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  • DRO power down memory

    I've just noticed on my Sino DRO fitted to my mill,that if I power down and then move the table,when I power back up the display reads the power down position but does not read the new table position...is this normal?

    It's very frustrating to have to reset the absolute zero back to the corner of the vice if I've moved the table whilst the DRO was switched off.
    I thought a DRO should remember the zero after being switched off regardless of where the table is when it's switched back on,not the position it was switched off at.
    I don't want to leave the DRO permanently switched on.

    Allan

  • #2
    In order to know where the corner of the vice is it has to know where the table is when powered up. To know that it has to have absolute position encoding.
    Free software for calculating bolt circles and similar: Click Here

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    • #3
      Some higher end DROs have battery backup and can keep track of where they are if the display is off.

      Simplest solution, dont turn the DRO off. Power consumption is negligible and its not going to hurt it one way or another. newer DROs use LED light sources in the scales unlike the old DRO scales with little incandescent lamps.

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      • #4
        I set the absolute position to the corner of the vise and zero the display,the DRO remembers this.If I power down and don't move the table at all the zero position is still there when I power back up.

        However,if I happen to move the table whilst it's powered down,when I then power back up the absolute position still reads zero which it was before I powered down,even though the table has been moved to a new position and shows the new position as the zero absolute.

        So basically what's happening is that the DRO forgets the absolute that I set when it's powered down if this makes sense.

        Allan

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        • #5
          Absolute position encoding means that the system is always aware of where it is regardless of the cycling of power. It is a hardware feature and is more expensive than a system that must be "taught" where zero is. A dial caliper is an absolute device whereas a cheap digital caliper is not. Good quality calipers usually have absolute position encoding, meaning they don't have to be zeroed.

          The same applies to DROs. I don't know anything about your specific DRO but is sounds as though it only remembers relative position and is not absolute position aware.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            Originally posted by Evan
            Absolute position encoding means that the system is always aware of where it is regardless of the cycling of power. It is a hardware feature and is more expensive than a system that must be "taught" where zero is. A dial caliper is an absolute device whereas a cheap digital caliper is not. Good quality calipers usually have absolute position encoding, meaning they don't have to be zeroed.

            The same applies to DROs. I don't know anything about your specific DRO but is sounds as though it only remembers relative position and is not absolute position aware.
            http://www.digital-caliper.co.uk/3_a...11crebonoqcvr5

            I assumed power down memory would remember a setting the same as my vernier and mic do when they're switched off.I certainly expected it for the price.

            Allan

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            • #7
              Allan

              In response to your initial question - yes, this is normal for many devices.

              In simple terms:-

              Your scale is made up of a long series of identical marks. so it can only determine relative position by counting them as they go past. Turning off the DRO stops the counting, so it loses track.

              An absolute scale would have markings which will indicate positon at all times, i.e. each mark is effectively unique so the position is known immediatley the device is powered up.

              Some relative devices only 'pretend' to turn off but keep the counter running...but then people complain that their batteries are running down even when they're off

              Cheers.

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              • #8
                It seems that my only solution is to wind the table out of the way to get out of my shed and then turn the DRO off and hope I don't accidentally move any of the handles before I switch back on.

                Allan

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                • #9
                  Like Macona said, the simplest solution is to just leave it on. It wouldn't cost more than .50 cents a month if that. You'd never notice it and it won't hurt the dro.
                  Jonathan P.

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                  • #10
                    My Acu-Rite does have absolute positioning. If I turn it off an on, then move each axis a couple of inches, it can figure out where it is. One of the reasons Acu-Rite and other high-end DROs cost more....
                    ----------
                    Try to make a living, not a killing. -- Utah Phillips
                    Don't believe everything you know. -- Bumper sticker
                    Everybody is ignorant, only on different subjects. -- Will Rogers
                    There are lots of people who mistake their imagination for their memory. - Josh Billings
                    Law of Logical Argument - Anything is possible if you don't know what you are talking about.
                    Don't own anything you have to feed or paint. - Hood River Blackie

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                    • #11
                      Or you could, move the table to the home/absolute 0 position then move the table an even number of inches to clear the door and 0 it there and turn it off. Then when you come back to use it turn it on, move it the same number of inches to home and reset 0.

