vibration generation.
The Maha Fluid Power Research Center is the largest academic laboratory in the United States entirely dedicated to research in fluid power. The research projects in the lab are supported by federal agencies as well as by industries. Vacca explains, “This is a unique experimental lab, with more than 40 researchers and numerous test rigs donated by industries.” Vacca’s research relates to hydraulic components, mostly pumps and valves, and optimization of the entire hydraulic system design. The research team supports several industrial players in solving engineering challenges in construction and agricultural machinery, aerospace and industrial manufacturing machinery.
The most important critical problem today is the ability to simulate the actual performance of a component in heavy equipment machines, considering all the physical aspects occurring at the fluid and structural levels. The traditional approach forces industries to conduct long and expensive experimental tests to refine the design. In general, this slows the progress of the fluid power industry to produce more efficient and human-friendly (silent, leakage-free) components. Fully aware of the internationally recognized competence of the research center, industrial companies typically contact Vacca’s team to take advantage of detailed numerical simulation of hydraulic components and systems.
“The fluid power industry is eager to move towards virtual prototyping design,” Vacca explains. “In the near future, all the components will be designed through simulation, thanks to progress in the manufacturing field, such as 3D printing.” Regarding this evolving environment, Vacca says, “The research team is particularly involved in studying this challenge and proposes modeling approaches for system or component design and optimization that can help the future industry in accelerating the progress of fluid power technology.”
To proceed, the Maha Fluid Power Research Center and Vacca have been working with Simcenter™ software such as Simcenter Amesim™ software for more than 10 years and LMS Virtual.Lab™ Acoustics software for seven years to support industries with modeling approaches. The use of the Simcenter portfolio of products helps to develop new strategies and design solutions to increase the efficiency and reliability of hydrostatic machines (pumps and motors). On the other hand, it enables the design of new layout architectures such as hydraulic hybrid and new control strategies to increase the controllability of the machines. Globally speaking, Vacca explains, “For this research particularly, we save a lot of time. Up to 50 percent of model implementation time can be saved by using Simcenter Amesim.”
The best example of Vacca’s use of Simcenter products is the current development of the hydraulic gear machines simulator (HYGESim), a numerical model for the simulation of external gear pumps and motors. The simulation tool consists of different modules: a lumped-parameter fluid dynamic model, a mechanical model for evaluating the gear motion (considering also the micro-motion of the gear axes of rotation) and a geometric model. The first two models are implemented within the Simcenter Amesim simulation environment, with proper submodels written in C language, while the geometric model is implemented by developing appropriate macros, capable of directly reading the 3D computer-aided design (CAD) models of the unit (pump/motor). This model is the basis of various research projects led by Vacca and his team at Maha Fluid Power Research Center.
There are numerous examples of mobile hydraulic machines used in agriculture, construction or load-lifting equipment; vibration on the load and the structure of the machine affects productivity and operator comfort and safety.
One of the goals of Vacca’s research team has been to develop an energy-efficient vibration damping control method which can be applied to a general mobile hydraulic machine by using an electro-hydraulic actuator.
The team proposed a control loop strategy that included two pressure sensors added to a classic hydraulic system configuration. The pressure sensor sends the feedback signal to the control unit, which accurately commands the actuators to achieve smoother operation of the mobile hydraulic machine. Simcenter Amesim was used to model the detailed hydraulic machine to capture the behavior of the machine and to design and validate the controllers with co-simulation between Simcenter Amesim and Simulink® control models.
“The vibration damping control strategy was successfully tested on a hydraulic crane displayed in the Maha research center lab, and achieved a vibration reduction of up to 78 percent,” says Vacca.
“The current poor efficiency of fluid power machines leads to high energy consumption,” explains Vacca, who considers green technology as one of the top-level priorities in fluid power research. “The solution of this problem is one of the main goals of the research projects we carry out at Maha Fluid Power Research Center.”
To address such a problem, it is fundamental to reconsider the current design methods for hydraulic systems and formulate novel design architectures that eliminate energy dissipation sources. “The counterbalance valves are a significant example of high energy dissipation in hydraulic machines with suspended loads,” Vacca explains. They introduce additional energy consumption in order to obtain satisfactory dynamic behavior of the overall machine.
