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Thread: Board encoder vs X motor encoder. Which is the master and which is the slave?

  1. #1
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    Default Board encoder vs X motor encoder. Which is the master and which is the slave?

    I was surprised to learn of two different encoders to control the X axis. Since both output pulse signals indicating direction and feed rate, which one is being used for the primary signal train? The controller tells the motor to move at a certain rate and direction and an internal PID feedback loop adjusts the speed of the motor to make this happen. I don't see why they felt the need of a redundant encoder to pull this off? Errors can be detected and corrected with just one encoder in the system.

    Why Carvewright chose to do this is a mystery and they only did it on the X axis from what I can see. I have not checked yet but with 4 wires out of the encoders I suspect they are quadrature encoders with an A and B output and probably run on 5 volts?

    Inquiring minds need to be satisfied.

    Brad

  2. #2
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    My crude understanding is as follows: The brass roller encoder is the accurate encoder. It is calibrated (and can be re-calibrated) to accurately measure the board position. The X motor encoder cannot really be accurate as it is too far removed from the actual board position through belt slippage, gear backlash, and so on. The machine does use the X encoder data to control board movement but continuously corrects it using brass roller data. When not under both rollers, however, the brass roller encoder data is ignored and only the X motor data is used. This is one of the hidden reasons to stay under the rollers. Another thing to note is that the machine only uses the brass roller to make very small corrections. If for one reason or another, the difference between the two encoders becomes too large, the machine rejects the brass roller as having failed and again defaults to only the X motor data. This when we can get the tracking problems.

    This, I guess, is the price paid to have the belt system with no board length restriction.

  3. #3
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    Thanks for the response. I think the best place for the encoder would be on the lower back roller. That location is after the gear train and fixed. A more accurate solution would be a tape marked with bars like a bar code that could be stuck to the edge of the board and read with an optical encoder sensor. The problem with that idea is we would have to keep the tape on hand and make sure we get it bonded correctly to whatever we are trying to cut. Then there is the chance the optical sensor could be fouled by chaff from the machining process but a small aquarium sized air pump with a small nozzle could blow it clean just ahead of the sensor. At least it could adjust for board slippage which is something the current system cannot do.

    All things considered, I think we would be better off with just the brass roller encoder and stay under the rollers and use a carrier board when the project is too short to do so. Tracking problems would be easier to diagnose and probably happen less often with the motor encoder taken completely out of the equation.

    In the future I'd like to see the cover that's over the X axis gear train molded out of a clear plastic so we could keep an eye out for backlash and missing teeth while still keeping the gear train sealed. I've still got the old two plate gear system. Old forum posts made it sound like it's a good idea to change them out at $5.00 dollars. I checked the price and that's no longer the case. A little inflation for prices is one thing but these gears are now more than a little pricey in today's economy. Same with the rubber belts. $200.00 dollars? I just ordered some sandpaper belts at $17.00 a pair and will stick with those until I find a vendor making aftermarket rubber belts at a more reasonable cost.

    Brad

  4. #4
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    Brad,

    I wish you would post this same question in the Facebook Carvewright Group. The reason is that the Owner of LHR sometimes posts some very interesting replies to our Technical Questions and that would be a good thing. Bergerud has a excellent handle on how the machine operated and I believe he is on track with the interplay between the Brass Roller and the X Encoder.

    I had to look up Quadrature Encoder.... And I have a long history in Electronics going back to Navy Analog Gun Fire Control Computers in 1976 to Fanuc Robots at SONY in San Diego in 1995. so you got me... Never heard of one... https://www.dynapar.com/Technology/E...ature_Encoder/

    So the CarveWright Encoder is a Simple Counter. The black lines seen in the Shadow Picture make and break the LED Light Beam. Resolution of the encoder is determined by the thinness of the line. Thicker Lines and less resolution. I have seen one encoder with finer lines then the CW Encoders.

