3rd and 4th Fingers struggling to close

[JonathanRaines]

There is one motor pulling against two elastics to close. They, therefore can't move their full range.

Possible solutions:

• Lengthen the relaxed length of the elastic in those fingers. The fingers are shorter and therefore less susceptible to wobble so it shouldn't be too detrimental.
• Thin the ligament for those fingers.
• Reduce friction on the tendon

[JonathanRaines]

Something else to consider is that the fingers clash when they close. This may be causing additional resistance to flexion.

[JonathanRaines]

Thinning the ligament certainly helped a bit. Waiting on printing other components to assess the impact on lateral stability.

The improvement was noticeable but slight. This implies the double elastic is mainly responsible. I'm currently printing the parts to assemble a hand with slacker elastic.

[JonathanRaines]

Testing flexion

Aim

To see how well the PQ12 could flex the fingers without any elastic spring back. Then to see how increasing levels of elastic pre-tension detriment flexion.

Method

The hand was partially assembled and the fingers were flexed and extended using a Chestnut board, controlled over a serial connection to my PC. The range observed for different levels of elastic pre-tension.

Equipment

The inner palm and fingers were assembled with one 12V 63:1 PQ12 (the one to be used for production) in the 4th slot. One tendon was used in the outer channels of the 3rd and 4th fingers. A new (vertical slit) actuator cap was used. The thinned ligament with altered phalanges was also used.

Results

The fingers could fully close when no elastic was used; but only just. Even just threading the elastic into position without securing it caused a reduction in range. The friction from pulling the elastic through its mounting block was enough to prevent full closure. In conclusion, these fingers are underpowered.

Recommendations

• Examine the tendon channels and see if altering them can reduce friction.
• See if the force to close can be reduced

[JonathanRaines]

Ligament thinned to two layers / 0.4 mm. The fingers can now close (tested using the same method as above). They can even handle a small amount of elastic tension.

It is not recommended that this is implemented on the 1st or 2nd fingers as it reduces lateral stability.

Reducing the thickness to only two layers raises concerns about the ligament snapping under high load. However, Cheetah is incredibly strong. The highly qualitative test of trying to rip it apart by hand was unsuccessful.