S2E3: How does Cruise Control work?
Introduction
If I remember correctly, the first time I rode in a car with cruise control was on the way to Tahoe with my friend Kaushik, a couple of years back. Many standard cars have this technology as we’re moving towards more advanced capabilities like full self-driving vehicles.
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. Once activated, the cruise control feature lets a car cruise at a constant speed without having you press the accelerator pedal constantly. Manually applying brakes gets your car out of Cruise Control mode. Here is how it works!
The basic operation of Cruise Control
The first step is to determine the car’s current speed. This is done using a speed sensor that is located on one of the wheels. The speed sensor sends this information to the ECU (Engine Control Unit).
The ECU then compares it to the set speed for the car to cruise at. If the car is traveling slower than the set speed, the ECU sends a signal to the throttle control system to increase the throttle opening. If the car is traveling faster than the set speed, the ECU signals the throttle control system to decrease the throttle opening.
How does increasing the throttle valve opening increase the speed of the car? Simple. 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.
Figure 1: Throttle valve animation
But how do we control the exact position of the throttle valve?
Let’s say our current speed is 10 mph, and that the desired cruise control speed is 100mph. To reach the desired speed, we need to open the throttle valve almost fully, as the difference is 90mph. This allows for a lot of air intake.
However, if the speed difference is only 5mph, fully opening the throttle valve can cause the car to overshoot the desired speed. It becomes difficult to maintain a steady speed. In such cases, we may only need to open the valve slightly to compensate for the small difference in speed.
How do these observations shape the algorithm that we can use that adjusts the throttle valve position accordingly?
PID control
The algorithm used to control the throttle valve position exactly is a PID control. It works by calculating the error between the current speed and the desired speed. And then adjusting the throttle valve based on three factors: proportional (P), integral (I), and derivative (D).
For example, if we use only the Proportional control in a PID, the throttle valve opens up to an angle proportional to the difference between the current and desired speed.
Conclusion
PID needs a separate blog post of its own. But if you’re hearing this for the first time and interested to know the specifics, here is a video explaining everything in a few minutes. This is how Cruise control works.
Thank you for taking the time to read my blog post. If you liked what you read, consider subscribing to my newsletter for exclusive content and understand how a gadget works in only 3 minutes, sent directly to your inbox every Wednesday. We'd love to have you as a part of our community. Also, if you found this post to be informative or enjoyable, please share this with your friends on social media. Thank you!
