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01/09/2007
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Pure electrical door latches without any mechanical connections to the handles are the ultimate solution to operating effort, styling and packing, writes Roger Bishop. However, because they also require mechanical links or back-up power in the event of power failure caused by a crash or battery discharge, they are not popular with OEMs.
ArvinMeritor Light Vehicle Systems has designed and patented a low energy release (LER) latch in which the operating effort, expressed as ‘work done’, is reduced by a factor of 50 compared with current designs. In a latch with a 500N seal force, this equates to a secondary pawl release force of 4N compared with 54N in a conventional design.
On a test rig designed to simulate the most extreme seal forces that can result from bodywork deformation in a crash, the additional release force required at the handle was barely discernable by the writer from normal latch operation (see graphs).
The LER is fundamentally a mechanical design even though all locking functions are software controlled. The electrical system is not safety critical. Key innovations are an additional short lever provided by an eccentric pawl pivot in the retention system that enhances the effect of the long lever to maximise the force reduction ratio.
The design and the prototypes currently being shown to the industry have not been produced with a vehicle manufacturer but are generic developments by ArvinMeritor that reflect the current industry ‘wishlist’ of desirable features. The pawl/claw geometry and interface are robust and well understood by the industry and the latch is directly comparable with existing designs in terms of strength, crash, noise and vibration performance.
The reduced operating effort allows steel operating cables to be thinner and less costly, door seal loads can be higher to reduce wind and road noise in the passenger compartment and handle design can be more innovative. In addition, the class-leading compact size of the unit leaves more space within the door for safety and comfort features.
A current fully featured latch may incorporate up to four electric motors to cover lock, double-lock, child lock and power release functions. LER achieves all this using just a single motor and an electromagnet.
If power is lost while the vehicle is being driven, including slicing of the harness, the latch defaults to a mechanical condition (unstable state) and is unlocked. In a park condition the latch is in a stable state and therefore remains locked even if the battery is flat or the wiring cut. However, a mechanical clutch allows the lock to be opened should the electronics fail. The critical importance of the electromagnet is that when energised it disables mechanical release. When de-energised, mechanical release is possible.
Software controls all possible combinations of functions and allows a single generic design to meet the regional requirements of markets globally. The control logic was based on a railway signal actuator dating back to 1907.
The latch is so exceptionally quiet that the design allows for ‘locking sounds’ to be added and played through the vehicle’s speaker system.
ArvinMeritor says LER designs have been developed for manual door latches, central door locking (CDL) latches, sliding door systems, boot and tailgate systems and powered closure latches.
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Author
Roger Bishop
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Copyright European Automotive Design.
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