The Details (for Developers)

 

All 3D printed parts are modeled in SolidWorks 2017, some using the student edition. Parametrization is achieved by using a combination of equations and design tables. Ideally we would use just equations, as they allows easier updating (just through rebuilding the part) than do design tables, but since our model is heavily based on imported and then altered parts, we use the scaling feature in SolidWorks. The scale feature does not allow the use of equations, but you can control the individual (x,y,z) scale factors with the proper syntax using design tables. (We could theoretically have avoided this issue but we believe in the future it will be invaluable to be able to import ready made models and then scale them as needed).

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Using design tables to update parts requires opening and closing the table, which would make this a very tedious process. To solve this problem, we use a macro designed to update the design tables of all open parts and assemblies in SolidWorks. This process takes a few minutes but succeeds in automating the procedure. You will find all of the equations .txt files along with the SolidWorks models. The design tables stored within each part are linked using functionality of Excel to a master design table. The equation files and the master design table are update automatically using the MATLAB code in the Dimensioning GUI. We have created special assemblies that can be used to group print the parts easily for a few different varieties of 3D printers.

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The process of customizing the design of the prosthetic is as follows:

1. Open Dimensioning GUI

2. Input dimensions

3. Open the Master Design Table.xlsx

4. Open the printing assembly

5. Rebuild the assembly

6. Run the macro: go to tools ---> Macro---> run :UpdateDesignTableMacro.swp

7. Rebuild again

8. Print!

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Not all of the parts are scaled. Most fixed dimension parts are smaller parts such as pins and joints. Scaled parts are:

1. Palm

2. Fingers

3. Forearm

4. Socket

Equation files included in the Design files include files for each of the above scaled parts. In each equation file there is one or two variables, usually at the beginning of the text. These variables control the rest of the equations in SolidWorks, and output values for the scale factors of each part. The scale factors are simply a calculation of the desired size (in the x,y,z) direction divided by the current size (imported model) . Before implementing design tables these variables had to be manually input in the .txt files and the scale factors had to be manually input in each SolidWorks part. You can see how with more than 30 individual parts this became very tedious. The dimensioning process described above is a much better solution. The variables input for the fingers are the length of the distal phalange of each finger, respectively. This length is determined by our MATLAB code based on an input of pinky length.

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The dimension ranges that we have tested our models on are:

1. Pinky Length - 60 to 75mm

2. Palm Width - 20-40mm

3. Wrist Width

4. Wrist Height

5. Forearm Width

6. Forearm Height

7. Forearm Length - minimum 85mm 

 

The fingers are designed using the skin and skeleton model (our goal is to have the majority of the hand this way). This allows you to develop new skins for the fingers. You can design a skin that you would like to use as a finger using whatever CAD program you choose, and as long as the phsyical design does not affect the mechanics of the skeleton, your skin will work with our prosthetic. This process will require some work with equations and design tables in SolidWorks to properly scale along with the skeleton models. If you do design your own skin, please share it with us!

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The motors used in the prosthetic are micro-gearmotors. Each has a gear and pinion system attached that transfers the rotational motion into the proper plane. Each gear has two strings attached to it, one on each side of the attached finger. Each string when pulled allows for motion in the corresponding direction This requires that the tension on the opposite string be released with an equal amount of slack. This system is designed so that when one string tightens the corresponding string loosens by the same amount, allowing for flexion or extension of the finger. The finger stops when the motor reaches its stall current or other current dictated by the algorithm and electronics, allowing for varying grip strengths. This whole setup allows us to have a relatively simple control system for the flexion and extension of the finger. 

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Fishing line is used to control the fingers because it has relatively low friction in its interaction with the printed plastic, and its elasticity allows for some flexibility and inaccuracies in the design.

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The statistical analysis mentioned above is based on images of hands collected in the lab that were analyzed to determine the distance between different points. We found that a second order polynomial dependent on the length of the pinky closely models the variations between different lengths of the hands.

 

Files

To download files only related to the design of the prosthetic, use this link. To download all files see BUILD/USE .

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Do not change the organization or names of files related to the Design of the prosthetic (unless you really know what you are doing). The assemblies and dimensioning process (mainly in the MATLAB GUI) require the files to be kept in specific locations relative to one another and with the correct names.

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Programs you will need to view, edit, or print the Design files.

1. SolidWorks 2017 or later

2. MATLAB or MATLAB Runtime 2017b, available here for free (to edit the dimensioning code you will need a MATLAB license. Parts can be dimensioned with the Runtime)

3. 3D printer and corresponding software of your choice (we use an UP mini 2)

4. Text Editor

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What We're Working On

• Skin and skeleton model for the palm

  • Creating a mechanically working skeleton for the palm itself that will allow easy design of skins while maintaining required mechanical characteristics​

• Moving motors to the palm

  • Will allow us to move the electronics to the forearm and/or reduce the minimum forearm size.​

• Improving the socket

  • In general it needs to be finalized​

• Alternate models to accommodate children and others with smaller hand sizes

• Alternate Skins

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