Analog engineers know what a servo system is. More specifically, they understand (probably intuitively) how a closed-loop servo system is defined and how it operates.
There are three parameters or constants that are used to define the loop: the proportional, integral, and derivative gain terms. One, two, or all three of these are used in a typical system. The system will compare the setpoint (the desired result) to some measurable process variable. The system will generate an error based on the difference between setpoint and process variable. This simplified description shows how straightforward the system design should be.
This drawing, taken from the Wikipedia page for PID controllers, shows such a straightforward system:
Except, apparently, in my cars.
HVAC (heating, ventilation, air conditioning) systems in cars have come a long way since the days of Henry Ford. Over the past 20 years, we’ve progressed from a slider knob that has its extremes labeled “Cold” and “Hot” to a digital readout with associated “Up” and “Down” buttons. This setup gives the impression to the user that there is a servo system hiding behind the dash that accurately and tightly controls the temperature in the cabin so that it matches the setting displayed on the readout.
Is this really a calibrated, closed-loop servo system? Or is this (as someone, somewhere else said) a design made by monkeys?
My experience on several vehicles indicates this is a system that is closed loop only if I am included as part of the loop. I need to characterize and quantify my annoyance, create a numerical value for the annoyance parameter, and write the corresponding loop equations for the control system. The reason for my annoyance? I have experienced systems that seem to have little regard to the outside temperature, the engine temperature, the cabin temperature, or the digital readout. Sometimes they seem to function as an on-off switch. Below a set-point of, say, 68, the heater is off; above 68, it’s on. I had better performance from my ’64 Falcon that used vacuum actuated baffles in the heater ducts.
Perhaps my cars use baffles that get stuck. This possibility points to a poor design. I don’t care to crawl under my dash and experiment with the control mechanisms to find out. Perhaps my cars use a simple proportional control to adjust the flow of hot water through the heater core based on temp-setting. Perhaps cabin temperature is not monitored at all. This seems like a distinct possibility.
Regardless, I just want better designs and better implementation. Comments welcome.