Motion transmission through a mechatronical wrist

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Summary
What could be a way to transfer a rotary motion within a (micro) system that have two axial DOF, which are the yaw and the pitch (aka a wrist within a mechatronic arm)?
Hello,

As a personal project,I am currently designing a mechatronic hand, with a number of DOF similar to our hand. I've choose to transmit the axial movement of a finger through a rotary motion, because cable where causing too many backslash an other linear transmission became severely more complicated due to the wrist complexity. I need to transmit four movement trough the wrist, so here two little schema to illustrate the situation:
Initial position Forum.jpg
Untitled Note - Oct. 16, 2019 16.24.jpg

Here, the green vector is the orientation of the forearm and the purple one is the orientation of the hand. The rotation on the blue axis has a limit of 90 degree ,from the purple vector orientation,toward the positive and the negative y axis. The rotation on the blue one have a limit of 25 degree toward the positive z axis, and a limit of 30 degree toward the negative z axis. In other word, these schema represent the orientation of your hand from your forearm.

The motion reception part is within the hand, so it have the same orientation, and it will always keep the same position within it.

The axial actuator is a servo motor. The motion can be translated in every possible way that you may think of. The only problematic is to transmit the motion within the volume of a quater of your wrist (because there is four finger that need it, I am excluding the ring finger).

If you have an idea, I would happily listen to it. Of course, I have been working on my project, so here is two idea that seem functional. Feel free to critique them, I am creating this thread exactly for this.

1-Transmission with miter gears:

Within this system, the transmission is made by a pair of miter gears. There is a 90 degree translation within a piece that is acting as a link between the arm and the hand. Of course, this system required a spacing between the red and the blue axis. I did a prototype of this system, but it too bulky and does not look good nor practical. I am considering this one as my last resort because it is functional, despite the default. Here is an illustration of the system and a picture of the prototype:
Prototype example - Page 2.jpg
Prototype example.jpg

As you see, it is bulky and does not seem to be an efficient way to accomplish the required transmission.

2-Universal joint

I've never work with those before, but it seem to be a better way to build my wrist than a bunch of gear. However, I do not know if u-joint are actually made to transmit a rotation between two shaft that does not have a predefined orientation. So here is another schema (yes, I love them too) that is explaining what I am planning to do.
Prototype example - Page 3.jpg
Prototype example - Page 4.jpg

However, this system seems to be too simple to work as intended and, with my inexistant knowledge concerning these u-joints, I cannot tell if this is an appropriate use of this method of transmission.
 

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A lot of pilots will smile at this. The word is yaw, not yawn. Roll pitch and yaw. I edited the summary, but I can't change your graphics.
 
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I can't change your graphics.
Well, lets hope that nobody is going to see it...
 

Tom.G

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Here are some ideas.
Take a look at: https://en.wikipedia.org/wiki/Constant-velocity_joint

Scroll down about 60% of the page to Double Cardan Joint. Click on the image, then click on the new image to see an animation.

BTW, finger flexure is sometimes done with a cable to curl the finger and an elastic band to straighten them. Takes up all the slop that way.

Cheers,
Tom

p.s. your axis labelling has me doing just that.
 
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Here are some ideas.
Take a look at: https://en.wikipedia.org/wiki/Constant-velocity_joint

Scroll down about 60% of the page to Double Cardan Joint. Click on the image, then click on the new image to see an animation.
p.s. your axis labelling has me doing just that.
Thank for your answer Tom, but my second idea is using a double cardant. Please, would you tell me if the schema was too simplified or unclear?

BTW, finger flexure is sometimes done with a cable to curl the finger and an elastic band to straighten them. Takes up all the slop that way
Yes, I did this for the finger. If we count the DOF in any finger (except the ring one), we can count 4 of them. Three are on the same plan, and I am actuating them with a cable-elastic system, as you said. However, there still another DOF on a second plan, perpendicular to the first one. Since my ultimate goal is to create a practical hand. Using a cable-elastic system would create issue when it come to apply a force on the non-actuated side of the articulation. This is why I must transfer a "complete" motion this fourth DOF.
 
