Open Cut » Health and Safety
This project sought to improve the ride in heavy off-road mining vehicles by replacing the spring-isolation system in a normal operator's seat with a hydraulic actuator assembly which responds actively to movement of the cab, moving the seat vertically in the opposite direction to cab to cancel out vibration resulting from road irregularities and loping. The seat is intended to protect the operator from vibration at low frequencies, which heavy mining vehicles are known to produce and which conventional vibration-isolation seats are known to be unable to attenuate.
This 'powered' seat suspension has become economically feasible due in part to the recent availability of low cost accelerometers and advanced hydraulic switching devices.
Normal vibration-isolation seats function by well-known dynamic principles, with the seat being connected to its base via a spring and damper system. Seats of this type isolate well at excitation frequencies well above the natural frequency of the spring and the mass supported by it. They are however much less effective under circumstances where there are significant excitations around the natural frequency of the system. At such frequencies, the seat actually magnifies the vibration by up to 40%.
Typical spring-isolated seats have natural frequencies around 2 Hertz, and it is not practical to build seats with lower natural frequencies, as the travel of the seat suspension becomes very large under such circumstances. This is unimportant for road trucks, where the significant vibration inputs are at their wheel-turning frequency of 4 to 8 Hertz. It is however critical on large off-highway mining trucks, where wheel-turning frequencies of 2 Hertz are common.
An additional problem with spring-isolated seats is that the suspension does not react quickly to shock loads because of the presence of hydraulic dampers. These dampers have a characteristic in which the reactive force applied by them increases with an increase in seat velocity. They are thus unable to react quickly to shock loads.
The powered seat successfully reduced the low frequency operator vibration and shock loads associated with the predominant spring and damper technique.