Understanding Dynamic Balancers: redux
Understanding Dynamic Balancers: redux
(OP)
Thanks to Greg Locock and Klyde for their posts in the original thread.
I have since made progress in my quest to understand the workings of the typical propeller/rotor dynamic balancer, but the original thread was closed, so I started a new thread.
I now have a desktop "rotor simulator" fabricated with a small variable speed AC motor, and an 8" flywheel to which I can attach trial weights at 10 degree increments. My simulator sounds much like Klydes' description of his.
Capturing scope-screens and Excel files from the accelerometer and Hall effect phase marker is now a snap.
The accelerometer signal is a beautiful, nearly noiseless sine wave. The phase markers are crisp and clean.
Initial data that I've captured, and rudimentary waveform analysis I've done, seems to indicate that the positive going velocity zero-crossing is different than the phase angle reported by the commercial instrument to which my simulator is attached.
The poccess I'm using is to generate an integral (velocity) curve of the acceleration waveform, via Sigview waveform analysis tools. The positive going zero-crossing of the velocity curve corresponds to 162 degrees from the zero degree phase marker.
The commercial instrument reports that phase angle at 320 degrees. A difference of a factor of 2. I'll test other samples at other imbalance to see if this holds true.
I've measured the phase lag "stack-up", due to electronics circuitry, of the commercial instrument between the input connector and the output of the anti-aliasing filter, and it appears to be precisely (and happily) 360 degrees. So electronics-induced phase lag error appears that it should not be a factor here.
The 90 degree shift due to critical speed that Klyde described in his post doesn't seem to apply here, although I do not yet know what the critical speed of my rotor simulator is.
I'd be happy to supply screen captures or Excel files if anyone cares to see them.
Can anyone offer some insight here?
I have since made progress in my quest to understand the workings of the typical propeller/rotor dynamic balancer, but the original thread was closed, so I started a new thread.
I now have a desktop "rotor simulator" fabricated with a small variable speed AC motor, and an 8" flywheel to which I can attach trial weights at 10 degree increments. My simulator sounds much like Klydes' description of his.
Capturing scope-screens and Excel files from the accelerometer and Hall effect phase marker is now a snap.
The accelerometer signal is a beautiful, nearly noiseless sine wave. The phase markers are crisp and clean.
Initial data that I've captured, and rudimentary waveform analysis I've done, seems to indicate that the positive going velocity zero-crossing is different than the phase angle reported by the commercial instrument to which my simulator is attached.
The poccess I'm using is to generate an integral (velocity) curve of the acceleration waveform, via Sigview waveform analysis tools. The positive going zero-crossing of the velocity curve corresponds to 162 degrees from the zero degree phase marker.
The commercial instrument reports that phase angle at 320 degrees. A difference of a factor of 2. I'll test other samples at other imbalance to see if this holds true.
I've measured the phase lag "stack-up", due to electronics circuitry, of the commercial instrument between the input connector and the output of the anti-aliasing filter, and it appears to be precisely (and happily) 360 degrees. So electronics-induced phase lag error appears that it should not be a factor here.
The 90 degree shift due to critical speed that Klyde described in his post doesn't seem to apply here, although I do not yet know what the critical speed of my rotor simulator is.
I'd be happy to supply screen captures or Excel files if anyone cares to see them.
Can anyone offer some insight here?
RE: Understanding Dynamic Balancers: redux
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Understanding Dynamic Balancers: redux
The signal from the Hall effect switch transitions right at this zero degree mark.
The actual weight is unknown. My digital postage meter is not sensitive enough to measure it.
RE: Understanding Dynamic Balancers: redux
I'm not neccesarily convinced that velocity is the obvious signal to use, I've always used acceleration.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Understanding Dynamic Balancers: redux
On further waveform analysis, I've learned that the neither the peaks nor the zero crossings of either the accel or velocity waveform correspond to the phase angle indicated by the commercial instrument.
RE: Understanding Dynamic Balancers: redux
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Understanding Dynamic Balancers: redux
Phase angle "from what" is currently the big question.
RE: Understanding Dynamic Balancers: redux
If so just try a balance with it, you'll soon see if it is making sense.
It does sound odd, I'd agree.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Understanding Dynamic Balancers: redux
The location of the dot on the polar charts, points to the specific corrective action required to reduce the imbalance and move the "next dot", acquired during the next acquisition, closer to the center of the chart.
I'll post an inquiry on the Vibration forum.
RE: Understanding Dynamic Balancers: redux
If I could segue off track a little, Greg, I noted your comment on IPS vs velocity, and I have also noticed the vibration pick-ups that were once described as "accelerometers" are now called "velometers" , at least with one system I'm familiar with. Is there a difference?
regards, Steve
RE: Understanding Dynamic Balancers: redux
"With some balancers I'm familiar with, working on aircraft, not experimentally, you would make a weight change, & observe the "move line" the weight change caused. If it didn't head right thru the center of the "clock", then you had a process called "correcting the clock angle" where you added or subtracted 'hours' from the clock chart, and continued with the balance procedure"
This procedure sounds like the process for creating the polar charts for a new machine.
I already have the polar charts for my aircraft.
RE: Understanding Dynamic Balancers: redux
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.