Cruise Control

 

 


Cruise control is far more common on American cars than European cars, because the roads in America are generally bigger and straighter, and destinations are farther apart. But it still finds its way onto luxury European cars. With traffic continually increasing, cruise control is becoming less useful, but instead of becoming obsolete, cruise control systems are adapting to this new reality soon, cars will be equipped with adaptive cruise control, which will allow your car to follow the car in front of it while continually adjusting speed to maintain a safe distance.

What it Does

The cruise control system actually has a lot of functions other than controlling the speed of your car. For instance, this cruise control pictured below can accelerate or decelerate the car by 1 mph with the tap of a button. Hit the button five times to go 5 mph faster. There are also several important safety features -- the cruise control will disengage as soon as you hit the brake pedal, and it won't engage at speeds less than 25 mph (40 kph).

The system pictured below has five buttons: On, Off, Set/Accel, Resume and Coast. It also has a sixth control, the brake pedal; and if your car has a manual transmission, the clutch pedal is also hooked up to the cruise control.

 


 

  • The on and off buttons don't actually do much. Hitting the on button does not do anything except tell the car that you might be hitting another button soon. The off button turns the cruise control off even if it is engaged. Some cruise controls don't have these buttons; instead, they turn off when the driver hits the brakes, and turn on when the driver hits the set button.

     

  • The set/accel button tells the car to maintain the speed you are currently driving. If you hit the set button at 45 mph, the car will maintain your speed at 45 mph. Holding down the set/accel button will make the car accelerate; and on this car, tapping it once will make the car go 1 mph faster.

     

  • If you recently disengaged the cruise control by hitting the brake pedal, hitting the resume button will command the car to accelerate back to the most recent speed setting.

     

  • Holding down the coast button will cause the car to decelerate, just as if you took your foot completely off the gas. And on this car, tapping the coast button once will cause the car to slow down by 1 mph.

     

  • The brake pedal and clutch pedal each have a switch that disengages the cruise control as soon as the pedal is pressed. So you can disengage the cruise control with a light tap on the brake or clutch.


How it's Connected


The cruise control system controls the speed of your car the same way you do -- by adjusting the throttle position. But cruise control actuates the throttle valve by a cable connected to an actuator, instead of by pressing a pedal. The throttle valve controls the power and speed of the engine by limiting how much air the engine takes in.

 


One of the cables is connected to the Accelerator pedal, the other to the actuator.

In the picture above, you can see two cables connected to a pivot that moves the throttle valve. One cable comes from the accelerator pedal, and one from the actuator. When the cruise control is engaged, the actuator moves the cable connected to the pivot, which adjusts the throttle; but it also pulls on the cable that is connected to the Accelerator pedal 

 


The electronically-controlled vacuum actuator that controls the throttle.

Many cars use actuators powered by engine vacuum to open and close the throttle. These systems use a small, electronically-controlled valve to regulate the vacuum in a diaphragm. This works in a similar way to the brake booster, which provides power to your brake system.

Controlling the "Cruise" Control


The brain of a cruise control system is a small computer that is normally found under the bonnet or behind the dashboard. It connects to the throttle control seen in the previous section, as well as several sensors. The diagram below shows the inputs and outputs of a typical cruise control system.

 


A good cruise control system should accelerate quickly to the desired speed without overshooting, and then maintain that speed with little deviation no matter how much weight is in the car, or how steep the hills you drive up. Controlling the speed of a car is a classic application of Computer control system theory. The cruise control system controls the speed of the car by adjusting the throttle position, so it needs sensors to tell it the speed and throttle position. It also needs to monitor the controls so that it can tell what the desired speed is and when to disengage.

The most important input is the speed signal; the cruise control system does a lot with this signal. 

Proportional Control


In a proportional control system, the cruise control adjusts the throttle proportional to the error, the error being the difference between the desired speed and the actual speed. So, if the cruise control is set at 60 mph and the car is going 50 mph, the throttle position will be open quite far. When the car is going 55 mph, the throttle position opening will be only half of what it was before. The result is that the closer the car gets to the desired speed, the slower it accelerates. Also, if you were on a steep enough hill, the car might not accelerate at all.

PID Control


Most cruise control systems use a control scheme called proportional-integral-derivative (PID) control. A PID system uses basic maths calculus in that the integral of speed is distance, and the derivative of speed is acceleration.

A PID control system uses these three factors

  • proportional
  • integral
  • derivative

calculating each individually and adding them to get the throttle position.

The integral factor is based on the time integral of the vehicle speed error. Translation: the difference between the distance your car actually traveled and the distance it would have traveled if it was going at the desired speed, calculated over a set period of time. This factor helps the car deal with hills, and also helps it settle into the correct speed and stay there. So if car starts to go up a hill and slows down. The proportional control increases the throttle a little, but it  may still slow down. After a little while, the integral control will start to increase the throttle, opening it more and more, because the longer the car maintains a speed slower than the desired speed, the larger the distance error gets.

Now add in the final factor, the derivative. Remember that the derivative of speed is acceleration. This factor helps the cruise control respond quickly to changes, such as hills. If the car starts to slow down, the cruise control can see this acceleration (slowing down and speeding up are both acceleration) before the speed can actually change much, and respond by increasing the throttle position.

Adaptive Cruise Control


Two companies are developing a more advanced cruise control that can automatically adjust a car's speed to maintain a safe following distance. This new technology, called adaptive cruise control, uses forward-looking radar, installed behind the grill of a vehicle, to detect the speed and distance of the vehicle ahead of it.

Last year, Mercedes-Benz became the first car manufacturer to install TRW's adaptive cruise control, called Auto-cruise, adding the device to its European S-Class Saloons. BMW followed Mercedes-Benz this year, adding the system to some of its European models. 

Delphi Automotive Systems has developed a similar adaptive cruise control system, already available on the 2000 Jaguar XKR in Europe.

Adaptive cruise control is similar to conventional cruise control in that it maintains the vehicle's pre-set speed. However, unlike conventional cruise control, this new system can automatically adjust speed in order to maintain a proper distance between vehicles in the same lane. This is achieved through a radar headway sensor, digital signal processor and longitudinal controller. If the lead vehicle slows down, or if another object is detected, the system sends a signal to the engine or braking system to decelerate. Then, when the road is clear, the system will re-accelerate the vehicle back to the set speed.

The 77-GHz Auto-cruise radar system has a forward-looking range of up to 492 feet (150 meters), and operates at vehicle speeds ranging from 18.6 miles per hour (30 kph) to 111 mph (180 kph). Delphi's 76-GHz system can also detect objects as far away as 492 feet, and operate at speeds as low as 20 mph (32 kph).

Adaptive cruise control is just a preview of the technology being developed by both companies.

These systems will be enhanced to include collision warning capabilities that will warn drivers through visual and/or audio signals that a collision is imminent, and that braking or evasive steering is needed. 

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