Sunday, January 29, 2012

Peristaltic Pumps

Peristaltic Pumps are not new, they are pumps that mimic peristalsis, a biological mechanism most commonly depicted as the muscle contractions of a swallowing esophagus. The vast majority of these pumps serve the medical and pharmaceutical industries. This does not bode well for hobbyists because the pumps are priced at an average of $300 each. When attempting to build a cocktail machine that dispenses over a dozen liquids, the cost becomes prohibitive.

"Why use a peristaltic pump in the first place?", you might be thinking. Well, for many reasons. The first and most obvious reason is maintenance. Since liquid does not pass through any moving parts, it becomes easy to swap out or clean hoses when necessary. In the same vein, reliability. Syrupy liquids like grenadine and Bailey's tend to gum up mechanical pumps and valves pretty quickly. Sanitation then becomes another issue when liquids become trapped in the crevices of a mechanical assembly and become impossible to clean. Finally, accuracy and simplicity are the final components that put the nail in the coffin on other approaches. While gravity valves and simple diaphragm pumps are a possibility, there is still a big issue with dealing with different densities and viscosity of liquids. Since peristaltic pumps are positive displacement, meaning they dispense the exact same amount of volume with every revolution of the motor and gearhead, it simply becomes a software problem to account for the number of motor rotations for accuracy. This is much easier and cheaper than fashioning some sort of flow rate meter.

When designing the pump, several factors need to be taken into account such as the occlusion of the tubing, the ease of assembly/dis-assembly of the tubing, maintainability and modularity. The picture shows the parts for the first batch of peristaltic pumps I made. There is a plate that the motor mounts to, a hub with sleeve bearing rollers, a shoe which is replaceable and provides the right amount of occlusion, and a support for the hub that prevents lateral motions from causing it to run to eccentrically. All in all they can be built for under $100 given the right amount of tools and resources.

Sunday, January 22, 2012

Bartendro 2 Frame and Enclosure Build

We built the frame using 1" aluminum square tubing, which seemed to offer the best balance between weight and strength. The individual pieces were cut to length on a milling machine and the edges that were going to be butt or T-joint welded were chamfered to reduce the amount of weld bead that would need to be ground down afterwards. Jerad TIG welded the frame together using his new beefy Bessey corner clamps, it only took him a couple of hours. 

The enclosure itself was made of 16 gauge 304 stainless steel. 12 and 14 gauge were considered at first because less structural members would be needed, but the cost of thick stainless was prohibitive. The 32" overall length emerged from the hassles of transporting the original Bartendro. Two people were required to move the machine, and it could only fit into a sedan that had its passenger seat removed, and still it would gouge chunks out of the interior in the process. The design criteria were simple: fit through a door frame, fit in the trunk of my Honda Accord, and be able to be carried by one person. Getting the sheet metal sheared and bent took about a week to get from the metal workers. Then the stainless steel tee fittings were post-machined and welded to the enclosure. Their precise outer diameters helped maintain a consistent and repeatable weld joint. 

The PVC coating on the sheet metal was really important in ensuring the finished product looked good and wasn't scratched up from the cutting and bending processes. I learned that it is really important to make holes in the stainless steel sheet first before making holes in the aluminum. Aluminum is really soft relatively, and is very easy and forgiving to make holes in later compared to stainless steel. Stainless work hardens quickly, meaning one cannot be timid when making a hole or cutting because it only makes matters worse.

Unfortunately, when the tee fittings were welded in place, the sheet metal warped a little on the narrow stretches, alternating between concave and convex. The additional members that were welded across helped to keep things uniform, but it was the use of panel clamps that made keeping things straight possible. They are quite useful tools, used frequently on auto body work, they allow sheet metal to be held in place yet still allow for minor shifts to help in the alignment process. Once the main panel was lined up, it was secured with  an abundance of clamps. Holes were drilled and tapped into the frame and the panel fastened with shiny button head screws. 

Sunday, January 15, 2012

Water to Wine. Sacrilegious fun for all.

Yesterday, Lauren Blomberg and Brian Matis came over to drink wine and help me with a photo shoot for Make magazine. Brian had taken dozens of shots in August '11 of the broken down components and assembly procedure from start to finish. The shots were well lit and well composed. Fancy white photo umbrellas go a long way apparently, when it comes to taking good shots.

The editor at Make recently started working on the article so it can go into their next issue, I believe volume 30. He found that he was missing some action shots though, with wine actually coming out of the machine. When we disassembled the machine for the August shoot, connectors, tubing and wires were cut and it sat that way for a while. Needing to take the action shots was good motivation to put it all back together. I used some new food grade tubing from the local beer brewing supply and put it all back together.

We took dozens of shots in the kitchen while spilling water and wine now and again as we tried to work air out of the lines. After about half an hour of fussing, everything was primed and ready. That's when the silliness began. As we drank more and more wine, we began to become more intimate with the machine. Finally, Lauren thought it would be a good idea to put her mouth up directly to the outlet and consume that miraculous goodness. It was priceless.