Single Plane Balance Problem 1

[dynaman]

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.

 

[electricpete]

"..can you turn it on it's side..."
(more specifically, turn it so the shaft is vertical)

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

 

[Strong]

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]

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:...MdNskh9B6NvVmaHvpAhtaQPvYlw9RDfr8g9I7nFYfIzdw

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]

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.