P08027: Wireless Assistive Control System
/public/References/testing/

strap

Because this project was to be used department demonstration tool, it required a certain level of aesthetics, a "coolness" factor. The customer did not want wires dangling all around the user or simply taped on to the arm. There was also a concern involving the risk of the EMG sensors coming loose during operation, so there would need to be some way of ensuring they maintained their placement while the user moved about. To satisfy these constraints a glove would need to be developed to fit as many users as possible. Since there are approximately 25 different anthropometric measurements on the hand and arm, no traditional glove can be truly universal. The customer did not want several size gloves, so a system of adjustable straps was developed. Out of several proposed solutions, three concepts were deemed viable enough to be worth evaluating

Matrix

Matrix

There were two versions of Concept C; one used Velcro while another used what are known as Ladder Locks to secure the strap to the user. Since both were so close in weighted score, supplies for both were purchased so that both could be developed simultaneously.

At this point anthropometric data was collected describing the 5th and 95th percentile male and female populations to determine the dimensions of the straps components. Data was obtained from the ANSUR database and a book titled Bodyspace by Stephen Pheasant and Chirtine M. Haslegrave. From these sources the dimensions were determined as follows:

public/References/testing/anthropometricsummary.jpg

public/References/testing/anthropometricsummary.jpg

The wrist to elbow dimension requires a maximum only since anyone with arms shorter than 41.1 centimeters will simply have a bit of slack between the wrist and elbow points.

public/References/testing/summaryofspecs.jpg

public/References/testing/summaryofspecs.jpg

The final column represents a conversion of the anthropometric data from a diameter to a circumference and converting the measure from centimeters to inches. The final lengths of each strap will need to overlap so an additional 3 inches was added for the Velcro overlaps. Next, drawing schematics of the strap system needed to be produced to provide a visual for demonstration and to simplify understanding of the concept.

public/References/testing/strap1.jpg

public/References/testing/strap1.jpg

public/References/testing/strap2.jpg

public/References/testing/strap2.jpg

A bill of materials was constructed for ordering purposes.

 BOM

BOM

Fifty foot rolls were purchased since this was a convenient shipping quantity. It was approximately the same price to purchase a fifty foot roll as to order it by the foot and only get thirty. One inch nylon webbing was chosen for its strength and durability. It is the same material that backpack straps are made of and any student can attest to the longevity of this material. A roll of two inch nylon webbing was also purchased. The thinking behind this is it might provide better resistance to slippage on the forearm if the one inch nylon proved to slide up or down too much. The one inch nylon tubing was perfect for the task of connecting the wrist and arm straps. It's essentially two piece of nylon webbing run together along the edges. It was perfect for hiding the wires from sight. The Velcro and ladder locks were purchased for the securing of the strap system. The Denier Cordura was purchased as a part of a back up design incase the strap system was a dismal failure or was not what the customer liked.

Nylon Webbing

Nylon Webbing

Nylon Web Tubing

Nylon Web Tubing

For construction, an amateur seamstress was employed. Each strap prototype was constructed using a conventional home suing machine and store bought thread. Prototyping began with the Velcro version of the strap. This prototype didn't pan out very well. The Velcro was difficult to maneuver in preliminary use and was almost impossible for the operator to put on without assistance. It was also noted that the slack between wrist and forearm strap was cumbersome and made wearing the strap awkward. It became clear that some adjustability was going to be necessary to make the system as aesthetically pleasing as possible. For the second prototype, the Ladder Locks were used for the wrist and forearm straps. This turned the wrist and forearm straps into loops which the operator would slide down their arm and then pull tight. Also, a second piece of strapping was used to sandwich the connecting tube between the forearm hoop and the extra piece. This created a hole for the forearm hoop to slide along the connecting tube. Two small holes were cut in the connecting tube and the forearm hoop to pass the wires. This prototype was far superior to the first. It was easier to put on, and was more adjustable. However, at this point, the need for the thumb strap was being called into question. The electrodes that had been selected were much stickier than originally thought and the thumb strap was somewhat cumbersome. The third and final prototype was a repeat of the second prototype, with one modification. The thumb strap was removed. This prototype became the favorite of the team. It allowed for easy use for the operator, both in hooking up the system and comfort during use of the system. It is also important to note that the original lengths of the wrist and forearm straps had to be increased in order to convert them in to hoops. Some material length was used up at both ends to secure the strap to the Ladder Locks.

Wearing strap

Wearing strap

Fully Hooked Up

Fully Hooked Up