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dynaman (Aeronautics) (OP)
4 Mar 12 2:14
Hi guys,

I recently built a circuit that interfaces with a MEMS accelerometer to perform a single plane balance. This was done on an electric motor and fan combination (5" diamater). The circuit interfaces with a CRO and generates a sine wave and also filters any unwanted noise coming from the motor.

I had originally placed the fan/motor configuration on a square aluminium beam supported by a fulcrum rod sitting on top of two rare earth magnets. A Velcro strap wrapped around the motor/fan to hold it to the beam. The beam ends have two pieces of silicon tubing that act as a spring/dampener to a ground plane. When the motor spins the whole unit oscillates about the fulcrum. The accelerometer is mounted on top of the fan shroud at a point furthest from the fulcrum point. This is a make shift set-up, crude but worked OK.

When I run this set-up I can perform a good single plane balance. I use a trial weight and measure phase shifts between weight and no-weight runs. All is good.

I decided to construct a proper beam mount with a refined fulcrum.  This setup allows the fan/motor to rock about the fulcrum with little oscillation normal to the fulcrum axis. However as nice as the beam mount is, it doesn't work very well. When I place the trial weight and then remove it between runs, I don not get a phase shift as I would with the crude setup. In other words the beam mount seems to be insensitive to changes in unbalance.

I'm not sure what is going on here? How can I build a mount that gives me consistent phase shifting?

thanks

Mark.
GregLocock (Automotive)
4 Mar 12 2:49
I'd guess that there was enough compliance in the first setup to allow the assembly to behave as if it was free free, whereas in the second case you've added too stiff a constraint.

A couple of photos would help.

Cheers

Greg Locock


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dynaman (Aeronautics) (OP)
4 Mar 12 3:14
OK here are the pics. The old setup is the black casing. The new setup is the wooden frame.
Strong (Mechanical)
4 Mar 12 12:01
Mark,

I suggest measuring vibration in the horizontal-radial direction on the new configuration. The unbalance force would cause a rocking motion about the fulcrum and be more sensitive to weight changes.

Walt
dynaman (Aeronautics) (OP)
4 Mar 12 16:12
Hi Walt,

The accelerometer was sitting horizontal on top of the fan shroud. I figured that the rocking would have caused a higher tangential acceleration at this location.

cheers

Mark.
dynaman (Aeronautics) (OP)
4 Mar 12 16:41
Just thinking about this again, I need to design a mount that is unstable which makes it sensitive to weight changes. Maybe the new mount stabilizes the fan/motor configuration too much?

On the crude setup the fan/motor is essentially sitting on two points that is one point using blue tack on the base of the fan and another point at the base of the motor (to stop it sitting from sagging). The motor hangs a fair way out the back of the fan shroud.

On the new setup the fulcrum is more rigid and allows motion about one axis. Also the pivot points are further apart.

Any thoughts?
Helpful Member!  GregLocock (Automotive)
4 Mar 12 17:59
I'll stick with my original comments.

One thing you can try is to put the trial mass at 4 or more  locations on the rotor. Then plot the vector diagram of the imbalance. This is far more robust than a single measurement.

Cheers

Greg Locock


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dynaman (Aeronautics) (OP)
4 Mar 12 18:35
Hi Greg, why is the 4 point more robust? Still trying to understand the mechanics behind this. Thanx.
GregLocock (Automotive)
4 Mar 12 18:43
The vector diagram that you can then plot shows you if the system is non linear, and also shows you the relationship between the measured phase of the vibration and the angular orientation of the trial weight.

Cheers

Greg Locock


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dynaman (Aeronautics) (OP)
4 Mar 12 18:46
OK makes sense thanks. Is there an Excel spreadsheet available for the 4 point method?
dynaman (Aeronautics) (OP)
4 Mar 12 18:58
I found this....

http://www.experimentalhelo.com/eh_Feature3-Meidell_Article.htm

I assume this is the method Greg recommended?

 
GregLocock (Automotive)
4 Mar 12 19:21
That's the basic idea, but if you know the phase of your vibration measurements you can jump to the answer much more quickly. The final plot is the same, you just don't need the amplitude circles, as he calls them.

Cheers

Greg Locock


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Strong (Mechanical)
5 Mar 12 18:11
Mark,

The softest support I can think of is to hang fan from two strings, one at fan shroud and the other at motor. One or two horizontal strings could stabilize the fan thrust. I am guessing that your vibration sensor and/or measurement system is not sensitive enough to measure the low vibration levels that you have on this fan.

