1　The principle and existing problems of manual stair climbing wheelchair crawling

Ordinary wheelchairs do not have the function of climbing stairs and crossing obstacles, which greatly limits the mobility of lower limb paraplegic patients and elderly and weak people who use wheelchairs as a means of transportation. To address this issue and considering the current economic affordability of most wheelchair users in China, crawling devices should not be overly complex. The Rehabilitation Research Center of the Department of Precision and Instrumentation at Tsinghua University was the first to develop a manual stair climbing wheelchair. It is made by adding a set of stair climbing actuators and drive systems to a regular wheelchair, which can travel on flat ground and use only the strength of the user's arms to make the wheelchair go up and down stairs. The structural diagram of the stair climbing wheelchair is shown in Figure 1. Its main feature is to add a set of stair climbing actuators composed of parallelogram mechanisms on each side of the ordinary wheelchair. The two equally long connecting rods (cranks 3, 8) in each mechanism are fixedly connected to a gear (4, 10) with the same number of teeth and driven by a chain 7. The four cranks on both sides are installed in the same phase, and the connecting rod (i.e. stair climbing rod 9) has one "leg" at each end. The four "legs" form two sets of support systems with the original four wheels of the wheelchair. When the shaking hand passes through a set of transmission mechanisms, causing the four cranks to rotate synchronously, the two support systems land in turns and lift the other support system up (or down) one step until a flight of stairs is reached. The seat and backrest of the wheelchair can be adjusted to maintain a horizontal position when climbing stairs. In order to reduce the center of gravity height of the wheelchair during stair climbing, increase stability and safety, a "facing downstairs" approach is adopted for both ascending and descending stairs, that is, a backward approach is used when ascending stairs (see Figure 2).
Figure 1 Schematic diagram of manual stair climbing wheelchair structure

Figure 2 Schematic diagram of climbing stairs

When driving on flat ground, this wheelchair only needs to lift the four "legs" of the auxiliary stair climbing pole 9 off the ground and lock it. The transfer of operating conditions is extremely simple. From this, it can be inferred that the theoretical displacement curve of the wheelchair's center of mass during the middle working section of climbing stairs is a pure circular arc shape. Due to the fact that the size of the stair climbing actuator is designed based on the statistical average size of the stair steps, in order to adapt to the needs of climbing stairs of different sizes, passengers need to adjust the position of the wheelchair relative to the stairs from time to time when climbing stairs. For example, before climbing up (down) a step, the large wheel should be tightly pressed against the vertical wall of that step. However, such frequent adjustments undoubtedly increase the complexity of the operation. If the specifications of the stair climbing actuator are made adjustable to adapt to changes in stair size, it will also complicate the structure of the mechanism. This contradiction needs to be resolved, otherwise it will affect the practical value of the stair climbing wheelchair. The author made the manual stair climbing wheelchair adaptable to different stair sizes by changing the type of climbing pole 9.

2　Improvement and effectiveness of crawling components

When using a wheelchair to climb stairs, the landing positions of the four "legs" of the stair climbing pole 9 may change due to the size of the steps and the uncertainty of passenger operation. This change in landing position requires timely adjustment of the wheelchair's position relative to the steps after climbing up (down) each step. For example, before climbing up (down) each step, the large wheel should be tightly pressed against the vertical wall of the step. This practice has become the standard starting requirement for climbing down the next step. Otherwise, the accumulation of crawling errors in consecutive steps will cause subsequent crawling failures and even lead to the wheelchair tipping over. Frequent adjustment of position can be very difficult for passengers, especially for elderly, weak, and paraplegic patients, which affects the practical value of stair climbing wheelchairs.

In order to improve this situation, the author experimented with changing the shape and size of the stair climbing rod 9 as shown in Figure 3. At this time, the shape of the stair climbing rod 9 is arched, and soft rubber with teeth is attached to the bowstring. In this way, during the middle of the climb, the stair climbing rod 9 no longer uses the four ends (both sides) to contact the flat surface of the steps, but the "bowstring" with rubber teeth contacts the tips of six steps (both sides) (see Figure 4). The 6-point contact is very stable, and the rubber teeth bite on the tips of the steps, with strong friction. In this way, when climbing stairs, there is no need to frequently adjust the wheelchair, simplifying the operation. It can also operate normally when crawling other paragraphs.

Figure 3: The shape of the new stair climbing pole 9

Figure 4 Schematic diagram of stair climbing action when installing a new stair climbing pole

The material of the bowstring rod can be selected from the same stainless steel pipe as the stair climbing rod 9, with a diameter of 40mm, and the two are connected by welding. Rubber with teeth, which can be made of natural rubber, is made into an open tubular shape with the same inner diameter and length as the outer diameter of the bowstring. The tooth shape is serrated and the tooth height is 10mm. After welding the bowstring, it is then put on and the opening is glued together. Wheelchair supplier