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  • Sparky_NY
    replied
    Originally posted by macona View Post
    I have driven a bunch of servo drives with single ended inputs with generic differential encoders. They usually output about 3.5v which is enough to satisfy the TTL single ended inputs I have used.
    You were lucky then, if they have a RS422 standard line driver the loaded output voltage can be as low as 2V. Look at the RS422 spec, which is the standard for encoder line drivers. http://www.rs485.com/rs485spec.html

    Again, I am speaking specifically of the BE2520AC drive the OP plans on using, if the encoder input voltage gets near 3V it will not function. This voltage CAN go as low as 2V if the encoder has a RS422 line driver. For some reason you insist on referring to generalities when the OP has a specific drive he plans on using. The RS422 spec is also quite clear, it CAN be 3.5V as you experienced but it is not GUARANTEED, the spec is as low as 2V loaded. I have personally ran into this using the BE25a20ac drive with a encoder meeting the RS422 line driver spec which resulted in a runaway due to loss of velocity feedback. (last specific time I seen this was with a fanuc redcap, incremental encoder and BE25a20AC drive, I have also seen it with other encoders and the BE25a20ac drive, its best not to play roulette with specs)

    I won't comment further, the OP's questions have been answered.

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  • macona
    replied
    I have driven a bunch of servo drives with single ended inputs with generic differential encoders. They usually output about 3.5v which is enough to satisfy the TTL single ended inputs I have used.

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  • Sparky_NY
    replied
    Its not a matter of single ended or differential, its a matter of the voltage levels of the RS422 standard. The drive model the OP listed has encoder input for velocity feedback that will not trigger at the voltage levels of a RS422 line driver. Some drives may have a RS422 line receiver in the drive, such is not the case with the BE25A20AC. The OP listed the drive he intends to use, other drives specs are irrelevant. The drive only has single ended encoder input for velocity feedback purposes and will not trigger if driven by a RS422 line driver. I learned this lesson the hard way.
    Last edited by Sparky_NY; 11-13-2016, 04:23 PM.

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  • macona
    replied
    Originally posted by Sparky_NY View Post

    The drive the OP is using accepts single ended encoder inputs only and they must be over 3.5 volts. A encoder with a line driver will not work with the drive unless you rig up a line receiver also. (fought that battle and had to ditch the line drivers) Line drivers usually meet the RS422 spec and are used with a line receiver.
    This depends on the drive. Some drives will work just fine with single ended encoders hooked to the differential inputs while others wont.

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  • Sparky_NY
    replied
    That should work ok.

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  • DICKEYBIRD
    replied
    Originally posted by Sparky_NY View Post
    Your plan presents a problem. Reversing the motor leads by switch will indeed reverse the motor BUT you will have a runaway condition unless you also reverse the tach input.
    Wasn't planning to reverse the motor lead polarity, just the input to the drive from the BOB. Won't that work or am I over-simplifying things?

    Mine actually threads fairly decently now with v.062 after I got the spindle speed down via jackshaft belt reduction but the larger sizes in steel (40mm & above ) still give me trouble as the spindle load goes up when the cut gets to its deepest & the speed starts the wobblies.

    My Mach threading problems have mostly been self induced or spindle speed related. Of course, I have always cut fairly short threads (1-2 dia's) and will always buy leadscrews if I need them.

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  • MaxHeadRoom
    replied
    Originally posted by Sparky_NY View Post
    I am guessing that by "BLDC type" you are referring to the encoders with commutation output capability along with the normal encoder outputs. As I am sure you know, those must be programmed with software they supply, the potential for errors in programming is huge without a deep understanding of BLDC motor theory.
    Most of the BLDC drives I use in servo applications, I have converted some Fanuc also, but if requiring BLDC commutation the encoder has to be changed as they have a proprietary method of commutation and are actually 3ph sinusoidal motor originally.
    As far as the AMT BLDC encoders, I purchased the required programming unit in order to program the correct commutation sequence when mounted and the pole count etc, I could not get consistent performance from them, the motor would run for a while, then suddenly reverse direction for no apparent reason.
    That is when I attempted the support contact, with no success.
    Max.

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  • Sparky_NY
    replied
    Originally posted by MaxHeadRoom View Post
    I have used the A-M-C fairly extensively on retrofits, many of the BLDC drives also have a Hall input tach feature on the mode-select dip sw, the one you have is one of the few that does not have that option, it is OK down to 300rpm for a 6 pole motor.
    I also was not impressed with the AMT capacitive encoders, I generally stick with the optical style, the AMC BLDC type was a disaster, I contacted their support line and was told the guy that designed it bailed on them or some such story.
    And were of no help.
    Max.

    I am guessing that by "BLDC type" you are referring to the encoders with commutation output capability along with the normal encoder outputs. As I am sure you know, those must be programmed with software they supply, the potential for errors in programming is huge without a deep understanding of BLDC motor theory.

    Leave a comment:


  • Sparky_NY
    replied
    Originally posted by DICKEYBIRD View Post
    Yup Mach3, hence the need for a steady speed on the low end when the lathe is configured in the belt reduced mode for threading. (No Linux gloating please.)