Walt
dynaman (Aeronautics) (OP)
5 Mar 12 19:57
Thanks Walt. I tried a new method yesterday by placing the front of the fan on the beam mount and the motor supported by a block of foam. Worked well for the one-shot and  4 point methods (got similar answers). The trick is to have soft mounting and allow enough DOF for the vibrating device. The sensor will pick up no problem if the supports are set up correctly.

cheers

Mark.
electricpete (Electrical)
5 Mar 12 19:57
Good comments from knowledgeable folks already. I'm trying to catch up.

What's a CRO?
How are you getting a phase reference to compare to your vibration?
You said you're not able to get an angle shift by adding weight... are you able to get a magnitude change? Did you try increasing the weight?
Is the measured vibration time waveform sinusoidal?
Do your pieces of tube remain continuously in tension during the vibration? (that would be my hope to help ensure linearity)
Out of curiosity, what speed is the fan?
 

=====================================
(2B)+(2B)'  ?

dynaman (Aeronautics) (OP)
5 Mar 12 20:01
CRO = oscilloscope

Reference from an optical pickup.

Yes I was getting a magnitude change (4 point method would work here).

Waveform is fundamentally sinusoidal. Need some filtering to remove noise.

Tubes are in compression most of the time.

Fan speed is 30~40Hz.  
electricpete (Electrical)
5 Mar 12 20:05
The spring is a linear restoring force.  The force of gravity on something resembling an inverted pendulum is a little unusual in the context of balancing rotating machinery.  I'm not sure how to think about that.  I guess if the spring force from those two tubes is much larger than gravity force at max angle of tilt, and again those springs both remain very far into tension, it should not be a problem. But if these assumptions are not met, seems like effect of gravity creates as you say an unstable system.. does not resemble a typical machine.

=====================================
(2B)+(2B)'  ?

electricpete (Electrical)
5 Mar 12 20:14
I was typing while you were writing.

Quote:

Tubes are in compression most of the time.
That sounds like a recipe for a possible non-linear system for 2 reasons:
1 - if the tube springs buckle, that would certainly be non-linear.
2 - most of the time? It seems like changing direction could create some problems when trying to replicate ideal spring with a flexible tube (does spring constant change when it changes direction).

It strikes me that in your first setup, the black velcro band played a big role and avoided inerted-pendulum effect and also lessened the role of those probably-non-ideal springs (the tubes).

Why is it that you want to create an inverted pendulum?
What are you trying to accomplish?

=====================================
(2B)+(2B)'  ?

electricpete (Electrical)
5 Mar 12 20:28
For a quick experiment to see if the inverted pendulum configuration is causing problems, can you turn it on it's side and re-try the balance?

=====================================
(2B)+(2B)'  ?

electricpete (Electrical)
5 Mar 12 20:29
"..can you turn it on it's side..."
(more specifically, turn it so the shaft is vertical)

=====================================
(2B)+(2B)'  ?

Strong (Mechanical)
6 Mar 12 9:36
Mark,

If you drive the unbalance low enough, then the 60-Hz electrical noise will be the dominant vibration. A CRO would not be the best tool for continued balancing using a waveform display. A tracking filter or spectrum analyzer would be needed.

Walt
Tmoose (Mechanical)
6 Mar 12 13:06
only one sensor?

I too-often manage to mount the first sensor on a node.  A second sensor, parallel to the first, somewhere offset along the rotating shaft axis will get a taste of whether there is some yaw going on.
http://t1.gstatic.com/images?q=tbn:ANd9GcRjPTzSCblzAhGdEMdNskh9B6NvVmaHvpAhtaQPvYlw9RDfr8g9I7nFYfIzdw

Another curious thing can happen when single plane balancing is done with a rig that pivots at one point along the axis of rotation (like yours, if the motor is mounted shaft vertical?). If the correction plane is not chosen by luck to be precisely right in line with "unbalance," the sum of the moments about the pivot may be zero, but the correction will not be equal to the unbalance.  If the "balanced" rotor was then mounted with the shaft horizontally in slippery bearings, a "static" unbalance equal to the difference of the original single plane unbalance and correction would be apparent.  Very embarrassing.
 
dynaman (Aeronautics) (OP)
7 Mar 12 1:11
Hi guys,

Some very good suggestions indeed. balancing the fan/motor in a horizontal position is a good idea. will need to try it. Supporting the unit so its articulated is a challenge. I'm getting a good balance with the original setup now.

The only error I'm seeing is due to the fan being canter-levered from the motor shaft. With some trial and error, I'm sure I can cancel this effect.

My sensor has two axes but the circuit only supports a single channel. Two channels would be better i.e. x-y plot on the scope.

cheers

Mark.

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