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Hyper-G a Hydro Pneumatic Catapult Type Sled

Sled testing is one of the most important test methods for the evaluation of vehicles interior with respect to crash safety. This method allows to simulate real crash conditions without destroying the vehicle structure.

In addition to conventional sleds which are accelerated smoothly using a bungy cord or other device and then impact a deformation element, catapult type sleds act in a reverse mode. The sled is thus accelerated within typically approximately 100 ms to the impact speed and then smoothly decelerated.

This type of sled guarantees, that the sled first moves, when the crash force already starts to act. Therefore accurate initial conditions especially regarding dummy positioning can be guaranteed.

To ensure good correlation with the full scale crash test an accurate modelling of the vehicles crash pulse must be guaranteed.

While conventional catapult type sleds mainly work on hydraulic or pneumatic modulation of the pulse Hyper-G combines a simple pneumatic spring with a hydraulic brake directly acting on the ram. While the pneumatic air spring provides a rather constant force, the pulse is modelled by changing the brake force.

As only little hydraulic flows are required to control the brake, standard valves with very fast response and small through flows can be used. In this way it was possible to use a fully closed loop system, which automatically compensates for moving masses. As the available brake force always exceeds the force of the pneumatic spring, positive and negative accelerations can be simulated.

The Hyper-G system gains the necessary force to accelerate the sled from a pneumatic high pressure cylinder, filled with pre compressed air, acting on a piston which directly pushes a ram. In this way a rather constant force is created. A cylinder with a typical length of 7 m is used to get a proper reservoir size. This is a similar approach like in the HYGE system, but inside the cylinder no valves or different pressure chambers are included.

The pulse is then modulated by a hydraulic brake, which is acting directly on the ram and controlled through hydraulic valves. The ram then directly acts on the sled.

In this way only rather small hydraulic flows have to be controlled and standard hydraulic components can be used. In addition the response time of the system is rather low, so that a closed loop regulation can be taken. This means, that the acceleration signal on the ram is measured in real time and used as a feed back signal to control the valve regulation.

The system is powered up by activating the ram-brake and filling the cylinder with pre-compressed air from a reservoir. In this phase the ram brake is fully activated to prevent the ram from motion. Through a controlled release of the brake the system is triggered and the sled will start moving. The hydraulic brake valves are then regulated to produce the proper pulse. When the target velocity is reached the brake is put to full brake force again to stop the ram. For normal tests ram and sled will separate at this point. The sled is than braked after a predefined time through a separate brake, mounted on the sled and acting on the rails.

After the test is finished, the system is de-powered by releasing the compressed air from the cylinder. In this way there is no energy left in the system after the test is finished.

The ram and the cylinder are moved back in the original position using a separate hydraulic cylinder. As the ram only heats up between two tests by a few deg C no cooling period is required between repeated tests. In this way a cycle time of only few minutes is required between repeated tests.

9 systems have been built and mostly installed, which show a rather simple usage, and a high accuracy of the pulse modulation. In addition repeatability has proven to be very good.

DSD- Dr Steffan Datentechnik
Salzburgerstrasse 34
A-4020 Linz
Austria