How Cruise Control Systems Work
Introduction
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 It Does
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. 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. 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.
How It’s Hooked Up
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 One of the
cables is connected to the gas pedal, the other to the vacuum 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 gas
pedal — this is why your pedal moves up and down when the cruise control is
engaged.
The
electronical
lycontrolled
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 hood 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 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. 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 most important input is the speed signal; the
cruise control system does a lot with this signal. First,
let’s start with one of the most basic control systems
you could have — a proportional control.
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-integralderivative
control (a.k.a. PID control). Don’t worry, you don’t need to know any
calculus to make it through this explanation — just remember that:
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 traveled and the distance it would have traveled 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. 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.
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).
Adaptive cruise control is just a preview of the technology being developed by both
companies. 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.
Advantages and disadvantages
Cruise control has many advantages but also some serious vices.
Some of those advantages include:
Its usefulness for long drives across sparsely populated roads. This usually
results in better fuel efficiency.
Some drivers use it to avoid unconsciously violating speed limits. A driver
who otherwise tends to unconsciously increase speed over the course of a
highway journey may avoid a speeding ticket. Such drivers should note,
however, that a cruise control may go over its setting on a downhill which is
steep enough to accelerate with an idling engine.
However, cruise control can also lead to accidents due to several factors, such as:
The lack of need to maintain constant pedal pressure, which can help lead to
accidents caused by highway hypnosis or incapacitated drivers; future systems
may include a dead man’s switch to avoid this.
When used during inclement weather or while driving on wet or snow- and/or
ice-covered roads, the vehicle not equipped with Electronic_Stability_Control
could go into a skid. Stepping on the brake — such as to disengage the cruise
control — often results in the driver losing control of the vehicle.
