I have a system that I need to shake using a sinusoidal motion from 2-20 Hz, 0.3-2 g's. Specs below...

500 lb load
Vertical orientation
According to my calcs...

SPEED AND AMPLITUDE
Hz	g's	Amp (inches peak to peak)	rpm
20	0.3	0.0146	1200
20	2	0.0978	1200
2	0.3	1.467	120
2	2	9.7798	120

That's a pretty broad range in terms of speed, displacement and load. I have to apply the force at 4 corners of a platform that is about 3'x5'.

My first thought is to build adjustable eccentrics that are connected by shafts and belts. When I got into everything that will be required I thought it might be asking if there is a common way to do this and if there might be a commercial shaker table available to buy or base the design on.

Thanks for any help,

Tony

 

This is a common application used by shaker tables for concrete moulding and also vibrating screens.
The common approach is to suspend a rigid table on resilient mounts (springs, rubber avm etc) and use a proprietary vibrator such as found here:http://www.vibtec.com/ or several other suppliers. Search for industrial vibrators with great care and possibly safe search filter on!!
The actual performance can be estimated using basic laws of motion, along with the degree of isolation afforded by the springs.
Be aware that anything placed on the table and not fixed will become detached and floating if acceleration exceeds 1g.

 

On second reading, you want to be able to continuously vary the acceleration over a range, not so easy to do mechanically.

I would suggest considering a linear motor to drive the suspended table using either analog or digital feedback to create the desired motion. This will require a fair bit of power to drive a large table, especially if the applied load is also large.

This firm is an authority on linear motor applications in the UK, give them a call for an opinion.

http://www.force.co.uk/

 

Thanks Chris.

In my original numbers I didn't add the 2g acceleration force when I calculated torque numbers so its even a tougher proposition than I thought.

Your post got me to thinking about designing a "tunable" counterbalance that has an adjustable resonant frequency so I won't need such a large motor to drive the system.

We have a large Bosch servo motor and drive in house that should be more than enough to do the trick with such a counterbalance system applied. If I can get it right I should only have to supply enough energy to overcome damping and friction losses plus a healthy fudge factor.

Thanks for the ideas. I believe a tunable counterbalance will make this a feasible job to do in house. Essentially, adjustment to the counterbalance will drive the frequency since without it the motor won't be able to move the load very quickly.

 

Hi Tony, changing acceleration at a constant speed is the big challenge. If you feel confident in designing a variable eccentric ( which isn't an insanely difficult proposal in engineering terms) it would be by far the most efficient method of actuation. I move hundreds of tonnes of rock an hour with only a few kW using imbalanced actuation which would be impossible with direct linear drive.

Spring suspended you should be able to exceed 90% isolation from the supporting structure or even higher if you use air springs.

Good luck!