Exoskeleton V.1 design (SMA Jacket) & review (project origins)

Primary engineer is a program which inspires children to think about if they were an engineer what would they do to solve a problem. Professional engineers enter the class room and speak to children and what they do and how they come to be in this job position, this is then followed with the children's idea. The program tends to work so well even the engineers often will walk away inspired.

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One of the 2018 uk north west entry winners ask a question "Why is there not something to help children move who are ill?" originating from personal experiences, the competition entry did pose an extremely interesting question and further interesting results.

Figure 1. Original Primary Entry

Figure 1. shows the original entry submitted to Primary Engineer 2018. The aim of this was to simply take the submission and produce a prototype that was to be unveiled at the winners event. Reviewed by the University of Central Lancashire the build team consisted of Dr Matthew Dickinson, a senior lecturer is mechanical engineering who's specialisms lie in design and simulation, Edward Sanderson, a PhD student whos work focuses of the use of smart energy systems and Morgan Jenkinson a MSc student (since then is now PhD candidate), her work focused on the control systems for this first proof of concept.

In an attempt to capture the essence of the entry, the first point of review was to examine the snake type structure within the drawing. As we had already concluded the entry was describing what we know to be a Exoskeleton. From reaching out and speaking with company's, it was learned that the reason these systems tend not have been developed for children, is the speed of their growth, by the time a custom system has been designed and built the children can easily of outgrown it. It was also noted that a exoskeleton can often come with quite a large price tag.

In review of the information and also relating to the entry it was decided that the use of a so called "Continuum" method could be used, almost segments that would act like spinal vertebrae, all guided by a Nitrile cord to allow for flexibility. By employing this method as a child grows further segments could be added. As this project was aimed at the assistance of a child, three pairs of 180 N linear actuators where sourced.

The design

Approaching this problem from a mechanical engineering perspective the decision was taken to identify key points on the body where the skeleton would assist the body in motion.

Figure 2. Key point locations

The aim of this exoskeleton system was to aid a person in the lying to sitting potion. Figure 2 denotes the areas which where deemed the ideal points of assistance as from further reading, we decided that the highest groups of muscles used during the sitting motion would be targeted.

Figure 3. First design iteration (dummy based on ratios of average 9 yr old)

Figure 3 shows the first iteration of the exoskeleton, this was designed under the idea that the skeletal structures would interact and allow flex. This would later prove to be not the best course to follow as each segment was squared and the plate iteration would be more motion restricting that what we were aiming to do.

Figure 4. Final design iteration

Figure 4 shows the design which met the requirements of our product design specification (PDS) consisting of segments that meet the requirement of continuum behavior as each part is geometrically the same, the hard sharp edges replaced with smooth rounded edges. Each active segment is scaled to ensure the most comfortable assistance around the muscle group. All active segments are attached by straps which attache around the body as like can be seen in a rucksack. So that the muscle did not receive a direct force from the linear actuators a small connector was developed that convert direct force into a moment, thus ensuring the muscle was given a aid to move rather than direct force which could easily damage the muscle group.

Figure 5. Connector design

The Manufacturing

Originally it was presumed that each part could have been produced through the use of billet option, hence pieces of aluminum material, under review it was found that each part would be on average £250 giving a weight of 30 kg, this left the team to reconsider options about how this could be produced, through some provisional testing we were able to confirm that the use of poly-lactic acid (PLA) for the parts had the possibility of supporting a low mass body, such as a children body. Averaging at around 50 p per component the skeleton was produced using an Ultimaker 2+.

Figure 6. Base skeleton design

Figure 6 shows the prototype base structure when first built. All connectors where produce using PET-C on the same Ultimaker 2+. Costing £10 for the material the skeleton proved to be extremely low in production costs.

The Team