Blog #3

During the work period of October 30 - Nov 7, the team finalized the electronics components and designed the electronics waist pack based on those components’ dimensions. We were able to model the components using Fusion360.

The electronics pack was designed to fit the electronic components needed and to minimize heat transfer and volume to eliminate discomfort to the user. Figure 1-3 show the 3D model of the electronics waist pack with the components such as Battery, motor, microcontroller, and motor driver in it. We decided to make the electronics case out of polycarbonate with an outer cover of soft silicone to provide comfort and reduce heat transfer. The final dimensions of the electronics pack are LxWxT : 172.64 x 129.46 x 80 [mm]

The electronics case will be attached to a nylon and neoprene band via industrial grade Velcro, which will hold the electronics pack on the user’s back at waist-level to minimize changes in the body’s center of gravity while walking.

We decided to scrap the full leg sleeve in favor of a smaller leg strap located in the upper calf. This consists of a rigid polycarbonate + ABS brace located in the front along the shin, with a nylon band looped through the brace to attach it to the calf. The purpose of the brace is to anchor the cable on the upper calf in order to avoid a direct pull on the waist pack by the Bowden cable. Figure 4 is an initial design of the leg strap. In the final version, the nylon band will loop through the entire brace to eliminate discomfort to the leg due to friction from the rigid brace.

For the foot strap that would hold the IMU sensor, we decided to design it such that it can be placed on the shoelaces of the shoe and collect gait data from the instep of the foot. A ring, as shown in the figure, would pass through the eyelets of the shoe. This ring would hook the Bowden cable and assist in lifting the foot when tension is applied through the cable. Materials we are considering for the foot strap are silicone rubber or elastic, and ABS+PC for the ring that the cables will hook through.

For the mechanical components, the main challenge was to design a layout for the electronics case. There were restrictions in arranging the electronics due to their varying dimensions and the placement of the motor shaft and battery was challenging since they would have to be accessible outside the case. However, the team discussed various ideas and came up with a final design that would meet all our size and user comfort requirements.

Currently Team 8 is in the process of conducting stress-strain and heat transfer analysis on the electronics casing, foot strap and the leg strap. A challenge that we have found is that we cannot analyze the behavior of our fabric materials using common stress analysis software such as Inventor or Fusion360 because fabrics have a nonlinear stress curve and behave like a membrane rather than a solid body. We are looking to use COMSOL Multiphysics software which has more advanced features in order to analyze the behavior of the system on the foot strap, waistband, and leg strap.

As for the electrical components, we have finalized to use the Maxon RE 30 motor along with the combination of GP 32A 166173 Planetary gearbox. This combination system provides enough torque (up to 4.5 Nm) required for our system, and its 190:1 gear ratio reduction stays allows us to stay within limits for our calculated RPM from our measurement stage. The gear ratio has comparatively smaller size and lower weight than the other bulky options available in the market. The Maxon group is a trusted name in the medical device industry and they have offered us a student discount and a possible sponsorship for our project. We have also selected the Talent cell PB120B1 battery which provides an output of 12V/6A and since our motor requires 12V/4A, for maximum constraints without the gear ratio reduction, it’s perfect for our application.

The Model DC Open-loop Equation for our Maxon RE 30 motor without the the GP 32 gearbox has the model equation (Eq.1.1). The variables with reference from the specification sheet [1] (column 3) are shown in the Table 1 below. They have been reduced to fundamental units. Moving forward, we plan to collect gait data from our finalized sensors and use those along with our model equation to develop the gait and motion control algorithm for our device.

References

1. Maxon Group [Brochure]. (2020). Retrieved November 13, 2020, from https://online.flippingbook.com/view/1042987/138/