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  1. #61
    Mfg Engineer Scott_bob's Avatar
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    I don't know guys, this is a lot harder than you think. I think...

    If anyone can do the control logic it would be:

    http://www.numeryx.com/cnc/index.htm
    Unbelievable smooth, motion control...
    You just can't believe it till you see it.

    Scott_bob

    Scott_bob


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    Gold Member chuckknigh's Avatar
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    I think I can see that idea, in my head. The only down side to it, is that I live in a small town, and unless I want to build it the size of a tractor, from actual tractor parts... Let's just say that gears, racks, pinions, etc are just not available out here...unless you want them to literally be 10" diameter for the pinion.

    I think you're right, though...that would be a very simple arrangement. I see two possible down sides...

    1) The racks will "stick out" at a virtually common point, on each of the 3 corners. There is the potential for interference...but that exists with allthread, too. The two corners will have to be *slightly* askew, in order to not interfere. Not sure how that'll affect the geometry, but I'm hoping it'll be negligible.

    Worst case scenario would be when they're fully retracted "out" of the machine, at which point they'd be almost horizontal. Move them back in, and you have a "scissor" arrangement, on the exterior of the machine.

    2) The angles of the struts will change, across a 60 degree arc...if the motor and pinion are mounted at the corner, and the whole corner "pivots," this might solve the problem. I don't think it's a problem, in reality.

    I wonder if I could do something similar, with parts I can get locally? Hmmm...

    -- Chuck Knight



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    Just think how solid it would be the size of a tractor, no vibration!

    Ok go back to the allthread and nuts. The nut is captured in the gear part of a worm gear assembly and the "worm" gear is driven by the motor through the splined universal joint. And the whole worm gear assembly floats with the nut on a bearing that would swivel as the arm changed angles. Ron



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    Default Tell Me if I'm looking at this wrong!

    I wouldlike someone to clarify some of this for me because I may be loking at this all wrong.

    It seemed as though a leap was made in this thread that the hexapod has six axises or six degrees of freedom because it has six diferent devises that cause motion in six different directions. (I'll call these actuators)

    To me, the axises are the driections that the tool can move in regardless of how many actuators it takes to cause that motion. If a tool can move in three perpendicular directions (X, Y, Z) while remaining verticle the whole time, this would be a three axis machine. If you throw in the ability to change the tool angle from verticle, this would be a 4 axis machine. And so on.

    A good example of what I'm talking about would be the gantry type machines that have been built that use two seperate motors and lead screws (actuators) to move the gantry along its axis. Just because there are two actuators here, there are not two axises here. It is only one axis controled by two actuators.

    Now as far as a hexapod is concerned, there is no doubt that it is capable of moving a tool along many axises. But it would seem to me that the best way of approaching developement would be to build it and develope it to opperate only as a three axis machine to start out with. Then, once developement of this mode of opperation is satisfactory more axises can be added.

    The construction of the machine itself would be the same regardless. The difference between 3, 4, or five axises would be in the controlling software. (A software upgradeable machine that could grow along with it's opperator?)

    It also seems that by doing it as a three axis machine first, existing code could be run on it without modification since it would be the controllers task to translate the three axises in the program into the six proper signals to move the machines actuators.

    To put it another way: If you only move the tool in those three axises then those are the only axises that need to be in the program. (The G-code should remain the same)

    The last benefit would be in the developement of the controller software. The algorythms for the three axises could be written and put into opperation before the remaining algorythms are completed. This allows us to have working machine sooner. I think the bussiness types call this time to market?

    I could be completely wrong about all of this though.



  5. #65
    Gold Member chuckknigh's Avatar
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    Default Re: Tell Me if I'm looking at this wrong!

    It seemed as though a leap was made in this thread that the hexapod has six axises or six degrees of freedom because it has six diferent devises that cause motion in six different directions. (I'll call these actuators)
    Actually, a hexapod had 5 DOF (degrees of freedom) which are X, Y, Z, pitch, and yaw. A special type, called the rotobot, adds a 6th DOF...rotation.

    To me, the axises are the driections that the tool can move in regardless of how many actuators it takes to cause that motion. If a tool can move in three perpendicular directions (X, Y, Z) while remaining verticle the whole time, this would be a three axis machine. If you throw in the ability to change the tool angle from verticle, this would be a 4 axis machine. And so on.
    Yes, though that isn't the only, possible arrangement. A CNC lathe might include X, Y, and tilt...no need for Z.

