Autopilot — Page 358, Lesson 421

I want to walk you through the very heart of what an autopilot actually is, because it's not just a machine that moves the controls — it's a control system built on something called control loops. Let's start with the most important idea: the inner loop. The inner loop is a classic example of a closed loop control system — that's why it's sometimes called a closed loop system. Now, what does "closed loop" mean? It means the system watches its own output and feeds that information back to correct itself. The basic elements of a closed loop control system are shown in Figure 26.1, and they comprise five things: the input, the error detector or signal processor, the output, the control element, and feedback. Let me take those one by one. The input is the command — what you want the system to do. The error detector, also called the signal processor, compares what you want with what's actually happening, and it produces an error signal — the difference between the two. The output is the actual result, the physical movement or response. The control element is what acts on that error signal to drive the output. And feedback is the crucial part — it takes a sample of the output and sends it back to the error detector so the system can see how far off it is and keep correcting. Now, contrast that with Figure 26.2, which shows an open loop control system. The key difference is that an open loop system does not have feedback. The controller in an open loop system may consist of a pre-determined programme or a human operator. But here's a subtle point: if a human operator is used, then the system, in effect, becomes a closed loop system — because the human is the one closing the loop, feeding back the output signals by watching the result and adjusting. Let me make this concrete with the domestic central heating example. A system with a timing controller but no thermostat is an open loop system. The pump would continue to send hot water round the house regardless of the room temperature, for the whole duration of the period set on the timer. It doesn't care about the result. On the other hand, a system with a thermostat would interrupt the circulation when the room temperature reaches the preselected level — that's feedback control. The thermostat senses the output, the room temperature, and feeds it back to stop the pump. Now, there's one more term I want you to know, because it's the professional name for what we're dealing with in aircraft. Feedback control systems used for positional control — like aircraft flying controls — are usually referred to as servo systems, or servomechanisms. And here's the formal definition: a servomechanism is a closed loop control system where a small input is converted into a larger output in a strictly proportionate manner. So a tiny movement or signal at the input gets amplified into a much bigger movement at the output, but the relationship stays strictly proportional — the output is always a faithful, scaled-up version of the input. So the whole picture is this: the autopilot's inner loop is a closed loop servo system that positions the flying controls. And above that, an outer loop interacts with the inner loop to give the aeroplane guidance, so as to achieve the required flight path. The inner loop handles the precise control of the surfaces; the outer loop tells it what flight path to follow. That's the architecture you need to hold in your head as we go forward.