The string

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I found a fishing rod in one of the online shops in Switzerland that stated it would be good for 89.6 kg. I ran a test but could tear it apart by only applying a part of my body weight - so it might be good for 50-60 kg.

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The string feels very smooth and a bit slivery - so I wondered about the knots I can use. If I pull it through my fingers it straightens out and this way it is not too hard to push it through the holes.

Cutting the strings

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Make sure you use your best cutter to get a somehow clean end on the string. Use your spit to get the end of the string straight.

The layout

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Cut yourself a rather lengthy part of the string - it will come handy and in the beginning I found it sometimes rather hard to try to add a knot into a seamingly too short part of string. I found it easier to add the strings when everything is still disconnected. Add a loose large knot to the finger tip end of the string. You will need to open that later so do not tighten this knot up (right side of picture).

Wrist side of finger closing string

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On the wrist side of the string for finger closing strings make a knot with a small loop in order to have enough space for the added spring to expand. For the hand opening strings just make them long enought to reach the servo pulley.

Knots

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I have experimented a bit with knots but found that the easy double overhand is good enough for me (and my string material) to connect to the springs and it never did slide.

String at the wrist side

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To avoid broken parts of stalling servos I use springs on the finger closing strings. I simply bought a set of springs and tried them out by hand to select them. This sping is 14 mm long (excluding the connectors) and is made of 0.7 mm steel. Cut off the unused part of the string ..

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then pull the spring as far into the wrist as it can go

Knot at finger tip

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On the finger tip side tighten the string (with all parts connected and axes in place) and try to place the knot as close to the finger as you can manage

Knot at finger end

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I use this slightly larger knot for the finger and and in addition add a nail for avoiding the string to slip into the finger

String stopper

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I shorten a small nail

knot and nail

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and place it next to the string into the hole. This way the string can not pass into the hole.

The pulley

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Add another string from the spring to the servo pulley. On the servo pulley I have drilled 2 additional holes and added M2 screws with washers. With this in place it is a snap to fix the strings to the correct lenghts for the streched out and bent fingers. Place the strings under the washers in the screw tightening direction to get a good result. If your servo has a wide rotation range you might need to add a mapping for the finger positions into the MRL script to be able to use the standard script scenes (e.g. full.py). With the lower arm still open use MRL to open and close the hand and adjust the strings for correct final positions.

What else

I dream of a micro switch or variable resistor that could fit into the finger springs and stop the servos when they start to strech out. I think this would be much easier to control the power of the fingers closing around an object than a pressure sensor in the finger tip and it would also be closer to the servos or an additional controller to modify the servo signal or cut off power to the servo for the closing motion. Haven't seen such a switch so far but will continue to look for it.

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