Announcement

Collapse
No announcement yet.

semi OT - help with some electronic circuit ideas

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • #16
    I once saw a balancing rig where the motor vibration problem was solved by mounting a balanced fan on the rotating shaft, and driving the fan with compressed air. Speed could be (roughly) controlled with a valve in the air line. No motor = no motor vibration.

    Comment


    • #17
      First of all, why filter everything above 5Hz? You didn't say how the shaft was turned, but if it is self-powered the most common vibration frequency is going to be around 1750Hz, right? I watched a guy balance a rotor from my drill press motor once. He did it on an IRD machine. The rotor was set up using its own bearings to spin on, and a rubber flat belt went to a variable speed DC motor with RPM detected and fed back in a closed loop, so he could accurately spin it at any speed.

      Second, it's Butterworth, not Butterfield. :-)

      Third, I bet you could go a long way just putting a dial indicator on it and reading the magnitude of the jiggling. It's the old KISS theory.

      I wish you luck. I have looked into balancing some and am very interested in it.

      BTW the drill press motor came off a 20" Taiwanese drill press. After that motor got balanced that machine behaved entirely differently. It was like a real machine tool. Not like a cheap drill press at all.

      If you're curious, see http://nwnative.us/Grant/shop%20articles/drillPress20

      metalmagpie

      Comment


      • #18
        Yeah, Butterworth, I know mechanical stuff not electronics so was thinking taps I guess...anyway you got drift. The filter was because there is a heck of a lot of noise/vibration on the signal out of the accelerometer, and I thought it would eliminate motor hum, motor bearings, etc obfuscating the signal I wanted. Currently I'm using a variable speed motor. What I've made is almost identical to the IRD you've shown in those photos, for safety (as its just a spinning rotor, nothing containing it) I had it running under 5 Hz. The trick so far has been the signal processing.

        I like the comparing phase with the square wave approach, although I did pick up bench strobe light recently and wanted to use it. The strobe might a lot less work and I don't think will be very rpm sensitive. Do you recall what sort rpm they were at?
        Last edited by Mcgyver; 04-24-2020, 03:23 PM.
        in Toronto Ontario - where are you?

        Comment


        • #20
          A related video from Mobius really shows a lot of detail about vibration analysis. They have a really cool simulator!

          http://pauleschoen.com/pix/PM08_P76_P54.png
          Paul , P S Technology, Inc. and MrTibbs
          USA Maryland 21030

          Comment


          • #21
            To make the problem a bit more descriptive, here's what I'm up getting. This is all straight off the accelerometer (1.5G) with no filtering or amps.

            First I added a big heavy clump of clay, recorded through an arduino then plotted in excel, there' a clear strong signal you could trigger a strobe from.



            Here's the same set up on the scope...



            That's great for something way out of balance, but if I remove the weight, there is still a bit of vibration. Its a decent enough rotor off a tool post grinder, but I'd like to make it better to improve the finish. I need turn the voltage sensitive way down and crank up the speed to see the imbalance but the signal to noise has deteriorated, and frankly I want to run it at slow speeds for safety.



            If I turn the speed down, basically just looks like noise, but I was hoping that amplifying and filtering would find a usable signal to trigger from. I can see how DSP holds some promise, lots of learning to do

            Last edited by Mcgyver; 04-24-2020, 07:04 PM.
            in Toronto Ontario - where are you?

            Comment


            • #22
              A simple RC low pass filter should remove most of that high frequency noise.
              http://pauleschoen.com/pix/PM08_P76_P54.png
              Paul , P S Technology, Inc. and MrTibbs
              USA Maryland 21030

              Comment


              • #23
                1. Before taking any measures to reduce the noise, it would be nice to know the source of that noise. He said he is using an inexpensive accelerometer. Then he filters and amplifies the signal. I would look at the output of that circuit with no rotation of the shaft to see if the noise is from the accelerometer itself or if it is coming from the circuitry. If it is coming from the accelerometer itself, then changes in the circuit will not help. If it is coming from the circuitry then changes there would be called for. If the noise is being picked up by cable(s) between various parts of the system, then cable with better shielding or the use of a balanced line cable may help (differential inputs and outputs). Also lowering the source and destination impedance of those cables would help. Again, there are pin-for-pin replacements for the 741 that would provide those low input and output impedances.

                I only suggested the 741 as a good STARTING POINT. From there you can change things as needed. A "good instrument amplifier" may be the ultimate solution but circuit design with them is a bit more involved and may take time and effort that is ultimately not needed. An accelerometer will pick up any and ALL acceleration within it's bandwidth and that includes any and all vibrations in the overall motor, bearings, belts or gears, etc. Heck, it will even pick up cars and trucks passing on the street and seismic activity if it is sensitive enough. I suspect that his noise IS coming from the accelerometer.

                2. Synchronous motor, yes. Stepper motor, perhaps not. Steppers use, well steps and there will be a step frequency that could easily be added to the already noisy signal he is getting. If you use a stepper I would add a nice, heavy flywheel to dampen out those steps. And a driver that uses micro-stepping to smooth them out; but then that just adds another, higher frequency step frequency. Perhaps that higher frequency would be outside of the band pass of the system.

                A simple tack pulse would probably be good enough for balancing a shaft, but if more resolution is needed then an encoder disk would be one answer.

                Also, considering the noise he says he is getting, an isolating base may help to reduce the effects of nearby vehicular traffic and seismic activity. A quick test for this may be to grab a heavy base plate and sit it on several layers of that non-slip shelf liner:

                ‚Äč

                That may not be a permanent part of the apparatus: I am just suggesting it as a test to get information on the source of the noise.

