Originally Posted by Machinbird
Temperature control is reactive rather than proactive.
Originally Posted by A33Zab
Uhhh...isn't that what is inherent to feedback controls? You need a certain deviation to correct the situation.
(we are awaiting RR_NDB Transient in Feedback Systems (II))
OK, Looks like I need to explain the situation better.
One form of current stabilization (old fashioned now but I have used it in equipment) is the old fashioned wire wound ballast resistor. As the current goes up, the temperature of the wire windings goes up, the resistance of the specially selected wire increases and limits the current. These things generally operate at a dull red glow. They are likely using this type system internally for pitot heat stabilization/regulation.
No moving parts, no electrical connection other than normal regulated voltage power supply, and you get a heat source. Properly selected, it is a simple means of generating heat for situations that are entirely predictable and slow changing. The aircraft climbs to altitude, the probe cools a bit, the wire cools a bit and the current increases which increases the power input. KISS to the extreme. To increase the power further, you need to increase the voltage.
The problem occurs when the heat demand changes rapidly. In the case of AF447, we had 19 seconds between the sound of ice crystals impinging and the AP disconnect. It took some time for the ice crystals to cool the probe internal surface and then the heating coils enough so that the power would then increase and melt the ice. When overloaded with ice crystals, this type probe goes through cycles of icing, melting, purging the melt water, and icing again.
If the icing had been detected in the first couple of seconds by a thermocouple, the power could have been cranked up then to meet the new demand before the blockage occurred, but this means that probe heating would be solely under the control of an external control system using feedback to sense and defeat icing. The dynamic control range would be much higher.
This is my take on what is likely the means of pitot tube heat control based on the evidence of the schematic you posted A33Zab. The externally controlled feedback system is probably what is needed to fix the problem.