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Automation & Control

Hybrid Hexapods Resolve Inherent Weaknesses of Conventional Hexapods

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Paul Sheldon
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Iron
Re: Weakness not inherent
Paul Sheldon   3/24/2014 10:46:40 AM
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Ah HA! A fellow traveler. Good to meet you. And now we can do some machine tool talk.

I'll give you a slightly different take. Hexapods didn't catch on, not because of inherent shortcomings, but because they are difficult to design properly and a plethora of perfectly putrid designs were put forth, discrediting the whole notion. But it didn't have to be.

Adressesing some of your comments, we can use Giddings and Lewis's Variax as an existence proof. It was built and was measured to have the followiing characteristcs. The stiffness was approximately 5 times that of a very good conventional machine, 1.2 million pounds per inch spindle housing to pallet. It dropped only 50% to 600,000 pounds per inch at the extremes of the work cube. The native work space was not the umbrella of some other designs but an "inverted turnip" into which a nicely proportioned cylinder or cube could be inscribed and defined. The volumetric accuracy was measured a 0.0004", about 3 times better than convetional machines of the time. It was completely sealed against the insults of chips and coolant. It was run in real time without preproccesing on on pc assisted control of 20 years ago.

And so forth. My main point is the hexapods' cited weakness are not inherent , but there were a great number of bad designs put on the market. Properly designed hexapods exhibit superiorities rather than weakness.

I'd like to continue this discussion, but this format is a bit awkward. I will attempt to contact you through your Alio Industries sales department via email. Please tell them I'm coming.

Bill Hennessey
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Iron
Re: Weakness not inherent
Bill Hennessey   3/20/2014 6:59:29 PM
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Paul I believe we have similar back grounds. I paid my dues from Precision Valley to the Queen City. I'm very familiar with Parallel Kinematics in the machine tool world and for the exact reasons the Hybrid has no equal with traditional Hexapods(( or serial kinematics with gimbals)) that I gave, is the reason Hexapods never made it in machine tools due to poor stiffness, poor joint precision, controllers ability to keep up with forward and inverse kinematics(( most Hexapods to this day still do not have these equations due to joint and controller issues)),  limited umbrella work envelope and poor equation assumptions of the link lengths and spatial pivot locations.  Placing the work area in the center of the Hexapod links is  definitely novel and it will increase stiffness while creating big issues on machine loading plus chip and oil degradation of the links and joints.  Moving from center to the limit and getting 50% stiffness for a Hexapod is unequaled and certainly novel when all normal Hexapod designs are orders of magnitude less stiff but you prove my point that the Hybrid Hexapod is still the platform champion!  Beyond stiffness a Hybrid has orders of magnitude more precision than traditional Hexapods especially flatness, straightness and curved profiles.

Paul Sheldon
User Rank
Iron
Re: Weakness not inherent
Paul Sheldon   3/20/2014 12:15:04 PM
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Bill, thanks for your thoughtful reply. Before we discuss this further and delve into details I should clarify. What you say is certainly true of the hexapod as drawn in the article, but there's no need for that morphology. If you look at the drawings in patent 5,028,180 you will see a fundamental configuration which can have the characteristics I described. Patent 4,998,224 shows a configuration that we built and tested extensively. It had equal stiffness in X, Y,Z in the center of the work area, and at the extreme limits of the work area the worst vector stiffness was half that, and still extremely hiigh. Please take a look before we get into the niceties of stresses, and joint design, and such.

Bill Hennessey
User Rank
Iron
Re: Weakness not inherent
Bill Hennessey   3/20/2014 11:23:22 AM
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Paul  your assumption is based on Z axis stiffness with a Hexapod at dead center which seldom happens in our three dimensional world.  The X and Y stiffness on a Hexapod at this center position is 10 to 100 times less than Z stiffness depending on the design. When the links move from center in simple x or y moves you now have a situation where links have varying loads and much less stiffness in all axes. As you mentioned flex links or joint flexures are even less stiff and all bets are off for constant stiffness even on center. With a Hybrid Hexapod® the precision and stiffness is greatly enhanced for many reasons such as with a  x or y move you do not move the links on the parallel kinematic device, one linear axis moves in x or y and not 6 parallel axes and Hybrid 3 axis parallel kinematic bottom link is constrained to only have one degree of freedom which moves into the other 2 links making it orders of magnitude more stiff than 6 links with each having no constraints.

Paul Sheldon
User Rank
Iron
Weakness not inherent
Paul Sheldon   3/14/2014 5:59:57 PM
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A hexapod can be designed have equal stiffness in all directions when in the center of the work space. In addition, the stiffness can be very high, as a result of using tension and compression, rather than bending, of the members.

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