# Analysis and Design of a Minimalist Step Climbing Robot
## 2 Working Principle  
This section explains the proposed mechanism of the stair-climbing robot. Figure 1a shows a schematic of a series of actions taken by the robot while ascending the stairs, whereas Figure 1b describes the descending process of the robot of the stairs. The proposed robot has two large front wheels and one small rear wheel. The rear wheel is connected to the body of the robot by a connecting link and a revolute joint. The angle between the connecting link and the body is varied to maintain the inclination angle of the body while ascending or descending the stairs.  
In Figure 1a, the left image represents the condition when both rear and front wheels are on the ground, while the middle figure refers to the situation when the front wheels  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Mechanisms** & **Advantages** & **Disadvantages** \\ \hline Tracked based [1] & Successful operation on stairs and irregular terrains & Slower in horizontal plane \\ \hline Articulated leg [4] & Stable design to climb stairs & Complex control design and slower in horizontal plane \\ \hline Tracked based hybrid [5] & Successfully climbs on stairs as well as fast on planes & Two separate mechanism results in heavier system \\ \hline Articulated leg based hybrid [6] & Stable in climbing stairs and fast in horizontal plane & Higher power consumption \\ \hline Two wheel cluster [23] & Autonomous operation on stairs & Required assistance during step climbing \\ \hline Three wheel cluster [27] & Successfully climbs on stairs & Unstable during operating on uneven stairs \\ \hline Proposed mechanism & Simple and minimalist design & Increased length of the system limits the system to operate in confined space \\ \hline \hline \end{tabular}
\end{table}
Table 1: Qualitative comparison of existing step climbing mechanism.  
Figure 1: Schematic showing different positions of the robot (it may be noted that the front wheel diameter is larger than the step height); (**a**) ascending the stairs; (**b**) descending the stairs.  
and rear wheels are on the step. The light shaded image of the robot represents the position of the robot in which the angle between the body and the connecting link is kept constant during the entire climbing operation. The right image on the figure shows the position of the robot after completion of the climbing operation. Similarly, Figure 1b illustrates changes in the posture of the robot while descending the steps.  
It may be noted, if the angle between the connecting link and the body of the robot remains constant, the inclination of the body with respect to the horizontal will vary as the robot progresses on the steps. During the ascent and descent, the inclination angle of the body will tend to increase and decrease, respectively. During the descent, depending on the configuration of the robot and steps, the inclination angle may reduce to a negative value which may cause the robot to topple. This situation can be avoided by controlling the angle between the connecting link and the body of the robot.  
In order to compensate for the change in the inclination of the robot, an additional servo motor is used at the joint between the body and the connecting link while climbing on the steps. This servo motor is actuated by a microcontroller whenever a change in the inclination angle of the body is detected. An IMU is used to track any change in angle during the motion of the robot. These signals are fed to a microcontroller, which in turn controls the actuator. As a result, the inclination angle of the robot is properly maintained. In the following section, the design and modeling of the robot are discussed.