Extra thermistors to monitor chamber
-
@fma:
Additionally, the thermistor which is linked to the chamber heater (a 200w 240v silicone heater with ac/dc ssr fotek 10A type, connected to an aluminium heatsink) is free in the air near the base of the chamber. Turning on this heater results in an instant fault as the air in the chamber does not increase in temperature quickly enough. Is there any way to make the thermal safety for the chamber heater much less aggresive? Or should I be attaching a thermistor directly to this heater and then determining by trial and error what temperature to set it to achieve the chamber air temperature (45-50 deg C) I am after?
I guess you can launch an auto-calibration?
I can try it, and I will. But I suspect the thermal safety parameters are too strict for this to work, they presume that the thermistor linked to the heater is in direct physical contact with the heater or object being heated. After my chamber heater has been on for 10 minutes (if it didnt go to fault almost instantly) the temperature rise might only be 10 degrees in the air in the chamber higher whereas normally the firmware is looking to see a 1-3 degrees rise per second on hotends and I'm guessing something slightly less strict for bed heaters, but still a lot more than I am going see with my "physically decoupled" thermistor scheme.
My chamber heater is a 200w heater helping to supplement the bed (460w) to get a volume of 1.3m3 to go from 20 deg C to 50 deg C. So it will be a slow process. Unless David knows how to reduce the thermal safety parameters for the chamber heater only (I would like to keep them switched on for bed/hotend) then what I will have to do is attach one thermistor directly to this heater, which will enable me to control the chamber heater temp, but I will have to use trial and error, to determine what effect on the chamber air temp different chamber heater temperatures have, I'd say that's a missed opportunity, if that's the only option available.
Unless there is a way to set a trigger based on a chamber air monitoring thermistor to switch the chamber heater on/off automatically at a predetermined temperature?
I sort-of presumed that given there is a specific chamber heater designation that it would work on the principle of measuring the chamber air temp to control the heater? Otherwise why have a chamber heater function? Or maybe it does and I am just not using it right.
-
Try auto tuning. If that doesn't work, you can configure manual parameters in the M307 command. You will need to use a low gain (A parameter), long time constant (C parameter) and long dead time (D parameter). At a very rough guess, A30 C1200 D300.
-
Thanks David. I haven't had much chance to experiment with it yet. Is it likely for it to work with the thermistor physically remote from the heater?
You may wish to consider using a slow fan to circulate air within the chamber, so that you get a more uniform chamber temperature and the thermistor responds more quickly to the heater.
-
I correctly tuned PID handles that. In most systems, temperature sensors are far from the heaters (a simple example is your house: sensors are not necessary in the rooms where heaters are).
-
That's true but in home heating systems there is still control directly on heaters themselves, i.e. boilers shut off when a set temperature is reached, electric heaters have their own thermostats, water tank heaters have thermostats in the tank, the room thermostat is a global control of the system, not local control.
What would be a great feature for heated chamber users with Duet is global control. So a heater that could be defined as "chamber air temperature" or "chamber thermostat", which could be set to turn on/off heater or heaters linked to it (which have their own thermistors if they require them as they are not inherently self-regulating for safety).
-
….......................
What would be a great feature for heated chamber users with Duet is global control. So a heater that could be defined as "chamber air temperature" or "chamber thermostat", which could be set to turn on/off heater or heaters linked to it (which have their own thermistors if they require them as they are not inherently self-regulating for safety).I'd have thought that should be easily doable. It's just like using a fan in thermostatic mode only instead of turning on a fan, you'd be turning off a heater. So some sort of electronic relay or triac thingy connected to a fan output with normally closed contacts that switch the heater. When energised (i.e "fan on") the relay\triac\electronic thingy switches from normally closed to open and cuts the power to the heater. As you can tell, electronics isn't my forte so I don't know what the "electronic thingy" would be but you get the idea.
-
That's true but in home heating systems there is still control directly on heaters themselves, i.e. boilers shut off when a set temperature is reached, electric heaters have their own thermostats, water tank heaters have thermostats in the tank, the room thermostat is a global control of the system, not local control.
