All control systems, including the DSM ECUs, can operation in one of two ways: open-loop mode and closed-loop mode. These are the terms most frequently misunderstood, so let's state what they mean.
Chapter 1: Open-loop Systems
Open-loop systems apply control signals without regard to the output signals being produced. To model one, take a piece of paper and draw a box on it. On the left side, draw a horizontal arrow leading into the box - this is the input. On the right side, draw a horizontal arrow leading out of the box - this is the output. Voila, an open-loop control system!
[Too busy to draw it? That's ok, I drew it for you. :-) Click here.]
The box is the controller or control system; the part that actually does the 'thinking', if you will. The controller can be anything - an electronic circuit, a mechanical system, a computer, whatever. It takes input signals (the leftmost arrow), processes them and generates output signals directing the action.
To get a little fancier, you can draw another box on the other end of both arrows to make 3 boxes. The leftmost box represents the input system, which provides the original input signals to the controller. The rightmost box is the controlled system - in other words, the part that the control system is running.
[I know, you can't find a pen. :-) Look at this diagram here.]
If you like, you can imagine the middle box on the page representing the ECU inside a DSM, or any other car. The input system can be any or all of the input sensors used by the DSM ECU, including airflow, throttle position, air temperature, barometric pressure, etc. The output signals consists of control outputs to fire the injectors, provide spark to the cylinders, and so on, which (naturally) run to the engine.
As you might expect, open-loop control systems are generally not very useful, because the controller does not have any way of knowing how the controlled system is behaving. As a simple example, let's make the input a temperature sensor, and the output a set of heating elements. For any given temperature, the heater controller 'knows' to make a certain amount of heat, and it will direct the heating elements accordingly.
The system will work fine when it is put together, and probably for some time thereafter. Let's just say, however, that something goes a little bit wrong with the heating elements - one breaks, or they just wear out a bit, so the heater is now putting out less heat than it should.
In this example, the air temperature is the same, so the inputs are unchanged. The control system is still producing the correct signals to run the heater at that air temperature. The heater could have lots of extra heating capacity left, which could easily compensate for the temperature drop. The problem is that the heater is not behaving like it should, and the control system has no way of knowing this.
There are some examples where open-loop control is fine. If all that is being controlled is an indicator light, or and LCD display, maybe it doesn't matter if they work because a person will notice the fault, and correct it. Systems that are easily predictable, in other words. This isn't always the case, however. Where open-loop control fails us, the solution to this is the closed-loop system.