Every minute, on average, at least one person dies in a crash. Air bags and seat belts save tens of thousands of people a year. But the ultimate solution and the only thing that will save far more lives, limbs and money is cruise control system. Cruise control was commercially introduced in 1958 as an option on the Chrysler Imperial. Cruise control is an invaluable feature on American cars. Without cruise control, long road trips would be more tiring, for the driver at least, and those of us suffering from lead-foot syndrome would probably get a lot more speeding tickets.
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. With traffic continually increasing, basic 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 is Cruise Control System?
Cruise control is a system, which automatically controls the speed of an automobile. Most cruise control systems don’t allow the use of cruise control below a certain speed. The purpose of the cruise control system is to maintain a constant vehicle speed despite external disturbances, such as changes in wind or road grade. This is accomplished by measuring the vehicle speed, comparing it to the desired or reference speed, and automatically adjusting the throttle according to a control law. The cruise control system actually has a lot of functions other than controlling the speed of your car.
It can accelerate or decelerate with the tap of a button. 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 a particular value (usually around 25-30 mph). We all know that the things that control the speed of the car are the gas pedal and the brakes. And the brain that normally controls the speed of the car is the brain of the driver. The driver senses the speed by looking at the speedometer and then adjusting the pressure on the gas pedal or the brakes to compensate for variations in the desired speed.
The cruise control system does the same thing with one exception. It only controls the gas pedal - it doesn't even know there are brakes in the car!! A Blind inventor and mechanical engineer called Ralph Teetor, invented cruise control in 1943. [pic] Modelling To understand the working of a cruise control system, we consider here a simple model of the vehicle dynamics, shown in the free-body diagram (FBD) below. The vehicle, of mass m, is acted on by a control force, u. The force u represents the force generated at the road/tire interface.
For this simplified model we will assume that we can control this force directly and will neglect the dynamics of the powertrain, tires, etc. , that go into generating the force. The resistive forces, bv, due to rolling resistance and wind drag, are assumed to vary linearly with the vehicle velocity, v, and act in the direction opposite the vehicle's motion. [pic] Figure 1 Summing forces in the x-direction and applying Newton's 2nd law, we arrive at the following system equation: (1)[pic] ---(1) Since we are interested in controlling the speed of the vehicle, the output equation is chosen as follows 2)[pic] ---(2) Taking Laplace’s transform for equation (1), we get m sV(s) + b. V(s)=U(s) ---(3) We find the transfer function of the cruise control system to be: [pic] [pic] Figure 2 – Block Diagram Controlling the Cruise Control The brain of a cruise control system is a small computer that is normally found under the hood or behind the dashboard. It connects to the throttle control as well as several sensors. The diagram below shows the inputs and outputs of a typical cruise control system. [pic] Figure 3 – Cruise Control Working Block Diagram
A good cruise control system accelerates 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 how steep the hill you drive up. Controlling the speed of a car is a classic application of control system theory. Since the cruise control system controls the speed of the car by adjusting the throttle position, 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 most important input is the speed signal; the cruise control system does a lot with this signal. The most basic control system is a 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. Most cruise control systems use a control scheme called proportional-integral-derivative control (PID control). The integral of speed is distance. The derivative of speed is acceleration. A PID control system uses these three factors -- proportional, integral and derivative, calculating each individually and adding them to get the throttle position. We've already discussed the proportional factor.
The integral factor is based on the time integral of the vehicle speed error. Translation: the difference between the distance your car actually travelled and the distance it would have travelled if it were 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. Let's say your car starts to go up a hill and slows down. The proportional control increases the throttle a little, but you 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 let's add in the final factor, the derivative. 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. pic] Figure 4 - Block Control of Cruise Control Model Setting of Cruise Control [pic] [pic] Figure 5 & 6– Setting of Cruise Control using buttons The system pictured above has five buttons: On, Off, Set/Accel, Resume and Coast. It also has a sixth control - the brake pedal, and if the 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. 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.
Adaptive Cruise Control System A more advanced cruise control is being developed that can automatically adjust a car's speed to maintain a safe following distance. Adaptive Cruise Control (ACC) is an automotive feature that allows a vehicle's cruise control system to adapt the vehicle's speed to the traffic environment. This new technology, called adaptive cruise control, uses forward-looking radar (or lidar – light detecting and ranging. But lidar based ACC’s are limited due to poor working in bad weather), installed behind the grill of a ehicle, to detect the speed and distance of the vehicle ahead of it. [pic] Figure 7 – Adaptive Cruise Control System 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 Autocruise radar system made by TRW 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 operates at speeds as low as 20 mph (32 kph).
These systems are being 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. In May 1998, Toyota became the first to introduce an ACC system on a production vehicle,luxury sedan. Main Parts & Working of ACC’s The main components of a typical radar-based ACC system are: 1. Fusion sensor 2. Headway control unit 3. Throttle 4. Brake 5. Dashboard display Fusion sensor: It is a combination of sensors and processors.
They are 1. Millimetre-wave radar 2. Stereo camera 3. Image processor 4. Fusion processor [pic] Figure 8 – Architecture of Radar Based ACC System Millimetre wave radar: It is a sensor which uses millimetre wave for detecting the position and velocity of a distant object. Range is calculated using the formula C = 2*R/T Where, ‘C’ is the velocity of light, ‘R’ is the range, ‘T’ is the time of flight of transmission. [pic] Figure 9 – Millimetre Radar Unit Stereo camera: The camera’s function is detection of cars and other objects in the roadway.
Image processor: It processes the images from the stereo camera and the data fed into the fusion processor. Fusion processor: The function of fusion processor is Data Fusion. Headway control unit: It has control on the brakes and throttle and uses dashboard for immediate warnings. Canceling Cruise Control Operation: • Brake pedal is pressed • 'Off' button is pressed • Vehicle Speed < 25 mph • An ACC system fault is detected [pic] Figure 10 - A car model with advanced cruise control system First, the vehicle speed sensor might fail. Normally the speedometer also fails so that's pretty easy to diagnose.
Next, the power to the brain can be interrupted (Here brain is referred to the cruise control system as it works similar to brain). A blown fuse or a corroded connector can prevent the brain from working correctly or at all. Next, the brains can lose its ability to function. A faulty component can prevent the brain from doing its thing. The brain is a pretty sophisticated box that contains a lot of electronic components including a microprocessor. Normally when the brains fail you need to replace the box. The vacuum diaphragm can develop a leak.
If that happens then the cruise control might set and hold the speed for some time however if the leak is larger than the supply line and modulator can add vacuum to the system the system will slowly lose control and the vehicle will slow down. This can also happen if the vacuum line to the diaphragm is cracked or loose. Finally, the linkage that connects the diaphragm to the accelerator linkage can fail. Some aftermarket cruise control systems use a short length of what looks like fat key chain - bead chain. I have seen several units fail when the chain simply breaks. Conclusion
But one drawback is that cruise control systems are dangerous in wet roads and slippery roads as they may cause some serious problems. Fully autonomous car is probably not viable in the foreseen future. Nearby vehicles would be in constant communication with each other and act co-operatively. It will probably take decades, but car accidents may eventually become almost as rare as plane crashes are now. References 1. www. howstuffworks. com 2. www. cars. com/features/adaptivecruisecontrol 3. www. autorepair. about. com 4. ctms. engin. umich. edu/CTMS/index. php? example=CruiseControl=SystemModeling