## 4 Control System Design  
### Hardware Design  
The automatic levelling system of the mobile platform is composed of a picking platform, controller, stepping motor driver, electric push rod, IMU, displacement sensor, pressure sensor, Voltage amplifying module (the model is HX711), level meter, and CAN communication module. The controller is the core of the levelling system [12] (which is a node in the control of the entire mobile platform. The controller chip is an Arduino Mega. It has enough memory to meet the levelling requirements.  
IMU adopts a 9-degree-of-freedom (DOF) IMU (the model is HWT901B) and a single-degree-of-freedom (DOF) IMU (the model is SINDT01), which are produced by Shenzhen Weite Intelligent Company, China. The HWT901B 9-degree-of-freedom (DOF) IMU and the  
Figure 4: Schematic diagrams of the inclination levelling range. (**a**) Schematic diagram of the \(X\)-axis. (**b**) Schematic diagram of the \(Y\)-axis. (**c**) Schematic diagram along the diagonal line.  
SINDT01 single-degree-of-freedom (DOF) IMU can achieve static accuracy within 0.05\({}^{\circ}\) and dynamic levelling within 0.1\({}^{\circ}\). Because the automatic levelling control system adopts three IMUs fusion, in order to facilitate the data transmission, the CAN communication module (the model is MCP2515) is adopted, since using the CAN module can reduce the connection between devices and the transmission pressure of the main controller. Therefore, the communication mode between the IMUs and the controller is CAN communication. The displacement sensor is selected to observe the displacement of each electric push rod after levelling. The linear accuracy of the displacement sensor is \(\pm\)0.15%, which can accurately observe the displacement of the electric push rod. Because the output signal of the pressure sensor is mV, in order to facilitate the controller to receive, the voltage amplification module (model HX711) is selected to cooperate with the pressure sensor, so that the mV signal can be amplified to the signal range that the controller can receive. The communication method between the displacement sensor, pressure sensor, and controller is serial communication. The levelling mechanism adopts an electric push rod, which has high precision and fast-moving speed. However, the electric push rod cannot communicate directly with the controller, so it needs to be equipped with a driver. After receiving the pulse signal from the controller, the driver will convert the pulse signal into an electrical signal, thus driving the electric push rod to rotation. The rotation of the electric push rod makes the electric push rod perform a lifting action. Through the number of pulses emitted by the driver, the angular displacement of the rotation of the electric push rod can be controlled, and the speed of the rotation of the electric push rod can be controlled through the pulse frequency emitted by the driver. Whether the pulse emitted by the driver is high or low, it controls the direction of the rotation of the electric push rod, and the precise control of the displacement of the electric push rod can be realized by driving the driver. The circuit connection between the electric actuator, various sensors, and the controller is shown in Figure 5.