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Hydro-pneumatic suspension systems: faster and more cost-effective development

Fig. 1 Schematic of a hydro-pneumatic suspension system

Fig.2 Modular control system

Fig.3 Basic module with 4/3 position control valve

Fig.4 Rod-side modules for setting the spring rate

Fig.5 Modular suspension manifold

Fig.6 ARGO-HYTOS load simulator

Fig.7 ARGO-HYTOS load simulator system mounted on the test tractor

Hydro-pneumatic suspension systems improve comfort and productivity of vehicles. The fact that these systems are currently utilized in only a small number of vehicles is mainly due to high development efforts and the additional costs of the suspension components. ARGO-HYTOS now offers a modular system, which features an improved cost-benefit ratio and a reduced development effort. Moreover, two specially developed hydraulic solutions offer distinct advantages over currently used systems.

Comfort, productivity, and profitability are among the most important qualities customers request from work machines today. Higher standards demanded with regard to comfort are impelled particularly by the Health and Safety directive 2002/44/EG, which defines the daily permissible vibration exposure to the driver. In other words, the less vibration the driver feels, the more comfortably the driver rides and therefore, the longer the driver is allowed to work. Especially during off-road work, this criterion may determine whether a driver is allowed to perform a certain job for the entire workday or whether the drivers must stop their work before the workday is done.

Hence, comfort is not merely a condition of personal well-being; it is an important factor when it comes to the actual permissible daily working hours. Therefore, it also influences profitability. Additionally, an increase in comfort will allow the driver to complete work processes faster and with more precision, which in turn makes the driver’s work more productive. This helps to enhance profitability, as well.

In order to achieve this kind of comfort it is necessary to have a suspension system that isolates the vehicle’s chassis, or rather, the driver from the unevenness of the ground. Hydro-pneumatic suspension systems can be located in various parts of a vehicle. Typically, there are three different applications – wheel or axle suspension, operator’s cab suspension, and boom suspension or suspension of payload. There is another important advantage with regard to wheel and/or axle suspensions – wheel load or ground pressure is equalized, which in turn enhances road-holding and wheel traction. This leads to increased efficiency and productivity, which consequently improves profitability.

Typically, vehicles subjected to frequent load changes have level-controlled suspensions. In many cases, this is a hydro-pneumatic suspension, i.e. a system consisting of suspension cylinders and accumulators as well as a position control system (schematic see fig. 1).

At present, usually only premium-class vehicles and vehicles that are produced in large quantities are equipped with such suspension systems. Various special applications, for which a suspension is indispensable, also use them. There are two main reasons for this – high development costs (hence, product costs), and long development times. These two reasons apply especially when an OEM introduces a suspension system into their vehicle for the first time or when the next evolutionary stage of suspension is to be launched. Often, these are exactly the obstacles that make OEMs decide against a suspension system, despite the clear advantages described above.

The target of ARGO-HYTOS is therefore to remove these barriers for vehicle manufacturers and to advance the technology of hydro-pneumatic suspension systems. Development costs and development time shall be reduced considerably. This is accomplished with a new system solution, which is characterized by three particular benefits:

  1. A standardized, modular control system for hydro-pneumatic suspension systems – including hydraulics and electronics
  2. A straightforward adaptation of  the  system  to  customer-specific needs simply by choosing appropriate modules and parameter settings
  3. Technical advice and support concerning the layout of the overall suspension system and its interaction with the vehicle

The advantages for OEMs:

  1. Prototype systems are available very quickly
  2. Standard modules that have been adapted for a specific application can be utilized as a series production solution for small to medium quantities
  3. For large quantities and/or specific requirements concerning installation space, a customer- specific hydraulic manifold can be derived from the prototype

The modular control system developed on this basis consists of a hydraulic manifold, which is connected to an electronic control unit, as shown in fig.2. The electronic unit is the command and control center; it coordinates and regulates all of the functions of the hydraulic manifold. The necessary input for these decisions is provided by data from the operating panel, various sensors and the vehicle’s bus system. The hydraulic manifold is connected to the suspension cylinder so it can control the position of the cylinder and the pressure within the cylinder’s rod-side chamber. Additionally, the connection between the cylinder’s piston chamber and the piston chamber’s pressure reservoir can be damped (damping control) or disconnected (blocking of the suspension) by a valve in the hydraulic manifold.

Moreover, there is a connection between the control system and the vehicle for hydraulic and electric energy supply. Load-sensing systems (standard and ‘common rail’) with fixed and variable displacement pumps are supported; supply voltage of 12 to 24V is possible.

The adaptation of the system to customer-specific requirements is achieved by selecting appropriate modules and their settings. With regard to hydraulics, there are various modules which can operate all types of hydro-pneumatic suspension systems (single-acting, double-acting, constant or variably preloaded, etc.)

