timed temperature controller......

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Fulthrotl

~Autocorrect is My Worst Enema.~
can someone who's got better control skills than i do point me in the right direction?

i need a controller that can accept a k thermocouple input, to control an output...
here's the function i'm looking for:

a timer will trigger an output that is settable from .01 to 1.0 seconds, every 30 seconds or so.

when an external dry contact is closed, the second output will be activated, and will vary
from .3 to 1.0 seconds every 30 seconds, with the amount determined by the thermocouple.

before i re invent the space shuttle here, i'm wondering if anyone knows of a panel mounted
or din rail controller that might be able to do this, or do i have to get a PLC with a thermocouple
input and code it that way?

thanks.


randy
 

Smart $

Esteemed Member
Location
Ohio
can someone who's got better control skills than i do point me in the right direction?

i need a controller that can accept a k thermocouple input, to control an output...
here's the function i'm looking for:

a timer will trigger an output that is settable from .01 to 1.0 seconds, every 30 seconds or so.

when an external dry contact is closed, the second output will be activated, and will vary
from .3 to 1.0 seconds every 30 seconds, with the amount determined by the thermocouple.

before i re invent the space shuttle here, i'm wondering if anyone knows of a panel mounted
or din rail controller that might be able to do this, or do i have to get a PLC with a thermocouple
input and code it that way?

thanks.


randy
Perhaps something like this...

http://www.omega.com/Temperature/pdf/CN3240.pdf

I'm uncertain if it will perform the exact functionality you are looking for. Would have to read the operation manual to determine that. ;)
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Perhaps something like this...

http://www.omega.com/Temperature/pdf/CN3240.pdf

I'm uncertain if it will perform the exact functionality you are looking for. Would have to read the operation manual to determine that. ;)
At first glance, from the lack of mention of a potential sales feature in the advertising, I suspect that the timer function cannot be made to change the interval based on TC input. Whether the control loop itself could handle that is the question I would ask.
An output pulse proportional to the TC reading, gated by the first 30 second period input, and an external RC network taking that pulse and shaping it to drive a control input might be one way to get what Fulthrotl needs.
 
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Smart $

Esteemed Member
Location
Ohio
At first glance, from the lack of mention of a potential sales feature in the advertising, I suspect that the timer function cannot be made to change the interval based on TC input...
I was thinking the proportional control could be set to manage the output "on" time relative to TC input. :blink:
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The Fuji PXR temperature controllers have this sort of capability, but I don't know if they will cover the range that you need with the resolution that you need.

A standard digital temperature controller will do most of what you need; adjust the duty cycle of something based upon the temperature sensed by a probe. Changing the cycle time is one of the standard settings; you need to be able to change the cycle period to match the load.

What seems to be less standard is the setting for limiting the duty cycle. On the PRX controller that I have use, there is a different settings page that you need to find which sets a minimum and maximum % duty cycle allowed. I don't know if all temperature controllers have the ability to set these limits.

http://www.americas.fujielectric.com/sites/default/files/ECNO1125s.pdf

The cycle time may be adjusted from 1 to 150 seconds. The lower and upper duty cycle limits (PLC1 and PHC1) can be adjusted by the 0.1%

-Jon
 

Smart $

Esteemed Member
Location
Ohio
Worth investigating. Depends on whether the period of the proportional control is adjustable.
Minimal investigating leads me to believe that series has been discontinued :eek:hmy:

May look into other options later, but done for the time being....
 

Smart $

Esteemed Member
Location
Ohio
I'm thinking most controllers you would come up will likely have PID capability, and why not use that feature?
I agree... but the question is whether it is possible. Proportional control will vary the duty cycle by percentage, but the cycle period (sum of on and off time) will have to be set to act in correlation to the 0.3 to 1.0 second on-time requirement.