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Tom.G

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Please, would you tell me if the schema was too simplified or unclear?
I certainly wouldn't call them bad. Considering all the information you needed to transfer I think they are fine. I'm not as adept reading them as many here are and made sense of all of them (I think!). The gear train I didn't try though, too complex, must have that 'in hand' and see it operate. :oldsmile:

For the second plane of finger motion, you may be able to get away with the 'cable & elastic' approach. The strength in that dimension is similiar to that of finger extension force.

Cheers,
Tom
 
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For the second plane of finger motion, you may be able to get away with the 'cable & elastic' approach. The strength in that dimension is similar to that of finger extension force.
Just to be sure that we understand each other about the use of this cable and elastic system, here is another schema, where E is the force creating a torque by the elastic, and T is the force applied by the cable. R and B are some force creating a torque on the red and blue axis. As for the purple-blue arrow, this is a repesentation of the finger.
defined orientation forum - Page 2.jpg


The blue ovale represent one of the three axis that can efficiently use the cable and elastic system, on the plane that is parallel to the finger, but perpendicular to the palm. Here, a cable can be used to actuate the rotation toward the palm an elastic to bring it back. On the red ovale, the axis on the second plane, parallel to the palm. On the drawing, we see the situation that I want to avoid. If a force is applied on a side that isn't actuated by the cable, the elastic would not hold such a load and the hand would not be able to accomplish any complex prehension.

If I am using the cable-elastic approach on this red axis, my hand would not be capable of reproduce any complex movement, unlike a human hand.
 

Tom.G

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My contention was that since in the Human hand (at least mine when I tried it), Er is similiar in strength to Eb, or at least the Moment is. Admittedly there is less room to implement Er than to implement Eb, which may make this approach impractical.

Anyhow, it was just a suggestion to consider, which you did. After all, you are the one that needs to implement all this, and are in a much better position to evaluate the trade-offs!

Would be interested in a video of the working model when you reach that point. Many of us here have an insatiable curiousity. :cat:

Cheers,
Tom
 
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Would be interested in a video of the working model when you reach that point. Many of us here have an insatiable curiousity. :cat:
I would gladly share such a thing as soon as I accomplish my project, which should follow to the creation of this damn wrist.

However, if you have knowledge of the following matter, do you know if a double cartan, such as the one you shared, can transmit motion as efficiently while their are "moving"? By moving, I mean that both the shaft that they are linking are changing of orientation.
 
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Tom.G

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I have no direct knowledge either way. However, as I understand it, the advantage of the cartan design is the rotational speed of the input and output shafts are the same, with the center link instantaneous speed varying.

Ping @jrmichler , can you help with this?

Meanwhile, here is an image of another approach.

Cheers,
Tom
 

jrmichler

Science Advisor
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do you know if a double cartan, such as the one you shared, can transmit motion as efficiently while their are "moving"? By moving, I mean that both the shaft that they are linking are changing of orientation.
I don't think that continuously changing shaft orientation will have any significant effect on rotary motion transmission. Just one thing to watch for; the greater the angle, the greater the velocity variation of the center piece. Not a vibration problem at the low speeds of a mechanical hand, but the internal forces increase at larger angles between shafts. Worded differently, the joint gets weaker at larger angles.
 
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I don't think that continuously changing shaft orientation will have any significant effect on rotary motion transmission. Just one thing to watch for; the greater the angle, the greater the velocity variation of the center piece. Not a vibration problem at the low speeds of a mechanical hand, but the internal forces increase at larger angles between shafts. Worded differently, the joint gets weaker at larger angles.
Allow me a quick reformulation:

When the angle is going up, more the the constraint on the mechanism is going up, and more the rotation of the unactuated part of the joint is phasing out. Is ist right?

And if it is, could a double cardan joint resolve this phasing issue?
 

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