Introduction: Exoskeleton for Paralytic Arm
Human beings have evolutionarily developed movement on pair of limbs which provides the coherent benefit of minute energy consumption. But this slick motion can also be hindered due to a diverse set of reasons like stroke, accidents and so on. The survivors are rendered with weakened limbs and require substantial effort in rehabilitation and regain routine gait. So, our objective in this project is directed at developing a novel type of exoskeleton to facilitate easy movement of the arm of a paralytic individual and also enabling them to work at their own efficiency involving daily chores. In the wake of that, we first developed a prototype model. for both arm and hand to check whether our concept is working or not. Our two concepts were wire technology and a link mechanism for providing motion to the exoskeleton. The finalized model was produced using 3D printing that gave strength to the model to act as a rigid body to hold high loads and equally easy to operate by the same individual or by any other. The high torque servo motor was used for providing torque to the whole system using a four-bar like link mechanism. The operating and controlling were done using Arduino and joystick. With the above course of action, the exoskeleton was able to meet the defined requirements satisfactorily.
Step 1: TYPICAL REASONS FOR LOSS OF GAIT
Medical science has benefited mankind immensely and continues to provide a cure as well as remedies against various diseases and physical problems as intensive research is going on in the field of medical science. One of the physical disorders is paralysis. Paralysis is the loss of muscle function in part of your body. It is a violation of motor activity, which is manifested by a complete lack of movement. It happens when something goes wrong with the way messages to pass between your brain and muscles. Paralysis can be complete or partial. It can occur on one or both sides of your body. If the movement is preserved but for some reason weakened to some extent, then such a violation is called paralysis. It can also occur in just one area, or it can be widespread.
Paralysis is ramified into various types depending place of injury
Paraplegia- Paralysis of the lower half of your body, including both legs
Quadriplegia- Paralysis of the arms and feet
Hemiplegia-Paralysis of the body from one side.
Step 2: CONCEPTUAL MODEL
The above 3D model was developed to conceptually understand how the arm portion is to be supported and the actuation is achieved for movement of the forearm with the help of cable mechanism. It consists of two long links joined by nuts and bolts. The links are of length 23 cm each. The joint between the two links acts like a hinge thus enabling movement of one link with respect to one another. The angle of rotation is approximated to be 270 deg.
Two models have been developed on the basis of research. Models have been primarily developed for the paralytic arm.
The glove prototype consists of threads and a glove with guides. Our primary focus is to achieve the grip by the fingers so the individual can hold objects all by himself. On the glove, the threads are fixed at the tip of each glove portion for the particular finger. The threads are fixed by stitching them and further passing it through plastic guides as it provides the way to threads to pass over the finger properly and uniformly resulting in correct functioning. Threading is done for index and middle finger and the thumb support has been designed and formed to let the thumb in a particular plane to hold the objects. When the threads are pulled the particular finger move to and fro. Through this, each finger can be independently moved as well as in combination forming a grip. This helped to reach the decision of using Bowden cable for the gloves as it will easily transmit mechanical force.
The initial model consists of the pulley for the movement of the support structures. The structures are formed using thermocole material. Precise shapes were cut out by taking measurements of the upper arm and forearm. The structures were joined using pulley of proper diameter and shaft is placed in inner diameter in pulley connecting both the arm supports. When the pulley is rotated the arm supports move accordingly. When the pulley is rotated in anticlockwise the supports attached with the arm retracts it and vice versa. The pulley is rotated using the threads. The arm supports provided with velcro strip helps in tightening around the arms to be intact and there should be no slippage while working. The problem with this prototype is in reflexing of the arm. The same motor cannot be used to provide motion to pulley in both clockwise and anticlockwise directions.
The second prototype has been developed using the thermocole material and threads passing around the pulley provides the motion. The supports were joined to each other using pulley in the same way as the first prototype but the pulley is attached to one of the arm supports and other support structure move with help of threads The one end of the thread is attached to support structure and passes through second arm support over the pulley. When on one arm structure is fixed and the thread is pulled the second structure moves to and fro. This proves the mechanism is working in the second prototype. This prototype provides better movement of the arm as well more degree of freedom which can be used to control different hand motions.
