Haptic Feedback Device for Prosthetic Hands
How do you convert the sense of touch into something else?
Background
This was for a semester-long project for Haptics for Human-Robot Interfaces with Dr. Jeremy Brown. The goal of the class was to come up with a useful haptic feedback device using what we had learned in class.
As a team, we decided to look into finding a way to regain the sense of touch. We found that people with peripheral neuropathy (a condition where people lose sensory function in their hands and feet) and those with prosthetics have a hard time gripping objects because they do not have the force feedback to maintain a grip. So, tasks like holding a cup can be difficult because they do not know how much force to apply to the task. They will either squeeze too hard and risk breaking the object or grip too weakly and it will fall out of their hands. To account for this, they need to visualize what they are holding at all times and get visual feedback that they are gripping hard enough.
One problem is that humans have a lot of nerve endings on their fingers, which means that they can discern much finer things. However, there isn't another place on the body to relay this sensation to other than your tongue and that alone might be a weird feeling. So, how do we go about translating the sensation of touch to the user?
The Idea
What if we converted the sensation of touching something into a squeeze on the arm? The arm has some touch receptors, not a ton, but if you squeeze it, you can feel varying levels of force. So we came up with the idea of devloping a coversion of the force exerted by the fingers into a squeeze on the arm.
The Design
The design is actually pretty simple. A force sensor is added to the index and middle fingers as well as the thumb. They are then wired to an armband that houses the microcontroller and stepper motor. The stepper motor has a spool of string wound to it that is connected to the band of the armband. When the user squeezes something, the armband will tighten a proportional amount.
Testing
Since there wasn't anyone we knew who could actually test out our device, we had to get a little more creative with the design. We could use people who still had their sense of touch by taking away their proprioceptive feedback. One idea was to use a rubber sleeve over the finger but another idea was to get a fake hand and have the user hold it. This would "take away" their sense of touch. The next thing was that we had to take away was their vision, this was easily done by making them move the hand around inside of a covered box. This way, they couldn't visualize what they were touching and instinctively modulate the amount of force they used.
We then put items in the box with varying levels of stiffness and had the users try to feel around the object. For something soft like a stress ball, they were able to feel a light squeeze but for something harder like an apple, they had to press a little harder and felt a harder squeeze. People lined up to try it out and even our professor had fun with it. We even got featured on the hopkins engineer site for our project!
Here's what the prototype looked like in action: