Are there prosumer machines that feature multiple PIDs for better intra-shot, or shot-to-shot temperature stability and control? Where should the PIDs be situated?

A pid is a control system.
What it controls, in an espresso machine, is the heater.
You can't have more than one PID battling it out over a single heater. A pid doesn't control a measurement point - it uses the measurements to control a heater.

If you have more than one heater, such as a separate group head heater, then this might make sense, but it would only keep the stability of the individual part of the system, not the system as a whole.

Better intra-shot stability is achieved through a variety of designs, but throwing more PIDs on a system won't make a poor design any better, and probably won't help a good design much, if at all.

What about having more than one sensor, at different points in the path?

There are commercial machines with multiple PIDs. As told to me by the rep from Nuovo Simonelli, a machine he demo'd for me (I don't remember the model) had a PID for brew temperature, one to control water preheating that went into the brew boilers, and one to control the temperature at the group somehow. It also had some sort of variable gicleur for water flow to the group so it would even extract properly with a half-full, untamped portafilter (I actually saw that happen and tasted the espresso and it was quite good!).

But in a home machine it can be done with less sophistication and a single PID such as with the VBM DD and other machines. Thermosyphon-heated group and preheating the brew water before it enters the brew boiler by passing the feed tubing through the steam boiler are but two ways to ensure temperature stability and control.

innermusic wrote:What about having more than one sensor, at different points in the path?

wikipedia is your friend

The more inputs, the more complex the control.
Imagine an automobile with 4 wheels, each with its own engine, throttle and braking system.

The more inputs, the more complex the control.
Imagine an automobile with 4 wheels, each with its own engine, throttle and braking system.

Actually, I was thinking more like an automobile with 4 wheels, each with it's own computerized traction sensor, making independent adjustments on the fly.

In a conventional automobile they work by reducing braking [cooling] to control skid, which is the opposite of adding power [heat]

With the emergence of electric vehicles new drive train configurations are possible. Multi-drive systems become easy to implement due to the large power density of electric motors. These systems, usually with one motor per driving wheel, need an additional top level controller which performs the same task as a mechanical differential.

from http://en.wikipedia.org/wiki/Electronic_differential [emphasis added]

It's the same in an espresso machine with multiple control points. If the group is actively heated, its controller needs to know the boiler water temperature. The boiler controller needs to know group and inlet water temps. The system needs to know whether 1 or 100 ristretto or lungo are on order in the next interval.

Multiple inputs work well on systems with quick reaction to input. Large water volumes and their metal containers are very slow responders.

An unconventional machine with laser / steam water heating at the group exit ONLY could work well with multiple sensors.
Send $10,000,000 to start the design.
http://www.engadget.com/2007/03/25/two- ... onlookers/

Send $10,000,000 to start the design.

Could I run a tab?

For what it's worth: multiple PID's can be hooked up in a control loop and programmed to interact. Maybe "watched over" by a PLC( programmable logic controller> an industrial computer) or PC. How to set that up and program it and what would be gained by all the expense and work is the big question.
KDM
PS: I think the PLC would be able to eliminate the need for PID's as they, with the proper input boards, can take thermocouple or RTD inputs directly.

By the time one adds input control, output display, sensor interface boards, programs and packages a PLC, an ots PID is a bargain. Multichannel PIDs exist if necessary.