Rotary encoder help

Discussion in 'General Electronics Chat' started by inwo, Jul 11, 2014.

  1. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    Found a couple unknown encoders.

    Had some time to check it out today. Hoping it will help someone in another recent thread.

    DRC model 77L


    Terminals marked :
    ENG
    GND
    MET
    CH A
    CH B

    Means nothing to me.:p
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I have worked with many types of encoders but so far have not come across MET or ENG?
    The GND is I imagine is shield/case GND, the ENG could be Encoder Ground, the A&B are typically two pulses 90deg apart, (quadrature) for position and direction.
    That leaves the MET as +ve supply?
    They can be open collector (up to 24vdc) or 5v line driver.
    If you google quadrature encoder it will show the signal out, to go any further without the data sheet, may need popping the cover off and reverse engineer it for power.
    Thats heck of a lot of terminals on the back?
    Max.
     
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  3. inwo

    Thread Starter Well-Known Member

    Nov 7, 2013
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    Thanks, the eng makes sense.

    I think I'll pop it open. Nothing to lose.

    I'm sure the MET makes perfect sense to someone.
     
  4. MaxHeadRoom

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  5. inwo

    Thread Starter Well-Known Member

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    Near as I can tell the 100 in the part number is 100 PPR

    Seems to check out.

    Pin 2 goes to pin 7 of a SN7406 buffer.
    Pins 1 and 5 (ENG and MET) both go to pin 14 of the SN7406 with one diode drop.

    No sign of regulator. Voltage on pin 14 continues to rise past 5vdc with increased voltage.

    Can read pulses on A and B (7406) with analog meter related to ground, so must be pull-ups on output.

    I picked this up to determine sensitivity for the irrigation sensor.
    http://forum.allaboutcircuits.com/showthread.php?t=99211

    Little confidence that it will work if geared up 1-3.:(

    I can easily hold shaft in a neutral position when near center of trigger points.

    When stopped on edge there is no room for vibration.

    This would not seem to change with lower PPR.:(

    Looks to me like some logic would be needed for an encoder to work.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    Usually a quadrature encoder is coupled to some sort of up/down counter to keep track of position. Some have a third output that happens only once per revolution. That output is used to lock out the counting process until the single pulse happens. That combination was used to establish an accurate zero position on automatic machines. A limit switch would stop the machine at extreme limits and as the machine moved off the limit, the counting would be inhibited until the single (zero) pulse came from the encoder. After that, just up/down counting kept track of position.

    Does your encoder have a glass disk or a metal gear?
     
  7. MaxHeadRoom

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    Jul 18, 2013
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    The counting is not usually Inhibited when using the marker or Z pulse.
    The method is that the Controller commands the servo to travel in rapid until the Home limit S.W. is made, it then reverses off the limit in slow , once off the limit it seeks the marker pulse, when it 'sees' the marker pulse, at that point either zero is entered or a pre-set position is entered into the controller.
    Max.
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    My reference point was the old Hughes MT3, 3 spindle milling machine with the Hughes control that was in service during the mid 1960's. I left out the point that the counter was set to zero (or some preset value) when on the limit switch. Thanks for catching that.

    When starting a program, the Hughes control, would load a counter from tape, sense that it was not zero and start moving in the Positive direction. When moved off the HOME limit and it saw the ZERO pulse, start to count the counter down to zero. Feed rate was controlled by a different value read from the punched tape. When the counter reached zero, a new block of data transferred in and the counter loaded with new data. Depending upon the required direction of travel, the quadrature data was either inverted direction wise or left in a normal relationship. It was not uncommon for these old machines to accumulate error counts but that position error was corrected upon each return to HOME.

    The digital controls were far ahead of the analog systems such as those used on the Fosdic Jig Bore. Thyratron drives on DC motors and the positioning sensed by multi-turn pots geared to the lead screws. Relay driven voltage dividers supplied the command voltage after decoding the punched tape.
     
  9. MaxHeadRoom

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    ....Ah the good old days!:)
    Max.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Normally if they specify a number for the encoder it is LINES. The number of lines on the disc.

    So a 100 line encoder will make 400 quadrature events per rotation.

    Normally the lines are a funky number, maybe it is the "254" number? That would be 1016 quadrature events per rotation.

    Or maybe it is the "77L" meaning 77 lines.
     