Not in all cases. My electric heaters do not have their own thermostats. They are controlled by room thermostats, which turn them on/off. When on, they heat at full power. They only have an over-run safety, which is never reached in normal operation (only if someone put some cloths on the heater, for example, which prevent natural convection).
Of course, if you heater burns surrounding parts when turned on, you need to limit its power. But this can be done in a simple way, with such device:
-
Hi Ian, thanks, that's true I'm sure I could do it that way, if I had duex2 or duex5 board I would, I'm using my 3 fan PWM outputs currently, maybe I can do it from the expansion connector directly, using an additional fan output to switch my SSR (would I need a mosfet? does an SSR draw enough current to be a problem?).
It's more my suggestion that having a specific system for enclosed chambers would be a nice feature request and I'm sure useful to many.
As I said above I'd propose a means of dedicating a temperature channel to chamber air temp. Then configurable triggers that could be set to turn on/off heaters and/or fans depending on a min or max temperature parameters. I'd like to have my 200w heater come on when the chamber was below 45 deg C, then go off, and then have a fan at the top come on if the chamber exceeded 50 deg C.
Frédéric - Thanks that's useful is that device something like that used in storage heaters which latches-off if over-temperature? That's not a bad idea but then it's not going to regulate the temperature it's a safety cutoff.
-
Okay so stuck the thermistor under the heater, in a more conventional way, and tuned it. Working fine now in this configuration. Documented it here http://forums.reprap.org/read.php?178,702599,790949,page=8#msg-790949
With the chamber heater at 80 deg C and the bed at 115 deg C the 1.3m3 volume heats to 45 deg C, which should provide enough heat to discourage ABS parts from warping.
Thinking about it a little more it will be problematic to have a global air temperature controller as both the bed and chamber heater are inputting heat into the system. But it would still be good to consider a more automated method of managing chamber heating.
-
Hi Simon,
As I understand it, you want to measure the chamber temperature and turn on a heater if it's below a set point. At the same time, you want to measure the temperature close to the heater to prevent it from getting too hot. Is that correct?
If so, could you perhaps use a fan to circulate the air within the chamber so that the temperature close to the heater is more or less the same as at the top of the chamber? Or would a draft, albeit a hot air draft, cause more problems? Forgive my ignorance as I don't print ABS - the smell can trigger my cluster headaches.
Ian
-
I have a heater in the chamber which has a little partially warmed air (some of the exhaust air from my electronics compartment) blown over it, this creates a convection system in the chamber. This heater has a thermistor now and is essentially the setup the same as an aluminium bed. I can turn it on with set chamber temp m141 s80 it sits happily at 80 degrees, from gcode only when selecting ABS, but it's just maintaining a temperature in a "dumb mode". What would be nice is if I could set triggers to turn this heater on or off to attempt to maintain 45 degree in the chamber based on my air temp thermistor rather than having to manually turn it higher or lower.
It's a challenge as I can see is not as simple as it appears. If the chamber air temp thermistor reads low because the lid was left open etc then this heater would continue heating (possibly dangerously although I think it would self limit at about 95 with the fan running) trying to maintain the chamber temp despite too much loss from the open lid. In any case it's basically functional now I'm printing a warpinator 5000 in abs to test it. Chambers are a challenge and a PITA as everything gets harder to get at, and much more complex, then your hotend jams as you're feeding it air at 45 deg C, luckily I only build them with no motors or electronics in the heated volume so mo issues there.
-
Frédéric - Thanks that's useful is that device something like that used in storage heaters which latches-off if over-temperature? That's not a bad idea but then it's not going to regulate the temperature it's a safety cutoff.
This is a simple on/off switch, with hysteresis. Yes, it is mostly used as over-temp switch, but you can use it as a power limiter for you heater. Another solution is to limit the heater power with max PWM, but having a real safety is a good idea, in case a FET stay closed.
