## 3 Materials and Methods
### Hardware System Design  
#### 3.2.1 Clutch Actuator  
The clutch is the main part of a tractor's powertrain, located between the engine and the transmission, which serves to transfer or disconnect the engine's power to the transmission [39]. Figure 1: Overall experimental procedure for developing the proposed tractor automated manual transmission (AMT) system. Figure 2: (a) AMT system layout including shuttle shifting actuator and clutch actuator; and (b) schematic of the AMT system combined with a manual tractor. The tractor used in this study is equipped with a single disc friction clutch, which is composed of components such as a release fork, release bearing, diaphragm spring, pressure plate, and clutch disc, as shown in Figure 3a. Clutch operation is largely divided into disengagement and engagement [40]. When the operator presses the clutch pedal, the central part of the diaphragm spring is pushed in the direction of the clutch disc by the release fork and release bearing. Thanks to the fulcrum ring, the outer portion of the diaphragm spring rises from the pressure disc, which causes the clutch to separate as the pressure plate is disengaged from the clutch disc. The engagement and disengagement of the clutches have opposite characteristics in the applied force and the behavior of each component. When the force applied to the pedal is removed, the pressure plate and clutch disc are connected by the elasticity force of the diaphragm spring to transfer power. A tractor AMT system automatically controls the clutch operation, instead of the operator using the clutch pedal. Therefore, to develop an AMT system, it is necessary to design and develop a clutch actuator which is suitable for the engagement and disengagement of the tractor clutch. In this study, an electronic actuator with excellent power transmission efficiency and fast control response was used, which was designed using an electric motor and reduction gear, as shown in Figure 3b, in consideration of the diaphragm load of the tractor clutch. In addition, as the engagement and disengagement of the clutch are performed by pushing or pulling the release fork connected to the clutch pedal, a screw was used to convert the rotational motion of the electric motor into linear motion. In this study, irreversible screw coupling was used so the clutch could remain in its original position when the motor was not controlled. The electric motor specification of the clutch actuator was calculated based on the force of the screw required for clutch control. The vertical force of the clutch pressure plate was 6174 N, and the lever ratio between the clutch pressure plate and the release fork was about 0.2. Considering a safety factor of 1.3, the required force of the actuator was thus about 1221 N. The input torque of the screw was calculated using the required force, screw efficiency, and screw lead based on Equation (1). The rotational speed of the actuator screw was calculated using the screw lead and screw feed rate based on Equation (2). The efficiencies of the screw and screw lead were 0.44 and 8.5 mm, respectively. The calculated torque of the actuator screw was 3.76 Nm. The feed rate of the actuator screw was 2.16 m\(\cdot\)min\({}^{-1}\) and the input rotational speed of the actuator screw was 254 revolutions per minute (RPM). The reduction ratio and motor specifications were selected to minimize the motor size using the calculated screw input torque and rotational speed. The reduction ratio from the motor to the screw input was selected to be 19:1. The motor specifications had a rated torque of 0.21 Nm, rated rotational speed of 4826 RPM, rated voltage of 12 V, rated current of 16 A, and stall current of 65 A at the maximum torque point. \[\mathrm{F_{s}}=\frac{2\pi\mathrm{\cdot}\mathrm{\eta\cdot}\mathrm{T_{s}}}{ \mathrm{I}}, \tag{1}\]  
where \(F_{s}\) is the force of the actuator screw (N), \(\mathrm{\eta}\) is the efficiency of the screw, \(T_{s}\) is the input torque of the actuator screw (Nm), and I is the length of the screw lead (m).