  11. MaxHeadRoom

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    There will still always be 100 quadrature events.;)

    Quadrature refers to the two pulses 90° apart, NOT the decision to use 1,2 or 4 pulse edges.
    The designer can opt to interpret these as 100, 200, or 400 counts per rev by using either 1, 2 or all 4 edges of the two pulses and is a controller design choice.
    Max.
     
  12. THE_RB

    AAC Fanatic!

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    Sorry Max, maybe I misunderstood, but your statements seem contradictory?

    The manufacturer label is almost always in lines. That remains constant and will not change because the user detects the pulses in any different way. (Which is why they specify lines).

    The number of individually detectable quadrature events is 4 times the number of lines. That is the max resolution of the encoder.

    Users might decode the 100 lines to 100, 200, or 400 events depending on their decoding hardware and software. But 400 is full quadrature.
     
  13. MaxHeadRoom

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    The point I was making is that there seems a general impression that the term quadrature refers to the 4x the natural or basic manufacturers line count description.
    Whereas the description refers to the fact that two pulses are produced at 90°, nothing to do with the multiplying factor, although they are derived from it.

    There are more than a few web sites that wrongly make the definition 'Quadrature encoders so named because of the x4 ability'.
    It is a quadrature encoder out of the box.
    IOW, An encoder with 100 lines is still a quadrature encoder when using 100 lines.
    The correct description when using four time the manuf line count would be quadrature count x4.
    Sorry if it appears a little pedantic.:(
    Max.
     
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  14. BillB3857

    Senior Member

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    Back in the day, the count on the lable WAS the count. Then some smarty pants found a way to multiply that to get greater resolution out of less expensive encoder. At least that's the way I remember it. Technical development is always about doing the same or better job with less money.
     
  15. MaxHeadRoom

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    True.
    The early Farrand linear scales I was originally involved with used automotive sized incandescent lamps.
    When the lamp was changed it required re-aligning to set the 90° (quadrature) using a Lissajous display.
    Max.
     
    Last edited: Jul 13, 2014
  16. BillB3857

    Senior Member

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    The Hughes rotary encoder I mentioned earlier had photo cells mounted to phenolic stubs pressed into a hole in the back plate. They had an O-ring on them to provide a high degree of friction and had to be rotated to get the proper 90 degree displacement.
    The only Farrand scales I worked with were inductive. The first were glass scales with printed patterns. One half was simply a printed repeating pattern like a square wave on a scope on a slab of glass 12" long. Multiples were wired in series and butted together to give any needed measurement range. The mate had two similarly shaped windings, but they were displaced by 1/2 the distance of one pattern. It sure was fun laying on my back in metal chips while adjusting those things while the mechanic sat up top reading the Vernac scale.
     
  17. MaxHeadRoom

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    We had Farrand LVDT and Ferranti-Packard glass scales, maybe I am getting them mixed up, that seems like eons ago now.
    The Ferranti-Packard was where I received instructional details on how they used the Moiré effect to detect otherwise unreadably fine gratings when using the photo cell method, still done this way today.
    Max.
     
  18. BillB3857

    Senior Member

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    Here is a document I got from Farrand quite some time ago. Interesting read, if nothing else...
     
  19. lightingman

    Senior Member

    Apr 19, 2007
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    Looks like you have cracked it. I have a couple of these.

    You can read these with a micro controller or you can use a JK master flip-flop such as the 4027 along with a two input or gate. This will give you clock pulses and the flip-flop will change state for up or down. If you need the schematic, let me know and I can try to post it here.

    Daniel.
     
  20. THE_RB

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    Thanks for the explanation, I get your point now. :)

    I'm still iffy on this one point of yours though. To my thinking, the encoder is still a "quadrature encoder" because of its construction (making your point right) but if the user is only sensing ONE output and using it for RPM:freq measuring etc, it is not being "used in quadrature", it's being used a simple frequency ouput.

    Also, some early count decoding systems using flipflops etc to detect the A output / edge to clock a flipflop, and the state of the B output a that instant determines if the count goes up or down. In that usage a 100 line encoder makes 100 counts per rev, and (to me!) that is not quadrature at all but is a usage where one output pin is the CLK and one output pin is UP/DOWN. It's a CLK and DIR system, as opposed to a full quadrature decoding system.
     
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