Forum

We investigated providing a bowing interface on and off for several years. The o-Box uses the same technology that optical mice use, and in fact you can near enough make one by butchering a mouse I believe. You could probably make one and fit it to an Eigenharp in this way, but you'd need to write an agent to read it and run another wire to carry the data.

The other option is to bow a percussion key directly. We thought seriously about making a different keytop available for these to allow this. It's a bit more complicated than just a different piece of plastic though, for reasons of resonance. The key system has a natural resonant frequency a bit north of 1kHz, and the percussion keys will be similar (they have twice the spring constant, but also a higher mass key). This natural resonance is damped in use by the carbon loading in the rubber light and dust seals that form part of the key assembly, but that damping, whilst sufficient to prevent key resonance in loud live situations, is nowhere near enough to prevent resonance if the key is excited by a device specifically designed to make it resonate such as a viol bow. Resonance is not to be desired in the keys for two reasons. Firstly the natural frequency is too close to the data sampling frequency for comfort, leading to potentially bad effects on the data stream due to aliasing near the Nyquist limit. Secondly, the key springs were a serious design challenge in that it took us two years of hard effort (including much computer finite element stress modelling coupled with countless new designs followed by lifetime testing on a specially built machine in what seemed like an endless task at the time) to get it to the point where I considered it acceptable. We succeeded to the extent that we have never seen a spring failure to date, but moving the system back and forth at 1000 cycles a second is a whole new fatigue regime and one I would strongly advise against as each second could represent quite a serious amount of the key lifetime - there wasn't much leeway in this aspect of the design and it is certainly not conservative enough for me to be comfortable advising you that it would be ok..

If you want to bow the percussion keys I think you'd have to arrange for the key to be mechanically damped so it doesn't resonate. We decided that we didn't have the time to investigate that further, but you could probably damp it fairly simply with the right elastomer (carbon loading is one such way its done) wedged round it, if you want to experiment. You'll know if it starts to resonate, you can normally feel that in a bow easily when using it.

Alternatively, it might be interesting to investigate a non-resonating bow, ie one with no spring in the system. I believe this is also known as a 'piece of stick', to be technical. This might work surprisingly well, if you choose your stick right. Just watch out for that resonance, you don't want it.

John

Actually, if you can suppress the resonance effectively it will work - the key would act as a force sensor, not a movement sensor. The friction of the 'bow' passing over the key surface produces a force, which produces a deflection in the key proportional to the speed and downforce of the 'bow' surface, with the addition of a datastream being available from the other axis, which would correspond to the angle of the force, so roughly equivalent to the effect of lying the bow over, which is a commonly used string technique.

The O-Bow works by measuring the movement directly and then producing a first differential to drive whatever parameter it drives, effectively producing the same signal as measuring force, making it analogous. Wooden bows on strings drive a Helmholtz oscillator (aka a 'stick slip' oscillator) in the combined system, which mixes both downforce and speed in a more complex manner than the O-Bow would be capable of as it stands but that the Eigenharp could possibly get nearer to by dint of the multiple axes in the sensor.

If anyone fancies doing some experimentation I suspect some fun could be had playing with this.

John

If resonance is adequately damped and the forces on the key do not exceed their normal values (+/- 8N for the percussion keys) then I would think not.

No, you are quite correct, and I was wrong. I was stupidly thinking of the work done in time, which does of course rise with speed, but that is dissipated as heat in the contact area and is not useful to us.

Using a key as a sensor will still give you two parameters of measurement, the angle of the bow from side to side, and the down force (which will both directly change the key pressure and also the key deflection as the frictional force will then rise as it is a largely linear relationship between the normal force and the coefficient of friction.

John