I’ve been strongly considering making a barbot (a.k.a. a drinkbot), even thought don’t usually drink at home. I haven’t given much thought to its cosmetics, instead I’ve been focusing on mechanics of the bot. I figure, the mechanics will dictate the form, and if one sprinkles enough LEDs on it, can look look fine.

Delivering and Dosing

Delivery

Broadly speaking, there are three main ways this is accomplished by the community of barbot makers: gravity, direct pumping, and air pressure.

Gravity

This is the simplest method. Typically, bottles are stored inverted and the liquid pours through the mouth of the bottle, with the flow regulated by a valve. (Although, one exceptional example is TipsyBot, which works by rotating the bottles.). Supposedly the main problem with this method is that the flow rate is dependent in the mass of liquid remaining in the bottle, and thus changes over time.

Direct Pumping

Pumping has an advantage over other methods in that the flow can be controlled without additional components. The main concern with direct pumping is that the pumps need to be food-safe. Mostly this means that lubricants can’t come in contact with the pumped fluid and that the pump must be easy to clean. Food safe pumps tend to be a more expensive than non-food-safe, but they’re really important.

The most commonly used pumps are peristaltic pumps. These work by having a set of rollers move across a hose to move the fluid. They’re self-priming, which is good, but they have drawback. Essentially there are two types of peristaltic pumps: fast and expensive, and excruciatingly slow and cheap. In the slow and cheap group we’ve got aquarium dosing pumps. These cost only a few dollars and have flow rate of about 1 ml / sec. If you want something faster, say 5 ml / sec, you’re probably looking at a few hundred dollars.

I did find one reasonably priced pump (or more specifically, a reasonably priced pump head), the 200 series from Williamson Manufacturing, but alas it cost $72 to ship a single pump from the UK, so that’s out. Nice looking pump though.

Some people have tried making tiger own pump heads out of laser cut acrylic or machined metal, but they’re still not cheap. Still, they combine delivery and accurate dosing in one simple package. If money or time isn’t an issue, peristaltic pumps are defiantly the way to go.

Air Pressure

instead if moving the fluid directly, a pump is used to pressurize the container, which in turn causes the fluid to flow through a relief port. Any sort of pump can be used with this method since nothing except air is in contact with the liquid, but air tight seals are required. Maintaining a constant pressure inside the vessels (and thus a predictable flow rate) might be problematic without a pressure regulator. Since not a lot of pressure is required to get typical liquids a barbot deals with flowing (5 psi seems to be a common operating pressure), a simple aquarium pump can be used to pressurize the containers.

Dosing

Measuring Cups

By “measuring cups”, I actually “mechanically determine a fixed volume”. Peristaltic pumps are great for this. The space between the rollers is fixed, and fluid fills it. Each revolution of the pump head delivers a known quantity. That’s why they’re also called “dosing pumps”.

The Inebriator uses bar shot dispensers (inverted bottle holders with a measured cavity and a spring loaded valve at the bottom) to measure the alcohol volumes. A servo motor releases a valve and one measured shot flows. This approach appears to work well.

Drink Making Unit v2.0 rather ingeniously uses Japanese deer scares to as a way to dispense known quantities in a visually unique way.

One approach that I haven’t seen that would probably work, is to have a cup on a rotating chain, so that on each cycle the cup dips into the liquid and the carries it up to a level where it is poured into the cup. The main drawbacks with this method is that it requires the liquid to be in a container with a relatively large mouth, and it couldn’t dispense liquids once the level dropped below the height of the moving cup. While this certainly isn’t an efficient method, it certainly would be visually appealing.

Flow Velocity

I have not seen anyone use this method, but it is possible to directly measure the flow rate of the liquid being delivered either by counting the revolutions of a water wheel or by measuring the Doppler shift of a a light or sound beam reflected off the surface of the liquid. If you know you’re velocity, and the size of the cross section of liquid, integrating over time gives you your volume.

Dead Reckoning

By far the most common method when measuring cups aren’t used. Here, we open a valve for a set time and if the flow rate stays predictable, we can estimate how much liquid was dispensed. This is a very simple method, and from what I read, works surprisingly well.

Mass

The old “Product sold by weight, not volume,” method. A digital scale provides feedback about how much liquid has been dispensed. Mass divided by density gives you volume.

A Word About Valves

If you’re not going with a measuring cup method for dosing, you’ll need valves to control the flow. Again food safety is paramount here. I’ve seen some bots try to use non food safe valves, but that just strikes me as a dumb decision, especially if you want to use you bot (or more specifically the valve) on more than one occasion. Solenoid pinch valves are the way to go. They’re electrically controlled, and the valve itself stays out of contact with the fluid. I’ve seen one bot that used a servo to pinch a hose, but that strikes me as a waste of a perfectly good servo.

My Own Bot

For my bot, I am considering going with an air pressure and solenoid valves with a digital scale for feedback as the primary dispensing system. I may want to use some aquarium dosing pumps (peristaltic pumps) to deliver ingredients measured in “dashes”, but maybe not. I’d love to peristaltic pumps for everything, but the pumps with a 5 ml/sec flow rate are simply way too expensive.