Editing IBM buckling spring (section)

==Capacitive==
{{Main |IBM Model F}}
[[File:ModelFdiagram.jpg|100px|thumb|right|Patent diagram of the pivoting hammer design]]
Patented as the '''"Buckling spring torsional snap actuator"'''<ref name="_PAT_4118611" /> by IBM in 1977, again designed by Richard Hunter Harris, this was a significant improvement upon his earlier design, and was used in IBM's Model F keyboards and some of IBM's electronic typewriters<!-- which ones? -->. It replaced the [[beam spring]] mechanism, having the advantages of being simpler, cheaper, and smaller, and allowing low-profile and low-cost keyboards (by the standards of the time) to be produced with the same desirable qualities as the beam spring.

The main problem with the mechanism described in the original buckling spring patent, and other proposed keyswitch designs based around the buckling action of a spring, was that in a mechanism where a straight spring was attached to two fixed points, and compression occurred (i.e. it was pressed down on) the direction in which the spring would buckle was indeterminate, and this meant that extra elements had to be added to the switch to ensure that it buckled in the right direction. This was undesirable as it added complication and potential sources of failure to the switch.

Harris' solution was to change the design so that the spring was tensed between the keycap and a pivoting hammer that could only move in the desired direction, and that the mounts for the spring were designed so that the spring was slightly bent forward in the desired direction of travel. These two measures ensured that the spring could only buckle one way.

[[File:act hmr.jpg|100px|thumb|right|Buckling spring with attached hammer and barrel module from Model F]]
But more importantly, instead of using the spring itself to actuate the contact mechanism, the purpose of the spring was to drive the hammer forward.  The hammer was made of the same material as the fly plate in the beam spring switch, and in terms of how they register key presses, they are somewhat similar – at rest, the hammer is raised over two capacitive contacts.  When the key is pressed, the spring buckles forward, pushing the hammer onto the plates, and causing a change in capacitance which is recognized as a key press. In fact, is is possible to use this mechanism on the PCB of a beam spring keyboard, and vice-versa, except for one major difference – they work in reverse. The beam spring registers a key press when the hammer is pulled away from the contacts, whereas this buckling spring mechanism recognizes a key press when the hammer lies right on top of it. 

When the key is released, the spring returns into its default state, which in turn pulls the hammer up and pushes the key back into rest position. The ingenuity of this mechanism is that a single spring provides the tactility, the movement of the actuation element, and the pre-travel/return travel of the switch.  In the beam spring switch, a different spring was required for each of these tasks.

[[File:Fig 2 harris keyforce-o.svg|250px|thumb|right|Force graph for the buckling springs on a Model F keyboard, as introduced with [https://www.google.com/patents/US4118611 US patent 4118611].]]
The mechanism is referred to as the '''capacitive buckling spring''' to differentiate it from the later buckling spring design that used [[membrane]]s instead of capacitive contacts. The capacitive buckling spring mechanism is popular due to the crisp tactility and loud feedback it provides, which is widely considered superior to that of the later membrane-type buckling spring. It also has a lighter actuation force of about 60–65g of force compared with 65–70g for later designs. However, these advantages come not from any intrinsic superiority of the design itself, but rather the superior construction quality of the keyboards that used it. In terms of inherent advantages, the capacitive mechanism is far more reliable than membrane or electrical switch based keyboards as there are no contacts to be worn out – the keyboard merely senses a change in capacitance caused by the movement of the hammer.  Thus the switch can take about 100 million keypresses before failure.