Editing Beam spring (section)

==Design==
The switch uses a capacitive contact mechanism on what IBM called a "pad card" – a two layer PCB covered with an insulating substrate.  On the lower layer of the pad card, there is a contact for each key on the keyboard, all of which are attached to an oscillator. On the upper layer of the pad card, there are two contacts for each key, one of which sits above the lower layer's contact, and the second is beside it, and is attached to a sensor. At the bottom of the beam spring key module is a so-called 'fly plate' – a piece of conductive plastic which sits on top of the pad card over the two contacts on the upper layer. The oscillator drives a signal on this arrangement of conductive pads, and three capacitors are formed – one between the contact on the back of the Pad Card, and the one directly above it, one between the latter contact and the fly plate, and one between the fly plate and the other contact on the top layer, which in turn is connected to a sensor. The capacitance between the lower contact and the one above it remains constant, and is used as a baseline by the sensor.

<gallery widths=250 heights=110>
File:padCardFront.jpg | Front of the Pad Card, attached to backplate.
File:padCardBack.jpg  | Back of the Pad Card
File:flyplates.jpg    | The underside of the upper half of a beam spring keyboard assembly, showing the fly plates and the switch modules.
</gallery>

Attached to each end of the fly plate are two metal arms, each with a slot near the top (the so called 'fly spring').  A flat piece of metal with tabs on either end slots into the fly spring. The metal is tensed upwards into an arch between the two arms, consequently forming a beam spring. The midpoint of the beam spring is attached to a plunger, on the opposite end of this the keycap is attached. There is a separate coiled spring used to return the mechanism (and the keycap) to its rest position when the key is released.

Pressing on the keycap depresses the plunger which in turn applies pressure onto the mid point of the beam spring, which flattens it. Eventually the beam spring inverts (from a dome shape to a 'u' shape) - this causes the tabs on the beam spring to push the two arms of the fly spring outwards, allowing the beam spring to pull the fly plate up. Once the fly plate is raised, the arms on the fly spring snap back into place, holding the fly plate in the raised position. Two studs inside the switch housing limit the upwards travel of the fly plate. The plunger has a rubber washer around it to absorb the force and cut down the noise of bottoming out. 

<gallery widths=250 heights=200>
File:Beamspring-side.jpeg   | Key module (left) and fly plate (right)
File:Beamspring-top.jpeg    | Top view of key module. Note the rubber damper under the keycap mount.
File:Beamspring-inside.jpeg | Inside of key module, showing the beam spring itself. (Propped up by fly plate)
</gallery>

As the distance has changed between the fly plate and the upper contacts, the capacitance has decreased. The sensor mechanism uses the change in capacitance to detect the movement of the fly plate, and when this occurs, the key press is registered.

Releasing the key causes the plunger to raise itself to its default position, which in turn pulls up the beam spring upwards, reverting it back to its rest position (making another audible snap in the process) which lowers the fly plate back into its rest position. The capacitance returns to its baseline level, and the keyboards ceases registering the key as being pressed.

The patent specifies that the switches had been shown to work after over 100 million key presses. The keyswitches were usually entirely covered over with a layer of insulating rubber (a ''contamination shield'' in IBM terminology), and the keycap slotted on to the mechanism over the rubber.