BIO-MECHATRONICS HAND

BIO-MECHATRONICS  HAND 

                               --HAND REPLICATING THE NATURAL HAND

INTRODUCTION :

          The objective of the present work  is to develop an artifical hand aimed at replicating the apperance and performance of the natural hand.the ultimate goal of thisresearch is to obtaina complete fuctional substitution of the natural hand.this means that the artifical handshould be felt by the user as the part  of his/her part (extended physiologicalproprioception (EPP)) and it should provide the user with the same functions of the natural hand like tactile exploration,grasping, and manipulation (“cybernetic prosthesis).commercially available prosthetic devices,as well as multifunctional hand designs have good (sometimes  excellent ) reliability and robustness, but their grasping capabilites can be improved.

The bio mechatronic hand is made up with the help of a FDM(fused deposition modelling) process.

Fig 1.man playing  using prosthesis hand 

                                  Fig 2. man running with his prosthesis leg.

Prosthesis :

          In medicine, a prosthesis is an artificial extension that replaces a missing body part. It is part of the field of biomechatronics. Prostheses are typically used to replace parts lost by injury (traumatic) or missing from birth (congenital) or to supplement defective body parts.

Mechatronics :

          Mechatronics is the combination of Mechanical engineering,Electronic engineering,Computer engineering,Control engineering and Systems Design engineering to create useful products.

          A mechatronics engineer unites the principles of mechanics, electronics, and computing to generate a simpler, more economical and reliable system. An industrial robot is a prime example of a mechatronics system; it includes aspects of electronics, mechanics and computing, so it can carry out its day to day jobs

Biomechatronics :

          Biomechatronics is the merging of man with machine -- like the cyborg of science fiction. It is an interdisciplinary field encompassing biology, neurosciences, mechanics, electronics and robotics. Biomechatronic scientists attempt to make devices that interact with human muscle, skeleton, and nervous systems with the goals of assisting or enhancing human motor control that can be lost or impaired by trauma, disease or birth defects.It focuses on the interactivity of the the brain with the electromechanical devices and the systems.

How It Works :

          Biomechatronics devices have to be based on how the human body works. For example, four different steps must occur to be able to lift the foot to walk. First, impulses from the motor center of the brain are sent to the foot and leg muscles. Next the nerve cells in the feet send information to the brain telling it to adjust the muscle groups or amount of force required to walk across the ground. Different amounts of force are applied depending on the type of surface being walked across. The leg's muscle spindle nerve cells then sense and send the position of the floor back up to the brain. Finally, when the foot is raised to step, signals are sent to muscles in the leg and foot to set it down.

Biosensors :

          Biosensors are used to detect what the user wants to do or their intentions. In some devices the information can be relayed by the user's nervous system or muscle system. This information is related by the biosensor to a controller which can be located inside or outside the biomechatronic device. In addition biosensors receive information about the limb position and force from the limb and actuator. Biosensors come in a variety of forms. They can be wires which detect electrical activity, needle electrodes implanted in muscles, and electrode arrays with nerves growing through them.

Mechanical Sensors :

          The purpose of the mechanical sensors is to measure information about the biomechatronic device and relate that information to the biosensor or controller.

Kinematics Architecture :

The kinematics of each finger joint is described in the following subsections.

1. MP joint: the proximal actuator is integrated in the palm and transmits the movement through a slider –crank mechanism to the proximal phalanx, thus, providing flexion/extension movement. The slider is driven by the lead screw transmission mounted directly on the motor shaft.

2. PIP joint: the same mechanism used for MP moves the PIP joint. Only the geometrical features in order that the size of the mechanism fits within the space available according to the strict specification of the biomechatronic hand.

3. DIP joint: a four bars link has been adopted for the DIP joint and its geometrical features have been designed in order to reproduce as closely as possible the natural DIP joint flexion. The mechanism has been synthesized according to the three prescribed position method.

Controller :

          The controller in a biomechatronic device which relays the user's intentions to the actuators. It also interprets feedback information to the user that comes from the biosensors and mechanical sensors. The other function of the controller is to control the biomechatronic device's movements.

Actuator :

          The actuator is an artificial muscle. It's job is to produce force and movement. Depending on whether the device is orthotic or prosthetic the actuator can be a motor that assists or replaces the user’s original muscle.

