Keyboard switch forces
Contents
Overview
“Force” in relation to keyboard switches refers to the force that exists between the operator’s finger and the key while the key is being pressed or held. The operator exerts a downward force on the key, and the key exerts an equal force in the opposite direction. The notion of switch forces is counterintuitive due to way that humans perceive them compared to what really occurs.
The force that exists between the operator’s finger and the key varies while the key is being pressed. Depending on the design of switch, as the key is pressed the force can increase, decrease, decrease and then increase, or increase and then decrease. The change in force during a keystroke, in conjunction with the switch’s travel distance—how far the key moves—is what gives each type of switch its characteristic feel. When measuring or documenting the forces of a switch, there are a number of specific points during the keystroke that are of some significance:
- Operating force
- Depending on the manufacturer’s terminology, the force required to actuate the switch (how far the key must be pressed before it registers a keystroke), or the force required to fully press the key.
- Total travel force
- The force required to fully press the key.
- Preload
- The force that the return spring exerts when the key is released. This is the minimum amount of force that must be pressed before the key will move.
- Tactile force
- The force required to clear the tactile peak of the force curve. This is often greater than the actuation force.
Broadly, switches can be classified by the way that the force changes during the keystroke:
- Linear
- The force increases steadily as the key is pressed: the force between the operator’s finger and the key is linearly proportional to how far the key is pressed. That is, if an increase in force of 20 cN causes the key to depress by 1 mm, then increasing the force by another 20 cN would result in a further 1 mm of movement.
- Tactile
- There is a non-linear change in force designed to signal that the keystroke has been registered; this can also be accompanied by an audible click.
Force graphs
The change in force that takes place when pressing a switch is universally presented using a force graph, also known as a force curve. These graphs plot the travel of the switch (vertical motion, from released to fully pressed) against the force present. Official force graphs can be found in some manufacturers’ literature as well as in a number of switch patents. These official graphs are indicative only and are often quite different from the genuine behaviour of the switch. Force graphs have also been measured by a number of people within the keyboard enthusiast community, including Jacob Alexander (see Jacob’s force graph listing and SPARC (see SPARC’s keyboard page).
The graph below depicts the official and measured force curves for Alps SKCLAR, the standard Alps linear switch in the SKCL range in the 1990s. The official graph comes from the 1993 and 1994 Alps catalogues, while the measured graph is Jacob’s Alps SKCL Yellow force graph. You will notice that the two correspond well.
Although SKCLAR is a linear model, the technicalities of the switch operation result in deviations in the force curve. In particular, there is a drop in force following actuation. The extent to which this is noticeable varies between models and batches, but generally it is not noticeable as tactile feedback.
By comparison, the Omron B2H-F graph published in JEE (Journal of Electronic Engineering, 1978) bears rather less resemblance to the measured behaviour of Omron B2H-F7W. Again, the measured graph is from Jacob: B2H-F7W force graph.
This is an intentionally tactile model, with a much clearer drop in force following actuation. In this instance, there is a strong audible click accompanying the tactile sensation.
Force classifications
Linear switches
Human perception can suggest that holding a key mid-way down involves no force, and only pressing the key further involves more force. In the case of “linear” switches—those with a linearly proportional increase in force with distance pressed—this would mean that the force does not change throughout the keystroke. In reality, the further down you press a key, the more force you are exerting against it. Holding a key mid-way down means that you are maintaining a constant force against the key. Increasing that force by a fixed amount causes the key to move until the spring or rubber dome inside is compressed to the point that its internal force—acting to push the key back up—is in equilibrium with the downward force.
The force required to press a key by a specific amount can be visualised by way of a force graph, a plot of the force exerted versus how far the key moves. An example graph of a linear switch is shown below:
Although not a scientific means of quantifying (measuring) the forces involved, stacking coins onto a key is a good visual demonstration of the forces involved. The collective weight of the coins is a downwards force, and the switch will depress until the compression of its internal spring or dome results in a corresponding upwards force.
Tactile switches
If the switch contains a mechanism that causes a reduction in force, the force being exerted against the key is suddenly much greater than the internal spring’s upwards force. The result is an overshoot, where the key drops rapidly. If there is no further point at which the switch is able to exert that much force, the key will seem to immediately drop to its fully-depressed position. The key is not pulled downwards; rather, the operator continues to exert an amount of force greater than the internal spring can match, regardless of how much the spring is compressed.
The graph below shows such a situation. The preload is 30 cN: the operator must provide at least 30 cN of force before the key will move at all, as this is the force resulting in the internal compression of the return spring or dome. (This force prevents keys from feeling too loose or being able to rattle.) A total of 70 cN of force is required to push the key a little under half way. Just before actuation, the internal mechanism releases much of the force; this could be due to magnetic release or from clearing a special tactile spring. By the time the switch registers, the force has fallen to only 50 cN. This leads to a point of confusion with manufacturer specifications, because a 50 cN “operating force” would be misleading: yes, a 50 cN holding force is needed to maintain 2 mm travel, but a higher 70 cN force was required to clear the tactile point.
