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Cheap o-scope recommendations?

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  • #16
    Originally posted by MaxHeadRoom View Post
    This is the one I have, although the Mark1 ver.

    https://www.syscompdesign.com/product/cgm-101
    Max.
    how much was that new? i cant find any price, is it discontinued?

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    • #17
      Originally posted by Sparky_NY View Post
      You can do a quick test with a digital voltmeter. When A is positive, B is at ground and vise versa. . . .
      Not so. Maybe you're thinking of the A and A/ or B and B/ pairs in a differential encoder.

      Signals A and B will be 90 degrees out of phase. Meaning that when the A signal rises or falls the B signal rises or falls 90 degrees later (I'm talking about the signal transitions around 1 line of the encoder, not 1 revolution of the motor). A and B will both be positive 1/4 of the time, both negative 1/4 of the time, and opposite each other 1/2 of the time.

      DICKEYBIRD: I think Centroid is specifying that type of encoder (differential A, B, Z) for rigid taping. In general you don't need an encoder that elaborate for measuring only RPM.



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      • #18
        Originally posted by DICKEYBIRD View Post
        I've wanted a 'scope ever since the 50's when I "helped" my father build a Heathkit that he used in his electronics side jobs. You'd think I'd be a chip off the old block electronics whiz but alas, the car bug bit me & that was that.

        I checked Amazon first & I saw a bunch of 'em but I don't know what bandwith/sample rate I need for the encoder testing & also simple auto tests.
        The bandwidth you need for your stated use depends on your spindle speed. With a spindle max RPM of up to a couple thousand even the highest pulse count encoder that Centroid recommends should be viewable with a 1 MHz bandwidth (10 MHz sample rate) o'scope. If it's an 18 or 20,000 RPM spindle you'll need higher bandwidth and sampling rate and/or/maybe a lower line count encoder. 1MHz BW should also be plenty for autos unless your going to be poking around on microcontrollers or data busses. You should also get at least two input channels so you can see their relationship to one another, and make sure probes are included.

        EDIT: Sorry, I was thinking of a milling machine with that 18-20000 RPM spindle. 1 MHz B/W should be plenty for your lathe.
        Last edited by genea; 11-29-2020, 08:23 AM.

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        • #19
          Originally posted by andywander View Post

          In what universe?

          Seriously, encoder quadrature outputs are supposed to be 90 degrees out of phase, so while the transitions of each output will never occur at the same time, there will be times when both A and B are positive, and times when they are both at ground.

          Or am I misunderstanding something here?


          You are correct, I misspoke. The idea is to see each the outputs at both high and low states, to troubleshoot deeper requires a scope. The digital voltmeter is a quick and dirty test only when a scope isn't available.

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          • #20
            Originally posted by Black Forest View Post
            In building a CNC machine what types of things would a scope be useful for to diagnose issues? Noise in the electrical circuits seems to be one of the biggest issues in CNC machines and tricky to isolate. VFD's being a big cause of noise. What scope would be good for this type of work? A scope that would save time in diagnosing issues and not just the most inexpensive scope. I would rather buy a better scope and save time which is my most precious commodity.
            Not being facetious here, the best path (fastest / least frustrating / least expensive in the long run) is to not design in noise in the first place. The chances of finding a noise spike that's ruining a part once a day, or week, or month with an oscilloscope is almost nil. The better drive, motor, VFD, and power supply manufacturers (Parker, Siemens, Kollmorgen, and others) publish recommendations for wire size, shielding, wire placement in relation to other devices, grounding, and so on in many of their product installation guides. Even if you're not using their products you should be able to get a good overview of what's required to put together a reliable system.

            Comment


            • #21
              Originally posted by genea View Post

              The bandwidth you need for your stated use depends on your spindle speed. With a spindle max RPM of up to a couple thousand even the highest pulse count encoder that Centroid recommends should be viewable with a 1 MHz bandwidth (10 MHz sample rate) o'scope. If it's an 18 or 20,000 RPM spindle you'll need higher bandwidth and sampling rate and/or/maybe a lower line count encoder. 1MHz BW should also be plenty for autos unless your going to be poking around on microcontrollers or data busses. You should also get at least two input channels so you can see their relationship to one another, and make sure probes are included.

              EDIT: Sorry, I was thinking of a milling machine with that 18-20000 PRM spindle. 1 MHz B/W should be plenty for your lathe.
              Thanks everyone for your the great info! I'm learning a lot.

              Genea's post gives we what I was looking for to help choose a scope to do the job. The RPM readout is fine to just see if the speed is matching what I've programmed in the code but threading & rigid tapping requires a clean signal at about 300 to 500 rpm.

              My Heidenhain encoder was purchased used (thanks Macona) and I have no reason to suspect it's faulty. I think the fluctuations I'm seeing in the RPM DRO are the normal results of the DC motor's speed controller and the encoder is just doing its job reporting it. I'd like to see the scope's real time view of the signal to verify that it is normal and hopefully see if I have any noise problems.

