Single Plane Balance Problem
Single Plane Balance Problem
(OP)
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.
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.





RE: Single Plane Balance Problem
A couple of photos would help.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Single Plane Balance Problem
RE: Single Plane Balance Problem
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RE: Single Plane Balance Problem
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
RE: Single Plane Balance Problem
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.
RE: Single Plane Balance Problem
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?
RE: Single Plane Balance Problem
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
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Single Plane Balance Problem
RE: Single Plane Balance Problem
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Single Plane Balance Problem
RE: Single Plane Balance Problem
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I assume this is the method Greg recommended?
RE: Single Plane Balance Problem
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Single Plane Balance Problem
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
RE: Single Plane Balance Problem
cheers
Mark.
RE: Single Plane Balance Problem
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?
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(2B)+(2B)' ?
RE: Single Plane Balance Problem
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.
RE: Single Plane Balance Problem
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(2B)+(2B)' ?
RE: Single Plane Balance Problem
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?
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(2B)+(2B)' ?
RE: Single Plane Balance Problem
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(2B)+(2B)' ?
RE: Single Plane Balance Problem
(more specifically, turn it so the shaft is vertical)
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(2B)+(2B)' ?
RE: Single Plane Balance Problem
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
RE: Single Plane Balance Problem
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.
h
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.
RE: Single Plane Balance Problem
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.