## 1 Introduction  
Active suspension systems (ASS) [1; 2; 3; 4] have been the subject of much development in the car industry over decades. To provide comfort ride and road holding for driving, ASS actively controls the vertical movement of the wheels when the road terrain changes. Therefore, ASS actively reduces road vibrations and gives a smoother ride experience for passengers than passive/conventional suspension systems because the design of a passive suspension system compromises between three conflicting criteria: road handling, road carrying, and passenger comfort [5; 6]. To improve road handling and passenger comfort, ASS uses actuators to provide additional force between the sprung mass and the wheels to control the attitude of the vehicle. ASS also reduces vertical vibration in cameras that are mounted on a vehicle, thus, stable camera sequences are ensured for image processing [1; 2]. Therefore, ASS is eminently suited to the needs of self-driving ground vehicles. The most-commonly used actuators for an ASS are pneumatic, linear electromagnetic, or hydraulic actuators. For a classical and conventional configuration, these actuators are commonly designed to be used in conjunction spring dampers to create an active suspension, which gives an additional vertical force between the chassis and the wheels [7]. A hydraulic actuator is one of the most commonly-used linear actuators for ASS, because it is tough and has a high power-to-weight ratio. It also maintains a constant force and torque without the need for extra fluid or pressure. [PERSON] et al. [7] designed a hydraulic-driven ASS. The hydraulic ASS uses a model-free adaptive sliding controller to suppress vehicle vibration due to a 20 mm-high bump on a test-rig. [PERSON] et al. [8] presented a hydraulic-driven ASS that is regulated using an enhanced fuzzy sliding controller, which effectively suppresses vibration. However, a hydraulic ASS creates environmental pollutions because of the possibility of fluid leaks. Recently, a serial active variable geometry suspension (SAVGS) [9; 10; 11] has been theoretically and practically demonstrated on its feasibility for active suspensions. SAVGS uses a device between the spring-damper and the adjacent body. The device is mounted in series with the spring-damper and contains an electro-mechanical actuator that controls the orientation and elongation of the strut. Pneumatic actuators are also eminently suited to use in an ASS. A pneumatic active suspension can be modeled as a system with a spring, a passive damper, and a pneumatic active component. A pneumatic suspension has advantages over a spring-damper suspension in that it gives a soft ride at low speed while driving on flat terrain but allows good control while driving on rough terrain and it provides a force that allows the height of the vehicle's chassis to be adjusted to suit particular terrain. [PERSON] et al. [12] designed a prototype of a pneumatic ASS for teaching students. The pneumatic ASS consists of a wheel, coil springs, a pneumatic actuator for active damping, and an alternating current (ac) motor to simulate various road conditions. [PERSON] et al. [13] presented an analytical model of a pneumatic suspension and tuned its behavior to operate an ASS. An air spring can also be used as a pneumatic actuator for an ASS. It uses an airbag as a spring to adjust the height of a vehicle's chassis. An air spring is both a spring and a pneumatic actuator, thus, it gives good control for load carrying, stance, tenability, and handling. [PERSON] [14] developed a quarter-car suspension system that uses an air spring active actuator to give improved vibration isolation in the low-frequency range. [PERSON] et al. [15] designed a pneumatic push-pull actuator to isolate vibration in a vehicle. The push- and pull-functionality is separated into three actuators: an active air spring for the push direction and two actuators that are fluidic muscles for the pull direction.