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Low cost prosthetic hand


Low cost prosthetic handThe prosthetic hand presented in this project is designed to be made, installed and used in developing countries: the purpose is to realize a functional and user friendly system, suitable for very young users, with a cost of less than 40€.

The hand consists of an aluminium gripper governed by an electronic circuit; it allows to practice the basic functions of the missing limb. With the electronic control system that we have developed, the patient can handle the strength and the speed of the prosthesis in function of the tension captured from the surface of a muscle by electrodes (Myoelectric control). The threshold activation can be easily calibrated by the patient; in this way the prosthesis can be immediately ready for use without complicate procedures or long physiokinetic therapy treatments.

The proposed project aims to make available to the populations of developing countries an existing but highly expensive technology (1000-20000€).

The number of children victims of landmines, mutilations and congenital malformations in third world populations increases every year: our idea derives from this consideration. In fact, key elements of this product are: a very low cost production and the possibility of maintenance in loco with components and simple instruments and facility to find them. The myoelectric control system allows managing opening and closing of the claw with lowest strain, from this it follows the chance of an easy use even by people having weak muscles like children.

 

Development

We have obtained the movement of the prosthesis with an electric motor that is usually used in aero modelling. The control system we have created is based on electronic circuit that doesn’t use advanced technological solutions but cheaper and easier to find circuit components: in this way we have tried to obtain the best compromise between performance, convenience and durability. Finally, the electronic board has been made by hand printing the circuital scheme on a tiny copper board, then immersed in acid. The project can be easily reproduced also in craft way in every part of the world thanks to its simple realization.

 

Electromechanical block

The mechanical part of the prosthesis (Fig. 1) is composed by light aluminium structures and acts as a clamp (a thumb-like and two opposing fingers) powered by a DC motor that imparts the only movement of grasp (one freedom degree). All mechanical parts are obtained from a single slab of aluminum. More precisely, the fingers and the two cover plates are made, thanks to a milling machine, from a 15X10cm aluminium sheet. 24 of these are obtained from a 60X60cm aluminium sheet.

Prosthesis total weight is 230 grams. The DC motor block (by MiniLAT) includes a linear actuator and it can exert a maximum force of 150N (about 15kg), a speed (at rated load) of 15 mm/s, its nominal supply voltage is 12V and absorbs a maximum current of 1.5A (specification and performance data in Fig.2 and Fig.3). When no current is supplied to the motor, the linear actuator and gears are designed to keep the position of the fingers: to maintain a hold no effort is required and muscle contraction is only used to vary the opening or closing of the clamp.

 

Fig.1 Electromechanical block

 

Fig. 2: Motor technical specs

 

Fig. 3: Motor performance data


Control system

The contraction of a muscle generates a myoelectric signal that is picked up by the skin electrodes. The signal is filtered and amplified in sensing block thanks to a instrumental amplifier and an active filter which amplifies about 39 dB. The filter erases the RF and EM noise and it is realized with an elementary circuit configuration to reduce costs and number of components.

The output signal is then processed by the control system thanks to a PIC microcontroller. This last realizes if the patient wishes to open or close his hand and, through a driver, controls the movements of the DC motor with a PWM signal.

Fig. 4: Block diagram of control system


In the following figures are shown: the electronic scheme of the control system, the PCB design and the circuit realization on a single side board.

Fig. 5: Scheme of control block

 

Fig. 6: PCB design

 

Fig. 7: The circuit layout is printed on a glossy paper (1) and it is fixed to a 12X9cm copper board thanks to an iron (2). The board is put into an acid bath (3). This last removes the copper from the board, excepting the areas protected by the resistant material. This process leaves the connections or wiring printed on the PCB. Subsequently, holes are drilled in the board to allow the components to be mounted on the board (4).

 

Fig. 8: final board


Costs

In the spread sheets below there are costs to construct a single prosthesis in case of a production of 100-200 units. The prices are in euro.

COMPONENT UNIT COST QUANTITY AMOUNT FOR COMPONENT TYPE RESELLER
Burr-Brown INA2126 2,43 1 2,43 Farnell
ST LM358 0,172 1 0,172 Farnell
Texas Instruments TL082 0,196 1 0,196 Farnell
Porta RJ45 femmina 0,5 1 0,5 Farnell
Microchip PIC18F23K22 2,2 1 2,2 Microchip
ST L293B 3,01 1 3,01 RS
Taiwan S. LM7805 0,2 1 0,2 RS
Resistor 20KΩ 0.25W 5% 0,005 11 0,055 RS
Resistor 1.6MΩ 0.25W 5% 0,005 2 0,01 RS
Resistor 2.7MΩ 0.25W 5% 0,005 2 0,01 RS
Resistor 5.6KΩ 0.25W 5% 0,005 2 0,01 RS
Capacitor 100nF 50Vdc 0,065 7 0,455 RS
Capacitor 220pF 50Vdc 0,05 2 0,1 RS
Kingbright green Led 3mm 0,06 4 0,24 RS
Philips BYD33D 0,15 4 0,6 RS
ON Semiconductor BZX35 0,05 2 0,1 RS
CAE multimedia FTP cable 0,62 1 0,62 RS
Eurocamina Ag/AgCl electrode 0,0584 5 0,292 RAM medicali
Single face 12X9cm board 5,14 1 5,14 Mdsrl



TOTAL AMOUNT



16,34
Tab.1: Costs of control system

 

COMPONENT UNIT COST QUANTITY AMOUNT FOR COMPONENT TYPE RESELLER
60X60cm 3mm aluminium sheet 2024T3 65 1/24 2,71 Onlinemetals
Screws 0,2 5 1 Hardware
Bolts 0,2 5 1 Hardware
Lock washers 0,05 10 0,5 Hardware
Promoco Minilat 12 1.5A 18 1 18 Commex



TOTAL AMOUNT



23,21
Tab.2: Costs of electromeccanical block


Total amount of final product is 39,61


There aren’t development costs because we have used freeware or open source development software tools. The assembly and installation costs will be very low because the system should be distributed as a kit and will be easily assembled and installed on patient by a technician directly on site. Finally, the costs of patient rehabilitation are close to zero because the prosthesis adapts itself to the patient and the control of the movements is very user friendly.

 

Video Dimostration

 


This is an open source project
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