## 3 Overall Structure Design and Force Analysis  
Through the analysis of the excellent entries in the previous six national undergraduate engineering training integration ability competitions over the years, the carbon-free car is mostly composed of six parts: frame, prime mover mechanism, transmission mechanism, steering mechanism, traveling mechanism, and fine-tuning mechanism. The main components and common realization mechanisms of the carbon-free car are shown in Figure 2[12, 13, 14]. This design is based on the control variable method, and the carbon-free car's length, width, and height are set to be 120 mm \(\times\) 140 mm \(\times\) 575 mm. The bottom plate is made of an acrylic plate with a thickness of 5 mm, which strives for a compact structure for improving the fault tolerance rate of the carbon-free car; the pulley mechanism was used as the prime mover, and three aluminium alloy tubes with a length of 400 mm were selected as the support frame of the pulley; the transmission mechanism was composed of 1\(\sim\)2 groups of gear transmission mechanisms according to the specific track; the steering mechanism adopted a cam mechanism, which could adapt to different tracks by changing the cam profile; the walking mechanism adopted a single-wheel drive, and the material was an acrylic plate with a radius of 65 mm and a thickness of 2 mm; the fine-tuning mechanism was to adjust the swing angle of the steering wheel by changing the length of the push rod and the connecting rod, for correcting the deviation caused by the influence of different venues. Figure 2: Main components and common implementation mechanisms of the carbon-free car. When designing the mechanical structure of the carbon-free car, the car was required to run smoothly in the process of obstacle avoidance, and its force analysis was required. Let \(m\) be the car mass, \(N_{0}\) be the supporting reaction force of the car on the ground, \(F_{0}\) be the driving force of the car, and \(G\) be the gravitational acceleration. The whole car is taken as the research object, and the simplified mechanical model is shown in Figure 3.  
\[mg=N_{0} \tag{1}\]  
The driving torque of the car is:  
\[M=F_{0}\times\frac{D}{2} \tag{2}\]  
where, \(\frac{D}{2}\) is the driving force arm. Then the driving force of the car is:  
\[F_{0}=\frac{2M}{D} \tag{3}\]  
The carbon-free car converts the potential energy of gravity into the mechanical energy of the wheels, ignoring the problem of transmission efficiency, and the driving torque M is converted from the weight gravity of the carbon-free car. \[M=G\frac{\phi}{2}=F_{0} \tag{4}\]  
where, \(\frac{\phi}{2}\) is the force arm of gravity. Then the driving force is:  
\[F_{0}=G\cdot\frac{\phi}{D} \tag{5}\]  
The carbon-free car is subject to running resistance when moving forward, which includes inertia resistance (especially in the car starting acceleration stage, set the acceleration as \(a\)) and static resistance. Where, the inertia resistance \(N_{a}\) is:  
\[N_{a}=ma \tag{6}\]  
Static resistance generally included the basic resistance \(N_{b}\) in the transmission chain and the friction resistance Ff between the wheel and the ground. The basic resistance \(N_{b}\) is:  
\[N_{b}=mgw \tag{7}\]  
Figure 3: Overall force analysis of the carbon-free car. \(w\) is the running resistance coefficient, and the empirical data obtained from the experiment is about 0.01. Therefore, the condition that needs to be met to make the carbon-free car start smoothly is:  
Figure 3.0  
Then the condition for a non-slip carbon-free car is:  
Figure 3.1  
where \(f\) is the friction coefficient between the car and the ground. Let \(S\) be the traveling distance of the car, and \(\eta\) be the total efficiency of the car. Then the work done during the falling process of the weight is:  
Figure 3.2  
Then there is:  
Figure 3.3  
As shown in the above formulas, the driving force should be reduced as much as possible to satisfy the starting requirements to increase the travel distance of the carbon-free car with no slipping.