# Design Considerations Concerning an Innovative Drive System for a Manual Wheelchair
## 3 Results and Discussion  
Figure 7 shows the dependency between the wheelchair velocity and drive torque relative to a selection of three ratios representing Gears 1, 4 and 6, for which the overall ratios are 0.592, 0.97 and 1.455, respectively. This illustrated the drive system's operation consecutively in different modes: reduction mode, an approximately one-to-one gear ratio and multiplication. Figure 6: A prototype wheelchair mounted on a dynamometer. Figure 7: Graph of the change in rotational speed and torque as a function of time, taking into account the gear ratio. (**A**) Gear 1, (**B**) Gear 4, (**C**) Gear 6. An analysis of the presented graph makes it possible to observe the cyclical nature of individual propulsion phases. It stems from the way the wheelchair is propelled. The user propels the pushrims with their hands at a certain rotation angle, thereby creating a driving torque. In the next phase of the propulsion cycle, moving the upper limbs to the initial position is necessary. This is dictated by the biomechanics of the human body. In the graph, this moment can be observed as a reduction in torque and velocity. In some cases, the cessation of propelling causes the torque value to drop to zero. At the same time, the cessation of propulsion generation does not cause the velocity to drop to zero, as the inertia accumulated in the drive system results in a gradual loss of speed. This can be observed in Figure 7a. This results from the fact that the successive propulsion phases could occur sufficiently quickly to prevent the occurring drag forces (e.g., friction) from acting long enough to brake the rotating wheels. Based on the results, it was found that a shift in the gears to higher ratios translates directly into an increase in the wheelchair's wheel speed. The individual incremental values for both wheel speed and drive torque correspond to pushrims' propulsion. The peak values, on the other hand, occur at the final phase of the upper limb movement, just before the user's hand releases the pushrim. The recorded wheel speed corresponds to the calculated theoretical values. The peak drive torques for the selected ratios ranged from 11.3 to 15.7 Nm. In turn, the recorded peak values for rotational speed were in the range from 29 RPM to 66.9 RPM. For the purpose of comparing the values for individual ratios, the root mean square (RMS) of the recorded waveform was determined due to its cyclical nature [33]. The calculated values for individual physical quantities are presented in Table 2.  
Analysis of the graphs presented in Figure 7 and the values in Table 2 allows us to observe that the root mean square values of the driving torque are not constant. The in-crease in the torque value for Gear 1 compared to Gear 4 is only about 3%, but for Gear 6 it is already 16%--which constitutes a significant value. This change is probably due to two factors. As the wheel rotation speed increases, the resistance to movement also in-creases due to friction but, above all, the inertia of the moving elements (both in the drive system of the wheelchair and the dynamometer itself). Moreover, in the experiment the controlled factor was the frequency of driving phases (by using a metronome), not the value of speed or driving torque. Therefore, the user could apply different amounts of force when generating the drive in the following propulsion phases. This is the influence of the human factor, which cannot be avoided in this type of research. The recorded data were then further processed to calculate the mechanical power in the drive system. As shown in Figure 8, an increase in the ratio results in an increase in power, leading to the conclusion that the designed drive mechanism is not a constant power system. This is mainly due to the increase in rotational speed values. The RMS value of the mechanical power was 15.8 W for Gear 1, 24.9 W for Gear 4 and 40.1 W for Gear 6--however, the peak values were much larger, as can be seen in the graph in Figure 8.