In particular, the passive brake consists of a magnetorheological fluid flowing between intertwined bell-shaped walls, whose viscosity can be tuned arbitrarily by controlling its phase transition from liquid to solid (and vice versa) by a magnetic field (from a built-in coil). While the liquid phase offers no resistance to the knob's rotation, the solid phase ensures a solid stop.
The prototype originated as follow up work from prof. Carlos Rossa's thesis "A hybrid actuation system for haptic interfaces", which already hinted at the device's construction with fluid gaps arranged between dome-shaped enclosures.
Interviewed by eeNews Europe, Moustapha Hafez, head of the Sensory and Ambient Interfaces Laboratory at CEA List gave us more technical details about the device.
"Because we can control the passive brake electronically, we can create virtual hard stops, to simulate a collision or provide progressive resistance, and anything in between. Due to the very nature of the magnetorheological fluid we use, there is hardly any friction in the off-state, something in the range of 2mN/m. But if we apply 1.5A to the coil, the magnetic field turns the fluid solid and this locks the knob with a force of 2N/m. That's three orders of magnitude between the "on" and "off" force couples" Hafez explained.
But how fast is the knob's response time? We asked.