Not So Fast!

Getting zapped by electricity is not exactly the most popular choice as a force feedback mechanism at present. However, since electrical muscle stimulation (EMS) miniaturizes force feedback systems a great deal when compared with other technologies, and does away with restrictive instrumenting of hands or wrists, it is emerging as a possible better path forward.

EMS-based force feedback has some undesirable qualities holding it back, however. It is imprecise, for example, in that it is not able to actuate a specific finger. Current runs through muscles adjacent to the target, causing them to also move. Further, it is not possible, with any degree of accuracy, to stop movement at a specific point. To attempt to do so requires the stimulation of opposing muscles in precise ways, and in practice, this leads to highly noticeable oscillations.

University of Chicago researchers have developed a device, called DextrEMS, that works with existing EMS systems to overcome these problems. DextrEMS takes the form of an exoskeleton for the hand that uses a braking system to independently lock fingers into precise positions.

A 3D printed mechanism was made with a clear resin, and hinges were laser cut from clear acrylic to minimize visual obstruction of the real world. Brakes, which are present at each finger joint, use a custom-made ratchet and pawl mechanism to lock fingers into position. A small DC motor moves the pawls into place to activate the brakes. The teeth on the current ratchet allow for locking of brakes in fifteen degree increments, but the precision could be increased as needed by designing new ratchets.

A Nordic Semiconductor nRF52811 microcontroller handles motor actuation as well as Bluetooth Low Energy communication with external devices. A custom printed circuit board with supporting components and a LiPo battery pack round out the remainder of the device.

DextrEMS was evaluated by using EMS to cause a wearer’s hand to perform the American Sign Language fingerspelling of the letter “K”. With EMS only, unintentional actuation of fingers adjacent to the targets was seen, and highly noticeable oscillations were present in the target fingers. Using DextrEMS, non-target fingers were locked, preventing the appearance of undesired actuations, while target fingers were held firmly in place once they reached their desired positions.

To demonstrate some applications of the technology, the team also created applications that allow the exoskeleton to perform as a training device for playing both the guitar and piano, assisting with proper finger placement and motions. In another demonstration, they designed a virtual reality whack-a-mole game in which DextrEMS gives the impression of changing shapes of mole heads through force feedback.

You may be wondering why one would go to the trouble of using an imprecise actuator such as EMS, then designing a braking exoskeleton to correct the imprecision. Would it not be easier to simply create an exoskeleton that actively moves fingers into position, without any need for a zap from EMS? Most likely, yes, that would be simpler, but with a tradeoff. An active exoskeleton would come at the expense of additional weight and bulk.

The DextrEMS team certainly achieved its goal of adding dexterity to EMS systems. It remains to be seen, however, what level of acceptance users will have for getting zapped, even if the system does perform well.