## 1 Introduction
In addition, the characteristics of a hydro-pneumatic suspension change according to current conditions, owing to the nonlinear characteristics of the hydraulic system. Thus, in order to improve the accuracy of the simulation model, it is necessary to accurately analyze the characteristics of the hydro-pneumatic suspension unit and to construct a model based on the characteristics of the suspension. Various methods have been used to test the characteristics of suspensions. [PERSON] et al. [7] developed a device that can simultaneously test the performance of active and passive suspensions for automobiles. A suspension was installed on a vertically fixed guide, and sprung mass was added to the upper portion and a motor was installed on the lower portion. This test device made it possible to convert the rotational motion of a disk into vertical reciprocating motion. [PERSON] et al. [8] constructed a reciprocating test device by fixing one side of a suspension and then connecting the other side to a rotating crank in order to perform characteristic tests on semi-active suspensions in automobiles. However, using a disk and crank in this manner is problematic because even if the input is rotated at a constant frequency, the output appears as a trigonometric function that reflects the mechanical characteristics; thus, the displacement, velocity, and acceleration of the output must be converted. [PERSON] and [PERSON] [9] developed a test device equipped with a hydraulic cylinder that can control the displacement, velocity, and acceleration of a suspension. In their tests, the upper end of the suspension was fixed, the lower end was connected to the hydraulic cylinder, and the spring stiffness and damping coefficient were obtained by measuring the low-speed reciprocating motion and high-speed reciprocating motion, respectively. The above-mentioned suspension characteristic tests were mainly performed on automobile suspensions, such as those in passenger cars and commercial vehicles. Because agricultural tractors drive at relatively low speeds under completely different road conditions than automobiles, an automotive suspension cannot be used as a tractor cabin suspension. In addition, an automotive suspension is located between the wheels and the vehicle body, whereas the cabin suspension of an agricultural tractor is located between the tractor body and the cabin. Thus, the operating conditions of the suspension are different, and the results of characteristic tests of automotive suspensions cannot be applied to tractor cabin suspensions. Therefore, in this study, a characteristic test of a tractor cabin suspension was performed according to tractor operating conditions. A high-performance exciter equipped with a servo valve was used for testing; this approach simulated load conditions more accurately than the conventional disk and crank method. To establish an accurate test method for deriving the spring constant and damping coefficient of a suspension, this study performed a characteristic test on a hydro-pneumatic, semi-active tractor cabin suspension under changing velocity and current conditions. The study also generated force-displacement and force-velocity diagrams of the suspension that can be input into a simulation model for control logic development. The test utilized an exciter with a servo valve for controlling the displacement and velocity of the hydraulic cylinder, and employed a method of fixing one side of the suspension and reciprocating the other side without sprung mass. In order to derive the spring constant of the suspension, a low-speed reciprocating motion test was performed to obtain the force-displacement diagram and to derive the damping coefficient; 48 tests were performed under 6 velocity conditions and 8 current conditions to obtain the force-velocity diagram for each result.