    Next WHY a X Motor Encoder? Well... I actually believed early on that is was not even used. Still on the fence about that... WHY... So the 3 Motors are interchangeable...

    The Brass Roller in my opinion was always the dominant X feedback. As seen when the board lifts off the brass roller the board keeps moving looking for the right counter number... Hence Expensive FIRE WOOD.

    So lets talk about homing of the machine when you turn it on. In the early days Z Axis has a metal fin and a LED Sensor when the head went up it blocked the fin and that was HOME. The System must have recorded the Z Encoder Number and made that HOME. Later LHR changed to the CURRENT Home Process.. The Z Bumps INTO the TOP PIN for HOME and that number is loaded in memory. IF you turn ON Z Encoder DATA on the LCD it will show 0000 as that is a COUNTER ZERO. Then move the Z up and down and the numbers will 0001, 0002 in one direction and 9999, 9998, 9997 in the other direction. That is why we recommend a Encoder TEST is to pull the Z to the top PIN and HOLD and then activate the Z Data on the LCD. Making the PIN 0000. Then we suggest to move the head up and down and left and right to also test the FFC Cable... Return to the PIN and if all is well you should have 0000 on the display. If you have a DIRTY Encoder Disk, Loose Encoder on the shaft, LOOSE Z Belt, or Bad FFC Cable it will show up.

    The Y bumps into the LEFT Side when Homing. The Z.... ??? Just accepts life as it is?

    I have never had the PLEASURE to OPEN and INSPECT a Brass Roller Encoder... Anyone have one that is broken that wants to donate it to SCIENCE? My belief is that the Brass Roller Encoder has a Different Resolution and might be a higher resolution.... A Finer resolution... ????

    So now time for Pictures....

    Picture 1. My Humble Beginnings in the Navy 75-95. A Analog Gun Fire Control Computer used to Aim the 5 inch Gun on Navy Ships with a 12 mile range. Dials, Hand Cranks, Gears, Synchros, Servos, Motors, Tubes and transistors... Not a single encoder....

    Picture 2. Seven years later... A Digital Version of the Analog Computer. They wrote a Program so the Digital Computer could Emulate what the Analog did but was more accurate and faster.

    Picture 3. The Money Shot in MY opinion. Showing the Shadow Lines of the Z Encoder. The Finer the lines the better the resolution.

    Picture 4 and 5. A Clean encoder and a DIRTY Encoder....

    Picture 6. Showing the Theory of a Pulse Encoder like WE Use and a ABSOLUTE Encoder like used in a Fanuc Robot. The Absolute Encoder needs to be Physical ZEROED upon Instillation then REMEMBERS where it is after a POWER OUTAGE.... The Lines and blocks represent a BINARY NUMBER. 0000 0001 0010 00 0011, 0100 = 0, 1, 2, 3, 4 by Bit Weights.

    Picture 7. Shows a Pulse Encoder Disk.

    Picture 8. Shows a Absolute Encoder. SEE at the 3 O' Clock Position.... See the 0000 Zero then 0001 or 1 then 0010 for 2 then 0011 for 3. It is all about bit weights... 0000000 to 1111111 or from Right to LEFT... 0, 1, 2, 4, 8, 16, 32, 64 So 1111111 or 7 1's... 64+32+16+8+4+2+1= 127
    https://www.calculator.net/binary-ca...ype=op&x=0&y=0
    Attached Thumbnails Attached Thumbnails zmk47.jpg   Zio1_WEB1.jpg   DSC06551a_WEB.jpg   DSC06552a_WEB.jpg  

    DSC06545_WEB.jpg   1 aCWecoder.jpg   1 aCWoptdsk.jpg   1 aCWAbsl.jpg  

    Last edited by Digitalwoodshop; 07-09-2019 at 04:51 PM.
    Favorite Saying.... "It's ALL About the Brass Roller"..... And "Use MASKING TAPE" for board skipping in the X or breaking bits.

