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Tactile feedback

Contents

Overview

Tactile feedback in pushbutton switches provides a physical sensation in one’s finger to that the keypress has been registered. It is widely used in buttons, such as your television remote control or computer power button. In many cases, the mechanism responsible also generates an audible click sound; the use of click–tactile buttons within computer mice has led to the use of “click” as a verb to describe pressing mouse buttons.

Rationale

Tactile feedback is widely used in computer keyboards. Historically, many switches offered a simple linear response (a smooth feel), but the vast majority of keyboards produced today are tactile. A key objective with tactile feedback is to provide feedback to the operator. Part of the idea is that you can stop pressing a key once you have received the tactile sensation, such that keys do not have to be pressed all the way down. However, with typical rubber dome over membrane keyboards, the overtravel distance is too short for this to make much difference; conversely, the soft landing of a rubber dome keyboard does at least soften the keystroke impact, which can be a hard stop in other switch types.

Linear switches can suffer from a “mushy” feel due to the continual rise in force as the key is pressed. Causing the force level to drop off during the keystroke, either suddenly or gradually, reduces the stiffness of the key. The “mushy” feel can be resolved instead by making the switch lower in force, but this brings with it the increased risk of unintended keystrokes.

Tactile feel classification

Tactile feedback takes a number of different forms. The most common tactile behaviours are illustrated below with force–travel graphs. The terms used to describe the various approaches are not official terms from any manufacturer literature or any other publications, and were chosen out of need to provide a name for each classification. These graphs do not generally represent any particular switch; rather, they simply illustrate the general principle of each classification. In reality, there is a continuous spectrum of variation across the many decades of keyboard development and production; the classifications and graphs below are merely broad examples to help understand the principles involved. For reference, the plot of a linear switch is as follows:

The y axis of these graphs represents the force exerted by the switch at this position in its travel (or conversely, by the operator’s finger on the key). The x axis represents the travel (motion) of the switch, from fully-released on the left, to fully-pressed on the right. Actuation of the switch typically occurs around the tactile point, although this depends on whether the switch mechanism couples these by design, or simply intends for them to coincide without any guarantee that they will. Few switches do make this connection, with the rest having a separate tactile feel mechanism intended to signal actuation at the same time that this occurs electrically.

The basis for the diagrams below is the force curve measurements taken by Jacob Alexander (see Jacob’s force curves on Plotly) and SPARC (see SPARC’s keyboards page). Please refer to these pages for examples of force curves measured from a considerable number of different switch types.

Obstructive

The most common tactile feel is the “obstructive” method: the keypress is impeded by a rise in force that must be overcome before the switch will actuate:

Although switches with a basic obstructive feel can be highly regarded (in particular “blue Alps”, Alps SKCMAG), these designs carry a drawback that the tactile event fights against the operator. Where the tactile ramp is steep, the sense in one’s finger of overcoming it can be jarring, almost as though the switch is stabbing back. The plunger can also come to hard stop and require a significant and potentially difficult or uncomfortable level of force to push past it. The revisions made to the design of Alps SKCM switches around the turn of the 90s undid some of the fine tuning of their tactile arrangement, with the final form of “white Alps” (SKCMAQ) having a more cumbersome tactile feel over its predecessor “blue Alps” (SKCMAG). In some designs, the tactile peak is a temporary deviation from an otherwise linear force curve, most notably with Cherry MX, which earned it ire from Matias Corporation, when compared with Alps-designed switches and in particular their clones thereof.

Switch types with an obstructive feel include Datanetics DC-50, Fujitsu FES-8 and Fujitsu FES-9 (tactile models), Micro Switch SD (tactile models), Cherry MX hysteresis types (including MX White and MX Blue) and some Alps SKCM. (The overtravel portion of Alps SKCL/SKCM force curves is non-linear due to the nature of the actuator leaf spring. This can result in some of their linear types (including SKCC series) feeling tactile.)

The tactile peak varies widely in steepness and duration. The peak may have a linear period before it drops off, or it may take a parabolic form. Rubber dome keyboards typically have a steep parabola peak.

The severity of the tactile peak can be reduced by removing or significantly reducing the lead-in period:

Some force curves for Alps SKCM switches show this characteristic. Due to age and wear, it is difficult at this time to be clear what Alps’s intentions were, but their 1993 and 1994 catalogues clearly show a very low lead-in for SKCM.

