TECHNOLOGY CASE STUDY -

CRUISE CONTROL

CHRISTOPHER ALLISON

Mr Tait 5th Year Techy

Introduction

Cruise control systems maintain the speed of a car at a level set by the driver, without having to use the accelerator.

The main developer of cruise control was Ralph Teetor, who was a mechanical engineer and had his first patent for the design of a speed cruise control device in 1945. Teetor was inspired to invent cruise control one day while riding with his lawyer. The lawyer would slow down while talking and speed up while listening. This rocking motion so annoyed Teetor that he was determined to invent a speed control device. It took him a while to make it suitable for commercial use and was first used in 1958 in a Chrysler.

Cruise Control is more common in American manufactured cars than European cars as the roads are straighter and longer. There is less need to break in America on the long journeys causing a cruise control system in a car to become more effective. But luxury European cars like BMW, Mercedes-Benz and Audi are developing adaptive cruise control which uses radar to follow the car in front while maintaining a safe distance from the car. Safety precautions are necessary to ensure the system fully works. Features like a manual override for the driver by accelerating or breaking to avoid immediate danger need to be available.

This project focuses on the manual setting of cruise control not the adaptive CC; this cruise control is a very good example of a feedback control system.

Analysis and Description

The cruise control system has a number of inputs, a control computer, and a mechanical output connecting to the engine throttle. The block diagram above shows the general arrangement of a cruise control system. Most updated cruise control systems have a feedback loop from the throttle position. The feedback signal is generated by the speed of the car and fed into the cruise control system through the vehicles speed signal input.

Driver Interface & Inputs

The controls for cruise control have to be easy to use whilst driving and are usually within range of the driver’s hands when steering.

The cruise control system actually has a lot of functions other than controlling the speed of your car. For instance, the 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.

The system pictured below has five buttons: On, Off, Set/Accel, Resume and Coast. Also the brake and clutch pedal are fitted with limit switches that detect pressure and will automatically disable 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 have 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. 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 shut off the cruise control with a light tap on the brake or clutch.

Mechanical Arrangement

The cruise control system controls the speed of your car the same way a person does - 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. When the cruise control system is active and is controlling the speed the accelerator pedal will move just like if the driver was pressing the accelerator pedal down.

One of the cable is connected to the accelerator pedal, and the other two the vacuum 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 - this is why the accelerator pedal moves up and down when the cruise control system is engaged.


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 control the vacuum in a diaphragm.

Cruise Control Software

The cruise control system is a small microcontroller computer that is normally found under the hood or behind the dashboard of the car. It connects as scene in the previous section to the throttle control as well as various other sensors. The diagram below shows the inputs and outputs of a cruise control system.

A cruise control system will accelerate aggressively to the desired speed without overshooting, and then maintains that speed with little deviation no matter how much weight is in the car, or big the gradient of the hill that it is climbing/descending. 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 it can tell what the desired speed is and when to disengage. The simplest control system to keep the car’s speed at a constant set level would be an on/off system. If the car’s speed was less than the set speed, full throttle (off) would be applied until the actual speed was the same as the set speed. At this point the throttle would be closed (off) and the car would begin to decelerate. The cycle would constantly repeat with the speed fluctuating above and below the desired speed.

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. As the speed increases towards the set speed the throttle will start to gradually close. The effect of this is that there is less fluctuation in the speed will be reduced.

PID control

Most cruise control systems use a control scheme called proportional-integral-derivative control (PID control). This is the most technologically advanced system as it takes in three main factors.

  • The integral of speed, is when the speed is increased by a minimal amount so to compensate with not accelerating it takes into accountthe distance the car would have travelled if it was going at the desired speed in comparison with distance it has done at the actual speed.It takes this distance into account and will increase the speed to compensate for this lost distance when needed.
  • The derivative of speed is the quickness the car responds to acceleration the car will experience at hills, i.e. opening/closing the throttle earlier.
  • The proportional control of the error described above.

A PID control system uses these three factors -- proportional, integral and derivative, calculating each individually and adding them to get the throttle position.

These factors all give PID control a huge advancement from the other cruise control systems giving it the smoothest and finest control; this is why it is used in all aspects of engineering, from industrial control through to aviation.

Adaptive Cruise Control

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. 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 safe distance between vehicles in the same lane in front of it. 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.

Social, Economic and Environmental Effects

Social: +++With the introduction of cruise control the driving experience has become more relaxed especially for long motorway drives where cruise control is most effective. This is a positive factor as it means that drivers will become less stressed and so can concentrate on other aspects of driving i.e. special awareness of other cars around. So this will reduce car accidents. Also there will be less cases of sore feet from pressing the throttle for long periods of time.

---But, drivers might take this relaxation as a negative and become less involved with their driving as they are not concentrating on accelerating and braking. This will increase the danger of drivers falling asleep and more accidents.

+++ When adaptive cruise control is introduces it will ensure that there is always a safe following distance between you and the car in front. This is positive as it will take into account reaction times when bad visibility and braking distance.

Economic: Jobs are being created for the research and development of adaptive cruise control but nothing has changed within the instalment of the electronics as it is all automated with robots. So no jobs have been lost due to the introduction of cruise control. Car manufacturing is constantly changing from the old mechanical engineering model to new electronically filled cars. This means that more of the manufacturing of components is done overseas.

Environmental: As the car will be keeping at a more constant speed there will be lower fuel consumption from constantly revving and burning fuel. So it will cost the driver less. Therefore the impact on the environment will positive as less fuel will be needed and used up, so less pollution given off. The electronics of cruise control is made up of lots of circuit boards and components so will no doubt have a small negative effect on the environment from the raw materials used up.

Conclusion

This report on cruise control is about the system and the main ideas about how it works. Cruise control is fitted to many American cars and the luxury European cars which makes the long motorway journeys more relaxable for the driver. The electronics of cruise control is linked up to the cars main computer during the main assembly line.

Cruise control is an example of a control system with Inputs, Outputs, Feedback Loops and software processing. The inputs and transducers are quite simple but the control software was very challenging to understand i.e. PID control.

Cruise Control is an example of the many new and innovative electronic ideas which are being introduced for cars to increase the safety and relaxation of the driving experience. Cruise control has a positive impact on driving with a benefit to the environment.

BIBLIOGRAPHY

WEB PAGES

  • http://www.cs.cmu.edu/~ModProb/CC.html
  • http://www.benefitcost.its.dot.gov

Christopher Allison