Compliant keyboard switches

2023-09-25

I am generally interested in input devices for computers. Earlier this year I made (assembled) an ortholinear 36 key split keyboard – a relatively common setup. Building these keyboards is expensive, especially when you have to source all the parts individually. In my case, the minimum viable keyboard required two controllers (RP2040s), two mainboards, 36 switches, keycaps and hotswap sockets ( you can emit the hotswap sockets if you a sure about the keyswitches), which all totaled to about £120.

I had some experience designing compliant mechanisms from uni, so I wondered if I could significantly rediuce the part count with a 3d printed keyswitch / keycap combo.

Rationale

Because 3d printing gives you ‘complexity for free’, you can make a very complex mechanism which is just as quick and easy to print as a simple one. If you could design an effective keyswitch, you could parametrize the model to make it work in a range of sizes and shapes. You could print them in place as apart of a larger structure – maybe a full sized keyboard or a game controller.

Requirements

A good keyboard makes typing more enjoyable, what makes a keyboard good is up for debate, but I think there are some basic things everyone can agree on:

  1. The key must have a constrained linear up and down motion with an elastic return
  2. The key must be easy to actuate from its corners as well as its centre, and must not bend (think cheap buttons that pitch when you press them from the edges)
  3. The key should not wobble

Mk 1

I replaced the pivots in a parallel mechanism with simple flexures. The key press is quite spongey because the flexure is too thick relative to the bars and feels quite plastic (we want elastic).

Mk.1

Mk 2

I increased the thickness of the bars and used a different hinge design. This flexure is made up of flexures layered perpendicularly to each other, similar to how flexure bearings work. This is a much stiffer hinge for the material used so we can make the flexures thinner and longer, meaning that we get less of a damping effect from the plastic.

Mk.2

Future Work

In order to be a switch, and not a fidget toy, we need some way of registering actuation. It would be nice if I could confine all the electronics to a single printed PCB that coud be glued or screwed to the bottom. There are two methods that come to mind: 1. Optical switches by integrating a light gate into the moving part of the switch, and placing a LED and a light sensor on either side 2. Hall Effect switches by placing a small magnet in the moving part and a hall effect sensor in the PCB

The mechanism is too thick, even for a desktop keyboard. The main thing preventing the design from being miniturized are the parallel sections, but I have a few ideas for a single layer mechanism.