Counterbalance for vertical stage...non-magnetic application

[chopraj]

I am designing an xyz positioning system in a non-magnetic application. I am using linear piezomotors on all axes (

http://www.nanomotion.com).

I am concerned about the z-axis. My reading tells me that I should ideally counterbalance the weight on this axis in order to cancel the forces acting on the load. There seem to be a number of solutions, but in the category of non-magnetic solutions it seems I could use a counterweight attached to the stage or a spring of the appropriate material.

Some relevant numbers: my travel in the z axis is 2", and the weight of the slide and the object I am moving is ~1000g.

I have seen constant force springs, but have not been able to find one made of a non-magnetic material such as beryillium copper. I have been suggested by someone that I could use a preloaded spiral spring to try to linearize the force associated with it to cancel the force due to gravity. I have seen non-magnetic spiral (compression/extension) springs on the internet.

Are there any other suggestions/reccomendations on how I should proceed. Am I likely to find a solution using springs for this application?

 

[israelkk]

Why not design a constant force spring made of beryillium copper yourself or with the help of a professinal.

http://www.webspawner.com/users/israelkk/

 

[chopraj]

Thank your for your interest...I spoke with a professional in my city and they were skeptical about the performance of a beryllium copper constant force spring. I suggested I prototype with a steel spring and then once I settled on the correct configuration, we could design a similar version in BeCu...again they thought this was risky. They then suggested the preloaded spring idea as a way to get approximately a constant force over my travel distance.

I guess I can follow up with some specific questions to you:
1) Do you agree with their diagnosis of the BeCu constant force approach?

2) Do you understand their alternative (based on my simple description) and is it a viable option?

3) Do you know of spring manufacturers who would be interested in desining & making me a constant force BeCu spring (in small quantities)? I am located in Canada.

Thanks again for your feedback.

 

[israelkk]

I meant, a professional engineer that is experienced with the design and analysis of various types of springs such as constant force springs, motor springs, or large deflection springs.

For a preloaded helical compression or extension spring that will give an approximate 1000 gr +/- 100 gr over a range of 2" travel, the spring should be pre-loaded/pre-deflected 20". This dictates that the spring free length will be very long.

I do not see any reason why a constant spring can not be manufactured from BeCu strip at spring temper or any other metal strip. However, without doing the calculations I can not say what will be the strip width and thickness and the drum diameter.

You didn't specify life cycle desired for the spring.

You didn't specify dimensional constraints for the spring.

I do not have a specific knowledge of a manufacturer in Canada but I am sure you will find some in the US. However, I am not sure they will design it for you. The most respected company is Associated Spring - Barnes

http://www.asbg.com/index2.html.

For every spring type you choose there are many design options. For example: for a constant force spring there can be many strip width, strip thickness and drum diameter combinations that can do the job. Therefore, to design an optimal spring the designer will have to check many spring options.

I do not believe that the spring manufacturer will do the design optimization. He may give you the first design option that he will find that can do the job but will not invest more.

http://www.webspawner.com/users/israelkk/

 

[chopraj]

Iraelkk,

I will take your advice and follow up with the company you mentioned. Our life cycles are rather short. This equipment may travel across 10000 cycles in its lifetime, and the speeds are very low...our focus is research, not production. Dimensional constraints are of course important, and I will discuss that with any serious party I find.

I'll let you know what I come up with. You implied that an engineer from a company will probably not do the design optimization. I'm assuming you could...perhaps we could discuss your services.

Rajiv.

 

[TVP]

I just wanted to say that I agree with israelkk on both points-- a constant force spring should be able to be made using beryllium copper strip, and that Associated Spring is an excellent spring manufacturer that you should contact.