Dear EE community,
For a friend that is paralysed in his underarms and hands, I wish to create 2 extra fingers, controlled by pressure sensors under his (very skilled) toes, so he can touch/push/grip. Much like the 2nd thumb project

Can someone give me hints about the right type of motors to use?
I would use the principle of motors winding a pull cable, and not so strong elastics to pull the fingers back straight when the motor is inactive. The finger motors should ideally be strong enough to grasp and lift about 2 kilograms, and this means that most of the time, they will be either off, or stalled in 'tight grip' mode. The motors should be able to handle being pushed in their opposite direction even when trying to wind the cable, for safety and since this will definitely happen with normal limbs.
They should ideally also be small and lightweight.
Which type of motors would be best suited, and why?
Links to similar projects or questions are very welcome too
Thanks a lot for your help!

 

@s@lome

I would not use 'motors' per se. Look at using 'artificial muscles' or fibers that respond to voltage/current. Here is one high-tech, lo-cost possible solution you can make:

https://hackaday.com/2018/07/11/robotic-muscles-from-fishing-line-and-nichrome/

You could use 42AWG Nichrome wire to heat it.

Best of luck to you

 

Research muscles created with Nitinol, aka memory wire.

The range of motion with Nitinol can be extended by forming Z springs… It’s main disadvantage is the recovery time once the muscle is activated.

 

Additionally, RC hobby servos could be used. Small position perturbations are corrected automatically by the motor. However, you have to be careful speccing the holding work required.

 

Hello there. welcome to AAC!
For clarification you wish a do-it-yourself components cables motors sensors? Or perhaps an exoskeleton for the hand with cable harness, terminating to sensors proximal to the toes, to assist in grasping tasks performed in everyday activities?
I have personal experience in designing such a device for my nephew.It was back when I was in high school,very crude but effective.
Superior components can be attained today off the shelf.as other insightful members have suggested there's a variety of options. But I must suggest you see a healthcare professional and I do understand the exorbitant prices and the possibility of a do-it-yourself option.
Off topic; How come we don't have that smiley!! The threat starter used?

 

Have a look at this pdf by Paul Ventimiglia at Worcester Polytechnic Instititute.

 

Research muscles created with Nitinol, aka memory wire.

The range of motion with Nitinol can be extended by forming Z springs… It’s main disadvantage is the recovery time once the muscle is activated.

Hmm. I have seen next to zero practical applications utilizing nitinol.

It's only used in highly specific applications that typically involve shape change, not mechanical actuation.
I have played with nitinol myself and found it to be basically useless as an actuator. Its force/stroke/power output vs power input is not at all favorable.

 

I have some experience in robot arm designs a long time ago and I would suggest that a simple linear actuator/geared motor would be the easiest and cheapest to make.These days they are smaller and cheaper than when we used them and many more are available now.I have attached some references .

Actuator types
1 : linear actuators
2 : DC motor/Gearbox
3 : Threaded rod/ motorized
4 : Pneumatic Muscle (Inflated balloon)
5 : artificial tendon
6 : Memory wire
I prefer 1 and 2 ,then 4,5

Gary

 

Dear EE community,
For a friend that is paralysed in his underarms and hands, I wish to create 2 extra fingers, controlled by pressure sensors under his (very skilled) toes, so he can touch/push/grip. Much like the 2nd thumb project

Can someone give me hints about the right type of motors to use?
I would use the principle of motors winding a pull cable, and not so strong elastics to pull the fingers back straight when the motor is inactive. The finger motors should ideally be strong enough to grasp and lift about 2 kilograms, and this means that most of the time, they will be either off, or stalled in 'tight grip' mode. The motors should be able to handle being pushed in their opposite direction even when trying to wind the cable, for safety and since this will definitely happen with normal limbs.
They should ideally also be small and lightweight.
Which type of motors would be best suited, and why?
Links to similar projects or questions are very welcome too
Thanks a lot for your help!

Flexinol actuator wire "muscle wire".

1. Cheap
2. Easy to use
3. Simple drive circuit
4. Fast response time
5. No noise
6. No mechanical moving parts
7. Durable

 

I have had a further look and found some very small and cheap stepper motors,which could be driven easily from a micro and also current sensing could be used to have enable touch control ! plus the speed could easily be varied .For a prototype this would be ok.
Gary

 

I doubt that a stepper if that size would provide anywhere near the needed torque. Have you calculated what you need?

A geared DC motor would provide more torque in the same size.

Bob

 

A geared motor and using differential winding on a drum for the cables. Doing it that way eliminates the use of an elastic cord to return to unflexed position and overcoming the elastic force, allowing a smaller motor too.
By differential winding, I mean when the motor turns one direction it pulls in the cable on one side and lets out cable on the other. Then by just reversing direction it does the opposite. Many car electric windows use this principle to lower and raise them.