## 4 Experimental Test Rig
### Multi-Axis Simulation Table (MAST)  
The MAST is a six degrees-of-freedom vibration simulation table supplied by Instron Structural Testing Systems (INSTRON, Buckinghamshire HP 12 3 SY, UK). It offers three translation motions as well as three rotations through hydraulic actuators. The MAST was employed as a vibration platform for testing the active seat suspension in the vertical direction only. The principle of hardware-in-the-loop (HIL) was used to control the MAST as the sprung mass in a simulated quarter vehicle model (QvM). The HIL simulation QvM was excited by an artificial road profile. Consequently, the active seat, which is mounted on the MAST, is subjected to an excitation equivalent to the sprung mass in a moving vehicle. The accuracy of the MAST's prediction of the QvM dynamics will inevitably be compromised by system friction, bandwidth limitations, and computational time delays. In addition, the active seat controller that requires preview information from the HIL suspension displacement and velocity depends upon the accurate prediction and generation of the system dynamics by the MAST. In order to improve the MAST dynamic response the principle of inverse dynamics as described in a previous study [34] was used. Figure 10 shows a block diagram of the MAST HIL and the proposed control strategies, as well as the experimental setup for testing the active seat suspension.  
The active seat suspension used in this study is schematically shown in Figure 11 and was previously developed at the University of Bath [14]. It consists of two main parts: the passive suspension and the active actuator. The passive suspension unit comprises of an ELka-stage-5 bicycle shock absorber that is composed of an adjustable damper and a coil spring. This unit is linked to the seat pan via a two-bar lever mechanism used to support the static load, regardless of any active force, as well as providing some small pitch rotation of the seat pan. The seat pan is constrained to move vertically by two linear rails and their corresponding carriages. Although the seat was designed to move in both pitch and heave, in this study only the heave motion is taken into account and the pitch motion is rigidly constrained. The active force is achieved through two electromagnetic linear actuators of type XTA-3806 (dunkermotoren, 79848 Bonndorf, Schwarzwald) that are fixed at the front and rear of the seat. Each actuator has a peak force of 1.12 kN, a stroke of \(\pm\)30 mm and a maximum speed of 3.8 m/s. It should be noted that these actuators were operated under current control mode. Their dynamics are very fast and may be approximated by a first-order lag with a time constant of less  
Figure 9: Simulated seat suspension performance of the passive and active systems when subject to a bump road input. (**a**) A1; (**b**) A2; (**c**) A3; (**d**) A4; (**e**) A5; (**f**) PID.  
than 1 ms, thus, the actuator dynamics are not significant when compared with the car and seat system dynamics and may be neglected.  
A test dummy with a weight of 542 N was used to characterise the dynamic response of a driver. Two piezoresistive accelerometers (Entran, EGCS-D1 CM-25) (StrainSense Limited, Milton Keynes MK19 6 FG, UK) were used to measure the vertical acceleration signals of the active seat suspension and the MAST. Further information about the experimental setup and the active seat suspension, as well as a detailed description of the HIL setup can be found in [14; 34].