    Now as far as a hexapod is concerned, there is no doubt that it is capable of moving a tool along many axises. But it would seem to me that the best way of approaching developement would be to build it and develope it to opperate only as a three axis machine to start out with. Then, once developement of this mode of opperation is satisfactory more axises can be added.
    We're in perfect harmony, so far. Take a look at the image I included, just above. You'll notice that there is an upper "tripod" and a lower "tripod."

    Either tripod is capable of motion in 3 dimensions...by linking them (an easy upgrade, in my mind) we can gain additional degrees of freedom.

    Thinking about it, keeping the router vertical with this geometry simply requires a "special case" condition, in which the upper and lower tripods have identical geometry. That's why I want to move 2 screws with a single motor...one motor driving both the upper and lower corresponding strut.

    I could do it with separate motors, and separate controllers, but at that point I'd have built a 5DOF machine...greater cost, greater complexity, and to be fair, greater capability. But NOT an entry level unit.

    The difference between 3, 4, or five axises would be in the controlling software. (A software upgradeable machine that could grow along with it's opperator?)
    Assuming the geometry supports the additional movements... In the example, above, it would. It would, however, require a hardware upgrade as well...an additional set of steppers and driver electronics, and disconnection of the link between the upper and lower struts. Not a big deal...

    It also seems that by doing it as a three axis machine first, existing code could be run on it without modification since it would be the controllers task to translate the three axises in the program into the six proper signals to move the machines actuators.
    Actually, there are standards for any number of axes, within G-code. You just tell your CAM program to generate code for a 3 axis, 4 axis, 5 axis, 6 axis...whatever machine.

    To put it another way: If you only move the tool in those three axises then those are the only axises that need to be in the program. (The G-code should remain the same)
    Assuming you're talking about the g-code interpreter, that's technically correct. However, since most software is designed in a modular way, it might not be as big a deal as you think to include support for multiple configurations.

    I could be completely wrong about all of this though.
    Sounds completely reasonable to me...

    -- Chuck Knight



  6. #66
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    Hexapods DO have 6DOF the only thing is that when it is the spindle that is moved it is a pointless one. i.e. It allows the spindle as a whole to be rotated.

    A tripod can produce 3DOF movement of it's hinge assembly but as it has only three legs the other axis are not "limited". By that I mean that although you certainly only need 3 actuators for 3 axis the structure still needs to support the tool in the other axes.

    So with this in mind you would have to build both top and bottom halves of the structure. But in order to ensure they move in unison yo will need to have 6 actuators.

    So your suggestion is to drive two screws with a single motor, but you have to make sure the screws have the opposite action, one leg shortens while the other lengthens. Coupling two leadscrews that can change there angles constantly is difficult enough but this would make it even more difficult. I might say impossible if I wasn't so against the word

    The other option is to have 6 motors but drive them in pairs with one of each pair having a wire reversed to give oposite motion.


    Graham



  7. #67
    Gold Member chuckknigh's Avatar
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    A tripod can produce 3DOF movement of it's hinge assembly but as it has only three legs the other axis are not "limited". By that I mean that although you certainly only need 3 actuators for 3 axis the structure still needs to support the tool in the other axes.
    True. To this end, I'm investigating two possible arrangements...one is the stacked tetrahedra model that I posted above, and the other is a tripod made up of parallelogram linkages. Either should restrain the motion in the other axes.

    So with this in mind you would have to build both top and bottom halves of the structure. But in order to ensure they move in unison yo will need to have 6 actuators.
    Almost correct...you have to have 6 active actuators. That doesn't necessarily mean 6 motors.

    So your suggestion is to drive two screws with a single motor, but you have to make sure the screws have the opposite action, one leg shortens while the other lengthens.
    No...to keep a router vertical, you need only extend the upper and lower struts the same distance. That means that they can be driven the same direction, if the screw thread is the same handedness.

    Coupling two leadscrews that can change there angles constantly is difficult enough but this would make it even more difficult. I might say impossible if I wasn't so against the word
    Ah, a man after my own heart.

    Here are a few thoughts. Assume the stacked tripod arrangement shown above.

    Drive the screws at the "common" point, located at the 3 corners of the stand. Their distance from one anothr is constant, but their angle is variable.

