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I need a mechanism to switch the vertical position of an assembly in as square a profile as possible - i.e. a normal crank mechanism produces a sine wave profile, to explain myself better. I've seen some quick return mechanisms (and this) - these obviously travel slowly in one direction and 'quick return in the other - I need the same thing, as quickly as possible, and in both directions.

Any ideas?

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    $\begingroup$ I assume you need to convert rotary motion to linear (i.e. use a motor). Otherwise its best if you use a linear actuator. $\endgroup$
    – NMech
    Aug 24 at 10:35
  • $\begingroup$ I imagine a crank mechanism with a spring and some locking mechanism that only releases once the spring is applying enough force - either triggered by the spring force itself or by the crank position. $\endgroup$
    – user253751
    Aug 24 at 10:45
  • $\begingroup$ @NMech As with most things, it should be small, cheap, low power - I've considered using a linear solenoid but 1 - I don't think thats going to be half as small, cheap and low power as a rotary motion with a mech, and 2. solenoids are problematic in this application for other reasons. Linear motors are completely out of space, money and power limits for this application. Any other ideas for using a linear actuator? $\endgroup$ Aug 24 at 11:36
  • $\begingroup$ @user253751 great thinking, I will try to complete the idea $\endgroup$ Aug 24 at 11:37
  • $\begingroup$ PLease provide a LOT more detail on the source power mechanism. If it's rotary, for example, take a look at actuating cams as used in race cars to drive the valves. $\endgroup$ Aug 24 at 13:07
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One of option might be the following

enter image description here

figure source with video

You can adjust the contours of the cam to create a more square profile.


square velocity profile

The following mechanism also has a squarevelocity profile. See video at 0.29 for an animation

enter image description here


linear actuators

There are linear actuators different versions e.g. with a solinoid or with a planetary gear system e.g. see.

However if they are applicable to your problem is depended on :

  • cost
  • the forces that you need to apply
  • the maximum velocity and acceleration profile.
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  • $\begingroup$ Cheers @NMech, just the thing I was trying to think of! $\endgroup$ Aug 24 at 13:19
  • $\begingroup$ I am not sure which one you are referring, however I am sure there are bound to be other possibilities with other pros and cons. $\endgroup$
    – NMech
    Aug 24 at 13:26
  • $\begingroup$ The first answer above with the star shaped cam I mean - the other idea below is interesting and a cool video, but too complicated and, the way its made here, actually a triangle wave rather than square (though it could be adapted to become a square - interesting idea will look into this too) - what you say about linear actuators very true, I will consider, cheers $\endgroup$ Aug 24 at 14:50
  • $\begingroup$ Yes. I think the first option is the best one in terms of complexity and velocity profile compromise. $\endgroup$
    – NMech
    Aug 24 at 14:53
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There is a four-bar linkage that will do the trick: it is the mechanism in an old-school movie camera which engages the sprocket holes in the film, jerks the film down suddenly to the next frame while the shutter mechanism is shut, and then resets as the shutter opens.

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  • $\begingroup$ @joojaa if the answer to this question is so easy to you, why didn't you put down an answer???? $\endgroup$ Aug 26 at 14:33
  • $\begingroup$ @niels nielsen can you find an image or video of this mechanism? $\endgroup$ Aug 26 at 14:34
  • $\begingroup$ @OliverWalters well consider that a quick rerurn fourbar mechanism is just a fourbar that is nonsymmetrical to give quick return. A symmetrical fourbar is just simply a forth an back just as quickly. But really i dont know if you want a dwell mechanism instead. $\endgroup$
    – joojaa
    Aug 26 at 14:52
  • $\begingroup$ @joojaa Yes it needs to be able to dwell at each 1 or 0 state - I don't see how a mechanism which produced short sharp back and forth movements couldn't have a dwell time? Anyway, please post a link or some kind of illustration of how a fourbar could achieve this $\endgroup$ Aug 26 at 19:06
  • $\begingroup$ @niels nielsen is it not the geneva mechanism which achieves this? I haven't seen a version which produces bidirectional back and forth movement though? I believe in a camera its unidirectional pulling each slide into place from a reel no? $\endgroup$ Aug 26 at 19:11
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Obviously you can not make a perfect square wave with any mechanism simply because they would need a infinite speed, acceleration, jerk (snap, crackle and pop). So no discontinuous functions and some finite raise time.

So from a design perspective there are many possible places to look at solutions. But the locations where I would concentrate my efforts are

  1. Cams
  2. Linkages
  3. Bistable mechnisms

First cams. Like said the canonical cam to start out with is the modified scotch yoke with a dwell because its simple to understand and easy to reason with. (not because its the best possible cam to do this, but gives you some insight into cam design in this case). The cam you got in @NMechns answer is okay but read a bit further when i talk a bit on the details of cam design. So the 2 dwell scotch yoke looks as follows:

enter image description here

Image 1: A modified scotch yoke. OUter black line is follower inner surface. the dashed circle the path the cam takes.

I have added an dwell in both the horizontal and vertical of the yoke. Just to show they are possible, other dwells could be built. each dwell consists of a offset base circle. The two dwell is not the one you are looking for but im just showing it for completeness, image 2 shows its movement function and its relationship to a non modified scotch yoke.

enter image description here

image 2: Graph if the x movement.

In reality you don't need the scotch vertical dwell you only need the horisontal one See image 3. In fact you never see the vertical dwell in practice because its problematic.

enter image description here

Image 3: Scotch yoke without the vertical dwell.

This should give the intuition needed to design the internal cam you need. If you need a faster raise time you can just slant the move lines a bit inward.

The role and shape of the blends

Remember when I said that you can not have discontinuities in your graph. Well if you do have discontinuities then your system is going to bounce. In general bad things happen if you bounce, either the cam follower loses contact or it causes a huge load in the rest of your system. The secondary result is that this causes the surfaces to erode reducing mechanism lifespan dramatically.

So for any high speed cam all of your shapes have to match:

  • Position
  • Velocity (first derivative of position)
  • Acceleration (second derivative of position)
  • Jerk (third derivative of position)

Now these functions are a bit tricky to work with. But literature has good candidates.

Other things to watch out are:

  • the friction between cam and follower if you rely on sliding. Choosing the material pairs is important. Here the Scotch yoke is a good candidate since the central pin can be changed to a bearing so it can have genuine rolling instead of sliding.

  • the load can cause the cam to bounce. Which is why many cam systems have a spring. The scotch yoke (especially one devoid of the vertical dwell) does not have a problem with forces that move against the cam, but can have problems with something pulling the cam along at the same direction the cam is moving. If you have this problem put a inner race inside the yoke too this way it cant move too much(This also applies to NMechs cam too which i would modify to be a grove cam), or use a spring load.

Could a four bar do the job?

Not alone. I tried to find a suitable four bar in my catalog of known paths. No suitable on hit my eye. Its still possible i don´t have easy access to a four bar curve fitter so who knows. But a six bar can do it.

However, you can take a bi-stable mechanism and drive with a crank rocker. Then you could do this. The plus side is that this could be really fast. I dont have time to sketch this out but imagine a rocker next to a electrical latching switch and the rocker has two pins that move it form state to another one when moving back and one when moving there.

Anyway i dont have much more time for this.

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