Fig. 4 Distribution of the actuator system. High temperature from each component can be cooled by two cooling fans.
The specifications of the motor with a gearbox were selected by analyzing the required piston head force with the simulated forces from the output torque. The gear ratio was changed from 66:1 in a previous study by 26:1 (planetary gearhead GP 42 C, Maxon Motor, Switzerland) to reduce the noise level, which is dominant in the noise level of the compressor operation. Following the gear change, the specification of the DC motor was also changed to a 60% higher level of nominal torque (0.177 Nm) and the comparable nominal speed (6940 rpm). The revised combination of the DC motor with the gearbox shows an output torque of 4.6 Nm and a velocity of 260 rpm, which satisfies the required performance of the compressor as the energy source of the pneumatic AFO. In addition, the energy efficiency of the new motor with gearbox increased from 60.9% to 73.3%. However, the weight of the motor with the gear increased from 534 to 840 g, which was compensated for by reducing the thickness of the compressor case to sustain the total weight of the system. The volume of the refined compressor was decreased by 11% compared with the previous one [13].
For the cooling of the portable compressor with pneumatic components, fans (EF80251S1-1000U-G99 SUNON, China) were installed in the package comprising these components for heat emission. The temperature change was monitored in six parts: the pump cylinder, DC motor, planetary gearbox, battery, master PCB, and pneumatic fitting. Before the installation of the fan, the temperature exceeded 100°C, which could damage the user and limit the long-term operation of the compressor. Operating both fans and the compressor simultaneously enabled the temperature of each component in the compressor to be maintained at 40°C for 20 min of continuous operation. Distribution of the actuator system is shown in Fig. 4.
Wearability improvement: By optimizing the AFO system, the volume of the system can be significantly reduced. In particular, the volume of the AFO was reduced enough to be hidden in the clothes, whereas the previous model was too bulky to hide. The use of the Bluetooth module instead of the tethering wire was also helpful for concealing the AFO.