Designing robust hydrostatic transmissions via system-performance simulation

  • 15-Oct-2013 10:03 EDT
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Poclain Hydraulics’ standard hydrostatic transmission offering.

In recent years, automotive suppliers have shifted from providing components to providing complete systems to their OEM customers. Poclain Hydraulics has also made that transition, implementing new competencies that enable it to perform simulations of the full transmission system of its customers’ machines.

The use of LMS Imagine.Lab AMESim software enables Poclain Hydraulics to simulate transmission behavior, helping its engineers develop answers to given issues faster and at a lower cost. “AMESim enables us to imagine new solutions because of a better understanding of the system,” says Gilles Lemaire, Poclain Hydraulics’ Scientific Support Manager. “In particular, the simulation enables us to focus on variables that cannot even be measured and test new concepts without the need of an expensive prototype. For vehicle simulation, even if we have a good knowledge of each component, the interaction between them is not always easy to evaluate.”

Poclain Hydraulics chose AMESim for this work because of the diversity of available libraries and the different levels of simulation provided, from detailed hydraulic components to hydraulic systems and complete vehicles. Another factor was the ability to address global system simulation with an explicit representation of components, such as pumps, motors, valves, etc.

With a new vehicle such as a hybrid commercial passenger bus, Poclain Hydraulics initially strives to understand the effectiveness of an existing bus, taking into account the vehicle’s duty cycle, driver, and local traffic laws. The second phase is to study new solutions and analyze their efficiency, not only on the actual duty cycle, but also with respect to the robustness of the solution under different load cases.

The main issue for the bus project is to how to store as much braking energy as possible and release this energy as efficiently as possible. To store the maximum amount of energy, the size of the system must be adequate to the requirements of the driver at any time during the duty cycle, under any condition of use. By grasping the efficiency of each component of the hydraulic system at any phase of use, the simulation enables global efficiency for the energy release. This is not easily measured on an actual vehicle, but readily simulated on computers.

The main contribution of the simulation is to analyze the impact of the system on the fuel consumption of the vehicle, as affected by the main mechanical transmission and the engine. For example, by decreasing the power demand on the engine during restitution phases, the impact on its efficiency was not always positive. “Using the simulation capabilities of AMESim, we were able to estimate the best way to release the stored energy by improving the control of the hydraulic mechanical systems,” said Lemaire. “We also particularly focused on the way the gearbox is controlled during braking to make all of the systems work together. This provides the best experience for the driver and maximizes energy regeneration.”

From this analysis, Poclain Hydraulics engineers adapt new components with better efficiencies while choosing the right size of components, keeping in mind the results for various duty cycles. “We end up with a good knowledge of the application, a good understanding of its functionality, and important clues for its improvement,” notes Lemaire. “At a higher level, this experience enables us to analyze a new application, such as ‘duty cycle + vehicle,’ to predict potential energy regeneration solutions.”

“At the same time, we take advantage of the different level of details offered by AMESim in order to investigate transient phases,” said Yohann Brunel, Advanced Studies Engineer at Poclain Hydraulics. “We are able to study behavior, such as engagement and disengagement of the motors from a complete representation of each motor piston, in order to synchronize them with the wheel speed.”

To make everything work together without conflicts or instability, Poclain Hydraulics needed to define different levels of control. The higher level of control helps ensure energy management by splitting power between both systems, while the intermediate level helps ensure each subsystem will provide the required power every time. The lower level manages the functional limitations of the systems. All of the basic controls were made by importing the signal and control library directly from AMESim.

To improve the system, i.e., choose the best components and the best way to control them, the analysis consists of observing the energy that transits into the system during deceleration and acceleration. This enables Poclain Hydraulics to track the lost or non-recovered energy. The main criteria is the fuel consumption on a given duty cycle and the return on investment. Brake wear can also be studied with this analysis.

“The online strategy is validated through AMESim,” says Brunel. “The main difficulty is to find a robust strategy, whatever the duty cycle. The strategy may also have to be adapted when the sizing of the system is modified.”

Lionel Broglia, Business Development Manager Off-highway & Mechanical Machinery, LMS, a Siemens business, wrote this article for SAE Off-Highway Engineering.

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