    Two possibilities spring to mind. One is a gear/pulley system that would drive both screws from the same motor. Remember...their distance is fixed, only their angle changes. With the right geometry, some gears could be made to mesh over a reasonable set of angles.

    A simpler and more obvious way to do it might also be a "flex shaft" feeding both screws. I think there'd be a lot of backlash in a system like this, but nevertheless it would drive both screws at once, with a minimum of fuss.

    Hardly impossible...doing it well is a challenge, though.

    The other option is to have 6 motors but drive them in pairs with one of each pair having a wire reversed to give oposite motion.
    Increased complexity and expense for a 3DOF machine...why not just build a 6DOF machine and be done with it, if you already have 6 motors and driver boards?

    -- Chuck Knight

    P.S. Update: I was at my local flea market, yesterday, and had a brainstorm about cheap ball joints. A vendor had some very nice "ball" drawer pulls, which are a very good ball form, and which come pre-tapped. Now, the thread is tiny...but it's a starting point. Some nylon washers, or maybe PVC pipe fittings, and I might have a cheap and workable ball joint.

    -- CK



  8. #68
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    >Almost correct...you have to have 6 active actuators. That doesn't necessarily mean 6 motors.

    That is what I meant.

    >No...to keep a router vertical, you need only extend the upper and lower struts the same distance. That means that they can be driven the same direction, if the screw thread is the same handedness.

    If you want to lower the spindle for example then for the lower set of legs they must be lengthened. For the upper set of legs they must be shortened by the same amount. Or is the upper tripod not fixed firmly to the shaft between the two tripods but allowed to slide up and down the pole so that it is only it's X and Y position that is important. This is what is done on a Tetrapod where I think the shaft it help vertical bya conventional x,y stage.

    >Two possibilities spring to mind. One is a gear/pulley system that would drive both screws from the same motor. Remember...their distance is fixed, only their angle changes. With the right geometry, some gears could be made to mesh over a reasonable set of angles.

    They would have to be plain pulleys rather than toothed not a good CNC solution. As for meshing gears the angle would be far more than reasonable.

    >A simpler and more obvious way to do it might also be a "flex shaft" feeding both screws. I think there'd be a lot of backlash in a system like this, but nevertheless it would drive both screws at once, with a minimum of fuss.

    Then you need a gearbox to produce two flexible shafts.

    Replace impossible with impractical.

    Graham



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    Hadn't been reading this thread until today. Instead of moving the workpiece, consider this approach.

    http://www.mmsonline.com/hsm/hsmevent/dm2bmach.html



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    Chuck:

    Concerning the problem of driving two struts from the same motor:

    This seems like the perfect application for the alternate geometry you mentioned earlier in this thread, I think you called it the "HexaGlide" in post #44 of this thread.

    Leave the strut length fixed (and equal) and attatch the ends of each pair of upper and lower struts to a common actuator. You will have three acuators in a horizontal plane, each pointing toward the center of the machine. On the end of each of these actuators will be two struts (an upper strut and a lower strut). All three of the upper struts will meet in the center above the horizontal plane and all three of the lower struts will meet in the center below the horizontal plane.

    Just imagine the machine in the picture in post #59 of this thread with solid, equal length struts. Each pair of struts connected to a single actuator sticking horzontaly out of each of the three corners of the machine.

    This method might solve a couple of problems such as the need for fancy ball joints for the struts to go through. I would think some simple heim joints would work well.

    Pat, the sober pollock

    Patrick;
    The Sober Pollock


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    sbrpollock,
    Actually that may be a very good idea. I have been leaning toward trying the Hexaglide because of the idea I think it would be easier to construct. Would transmit screw error less than actuators that shorten and lengthen directly I think. Build the trolly rails out of C channel with the screw down the center and roller blade bearings to run on. Also use square aluminum stock to machine out nice little universal joints using skate bearings to swivel on. I was also thinking about running the trolly rails out from the center on a triangle or hexigon configuration instead of parrallel to each other to make calculations for control software easier. One could actually build it with six trollys and rails and use belt or gear drive to run two trollys the same if you wanted to have just the 3 axis machine and be able to expand to the five axis later with six motors and controllers.
    Not sure about getting software to interpet g-code to run the actuators though. Maybe bb99 or anyone actually might have a thought or two about that. Any thoughts from anybody? Ron



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    Although I am not certain I've heard that Linux EMC can control a hexapod. At least that's what I am hoping cause I too would like to build one of these instead of a big gantry.

