How to measure and analyse the texture of food, cosmetics, pharmaceuticals and adhesives.

Tuesday, 7 August 2018

Testing the Behaviour of a Keyboard using a Texture Analyser

Testing keyboard actuation force
When performing an objective test on a keyboard, the main components under investigation are the switches, actuated by pressing keys.

From keyboard to keyboard, there is little a given manufacturer can do to affect a switch's performance. The switches come in batches from switch makers, and keyboard manufacturers mount them onto printed circuit boards and do not have the ability to alter them. Seth Colaner at Tom’s Hardware has developed a procedure for testing mechanical keyboard switches using a TA.XTplus Texture Analyser.

Previous attempts at testing switch performance has involved the use of small weights, but this is not an accurate technique. Maintaining the balance of multiple weights placed on top of keys with varying geometry affects the measurement as it is easy for weights to move off-centre. Additionally, the forces required to actuate a switch can be very small (on the order of 50g), so the way even small weights are placed can fluctuate the actual load and throw off the measurement.

Other attempts have involved the use of callipers to measure the distance to actuation, switch travel and depth of the rebound. This can be more accurate than weight dropping, but the distances involved are so small, human error plays too large a part. As well as this, callipers cannot always reach all the keys on a keyboard.

A disadvantage of both of these techniques is that one only measures force, whereas the other only measures distance. To fully characterise the mechanical performance of a keyboard, data for both these parameters is necessary. This is where instrumental testing using a Texture Analyser steps in. Texture Analysers are used across almost every industry involved in manufacture or R&D, but they are also ideal for keyboard testing. 

Having previously had success in the measurement of the actuation of personal hair products with a finger pump, it is a simple transition to keyboard actuation. Both applications are simply measuring the force, distance and time of a finger’s movement when pressing down on an object, and the point on the curve at which either a product is expelled from a container or when a key press is registered on the computer.

To perform the test, the keyboard is placed on the instrument base of the Texture Analyser and a cylindrical metal probe with a slightly domed end is moved down to press a key and then retracts. Exponent records this data into a force/distance/time curve of both the press and release for further analysis. The test can be run on every switch on the keyboard, and at different points along the key if there are stabilisers (such as the spacebar).

It is important to position the keyboard so it does not move around during the test. Additionally, it is a good idea to press each key manually multiple times before running any tests to ensure that all key caps are firmly seated – sometimes key caps on shipped keyboards are slightly loose, and loose key caps return test results that appears as outliers.

Finding the correct test parameters can take some trial and error. The target force and distance must be set so that the key travel completes, but the force should be limited so the keyboard is not damaged. The test speed also requires some consideration – a moderate speed that allows for all details of the travel to be recorded is ideal. Too slow and the noise on the data will be higher and the test will take too long. Too high and some detail may be missed.

There are some key terms used to describe switches:

Pretravel: the distance from the beginning of a keypress to the actuation point.
Actuation point: the point at which the switch triggers a keypress event. The pretravel ends at the actuation point.
Actuation force
: the amount of force required to push the switch down to the actuation point.
Tactile point (depending on the switch type)
: the point in the key travel where the tactile bump is located.
Tactile force (depending on the switch type)
: the peak force required to push the switch past the physical “bump” in the key travel.
Full travel
: the distance from the beginning of a key press to the bottom of the key travel.
Bottom-out force
: the amount of pressure required to push the switch to the bottom of the key travel.
Reset point
: the point at which the switch stops registering a keypress. The switch must cross this point after a keypress before the switch can be actuated again.
Key feel
: a description of the relative sensations when operating a switch. For example, if it feels smooth or scratchy, light or heavy, or if the tactile bump is sharp or gradual, strong or weak, etc.
: the distance between the actuation point and reset point on return.

There are some limitations to the measurements made by Texture Analysers as they are unable to measure some aspects of the rebound or switch lifetime. The actuation point (triggering of a digital event) is not necessarily at the same point on the curve as the peak in force corresponding to the tactile bump, so the force peak cannot be relied on to record actuation. Actuation can happen at almost any point in the key travel. However, Exponent can be set up to mark an Event when the key is actuated; this is a point that is marked on the curve when actuation occurs, and can be used in the analysis to measure pre-travel and to compare with the tactile bump.

Switch consistency is important to keyboard manufacturers as it is beneficial to have the same key behaviour with each press. A perfectly consistent key would have the same force curve with each press. The variation of behaviour of each switch (or even variation between switches) can be tracked by comparing key graph parameters of each switch over a set of tests (such as the maximum force of the tactile bump or the pre-travel). Exponent provides automatic sorting of tests into their sample batches in a spreadsheet along with statistical analysis to quantitate the behaviour variation.

As with all products destined for consumer interaction, it is very important to determine a range of acceptability of force, travel and the other keyboard parameters by using a panel of testers. These acceptability limits can be inserted into the spreadsheet in Exponent to automatically specify whether a test was a pass or a fail, and will help with the choice of key types for the keyboard manufacturer for each intended use.

This article was adapted from an article by Seth Colaner | 21-04-2017 |
Tom’s Hardware

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