How does pressing something on the steering wheel maintain the speed? Is it the same mechanism as pressing the accelerator pedal?
In: Engineering
Cars are mostly computer-controlled these days. Pushing the gas directly controls the throttle via a cable. With cruise control, there is a mechanism that controls the throttle/pedal through signals from the computer that adjust it to maintain speed over different terrain/inclines and bring it back up to speed if you brake.
It uses a [PID controller](https://en.wikipedia.org/wiki/PID_controller). This is what is actually a simple feedback mechanism to bring an input in line with a reference through an output. For simplicity sake, your desired RPM is a given voltage, and your current speed is another. There is some output to your throttle that can adjust your current speed signal, up and down, your speed faster or slower, to match what you want. PID is used everywhere something is trying to maintain a steady state, like a thermostat, or a fan speed, or tied to a regulator valve and some flow rate sensor, etc… I know a guy who hooked one up to his smoker to maintain both heat and feed rate of his wood chips.
Today all of this is the same mechanism. On modern cars the accelerator pedal is no more then another electronic positional switch. There is no mechanical linkage between the accelerator pedal and the engine and everything is handled by the engine management computer. The cruise control is just a simple software system where the computer makes sure to maintain the speed using the throttle.
On old cars however this was all mechanical. The key feature of a cruise control is called a governor. This is a device first invented for steam engines which converts rotational speed into a linear position. This can then be used to control a throttle. If the wheels go too fast the governor will reduce the throttle. If you combine the output of the governor and the position of the cruise control then you can set the point at which the system is stable. The engine might even have a governor by default, all diesel engines have them, and the accelerator pedal might already adjust this instead of controlling the throttle directly. In which case a simple cruise control is just keeping the accelerator at the same position even if you do not press it.
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