    Brian



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    BrianS,
    Yes the guys over at the EMC site says there is a basic setup in EMC for Hexapod. I have not tried it yet cause I don't have a computer to didicate to Linux and at the present work has been cutting into my play time way too much. If you have read this whole thread you will find how to find the EMC site and you can join it. If you don't find it, I think if you do a search on yahoo it will come up, I think, been a while since I joined it. Someone here will surely be able to tell you where to go. Keep us posted Ron



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    Default Keeping the router upright.

    Hi all. Newbie here.
    I think this might be a simpler approach to a 3-axis design. It uses a single parallelogram and doesn't need any additional drives to keep the router upright.
    The double tetra is a great idea for more degrees of freedom. For now tho I don't know about you but I'd rather start with the simple design.
    Any thoughts?

    Hope the attachment works....

    Red dots are ball joints with linear slide, black dots are plain ball joints. It works. The parallelogram needs to be kept vertical though, and not allowed to twist, so it'd have to be beefy.
    Oh, the vertical links shown are fixed, one on the base and one on the head.



  15. #75
    Gold Member chuckknigh's Avatar
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    I don't see the attachment, and I'm curious now. Do you have a direct URL?

    -- Chuck Knight



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    Ok, I can't seem to get attachments working. No URL unfortunately, but if anyone wants I can email a bmp or slasm file.

    I'll explain the thing. 1000 words, LOL.

    Basic parallelogram on one strut only. The strut is longitudinally extendible. Lets call it a pneumatic piston for sake of visualization. Oh, and lets pretend that the piston rod can't rotate relative to the cylinder. Bear with me..!
    Might be easiest if you try to sketch it.

    Base and head are triangles.
    The piston is attached by ball joint to the base.
    The end of the piston rod is attached by bearing to the head, allowing rotation in one plane only (vertical).
    The base has a fixed vertical extension that makes up one end of the parallelogram.
    The head also has a fixed vertical extension, makes up the other end.
    Stack another piston (but this one is undriven) on top of the first to make up the top of the parallelogram. It's mounted the same as the first, with ball joint to the base (top of base extension) and bearing to the head (top of extension).

    So far we have a basic parallelogram, which can extend in and out. But the head can still rotate away from vertical around the axis of the strut bearing.

    Here comes the trick.......Put a fixed length link (same length as the other two vertical extensions) between the top and bottom piston rods. Mounts by bearings, not ball joints. The ends of the link are fixed to the rods, but free to rotate.
    Now the head is fixed vertical. If you've sketched this assembly, try moving the driving piston rod in and out. The link will always stay vertical. No proof is required.....the vertical extensions are the same length as the link, so it too will always be vertical as its length is fixed. It will push the top piston rod in and out the same distance as the bottom piston rod, no matter what the angle. Awesome.

    Now the other two struts are connected as usual, to control the thing.

    Okay, not too concise, but hopefully you get the gist.

    Maybe someone can help me get a picture up, cos I can't work it out. I can upload a file with the "Manage Attachments" button, but it doesn't stickto my post. I can click the "Insert Image" button, but I don't know the URL for the uploaded files.



  17. #77
    Gold Member chuckknigh's Avatar
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    Just send the picture to me, and I'll post it for you. I have very little trouble with attaching pictures, as this thread proves! :-)

    Here's your picture...

    -- Chuck Knight

    Attached Thumbnails Attached Thumbnails Hexapod designs?-tetra-gif  
    Last edited by chuckknigh; 11-13-2004 at 07:25 PM.


  18. #78
    Gold Member chuckknigh's Avatar
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    OK, your picture is available in the post, above. Let's open the floor for comments.

    -- Chuck Knight



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    I don't quite follow the idea in posts 76 and 77. If the head is to remain vertical, wouldn't the upper link of the parallellogram also have to shorten and lengthen in concert with the lower one ?

    any other recent developments or bright ideas about tetrapods or simplified 3D hexapods ??



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    Andy, that's right, this is what the link between upper and lower struts is for.
    I'm still thinking and planning on this idea. Brain strain on the geometry of an elegant solution. Maybe there isn't one. Might post something further soon.



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Hexapod designs?

Hexapod designs?