It would help to know more about process, the purpose of the timing requirements and how they were established. The heating element must be rather fast acting and heat retention of the process quite high to maintain a set temperature cycling on only once every 30 seconds for a second or less.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
As I said above, the Fuji PXR temperature controllers have this capability, and to make it clear, I have used this feature.

I have a Fuji PXR3 controlling a SSR which runs a commercial food warmer that I am using for chocolate tempering. The sensor is a type K thermocouple.

The food warmer has a 1KW heating element, and if I allowed the temperature controller to use a 100% duty cycle during the initial heatup, the chocolate would burn at the hot spots.

So I have the controller set to do standard PID control (with a 4 second cycle time), but I have the output limits set to permit no more than a 10% duty cycle. So as I am using the unit, the heating element is 'ON' for a period of 0 to 0.4 seconds every 4 seconds.

The unit can be set up with a 30 second cycle time, a minimum duty cycle of (say) 1%, a maximum duty cycle of 3.3%, and you can use P, PD, or PID control to translate the measured temperature into the output duty cycle. You can manually adjust the parameters, or use an auto-tune function.

-Jon
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Not doubting. My reply to kwired was based on his reference to "most controllers".

Exactly, and I agree with you. You would be hard pressed today to buy a digital temperature controller that didn't have PID capability, but I don't know if all/many/most/only a few such controllers have the ability to adjust duty cycle limits. I know that one controller that I looked at didn't have this capability, and one that I am using does....and the one that I am using just might fit the OP's needs.

-Jon
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I suspect you are trying to reinvent the wheel here, but it is hard to tell with any certainty since you have not specified why you want to do what you are suggesting.

What you are suggesting appears to be essentially a PID loop with no I or D setting.

Most temperature controllers with digital outputs can do this.

Most temperature controllers have a timer in them that determines how often the loop is solved, and most have some kind of timer function that can be set for the duty cycle period.
 

Fulthrotl

~Autocorrect is My Worst Enema.~
I agree... but the question is whether it is possible. Proportional control will vary the duty cycle by percentage, but the cycle period (sum of on and off time) will have to be set to act in correlation to the 0.3 to 1.0 second on-time requirement.

It would help to know more about process, the purpose of the timing requirements and how they were established. The heating element must be rather fast acting and heat retention of the process quite high to maintain a set temperature cycling on only once every 30 seconds for a second or less.

let me clarify what this is going to be used for... as usual, i'm looking to fix something that isn't broken.:p
and it isn't even for profit, or honest work. i was trying to shield you folks from the extent of my obsession
with BBQ and smoking food, but you are all in now....

i've got a pellet fed smoker. there is an existing controller that maintains a set
point quite well, but is not able to be changed remotely. it controls two things,
the electric ignitor, and the pellet feed screw. combustion airflow is constant when the unit
is running, from a muffin fan. it also won't compensate for ambient temperature,
so you have to keep an eye on it.

the unit has a TC mounted at cooking level for control.
it maintains the electric ignitor until temp hits 140 degrees,
then maintains the fire and controls the temp with two presents,
low heat, and high heat.

low heat keeps the fire from going out with a small amount of pellets (1.5 seconds, adjustable)
if TC is showing below setpoint, the auger turns a longer period of time (up to 12.0 seconds, adjustable)

sample rate is every 30 seconds.

and, it works pretty well, but some tweaking is necessary to get it to hold temperature.
if your high heat setting is too low, obviously you won't hit your set point, and if you
are overfueling the smoker, the temp will fluctuate widely.

usual high heat setpoint is from 3.0 to 10.0 seconds, depending on pellets used, and ambient
temperature.

so, i bought a wifi controller for monitoring temperature of the cook. the unit has addressable inputs,
and addressable outputs so you can drive a blower on a uncontrolled smoker, and control your
cook... and i thought... hmmm... it'd be cool to be able to control the setpoint of the unit with
this wifi deal as well.....

so the first idea was simply to set the factory control to 500 degrees, so it was always calling
for high heat, then interrupt the feed to the auger motor with a ice cube relay, so when the
new controller was sending an output calling for high heat, it closed the relay, and the pellets
got fed, etc.

