mechanism for angular stroke 1

[hobbs101]

Hi. I'm developing concepts for a mechanism that I need to design. The requirements are

- move a metal block (4"x4"x8") along a circular path
- stroke angle = 11.5deg
- radius of circular path = 0.5m
- needs to be accurately positioned at any intermediate or end angle
- manual adjustment, i.e. very low speed.
- has to work in all orientations, i.e. changing gravity vector.

The problem is I can't use the centre point of the circular path as this is inaccessible. So, what I'm looking to do is use a clever design that gives me movement along a circular path in a compact envelope.

I was hoping a 4-bar linkage of some sort would be the solution, but as far as I can see this type of linkage would also give me a parasitic motion away from the virtual pivot point. This may not be an issue, but I don't think it's ideal. I'm not experienced in designing linkages, so perhaps I've got this wrong.

Another option is to use roller guides moving along angular slots. I was hoping for something more simple and elegant than this. Also, maintaining position with changing orientation is a challenge.

Perhaps a combination of linear and rotary motion might be an option? A linear slide with a coupling to a rotary mechanism?

Any thoughts on the 4-bar linkage? Any other possible solutions?

Thanks in advance for your help.

 

[gruntguru]

One of these? Link

je suis charlie

 

[hobbs101]

They look like a good option. I'd be concerned of the block crabbing, but assuming the carriages are stiff enough I guess that might not be an issue. I would probably need to drive it from one end only. Ideally it would be driven from the centre.

I'd be interested to hear anyone's thoughts on linkages. Thanks.

Hobbs101
Mechanical Design Engineer

 

[jlnsol]

How much movement of the circle centre point is allowed if any?

 

[IRstuff]

Bigger question is what do you mean by "accurate?" An inch, a millimeter, a micron?

TTFN
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[Occupant]

Maybe this might work -->

 

[btrueblood]

Any large-diameter ball bearing should be able to work, based on your description. Or are you limited to only a small angular segment as well?

 

[jlnsol]

The number of mechanism to do this is unlimited. Here are the first two;-)
One exact and one approached circle.

 

[Jboggs]

I have used these items with very good results. Heavy duty, easy to assemble, solid.

http://www.bwc.com/products/rotary-guide-systems/prt2.html

http://www.bwc.com/products/rotary-guide-systems.html

 

[MintJulep]

Double Parallel Crank.

 

[hobbs101]

Thanks for all the posts.

In terms of accuracy, the block needs to be positioned and maintained at an angular accuracy of 0.25deg. Other tolerances are TBD, but will be in a similar ballpark.

I'm not sure any of the suggestions will fit into my envelope, other than the BWC ring segments. I need to clarify, I need to minimise the envelope in which the block is housed, so not only is the centre point of the arc inaccessible, but I don't have room for other large linkages or mechanisms. I need something that is compact.

I like the idea of the circular track segments, but what I really need to know is ... is there a compact linkage arrangement that would also do the job?





Hobbs101
Mechanical Design Engineer

 

[arbreen]

It sounds like you're looking for a goniometer stage:

https://www.google.com/search?q=gon...v&sa=X&ei=WhmdVfiEOozfoAT7y7uQBg&ved=0CDQQsAQ

 

[hobbs101]

I've considered goniometers as I've used them before, but I need a specific circular path radius and I'm not sure bespoke goniometers are easily available.

It's the linkage option that I'm trying to explore. Is there a linkage solution or can I discount it?

Hobbs101
Mechanical Design Engineer

 

[arbreen]

Does this linkage need to control both position and angle? At least it would be possible to use a pantograph linkage to translate circular motion, but it would be necessarily very large. It does appear that any linkage that traces an arc will not also point your block at the correct angle. Can you use a 1- or 2-sided 90-degree spherical link (gimbal) to offset from the center point, rather than placing the hinge directly in the middle?

Is it going to be actuated? Under NC control? You could use a COTS gonio stage in concert with a linear stage to keep the block facing the center point but not move along an arc, or you could use an x+y+rotary stage and computer-control the radius and angle. You could use 2 or 3 linear actuators to produce the same motion in a different footprint, like a 2D stewart platform. THK for example sells profile rail (model HCR) with a .5 m radius of curvature, though you would have to buy a whole 60-degree segment and cut it down.

Many of these concepts are pretty expensive. A simple and cheap option would be to put rollers on the bottom of your block and pre-load it against an arc-shaped track.

 

[hobbs101]

Hi Arbreen

Yes, both position and angle. The attached shows the layout. The block moves from the 0.5deg position to the 11.5deg position on a radius arm of 500mm. The red box shows the approximation envelope that I have to fit the mechanism.

Note, if the block translates along the radius arm during the rotation, i.e. the 500mm changes, then this is not a problem.

I agree, all the linkages I've looked at don't pure radial motion. There always seems to be some parasitic motion, either a rotation of the block or translation.

The pantograph would be too big. I'm not sure I understand the '1- or 2-sided 90-degree spherical link (gimbal)'. Do you have any examples?

The first machine will be manual. The subsequent machines will be powered. So, I'm thinking that I'll design it for motor/gearhead actuation, but with this replaced by a handcrank for the manual version.

I like the rotation/linear stages combination, but I fear it'll be too expensive and won't fit within my envelope. I've seen the THK HCR rails, and they are currently my favoured solution. I just need to discount the linkage concept first.

Yes indeed, cost is a big factor, so a simple arc shaped track is probably the way to go. I'm thinking that a gear segment / wormwheel would be a good solution for the actuation as this will prevent any backdriving when the orientation of the mechanism changes.

Hobbs101
Mechanical Design Engineer

 

[arbreen]

It looks like a gimbal arm would also be too big. The arm would be something like the attached picture, coming into or out of the page w.r.t. your diagram so as to dodge your target, but the overall envelope of the machine still have to go to center, just on a different plane. You won't need to get special hardened, ground steel worm gear sections. Consumer telescope mounts use an aluminum gear and brass worm, and achieve sub-arcsecond positioning with no life problems under small loads. Your loads and precision requirements appear to be orders of magnitude less (.25 deg @ .5 m is >2mm), so coarse pitch plastic would likely work fine for your application, and even a 3D print could be a functional prototype.

 

[hobbs101]

Yes, I like the idea of the telescope mount mechanism. I'll have a look at those. Thanks.

Hobbs101
Mechanical Design Engineer

 

[raz2]

Having a rigid body rotate around a point is not possible with a 4 bar linkage if you cannot use the center point as a pivot. However this can be done with a 6 bar linkage. It is called a remote center mechanism. I'll attach a video to illustrate and a paper describing the construction.

 

[raz2]

construction instructions

 

[tbuelna]

The link from gruntguru showing semi-circular opposing V rails with preloaded rollers supporting the base seems like it would provide the best combination of stiffness at all orientations, precision and minimum number of parts. A linear actuator attached to one end of the base, using a roller screw and motor/brake, would provide efficient, precise and low backlash positioning.