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  5. #5
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    Quote Originally Posted by Digitalwoodshop View Post
    I have never had the PLEASURE to OPEN and INSPECT a Brass Roller Encoder... Anyone have one that is broken that wants to donate it to SCIENCE? My belief is that the Brass Roller Encoder has a Different Resolution and might be a higher resolution.... A Finer resolution... ????
    In fact, the brass roller encoder is the same as the motor encoders. (All four the same!) The circuit board is not soldered on and so it provides perfect repair parts for a motor encoder.

  6. #6
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    Quote Originally Posted by bergerud View Post
    In fact, the brass roller encoder is the same as the motor encoders. (All four the same!) The circuit board is not soldered on and so it provides perfect repair parts for a motor encoder.
    Did not know that...

    AL
    Favorite Saying.... "It's ALL About the Brass Roller"..... And "Use MASKING TAPE" for board skipping in the X or breaking bits.

    Follow ME on Facebook http://www.facebook.com/pages/Accoun...50019051727074

    www.PoconoDigitalWoodshop.com

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  7. #7
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    One of the best videos for breaking down how encoders work and how they can be used to control motors is here: https://www.youtube.com/watch?v=XIUrnR8bLAI

    If you want to know even more I suggest you subscribe to his YouTube channel and also check out his many videos on various aspects of cnc and motor control.

    I hooked up my servo to power today to verify I had figured out the wiring. The white wire and the black wire in the row next to it are the power wires for the motor. The other 4 wires are for the encoder signals. If I find time I'll hook it up to the oscilloscope I built and verify the pin out connections for the encoder signal.

    Al, you mentioned the finer the line the better the resolution. If your talking about a fixed diameter circle that's true. A bigger circle has more space at it's diameter so you can get more lines and spaces on the disc without changing the line width. In other words; resolution is lines per revolution.

    In the video I linked to he's using an Arduino Uno to control the servo he built. I was browsing the circuit board inside the controller and noticed one of the chips is an 8 bit micro controller with 2K bytes of flash. Those chips are programmable very much like an Arduino. (Atmel 0650 attiny26-16 pu) I also noticed some popular chips for reading Quad encoders. (74HC86D)

    As far as Face Book; while I would enjoy the information, I don't do Face Book. I signed on in the very early days and probably was one of the earliest to see a problem with the way it's coded and ended my account there.

    I mentioned using printed tape as an encoder. While there are several cnc's out there that use such a system, it does have it's down side. If you stretch the tape while installing it you change the resolution. Another idea I am thinking about is a retractable reel with a non stretchable cord. A tape measure for example. It could be mounted to the machine at the front or back. You could pull out the cord or metal tape and attach it to the end of the project board or sled with a small screw. The encoder would be attached to the reel and would be able to accurately track board position as it moved in both directions. The encoder lines could be printed onto the steel tape measure in the desired resolution and could be replaced if damaged like the replacement tapes you can get for conventional tape measures.

    Encoders don't have to be optical either. Anyone remember the old wire tape recorders that predated tape recorders? A piece of that wire on a retractable spool would also work. A tone recorded on the wire would serve as the encoder lines and more or less resolution could be adjusted by changing the frequency of the square wave signal recorded on the wire. A regular magnetic head could read and write the signal and an analog to digital converter could get the signal in the format for the computer to understand. The Arduino Uno for example can do all of this and they are easy to program and very cheap to buy. I paid less than $2.00 dollars for the last Arduino boards I purchased. The nice thing about using a piece of recording wire is cost and it cannot be effected by dust and dirt like optical encoders can.

  8. #8
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    Interesting take on Encoders.

    Great Video too.
    Last edited by Digitalwoodshop; 07-11-2019 at 08:56 AM.
    Favorite Saying.... "It's ALL About the Brass Roller"..... And "Use MASKING TAPE" for board skipping in the X or breaking bits.

    Follow ME on Facebook http://www.facebook.com/pages/Accoun...50019051727074

    www.PoconoDigitalWoodshop.com

    www.AccountabilityTag.com


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