Force feedback devices resist miniaturization, because they require physical motors and mechanics. We propose mobile force feedback by eliminating motors and instead actuating the user’s muscles using electrical stimulation. Without the motors, we obtain substantially smaller and more energy-efficient devices. Our prototype fits on the back of a mobile phone. It actuates users’ forearm muscles via four electrodes, which causes users’ muscles to contract involuntarily, so that they tilt the device sideways. As users resist this motion using their other arm, they perceive force feedback.
Figure 1: Our prototype electrically stimulates the user’s arm muscles via the shown electrodes, causing the user to involuntarily tilt the device. As he is countering this force, he perceives force feedback.
The device renders force feedback by tilting itself against the user’s will. It achieves this by stimulating muscle tissue in the user’s arm though the electrodes, triggering an involuntary contraction. This causes the user’s arms to tilt sideways and thus the device to tilt. In a videogame where the player's airplane is controlled by tilt (Figure 2), the involuntary tilting represents winds that threaten to derail the airplane. To stay on course, players counter the actuation using the force of their other arm (Figure 3). As we find in Study 2, players perceive this as force feedback.
The prototype is based on a medically compliant signal generator and use is pain-free.
Figure 2: Close-up of the video game from Figure 1: wind turbines create side-winds that derail the airplane. This is rendered as force-feedback by the device.
Figure 3: (a) As the user is playing (b) muscle-propelled force feedback kicks in, causing the user’s left wrist to tilt the device. (c) The user responds by countering the forces, steering the plane against the wind.
Figure 4: a participant from our user studies experiencing the force feedback sensations delivered by our prototype.