Ok as promised, here’s some things I came up with on my first day reunited with my notes in almost 2 months, I’d like to see all of these things be accounted for….chop,chop.
Automatic control brings to the table some interesting challenges, people are going to expect to be able to set a number, representing a desired cabin temperature, and have the climate control sustem do whatever it takes to achieve and maintain it:
First, it can be all but guaranteed that linear control of the blend damper will not provide linear control of the the cabin air temperature.
Second, perceived cabin air temp will not always (or often) feel like actual, as sensed by its designated sensor. When the sun is out, it will frequently feel much warmer. Ambient temperature will change the perceived temperature as well. The effects of these factors on perceived temperature are not insignificant, and must be accounted for. So there needs to be a curve, or a map for the error signal driving the blend damper and fan that takes sensed cabin air temperature and corrects it with sunload and ambient temperature. More on this in a paragraph or two.
Due to high heat transfer rates from the vehicle in moderate to very low temperature, fan speed should be increased in addition to the driving signal above solely due to outside ambient air temperature. The same is true for periods of high sunload and high ambient air temperature.
All of these factors must be weighted and the amount by which they are weighted will vary greatly based on the different models of climate control systems (air box/ damper geometry, fan curves, etc.), and models of vehicles.
Now, in addition to the above, one final factor should be added, or multiplied to this error signal before it drives the blend door actuator. That is a factor representing the non-linearity of the dampers effect, through the fans airstream, on cabin temperature.
Where do we get these factors from? Auto manufacturers have the luxury of all their testing, using the automobile itself, the airbox, special wind measurements, heat transfer data, and years of experience. We obviously don’t.
My first idea paralleled this however; come up with a simplified, general way of determining ft3 and glass ft2 fan efficiency, etc. and way to gather this data in a software wizard. There were some holes I couldn’t fill with this method in my mental walkthrough of the occasion.
Empirical Data:
In my years of experience in industry, I’ve found that my theoretical knowledge can not usually compete with more experience and empirical data. With this thought in mind, I turned to the idea of a calibration sequence that uses the things it knows (temperatures) and finding things it doesn’t know (factors). With some assumptions and generic models, I think that can really complete the equation for most applications.
And since I’ve touched on extreme temperatures, I should also say that many behicles have an evaporator (anti-frost) thermostat which interrupts power to the compressor when close to freezing temperatures exist there. Refrigeration pressure switches, as with manual systems, are obviously required still.
To round out compressor shut offs, the compressor should shut off when temp gets too high, or on full acceleration. (which brings to mind another dilemma in determining which functions like these or parts thereof are performed by each vehicles engine controller and which are performed by the climate controller. Where is the line drawn?)
Regarding compressor shutoffs: Sure these switches can be wired in series with the compressor, but:
When we install these new controllers, we become the only qualified technicians for them. This introduces the concept of error codes to this application. We’re not going to send them to the diagnostics port anymore, like current automotive auto climate controllers do, but there’s no reason we shouldn’t have them available. Hell, we’re taking the diagnostic information already there, and putting that on the carPC anyway.
A few more quickies:
How are we going to get this thing to tell the ECU to make the engine idle faster when the AC compressor clutch energizes?
High end systems will also redistribute air based on cooler (face) hotter (floor) More than 4 fan speeds are needed for that smooth ramp up and down, a must for precise control anyway.
The signals to these motors is varied, how do we account for all?
I have more, but I'm going to try and get back in the groove of this for a little while first. Goodbye real world
There goes that dream
Linear Mode
