[1] These test systems consist of 4 hydraulic actuators on top of which the wheels of the vehicle are placed.
During the research phase of a vehicle, a 4-poster system is used to test newly designed suspension systems and their durability.
In production, 4-poster systems are typically used to test every vehicle at the end of the production line on squeak and rattle, which is a check to make sure the vehicle doesn't have any loose parts.
Multiple kinds of tests are possible: sine test (sweep in frequency), simple wave form on each actuator, open loop test (driven by an external tension signal).
There is also a particular testing method called ICS control, which allows to reproduce on a car an actual service environment, starting from those data coming from the outdoor acquisition sessions.