I'm a refrigeration mechanic and I'm looking to do something funky. It's a device that takes a pressure and temperature reading, compares it to a saturation pressure table and outputs a stepper motor command to try and keep the calculated value within range. I'm also looking to control water flow on the same board, also by a step motor driven process using a temp sensor.
I'll be happy to answer questions about the physical requirements of this, but very briefly:
I need some guidance on what additional components (beyond the human interface portions) that I need to add to an Uno or Nano to obtain my readings and generate outputs.
I know I'll need a way to control a stepper motor (possibly 2) and likely a way to generate a 4-20ma signal.
The board needs to "see" two 10k NTC thermistor sensors and a pressure transducer (0-5v or 0-10v signal with a filter)
I'm getting a bit lost in all the available shields and add-ons, so any help getting there will be extremely useful!
Is the stepper controlling pressure (valve or pump)?
I made a simulation to control a servo depending on temperature and humidity... in your case, a pressure reading and thermistor... the code for the stepper would need to be changed, but it might give you ideas (or answers). Right-sized power supply/supplies will be important.
Shields... if you can make a solid prototype, just solder on a perforated breadboard. You might consider creating your own PCB design and ship it to fabricators like JLBPCB (and some other fabricators... usually sponsors of well-known makers on YouTube).
Get some real-time readings and put them in a table with actual output and desired output, and post that here... it will help in defining the desired outcome.
A superheat controller is exactly what I'm building, but I'm adding more user-friendly HMI and some additional features to control condenser/evaporator throughput and so forth too.
I'm pretty familiar with Danfoss, Sporlan and Emerson controllers; probably cheaper to use one of the less expensive options that what I'll end up building but the point is the exercise of building and programming
What I'm actually building is a portable half horsepower water source chiller that has tight control of the process stream and minimizes water usage on the condenser side via flow control to optimize the condensing temperature. I knew I'd likely need to build from the ground up, but I've been surprised with Arduino shield offerings so far.
The stepper controls an EEV (electronic expansion valve) that directly controls the flow of liquid refrigerant to the evaporator. On the second side I'll likely use an off-the-shelf modulating valve that accepts a 0-10vdc or 4-20a signal to control a valve for condenser throughput. I do believe I have a decent pairing for the EEV stepper to a TMC driver already.
The point of this exercise for me is more of an "I can do it" than anything - there are plenty of off the shelf solutions and I work in PLC cabs that incorporate lots of these features - it's more that I want to build a single controller with an HMI that manages the full device.
Basically I've not tried to return a pressure transducer or thermistor signal to Arduino yet, so I think your simulation will be very useful in tackling that aspect, I really appreciate you sharing this!
Look at the DS1B20 sensor, it is easy to use and fits nicely in the range you will be working with. Caution with the pressure transducer, some are not compatible with refrigerants. The UNO or Nano would be a good choice. I would recommend you power them with about 8V via Vin and place in a grounded metal box. Consider a zero cross solid state relay for the compressor, that will help keep noise out of your system.
One thing I did years before Hot Gas Bypass on water chillers was use a small tube ( like a cap tube but bigger) to bypass the evaporator and by opening the valve that control it it cut the refrigeration capacity. I could cycle this a lot without any problems. These ranged from about 5 ton up. You have to be careful not to pass two much or you will heat the compressor. This gave me very fine control.
My preference is for modules rather then shields, they give more versatility.
I'm at the design stage right now, so capacity control is something I'm currently working out, which is going to play heavily into what I settle on for my sensor suite.
I did actually look into HGB via a pair of solenoid valves, cycling hot gas into the evap while allowing a small bypass of liquid refrigerant into the suction accumulator through a cap tube to ensure sufficient cold vapor return - kinda like a pseudo heat pump.
I do think that most of my control is going to be possible through use of the EEV, but I I really appreciate the engineering input; it's reassuring to hear that someone else had a very similar idea.
I did it with a National SCMP processor in the late 70's. It did very well but the systems were a little on the slow size because of the size and heat transfer into water. It was PID with about a 120 second cycle. Slow but it worked. The pressure transducer was the hardest to find.