                      Why don't you just reposition the machines so the doorway is clear at all times. It seems to be a safety issue having it set up the way it is.
                      It's only ink and paper

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                      • #12
                        Depending on your DRO, it may be able to recover the zero with a little help from you. There's a video at DROPROs showing how to do it and my Meister Top10 has the same capability but implemented a little differently.

                        Scroll down to "Power Off Memory" for the video:
                        http://dropros56k.com/Videos.htm

                        Then have a look at the manual for your DRO to see if it has a similar feature.

                        John
                        Location: Newtown, CT USA

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                        • #13
                          Need absolute scales.

                          Your DRO will rezero on the power up position unless it uses absolute scales. Most inexpensive DROs use an incremental encoder - has 2 signals - typically called A & B - pattern goes like this - A off B off, A on B off, A on B on, A off, B on and back to A off B off - when turning in one direction and - A off B off, A off B on, A on B on, A on B off, and back to A off B off - you can get the neumber of counts and direction from this pattern. (Called a GRAY CODE, there are also 3, 4, 8 and more bit GRAY CODES) A 2 bit GRAY CODE is very easy to decode and works on a huge number of machine encoders.

                          Absolute encoders require more bits (and some require batteries - like on a Mitsubshi robot). And because of the more bits they use a different interface and are more expensive.

                          You could make a hard stop at one end of your table travel and always run the table up against that stop before powering things down. Want to be careful how hard you tighten up against the stop. Not knowing what kind of encore system your DRO is using - some encoders have a Z signal - only changes state at one point in the encoders feed back -
                          A B Z
                          0 0 0
                          1 0 0
                          1 1 0
                          0 1 0
                          0 0 1
                          1 0 1
                          1 1 1
                          0 1 1
                          0 0 0
                          1 0 0
                          etc...

                          If you could see the different signals, move against your stop and with the Z signal ON you could then zero the DRO.

                          (Probably don't have any access to the individual signals, and Z probably isn't there any way...

                          Do you know what type of signal your scales send to the readout? Might be able to hook up a couple of LEDs and decode A & B and use them to set zero.

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                          • #14
                            (Called a GRAY CODE, there are also 3, 4, 8 and more bit GRAY CODES) A 2 bit GRAY CODE is very easy to decode and works on a huge number of machine encoders.
                            The 2 bit pattern is quadrature encoding. The encoder is arranged to turn each bit on and off 90 degrees out of phase from the last transition giving 4 different possible values than can be represented by 2 bits, hence the name "quadrature". That isn't Gray code although it shares the characteristic of only changing one bit at a time. The reason for using Gray code with larger word lengths is that it only ever changes one bit of value as it increments or decrements. This means that if the value produced by the encoder is jittering it jitters one count only in the least significant digit of the decoded value. An encoder that uses standard binary code has the potential for a 1 bit change to cause a jitter from min range to max range if using complementary binary codes.

                            Also, a binary encoder may change as many as wordlength-1 bits at once. If they don't all change at precisely the same instant intermediate error values will be produced. That can't happen with Gray code.

                            The main thing that separates quadrature encoding from Gray code is that it cannot provide absolute position information, only direction information.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              Originally posted by GadgetBuilder
                              Depending on your DRO, it may be able to recover the zero with a little help from you. There's a video at DROPROs showing how to do it and my Meister Top10 has the same capability but implemented a little differently.

                              Scroll down to "Power Off Memory" for the video:
                              http://dropros56k.com/Videos.htm

                              Then have a look at the manual for your DRO to see if it has a similar feature.

                              John
                              Thanks for that link,unfortunately it doesn't apply to my system and I can't find anything in the manual either.

                              Thanks kf2qd and Evan,but your replies are a bit like me asking you the time and you start to tell me how a watch works.

                              Allan

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