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Render of Full Design
The final design for TrueStep consists of a custom electronics pack connected to a prefabricated waistband via industrial-grade Velcro™ fasteners. The bowden cable is attached to a bar connected to the gear shaft, guided through a 3-D printed leg brace, and attached to an structure that goes through the eyelets of the shoe. IMU sensors clipped onto the laces will allow us to track the stage of the gait as the user walks. We plan to develop algorithms to detect when the user lifts the foot and send instructions to the motor to move the gear shaft, rotating the bar and pulling the cable, thus lifting the foot.
This blog will detail the analysis of the electronics pack, leg strap, and the motor and gearbox system.
Thermal Cross-section of Electronics Pack (Air Invisible)
Thermal Cross-section of Electronics Pack (Air Visible)
The electronics pack, was designed according to the size of the electrical components such as the battery and motor. We had to ensure that the heat generated by the motor and battery did not affect the functionality of the other parts or transfer over to the patient. In the heat transfer analysis of the electronics pack, we simulated heat generation from the motor and battery conducting to stationary internal air and the rest of the internal structure. The outer walls of the enclosure were modelled with natural convection occurring on all exposed faces to show the heat dissipated by the ambient air set at a temperature of 20°C.
As for the internals, we found that the maximum temperature that occurs is in the motor at 222.8°C, which is within its working temperature range. The maximum temperature of inside surface of the case reaches about 60°C. We plan to use polycarbonate as the material for case, which has a working temperature range of -40 to 140°C. This indicates that the heat output by the electrical devices will not affect the integrity of the case. Components, such as the Arduino Uno microcontroller, Cytron MD10C motor driver, and breadboard, reach a maximum of 40°C, which falls in their working range.
The results of the analysis shown in the figures indicate that the external temperature of the case under standard operating conditions reaches 23.8°C, which is close to the temperature of the air. This satisfies our requirements for the outer casing to be around the room temperature of 25°C.
Furthermore, in practice, we can expect the actual temperatures to be lower than our results indicate because there will be some degree of natural convection within the case. Based off our results, we do not expect the heat generated by the device to be problematic to the functionality of the device or to the comfort of the user.
The leg strap is comprised of a 3-D printed ABS brace attached to an adjustable nylon band. The bowden cable passes through the hole in the center of the brace as shown in the figure. Because the outer shell of the cable is stationary relative to the strap, there is relatively little force acting on it. However, for analysis, we simulated the full cable force pulling up on the brace. Our results indicate that the brace deforms very little under the load and remains virtually stationary. This indicates that the brace can anchor the cable to the shin, allowing us to control the slack of the cable.
Table 1: Gear Ratio Determination
The analysis and calculations shown in Table 1 were used to determine the required gear ratio and torque in the system to lift the foot of our user. The upper limit measurement torque and RPM were calculated based on the upper limit of our user weight demographics which is 95 [kg]. The last row shows the torque output of the Maxon RE 30 DC motor and GP 32 gearbox system which is 3.72 [Nm]. This torque outputs meets the upper limit criteria of our system as calculated in the first row. Hence the team has decided to go with the above mentioned motor + gearbox system from Maxon group.
Table 2: Power Supply Determination
Table 2 shows the analysis and calculations performed to finalize our power supplies for the system. The maximum power would be required by the motor + gearbox system so we have taken that into account in our calculations. The first row in the table lists the nominal voltage and current requirements for the motor + gearbox to operate. Using the torque and RPM constants, we calculated the torque and RPM produced at nominal power. Comparing these values with our required values from table 1, we concluded that the torque needs would be met. Hence, we decided to finalize the TalentCell PB120B1 battery supply which has a 12V/6A output, and hence meets our power requirements for the system.
Regarding future steps, we plan to acquire all the components of the device before Spring 2021. During spring 2021, we will assemble all the mechanical and electrical components in order to create a prototype of the device. The team will also be working on developing motion control and gait detection algorithm for Spring 2021. We expect the algorithm development and refinement to take a significant time due to the precision needed for the motor control.
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