Tech In Plain Sight: Car Doors [Hackaday]

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There are a lot of common phrases that no longer mean what they used to. For example, you may have used the term “turn on the lights.” What are you actually turning? Where does this come from? Old gas lights had a valve that you did physically turn, and the phrase simply stuck around. Kids of the 90s have no idea why they “dial” a phone number. What about “roll up the car window”?  You don’t often encounter old-fashioned car doors with manual locks or a crank to roll up the window. These days it is all electronic. But have you ever wondered what’s going on inside there?

Let’s take a look at car doors, how they keep you safe, and how that sheet of glass slides into place, sealing against wind, rain, and noise. Of course, there are fancy car doors like suicide doors or sexy-but-impractical gull wing doors. At least one concept car even has a door that disappears under the vehicle when it opens; check out the video below. But even garden-variety doors are marvels of mechanical engineering. A compact structure that is secure and — mostly — reliable. Let’s look at how they do that.

Conversion

When you think about it, a great many things we build convert something to something else. Energy to motion, for example. Or one type of motion to another. Car door mechanisms are no exception. Of course, the lock actuators and the window actuators both convert an electrical command to some kind of physical motion. But there is also the problem of getting the right physical motion in a confined space.

One of the interesting things about most car doors is that the locks have to be secure but offer many ways to open them. Electronic control alone is usually not sufficient: there is often a way to unlock the car manually. And you certainly have to be able to operate the door from the inside even with no power, to avoid trapping occupants. Some cars, like the Boxter or some Lotuses, have no obvious way to unlock the car without power. These cars usually have a hidden emergency keyhole or a set of hidden terminals that allow you to temporarily power up the system to unlock the vehicle.

Lock Up

The motor that controls the power locks in a car is known as an actuator. You can get universal actuators that can be made to fit most cars, even those that don’t normally have power locks. Because the door latch must be workable from multiple sources, the actuator usually drives a rod, and that rod often appears near the window so you can manually operate it as well. The rod engages and disengages the latching mechanism, and when disengaged you can’t open the door. In fact, often if the rod is disengaged and the latch is closed, you won’t be able to close the door if it is open.

A typical actuator is a simple electric motor that rotates. Of course, we need a different motion, so gears not only reduce the motor speed and increase the torque but a rack and pinion gear converts the rotation to a linear motion. Because you usually don’t want the motor and the occupant fighting for control, there is often a clutch that does not transmit manual motion back to the motor. The motor only runs momentarily. Some cars have feedback and will try a few times if something goes wrong.

Older cars used vacuum pumps to suck or blow air to operate doors because electric motors were noisy and prone to failure. Mercedes, for example, used air-operated door locks until around 2003.

Windows

Windows are a bit easier because you don’t normally need manual control. There are some special concerns, though. For example, have you ever noticed that your window stays perfectly level? In addition, it should be almost impossible to force the window down for security reasons. This is often handled by using a worm gear. Because of the angle of contact between the worm gear and the spur gear, the worm gear can turn the spur, but not the other way around because of friction between the gear teeth.

Since the window has to go up and down, there is wiring similar to an H-bridge that can run the motor in either direction. For a car with a sophisticated controller, it probably does use an H-bridge and your window buttons are just inputs to the computer. But basic cars may simply have wiring that allows either the driver or passenger to rotate the window motor in either direction.

Very old cars actually used hydraulic systems to operate the windows. This had the advantage of allowing the window to roll down with no power, but you did need power to operate the pump to raise it again. However, hydraulic lifts fell out of favor long ago in favor of electric motors.

The actual mechanical part can be in several different configurations. With direct drive, there will be either a scissor-like mechanism or a single arm with teeth. Both of these configurations often use a spring to maintain tension.

Many modern cars use a thin steel cable on a reel. By playing out the cable or taking it up, a roller along the bottom of the window can move up or down. A thick plastic tape with sprocket holes serves a similar function in some systems. Altogether, these are known as a regulator. Like the power locks, you can find universal regulators that will repair any power window system or add power windows where you previously didn’t have any. However, as you might expect, these are more complicated and expensive than the comparatively simple door locks.

So What?

This is one of those things where you probably use your car door hundreds of times a year, but you don’t think much about what is in there until it breaks. But even if you never need to repair a door, consider this: universal power lock actuators make excellent linear actuators for all sorts of projects. If you are doing a one-off, you might even be able to harvest one from a junkyard for nearly free. Even if you buy them new, you can find them for $5 or $10 online.

Power window mechanisms cost more. While there are also universal windows available, they are probably harder to find a use for unless you really need something to pop up and down like a car window. If you do a project with either of these, don’t forget to let us know so we can share it with everyone.

Of course, these car parts are pretty beefy. If you need something more modest, you have options. You can even convert a servo.

[Banner image: “Car door sides” by David Rosen, CC BY 2.0. Thumbnail image: “Muscle Car Door 47” by Steve Snodgrass, CC BY 2.0.]