To understand this problem, Vacca uses the Simcenter Amesim simulation environment to model the complete system as well as the details of the hydraulic components. “The hydraulic libraries of Simcenter Amesim are welldeveloped and reflect the state of the art for hydraulic components modeling,” Vacca says. His team designed various valve architectures and assessed the system dynamic performance related to these components, determining the best parameters such as the pilot ratio and the setting pressure. “For example, with our control strategies we were able to reduce the energy losses introduced by counterbalance valves in hydraulic cranes,” Vacca adds. “Experiments demonstrated a potential for the reduction of fuel consumption by up to 40 percent.”
The operating features of hydrostatic units are sources of noise in fluid power systems. Noise in the working environment can induce fatigue or hinder workers’ productivity. In the worst scenario, it can jeopardize safety and work ergonomics.
Modeling the propagation of noise from the fluid source to the external surroundings is a complex multiphysics problem. Successful noise reduction can only be achieved through a deep understanding of the noise generation and propagation mechanisms for both fluid- and structure-borne noises. Accurate simulation requires a coupling between fluid, vibration (structural) and acoustic domains to correctly capture all phenomena involved.
Noise and vibrations of fluid power systems mainly originate from the structural excitation of the gear teeth, gear and the casing, induced by the flow oscillation at the outlet port. The structural deformation on the outer surface of the casing causes the air around it to move, generating pressure variations perceived as noise. The first step is to calculate the fluid pressure acting on the gear and the casing. The HYGESim model is used to compute the loading force from outlet pressure ripple. This pump model combines several approaches in different domains; it utilizes a robust pump geometric model in C code, a lumpedparameter fluid dynamic model for the main pump features, and a computational fluid dynamics (CFD) model for the lateral gap. The fluid dynamic model is applied at the oil level to compute the forces.
Next, to accurately capture the dynamics of the pump structure in the desired frequency range, a structural finite element (FE) model of the pump is created. The simulation model is correlated to the tested results. By combining the structural modes of the updated FE model with the mapped fluid pressure loads, modal-based forced response analysis calculates the housing vibrations. These are used as boundary conditions for acoustic simulation. The structural vibrations on the surroundings are captured using the boundary element method (BEM) solver of LMS Virtual.Lab Acoustics.
In addition to assisting the fluid power industry by leading a team of 17 researchers at the research center laboratory, Vacca is also an associate professor at Purdue, a public university that is highly ranked in engineering sciences. He notes, “Purdue has clear strengths, as it is one of the top 10 engineering schools in the U.S., with high rankings in the programs such as agricultural engineering (number one), mechanical engineering (number eight) and aerospace engineering (number six).”
Vacca teaches both graduate and undergraduate level classes in fluid mechanics and fluid power technology. For educational purposes he uses the Simcenter Amesim academic bundle to enable the students to build and understand the behavior of simple hydraulic systems. This helps students to apply their fluid power knowledge to these projects.
“Simcenter Amesim is particularly suitable for academic use, and a great source that enables us to effectively teach the operation of entire hydraulic systems to engineering students,” Vacca says. “The students in my classes use the software also for their homework and they are able to download the Simcenter Amesim Student Edition as well. This also gives us the opportunity to approach problems of a certain complexity. This would not be possible without software such as Simcenter Amesim.”
“In the near future, simulation will dominate the R&D practice for complex systems such as hydraulic machines,” says Vacca. “Virtual prototyping of components as complex as high-pressure hydrostatic pumps and motors will become a viable possibility.” For students to become engineers, it is crucial to begin their careers with the right skills for mastering system simulation software.
“System simulation will certainly constitute the basis of the design of engineering products,” Vacca affirms. “Still, the engineers will need to be able to elaborate and understand the basic physical principles that affect the design of a product. Even though the software and the mathematical models do a great job in understanding system behavior, we must not forget the basics and completely rely on these tools. The academic preparation of future engineers to face challenges depends as much on their basic fundamentals as on their usage of tools like Simcenter Amesim.”