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Latest revision as of 12:09, 19 October 2016
Hardware
ACME Stomach Masher
Introduction
According to Kozu, et al. a simple compressing, rolling motion adequately simulates the stomach’s mechanical stress. With that in mind reusing the internals of a printer came to mind as rollers and motors would be among the recovered parts. Not only was this an economical option but also a environmentally friendly one. The contents of the stomach would be put in a plastic bag that would be passed back and forth through the recovered rollers. To put it all together the Dundee Maker Space laser cutter would be put to use extensively.
Planning
The stomach was modeled in OnShape, a free cloud based CAD suite. Initially the design took advantage of the printer’s linear scanner drive since linear motion was required for the stomach. This choice is reflected in the image above. Unfortunately the motor for the scanner motion was not powerful enough to properly move the contents of the stomach.
Building
As mentioned, initially the motion was to be provided by the scanner motor. Using this would have simplified the design since it already created a linear motion. Also, controlling it would have been very simple. A double pole double throw (DPDT) switch can easily be used as a mechanical H-Bridge (bi-directional motor controller). When in one position the motor will rotate clockwise, and when flipped to the other position it rotates counter-clockwise. The red end-stops in the image above were positioned to actuate the switch when it reached each end. As soon as it hit the end of travel it would immediately reverse direction. The dead-spot in the switch where it is in neither positions and therefore not conducting electricity was too large and resulted in no motion.
With the scanner not working, a spool and thread was turned to. Printers also contain several gear trains, which reduce speed and increase torque, something the scanner motor could have used more of. The string had trouble staying positively engaged with the spool unless under extreme tension. At that point though the motor needed to do extra work to overcome the added friction. Another issue was that the string would always jam up on one side or the other, depending on the direction of rotation. Looking back, both problems could have been solved by a much wider spool with the string fastened in the middle. As the string unspooled from one side, it would be taken in on the other. So long as the width of the spool is sufficient, the string will never encounter the edge. Without the benefit of hindsight in the moment, a new method of motion was devised.
References
Hiroyuki Kozu, Yuki Nakata, Mitsutoshi Nakajima, Marcos A. Neves, Kunihiko Uemura, Seigo Sato, Isao Kobayashi, Sosaku Ichikawa “Development of a Human Gastric Digestion Simulator Equipped with Peristalsis Function for the Direct Observation and Analysis of the Food Digestion Process”, Food Science and Technology Research, 20 (2), p.225-233