Editing Force (section)

==Force curve==
As mentioned earlier, the force present between your finger and the switch (the force you feel is of the same magnitude but opposite direction of that felt by the switch, per Newton's third law of motion) changes as you press down on a key. This change can be plotted on a graph as a "force curve", with force as the '''y''' axis and [[travel]] (how far down the key has been pressed) as the '''x''' axis. Many reputable manufacturers publish such graphs, either in their documentation or in their patents. (In German, key travel is "Tastenhub", but in German documentation, the graph is "Kraft–Weg", not "Kraft–Hub".)

The downstroke (press) and upstroke (release, or return) curves are typically different, sometimes by a large amount, depending on the switch mechanism. This is particularly the case with switches that provide [[hysteresis]]. The force curve graphs below give both the downstroke and the upstroke; in each graph the solid black point represents the actuation (make) point, and the hollow point represents the reset (release) point.

The way that the force changes with travel distance can be grouped broadly into specific types of switch, as explained below.

===Linear===
A switch with a [[:Category:linear switches|linear]] force curve requires a uniform increase in of force to depress the switch. If it requires ''x'' cN to depress it by 1 mm, it will take 2''x'' cN to depress it by 2 mm, and 3''x'' cN to depress it by 3 mm. There is no tactile indication given that the switch has actuated. Vintage switches are frequently linear, before switch manufacturers devised ways to generate feedback to the user.

[[File:Force graph--linear--Cherry MX Black.svg|326px|centre]]

Linear switches are still widely favoured by gamers, and the [[Cherry MX Red]] switch is considered to be broadly suitable for both typing and gaming.

See: [[:Category:Linear switches|Linear switches]]

===Tactile===
Tactile switches use an uneven force curve. Quality (mechanical) tactile switches use build-up and then a fall in resistance to indicate where the switch actuates. Membrane [[rubber dome]] switches maintain this same force curve, but for operator satisfaction alone: the force curve has no bearing on actuation as membrane dome switches actuate on bottoming out.

[[File:Force graph--tactile--Cherry MX Brown.svg|326px|centre]]

Many tactile switches also emit an audible click during actuation; such switches are referred to as "[[:Category:Clicky switches|clicky]]", with "[[:Category:Tactile switches|tactile]]" generally referring to a switch that has a tactile force curve but no audible feedback upon actuation. There are also keyboards that produce audible feedback for linear switches, using a relay or small speaker.

[[File:Force graph--clicky--capacitive buckling spring.svg|326px|centre]]

The [[Datanetics elastic diaphragm array]] actuator patent cites two reasons for the switch having a tactile force curve:

#To provide "unconscious assurance that the key stroke or displacement has been adequate for actuation"
#To "urge the key away from the exact point of actuation to prevent hovering at this stroke position which could cause switch jitter and unintended multiple actuation"

[[Cherry MX Brown]] in particular is noted for its weak tactility, but the patent in question (US 3773997) suggests that tactility does not have to be consciously observed.

The patent also suggests a role for a tactile force curve that is not related to the operator: to prevent the key from being accidentally held at the point of actuation (risking jitter), as an alternative to [[hysteresis]].

See: [[:Category:Tactile switches|Tactile switches]]

===Parabolic===
A parabolic switch is a type of switch that follows a parabolic weighting curve. After reaching the peak of the parabola, the force to continue pressing down on the switch will actually decrease. This gives a very unique and consistent keyfeel as the low grade parabolas will provide an almost constant amount of force to be applied throughout the throw of the key switch.
The actuation point can be anywhere on the parabola, but is usually placed at 55–60% of the travel. Unfortunately this is usually after the crest of the parabola, so it becomes extremely difficult to avoid bottoming out in comparison to a linear switch.

See: [[:Category:Parabolic switches|Parabolic switches]]

===Progressive rate===
Strictly speaking, "progressive rate" appears to denote a continual change in the rate of increase of force. The term is also used to denote a sharp change in the gradient of a linear force curve. The most obvious example is [[RAFI RS 76 M (illuminated)]], where the bridge contact system clearly introduces a sharp change of force at the point of actuation.

See: [[:Category:Progressive rate switches|Progressive rate switches]]