It's really cool to see PID becoming more common in controlling espresso brew boiler temperature. PID brings substantially smaller variation in boiler temperature than simpler thermostat devices. In engineering terms, the "deadband" is smaller meaning the designed, allowed variation is reduced. So, you invest more money in parts to create more accurate temperature control.
PID is great. However, PID is really not suited to the job of controlling temperature during an espresso shot. Why is that? Because PID is designed to provide steady-state control (of the temperature in this case). Is the brew boiler in an espresso machine in a steady-state condition during an espresso shot? NO! During the shot, to get the nicely controlled temperature water out you have to put something into the boiler. The pump injects water into the boiler to do that. Because a PID-controlled heating element can only add heat AFTER the water has cooled down it means there must be a time lag before the temperature comes back to the control point.
Preheating the water that enters the brew boiler is one way to help with this because it reduces the temperature delta between the new water and the controlled-temperature water. Controlling the water flow within the boiler can help with this (see my other thread Water Flow within Brew Boiler and Temperature Stability
). However, with the trend towards smaller brew boilers these have less impact. In general, I think that the move to small brew boilers helps because it reduces the thermal mass and makes the boiler more responsive to heat input.
So, we have a PID control system trying to control temperature while a pulse of cooler water is being injected into the boiler. What could be better? Just using PID ignores the fact that we KNOW that cooler water is going to be injected into the brew boiler. If we really want to provide the best temperature control, we'll measure the temperature of the incoming water, we'll plan for the amount of water being injected, and we'll adjust our controller to start providing heat to the boiler as soon the pump starts. This "modern control system" would reduce the lag of when the heat begins counteracting the addition of the cold water.
So, the best temperature control will come when we move beyond PID to begin using the other factors we know about an espresso brew boiler to best advantage. This will add more cost but will bring better temperature control.
I've been thinking about this for a while and today is the day it spewed forth.