Comfort Controller : ma In to V Out direct conversion
I have been trying to get a comfort controller(actually 3) to take a 4-20ma demand signal and change it directly to a 2-10v out for CWV & HWV of three identical AHU's. The demand is coming from another source that we cannot bypass. The "house" system gives demands only referencing the space temp. The CC6400 only sees the demand. We have supply and return sensors in place but they are not being used to control the system. Conventional wisdom says to let the CC6400 control the building, but the powers that be will not allow that. I have been working closely with the local Carrier technical support guy.( who is great to work with and a nice guy ) but we can't seem to get a direct input to out to the CWV , HWV AOut. We are using custom Ma in and the like , but seem to only be able to speed up and slow down the PID reacting to the demand signal. ANY IDEAS WOULD BE APPRECIATED. I know it would be easier to direct wire the demand to the valves with resisitor but then the safeties(esp. frzstat) would also have to be reworked and the issues could get bigger.
Last edited by jackleg in NC; 10-24-2008 at 09:51 AM.
Reason: Additional info
I'm going to take a stab at this realizing that I may not completely understand your canundrum....
Are you saying that your controller receives signals from multiple thermostats and converts that to a 4-20ma demand signal? The demand signal then allows the controller to output a 4-20ma basically a stand alone signal based on building temp?
Assuming that I got this right....
Have you looked at ACT converters (advanced control technologies)? They make boards that can convert analog signals such as 0-10v to 4-20ma and vice-versa, I believe that you can jumper them direct or reverse acting too. They come factory calibrated as a 1-1 input/output but they have pots that you can recalibrate them, you can also request custom calibations. What if you took this 4-20ma demand signal and converted that to a 0-10V signal? Then fed that into 2 converter boards, I say convert to 0-10v because I don't think that you can parallel (2) 4-20ma signals. Feed this signal into two converters that you calibrate to 0-5V = 2-10v for heating and 5-10V = 2 -10v for cooling control. Older controls did that pneumatically so that one signal controlled HTG and CLG. As demand increases the cooling ramps up, as demand drops heating increases after you cross over the center. You can reverse the startegy if necessary but I think you get the point.
Look up ACT (advanced controls technology) part number AIM1 or AIM3, the same module except for input power. They are available from ALPS Controls.......www.alpscontrols.com, they run about $85 or so each.
Update: Maybe I read too much into this, if all you're looking to do is convert the demand signal of 4-20ma to a 0-10v, AIM1 is your answer and you can ignore my rambling...LOL
Controls..some days your a hero, some days your a zero. Direct acting since 1992.
Not sure if I totally understand, you have a single 4-20mA input that needs to be directly converted to a 2-10vdc for one valve or the one signal for both the HWV & CWV valves?
You will probally not get direct control with any of the built in algo's as they all have PID loops, with the calculations & task timers you will have delays that will probally never have the input equal the output.
Using BEST++ should work. Hopefully you wont have to have precise control down to 0.01 of a mA as that would make a long program. If 1mA change can equal a 0.5vdc, then you are only looking at 16 changes which should be fine for water valves.
Originally Posted by Qui-Gon Jinn (Star Wars Episode 1)