S2E4: How does Adaptive Cruise Control work?
Introduction
I have traveled in a car with Adaptive Cruise Control in the past, but have never experienced the essence where the car slows down automatically once the one in the front gets too close to ours. Several car models, including the BMW 3 series, Volvo S60, Nissan Altima, and Toyota Corolla Hatchback, are equipped with Adaptive Cruise Control (ACC) technology. The entire process doesn’t need any user intervention other than using the steering wheel; and brakes when you want to get out of ACC. Let’s see how they work. But before proceeding, let me clarify that I am referring to cars that run on gasoline as I can explain the entire sequence using a particular type as an example. Though the essence is not different in an electric one.
Why do we need Adaptive Cruise Control?
Let’s say we’re driving at a constant speed of 60mph with the Cruise Control activated. What would happen if a car appeared in front of us and then traveled at 40mph? Intervening and applying brakes becomes necessary to slow the car down, as maintaining a speed of 60mph will lead to a collision.
This is solved by Adaptive Cruise Control (ACC) - It constantly measures the distance between your car and the one in front of you using Radar. Once the other car gets closer to yours beyond a safe distance, the speed of your car is reduced automatically to follow the one in front of you at a constant distance until it’s safe to cruise again at 60mph. Leaving a constant distance between your car and the one in the front makes sure that the ACC can safely apply brakes when needed, and avoid a collision.
What is a Radar?
RADAR (RAdio Detecting And Ranging) is a radiolocation system used to determine other objects' distance, angle, and velocity relative to the source. A radar system in an automobile produces electromagnetic waves with frequencies that have wavelengths between 1mm and 10mm (AKA Millimeter Wave Band). These are microwaves instead of radiowaves as we can more accurately measure the distance between the source and the object.
Figure 1: Adaptive Cruise Control in Action
How is Radar used to achieve Adaptive Cruise Control?
The radar is typically placed in front of your car behind the grill. Short pulses are sent out as waves and are reflected back to the radar when they encounter a car in front of them. This concept is similar to hearing an echo.
The faster the pulses return to your car and the radar detects it, the closer the distance between the two cars. So once the radar detects that it received the pulses back in a short time, it sends a signal to the ECU which then commands the throttle valve to close further and restricts the air intake to the engines, thereby reducing the speed of your car.
If you have read my blog post-S2E3 on how cruise control works, we discussed how throttle valves are controlled. Here is how! The more the valve opens up, the more air is sucked in through the throttle body, resulting in more fuel intake into the engine cylinders to maintain the air/fuel ratio. Consequently, the faster the crankshaft rotates due to combustion, the faster the car travels.
Conclusion
The shorter the time it takes for pulses to reflect back and reach the radar in your car, the closer the car is in front of yours, the slower we need to go, so the more the throttle valve closes to adjust the speed accordingly. Once the car in front of you moves away beyond a safe distance or to the adjacent lanes, your car speeds up again to travel at a constant speed in Cruise control mode.
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