Final Design

3D Printed Robotic, Upper Limb Prosthetic 

    The fingers are composed of 3 digits with custom machined delrin pins that allow rotation with respect to each other. The delrin pins are secured via a e-clip as can be seen in the figure on the right. 

    The hand is based on human anatomy where 'tendons'  (200 lb. test braided fishing line) are pulled through guides via servo motors located on the forearm and exert a torque about each pivot (knuckle) of the finger creating the extension and closure of the hand thus enabling the grasping ability of objects. 

Above Left: CAD image of Final Design Assembly

Above Right: Image taken of the Final Design Assembly

Brief Descriptions of Main Components (MECHANICAL AND ELECTRICAL):

MECHANICAL

Fingers and Palm (includes tendons):

Wrist:

    As can be seen in the image on the right, two ball nose spring plungers are inserted to the male end of the wrist (on hand) and serve to lock the hand in place by plunging into the holes on the female end of the wrist. This allows the hand to maintain a palm down or thumb up configuration.

See video in multimedia tab labeled "wrist Function" to see how the two components interact with each other. 

Forearm Packaging:

    The tendons are optimally routed through the palm with teflon tubing in order to reduce the internal friction of the system. The teflon tubing is the same that is commonly used and found among desktop 3d printer filament housing. The geometry of the palm has been designed to optimize the pinching and grasping force of the fingers. 

    This component houses the servo motors controlling the fingers and serve as an attachment point for the hand to the overall solution. It is 3d printed. Aluminum wrist insert can be 3d printed, but the aluminum insert serves to increase the life of the device by reducing the wear cause by the male end of the wrist rotating  

Elbow:

Cast (socket):

    The elbow assembly attaches the lower and upper parts of the prosthetic and rotates the respective two parts about the alignment shaft's axis to create elbow rotation. This ultimately serves to help Gabriella move the bionic hand and help her overcome the weight of the entire prosthetic. Additionally, this assembly serves to protect Gabriella by way of concealing the moving gears, and (not seen in the image are the wires) routing/concealing the wires from the forearm assembly to bicep assembly with limited outside exposure. Due to the rigidity and strength required by this assembly, an aluminum shaft was machined to ensure the gears would constantly mesh without slipping at the given loads (appx. lift up to 2.5 kg). The aluminum gears were used as opposed to 3d printed ones because a relatively high gear ratio (7:1) in a small area was required and they are not custom (came from ServoCity.com).

 

             

    The socket was professionally created by our friends at Hangar Clinic and is made from a heat formable thermoplastic custom shaped based on a mold of Gabriella's arm. This two piece component is rigidly connected by way of the elbow and serves to comfortably house the robotic solution onto Gabriella's arm. This is the only component that will need to be changed as Gabriella herself grows. Ideally, a new socket (cast) can be made with the correct mounting holes, and the robotic elbow/hand will still be functional, allowing the entire solution to last as long as possible.

ELECTRONICS

Adafruit Feather:

    The brains behind this robotic solution lie in the Adafruit feather MO with RFM95 radio board.This micro controller runs off Arduino code which allowed for rapid prototyping and had built in radio communication allowing the wireless joystick to be easily implemented. Its small packaging, ease of programming, and sufficient amount of GPIO pins along with the RFM95 LoRa radio built in made it the best choice for controlling this robotic arm.

User Input Method:

    Through much trial, tribulation, and deliberation, it was decided that the best way to actuate this solution would be by way of a wireless (RF) contact joystick with an additional red button for added functionality attached to the patients opposite arm.

    The joystick's vertical movement control the elbow's bending and the horizontal movement controls the opening and closing of the hand. The joysticks signals are sent to the bicep (housing the Adafruit feather board) via radio waves from another feather board located with the joystick. The red button is used to send the arm to 'sleep mode' so that accidental signals will not be sent to move the bionic arm, thus making the 'taking off' and 'putting on' aspects easier. There is also a red LED battery indicator light.

    A 3d printed enclosure houses the joystick, controller board, button, LED and battery with an elastic strap to ensure the components remains on the patients arm. Conveniently, there is also a charging port hole for easy charging.

Above Left: The wireless contact joystick ~user input device (not pictured is elastic band)

Above Right: Exploded view showing assembly of wireless contact joystick

Sensors and Control: 

    The elbow and hand have feedback sensors to ensure safe operation and allow software safety protocol's to be put in place. 

    Each of the three motor's that were responsible for opening and closing and hand were connected to current sensors. Through testing, a thresh hold current was determined (~350mA which was ~1/4 the rated stall current of the motors) to ensure sufficient torque without risk of burning out the motors. Below is an image used to determine the appropriate current values from the current controller.

Above: Data was acquired using 2 different types of multimeters to confirm results. Values also confirmed with current sensor that was implemented into design solution.

Note: 0 degrees corresponds to a closed hand and 120 degrees corresponds to an open hand.

    This (on the right) is an image of the high-side current sensor used for each finger motor. [Appx. size ~a quarter] High-side refers to measuring the current between the load (motor) and power source (battery) as opposed to low-side current measuring which takes place between ground and the load (motor). The method of current sensing was done by way amplifying the voltage drop across a shunt resistor (1% sense .1 ohm) using an op-amp with a gain such that 1volt = 1 amp. 

    The elbow has two hard mechanical stops built into the linkages, and recessed in those stopping surfaces on the linkages are limit switches that send signals to the controller to let the controller know that the elbow has reached one of its limits of travel.

    With information about the user input, current draw of each motor, and weather or not a limit switch had been activated, the controller sends an appropriate signal to the motors.  

    The fingers are actuated by 3 Hitec hs-5070 servo motors which have a range of mobility of 120 degrees. This motor was selected because it was the smallest, lightest servo motor available that satisfied the required necessary torque of ~1.2 kg-cm (~12 N-mm) (two fingers, one motor) with a reasonable factor of safety to account for un-modeled friction and satisfy the desire to not constantly run the motor at stall torque as they would burn out quickly.

Above: A simple flow diagram describing the operation and control of the final design

Motors:

    This is a position problem. In order to achieve opening and closing of the hands and being able to, with low power consumption, hold position with sufficient torque, it was decided that servo motors were the best choice for this application.

    

    The elbow is actuated by a single hitec 2645 continuous servo which as the name implies, is continuous due to the fact that the large gear reduction necessary to provide sufficient torque at the elbow requires multiples rotations of the motor's pinion gear. This motor was chosen because of its high-torque, relatively small form factor, and ease of programability. It also satisfied the torque requirement of ~3.2Nmm ~ 32Kg-cm with a reasonable factor of safety. 

Above Left: Hand calculations used to determine necessary torque of the finger motor

Above Right: Hand calculations used to determine necessary torque of the elbow motor

Battery:

    Joystick: 3.7V, 1200mAh single cell LiPo battery from Adafruit. The feather board inside the joystick housing has a charging port and because it is a single cell LiPo, no balancers are required. This means when the battery gets low on the joystick, it can just plug into any USB (via mirco USB) charging device.    

    Everything Else: 7.4V 1800 mAh dual cell LiPo battery from MaxAmps. Since this battery is a multi-celled LiPo battery, the battery will need to be removed and charged on a balancer for safety measures. 

Above Left: 7.4V battery (from MaxAmps) powering all motors and housed in bicep measuring ?x?x?

Above Right: 3.7V battery (from AdaFruit) powering the joystick in joystick housing measuring ?x?x?

OVERALL PERFORMANCE METRICS:

 [Video of Gabriella drinking juice box with her new hand]