Civil Engineering Reference
In-Depth Information
- a loading system, with 1 or several degrees of freedom;
- a system generating pressurized hydraulic oil;
- a servo-control system;
- a driving system;
- a reaction basemat;
- acquisition and processing means.
The table plate is generally square. It is made of welded assemblies with
reinforcements in order to have free-free frequencies as high as possible (over 50
Hz).
The table can be loaded in a horizontal mono-axial mode, in a vertical-horizontal
bi-axial mode or in a three-axial mode. It is guided by sliding rail systems or directly
by transverse jacks. In the case of a sliding rail system, the maximum loading
frequencies are lower than 100 Hz because of the operating gaps.
As for multi-axial tables, these are driven directly by jacks in all directions.
There are geometrical compensations between the various axes in order to have
perfectly rectilinear displacements in the three directions. All the jacks are equipped
with hinges. The hinges limit the stiffness of the table as well as its rotations.
The hydraulic power system allows us to supply the jacks with pressurized oil
(210 bars) with outflows varying according to the number of pumps in service.
In order to control the table, electronics bays are needed to drive the jacks. The
jacks are controlled with closed loops. The basic closed-loop control system consists
of:
- an amplifier for the control of the servo-valve and the servo-controller;
- feedback of a LVDT displacement sensor to measure the position of the piston.
A servo-controller compares the feedback signal of the LVDT displacement
transducer with a reference signal (control or program). For any difference between
the feedback signal and the reference, the servo-controller injects an error signal into
the servo-valve in order to bring the difference back to zero. The performance of the
basic control is limited because of the frequency response of the different
components and the maximum outflows of the servo-valves.
Whenever the tested specimens are very big and heavy and the oil outflows are
quite important, it is necessary to use multi-stage servo-valves using two closed-loop
control circuits: an outer loop and an inner loop.
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