- #1

Jesse Millwood

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Hello,

I am an electrical engineering student and I was hoping some body here could help me out with a cantilever question.

I want to model a vibrating cantilever with a mass at the end. I am doing this for a project where I wanted to model a Wurlitzer 200 Electric Piano. The way they produce the oscillations is kind of neat where there is a steel reed that is grounded and when it is struck by the key mechanism it vibrates. There is then a pickup that is kind of like a comb, where the teeth of it go in between all of the reeds corresponding with different keys. The comb pickup is pulled up to around 150V by a 1Meg resistor, since there is a difference in voltage across a distance, there is capacitance and with the vibrating reed, it makes a variable capacitor. I am fine with the capacitance calculations and modeling the rest of the circuitry but I am getting kind of bogged down with some of these (relatively basic?) calculations with the beam vibrating.

My idea to model the vibrating reed as a cantilever with a point mass on the end is to treat it as a spring mass system. I (quickly) read through some texts on the Euler-Bernoulli Beam Theory but opted to model it as a spring because in

What I have so far for an example calculation is something like this:

^ y

| ///|________

| ///|________|* <-Point mass

| ///|

|-----------------------> x

Length : ##36.83 mm##

Thickness : ##0.64 mm##

Width : ##3.83 mm##

Density (##\rho##) : ##8050 g/m3##

Youngs' Modulus (##Y##) : ##210e9 GPa##

2nd Moment of Inertia (##I##) : ##\frac{width \cdot thickness^3}{12} = 83.67e-15##

Spring Constant (##k##) : ## \frac{3\cdot Y \cdot I}{L^3} = 1055##

Damping Ratio (##\zeta##) : ##\frac{\pi}{L} \cdot \sqrt{\frac{1}{k\cdot \rho}} = 29.27e-3##

Initial Displacement (##\delta_0##): ##2mm##

mass at the end(##m##): ##1.33g##

I want to be able to calculate the vertical displacement at a particular x value along the beam at a particular time value. I am doing it this way:

##y(x,t) = \delta_0 \cdot e^{\frac{-\zeta}{m\cdot t}}\cdot sin\left(\pi \frac{x}{l} \right) sin\left( \omega t\right)##

where ##\omega =\sqrt{\frac{k}{m+0.23m}}##

I have pieced together equations from various texts that I can find for free and other websites so my main question here is :

My reason for asking for a second set of mechanically inclined eyes is when I model it this way, the oscillation does not seem to die out as I would expect. I don't know why I expect it to die out sooner but I just have a feeling that something may be miscalculated.

As it stands my simple python script plots this oscillation at 174Hz (which is what I want) but the oscillations seem to die out around 4 minutes (the plot is attached). Also When I change my ##x## value to be half of the length the oscillation at the shorter distance down the beam has a higher amplitude, I feel that it is from the ##sin\left(\pi \frac{x}{l}\right)## term in my ##y(x,t)## equation and now I can not find the source of that equation.

Thank you for any help, guidance or a friendly point in the right direction/material,

Jesse

I am an electrical engineering student and I was hoping some body here could help me out with a cantilever question.

I want to model a vibrating cantilever with a mass at the end. I am doing this for a project where I wanted to model a Wurlitzer 200 Electric Piano. The way they produce the oscillations is kind of neat where there is a steel reed that is grounded and when it is struck by the key mechanism it vibrates. There is then a pickup that is kind of like a comb, where the teeth of it go in between all of the reeds corresponding with different keys. The comb pickup is pulled up to around 150V by a 1Meg resistor, since there is a difference in voltage across a distance, there is capacitance and with the vibrating reed, it makes a variable capacitor. I am fine with the capacitance calculations and modeling the rest of the circuitry but I am getting kind of bogged down with some of these (relatively basic?) calculations with the beam vibrating.

My idea to model the vibrating reed as a cantilever with a point mass on the end is to treat it as a spring mass system. I (quickly) read through some texts on the Euler-Bernoulli Beam Theory but opted to model it as a spring because in

*Harris' Shock and Vibration Handbook*there didn't seem to be that big of a difference between the Rayleigh method and others when compared in chapter 7. If I have time at the end I will go back and get more complicated but for now I would like to just model the beam in a simple manner. I haven't had a mechanical class that dealt with cantilevers in a while and I looked at some old notes but we never covered vibrating structures, as it was a statics class. So I was thinking that I could model the cantilever as a spring mass system.What I have so far for an example calculation is something like this:

^ y

| ///|________

| ///|________|* <-Point mass

| ///|

|-----------------------> x

**Dimensions of the beam (this is for one of the F# reeds):**Length : ##36.83 mm##

Thickness : ##0.64 mm##

Width : ##3.83 mm##

**Steel Properties:**Density (##\rho##) : ##8050 g/m3##

Youngs' Modulus (##Y##) : ##210e9 GPa##

2nd Moment of Inertia (##I##) : ##\frac{width \cdot thickness^3}{12} = 83.67e-15##

Spring Constant (##k##) : ## \frac{3\cdot Y \cdot I}{L^3} = 1055##

Damping Ratio (##\zeta##) : ##\frac{\pi}{L} \cdot \sqrt{\frac{1}{k\cdot \rho}} = 29.27e-3##

**Cantilever (reed) Properties:**Initial Displacement (##\delta_0##): ##2mm##

mass at the end(##m##): ##1.33g##

I want to be able to calculate the vertical displacement at a particular x value along the beam at a particular time value. I am doing it this way:

##y(x,t) = \delta_0 \cdot e^{\frac{-\zeta}{m\cdot t}}\cdot sin\left(\pi \frac{x}{l} \right) sin\left( \omega t\right)##

where ##\omega =\sqrt{\frac{k}{m+0.23m}}##

I have pieced together equations from various texts that I can find for free and other websites so my main question here is :

**Does anyone see any glaring inconsistencies or anything that is very wrong here?**My reason for asking for a second set of mechanically inclined eyes is when I model it this way, the oscillation does not seem to die out as I would expect. I don't know why I expect it to die out sooner but I just have a feeling that something may be miscalculated.

As it stands my simple python script plots this oscillation at 174Hz (which is what I want) but the oscillations seem to die out around 4 minutes (the plot is attached). Also When I change my ##x## value to be half of the length the oscillation at the shorter distance down the beam has a higher amplitude, I feel that it is from the ##sin\left(\pi \frac{x}{l}\right)## term in my ##y(x,t)## equation and now I can not find the source of that equation.

Thank you for any help, guidance or a friendly point in the right direction/material,

Jesse