Coffee Cup
"Espresso! My Espresso!"
An Ongoing Internet Novelette
by Randy Glass - Copyright 2003 - All rights reserved

Coffee Cup
63
Me Be PID

      PID- Possibly second only to Silvia and Rocky, few hardware items have been discussed more on alt.coffee. With over 2,560 messages going back to the year 2000, it has been a popular discussion topic. For the one or two of you who may not know, PID stands for "proportional, integral, derivative. I am not even going to try to explain what that means, but basically, a PID is a digital, computerized thermostat. It "learns" about your device and can accurately control any temperature you program it to reach. There are a variety of them available, but generally speaking, and depending on how fancy you get and how full your box of junk electrical stuff is, by the time you install one, figure on spending about $175 and a number of hours of labor to get it all working.

      Here you see how I set mine up. Although I could have used a smaller unit, this one was available to me on a one-time, not-to-be-repeated offer at a VERY low cost. The smaller ones can be installed in Silvia, but I was not at all motivated to cut her up.

      It is mounted without modifying Silvia at all. The project box is from Radio Shack, and the bracket on which it hangs is a homemade, heat-bent piece of black Plexiglas. The cable created by all the connections goes through the split cable loom and into the bottom of the machine, into the bottom cavity under the water tank. In that area there are a number of small holes. I put grommets into four of those holes and fed two or three wires through each hole. The SSR is mounted under the water-tank separator panel, to the right of the pump (facing the machine). A few tie wraps to keep everything in place and the easy part was done. The box was a little too close of a fit, and getting the mounting brackets which secure the PID in the box was a bit of a challenge. It was easier the second time when I had to correct some wiring mistakes! I found that the alarm function on this was just switched contacts of a relay and they were not powered as I had assumed.


The basics of wiring most any PID for an espresso machine go like this:

- The "Thermocouple" is attached on the boiler of the espresso machine. I used point F which was the attachment point for the hold-down bracket of the brew thermostat A. I used a little heat transfer grease as would be used on a CPU or other electronic equipment. The two wires G are polarity sensitive with White being positive and Red being negative (go figure).

-The original wires B are removed from the brew thermostat A and attached to the output side of the Solid State Relay (SSR) D using wires C. The relay is needed because the heating element draws more power than the PID itself can safely switch.

If you have no experience with relays (solid state or not) it works this way. The two wires at E are carrying low voltage. The PID turns the voltage on an off through these wires. The SSR is nothing more than a switch. When the voltage goes through E the two wires C are connected. When the voltage at E is off, then the wires C are disconnected from each other. The point to remember is that the SSR does not supply any electricity to the C wires.

-The controlled side of the SSR is wired to the PID with a + and - wire E. This is low voltage DC current, so polarity counts here.

-AC line voltage powers the PID using the wires at I. I used piggyback connectors and connected the two wires directly to the power switch on Silvia. When Silvia is turned on it powers up the PID. A common ground is a good idea so I connected the existing ground in Silvia at H to the ground terminal of the PID. Alternately, you can power the PID with a line cord of its own plugged into the wall, but then you need to also include a separate power switch.

     Some PIDs have dual set points. They can be used to adjust the device to two different temperatures. My PID has the second setpoint selected by an external toggle switch. I wired mine to the steam switch on Silvia (after disconnecting all the original wires to the steam switch) so that the PID controls steaming as well as brewing temperature. NOTE: If you are going to use the second set point for steaming you will notice that the system is wired through the steam thermostat! Even though the brew T-Stat had been removed from the system, regardless of how high you set the PIDs setpoint, the steam T-stat will open at around an indicated 288. I found this when trying to get the autotune to set a 300 temp and it couldn't do it. What you will need to do is disconnect the steam T-stat and connect the two wires you remove from the thermostat together. This is not a problem for people who only use the PID to brew, but if using dual set points on the PID, then the steam T-stat must be bypassed.

      When you first start the PID you program the digital device with a set temperature- the temperature you wish the boiler to reach. This is usually a bit higher than the 203-205f. Temp recommended for espresso brewing. You then enter the "Auto Tune” mode. This allows the device to "learn” how the boiler and heating element work. It figures out how fast (or slow) the temperature rises, the level of overshoot the boiler displays, heat soak, and a bunch of other stuff us art majors didn't study. The better PIDs also have fuzzy logic so that they 'think' and learn a bit better and are more adaptive to the entire process. In a few minutes the PID has figured out the device and has learned how to best keep the machine at your predetermined temperature. The benefits of all of this include:

      The ability to play with one degree increments in brew temperature. Just the ability to adjust brew temperature is a real boost in the ability to control another variable in the espresso-making process.

      The PID also eliminates the entire temperature or time surfing process completely. No more pumping water then waiting thirty or forty seconds before you can pull a shot. This definitely speeds up the process of making espresso, particularly if you make successive shots.

      Repeatability- It is a lot easier to get repeatable results because one of the most difficult to control factors is now under the control of a little computer.

      Anyone with a reasonable mind and as tight of a hold on their wallet as I seem to have would ask, "Is it really worth $175 just to add an electronic temperature control to my espresso machine?" You'll have to wait for the next chapter or two for the answer to that one!

Here are a few websites that may also be of interest to you if you are contemplating a PID installation

Murph's Silvia PID Page - Murph has done a great job putting together this site with lots of details on actually installing a PID including parts lists and wiring details. It is partly why I didn't go into any more detail here in installing.

PID Tutorial - This page is aimed more at the electrical or thermal dynamics engineer, so if sentences like, "A proportional controller (Kp) will have the effect of reducing the rise time and will reduce, but never eliminate, the steady-state error. An integral control (Ki) will have the effect of eliminating the steady-state error, but it may make the transient response worse." are to your liking, check out this site.

Paulo's Silvia with a tail. - Paulo came up with a novel mounting system for his Fuji PID. He used a gooseneck that allows the PID (mounted in an aluminum enclosure) to 'float' next to the machine. If you are thinking about adding a PID to your Silvia (or any machine) you NEED to see this!


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