A variation on this general principle is the obstructive feel with dip. Here, following the tactile peak, the force drops below where it would be on a plain linear plot, before rising up and returning to a regular linear path:

This approach is used with Cherry MX Brown and Clear, and it increases the tactile drop. It could be argued that this approach is also used with Cherry ML, although the ML force curve is quite unusual and does not follow standard patterns. The underlying ML tactile feel is superlative, but the high level of friction seemingly inherent in the design undermines what would otherwise be one of the best tactile designs.

In general, obstructive feel is not an inherent poor design, but careful attention is needed to balance out the forces, so that the switch does not feel like it is trying hard to stop itself from being pressed, and does not have a jarring feel.

Stepped

“Stepped” feel is most associated with stacked-spring switches, in particular Futaba MA. It is also the behaviour of Alps KFF, IBM buckling spring, snap-action Fujitsu leaf spring, Olivetti snap action and the unintentional behaviour of linear Alps switches using their so-called “switchplate” contact modules. In “stepped” switches, the force increases linearly up to the actuation point, at which it drops instantly, and then resumes increasing linearly. The rate of increase may be greater after the actuation point that it was before it, as is the case in Alps KFF and Futaba MA.

Stepped feel tends to have a comparatively lower level of tactility versus other methods.

Dip

“Dip” feel involves a temporary reduction in tactile force. It is similar in princple to stepped feel, especially if there is a permanent reduction in force following actuation:

However, its scarcity makes understanding its relationship to other types impossible. This behaviour is known from Marquardt’s unidentified tactile type. It may also apply to Datanetics DC-60. Virtually nothing is known about the tactile form of DC-60: only the tactile type was advertised, but all known customers—in particular HP and Fluke—always ordered the undocumented linear types. The one tactile sample provided by Meryl Miller of Datanetics has a very strong and smooth non-impeding tactile feel, and it seems to be significantly more tactile than the force curve shown in the catalogues would suggest. Without an independently-measured graph available, or any other tactile examples, there is no way to determine whether this one switch has a force curve that matches that shown in the catalogue, or what the production switches felt like.

Parabolic

The parabolic force curve is one where the force reduces in a downwards curve, instead of increasing. Burroughs Opto-Electric is the only known switch type with a pure parabolic force curve:

Other parabolic switches have a linear overtravel curve. IBM’s beam spring, Marquardt “butterfly” and Omron B2H-F7W have a stepped drop in force as with pure stepped designs:

ITT ETL18 instead has a knee point into the linear overtravel:

Having the force remain continuous (non-increasing) or decrease during travel causes it to feel like it is “pulling in” the operator’s finger: the switch works with you rather than against you, compared to obstructive designs. This yields a “bouncy” feel. Parabolic force curves are rare, perhaps because fewer switch designs can produce it. It is not known to be used by any current designs (as of November 2020) but Input Club’s forthcoming Silo Beam switch is designed to replicate the feel of IBM’s beam spring, and will reintroduce parabolic feel into modern keyboard manufacturing.

Progressive rate

Progressive rate is not generally an intended tactile feel, but simply the effect of certain designs of switch. In particular, RAFI RS 76 M illuminated exhibits this behaviour. At the point of actuation, the rate of increase of force itself increases:

Detent

A small number of switches have a hard stop during switch travel, after which the level of force is distinctly higher than it was before. These include Tokai MM9 series - Deskthority wiki, Robotron dual-spring contact, and RTA 80 Teletype (all determined from force curves measured by Jacob Alexander):

It is not known whether this type of tactile feel was intended, or was simply a consequence of the design of the switch. Jacob’s Stackpole Lo-Pro chart shows it to be double-stepped.

Wave

“Wave” feel is represented by a shape resembling a sine wave:

“Wave” feel is used by Topre electrostatic capactive, as well as being the officially-documented force curve for Cherry’s NTK, ATK and LPK rubber dome types. More typically, the “positive” portion of the curve is steeper and wider, and the “negative” portion shorter. Very few independently-measured graphs exist for rubber dome keyboards, so it is hard to understand these in the same level of detail.