 The biomedical has three fingers to provide a tripod grasp

1.Two identical fingers( index and middle fingers)

2.Thumb for achieving a tripod pinch grasp and cylindrical grasp.

The two micro actuators present in the finger design help it to drive the PIP andMP joints.The index/middle fingers of an artifical hand will comprise of palm housing and three phalanxes.The thumb is the most important finger,because the absence of this finger,makes grasping tasks very complicated. The design of thumb finger is almost same as the two fingers. The only difference is that it doesnot have a distal phalanx.

Architecture of the biomechatronic hand :

At least three fingers (non rolling and non sliding contact ) are necessary to completely  restrain an object.

The  hand performs two grasping tasks :

1.Cylindrical grasp.

2.Tripod grasp.

The finger actuation system based on two microactuators which drive meta carpophalengal (MP) and the proximal interphalengal(PIP) joint.The thumb actuation system is based on microactuators and has two DOF(degree of freedom) at the MP and te interpalengal (IP) joint ,respectively

    The grasping task performed by the hand compromises two subsequent phases:

1.Reaching  and shape adapting phase

2.Grasping phase with thumb opposition

          In phase one,the first actuator system allows the finger to adapt to the morphological characteristics of the grasped object.In phase two ,the second actuator system provides thumb opposition for grasping.

Natural Hands :

          Natural Hands are the chief organs for physically manipulating the environment, used for both gross motor skills (such as grasping a large object) and fine motor skills (such as picking up a small pebble). The fingertips contain some of the densest areas of nerve endings on the body, are the richest source of tactile feedback, and have the greatest positioning capability of the body; thus the sense of touch is intimately associated with hands. Like other paired organs (eyes, ears, legs), each hand is dominantly controlled by the opposing brain hemisphere, and thus handedness, or preferred hand choice for single-handed activities such as writing with a pen, reflects a significant individual trait.

Biomechatronic Hand :

          An “ideal” artificial hand should match the requirements of prosthetics and humanoid robotics.It can be wearable by the user which means that it can be perceived as part of the natural body and should replicate sensory-motor capabilities of the natural hand.This means that the artificial hand should be felt by the user as the part of his/her own body (extended physiological proprioception(EPP) ) and it should provide the user with the same functions of natural hand: tactile exploration, grasping , and manipulation ("cybernetic" prosthesis).

The CyberHand :

Kinea Design LLC :

          Kinea Design's biomechatronic technology forms part of the research being undertaken for the multinational Revolutionizing Prosthetics 2009 (RP 2009) initiative. The firm's research and advanced electromechanical prosthesis project is sponsored by Defense Advanced Research Projects Agency (DARPA)

DESIGN OF HAND PROTOTYPE :

          In order to demonstrate the feasibility of the described biomechatronic approach,we have developed a three fingered hand prototype with two identical fingers (index and middle) and thumb. Actuators, position sensors and a 2-D force sensor are integrated in the hand structure.

The index/middle finger has been designed by reproducing, as closely as possible, the size and kinematics of a human finger. Each finger consists of three phalanges, and a palm needed to house the proximal actuator.

Actuator System Architecture :

          In order to match the size of a human finger, two micro motors have been integrated within the palm housing and the proximal phalange of each finger.

          The selected micro motors are Smoovy (RMB, eckweg, CH) micro drivers (5mm diameter) high precision linear motion using lead screw transmission

The main mechanical characteristics of the linear actuators are listed below

Nominal force	12newtons
Maximum speed	20mm/sec
Weight	3.2 grams
Maximum load(axial)	40 newtons
Maximum load(radial)	25 newtons
Transmission rate	1:125
Gear stages	3

The selected actuator fulfills almost all the specifications for application in the prosthetic finger: small size and low weight. The main problem encountered is related to noise which turns out to be relatively high, at least in the current implementation. Despite of this limitation, we decided to proceed with the application of the linear actuator in order to investigate integration problems and global performance.

          The shell housing provides mechanical resistance of the shaft to both axial and radial loads system. This is very important during grasping task, when the forces generated from the thumb opposition act bon the whole finger structure.

