## 4 Experimental Results and Discussions  
To implement the proposed controller, an experiment setup is shown in Figure 6, in which several sensors such as linear variable differential transformer (LVDT) transducer, accelerometers at the seat and driver were used to get the feedback signals to the controller. Detail information for the sensor is shown in [9]. In this experiment, two comparative controllers were adopted for comparison: Comparative Controller 1 [22] and Comparative Controller 2 [9]. These comparative controllers are the type of hybrid controller including the Riccati-like equations and the combination of sliding mode control and fuzzy control in their design. This choice is valuable for evaluating the proposed controller, which is also a kind of hybrid controller. As in the computer simulation, the random-step-wave road  
Figure 4: Simulation results of displacement of seat under random step wave road excitation: (**a,b**) general view; and (**c,d**) large view. Figure 5: Simulation result of power spectral density of displacements under random step wave road excitation. excitation was chosen for the experiment. This excitation was embedded in the shaker system, as shown in Figure 6. It is noteworthy that the computer system included two part: one for the hydraulic shaker control and the other for the proposed controller with Dspace 1104. The applied current for MR damper was used with maximum value 2 A for three controllers. The limitation of the maximum value of the applied current was reasonable to evaluate three controllers showing their best in control performance under severe vibration environment. The measured results of the controllers are presented in Figures 7-9. In Figure 7, the responses of the controllers including displacement, velocity, and acceleration of seat and driver are shown. It is clearly seen that the proposed controller provides better vibration control performance than the comparative controllers [9; 23]. The applied current of three controls are shown in Figure 8. In this figure, the input controls appear to be very complicated, but it is clear the maximum input current has been limited by 2A. Now, to more clearly observe the benefits of the proposed controller, the power frequency responses are presented in Figure 9. It is remarked here that, according to the standard ISO 2631-1, the acceleration at the driver position should be less than 2.5 m/s\({}^{2}\). It is seen in Figure 9a that three controllers are effective in control vibration, but the proposed controller clearly provides lower values than the other controllers [9; 23]. Based on the decrease of the acceleration of the seat, the acceleration at the driver position is also decreased, as shown in Figure 9b. The acceleration of the proposed control and the comparative controller 2 [9] is nearly similar. This is originated from the damping ratio of the seat frame which is nonlinear. To compare three controllers, quantification was undertaken in this work: the performance of three controllers was continuously evaluated using the ToD (Transmissibility of Displacement) and ToA (Transmissibility of Acceleration) values of the seat [9]. The ToD values of the random step wave excitation were 0.213565, 0.358171, and 0.246641 for the proposed controller, Comparative Controller 1 [23], and Comparative Controller 2 [9], respectively. Similarly, from the acceleration responses, the ToA values of random step wave excitation were identified as 0.170182, 0.396618, and 0.257991 for the proposed controller, Comparative Controller 1 [23], and Comparative Controller 2 [9], respectively. From the above results, the proposed controller is the best, showing outstanding vibration control with the stability of the system under severe road disturbance. Figure 6: Experiment setup of seat suspension with MR damper for a commercial bus.