threephase effects - diglet48/restim GitHub Wiki

To use the tricks described here you need hardware capable of generating pulses on both channels with identical phase. Most pulse-based devices cannot do this.

Precise position control

We will look at a simple model of a hexagonal body with three flat electrodes.

When sending a pulse on the left channel, current flows between the neutral and left pad.

There are multiple current paths. Most current will flow along the path with the least resistance. This will result in hot spots, in this case concentrated along the corners of the pad.

But if we send a pulse on the left and the right channel at the same time, the current density on the top pad becomes more uniform.

By playing around with this effect the sensation can be moved around much more precisely than is possible on devices without overlapping pulses.

I implemented a special mode in Restim to disable these tricks for a side-to-side comparison, the improvement was evident.

Pulse-based devices

Restim uses an advanced signal generation algorithm to generate a moving sensation. This algorithm only works with sine-wave signals. We believe a pulsetrain of sufficiently complicated monophasic pulses might be able to imitate the sensations from a threephase setup. However, this needs more research.

Differences in sensitivity

In most cases we won't have a perfectly hexagonal body, with pads of identical size, attached to three positions with the same nerve sensitivity.

Let's suppose that the nerves at electrode 'L' are much less sensitive than at the other pads. Like before we will send a pulse on the left channel:

See the problem here? If the nerve sensitivity at the left pad is low, then we will reach the pain threshold at the neutral pad before we will feel strong sensations on the left pad.

The solution is again to send pulses on both channels at the same time. With three electrodes, the maximum difference in nerve sensitivity that can be overcome is 1:2. With four electrodes, 1:3, etc.

Electrode size is also a factor.

Calibration

Differences in electrode placement and nerve sensitivity make scripting very difficult. Restim uses a calibration process to correct for these difficulties, when writing scripts for Restim you can assume each pad has identical area and nerve sensitivity.