          Due to the high transmission rate (planetary Gears and lead screw transmission) friction is high and, thus, the joints are not back-drivable. This causes problem in controlling accurately in hand. However the positive side effect of friction is that the grasping forces can be exerted even when power supply is off, a very important function for hand prostheses.

POSTION AND FORCE SENSORS  :

Sensors :

          Sensors are used as peripheral devices in robotics include both simple types such as limit switches and sophisticated type such as machine vision systems. Of course sensors are also used as integral components of the robots position feed back control system. Their function in a robotic work cell is to permit the robotic activities to be co-ordinate with other activities of the cell. 

1) TACTILE SENSOR: These are sensors, which respond to contact forces with another object; some of these devices are capable of measuring the level of force involved.

2) PROXIMITY AND RANGE SENSOR: A proximity sensor that indicates when an object is close to another object but before contact has been made. When the distance between the objects can be sensed, the device is called a range sensor.

3) MISCELLANEOUS TYPES: The miscellaneous category includes the remaining kinds of sensors that are used in robotics.

4) MACHINE VISION: A machine vision system is capable of viewing the workspace and interpreting what it sees. These are used in robotics to perform inspection, part recognition and other similar tasks.

Use Of Sensors In Robotics :

          The major use of sensors in robotics and other automated manufacturing systems can be divided in to four basic categories.

1) Safety monitoring.

2) Inter locks in work cell control.

3) Part inspection for quality control.

4) Determining position and related information about objects in the robot cell.

Position Sensors :

          A position sensor, based on the Hall Effect sensor is mounted at each active joint of the hand. The main advantages of Hall Effect sensors are there small sizes and their contact less working principle. In each finger, the hall sensors are fixed, respectively, to the palm and to the proximal phalanxes, where as the magnets are mounted directly on the sliders of each joint.

          In this configuration the sensor measures the linear movement of the slider, which is related to the angular position of the joint. In each MP joint, the linear range of the sensor is 5.2mm, where as in the PIP joint the linear range is 8mm.

          Using a micrometric translator stage we found optimal configurations for the position sensors. In the first optimal configurations two magnets are used at a distance of 3.5 mm this configuration has a working range of 5.4mm with a linearity of 5.34%. The second optimal configuration (suitable for MP joints) has six magnets and a working range of 8.4mm with a linearity of 3.81%.

Hall Effect Sensors :

          When a beam of charged particles passes through a magnetic field, forces act on the particles and the beam is deflected from its straight line path. A current flowing in a conductor is like a beam of moving charges and thus can be deflected by magnetic field. This effect is known as HALL EFFECT.

Hall effect sensors are generally supplied as in integrated circuit with the necessary signal processing circuitry. These are two basic forms of such sensor, LINEAR where the output varies in a reasonably linear manner with the magnetic flux density and THRESHOLD where the output shows a sharp drop at particular flux density

2D Force Sensors :

          A 2-D force sensor, based on strain gauge technology, has been developed in order to sensorize the distal phalanx of the index and middle fingers. The sensor design has been optimized using the Pro/Mechanical structure software.

force sensor :

The force sensor was characterized using an INSTRON 4464 testing machine.

A traction-compression loading cycle (0N-10N-0-N) was performed for each direction. Results are presented in fig 11, for the normal loading direction and the tangential loading direction respectively. Diagram show a linear behaviour of the 2-D force sensor.

FINGERED TIP FORCE ANALYSIS :

          A first set of experimental test has been performed in order to evaluate the force that the index /middle finger is able to exert on an external object. To this aim we are measuring the force resulting when the finger is pressing directly on the high accuracy piezo electric load cell corresponding to different configurations of the joints. Two “pressing “task were identified in order to evaluate separately and independently the force generated by actuators of the fingers.

TASK 1: The pushing action is exerted only by the distal actuator.

TASK 2: The pushing action is exerted only by the proximal actuator.

Then ten tests were performed for each subtask . The results obtained are illustrated in the fig

ADVANTAGE :

1.Handicapt can lead an independent life.

2.Noiseless operation for not disrupting social interactions, cost suitable for support by the health insurance system.

3.Light weight

4.Large contollers are absent

DISADVANTAGES :

1.Implementation cost is highly expensive.

2.It can’t withstand a load of more than 40 N.

3.Causes problem in controlling accurately in hand