# Development of a Motorized Hospital Bed with Swerve Drive Modules for Holonomic Mobility
## 3 Results and Discussion
### Velocity Response, Error Characterization, and Demonstration  
To assess the performance of the system, the velocity responses of the propulsion motor were measured and then compared with the values generated by the quintic trajectory generator. Two different conditions were considered: no load, as shown in Figure 5a, and with load, as shown in Figure 5b. The former was achieved by turning the prototype upside down such that the wheels rotated without friction to the surfaces. However, the latter refers to the condition of the prototype receiving load from its own weight while making a forward motion at a constant velocity. For this particular performance evaluation, only data from motor no. 1 (refer to Figure 3b) were shown. During the data collection for both conditions, the swerve drive module was subjected to three different motion phases, as depicted by the shapes of the graphs (also indicated in Figure 5b), i.e., accelerating, constant velocity, and decelerating. From Figure 5, it can be inferred that the developed control system performed well. However, further assessment is necessary, especially to quantify the errors and evaluate them for each motor under different motion types. Therefore, the same experiments were conducted again, and data were rearranged to show the error values, defined as the summation of the differences between the angular velocities obtained from the quintic trajectory generator and the actual values for each sampling time during a single experimental run. Figure 6a depicts a comparison of the average errors for the conditions of no load and with load for motors 1-4. In this graph, the values include all the motion phases of accelerating, decelerating, and constant velocity. Therefore, in Figure 6b, a breakdown of the errors based on the motion type is shown. Figure 4: Response of the propulsion motor with PID control. From Figure 6a,b, several observations that provide further insight into the performance of the developed prototype can be made. First, from Figure 6b, it can be inferred that the error can be considered small for the condition of constant velocity; the values for all motors are below 3%. However, the error for the condition with load for each motor shows a huge difference, as depicted in both Figure 6a,b. The error values for some motors can reach double the values of others, e.g., motor 4 in comparison with motor 1 in Figure 6a. Furthermore, when the error is broken down into the motion types, the difference between each motor is still evident with a trend that seemingly shows that motors 3 and 4 (rear side) generally have larger error values than motors 1 and 2 (front side). As an initial hypothesis, several factors can pose as  
Figure 5: Comparison of the angular velocities for the conditions of (**a**) no load and (**b**) with load. Figure 6: Average error (**a**) as a whole and (**b**) broken down based on the motion phases, i.e., accelerating, decelerating, and constant velocity. the possible root causes, such as unequal load distribution; different torque requirements between the front and rear sides and between the right and left sides; and the steering angle effect, which is not completely straight. Especially on the torque requirements, this factor is significantly affected by the choice of material for the traction surface area. Its contact with the floor surfaces would affect the traction characteristics which eventually influence the errors. In the future works, this aspect shall be studied thoroughly to offer more insight to the evaluation of performance. Additionally, another finding from Figure 6b is worth mentioning: the error tends to be larger for the condition of accelerating than for the condition of decelerating, which is true for all motors. This can be caused by the gap settling time, which is present in any system with PID control. In addition, for all the motor cases, the error for accelerating and decelerating was generally larger than that of constant velocity.