Multilevel Temperature control

Question

I would like to control oven temperature with a Raspberry Pi. I have a simple on-off control with deadband working well now. My challenge is I need to control the oven to different temperatures for different time periods: ie: 0 to 200F over a period of 6 hrs, cool to 125F over a period of 4 hrs, then back to 280F for 3 hrs. I hope to do this with a while loop but do not really understand how to proceed once my first setpoint and time has completed. I will count down the time with the time function, so once the first one zeros out, how do I proceed to the next setpoint/time ramp for the cooling phase? Any suggestions or if you could direct me to some code that is already built it will be great!

rew · Answer

Hi Terry, Good to hear that it works. :-) I have a little project with a MAX38155. I have not looked into the compatiblity with the older 6675 chips. But Maxim is inclined to keep the protocols the same as much as possible. Many people who for example know the 6675 chip, will decide: "We can just as well use the TI chip instead of the Maxim one, if we have to rewrite the software". So Maxim is interested in keeping the protocol the same. "only minor changes" or "no changes at all". So find the datasheets and try to see for yourself if they are the same. After you've made a decision, you're welcome to post again including your verdict and with links to both datasheets. Then it's easy for me to verify your claim.

terryaris · Answer

I really like your approach Roger and I thank you for getting back to me so quickly. I have had a couple of issues with your script and wonder if you could help me out. I have the following questions: 1 When I put your script into Python 2, it did not like the $ on the the variables, so I removed them and all went well. 2. it does not like the -le, just after the while loop, changing that to curtime-tottime lets it work. 3. should the curtemp statement be all in quotes, or should we be allowing that calculation? 4. I cannot get this one working in any way. curtemp=`echo "$starttemp*($tottime-$curtime)/$tottime + $endtemp*$curtime/$tottime" | bc` echo -en $curtime $curtemp '\r' echo $curtemp > the_temperature_file sleep $step I have tried -e, -en, -ne, brackets, ., and all I can think of but I cannot get python to accept it in any manner. Please let me know where I am going wrong, as I believe this solution should work well. Once again, thanks for your help.

mconners · Answer

Roger's script was in bash, not python, and I agree with his approach. I misread your original question and thought you had completed the temp control part, but just needed a way to accurately trigger the execution. I was trying to accomplish the same thing roger was, separating execution from logic. BTW, bash is the Bourne Again Shell, and it is the default command line interpreter when you login to your pi. So you could copy the code roger gave you when saved in a text file will work just by changing the execute permissions on the file and typing it's name on the command line. the first line #!/bin/sh or #!/bin/bash tells the CLI to execute the program as a sh script or a bash script, it's just as if you were typing the commands one at a time on the command line. Mike