                Originally posted by PStechPaul View Post
                A few observations and suggestions:

                1. For low level signal amplification, as well as common mode noise rejection, a good instrumentation amplifier such as the AD620 or INA118 will work better than an LM741. You can also make an instrumentation amplifier with full differential input using a quad op-amp such as LM324 or TL084.

                2. You might consider driving the shaft of the balancer with a synchronous motor or a stepper motor, so you can precisely set the speed and even know the angular position if you use a sensor to detect a zero degree reference point.
                Paul A.
                SE Texas

                And if you look REAL close at an analog signal,
                You will find that it has discrete steps.

                Comment


                • #24
                  The noise always stays about the same, it is a physical property type thing, the signal gets smaller. OK

                  Essentially, you need two things.

                  First, you need to have more noise filtering, so that the noise is cut off not too far above the signal, frequency-wise. A "steeper" filter, meaning a "higher order" filter. I do not know what, if any, filter you have in that, the second pic (first one of the scope screen) still has a good bit of noise in it. The filter can be 3 or 4 times higher in cutoff frequency than your signal, or more, with what you want to do.

                  Second, you need that filter after you have "boosted the signal out of the noise", in other words, after the amplification, because the amplification circuitry adds noise as well as amplifying it.*

                  Those two things should cut the noise you see quite a bit.

                  Then also, you probably want to slow the sweep, so that the long low waveform becomes shorter and steeper in appearance. OR increase the gain ('scope setting). This is nothing to do with the signal, it is just presenting it in a way you can see easier. On a meter, with reasonable filtering, it would still be the same.

                  About the filter.... "white" noise is generated as so many volts per Hz of bandwidth (never mind the exact formula). So, if you are at 5 Hz, or probably 300 rpm, you can have a filter at 30 Hz or 50 Hz, and there will not be a lot of difference. However, if you had the filter at 1000 Hz, you might begin to see a bunch of junk in the waveform.

                  I suspect you would be OK with no more than a "2-pole" filter, which is one with 2 R-C elements (or one L-C). In the frequency range you are working, you will not be likely to need a tracking filter, or other refinements, although they would not hurt anything except the complexity.

                  * "Noise" is a property of resistors. They "generate" thermal noise whether they are ones you put into the circuit, or ones that are a property of the signal sensor, etc. The larger the resistance, the more noise voltage-wise there will be.
                  Last edited by J Tiers; 04-24-2020, 07:31 PM.
                  CNC machines only go through the motions

                  Comment


                  • #25
                    This makes me wonder if you could spin up the shaft and then disconnect the motor to take your readings. Perhaps even have a fast acting brake to completely stop the motor while the shaft continues to rotate.

                    Just spit-balling. But that would completely eliminate anything from the motor.



                    Originally posted by alsinaj View Post
                    I once saw a balancing rig where the motor vibration problem was solved by mounting a balanced fan on the rotating shaft, and driving the fan with compressed air. Speed could be (roughly) controlled with a valve in the air line. No motor = no motor vibration.
                    Paul A.
                    SE Texas

                    And if you look REAL close at an analog signal,
                    You will find that it has discrete steps.

                    Comment


                    • #26
                      I've been wondering if the SAMS schematics are available for commercial tire balancers
                      25 miles north of Buffalo NY, USA

                      Comment


                      • #27
                        If you are driving the shaft with a brushed DC motor, that noise could be brush noise. You might try spinning the rotor by hand to see if it improves. As shown in the last video above, bearing noise also looks a lot like that.
                        http://pauleschoen.com/pix/PM08_P76_P54.png
                        Paul , P S Technology, Inc. and MrTibbs
                        USA Maryland 21030

                        Comment


                        • #28
                          Originally posted by Mcgyver View Post
                          Yeah, Butterworth, I know mechanical stuff not electronics so was thinking taps I guess...anyway you got drift. The filter was because there is a heck of a lot of noise/vibration on the signal out of the accelerometer, and I thought it would eliminate motor hum, motor bearings, etc obfuscating the signal I wanted. Currently I'm using a variable speed motor. What I've made is almost identical to the IRD you've shown in those photos, for safety (as its just a spinning rotor, nothing containing it) I had it running under 5 Hz. The trick so far has been the signal processing.

                          I like the comparing phase with the square wave approach, although I did pick up bench strobe light recently and wanted to use it. The strobe might a lot less work and I don't think will be very rpm sensitive. Do you recall what sort rpm they were at?
                          They asked me what rpm the motor was. Remember, all they saw was the rotor. I told them 3450 even though it runs at 1750. Then they just ran at that rpm the whole time.

                          metalmagpie

                          Comment


                          • #29
                            interesting. last time i had a rotor dynamically balanced they ran it very slow, 300 rpm i guess.

                            Comment


                            • #30
                              I had a lot of success with this today. My conclusion is I had bad accelerometer, or maybe I damaged it, man I wasted a lot of time fighting that. A new acclerometer with two simple RC filters (high pass to get rid DC offset from accelerometer, then low pass, cut off 10 Hz) then into a simple amplifier and I got the following signal - this is the same mechanical setup as the last photo in post 21 - quite an improvement! Its with a lot of gain as well, so I'm pleased at what I'm getting.....now if I can get the strobe hooked up I'll get some results!

                              This is balancing the rotor & pulley off a Wolf TP grinder which should have been decent to start with so I think I'm getting pretty good sensitivity

                              Last edited by Mcgyver; 05-02-2020, 03:04 PM.
                              in Toronto Ontario - where are you?

                              Comment

                              Working...
                              X