Achieving Varying Damping and Stiffness Profile - Need some ideas! 1

[cHollingbery]

Hi guys and gals,

I'm trying to achieve 2 profiles for stiffness and damping that you can see here: (Copy and paste into browser:

http://img399.imageshack.us/my.php?image=profilespk9.jpg

)

The max initial velocity will be 4.44 m/s for an object of about 150kg that will be sliding downwards at 30 degrees to the horizontal (in a straight path) without any other propelling force.

I'll only be taking the displacement to 6cm and have roughly 30cm (in the direction of the motion) x 20 cm x 5cm (height) of space for both damping and stiffness elements.

I have a few ideas but don't really have any experience with the kind of components that will or could be used. What I am essentially asking for are a few ideas!

Thanks for your time - it's much appreciated!
Chris

 

[drawoh]

cHollingbery,

Read up on cars with rising rate suspensions. That might give you a few ideas.

I designed a custom rotating solenoid with a descending rate spring. The spring force declined as it extended. I did this by attaching the spring to a rotating base such that in the open position, most of the spring force acted normal to the radius, not tangentially.

Does your force profile have to be exactly as-shown, approximately as-shown, or sort of as-shown? If it is sort of as-shown, it should not be difficult to draw and analyze some linkages.

How critical is accurate damping? This sounds like a challenge.

If you want exactly as shown, you may have to design a servo-motor system with a feedback loop, and use a microprocessor to create your force profile. I think you should even be able to model a damping profile.

JHG

 

[cHollingbery]

Thanks drawoh - The rising rate idea looks extremely promising! I've been looking for some documentation on design of the linkages & spring selection but have found nothing - know of any? If not could you briefly outline the theory and calcs needed?

Luckily both the profiles don't have to be exactly as-shown - more 'as near as is realistically possible'.

Yes - the damping has been my main worry too! Could it be possible to incorporate both damping and stiffness elements into the same linkage system and still achieve the 2 profiles?

Thanks again

 

[drawoh]

cHollingbery,

Force is a function of spring displacement. Damping is a function of damper velocity. Work out the behaviour that you want, and work backwards to your spring and damper. Judging from a quick look at your graph, you need two separate linkages. You are going to have to spend a few hours trying stuff out. You should analyze more than one concept. It is not just the force and damping characterstics. It has to fit in your available space, it has to remain intact, and you have to be able to fabricate and assemble it.

3D parametric CAD like SolidWorks probably will make this easier. Hopefully, you have taken mechanics of machines, machine design, and vibrations as part of a mechanical engineering course, and you understand all this stuff.

You should consider doing all this electronically, and keep your mechanical structure simple. A simple structure is probably a sturdy, well understood structure. I would not want any 150kg items hurtling around me at 4.44m/s (10mph?).

JHG

 

[zekeman]

Please post what you are trying to achieve. Is stopping the mass the idea or is this an academic exercise?
If you are trying to stop the mass, you don't spec both force and damping.