Weight of Sphere Covered in Piezo Force Sensors 3

[ConvexitySeller]

Hi all - new here and new to piezoelectric force sensing technology

I need to measure the weight of a sphere dropped onto a flat plane from a height of 1 foot.

[highlight #FCE94F](Im sure there is an easier way to do this - but please entertain my hypothesis)[/highlight][highlight #888A85][/highlight]

I want to cover the sphere in small circular piezoelectric force sensors so that ~99% of the surface area is covered. I will then drop the sphere and measure the resulting dynamic forces. After the initial impact force (and any forces incurred as the sphere rolls on the plane), could I effectively measure the weight of the sphere by measuring the final semi-static force of the sphere resting on the plane after it comes to a complete stop?

Sphere weight is anywhere between 50lbs and 250lbs

 

[ConvexitySeller]

[highlight #FCE94F]Sorry - height is actually variable, but around 1ft. I also have no way of knowing what orientation the sphere will be in when it makes contact with the plane.[/highlight]

 

[GregLocock]

Piezo transducers aren't very good a measuring static (0 Hz frequency) properties. I would have thought compared with the enormous expense of what you are proposing that a separate weighing platform would be a cheap and easy solution.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[gruntguru]

I will take a stab and say yes, depending on your accuracy requirement. You will need to compensate for the low frequency roll-off characteristic of the sensors. You may have more success analysing the data during the "rolling" phase (if there is one.)

I recall calibrating a piezo pressure sensor using a jury-rigged step input (3-way valve manually toggled between a pressure source and atmosphere). Worked well with some extrapolation of the results.

je suis charlie

 

[racookpe1978]

No. It sounds rather that you are trying to measure the IMPACT forces of a sphere covered with sensors weighing between 50 and 250 lbs on to a flat surface of water.

Initial weight of the sphere must be weighed 'dry". Dropping it in water will not give you weight, but only bouyancy (amount of water displaced by whatever part of the ball is below the water level.)

 

[IRstuff]

The piezo sensors will, at the minimum, distort whatever the true impact forces might be, since it will be measuring the impact force of a marge mass behind a pizeo sensor, and not the impact force of the large mass.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[hydtools]

It would seem that you could calculate the weight using impulse = momentum.
The transducers would measure impulse force and impulse time. If you know the drop height, you can calculate velocity at impact. Calculate the unknown mass.
F x t = m x v

Ted

 

[KENAT]

Could you put the sensors on the plate instead of the ball? I'm not sure I entirely follow what you are trying to do.

Or as others say, what does the weight (mass?) have to do with the drop as such that you can't just measure it on conventials scales of some kind? Before & after drop if something in the process may cause a change.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[chicopee]

I think that the OP is actually more interested in measuring the impulsive force than the weight suggesting that Hydtools is on track with his comment.

 

[rb1957]

maybe its "ESL" but i'd've expected engineers to know the difference (between force and weight).

another day in paradise, or is paradise one day closer ?

 

[ConvexitySeller]

Hey everyone, thanks for the replies. I want to measure weight, not impact force, just resting weight when the sphere finally comes to a complete stop. I need to know if piezo works for this, and if not then why. I don't need alternative solutions.

The sphere must be the measurement device. I do not know what orientation it is dropped at. I do not know exactly what height it will fall from.

So, will the final force exerted on whatever sensors the sphere stops on be detectable such that I can isolate the electrical charge produced at that point (isolate it from previous noise produced by the sphere while in motion) and convert the signal into weight.

My apologies in advance - trying to be as clear as possible. This is not at all my area of expertise.

 

[IRstuff]

a> the impact will most likely shatter the piezo, which is typically like a ceramic material
b> as already mentioned, piezo sensors don't do DC very well, which is what you are essentially asking for

I will refrain from any further input, as it's clear you don't seem to be reading the posts anyway.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[GregLocock]

At 0 hz th the capacitors tends to drift in response to things like temperature, odd electrical currents and other stuff. If you look I doubt you'll find a capacitative piezo amplifier that goes to 0 Hz for this reason.

