Before I go into details regarding the automation, let’s first take a peek at a cross section of a la Pavoni below. This diagram by Lindsay Wright (http://www.linz.wright.name/PavoniExpress/index.html) is fantastic! The La Pavoni Boiler brings the water to around 110 Celsius and ~0.9 Bar. The pressure stat, discussed in my prior article, maintains this setting. After heating to temp, coffee beans are ground and placed in a Basket inside of a Portafilter. At this point, heat from the boiler has heated the Group head to around 90 C. The Portafilter is then locked into place at the bottom of the Grouphead. Once that occurs, the user lifts the Lever, and water from the boiler fills the Sleeve. The thought is that the Grouphead cools the water from 110C down to about 98C. The ground coffee acts a clog, building pressure in the Sleeve. Once the Sleeve is full, the user pulls down on the lever, and pressure builds. The optimal pressure is around 9 bar, or around 4-5 kg of force on the Lever. This simple process of controlling the movement of the Lever drastically changes the taste, texture, and temperature of the espresso. The unique thing about this system, is that the user controls the volume of water by lifting the Lever, and the pressure by pulling down on the Lever.
What I described above is that the Lever controls both the pressure and the volume of the brew, which means that the entire brewing process can be controlled by one variable: lever position. So let me first start out with a few base assumptions that I made when I began designing this system.
My main thought process is that I can tune values for assumptions 2 and 3 after creating something that I can reliably use to control the lever or the piston rod’s position. So without further procrastinating, here is a simple set up:
Look at all these electronics!
So this is the basics, it’s made from 2” square by 1/4” aluminum. The main portion of the frame holds a Thompson Linear Actuator. There are 4 plates ½” thick to act as spacers and a standoff. Since this is a personal project, I found a refurbished actuator, powered it and provided framing. This actuator moves 500 lbs at ~2 inches/second, so it is complete overkill for the La Pavoni, but it was easy to find, and its always easier to work with an over speced item, and then simplify for production. There is a linear potentiometer that moves in sync with the linear actuator to actively track position. Most of the metal is in place so that the linear actuator doesn’t crush the espresso machine, and a commercial version could be produced with 1/2 to 1/3 of the metal used as it would have a properly sized and specced linear actuator. All in all, as a first prototype, this servers its job. I will be including more information regarding builds