Step 3: ACTUAL MODEL FABRICATED
The model consists of a hand glove, which is worn in the palm of the patient. The guides designed to hold the cables during the expansion and retraction of the hand are fixed in the glove by stitching it to the glove.
The guides were designed in Solid-Works 3D Modelling software according to the needs and requirements of the designed model. The solid model of the guides were 3-D printed. The guides apart from holding the cables in position also give direction to the cables, guiding the direction of extension and retraction of the fingers attached to the cables. The upper end of the cable is fixed at the tip of the finger inside the guide. The wire passes from the tip of the finger towards the wrist. The upper end of the cable is open, while the part of the cable below the fingers is kept inside a plastic sheath or tube. The cable along with the sheath forms the Bowden-Cable combination. In such a combination, the steel cable moves inside the sheath while the sheath remains stationary or fixed. Such a mechanism is generally used in the braking mechanism of the bicycles. The Bowden-cable moves all the way down and the other end of it is fixed in the housing.
The housing is designed to house the servo-pulley arrangement. This housing is also designed in solid works according to the requirements and 3-D printed. The housing consists of a servo-motor (MG996R) fixed with screws to keep it rigidly fixed in position during the actuation. The pulley is attached to the servo via a screw. The pulley is also designed in accordance with the needs of the fabrication using solid works and is 3-D printed. The Sheath is fixed in an orifice and the cable is allowed to pass inside the housing. The cable is wrapped around the pulley and is fixed using a screw. To hold the pulley tightly to the servo, super glue is used to keep it in position. The servo is attached to an Arduino-Uno and shield (REES52 16 channel board) combination. The power supply to the arrangement is provided by a 12V Lead-Acid battery.
The code was created and dumped into the Arduino to provide the command to the servo for the purpose of actuation. A joystick is also attached to the Arduino. On moving the joystick, the servo rotates and thus making then pulley rotate along with it. The cable wrapped over the pulley gets stretched and slips inside the plastic sheath. This action creates a tension in the finger pulling it towards the palms or similar to close fisting position. When the joystick is moved in the opposite direction, the servo also rotates in the opposite direction. This time the cable is pushed outside, causing the fingers to retract or open up. Thus, using the simple command from a joystick controller, a person can open and close the fingers enabling him to hold objects. The whole housing and Arduino are rigidly fixed on a steel mount using a screw.
For mounting the pulleys and the motors a casing as shown above Fig 4.8 was made. Clamps like structures were provided at the base with a notch to hold the motor firmly. On the shaft of the motor, the pulleys were fixed using screws. Thus, when power was given the fingers achieved flexion and extension due to winding and unwinding of the Bowden cable. After that, the individual components were placed and mounted on a metallic frame forming a rectangular base using nuts and bolts. Acrylic Sheets were put around the base to form a box in the shape of a cuboid in which houses the whole setup.
Step 4: Fabrication of Exoskeleton for Arm
Since the wire technology requires very high torque servo DC motors as well as a rigid heavy weight mounting, we switched to a link actuation method which together with the arms constitutes a 4-bar mechanism. This mechanism is made in such a way that the overall weight doesn’t exceed more than 400 grams.
The links are made of PLA (Polylactic Acid) using the Additive Manufacturing process of FDM (Fusion Deposition Modelling). It principally consists of seven parts. There are two links of one of length 21 cm and other 24 cm. On the links are present four structures resembling small arcs which are affixed to the links by nuts and bolts. On those structure is wound wrist supports so that the patient can wear the arrangement comfortably. The two links are joined via a circular part with the help of screws. Mounting is made of steel to attach the motor to the arm. The motor used is MG996R servo motor of 17kg-cm. The motor drives the arm via two links. The whole arrangement is controlled by Arduino Uno in arrangement with a servo shield.
The input to the exoskeleton is given via joystick. When the joystick X-axis value is fed with a signal value above 570, the Arduino microprocessor processes the value and sends a PWM signal to the servo motor via servo shield. The servo makes positive 1800 turn thereby moving the links. When the value fed is less than 450 the servo rotates in the opposite direction. All the components of the control system are connected by multi stranded wire of suitable length and soldered at appropriate places. The system is powered by 12V Pb Acid Battery which is rechargeable. With this the arm is actuated so that flexion and retraction is possible.