WHY ONLY DYNAMIC FORCE CAN BE MEASURED WITH PIEZOELECTRIC FORCE SENSORS
The quartz crystals of a piezoelectric force sensor generate an electrostatic charge only when force is applied to or removed from them. However, even though the electrical insulation resistance is quite large, the electrostatic charge will eventually leak to zero through the lowest resistance path. In effect, if you apply a static force to a piezoelectric force sensor, the electrostatic charge output initially generated will eventually leak back to zero.

The rate at which the charge leaks back to zero is dependent on the lowest insulation resistance path in the sensor, cable and the electrical resistance/capacitance of the amplifier used.

In a charge mode force sensor, the leakage rate is usually fixed by values of capacitance and resistance in the low noise cable and external charge or source follower amplifier used.

In a force sensor with built-in ICP electronics, the resistance and capacitance of the built-in ICP electronics normally determines the leakage rate.

When a rapid dynamic force is applied to a piezoelectric force sensor, the electrostatic charge is generated quickly and, with an adequate discharge time constant, does not leak back to zero. However, there is a point at which a slow speed dynamic force becomes quasi-static and the leakage is faster than the rate of the changing force. Where is the point at which the force is too slow for the piezoelectric force sensor to make the measurement? See the next section on Discharge Time Constant for the answer.

Taken from

https://www.pcb.com/TechSupport/Tech_Force.aspx

Rather crucially it looks as though with a built in ICP amp you may be able to get a reasonably accurate weight reading for minutes after it comes to rest.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[gruntguru]

Yes, but not accurately. You seem to appreciate that piezo sensors cannot be used to measure DC (static) values, but the frequency response (with appropriate instrumentation) does extend quite low. This will give you a "glimpse" of the static value - a weight which approaches the true value but is always decaying towards zero. Pre-determining the frequency response of your measuring system will allow you to correct somewhat and improve the accuracy of your measurement. If there is a true rolling phase, (no bouncing oscillations) you will get a more accurate weight (by looking at the peaks).

I assume you are not going to actually do this - it sounds more like a theoretical problem?

je suis charlie

 

[Occupant]

It seems like, I am the only one at a loss here. Maybe somebody can educate me. You're hooking up a piezo-electric sensor to a sphere and then drop it from a given height. When the sphere hits a solid plate the mass will decelerate. That deceleration will be measured by the sensor to be theoretically ∞ at the point of impact depending on the coefficient of restitution. What is that going to tell you?

 

[GregLocock]

No he's measuring the weight of the sphere after it has stopped bouncing and rolling.

Probably there are other types of transducer that would be much suited to this application, for example fibre optic force gages, but given that the OP seems to be keeping all details close to his chest there isn't much we can do.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[tbuelna]

Is it even geometrically possible to cover 99% of a spherical surface using circular sensor pads without overlapping them?

And why not attach the force sensors to the flat plate rather than the sphere?

 

[RolMec]

The location & extension of the sensors (whichever) are discrete, extending on the surface with a definite boundary. Whereas the final resting point is probabilistic --> You would need LOTS of tests to come to a result, and even so I'd doubt it.
As the sphere shall touch the plane in a singularity = a point, the only one who could know its weight is the plane. But then, in order to get a message, you'd need to 3-foot it with a load cell for each foot. A piezo is perhaps not the remedy of choice here (see Gregs post), so why stick to it at all?
If the sphere shall truly roll off from impact a bit, and your accuracy & repeatability requirements allow it, why not use a sort of soft cover which let the sphere sort of sink in depending on weight. This would come out to a height measurement.

Regards
R.

 

[rb1957]

"I want to measure weight, not impact force" ... "just resting weight when the sphere finally comes to a complete stop." ?

weight is weight, a basic property of mass. it weighs the same whether you drop it from a height, or carefully place it on the surface ?? why will the weight change ???

another day in paradise, or is paradise one day closer ?

 

[IRstuff]

And, let's not forget that if the piezos are large, relative to the sphere, then landing on the boundary between two piezos would result in erroneous answers anyway.

Is this for school? Student postings are not allowed.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

Of course I can. I can do anything. I can do absolutely anything. I'm an expert!