Electromagnetic taxels

(extract from EPFL’s LMTS webpage on haptic displays and from the project final report)

We have developed a scalable and manufacturable fast haptic display with 192 taxels on an 8 mm pitch with a 10 ms refresh time per taxel. The device consists of an array of 12×16 latching electromagnetic actuators using a novel magnetic shield concept, that enables high fill factor, and eliminates cross-talk.
Key parameters of our portable EM haptic display:

  • 12 x 16 independently controlled taxels
  • 10 milliseconds refresh time per taxel (enables vibration and rapid update of static images)
  • 8 mm pitch between taxels
  • 0.8 mm vertical travel
  • 200 mN holding force (ie, easy to feel)
  • latches in both up and down states

The user explores the pattern on the haptic display using his fingertips. Our goal is not oriented to Braille reading, but to provide graphical information such as geometrical figures, maps or even artwork.

Diagram and key elements assembly of the 4×4 haptic display. a) Schematic view of a single taxel and the main actuation components. b) Photo of the 6-layer PCB containing the array of planar coils. The PCB is placed on an aluminium plate supported by four standoffs. c) Top view of the magnetic layer. It is formed by the 16 moving pot-magnets, a perimeter line of fixed pot-magnets, the top and bottom elastomer membranes (not visible) and an acrylic holder. d) A 3D printed pin interface completes the device as a final layer, and is what the user touches.

 

Load force vs. displacement curve for one taxel, for the two studied configurations. In the pulling configuration (blue lines (1) and (2)), the up-taxel state is dominated by the spring effective constant of the membranes. In the pushing configuration (red lines (3) and (4)), up-taxel state reflects the magnet/coil repulsion force. The colored zone around each experimental curve indicates the measured dispersion for the 16 taxels of the array.

 

Image of the six symbols used for the perception test. The images correspond to the device set to the pushing configuration, meaning that non-actuated taxels are in their down state, while actuated taxels are in the up state.

References:

Zarate and H. Shea, “Using pot-magnets to enable stable and scalable electromagnetic tactile displays”, IEEE Transactions on Haptics (2016) doi: 10.1109/TOH.2016.2591951