hammer implemented with accelerometer 2

[Dmitry]

Hi, All
I'm using for modal tests a shock hammer implemented with accelerometer, established on its top. On how many I know a force cell is common choice for hammer implementing and it is always established on the hammer tip.
I want to find out, whether there will be errors in estimations of transfer function?

Best regards,
Dmitry

 

[GregLocock]

Hi Dmitry,

We use hammers all the time. Ours have force gauges in them, as you describe.

The purists will dislike your arrangement because you are coupling the mass of the hammer in to the mass of the structure at each excitation point, but removing it when you measure the response at that point, if you are exciting elsewhere. This could be a big problem if the mass of your hammer is similar to the local mass of the system.

You can check whether this is a problem by loading your structure with a hammer at each measurement location and seeing if the response is affected.

The same will happen with stiffness- but I assume you have a soft tip on the hammer? Are you assuming that your excitation is a force given by mass*acceleration, or k*displacement, or what?
Cheers

Greg Locock

 

[Dmitry]

Greetings Greg! As always your answer has considerably cleared the problem.
Now we work with the first bending form of a cylinder (mass is more than 80 tons), therefore I assume, that the local mass of the system in this case is equal to full weight of object (80 tons). As the weight of a hammer makes 0.9 kg, that, perhaps, it is possible to count, that influence of additional weight extremely small.
As to rigidity it is similar that it is not too influenced, as impact is put through a rubber lining by thickness of 3 mm. For definition of transfer function the spectrum of acceleration is used, so it turns out, that we have implicitly made the assumption, that our excitation is a force given by mass*acceleration.
Well, in this case my hammer can be used!

Yours faithfully,
Dmitry Semenov

 

[ANTOINE]

Dmitry,

You shall not consider the total mass of the structure, but the modal mass,i.e. the amount of material that "moves" at the mode you are considering.
Typically, the mass of your measuring equipement,i.e. accelerometer, or in your case modal mass of the hammer shall not exceed 10% of the modal mass of the structure.

as a swag, if you are looking at the first global modes of a 80 tons structure, you should be OK.

Tom