【正文】 damper by adding a bypass assembly and a controllable valve and demonstrated significant improvement in both handling and ride fort. Giliomee and Els described the development and characterization of a semiactive hydro pneumatic suspension with a twostate pneumatic spring and a twostate hydraulic damper. Tests were conducted on a single DOF test rig and the semiactive damper control strategies were evaluated. It was shown that the acceleration levels were much lower with the semiactive mode. Els and Holman developed a twostate discrete adjustable semiactive rotary damper for heavy offroad wheeled vehicles. They carried out a full vehicle simulation using DADS mercial software and demonstrated that the semiactive rotary damper performance was better than both the passive rotary damper and traditional damper. Use of electrorheological (ER) fluids in dampers have also been attempted. Choi et al. have proposed ER dampers for ride fort improvement in tracked vehicles. Initially the damping characteristics are established with respect to the electric field using a Bingham plastic model and this is incorporated in a linearized statespace model of the vehicle dynamics. An optimal controller with a Kalman filter has been used to demonstrate the reduction in vertical acceleration of the vehicle. Most of the semiactive dampers reported in the literature for tracked vehicles are twostate dampers (on–off type) which require plex control strategies. In this paper, the skyhook damper strategy (on–off type) is used in a quartercar model to arrive at an optimal damping coefficient. Based on this a continuously variable damper, with a standard PID controller, is proposed for damping control. Using an analytical model of the suspension, validated with experiments on a suspension test rig, an inplane simulation model is prepared. Comparisons with the passive suspension clearly demonstrates the superior performance of the semiactive damper. 2. Passive hydrogas suspension Tracked vehicles fitted with torsion bar suspensions are limited in their ability to achieve high mobility due to their linear characteristics. Hydrogas suspensions due to their inherent nonlinear behavior can provide higher mobility and better ride fort performance. The hydrogas suspension model has usually been developed from experimental force–displacement characteristics, which requires availability of suspension hardware. A typical hydrogas suspension system, shown in Fig. 1a, consists of a stationary accumulator and accumulator cylinder, damper, crank pin, road wheel and axle arm. The crank, connecting rod and piston form a four link slider – crank mechanism as shown in Fig. 1b to convert the rotary movement of the axle arm to linear piston displacement for pressing the gas medium. A damper is animportant subsystem of a suspension located between and linking the actuator and accumulator cylinders. Applying loop closure along the X0 and Y0 axes for the slidercrank mechanism as shown Fig. 1 b yields for rebound position: velocity of the piston, with x being the angular velocity of the axle arm (crank), is given by To obtain the stiffness of the suspension, it is assumed that the gas pressure under pression/expansion is given by a polytropic law, pVn = constant, where n is the polytropic coefficient. The volume of the gas V is calculated from the slider position (Eq. (3)) and the force on the cylinder block is puted by multiplying the pressure p with the piston cross section area. The force–deflection relationship yields the gas spring characteristics corresponding to the permissible travel limits of the piston. To validate the stiffness calculations using the polytropic law, an experimental setup, as shown in Fig. 2, is used. Note that the same setup is used for both passive and semiactive suspension experiments. The suspension test setup has a capability to test a single station suspension with a maximum stroke of 600 mm. The displacement is sensed by a LVDT and the load is measured using a load cell with a rating of 250 kN. The test rig is capable of carrying out both static and dynamic tests at various conditions. The wheel is on a platform connected to an actuator. For varying the spring characteristics, the platform is moved from 0 to 500 mm in steps of 50 mm. the force at the fixed end of the suspension is measured for each position. The force–deflection curve obtained from experiment is pared with the analytical model. As seen from Fig. 3 the agreement is excellent indicating the validity of the theoretical model. . Damper characteristics using electrical analogy The hydraulic conductance of the damper is calculated from the pressure drops for different piston velocities using an electrical analogy. Kirchhoff?s laws are used to relate the flow rate and the pressure drop across an orifice, analogous to an equivalent electric circuit. The analogy maps flow rate to current, pressure to voltage but hydraulic resistance is used with care, as it is nonohmic. Therefore, hydraulic conductance (gi) of an orifice, proportional to the orifice area, is defined. The instantaneous rate of flow through a damper orifice can also be written as where Cd is the coefficient of discharge, q is the mass density of the fluid, Ao is the area of the fixed orifice. Eq. (4) can be written as, Conductances in parallel add like electrical capacitances. However, conductances in series add as reciprocal squares, with the effective conductance ,gp given by where g1, g2... gn, are the conductances of the various orifices/passages in the damper. The hydraulic circuit of the damper with plex passages and orifices can be visualized to be arranged in parallel, or series, from which the effective conductance can be calculated. This a