Keyswitch issuesKeyswitches are central to the feel of computer keyboards.The Kinesis keyboard and the X-Keys devices both use Cherry MX soft tactile feel switches. These switches activate before the bottom of their travel is reached. The ML keyswitches (designed for laptops) appear to have reduced activation travel and reduced activation force - compared to the MX versions.
The upper lines in the force vs distance diagram show how the key displacement changes according to the applied activation force. The height of the first peak gives an indication of how hard you need to press a key in order to activate it. The both keyswitches 'bottom out' with similar forces to those that are required to activate them in the first place - which is not ideal. Both designs bottom out in a manner resembling running into a brick wall. Not all keyswitches do this. IMO, Cherry need to rethink their approach here - if the keys are going to bottom out so close to the activation point. From these force diagrams, the ML key switches look as though they would be better to use in a health-conscious keyboard - due to the lower activation force and shorter travel - and despite the fact that they bottom out even earlier than the MX ones. However, having tried them, they seem to be slightly worse than the MX keyswitches. The constraint of small size has resulted in a number of compromises. The ML keyswitches have incompatible keycap interface to the MX ones - and need to be board-mounted. Some keyboard manufacturers seem to be using this type of laptop keyswitch in their products - e.g. the [Plum keyboard] and the [Devlin SpaceSaver range]. The [IceKey] keyboard also uses low profile laptop-style keyswitches as does the [TypeMatrix] keyboard - though these are both scissor/membrane type. Both types of Cherry keyswitch have activation forces which seem to be harmfully high. The forces involved to activate them are far greater than what is necessary to prevent accidental keystrokes. Yet these are supposedly some of the best keyswitches on the market. Keyswitch designers: please provide us with the option of reduced prestress and lower activation forces - without the users having to dismantle the switches and perform surgery on their springs.
Keyswitch activation forcesI've measured the average activation force for a number of my keyboardsThey are ordered by activation force in the following table:
Minimising activation forcesThe activation force is not the only factor that causes tissue damage with repeated use - but it seems to be a very important factor.Other factors include: the activation travel distance, the maximum travel distance and the way in which the force changes as the key reaches the end of its travel. This paper explains the situation reasonably well: [Computer key switch force-displacement characteristics and short-term effects on localized fatigue] [PDF]. The basic idea is that damage is caused by the tendons sawing into the flesh that surrounds them, and - as with a saw:
damage = repetitions x sin theta x distance x force
All the commercial keyboards I have used ship with activation forces which I find to be much too high. It is clear that some people realise the significance of high activation forces in causing fatigue and cumulative tissue damage disorders. [Datahand] produce low force devices, and [Kinesis] advertise a number of their devices as having low activation forces - e.g. see [here]. However it is equally clear that others do not understand. This essay on [The Cause of RSI] suggests low activation forces are the cause of RSI. Also, Cherry write on their web site:
Some specialists still believe that typing on an electronic keyboard is less damaging to health than typing on a mechanical typewriter since strokes need less force. This has proven to be wrong. Especially, work at a computer screen has shown that the force required for a movement is less of a problem - it is the high movement frequency that is a principal reason for RSI complaints. The minimization of the force needed to operate an electronic keyboard has enormously increased typing speed, and there is no longer a pause to execute a carriage return or insert a new sheet.This kind of material makes one wonder whether they understand the importance of minimising activation forces. Low force microswitchesI discuss the use of low force microswitches as keyswitches on a separate page.
Tactile feedbackI rate tactile feedback highly. Auditory feedback seems to be of markedly inferior quality.I hypothesize that direct spinal circuits can be formed to deal with relaxing upon encountering tactile feedback - based on existing withdraw-on-finger-prick circuitry. By contrast, auditory feedback necessarily has to travel via the brain - which is slower - and thus less effective. Often good quality tactile feedback seems to require high activation forces.
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Other low force switchesDataHand keys are reputed to be low force (that produced by a 18-22g weight).The Touchstream keyboard was low force (that produced by a 5g weight). However there was also very little tactile feedback. The "Bloorview Miniature Keyboard" had a low activation force (that produced by a 30g weight). The [CD Switch, model 90] has a low force (that produced by a 40g weight). A membrane sensor called a [MicroNavRing] has a low activation force (that produced by a 20g weight). A membrane sensor called SensorButton15 has a low activation force (that produced by a 35g weight). Some IBM Model M keyboards are reputed to have some of the lowest activation forces available in a full-travel keyboard (that produced by a 35-40g weight). However, my own IBM "buckling spring" keyboard does not bear witness to this positive reputation. Some of the best switches I've seen in a commercial keyboard were those in the keyboard used in the Apple MacBook Pro laptop computer. It seems to me that touchscreen devices - such as [Apple's iPhone] have considerable potential for decreasing activation forces further.
Spring surgeryI dismantled some of my keyboards and tried my hand at some spring surgery - in an attempt to decrease the keyswitch activation forces manually.The surgery was successful. More details are on the keyboard surgery page.
LinksThis siteFrom here you can go back to Tim's keyboard page.The page about low force microswitches is related to this one.
KeyswitchesCherry MX keyswitches (Cherry site)Cherry ML keyswitches (Cherry site) Photos of Cherry MX keyswitches Photos of Cherry ML keyswitches Photos of IBM buckling spring Maltron on Cherry MX keyswitches Spring surgery for Cherry MX keyswitches Caps for Cherry MX keyswitches Cherry catalog - with many keyswitch force diagrams Keyswitch discussion Keyboards with microswitches
General switchesSwitches for sale - UKLCD switches Cherry distributors Cherry switches - UK OtherCherry Cymotion - Cherry ATKCherry Office XPress - Cherry ATK Kingston Studioboard Mechanical USB - UNK DatadeskTech keyboards - UNK Mouse microswitch surgery |
