No, there is a large difference between a negative feedback system and a positive feedback system. Tell us how to make your temperature controller hold a constant temperature with positive feedback and a loop gain of greater than 1. If you do, you might find that it matches my model.
Dave -----Original Message----- From: Andrew <[email protected]> To: vortex-l <[email protected]> Sent: Sun, May 26, 2013 8:05 pm Subject: Re: [Vo]:Re: Constant temperature Operation of ECAT? See my follow-up on this. There's always going to be a tracking error, no matter how sophisticated the regulation algorithm. I think the prime objective here is not to have absolutely constant temperature per se; rather, it's to guarantee that thermal runaway cannot occur. Andrew ----- Original Message ----- From: David Roberson To: [email protected] Sent: Sunday, May 26, 2013 5:00 PM Subject: Re: [Vo]:Re: Constant temperature Operation of ECAT? How many of these controllers use positve thermal feedback to keep the sink at a constant temperature? Dave -----Original Message----- From: Andrew <[email protected]> To: vortex-l <[email protected]> Sent: Sun, May 26, 2013 7:52 pm Subject: [Vo]:Re: Constant temperature Operation of ECAT? Seems to me that if active cooling control is used as the only control input, thus satisfying the "unplug it!" sceptics (and I'm one of them), then it only has a chance of working if there is good thermal contact and good thermal conductivity and substantial enough heat capacity in the active cooling implementation. I don't know why this is supposed to be hard. Gaming PC's of the high-end variety use this all the time. Prompt temperature feedback to the cooling pump is all that's needed, plus a simple PID controller. This is very well-known technology. Andrew ----- Original Message ----- From: David Roberson To: [email protected] Sent: Sunday, May 26, 2013 4:44 PM Subject: [Vo]: Constant temperature Operation of ECAT? My model demonstrates that constant temperature operation of the ECAT is not going to work under normal conditions. The relatively high value of COP when temperature control is used depends upon operation in a positive feedback region. This can be thought of as related to the question that always arises about why the device does not supply its own drive and therefore run continuously in SSM. Once the loop gain becomes greater than 1, the device will tend to move in the direction that it is currently heading. This allows it to heat up to a relatively larger temperature than that due to the drive alone. When rising in temperature, the device begins to put out additional heat, more with time. The trick is to turn the process around at a good point before it goes too far. The best turn around temperature is well defined and shows up as a tendency for the device to continue putting out power at a constant rate with time. Unfortunately, this exact point would be impossible to achieve while maintaining control. It is a balance between how long you want the temperature to remain nearly constant and the risk of loosing control. Rossi chose a relatively safe turn around temperature for the last test which caused the COP to drop below his desired value of 6. I suspect he chose this because a COP of 3 well demonstrates that the process is real and also has enough margin to keep the device safe from melt down. I think I would have done the same under the same constraints. Dave