The basic module consists of a minimum of required hydraulic components. It is a manifold that provides position control of the suspension: it feeds or drains oil to/from the suspension cylinder’s piston chamber. A special feature of it: it needs just one proportional valve with only one solenoid (see fig.3), for upward and downward leveling. There are two advantages here as opposed to suspension manifolds commonly used at present; one solenoid can be eliminated, additionally the position adjustment can be done proportionally. This means that the valve can on react very sensitively to small changes in position and open fully (control range 2.5 to 25lpm at 20bar ?p). Therefore the required position can be reached faster, for example while under extreme load changes or in cold temperature conditions with high oil viscosity. Patent applications have been filed for the valve and its integration in the circuit of the position control manifold.

Additionally, there are rod-side control modules for the setting of the rod-side pressure in preloaded suspension systems (fig.4). Using these, the spring rate of the system can be varied in a wide range. A first module is equipped with a hydro-mechanical pressure control and is available in two versions – one with a constant rod-side pressure, and one with a rod-side pressure following a characteristic curve depending on the suspended load. The latter is especially designed for tractors, a patent has been granted in Germany. In comparison to a two-step variation of the rod-side pressure, it offers the benefit of a smooth, continuous transition of the rod-side pressure in the switching point, hence an increase in spring rate continuous to the load reduction. A second module utilizes the technology of the 4/3 directional prop valve mentioned above for electro-hydraulic rod-side pressure setting; hence, combined with a pressure sensor, it provides fully variable pressure control. Consequently, the spring rate can be optimally adjusted to the given working conditions.

Furthermore, the piston-side module offers the possibility to control the oil flow between the piston chamber of the suspension cylinder and the accumulator. This allows for a specifically adjusted damping or the complete hydraulic blocking of the suspension. Correspondingly, proportional or switching valves are used here.  A possible in-line arrangement of the piston-side modules also allows for the selective deactivation of the piston-side accumulators.

Instead of the piston-side module, in the same flange mounting face a multi-cylinder module can be used.  It allows the separation of two hydro-pneumatic springs, thereby providing roll stabilization, for instance.

The individual hydraulic modules are bolted together. They are easy to handle since the individual surfaces of the manifolds follow a strict separation of functions (see fig.5): module flange surfaces left and right, hydraulic supply in the rear, suspension hydraulics in front, valves on top, and mounting at the bottom. The whole manifold can be mounted in any orientation. Pressure accumulators, varying in size and pressure range according to the design of the given suspension system, can also be supplied. Due to their excellent cost-benefit ratio, electron-beam welded accumulators are the best choice here.

The electronics needed to operate the manifold can be supplied as well should the customer request it. They consist of a position sensor and a position control unit in its most basic form. It is preconfigured according to the given hydraulic setup and can be adapted to an application by selecting algorithms and parameters. The electronics can be also equipped with operating elements, which enable the operator to manually adjust the normal position or the suspension characteristics (damping/spring rate). The control unit is able to receive further data from the vehicle’s CAN bus. At present, it is available for the CANopen protocol, other protocols are possible. There is a touchscreen display available for simple parameter setting, maintenance and diagnostic work, as well as for special, large-scale suspension solutions.

The electronics provide a proportional position control, and a spring rate and damping control. In order to improve comfort and ride quality, the suspension characteristics can be automatically adapted to the given riding and working situation with the help of our adaptive control technology. Manual adjustment is also possible. A semi-active damping control based on a proportional valve is currently being developed.

The complete system is developed to series maturity. Tests with the ARGO-HYTOS load simulator (fig.6) in the hydraulic lab have been conducted successfully. Using the program DSHplus, a hardware-in-the-loop-simulation was performed and the control unit was initially put into operation and tested in a virtual suspension system. Afterwards, a first real-life operation of the modular control system was performed on a commercially available tractor. The suspension control was replaced with the suspension control of ARGO-HYTOS (fig.6). All functions of the system have been tested properly.

When starting customer projects, it is convenient to use the modular system for first prototypes due to its quick availability and adaptability. As a first step, ARGO-HYTOS develops suggestions for the layout of the suspension and suitable combinations of modules – this step is simplified and accelerated by dedicated calculation and simulation programs. Subsequently, the combinations of modules are tested in the vehicle in order to find the best suitable solution and the optimum control of the respective parameters. The next step is for the customers to decide whether they want the modular solution directly for the serial production or whether they prefer a customized hydraulic manifold. The latter option will most likely be chosen when there are specific limitations concerning installation space, or to further reduce costs for large quantities.  In this case, ARGO-HYTOS will design a new, custom-made hydraulic manifold - using exactly the same components and valves as in the modular setup for the serial solution.

Hydro-pneumatic suspension systems improve comfort and  productivity of  vehicles. ARGO-HYTOS offers a modular system, which features an improved cost-benefit ratio and a reduced development effort. 

Dr. Wolfgang Bauer is part of the Advanced Engineering of the ARGO-HYTOS Group and in charge of systems technology

Dr. Marcus Fischer is Chief Technical Officer of the ARGO-HYTOS Group


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