## 1 Introduction  
It is well known that the human body is particularly sensitive to whole body vibration (WVB) over a frequency range of 4-8 Hz in the vertical direction caused by road roughness transmitted via the vehicle body. This will affect the ride comfort and can be harmful if experienced over a prolonged period [1]. Much research has been undertaken in this area, covering secondary spring/damper vehicle and seat suspensions. The latter option of a seat suspension has some advantages as it can directly reduce the WBV, is cost-effective and can provide practicable solutions. There are three types of seat suspensions, namely, passive, semi-active and active. With passive seat suspensions, the vibration attenuation performance is limited due to the fixed characteristics of the suspension elements and, thus, they are only effective over a limited frequency range. In contrast, the suspension characteristics of semi-active systems are controlled by using adjustable suspension elements, such as magnetorheological dampers (MR) [2] or electro-rheological (ER) dampers [3]. Whilst, semi-active systems are safe, cost-effective and require low power consumption, the isolation performance is still limited, as a semi-active suspension can dissipate energy and re-distribute this within the system but cannot add energy [4]. An active system can dissipate and/or release energy from the system by applying an external force. This increases the attenuation performance of this system over a wider frequency range when compared with passive and semi-active seat suspensions. However, the performance of these systems depends on many factors, such as the actuation and sensing systems as well as the control approach that is employed to generate the required control force [5]. In addition, there is a compromise between improving the ride quality while also limiting the seat suspension travel within its allowable physical limits and consequently, comprehensive research in the literature has been conducted regarding a range of control approaches. [PERSON] and [PERSON] [6] applied optimal control theory in designing an active seat suspension for a heavy truck using an electric servomotor and ballscrew mechanism in which the system state's variables were obtained by integrating the corresponding associated measured acceleration signals. An active vehicle suspension using a robust Hoo and output feedback controller based on a three degree of freedom (DOF) quarter vehicle suspension with a human body model was investigated by [PERSON] et al. [7]. Although the controller claimed to use available states, the absolute velocities of the sprung and unsprung masses were acquired by integrating the associated acceleration signals. In a paper by [PERSON] [8] a hybrid feedforward-feedback controller based on the sky-hook principle was investigated in order to attenuate the vertical vibration of a driver's seat using an electropneumatic active seat suspension. [PERSON] et al. [9] modified a dynamic output feedback Hoo controller over a limited frequency range for an active seat suspension. [PERSON] and [PERSON] [10] studied experimentally and in simulation the application of a hybrid control approach that comprised of an adaptive controller that combined a filtered-x least mean square (Fx-LMS) and a robust Hoo feedback controller for an active seat suspension. Beside the complexity of the proposed controller it failed to significantly attenuate the vibration at the driver's seat in the case of a random broadband vibration disturbance which is the most common type of road input. [PERSON] et al. [11] experimentally investigated an active pneumatic seat suspension using a triple-feedback control algorithm that utilised available and measurable system states. As a result of the complexity associated with the pneumatic system and its limited frequency response, the active system was only effective in reducing vertical vibration at low frequencies, less than 4 Hz and did not perform well at higher frequencies that can cause significant discomfort.