the problem is, when the new controller was satisfied, it'll open the power to the auger,
and there won't be any trickle of pellets to keep the smoker afire until you next need heat.
so that won't work well.

so then, that leaves a micro PLC to replace the factory controller entirely. so i can put
one TC in smoker to control the ignitor, use the call for heat from the new wifi temp
controller to toggle between high heat, and low heat. same basic functionality.

then the real problem with that, is that i continued thinking...... and why not put a second
thermocouple in ambient air, so it can correct automatically for air temperature, and
save having to fiddle with the high heat duration manually. i mean, as long as i've had
to go and buy a PLC to do this...... why not?

that's when i decided to post here, and see if anyone has a simple solution to this,
or can recommend a medication to keep me from having these thoughts.

either one will do, 'cause while i was thinking about this, the thought popped into my
head... "you are so batchit crazy, why don't you also put a wind speed indicator
on top of the smoker, so you can compensate for wind chill as well..."

it's a slippery slope for me, as you can see.....
 

Smart $

Esteemed Member
Location
Ohio
Thanks for elaborating...

As with any process like this, even with a capable controller, it will require a fair amount of tweaking.

Don't think I can provide any more assistance at this point...
 

__dan

Senior Member
Sorry i don't have a neato solution, but I can add to the perplexity.

The pellet fuel is 67% volatile gasses and 33% charcoal, neglecting moisture content. So there's a variation in heat output as the volatiles flash off and combust first, then the charcoal is left to burn.

The burner control probably accounts for this by adding pellets relatively constantly at a slow rate (add a few, wait, add a few) rather than batch burning (adding a lot and waiting longer).

The advanced pellet boilers watch boiler water temp and flue gas temp, or boiler water temp, flue gas temp, and lambda (excess O2 in the flue gas, targeting ~ 9% excess O2 for the burn), controlling primary combustion air, secondary combustion air, fuel feed rate, and draft fan speed.

It's not clear, but I'm assuming you want to hold a fixed, adjustable, cooking temp over a long period. The easiest solution would seem to be, try to make the factory control work with some quantity of operator experience and monitoring to see what it is and is not doing correctly. There will be a big difference in heat output, burn rate, between different pellet brands.

If you stabilize with a fixed, preferred pellet brand and fixed augur screw rate using the existing controller, the factory control scenario would seem to be high heat on a call for heat and low heat when the unit is on but not calling for heat.

Fixing pellet brand, high heat and low heat augur feed rates, the next parameter to control for variability of heat output is combustion air supply, either draft or a damper at the combustion air input.

That's probably a fixed, adjustable damper, which you could motorize with a PLC and Belimo operator on the damper, but the easiest solution would be to try to manually adjust the combustion air damper so the factory control works correctly in the high heat mode and can cycle between high and low heat to maintain even cooking temp.

What you want is a data logger to see when the cooker is out of spec and tweak the factory provided adjustments, air and fuel feed rates, to see if the factory control can keep the unit in the cooking temp range.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
While I think that the PAX units just might work (they take digital inputs that allow remote selection of different set points), seeing as this is an open ended tinkering project, I'd actually suggest using something like an Arduino system rigged as a PID controller, and also suggest moving over to a different board for the discussion, eg 'cooking for engineers' or the like.

People have already made Sous Vide controllers using the Arduino (eg. http://learn.adafruit.com/sous-vide-powered-by-arduino-the-sous-viduino ) so there are PID libraries out there for programming. Adding multiple temperature sensors would be trivial. The biggest problem would be the high temperature sensor.

You could hack away as much as you wanted, including doing things like adding fan speed control to adjust airflow. I recall someone who had gotten very precise temperature control by adjusting air flow to a smoker.

Actually I just looked: there are many people building Arduino controllers for smokers and barbecues. Lots of ideas about how to best proceed out there.

-Jon
 
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