Design of vibrating amusement ride 2

[RobertCasey]

Hi. Im designing an amusement device which consists of a platform that will be suspended on 4 inflated vibration isolators. Underneath the platform is a motor with an eccentric weight. Passengers (up to 12 people) will stand on the platform and the motor will be driven in bursts, at various frequencies between 5Hz - 20Hz.

I have modelled the platform in Solidworks and have used this with Ansys to determine the modes of the platform and to ensure that the frequency of the platform isnt close to the driving motor frequency, but I have assumed rigid supports instead of springs and I have ignored the weight of the passengers. Is this reasonable do you think? The company that manufactures the motor (these things are normally used to drive industrial sieves) says that they usually ignore the bulk mass of the product when calculating the supporting structure frequencies because it is not directly coupled to the platform.

I have also modelled the platform in visualNastran assuming it to be a completely rigid construction, but using spring mounts. From that I can vary the motor speed and can take whatever accelerations I want.

My next question, is how to take the data from the simulation, and apply it in a meaningful way to a structural analysis of the platform. I can measure the lateral accelerations of the platform which peak at 5g for short durations, but would I be correct in applying this acceleration to the platform mass alone assuming that the motor was the fixing point (rather than the spring supports)? For the loading in the vertical direction, again I think that I should use the accelerations in that axis acting on the platform alone (again fixing it at the motor), and only consider the full weight (ie including the passengers) for a static analysis of -1g vertical.

I've have never done anything like this before (its always been nice straightforward stuff that doesnt vibrate), so any input would be greatly appreciated. Thanks.

 

[CESSNA1]

ROBERTCASEY: It sounds like what you have is a classic spring-mass vibrating system. Did you model only the platfrom, or the overall system, or both? The mass(platfrom/people) are suspended on air springs. You need to replace the solid mounts with the spring constant of the air springs. (They probably are non-linear, but may be linear over the range of interest. 5G lateral vibration sounds like a lot with people on the platform (are they restrained?). Bear in mind at 180#/person average, 12 people will add 2160 pounds or 67 Ft-sec^2/# to your system. That will lower the resonant frequency if it was not included in the calculation.

Can you post a sketch?

Regards
Dave Hall

 

[Vibac]

Actually, people do not only act as added masses to your system, they themselves correspond to mass-spring-damper systems. A simple model of a standing person just assumes one degree of freedom with a natural frequency between 4 and 8 Hz (should be checked). I remember having seen papers discussing the influence of people on the dynamic properties of building decks. In that case just adding the mass of the people doesn't comply with measurements.

 

[RobertCasey]

Thanks for your replies.

The platform is a simple construction. Its 13ft x 6.5ft and is supported with a Firestone Airstroke airspring at each corner. The motor is located midway along the platform . The passengers stand on the platform in two rows of 6.

The airsprings indeed have a non-linear spring rate but I can assume that its linear for the almost constant pressure that the springs will be working at.

With ANSYS, I have determined the modes of the platform using a detailed model direct from Solidworks, and with the passengers modeled as individual blocks 20" x 8" x 4" high rigidly fixed to the deck. The platform supports are modeled with a combination of fixed and sliding restraints. The lowest frequency is higher than the forcing frequencies so I don’t think resonance should be a problem.

Using VisualNatran, I have modeled the whole platform as a rigid block with the appropriate spring stiffnesses for the supports. The passengers are modeled as tall blocks sat on the platform surface with only friction between the contact faces (only held down with gravity).

When the simulation is run, the acceleration of the platform shows a peak lateral acceleration of +/- 2g at about 16Hz. The platform vertical acceleration has a maximum downwards acceleration of about 6g at the bottom of the stroke, and about 2g upwards at the top of the upwards stroke, again at about 16Hz. However, measuring the accelerations of a typical passenger block, shows very low vertical accelerations (between 1g and 0.8g), and almost negligible lateral accelerations. Viewing the simulation close-up clearly shows the passenger blocks bouncing about on the surface of the platform and if left long enough, vibrate themselves to the edges of the platform and fall off.

I feel it would be erroneous to design the platform at 6g including the weight of the passengers (indeed the platform would not be able to take it from a stress point of view), so I was going to design the platform with its self weight only at 6g, and only include the passengers in a separate 1g vertical acceleration load case.

My problem is, as I said, that I have never done anything like this and so i'm not completely comfident of my approach. However, does this sound a reasonable way to proceed? I know it would be nice to model the passengers properly as individual spring/damper systems, but I dont have the time to be able to do this.