Dynamic load sensing allows for safe electro-hydraulic joystick steering of wheeled excavators while increasing productivity and reducing driver fatigue

Eaton has developed an innovative electro-hydraulic steering solution for Liebherr´s new generation of wheeled excavators. This steering solution is a feature to a traditional hydrostatic steering system. Because of the dynamic, intelligent load sensing system, the electro-hydraulic steering can now be operated by a joystick at limited driving speed, thus increasing maneuverability and productivity while reducing driver fatigue. However, as soon as the driver rotates the steering wheel, joystick steering is disabled and the system is back to traditional steering system control. This necessary safety feature is incorporated via a tailored valve block.

Compact wheeled excavators like Liebherr´s 914 Compact Litronic (photo 1) often operate in tight spaces of urban building sites. In these instances, the driver prefers to steer the machine with a joystick.

"Steer by joystick" as an option
The electro-hydraulic "steer by joystick" systems allows a driver to execute all working and driving functions with just two joysticks, without ever touching the steering wheel. With this steering mode, a driver is more productive and safer when maneuvering in tight spaces. However, as per various regulations and directives joystick steering mode is allowed within a limited driving speed range only – thus requiring priority of the traditional steering system over electro-hydraulic steering in any event of system failure.

This is exactly what Liebherr requested  when it started working with Eaton. The German sales engineers passed this request to Eaton´s mobile applications engineering team based in Pessano/ Italy. [JEC1] Eaton's mobile application engineering team designed and delivered the appropriate solution in just three months, with just three more months to build, field and test samples.
The design process was supported by Famic´s AutomationStudio software for system work, troubleshooting and simulation. Once the concept had been defined, AutomationStudio provided the tool for finetuning the system for proper performance and therefore significantly reduced the time needed for adjustments on the test bench.
The hardware in the system is based on Eaton´s proven "Cetop" valves as well as on Eaton's superior "IH" cartridge valves.

Safeload sensing
The electro-hydraulic steering system, (photo 2) designed by the applications engineers, is a dynamic load sensing circuitry with the LS signal deployed as a safety signal as well. The system architecture is based on an open-center circuitry, with gear pump and unloading stage that is dynamically piloted by the electro-hydraulic block (photo 3).
Proportional flow is performed by Eaton's proven KDG4V3 proportional valve. An adjustable restrictor limits the maximum flow to prevent excessive steering speed. In accordance with relevant directives, the proportional directional valve is pre- and post-locked with three independently adjustable sequence valves on P, A and B ports. These valves can completely lock flow passages from both pressure supply ports to service and from service ports to tank- and therefore disable the EH steering system.

A perfectly tuned steering
The uniqueness of this system is given by its capability of fine tuning for perfect adaptation to the pressure rise rate of traditional hydrostatic steering control units. This ensures a very fast response when turning the steering wheel in the event of emergency; even at a very marginal input on the steering wheel: Just 0,5 cm of steering wheel rotatory movement will completely lock the parallel electro-hydraulic steering valve.

An additional safety signal is provided by the pilot pressure to the propel drive control valve. This allows for detection of  the maximum vehicle speed. The electro-hydraulic steering function is interlocked beyond that speed. This is an important requirement for approval and homologation of On-Road applications, which is certified by relevant supervising organizations such as the German TÜV.

Result: Optimum maneuverability and excellent safety
Tests with Liebherr engineers have confirmed that the new kind of safety steering in "dual mode" is extremely responsive – and safe. Following his or her preference, the driver can make use of the electro-hydraulic joystick steering mode when working on-site (photo 4). As soon as driving speed exceeds a defined level or immediately by turning steering wheel slightly, the steering wheel "overrides" the joystick operation and the excavator, in effect, switches from "steer by wire" hydraulics to traditional hydraulic steering system.

A scalable system
As a result, all safety requirements are fulfilled and the system – which is able to communicate with other control systems – is road circulation compliant. Furthermore, it can be fine-tuned to match the needs and expectations of the OEM.

The benefits of this design are starting to hit building sites. Serial production of Liebherr's new generation of wheeled excavators has started, and many end users have started selecting its innovative steering option. Now that Liebherr's needs are met, Eaton is turning its attention to how other customers can benefit from the new electro-hydraulic joystick steering in extended applications. Adjustable valve setting, eases adaptation to different SCUs and ensures premium response and great flexibility in case the joystick steering block is used on different machine platforms and sizes. The solution is scalable from 10 to 30 LPM flow and therefore covers an entire range of excavator types and sizes.
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Captions
Photo 1 (product photo A 914 Compact Litronic)
Compact power: The A31X excavators are often working in tight urban sites.

Photo 2 (3D drawing)
All hydraulic control functions were integrated into one compact valve block.

Photo 3 (Automation Studio model)
CAD drawing of the hydraulic circuit design

Photo 4 (A 914 Compact Litronic in operation)
Joystick or steering wheel? Drivers choose – and all safety requirements are fulfilled

Contact

Emil Berning

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011 961 2400

NOTES FOR THE EDITOR

Eaton's Hydraulics business is a worldwide leader in the design, manufacture and marketing of a comprehensive line of reliable, high-efficiency hydraulic systems and components for use in mobile and stationary applications. Mobile and stationary markets include agriculture, alternative energy, construction, forestry, manufacturing, material handling, mining, oil and gas, processing, transportation and utility equipment. Eaton's Hydraulics Group provides customer-driven solutions under product names such as Aeroquip®, Boston®, Char-Lynn®, Eaton®, Hydrokraft®, Hydrowa®, Hydro-Line®, Integrated Hydraulics®, Synflex®, Vickers®, Walterscheid™, and Weatherhead®. For more information, visit www.eaton.com/hydraulics 

Eaton Corporation is a diversified power management company with more than 100 years of experience providing energy-efficient solutions that help our customers effectively manage electrical, hydraulic and mechanical power. With 2011 sales of $16.0 billion, Eaton is a global technology leader in electrical components and systems for power quality, distribution and control; hydraulics components, systems and services for industrial and mobile equipment; aerospace fuel, hydraulics and pneumatic systems for commercial and military use; and truck and automotive drivetrain and powertrain systems for performance, fuel economy and safety. Eaton has approximately 73,000 employees and sells products to customers in more than 150 countries. For more information, visit www.eaton.com