The current trend and EU directives towards higher-performance braking systems in mobile machinery and towed vehicles, require intelligent valve technology, which have been successfully developed by HydraForce, part of the Bosch Rexroth group. However, the hydraulics experts are certain that this development will enable further potential to be leveraged, particularly in regards to electrified and highly automated vehicles.
Since 2021, the EU brake directive 2015/68 requires newly developed tractors, and all other agricultural and forestry machinery and trailers, to be driven at speeds of 40-60 km/h (or sometimes even higher), and to have a redundant braking system, so that in the event of an emergency, braking is still possible if the service brake fails. This can be achieved by means of the parking brake. However, it must ensure that the braking force of the emergency braking system can be modulated, so that the emergency braking system is not triggered accidentally if the control signal is lost.
Damien Bonenfant, Senior Application Engineer at HydraForce Hydraulics Ltd, is confident that he knows the right approach to meet these requirements: "Electrohydraulic braking systems offer greater reliability because their operating status can be monitored. They also have the advantage that they can be integrated into the driver's cab design in a more ergonomic, space-saving and user-friendly way, compared to a classic mechanical parking brake lever."
HydraForce offers high-performance components to achieve brake-by-wire solutions for these types of demanding applications. For example, proportional valves can be combined with a mobile controller and pressure sensors to monitor and regulate the pressure in the brake lines. These electro-hydraulic systems make it not only easy to safely detect leaks or faulty components, but the solution is also very user-friendly for machine manufacturers to scale and integrate into the machine design - especially in comparison to mechanical solutions, which are prone to failure in harsh working conditions. Such intelligent systems are also the best solution for trailed machines to comply with the EU brake directive, as the hydraulic pressure between tractor and trailer can be modulated to ensure optimum braking performance. Compatibility problems between tractors and older trailers with 1-wire braking systems can also be eliminated in this way.
Intelligent driver assistance systems
Markus Bissbort, Application Engineering Manager at HydraForce Hydraulics Ltd, has recognized further advantages of intelligent braking systems, in addition to the safety aspect: "Based on smart braking systems, stability assistance systems such as ESP can be integrated onto the machines. On one hand, this allows the vehicles to be driven faster and more safely from field to field. On the other hand, these support systems could also be used by less experienced drivers on appropriately equipped machines. This poses a competitive advantage in the face of the increasing shortage of skilled workers." Smart braking systems can also be used to achieve narrower turning circles during operation by automatically braking the inner wheels during steering maneuvers.
However, greater potential can be tapped into with the integration of intelligent braking systems. Current human and object detection systems only warn the driver with an acoustic or visual signal, thus giving him the indication to brake. In contrast, the HydraForce system enables a higher degree of automation at this point, as Markus Bissbort explains: "By combining the signals from the machine's proximity monitoring system with our intelligent electrohydraulic braking solutions, self-braking vehicles can automatically react to obstacles or people in the path of travel and brake automatically, as we are used to with modern passenger cars."
Braking systems for autonomous and electrified vehicles
There is also potential for mobile machines with electrified drive systems, to have the possibility of recovering energy through recuperation in the braking process, which feeds into the vehicle's battery storage system, thus making the machines more energy-efficient. However, for this to succeed, an intelligent braking system is needed that distributes the energy evenly, without negatively affecting the driving experience. In addition, emergency braking must still be possible if the electric drive fails, particularly in the case of electrified drivetrains which do not have a conventional engine brake; as is the case with classic transmission/combustion engine combinations. HydraForce offers suitable proportional valves for these demanding requirement profiles. In addition, customers can also rely on the internal expertise of Bosch Rexroth experts who specialize in electrification and can help customers source the best possible solution (since HydraForce became part of the Bosch Rexroth Compact Hydraulics Division in 2023).
Mobile robots and autonomous machines need to meet much more complex safety level requirements and therefore need powerful redundant braking systems, for example with an additional safety valve. However, HydraForce have the expertise to draw on extensive experience in this area and support machine manufacturers individually in solving their respective challenges.
High-performance valve technology
HydraForce has developed the new HTS38-34 direct-acting proportional valve specifically to meet the requirements of redundant safety brake systems in agricultural and forestry machinery. This pressure control valve can be operated with an inlet pressure of 350 bar and offers additional benefits including low leakage and fast response time. The maximum control pressure can also be limited on a customer-specific basis. For high flow rates, the TS90-31 is a pilot-operated proportional pressure/reducing control valve with comparable performance. In contrast to standard pilot-operated valves on the market, it enables pressure control from 0 bar. This makes the valves ideal for intelligent braking systems, both for tractors and towed machines.
Another drop-in variant of the pilot-operated pressure relief valve is the TS90-G34, which is available in variable pressure control ranges between 0 and 30 bar, and the same flow rate as the TS90-31 previously mentioned. For this valve, HydraForce has developed a product variant with particularly low leakage, which makes it possible to combine the advantages of spool valves and direct-acting valves, i.e. high flow rates with particularly low leakage. This combination enables the hydraulic pump and the hydraulic accumulator solutions, which are often required in braking systems to be downsized, thus saving investment costs and improving the efficiency of the system.
The EHPR98-G37 proportional valve with inverse logic provides an elementary feature for safety applications, as Damien Bonenfant explains: "The stronger the current acts on the valve coil, the lower the control pressure becomes. This approach means that the maximum hydraulic pressure is always available in the event of a system failure and the braking process can be carried out in an emergency." This is further enhanced by the fast response time of the direct-acting valve. This product family also includes special valves for high-pressure applications, with low hysteresis and leakage.
The SVPR98-G38 screw-in valve is unique on the market, and combines the functionalities of a 3/2-way solenoid valve with those of a mechanical pressure compensation valve. This combination saves valuable installation space and investment costs. It was developed for use in parking brake systems, among other things.
Markus Bissbort concludes: "We support our customers not only with high-performance hydraulic components, but above all with our advisory and integration expertise. We provide individual and holistic assistance in identifying the best solution and actively support the implementation of these solutions. And we are always close to our customers, thanks to our global network of application engineers."