              I would also like to use it to view various auto signals to help diagnose faults. Things like alternator diode leakage, fuel pump issues, crank/cam sensor faults, etc.

              Paul's links gave me things to choose from and I think I'll save up for a 2 channel, 50 or 100 mhz model.
              Milton

              "Accuracy is the sum total of your compensating mistakes."

              "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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              • #22
                Originally posted by dian View Post

                how much was that new? i cant find any price, is it discontinued?
                I see that?
                I will contact my source and see what the issue is.
                My original Mk1 of this unit when they first came out was around CAN$140.00

                Incidentally, concerning the practice of using a quadrature (90°) spaced waveforms, with all of the common optical head unit, the detected signal starts out as a sine wave.
                Usually squared up before the output.
                And it is virtually impossible to read a grating directly this way that is over ~100 p/rev, so in order to read up to very high resolutions, a method using something called the Moiré effect, effectively enlarging the grating resolution many times.
                To achieve this, a piece of the same grating is carried by the read head and skewed slightly to effectively magnifying & create a much enlarged grating using this Moiré effect
                In some cases when looking at the encoder disc, in the right light, it can be observed with the naked eye, quite neat to see.
                Incidentally the 'shutter' produced rotates up or down, perpendicular to the direction of grating!.
                Encoders that do not square up the sine wave, use the two sines waves in a co-tangent form to read in absolute values.

                The old original optical scales we used, had an automotive looking bulb to read the scale, when the lamp was changed, one had to realign the read head using a scope and lissajous figure to set the 90° spacing!!
                Max
                Last edited by MaxHeadRoom; 11-29-2020, 12:29 PM.

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                • #23
                  Originally posted by genea View Post

                  Not being facetious here, the best path (fastest / least frustrating / least expensive in the long run) is to not design in noise in the first place. The chances of finding a noise spike that's ruining a part once a day, or week, or month with an oscilloscope is almost nil. The better drive, motor, VFD, and power supply manufacturers (Parker, Siemens, Kollmorgen, and others) publish recommendations for wire size, shielding, wire placement in relation to other devices, grounding, and so on in many of their product installation guides. Even if you're not using their products you should be able to get a good overview of what's required to put together a reliable system.
                  Best advice in this whole thread.
                  3751 6193 2700 3517

                  Keep eye on ball.
                  Hashim Khan

                  If you look closely at a digital signal, you find out it is really analog......

                  Comment


                  • #24
                    ditto.. Are you saying the treading/rigid tapping isn't working correctly?


                    Originally posted by J Tiers View Post

                    Best advice in this whole thread.

                    Comment


                    • #25
                      Originally posted by genea View Post

                      Not being facetious here, the best path (fastest / least frustrating / least expensive in the long run) is to not design in noise in the first place..
                      There was a paper published by Siemens on equi-potential grounding & bonding aimed at defeating ground loops and general machine 'noise'.
                      They changed their site and it is no longer on there unfortunately, but I did save it as a PDF file, not sure if it will be too large to post here.
                      One chapter shows how shielded cables can now be grounded both ends when the above grounding bonding method is used.
                      Max.

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                      • #26
                        The usual old rule is ground carefully, or ground everything. Both work.

                        Grounding carefully often means you are in danger if making "offsetting mistakes" that work until something changes, then suddenly the cancellation no longer occurs and everything is horrible.
                        3751 6193 2700 3517

                        Keep eye on ball.
                        Hashim Khan

                        If you look closely at a digital signal, you find out it is really analog......

                        Comment


                        • #27
                          There are generally two schools of thought on bonding commons or complete isolation between parts of the system, the latter especially where the system is made up of individual devices from separate companies.
                          I have always conformed to the bonding of system commons to the star point earth GND regardless of the device as long as this is viable.
                          So far it has worked out fine.

                          Unfortunately the Siemens PDF is too large for this site!!
                          .

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                          • #29
                            [QUOTE=PStechPaul;n1912963][URL="https://assets.new.siemens.com/siemens/assets/public.
                            [/QUOTE]

                            Unfortunately that is not the complete publication of theirs on equi-potential bonding.😉
                            Max..

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                            • #30
                              Originally posted by skunkworks View Post
                              ditto.. Are you saying the treading/rigid tapping isn't working correctly?
                              No, I'm not. I admit (sheepishly) that I haven't even tried it yet. I've been doing so much part time side work that I haven't had time to use the Orac other than simple stuff, bushings, shafts etc. No threading yet. It should work fine once I get back on it diligently.

                              I'm also trying to finish the enlarged, ball screwed Sherline mill...and then the super low price Glarry Telecaster clone reached out across the internet & grabbed me last Friday...and, and, and.
                              Milton

                              "Accuracy is the sum total of your compensating mistakes."

                              "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

                              Comment

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