    So you don't think that an encoder would be quite a bit more accurate & noise free than the tach-gen? It would be a little easier to stick with what I already have on it; couldn't hurt to try it that way first I suppose.

    The C-11 BOB is definitely has a 0-10v spindle speed output. The reversing plan is to cobble up a DPDT polarity reversal relay to trick the drive into running the motor backwards upon M4 command. Shouldn't be too hard I hope. (?)
    Your plan presents a problem. Reversing the motor leads by switch will indeed reverse the motor BUT you will have a runaway condition unless you also reverse the tach input. (same hold true if encoder feedback is used) The polarity of the feedback signal has to match the motor polarity or you get a runaway (motor comes up to full speed, in the case of a BE25...AC drive, 170 volts to the motor !) IF you switch the tack polarity at the same time, you are good to go. I don't think there is a easy/cheap way to get +/- 10V control signal out of a mach setup. Either a multi-pole switch or a relay with multiple contacts could be used. The only BIG warning is that this would have to be done with the drive not enabled, if Mach energizes the relay and with the drive enabled, expect a boom.

    As for accuracy, yes encoder feedback is more accurate but in the real world, it only comes into play at extremely low speeds, say 10rpm and under.

    Don't be too surprised if using the AMC drive does not result in good threading, the problem isn't spindle speed variation, its the approach Mach takes. Spindle speeds will vary a bit, its a fact of life, others thread perfectly despite that fact.
    Last edited by Sparky_NY; 11-13-2016, 11:56 AM.

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  • MaxHeadRoom
    replied
    I have used the A-M-C fairly extensively on retrofits, many of the BLDC drives also have a Hall input tach feature on the mode-select dip sw, the one you have is one of the few that does not have that option, it is OK down to 300rpm for a 6 pole motor.
    I also was not impressed with the AMT capacitive encoders, I generally stick with the optical style, the AMC BLDC type was a disaster, I contacted their support line and was told the guy that designed it bailed on them or some such story.
    And were of no help.
    Max.

    Leave a comment:


  • DICKEYBIRD
    replied
    Yup Mach3, hence the need for a steady speed on the low end when the lathe is configured in the belt reduced mode for threading. (No Linux gloating please.)

    So you don't think that an encoder would be quite a bit more accurate & noise free than the tach-gen? It would be a little easier to stick with what I already have on it; couldn't hurt to try it that way first I suppose.

    The C-11 BOB is definitely has a 0-10v spindle speed output. The reversing plan is to cobble up a DPDT polarity reversal relay to trick the drive into running the motor backwards upon M4 command. Shouldn't be too hard I hope. (?)

    Leave a comment:


  • Sparky_NY
    replied
    The tach you have will work just fine, save your money for other toys. That is, unless you happen to also need a spindle encoder, the encoder could do double duty in that case if you mount it at the spindle.

    I have used those drives with both brush and brushless motors, even did a Fanuc redcap with one for a friend earlier this year. You will be pleased with the results.


    Isn't that lathe you have running under mach 3? If so, the motor control is probably 0-10V along with fwd and rev signals. Have you thought about how you will handle reversing the motor? Normally, + and - 10V is used for such purposes with that drive. Opposite polarity on the control voltage reverses the motor. IF, you are just going to use a speed control pot then its not a issue, if the breakout board supplies 0-10V then you have to come up with a plan.
    Last edited by Sparky_NY; 11-12-2016, 06:18 PM.

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  • DICKEYBIRD
    replied
    Originally posted by Sparky_NY View Post
    I initially wired it up using encoder feedback to the drive but then switched over to tachometer feedback because of instabilities when the motor was stationary.
    The reason I'm doing this George is for low spindle speed stability. Jitter at rest won't be an issue I think. The AMC data mentions possible noise/stability issues with tachs at low speeds & my tach is the lower range type (7v/1000 if memory serves) so I'm thinking an encoder is the way to go.

    I'm real glad you've used these drives successfully, it gives me hope that even I can get mine working well!

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  • Sparky_NY
    replied
    Originally posted by macona View Post
    Those AMT encoders have never given me anything but trouble on servo applications. Go to US Digital and get an optical encoder from them. The HP/Agilent compatible ones work great. Probably something around 500 line should do it. Differential is usually only about $10 more. A lot of single ended encoders
    Are actually differential internally and they just connect wires to the A/B output lines of the driver cheap.
    I have used them on 3 mills and a lathe, all with excellent results, no complaints or issues. A friend has also used them on a lathe and a mill with similar results. I recall you had some problems with the AMT encoders but that is certainly not the norm, they are widely used.

    The drive the OP is using accepts single ended encoder inputs only and they must be over 3.5 volts. A encoder with a line driver will not work with the drive unless you rig up a line receiver also. (fought that battle and had to ditch the line drivers) Line drivers usually meet the RS422 spec and are used with a line receiver.
    Last edited by Sparky_NY; 11-12-2016, 05:11 PM.

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  • macona
    replied
    Those AMT encoders have never given me anything but trouble on servo applications. Go to US Digital and get an optical encoder from them. The HP/Agilent compatible ones work great. Probably something around 500 line should do it. Differential is usually only about $10 more. A lot of single ended encoders
    Are actually differential internally and they just connect wires to the A/B output lines of the driver cheap.

    Leave a comment:

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