Wednesday, April 14, 2010

Quest for the best espresso Pt. 4

With this part of the espresso machine build, I will bring you up to the current state of the project. From here on out, my posts will be on Weds and will follow where I go from now in real time. The wiring isn't much fun to look at, so consider this a light post. Finally, I might be revisiting the wiring once I finish all three phases of the project and replace all the lower current lines with a simple circuit board.

With the mechanical parts assembled I set out to complete the wiring needed to supply the control and life giving electricity to all the components of the espresso machine. I used a wiring scheme that allows the user to determine when they want the pump and the solenoid to be activated which allows pre-infusions as well as separate pressurization of the the main boiler. This wiring will be important in the in the final stages of Phase three when I start programing the steps so the machine can be run either full manual, or semi automatic.
This took me longer then it should have, not because the wiring was difficult, but because understanding how you want to electricity to be supplied and how the wiring actually needs to be done are two different things. This resulted in many tripped GFI outlets or breakers (editors note: not happy about that one!), several shocks from ungrounded wires and bad connections, as well as a melted Solid State Relay in the main PID due to crossed wires.

Looks bad already an this is only partially complete

Once I got the PID repaired and all of the wire end properly terminated, I was presented with an espresso machine that has good visibility of the internal temperature of water in the boiler as well as good visibility / control over the pressure. To obtain the control I wanted over the temperature in the boiler, I needed to program the PID. You can go about doing this using to methods; you can use the system of equations and solve for the values for Proportional , Integral, and Derivative or you can "Tune" the loop by using a general values biased on the type of system you are using and altering the values biased on the way you want to perform. I chose the later of the two options because I had a good idea of how this system was set up. For this type of system high density resistance 800 watt heater is designed to heat up and quickly reach the set point. Unfortunately there are a few draw backs to this system; mainly the heating coil has a fairly long warm up time, the coil puts out way too much heat for the boiler size when trying to maintain a single temperature, and there is a high barrier for the electricity that is running through the coil so that only pulse type PID control can be used as a flow of 20% or even 50% of the electricity flow would not pass through the heating coil. With that in mind we have reached the current state of the espresso machine.

The espresso machine is able to pull a shot a the correct temperature and pressure with a little coaxing because the PID controls that I am using are still not stable. The current espresso machine does not have steaming capabilities and will not till the completion of phase 3. Next week will be the start of phase 2, the heated reservoir. This will allow the PID to be more stable in the long run. So see you next week on Wednesday!


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