DECLARATION 2 PUBLICATIONS
2.1 MECHATRONIC DESIGN OPTIONS
In order to design a mechatronic prosthetic hand that can be operated autonomously by an amputee one needs to look at a few options from a mechanical perspective. Concepts of finger actuation – how the fingers will move; wrist motion – turning the wrist; motor selection;
2.1.1 Finger actuation:
Finger actuation is one of the main contributing factors to the success of the design. Initially there were many thoughts and brainstorms on options.
The Touch hand 2, which was actuated by DC motors for each finger and cable and pulley systems. This hand was quite capable and a great improvement on the Touch hand 1. It had a final power grip strength of 60.6 N for the Touch hand 2 (Jones; 2015) versus 19.5 N for the Touch hand 1 (van der Riet; 2014). Figure 2-1 shows the actuation system used in the Touch hand 2.
Figure 2-1. Touch hand 2 finger drive system.
An initial test design was done using Nylon fishing line which was fed through guides in the fingers so that the finger joints could be rotated relative to each other. This was decided against as there was too much friction.
Comparison was done between using cables as tendons and linkages and the result was chosen to rather use fixed linkages to actuate the fingers than a cable system. The thought here was that in using linkages besides the strength, one could better positon the fingers if a driven actuator was used. The concept in Figure 2-2 was used to base the final design on.
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Figure 2-2. Four-bar finger linkage concept.
Research was done on other options including a leadscrew. Leadscrews can generate a large amount of force. It was found that the available leadscrews on the market, which needed to be attached to a DC motor gearhead, were rather expensive and would not suit the purpose in that they never fitted the budget and would not never fitted the dimensions needed in the hand. The leadscrew however could be a good option and is what is used on the Bebionic hand. A leadscrew system would work similar to Figure 2-3, an initial design that was used in closing the finger joint. Figure 2-4 shows a leadscrew and stepper motor combination.
Figure 2-3. Initial design concept: leadscrews.
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Figure 2-4. Stepper motor/leadscrew.
2.1.2 Motor selection
An actuation method for the fingers needed to be the next step in the decision making process.
The Touch hand 2 used the pulling force generated by pulleys to open and close the fingers, this has its inherent flaws in that there could be slippage between the cable and pulley system.
A linear actuator was the next concept. Research was done into the various types available and micro linear actuators were researched due to the small size and fit required in the hand. A linear actuator would provide the ideal system for the finger motion as rotational motion was already converted to linear motion in the gear system internally hidden by the housing so that all that needs to be done was place the actuator in the correct position in the hand.
In order to select the correct motor and actuating system for the design many factors were considered including the price of the motor or actuator, the speed and time the fingers would move at to open and close, the force and torque that was needed to be generated by the fingers opening and closing, the weight of the motors, and the reliability of the motors. One would also consider how the motors would interact with the electronics and the dynamics involved in the different motors – i.e. what kind of gear system or actuating system would be needed. There were various factors that would affect the complexity of the design and along with the decision made already for linkages over tendons for finger actuations also affecting the type of actuator chosen. In essence what is needed to close the fingers was a downward stroke of an actuator and this was much easily completed if it was already the actuators function rather than converting the motion from angular motion to linear motion. The table below shows the selection criteria on which the final motors for finger actuation, the Firgelli PQ 12 (100:1) were chosen.
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Table 2-1. Comparison of various factors influencing actuator choice.
The Firgelli PQ12 (100:1) is a type of micro linear servo-driven linear actuator and is often used on RC cars and the like to produce linear motion. The actuator whose specification is given in part A of the appendices is small enough for the application and produces a force of 50 N with a travel of 20 mm. This is then ideal for the mechatronic hand and one can be used per finger to get close to the grip strength required.
2.1.3 Mechatronic design specification for prototype and final design
For the prototype mechatronic design the following specifications were developed:
EMG sensors: Electromyography (EMG) is a used for evaluating and recording the electrical activity produced by skeletal muscles, the instrument used is an EMG sensor, and electromyography detects electric potential developed by muscle cells when the cells are neurologically activated. The EMG signal is used as the control signal for the prosthetic hand.
Microcontroller board: The Microcontroller board selected for the design was initially the Arduino UNO microcontroller.
Electronic board: Two separate electronic boards would need to be developed – one for the motors and one for the EMG sensors (input sensors). designed by the electronic engineer (Mangezi, A.; 2016)
Pressure sensors: The final motor electronic board was designed such that pressure sensors could be plugged in to the board and then routed to the finger tips.
Temperature sensors: Temperature sensors were also included in the final motor electronic board and these were also incorporated into the final finger design. Although provision was given for both types of sensor they were not used in the final testing.
Travel mm/s, max speedEstimated travel distance(mm)Closing time est (s)F1 Max force:(N) per finger possibleGrip Strength (N) * possible Max force x5 fingersPrice per eachx5: approx no vatSize of body Stroke lengthMass(g) total mass
Firgelli PQ12(35:1) 28 25 0,892857 15 75 R5 200,00 36,5x21,5 20 15 75
Firgelli PQ12(63:1) 15 25 1,666667 30 150 R5 200,00 36,5x21,6 20 15 75
Firgelli PQ12(150:1) 10 25 2,5 40 200 R5 200,00 36,5x21,7 20 15 75
Firgelli L12(50:1) 23 25 1,086957 12 60 R5 200,00 37x15 30 28 140
Firgelli L12(100:1) 12 25 2,083333 23 115 R5 200,00 37x12 50 34 170
Firgelli L12(210:1) 5 25 5 45 225 R5 200,00 37x12 100 56 280
Firgelli L16(35:1) 32 25 0,78125 50 250 R5 600,00 53x18 67,5 56 280
Firgelli L16(63:1) 20 25 1,25 75 375 R5 600,00 53x19 67,5 74 370
Firgelli L16(150:1) 8 25 3,125 175 875 R5 600,00 53x20 67,5 84 420
*Touch Hand 2 *final testing value 1 60 R5793(R965,5 each) 450
* Touch Hand 1* final testing value 2 20
Portescap p110 015 12 with R16 gearbox Torque 0,3N 0,002
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