1. Bionic Hand – an Overview

Bionic Hand – an Overview

The first general impression about robots – this is some artificial machine, similar to a human body, and acting similar to us. We even expect talking to us and understanding us. Therefore, such robots are called humanoids. Actually, there are lots of much simpler mechanical robots used in industry. They usually perform only one or some actions (e.g. transferring an item being manufactured from one worker to another), but general principles of operation are the same. 

^{Figure 1. Example of a Bionic Hand}

The situation does not look so complicated, if broken down into parts and looked from inside – recent technological achievements allow us to create a bionic hand ourselves (DIY = “do it yourself”): 

• The “body” parts of a bionic hand could be produced by the use of a 3D printer; 
• There should be some space for mechanical and electronic components inside it;  These “body” parts are usually designed by employing 3D modeling software; 
• The motions are realized by small motors (“muscles”), pulling lines/strings (“tendons”) attached to proper parts of the built hand; 
• The motors are driven from a software program, that runs on the microcontroller.

^{Figure 2. Robot humanoid}

To enhance understanding, students can use Augmented Reality (AR) to explore the internal structure of a bionic hand. Through AR models, students can visualize how motors, tendons, and sensors interact to replicate human hand movements.

• The motions could be initiated by electric pulses existing inside a human body – these pulses are detected and transferred to the microcontroller.

^{Figure 3. Main parts and components of a bionic hand}

The following topics will help us to better understand how these steps are realized, and even to try them ourselves.

Modelling and design

Initially, we shall limit ourselves to simpler movements – only a rotation between two adjacent finger parts. There should be enough space without obstacles for moving strings inside the body, and an opportunity to install them (any internal parts) conveniently. Therefore, “a body part” is expected to be composed from two pieces, having axis holes and space shifts for joining different “body parts” altogether. 

The initial shape for modelling fingers are half-cut cylinders. We need parts to overlap partially in order to be fixed one to each other or make a rotating connection.

^{Figure 4. Composition of a finger} 

The space inside fingers is not enough for motors to be fitted inside (or such micro-motors might cost too much), therefore finger parts are moved by pulling strings, and the motors are usually placed in the wrist or the arm part of a bionic hand. 

 ^{Figure 5. Separated parts of a finger}

So, initially, we need to manufacture all necessary parts of a bionic hand. For this, the first step is to create models of these parts. And after that, the obtained models are used to generate instruction files for 3D printers which help us in the manufacturing process.

3D printing

In general, 3D printing technology allows us to create an object of almost any shape. The technology evolved from ink-jet printing: melted plastic (instead of ink) is sprayed layer by layer according to the instruction file and becomes solid at room temperature. Besides the mentioned way of 3D printing, there are several other technologies that allow customers to produce parts of different materials such as metal, concrete, rubber, etc. 

For example, in the special 3D printing technique to produce metal parts the metal object is generated in the sand dust; the metal is melted into a solid object heated by a laser.

^{Figure 6. 3D printing technology enables to create object of almost any shape}

“Muscles” pulling “tendons”

The movement of a robotic hand is very similar to that of a human hand. The muscle contracts when an electric current passes through it¹. It pulls tendons attached to an adjacent body part. That rotates around a joint connecting these two body parts. 

In a case of robot, it looks similar – a motor pulls a string. In order to deploy a reverse movement, two alternatives can be used: 

• a spring, forcing a body part to an initial position; 
• two-string system, pulling the body part both directions. 

In any of the cases, the motion is realized by special kind of motors, called servo-motors (or servo-actuators). These digital devices move to and stop only in certain positions, - differently to all-time rotating motors.

^{Figure 7. Mechanics, that moves a finger of a bionic hand}

¹ The muscle contractions are controlled by very small electrical pulses, that might be measured by special sensors. These pulses are comming from brain and might be used to contol a bionic hand, attached as a replacement of one, lost during an accident.

Programming and controlling the motion

The target position for motors of a bionic hand is set by a microcontroller – a small “computer” with a software inside. 

There are several important things how the software controls the motion: 

• the program is repeated almost endlessly (e.g. until a special condition is met or power supply is switched off); 
• the program checks the mio-sensor (a muscle sensor) to know when to start pulling string (and which one); 
• if we know in advance what object we would like to take by our bionic hand, then we can instruct the motor about the target position; 

^{Figure 8. Bionic hand holding an object} 

• In most of cases the shape of an object is not known, then we need another sensor at the end of a finger to allow force be checked, not to squeeze the object in our bionic hand. 

  ^{Figure 9. Force sensors placed at fingers‘ end}