9.4 Verification of the compensations by using virtual substructure test
9.4.1 Control and measurement system for virtual substructure tests
The first diagram (Figure 9.8) is used for VSTs without compensation while the second and the third diagrams (Figures 9.9 and 9.10) are used for virtual substructure tests with the error force compensation and with the phase lag compensation.
Figure 9.8: The test system for virtual substructure tests without hydraulic system (S1 is set at the position 2) or with hydraulic system (S1 is set at position 1) and without compensation, with
or without noise of force measurement (S2 is set at position 1 or 2)
fcs
1
2 S1 +
+
fnoise
um
fl
fc
uc
Numerical
substructure Experiment
(simulated) Hydraulic
system
free
load cell 1 2 S2
With the control system in Figure 9.8, the effects of the displacement error in the hydraulic system and the noise of force measurement can be adopted or rejected by selecting the positions of the switches S1 and S2.
If the switch S1 is set to be at position 1, the measured displacement um of the hydraulic actuator is used to simulate the response of the experimental substructure.
Based on the measured displacement of the hydraulic actuator and the response of the numerical substructure, the coupling force fcs is calculated at each sub step. To cope with the noise phenomenon in force measurement, a realistic noise signal fnoise coming from a free load cell with its amplifier is added to the simulated coupling force fcs
(when the switch S2 is set at position 1). In the case that both switches, S1 and S2, are turned into position 2, the calculated displacement uc is used as the input of the experimental substructure and there is no noise of force measurement (fnoise =0) in the coupling force. This case is used in order to produce substructure solutions without error of displacement control and without noise of force measurement.
It should be aware that there are few differences between VST and RTST.
• The first one is that there is no specimen being coupled with the actuator in a VST while a real substructure test has a specimen and it may affect on the displacement response due to certain nonlinear features in the hydraulic system.
However, this kind of effects is not considered in this thesis. It is important that the major errors of a realistic displacement control system (including phase lag and amplitude error of the actuator as well as the measurement error) be included in the virtual tests if the switch S1 is set at position 1.
• The second difference is that the feedback force in a RTST is a measured signal coming from a measuring system (including a load cell, amplifier and digital converter) with a certain noise while the coupling force in a VST is a simulated force without noise. This is not negligible because the noise of force measurement can affect significantly on the estimator of the error force compensation. Thus, a free load cell is used to add a noise of force measurement to the simulated force if the switch S2 is set at the position 1.
In Figure 9.9, the error force compensation is used in VST with noise of force measurement by using a free load cell. In VST with phase lag compensation in Figure
9.10, the measured displacement of the hydraulic actuator is compared with the computed displacement and then, the estimation mechanism calculates the compensating displacement and adds the compensating value into the displacement controller. In addition, since each compensation method works independently from each other, VSTs with either only one or both compensations will be carried out in order to demonstrate the effectiveness of the compensations.
Figure 9.9: Diagram of a VST using a hydraulic system with the error compensation and noise of force measurement
Figure 9.10: Diagram of a VST using a hydraulic system with phase lag compensation and noise of force measurement
The hardware installation for RTSTs is presented in Figure 9.11. The substructure algorithm, both error compensations and a displacement controller are programmed on a computer and then they are loaded into the real-time control system: ADwin. The
+ +
fnoise um
fl fc
fe
(residual e)
uc
Numerical substructure
Experiment (simulated)
Estimator
measure error force
Hydraulic system
free load cell fcs
The loop with dash lines works at each sub step.
The loop with doted lines works at each time step to measure and to compensate the error force.
The loop with doted line works at each estimation step to estimate the compensating displacement.
The loop with dash line works at each sub step.
+ um +
+ fl
+
fc
e
∆u uc
Numerical substructure
Experiment (simulated)
Estimator Error measurement
Hydraulic system
free load cell fcs
fnoise
displacement controller includes an Adwin system, an U/I converter (voltage-to- current converter), a servo valve, a hydraulic cylinder, a displacement transducer and an amplifier numbered as 1. In this control, the servo valve of hydraulic block is controlled directly by the software in ADwin. The control technique used in this application is a standard PID control. Further information of the components in this control system can be seen in Appendix A7.
Figure 9.11: Diagram of the control and measurement systems for VST