When the servo rotates the resultant torque is transmitted through the apparent four bar mechanism formed between the steel link and that of the 3D printed parts. The link enables the torque to be increased with the present motor configuration. The angle of rotation of the exoskeleton is almost 1800.Thus with resultant movement the concerned person can move his arm normally to carry out their day to day chores.
4.5.1 The Glove Model
The materials used in the fabrication of the glove prototype was a white cotton glove. The guides fixed along with the fingers and the palm was made using plastic straws (used for drinking juice), the guides were designed to allow the cable to pass through it. The guides were fixed to the glove using a glue gun making it stick to the glove tightly. The cable used was of cotton thread. This cable was fixed to replicate the actions of the Bowdon cable mechanism.
126.96.36.199 Original Model
The original model comprised of Kobo Cross Fitness Hand Protector Gloves. The guides were designed according to the requirements by using solid-works software and then 3-D printed. The material used to print the guides was Nylon and the method used was Stereolithography (SLA). The cable used for the Bowdon cable mechanism was made of stainless steel and the sheath used was of plastic. The Pulleys and the housing of the pulleys used were also designed using Solid-works software and 3-D printed. The material used for 3-D printing was Polylactic acid and the method that was used for printing was Fusion Deposition Modelling.
4.5.2 The Arm Support Model
The first prototype was made using cardboard and Polystyrene (thermocol). The thermocol was sandwiched between two cardboard sheets to make it strong and durable. The guides were made using the plastic straws stuck using glue from the glue gun. The cable used was a cotton thread.
The second prototype was fabricated using aluminum channels. Cut and fixed as per the design. The bearing used to create the joint was of mild-steel. The cable used for the Bowden cable mechanism was of stainless-steel and the sheath was made of plastic. Velcro was used to fix the model with the arm. Velcro used was made of Nylon.
188.8.131.52 Original Model
The entire arm support model was designed using solid-works software and 3-D printed. The material used for 3-D printing was Polylactic acid and the method that was used for printing was Fusion Deposition Molding. The link mechanism and the housing to store the servo-motor were fabricated using mild steel. Tennis elbow support made out of puff fused matty fabric was used to fix the model with the arm of the person.
Step 6: Circuit Diagram
The circuit consists of an Arduino UNO, 2 servo motor, battery and joy stick. The joy x pin is connected to an analog input of Arduino. The signal pin of the servo motor is connected to the PWM pin of Arduino. The battery is attached to the other pins of the servo as well as the Arduino
Step 7: Codes
Adafruit_PWMServoDriver pwm = Adafruit_PWMServoDriver();
#define MIN_PULE_WIDTH 650
#define MAX_PULE_WIDTH 2350
#define DEFAULT_PULE_WIDTH 1500
#define FREQUENCY 50
#define GROUND_JOY_PIN A3 //joystick Gnd pin will connect to Arduino analog pin A3//
#define VOUT_JOY_PIN A2 //joystick +5 V pin will connect to Arduino analog pin A2//
#define XJOY_PIN A1
#define YJOY_PIN A0
pinMode(VOUT_JOY_PIN, OUTPUT) ; //pin A3 shall be used as output
pinMode(GROUND_JOY_PIN, OUTPUT) ; //pin A2 shall be used as output
digitalWrite(VOUT_JOY_PIN, HIGH) ; //set pin A3 to high (+5V)
digitalWrite(GROUND_JOY_PIN,LOW) ; //set pin A3 to low (ground)
int pulseWidth (int angle)
int pulse_wide, analog_value;
pulse_wide = map( angle, 0, 180, MIN_PULE_WIDTH, MAX_PULE_WIDTH);
analog_value = int(float(pulse_wide)/1000000*FREQUENCY*4096);
int joystickXVal = analogRead(XJOY_PIN) ;
int joystick Val = analogRead(YJOY_PIN) ; //read joystick input on pin A1& A0
Serial.print(joystickXVal); //print the value from A1
Serial.println(" = input from joystick"); //print "=input from joystick" next to the value
Serial.print((joystickXVal+520)/10); //print a from A1 calculated, scaled value
Serial.println(" = output to servo"); //print "=output to servo" next to the value
pwm.setPWM(0, 0, pulseWidth((joystickXVal+520)/10));
pwm.setPWM(1, 0, pulseWidth((joystickXVal+520)/10));
pwm.setPWM(2, 0, pulseWidth((joystickYVal+520)/10));
Step 8: Calculations
SELECTION OF MOTOR FOR THE ARM
The principle formulae used in the wake of the selection of Motor to be used in the exoskeleton arm is the rudimentary torque equation
Where T=Torque required to rotate the forearm without any extra weights on it.
