## 1 Introduction  
To reduce ride vibration, existing tractors mainly use rubber mounts between the tractor body and the cabin in which the operator is seated. These rubber mounts have a large spring constant and a small damping coefficient, which makes them suitable for supporting weight but unsuitable for absorbing vibration transmitted to the cabin [1]. Recently developed large tractors are increasingly equipped with cabin suspension instead of rubber mounts to reduce cabin vibration. Because the maximum driving speed of a 100 kW-class tractor recently developed in the Republic of Korea can reach 40 km/h, there has been increasing demand for cabin suspension that reduces the impact from the ground during high-speed driving. Studies have shown that vibration is reduced when rubber mounts are replaced with cabin suspension [2]; however, there has been minimal research analyzing the characteristics of cabin suspension and the optimization of design factors. Cabin suspension is functionally a combination of a spring that supports the weight by also acting as a low pass filter, and a damper that dissipates energy by reducing motion speed. Springs are divided into mechanical, pneumatic, and hydro-pneumatic types according to their operation method. In general, mechanical springs are used in systems with lighter weights, whereas pneumatic and hydro-pneumatic springs are used in systems with heavier weights. The damper reduces the speed by dissipating kinetic energy into heat energy through resistance generated when hydraulic oil passes through a narrow cross section. Dampers can be classified as passive, semi-active, and active depending on the damping force adjustment function and the presence or absence of an external energy source [3]. Tractor cabin suspensions should be capable of level control in inclined operation environments; further, hydraulic springs are suitable for use in rough terrain because of the heavy load [4]. Tractors perform a wide variety of tasks and can drive at a wide range of speeds, and thus experience varying types of vibrations coming from the ground. Therefore, a semi-active damper is considered suitable for adjusting the damping forces depending on the circumstances. Because of the wide variation in tasks and driving speeds, it is thought that adjusting the damping force according to the operation environment is suitable. In this study, a semi-active hydro-pneumatic suspension was selected as the suspension to be used for the tractor cabin, and the characteristics of the suspension were analyzed through a single unit test. [PERSON] et al. [5] measured acceleration data while driving a tractor with a passive cabin suspension system according to ISO 2631(1985), and developed a finite numerical analysis model based on the test results in order to perform parameter optimization. [PERSON] [6] developed a system in which semi-active suspensions were mounted on a tractor's seat, cabin, and axle in order to improve ride comfort. Virtual ground vibration was applied to the developed tractor using a road simulator, and the effectiveness of the suspension system was verified by measuring the rotational acceleration in the roll and pitch directions and the acceleration in the vertical direction. Recently, owing to the installation of proportional control valves in hydro-pneumatic suspensions, research has been conducted to develop optimal damping force control according to operating conditions. [PERSON] [3] modeled a semi-active hydro-pneumatic suspension system with a simulation program and verified the model by comparing it with actual vehicle test results. The ride comfort was analyzed by applying a suspension system and control logic to a tractor simulation model with 14 degrees of freedom. These previous studies on tractor cabin suspensions analyzed how these suspensions could improve ride comfort and reduce ride vibration, but did not analyze the characteristics of the tractor cabin suspension unit.