String Theory Applied to VR

Tying a string around one’s finger is traditionally done to serve as a visual reminder of something important. But a pair of researchers at Carnegie Mellon University believes that string may be the key to better and richer haptic experiences in virtual and augmented reality. The team is working to address a major limitation of existing haptic interfaces — they typically can only reproduce one type of sensation, such as the feeling of textures. That makes for subpar virtual experiences that are anything but fully immersive.

The best way to get multi-dimensional haptic effects today would be to combine several different systems and use them simultaneously. However, in practice, this type of solution is very cumbersome, not to mention expensive. And for many applications, it is impossible to do at all. As a better solution, the researchers have introduced what they call Reel Feel. It is a wearable haptic interface that attaches an individual string to each finger. By pulling on, or loosening, each of the strings independently, a wide range of sensations can be simulated.

Mounted on the user’s shoulder, the Reel Feel system uses high-torque brushless DC (BLDC) motors to pull strings tethered to the fingers. By adjusting motor torque and speed programmatically, the system can convincingly render five distinct types of haptic effects: rigid geometry, haptic animations, impulsive forces, surface compliance, and fine-grained spatial effects. This modular approach allows these sensations to be felt individually or blended together to simulate more complex tactile experiences.

One of the key design goals was to eliminate the bulky, glove-based wearables common in prior haptic systems, which can be uncomfortable and fatigue-inducing. Instead, Reel Feel offloads nearly all of the system's weight to the shoulder — a location found to be ideal for wearable devices due to its comfort and support. Each of the five strings extends from a motorized spool housed in a compact enclosure weighing 710 grams.

For motion control, the system uses GM3506 gimbal motors, which are a type of BLDC motor popular in drones and camera stabilizers. These motors offer a strong torque-to-weight ratio while weighing just 64 grams each. The motors are capable of continuous operation with smooth motion, which is critical for producing realistic haptic effects. The fishing line used as string is two meters long, ensuring a wide field of motion without restriction.

Each motor is paired with an AS5048A magnetic encoder to monitor spool rotation and maintain accurate force feedback. In earlier prototypes, the system relied on mechanical limit switches to detect slack in the strings, but these were abandoned in favor of tracking the user’s fingertips using cameras on the VR headset. This improved both comfort and performance, ensuring that the string tension remains consistent without introducing distracting tactile artifacts.

Driving the hardware is a custom-built control board featuring a Teensy 4.1 microcontroller running at 625 MHz. The microcontroller communicates with a PC over USB 2.0 and handles high-frequency updates from VR applications developed in Unity, running at 70 frames per second. The microcontroller interfaces with DRV8313 motor drivers, each capable of controlling three-phase BLDC motors.

Altogether, the prototype costs approximately $240 in parts, with the majority of the expense coming from the motors and encoders. The researchers estimate that, with scaled production, the cost could be reduced to under $200 (and possibly even below $100) making the technology a promising candidate for consumer-grade VR haptics.