F=Weight of the forearm
R=Average Length of Forearm of a normal human being.
We approximated the F value as 3kg (Average Weight of forearm of a healthy person) multiplied by 9.81 m/s2 (Acceleration due to gravity).
The value of R was taken as 26 cm; the average length of the forearm of a healthy individual.
The amounted torque was 78 kg-cm. The average value varies from person to person and may be high in the case of certain individuals. Using average value allows a full-proof selection of equipment. It was taken so that the motor of correct Stall torque is chosen so that it could provide sufficient torque to the arm to lift the required weight. High torque motor MG996R of 17 kg-cm torque was used and the link length of 9 cm was taken.
Using same formula 5kg-cm servo motors were selected and chosen for finger actuation and was found to fetch satisfactory results.
CALCULATING DIAMETER OF THE PULLEY
For finding out the diameter of the pulley which will multiply the torque produced by the motor we used the basic equation
Where, S=Length of finger around which the cables are wound
R=Radius of the pulley
taking the value of s= 7 cm we get the R= 4.45 cm
so, we took R= 4.5 cm and designed pulley of diameter 9 cm
Torque Generated by a 4-Bar mechanism
Considering the whole exoskeleton arm to be a 4-bar link mechanism. Assuming a situation when the link (connecting servo link to the forearm) is the longest, comes in a position as shown in the Fig and the servo link makes 900 with the upper arm. The Fig represents the simplest form of the whole mechanism in the form of lines. The arrow represents the direction of the force acting on each link. Considering only aa^ to be the rotating on which a net torque of 153kg-cm acts on the link.
Therefore, the net distance of the force Fa and Fa^ is =9 cm.
Fa = 155.66N = Fa^
Similarly, for the link bb^, force obtained is given as
Fb = 155.6N = Fb^
Therefore, for the link cc^, the force acting at c^ is 155.66N. The radial distance between the forces is R=82mm.
Hence the torque on cc^ is:
T= 12.76 Nm
In this case the longer link transmits force to the forearm where CG of the arm is situated. The force acts slightly offset and not directly below the servo link. The 17kg-cm torque of the servo gets amplified to nearly 130kg cm torque. To increase the torque, we need to reduce the length of the longer link (connecting servo link to the forearm) by more than 30mm so that it increases the torque more than 153 kg-cm.
Step 9: Upgrades
The current Arm Support Model of this exoskeleton model uses link mechanism to raise and lower the arms, which requires the servo-motor to be mounted on the support. The link mechanism increases the torque produced by the servo-motor enabling it to lift the arm. In the present work the complete link mechanism can be replaced with same cable mechanism that is being implemented on the exo-glove. Instead of links, Bowdon cable mechanism can be used to raise and lower the arms. Currently a joystick is used to operate the exo-glove. On moving the joystick, the actuation signal is sent to the Arduino which moves the servo-motor. This can be replaced by the EMG. The sensor along the diodes, can read the electric potential produced by the muscles of one hand that is functional. This data can be feed to microcontroller connected to the EMG through wires. This will enable the exo-glove to replicate the movements of the other hand, removing the need of a joystick. The current model doesn’t provide any solution for wrist movement. A simple wrist rotating mechanism can be embedded to allow the patients to move their wrist as well. This will increase the degree of freedom of their hand making them use their hand in a better way and making them feel more normal psychologically. In this case actuation is being controlled using a joystick or may be an EMG as an enhancement but this can be totally replaced by neuro-sensor. These sensors can enable the patient to control and actuate the exoskeleton directly from brain pulses, making it work similar to any normal limb. The sensors will be attached to the brain of the patient and receptors will be available inside the exoskeleton. When brain sends a signal to move the limb that signal will be received by the neuro-sensors and transmitted to the exoskeleton. Thus, creating movement of the non-functional limbs similar to that of a normal limb. However, a lot of research is required to enter into the sphere of Neuro bioengineering.