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Power
Windows
BY KARIM NICE
How about the power windows
with the automatic-up feature that raises the windows by themselves, but stops
raising them if there is an obstruction?
Or maybe you've seen the
Volkswagen TV commercial where the guy opens the windows by turning the
key in the door lock.
In this article, you'll learn
about what's going on in all of these power-window features and more!
The Lifting Mechanism
Let's start with the lifting
mechanism. This cool device is the heart of a power-window system.
The
window lift on most cars uses a really neat linkage to lift the window
glass while keeping it level.
A small electric motor is
attached to a worm gear and several other spur gears to create a
large gear reduction, giving it enough torque to lift the window.
An
important feature of power windows is that they
cannot be forced open -- the worm gear in the drive mechanism takes
care of this.
Many worm gears have a
self-locking feature because of the angle of contact between the worm and the
gear.
The worm can spin the gear,
but the gear cannot spin the worm -- friction between the teeth causes the
gears to bind.
Animation of window lifting mechanism at work, with inset of
motor and gear reduction
The
linkage has a long arm, which attaches to a bar that holds the bottom of the
window. The end of the arm can slide in a groove in the bar as the window
rises.
On the other end of the bar
is a large plate that has gear teeth cut into it, and the motor turns a gear
that engages these teeth.
The
same linkage is often used on cars with manual windows, but instead of a motor
turning the gear, the crank handle turns it.
In the next section we'll
learn about some of the neat features some power windows have, including the
child lockout and automatic-up.
The Wiring and Switches
Car doors are wired in many
different ways, depending on which features are incorporated.
We'll go through the wiring
on a basic system -- one that allows the driver to control all four windows on
the car and can lockout the controls on the other three individual windows.
A
Basic System
On
this system, the power is fed to the driver's door through a 20-amp circuit
breaker.
The power comes into the
window-switch control panel on the door and is distributed to a contact in the
center of each of the four window switches.
Two contacts, one on either
side of the power contact, are connected to the vehicle ground and to the
motor.
The power also runs through
the lockout switch to a similar window switch on each of the other doors.
A simple power-window circuit
When
the driver presses one of the switches, one of the two side contacts is
disconnected from the ground and connected to the center power contact,
while the other one remains grounded.
This provides power to the
window motor. If the switch is pressed the other way, then power runs through
the motor in the opposite direction.
An
Advanced System
On
some cars, the power windows work in a completely different way.
Instead of the power for the
motor going through the switches directly, the switches are connected to one of
the many electronic modules in the car (the average car contains 25).
Some cars have one in the
driver's door, as well as a central module called the body
controller.
Cars
that have lots of controls on the door are more likely to have a setup like
this.
Some cars have the
power-window, power-mirror, power-lock and even power-seat controls
all on the door. This would be too many wires to try to run out of
the door.
Instead
of trying to do that, the driver's door module monitors all of the switches.
For instance, if the driver
presses his window switch, the door module closes a relay that provides
power to the window motor.
If the driver presses the
switch to adjust the passenger-side mirror, the driver's door module sends a
packet of data onto the communication bus of the car.
This packet tells the body
controller to energize one of the power-mirror motors.
Interesting Features
· Automatic Up/Down - The automatic-down feature is
fairly common on cars with power windows.
You
tap and release the down switch and the window goes all the way down. This
feature uses a circuit that monitors the amount of time you hold the switch
down.
If the switch is down for less than
about half a second, the window will go all the way down until it hits the
limit switch.
If you hold the switch down for longer
than that, the window will stop when you release the button.
Automatic-up windows are less common.
The problem with automatic-up windows is that if anything gets in the way of
the window, such as a child, the window has to stop moving before it hurts the
child.
One way that carmakers control the force
on the window is by designing a circuit that monitors the motor speed.
If the speed slows, the circuit reverses
the power to the motor so the window goes back down.
· Window Control From Outside - On the Volkswagen in the TV
commercial, the windows can be lowered by inserting the key in the driver's
door, turning and holding it.
This
feature is controlled by the driver's door module, which monitors a switch in
the door lock.
If the key is held turned for more
than a set amount of time, the driver's door module lowers the windows.
· Courtesy Power-On - Some cars maintain the power to
the window circuit after you turn your car off, which saves you from having to
stick your key back in the ignition if you forget to roll your window up.
The power-window circuit will have a
relay on the wire that provides the power.
On some cars, the body controller keeps
this relay closed for an extra minute or so. On other cars, it stays closed
until you open a door.
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