P15041: Smart Walker III
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Systems Design

Table of Contents

Load Support Structure

There are several problems with the way the current load support structure was designed.

The first is the fact that the bearing plates for the wheel axle do not support themselves, let alone the weight of the walker or user. When the weight is applied, a moment is created which due to the lack of support, causes the motor axle to turn. We added bolt holes to the design to make the braces much more sturdy, so when the weight is applied, they are strong enough to resist the added moment. Due to this lack of support, the current gears have been supporting the weight. They are damaged and will need to be replaced. The bearing plates are ready to be remanufactured.
public/Photo Gallery/currentplates.JPG public/Photo Gallery/topplate.JPG public/Photo Gallery/bearingplates.JPG

The second is that the wheel was modified in such a way that it is too far from the center of gravity. This also creates a moment that provides problems for our walker. We will cut the axle down to bring it 0.2'' closer (all we can afford).

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A third problem is the wheel housings. In the current design they provide support for some components. However the walker performs much better when the housing is removed indicating that these support pieces cause more problems than they solve. They were removed from the design and the housings will be re-printed as shown below.

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A fourth problem has to do with a twisting motion in the wheel. This was caused by the way the walker legs are attached. They are able to rotate slightly which causes the wheel to turn when weight is applied. Rivets were placed in the open holes which provided a sufficient block to this twisting motion.

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Clutch System

The current clutch system was designed in such a way that when the solenoid is activated, it creates a moment on the axle. This causes the axle to rub against the mechanism designed to transfer the motor motion to the wheels, which causes the system to not operate as it should due to friction. To reduce this friction, bushings were added. The system was also redesigned.

Mechanism Morphological Table

In efforts to minimize the moment caused by the current clutch system

Evaluation Criteria

Feasibility

public/Photo Gallery/clutchtape.jpg

The above picture shows a proof of concept we did for a possible new design that would significantly reduce the moment that is created. The test was successful, and we learned that the system was feasible.

System Test

Once we knew the concept was feasible, we did a full test with the walker included. The piece we used was machined from a piece of stock in the machine shop. The test was successful. The redesign and the second iteration of the redesign were 3-D printed (the models are shown below).

CAD Models of Redesigns

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BMI

One of our requirements is to incorporate a BMI analyzer into the handles of the walker. After researching potential BMI's, we found the best one to use was the Omron HBF-306.

We purchased the item and dissected it to see how the system worked. After scrutinizing the design, it is unclear whether it would be better to incorporate the purchased BMI into the systems, or to design a new one.

Regardless of what decision is made, the metal electrodes will be incorporated into the handles and will be supported by a 3D printed slot. Work for this is currently on hold until a final BMI design is completed so that no unnecessary alterations are performed to the walker.

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Load Cells

The load cells are connected to a plate under the seat. They are a set of four individual load cells, one in each corner of the seat. The goal with these load cells is to collect four individual readings from the cells and use these to determine the center of mass. The method of analysis of the points had already been determined in Smart Walker II.

Currently the load cells have been tested but not calibrated. They read voltages indicating that they work, but need to be calibrated so that the voltage corresponds to a correct load.

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The data sheet for the load cells can be found here.

XTION Housing

The case was designed to enclose the camera and set the angle required for the SLAM software. The walls are 0.25” thick. The connecting piece was taken from the previous design, as this was modeled correctly. It is incorporated directly into the case at a fixed angle preventing any rotational motion accidentally caused by the user. The angle is yet to be determined, but will be chosen such that the wheels are not in the cameras view. Once the angle is determined, one dimension will be fixed and the case will be ready to be printed.

public/Photo Gallery/xtionfront.JPG public/Photo Gallery/xtionback.JPG

PCB Design, Rev. 1

We have created a PCB design and ordered it. The image shown below is the PCB layout using PCB Artist. The left hand side of the PCB is the schematic for the strain gauges. The middle is our design for the ADC, but we'll be using another groups ADC design. The upper right hand side is our power circuit and the lower right hand side is the circuit to drive the solenoids.

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The link to the files are here.

When the PCB arrived, we populated the strain gauge circuit, power circuit and solenoid circuit. **The image is shown below**

Strain Gauges

The strain gauge PCB is shown in the picture above. The schematic of the strain gauge is shown in the PDF link here. The strain gauge circuit is shown on page 2 and 3. Page two is the handle strain gauge circuit and page 3 shows the load cells circuit.

ADC and Data Conversion

The ADC was originally going to be from the Tiger Bot V team. In Rev 2 of the PCB's, the ADC chip was integrated into the strain gauge PCB (SW3-03). This eliminated the need for the Tiger Bot ADC board.

The documentation for the design can be found here. The component datasheets and user guides can be found here.

Arduino and Motor Control

The motor encoder flow chart is shown in the visio documentation, here.

The software reference for communication between the Arduino and the Pandaboad is shown in the reference documentation here.

Webcam Heart Rate Sensor

The webcam will be used to extract the heart rate of the user. There is existing code from Smart Walker II found here. The code was tested for bugs and errors, and it was deemed that creating our own code was more worthwhile.

The webcam is currently cased into the back rest of the smart walker shown in the picture below.

here

Xtion and SLAM

There is code to extract data for the Xtion from Smart Walker II shown here. The current status of this code is debugging Smart Walker II's Navigation code shown in the link above.


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