If the operator were to immediately reduce their exertion from 70 to 50 cN immediately after clearing the tactile point, such as by pressing the key very slowly, they would barely perceive a difference: tactile switches pressed slowly feel linear. However, human reaction time is not fast enough to respond in this manner: the operator will continue to supply 70 cN force for long enough that the key will follow the blue line on the graph: it will drop rapidly until the key’s internal spring is providing 70 cN of force in return. This overshoot gives the user a tactile sensation that the key has registered, although in most switch types there is no mechanical correlation between the two, and the switch can even register before this point.
Tactile feedback takes many forms, and is covered separately.
It is worth mentioning that the term “tactile” has more than one connotation. To keyboard enthusiasts, strictly speaking “tactile” describes a switch that provides tactile feedback with a minimum of additional sound. For example, Cherry MX Brown is a tactile switch, as the tactile feedback does not involve additional sound, while Cherry MX Blue produces a click sound during operation and is thus deemed a “clicky” or “click” type. Click-feedback switches are by nature tactile due to the extra force required to arm the click mechanism. (Cherry MX White sits on the fence, as it is a clicky switch with the click feedback suppressed to leave only tactile feedback and hysteresis, but the click sound is not completely absent.) In contrast, switch manufacturers historically used the term “tactile” (and related terms) to refer to switches with a tactile feel regardless of whether a click sound results from the tactile feedback mechanism.
Force ergonomics
One design consideration that has never been settled, is just how much resistance should keyboard switches offer against being pressed. Like many other considerations, it is a matter of personal preference and accommodation. Keys that are too stiff to press can cause fatigue, while keys that are too soft can end up accidentally pressed. Some people rest their hands on the keyboard, and they expect a certain level of stiffness in the keys to avoid unintended keypresses. Higher forces, especially tactile forces, can be an impediment to fast and efficient typing: the key is trying too hard to resist being pressed, which is not necessarily optimal.
In their article Hall IC Keyboard Switches Become the Leading Type (Yoshikazu Kitao, JEE, August 1978), Omron explain:
Generally, a stroke force from 55 to 65g is desired for equipment utilized by unskilled operators who do not normally operate keyboards. However, a stroke force from 35 to 45g is desirable for operators who have achieved proficiency in operating keyboards, for billing data entry equipment (slip issuing) and for card punching.
Omron’s article in JEE (Journal of Electronic Engineering) describes Omron B2H, a curious design of Hall effect whose tactile feedback is generated magnetically. Indeed, the tactile version of B2H provides comparatively low tactile and operating forces for the time.
Force specification
Historically—prior to the widespread adoption of rubber dome keyboards—basic keyboard switches are linear in nature. That is to say, the force required to press the key can be plotted as a simple y = mx + c graph. Although various designs deviated from a straight line for technical reasons, a linear increase in force during keystrokes was the goal. A few designs such as buckling spring and rubber domes are inherently tactile, while most other non-linear types involved special mechanisms to provide tactile feedback.
The equation of a straight line requires three variables: the gradient (m), the y intercept (c) and x. In a force curve for a pushbutton switch, y represents the mutual force exerted between the operator’s finger and the key, the y intercept c represents the preload (the static force in the return spring when the key is released) and x represents the travel of the switch. Describing the line requires two co-ordinate pairs to be specified, or one co-ordinate pair plus the gradient. For a keyboard switch, the two co-ordinate pairs could be any two of three obvious ones: preload (force at 0 mm travel), actuation force (force at the actuation point) and total travel force (force when the key is fully pressed). Unfortunately, a significant proportion of manufacturers only provide one co-ordinate pair, described as “operating force”. Unfortunately, the term “operating force” is not consistently defined. Some manufacturers use it to mean the force at the point of actuation—frequently but not always at half travel—and others use the term to mean the force at full travel. The reader is generally left to guess which of these two terms is meant.
The graph below demonstrates the difference in gradient resulting in changing the assumption of whether “operating force” is the actuation point or total travel point. Preload has been assumed to be 30 cN; quite likely this would not be known.
Different assumptions about the preload will also result in a difference in switch stiffness, as illustrated in the next two graphs:
Even knowing all three forces (preload, actuation and total travel) are not sufficient without knowing the switch’s total travel:
The graph above demonstrates that the force figures alone are not necessarily comparable between different models of switch, as manufacturers provide a range of travel distances from as low as 2.5 mm up to 4.5 mm or more. Laptop keyboards can even have travel distances below 2.5 mm.
For linear switches, it is preferable that the manufacturer publish a force graph or provide two complete co-ordinate points on the force graph. For tactile switches, the manufacturer force graphs are not always accurate, and third-party measurements of force curves provide a significantly clearer picture.