# BIZON-UGV for Airport Pavement Testing: Mechanics and Control
## 3 Modelling and Control
### Control System and Autopilot Design  
The autopilot for the UGV Bizon [23; 24] is depicted in Figure 8. The main purpose is to follow the reference trajectory or path and speed commands issued in the path planner. The structure consists of a path planner, reference path (reference motion position), speed converter, feedback loop controller of the heading angle \(\psi\) rate, and wheel torques based on the PID technique. The autopilot also includes the feedforward component and a reference model that specifies the desired response to a step command. Finally, the turn angle and wheel torque signals are fed to the mixer, which converts it to the right and left wheel torque or speed scaled in the domain of inverter frequency. Hence, the presented block diagram generally represents the implemented control algorithms. Figure 8 shows that the control portion consists of a block named \"Path Converter\" and \"PID+FF Controller\". In simplified terms, feedforward control is determined based on set values defined from the current platform position, desired speed, and cross-track error (XTE). The control of the platform's deviation angle is cascaded through the control of angular velocity \(\mathbf{r}\). This velocity is limited, and these limitations, as well as the XTE error, allow for determining the initial control value. This value is then fine-tuned by an additional PID regulator (Distance To Heading), which determines the course correction based on the distance from the route. Additionally, the BIZON platform is equipped with inverters that maintain the desired wheel rotation frequency regardless of external conditions, which directly affects the speed. Knowing the inverter parameters, gear ratios, and wheel parameters, the set frequency value is determined as feedforward feedback in relation to the desired speed. Subsequently, the external regulator enables real-time speed control in response to the actual speed error determined by a state observer based on GPS receiver data, acceleration sensors, and encoders. Based on introduced waypoints and current platform position, the autopilot determines trajectory in two modes: with and without tightening (a special function to obtain better mission accuracy, which means tightening to the reference trajectory, minimizing tracking error). When the tightening function is off, only the actual position and reference position are considered in the control process. When the tightening mode is on, the cross-track error (XTE) is considered a function of the distance from the reference trajectory, in this case, the line between two waypoints. The sum of the position error and XTE is considered to determine the motion trajectory and heading angle of the platform. In the case of a very slow platform speed, the motion trajectory computation is problematic and inaccurate. As shown in Figure 8, one of the simplified modules in the autopilot structure is the PID and FF controller. The PID controller performs UGV control and stabilization in the context of heading angle \(\psi\). It is additionally equipped with the FF component to obtain a quick response due to the reference values. Figure 9 shows the structure of the PID-FF controller and explains its role in the control system. Figure 8: The autopilot design. PID-FF controller. PID control stands for proportional, integral, and derivative control and is the most commonly used control technique in industry [25; 26]. It works in a closed-loop configuration to minimize control error \(e\). Feedforward control (FF) is a strategy used to reject persistent disturbances from \(\psi_{ref}\) that cannot adequately be rejected with feedback PID control. Feedforward control is added to feedback control and is not implemented alone. In this situation, the performance of control systems can be greatly enhanced by applying feedforward control.