Mounting of accelerometer to measure acceleration

[StathPol]

Hi guys,

I am trying to mount somehow an accelerometer on a bullet (diameter = 100 mm) to avoid any vibrations.

The bullet is made of high tensile steel and it will experience rigid body motion.

I was thinking of cutting the middle of the bullet, mount the accelerometer and then cut a circle to pass the wire on the amplifier. The magnet will be bolted on the top to be able to be lifted in a predetermined height. Will the magnet produce any vibrations because it is bolted? Or is there any other configuration that I could go with?

Thanks.

[johnsold]

The question is not how to avoid vibrations. The fact that the bullet is made of high tensile steel guarantees that it will vibrate. I am not sure what you mean by "rigid body motion" but the bullet will definitely compress and then relax (vibrate) when the impact occurs.  It takes a finite amount of time (related to the speed of sound in the material) for  the effect of the impact to propagate across the height of the bullet.  While that is happening, the lower part of the bullet is compressing because it is stopped or moving more slowly while the top of the bullet does not yet "know" that the other end is slowing down.

The question you may want to ask is what, exactly, do you want to measure.  You are now measuring the motion of the top of the bullet.  Most likely you are really interested in the forces and motion in the concrete target.

Lynn

[StathPol]

Lynn,

I hope you are well - the question is a following up of the question posted here (I couldn't upload the question there, don't know why):

http://forums.ni.com/t5/LabVIEW/Impact-force-measured-using-an-accelerometer-mounted-on-the/m-p/3329...

We fabricated a new rigid bullet made of steel to avoid the bouncing of the two masses that caused damage in the results. I agree that the bullet will vibrate once it hits the concrete. By saying the bullet is 'rigid body ', I mean that the deformaiton of the steel under impact will not be significant (neglected).

I am interested to measure the acceleration of the bullet to calculate the impact force in the concrete target, and then carry out energy calculations. Also, I am interested to integrate the acceleration-time graph based on the initial condition to get the velocity, then displacement. Therefore, I am wondering how to mount the accelerometer without causing any changes in the bullet. 

[Henrik_Volkers]

For a coarse overview what resonances are to be expected, use a FEM program (from ANSYS to freeware like Calculix) , your body is a simple rotational part, so it shouldn't be hard to get the needed information, like the first x modal resonances.

A solid ball isn't that bad, and if one part of the surface is ... say more flat, due to impacts, just rotate it.

Simply glue the accelerometer and the magnet/lifting aid ...

[StathPol]

Thanks guys.

I will do the FEM in abaqus (only available), but will I restrain the top flat surface in all degrees of freedom except the vertical movement.

I am not quite sure if we can glue the accelerometer because it has a height of 40 mm after the cable is twisted and i need to configure the accelerometer asap. This is the reason I thought to drill the steel and mount the accelerometer on it, then extract a small hole to bring the cable out. However this will alter the bullet :((((

[Henrik_Volkers]

using the bold/screw is OK to fix the sensor.

Just make shure that the mounting surface is flat and has a rougthness according to the spec of the sensor manufactor (common value <1µm) 

Mount with the specified torque (2.5Nm?? again see Spec)

And use vaseline or oil while mounting , you don't want air (springs) between your mass and the sensor 😉

[StathPol]

How can I set up this bullet to find the natural frequency of this steel piece with the accelerometer mounted on the top surface? The point of contact is the bottom hemispeherical head. In which point of the bullet shall I hit with the impact hammer? If I leave it as it is on a steel plate, it will fall down. 

Thanks.

[Henrik_Volkers]

Place it on a peace of styrofoam or hang it free with some thin (but strong 😉 ) wires (I do archery and would use dacron)

and hit it at the contact point with an impuls hammer (or an allan wrench 😉 )