My gripper design, need a little help.

[corrado33]

Hey guys, I'm currently thinking of making a robotic gripper, and I've been playing around with a design in CAD, but I'm having a bit of trouble.  I've tried searching, but got nothing fruitful, as I don't REALLY know what this type of gripper is called.  Anyway, here's my design so far.  



Here's a pic with the jaws slightly open.  Everything looks fine here, this is with the 8mm linkage bar at A in the prev. pic.


Here's a pic with the jaws more open.  You can see that the 8mm linkage bar at A is not straight up any more, and that's where this design fails, because if I attached the other side, the hand wouldn't open.

What I can't figure out is how long to make the remaining bars.  I THINK that the bar going up from the middle (at the top of the triangle, point A) has to be as long as the pegs on each gripper half are apart.  The pegs on the actual grippers are 5mm apart, from middle of peg to middle of peg. (Yes it's a small gripper).  So therefore the bar going up from the middle, point A would also have to be 5mm, however when I try this my linkage bars hit each other.  So, if I'm right in assuming that that the bar going up has to be 5mm, then I would have to make my linkage bars "thinner", meaning they'd look more like cylinders than... flat things...  I've also tried an 8mm "linkage bar" in the middle, but that didn't work either.  The other linkage bars didn't hit, but... it just didn't work.

Any help of links to this type of mechanism would be useful.  I haven't really found anything yet, I just remember seeing a gripper that had this type of mechanism.  I'm sure that the lengths of the linkage bars have to be in some sort of ratio.  Here's the link to the solidworks file, but I'm not sure if it'll work considering you don't have the individual "parts", well n/m I'll just zip them all together and you can unzip and go from there.

Here's the .zip link.   http://www.box.net/shared/k4siteyjye

Thanks in advance.

EDIT:  I close the gripper by pulling up on the linkage bar at A, therefore closing the gripper.

[little-c]

5mm, may not be enough clearance for the bars.


I think you idea is correct(ages since I played with this, but it makes enough sense)

maybe make one bigger in cad, and scale it down? i.e, if it works on a cm scale, work out the max thickness of bar before it jams, then scale that down to mm scale.

[waltr]

My thought is to move the top arm pivot point on the jaw up a bit so the two almost parallel arms don't interfere.
Image shows the new pivot point and jaw extension in red. the new are location in blue.

[nottoooily]

What I can't figure out is how long to make the remaining bars.  I THINK that the bar going up from the middle (at the top of the triangle, point A) has to be as long as the pegs on each gripper half are apart.  The pegs on the actual grippers are 5mm apart,

The pins connected to the grippers, put them vertical from each other, instead of horizontal. Then everything remains parallel when it moves. Length of link A has to be the same as the pin spacing at the gripper.

It can give a lot more insight to plot the motion on paper using a compass. You might come up with better ideas than just plonking numbers into CAD and hoping it works.

Having said that, why make it so complicated? What purpose do all the extra links serve?

[corrado33]

Having said that, why make it so complicated? What purpose do all the extra links serve?

Thanks for the advice, but, I didn't see any other way to make it MORE simple?  If you only have one set of linkages connecting the two, wouldn't the force required to close the gipper with any strength at all be high considering you're converting vertical motion into horizontal when when the gripper is almost closed the one set of linkages are almost vertical? It's really hard to put these things into words so I hope you understand. 

[Soeren]

Hi,

It seems like you're mixing up two types of grippers?

The one with two arms each side doesn't need the pin/bar to slide on, but the two arms need to be equal length, so as to make a parallelogram.

If you want it simple, you could take a glance at a few types of bicycle handbrakes.

[corrado33]

Hi,

It seems like you're mixing up two types of grippers?

The one with two arms each side doesn't need the pin/bar to slide on, but the two arms need to be equal length, so as to make a parallelogram.

So... you're saying that the arms should look something like..... this...



Correct?  I have to mess with the spacing a bit with the bars to allow it to open completely, but that's not much of a problem. EDIT2: Yeah... can't get it to open completely, I wasn't thinking, but that's nothing that can't be fixed with longer linkages to make the gripper open more.

EDIT:

My thought is to move the top arm pivot point on the jaw up a bit so the two almost parallel arms don't interfere.
Image shows the new pivot point and jaw extension in red. the new are location in blue.

I tried this idea... it didn't work, it fixed the interference problem, but the distance from the middle pin going up from A and the pin on the gripper part constantly changed (more than it did before.)  But thanks!

[nottoooily]

you only have one set of linkages connecting the two, wouldn't the force required to close the gipper with any strength at all be high considering you're converting vertical

Yes the force can change dramatically, see "toggle mechanism", but you can't solve it with two mechanisms operating at the same time. That's analogous to driving a shaft with two gear ratios at the same time, it's always jammed.

To keep the force reasonably constant, just adjust the lengths to the angles never change too much.

[corrado33]

Yes the force can change dramatically, see "toggle mechanism", but you can't solve it with two mechanisms operating at the same time. That's analogous to driving a shaft with two gear ratios at the same time, it's always jammed.

To keep the force reasonably constant, just adjust the lengths to the angles never change too much.

I see, I was debating that, and it's still an option, however I'd have to move the linkages to the outside of the "claws", so that they aren't as vertical when the gripper closes, basically so that they still have an angle to them (if you understand what I mean).  I'll test both ways, and see which way is better, mainly which design requires less vertical force to produce the greatest horizontal, or closing, force for the gripper.  Your idea does require a lot less parts however, which is always good. 

[nottoooily]

I see, I was debating that, and it's still an option, however I'd have to move the linkages to the outside of the "claws", so that they aren't as vertical when the gripper closes, basically so that they still have an angle to them (if you understand what I

Yea it might end up a bit of a compromise. What requirements do you have? Specified opening range? Force? Actuator force? Do the jaws have to remain parallel? How will the actuator be attached? No point making something that feels really nice if it's better than it needs to be.

[Soeren]

Hi,

This, while not ideal, could perhaps be an inspiration.
And bottom of this page is some photos that might help as well.

[corrado33]

Yea it might end up a bit of a compromise. What requirements do you have? Specified opening range? Force? Actuator force? Do the jaws have to remain parallel? How will the actuator be attached? No point making something that feels really nice if it's better than it needs to be.

Hi again,
I do have some very loose requirements.  I would like it to open at least 2 cm, and I would like the jaws to remain parallel.  And well, as for the force, I would like it to be able to pick up pretty small things, as this gripper is small to begin with.  I haven't really decided how powerful I want to make it, but I think it's going to depend on WHAT I can actually build, and the actuators I am actually going to get.  I think I'm actually going to use an actuator rather than a servo, any advantages of one over the other?

[nottoooily]

Oh yea I remember the days of not having requirements. For me it usually meant never being satisfied and never getting it finished

Sounds like you won't have to worry about the stiffness/strength of the parts and the biggest factor is going to be if the actuator/servo (what's the difference?) can provide enough force and speed. You should do some calcs to check that or it might end up really wimpy and annoying.

If by actuator you mean something that's just on or off, that definitely sounds like a good idea because it's so easy to control. I can't really imagine needing to set the jaw position to some specified number of mm, just grab and release yea?

Hi again,
I do have some very loose requirements.  I would like it to open at least 2 cm, and I would like the jaws to remain parallel.  And well, as for the force, I would like it to be able to pick up pretty small things, as this gripper is small to begin with.  I haven't really decided how powerful I want to make it, but I think it's going to depend on WHAT I can actually build, and the actuators I am actually going to get.  I think I'm actually going to use an actuator rather than a servo, any advantages of one over the other?