Is this a tachometer?

I am reading about AMC drives and how they are supposed to be connected to tachometers for feedback.
And my question is, is this thing with brushes and wires a tachometer?
http://tinyurl.com/25zwke9
i
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you're looking at the motor brushes.
You don't use a tach with EMC. Encoders go to control. Conrol sends -10v to +10v to amps set in torque mode. Only connections on your amps are DC+ and DC- in, Power+ and power- to servo, your analog reference signal and analog ground. Also strongly recommend servo fault output and inhibit input if your control will do these.
Karl
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OK... They look kind of small...

That's "power in", got it

That's "power out", got it

Is that the +/- 10v control signal?

OK, I am not there yet to understand this properly. I did not know that servos had a fault output.
By inhibit input, do you mean things like limit switches, estop etc?
So, the amplifier's tach input simply remains completely unconnected, right?
i
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yep
Yes your servo will fault on a number of out-of-bounds conditions. If the control doesn't know, it keeps trying to move.

No, the control sees the limits. I use the inhibit for machine stop, tool changes, etc. any time the operator is in the machine like with a door open, on and on. Some use it for E-stop. My training as a corporate engineer says this is a no-no. Estop physically kills all power to any device that moves in my equipment. Estop should kill a broken or misprogrammed machine. I had to have idiot proof equipment and they had some really ingenious idiots where I worked.

Yep, the tach isn't used at all.
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I inspected the motor more closely. There is another set of larger brushes that clearly are for actually running the motor. These tiny brushes are likely to be a tachometer.
i
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    Great! And you have a servo amp which accepts the tachometer wires.
    Note that the easiest thing to get wrong is to wire the tach (or the motor) wires backwards. If that is done, it will shoot to full speed on one direction or the other no matter what the command voltage is.
    Just test this with no mechanical connection to the machine to make sure it is right before you let the amp and motor run the machine.
    If you have a lab type power supply which can put out any voltage between 0V and 10V, you can run at any speed within the motor's range. If the amp is not the one which came with the motor and machine, you may need to adjust the feedback gain so your 10V is truly the maximum RPM you want. The motor's label should tell you both the maximum speed *it* can take happily, and the volts/1000 RPM from the tach, so you can use a DC meter on the tach leads at the amp to see what speed you are really getting.
    Anyway -- the lab supply will give you all possible speeds in *one* direction, and you will have to swap ground and + to the servo amp inputs to get it to run in the other direction.
    Or -- you could set up a pot between +10V and -10V with either a pair of resistors defining a 0 voltage reference and use the pot to get all speeds within the range of the motors.
    Of course, again while playing with this, you want the toothed belts off the motors so you don't drive anything hard against a stop.
    Set up the travel limit switches to shut down the amp if it hits one of them, and make sure that always works before you let the motors actually move the machine. (This applies, of course, whether you have tach feedback or not. :-)
    I would suggest using relays on the limit switches, so each one will stop the servo motor moving in that direction, but allow moving in the other direction, but other contacts will also hit the E-stop on the controller as well, since if you hit a limit switch you want to stop all other motion as well. For convenience, given how buried some of the belts are, you might want to provide a pair of low voltages (perhaps +/- 0.5V or so) to feed to the servo amps to back off a limit switch while the computer is shut down. The Anilam conversion which I used at work had handwheels, but your (and my) Bridgeports do not. If you can find them, set up military style pull-to-unlock toggle switches (the kind which arm weapons on aircraft) for connecting the slow motion voltages into the servo amps. This way, someone has to *think* before they interrupt the control signals to the servo amps.
    Enjoy,         DoN.
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    They are rather small.

    Plus there *may* be provisions for using the tach feedback. It will be a function of the servo amp in use -- not EMC itself.

    Yes -- to totally shut down power output to the motors from the servo amp -- so while you are fixing whatever went wrong, you don't suddenly get a hand trapped between a (non-rotating_ milling cutter and the workpiece or something else rigidly mounted.

    Look up the data for your servo amps. Don't take the "EMC does not use tach feedback" as true. It is not universally true. It is true when using the Gecko drives, which are emulating a stepper motor, but not so with many servo amps.
    Enjoy,         DoN.
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    [ ... ]

    That depends on the servo amplifiers. If you are using the Gecko drivers for DC servos, then yes, the tach is not used. But the Gecko is made to make the servo act like a stepper motor.
    However -- other servo amps may well accept tach feedback.

    Which servo amps are you describing? It is not true of *all* possible implementations of EMC. Many DC servo amps accept tach feedback and automatically adjust the power to the servo motor so the speed indicated by the tach feedback matches the speed commanded by the control. I have similar SEM servo motors which do provide tach feedback, and the AeroTech 4020 (+/-40V +/-20A) amplifiers which I have utilize that tach feedback. You can command speeds which are slower than the second hand on a clock with that -- even without the encoder feedback. Just a servo amp, and a power supply which can output little as a stable 0.001V or less.
    Enjoy,         DoN.
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Don't make life hard for iggy. It's his first one. Or that is, he's a virgin or at least he used to be.<VBG>
Iggy has some brush type analog servo amps made by AMC. They work extremely well with only +/- 10 volt from the control. Simple to install and reliable.
Karl
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I am reading the drive datasheet right now.
The drive does accept the tachometer signal.
Also, I do think that the object that I showed on the picture, is actually a tachometer. The wires are too small for carrying motor power.
I received Jon's PPMC today and I will try to hook it up to the PC (but not to the mill yet).
i
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On Tue, 08 Jun 2010 20:50:12 -0500, Ignoramus8975

One should note that many servos/encoders have a number of different outputs. Some you may need, some you dont need.
Counts AB/BA/Z Home position Secondary Home Velocity Tach (different than velocity)
And others
Here is a typical wiring diagram of the servos that I use on OmniTurn CNC lathes http://www.glentek.com/glentek/dcconnt.aspx
btw..have you seen this?
http://www.homecnc.info/servo%20box.pdf
Gunner
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    [ ... ]

    [ ... ]

    Indeed.
    Useful information.

    Hmm ... that looks like it is using the Gecko servo drivers -- the ones which make the servos look/act like steppers. Note the presence of the step and direction inputs -- no tach input, but encoder inputs. You feed it a step signal, and it goes full speed (or perhaps a speed which is a function of how many steps behind it happens to be) until the internal step count matches the number of step pulses received. Which means that you lose the ability to have a very slow speed on one axis without steps -- so a lathe would turn out a stepped Morse taper instead of the proper smooth one. How bad that would be is a function of the resolution of the encoder, of course. I know that it is terrible with my Compact-5/CNC lathe, which has a minimum step (set by the computer) of either 0.001" or 0.01 mm *radius*, not diameter. This turns out a very visible step in diameter when attempting to make a Morse #2 taper.
    So -- this is throwing away one of the benefits of servo motors for the convenience of interfacing to a version of EMC set up for steppers, instead of one with an interface board for producing velocity commands to servo amplifiers. The interface board should be able to put out an analog voltage between +10V and -10V to command the desired speed from the amp/motor combination
    I also see the possibility of resonance with the step rate and the mass of the machinery being controlled. With a true servo amp. you can add damping to control this.
    Also -- with only 36V, you are throwing away quite a bit of the speed that the servo motors are capable of.
    Enjoy,         DoN.
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You won't need a tach input on the drive.
A servo is just a dc motor. Put the leads on your battery charger. It should turn slowly.
Hope you're having fun and learning.
Karl
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    Ignoring AC servo motors, of course.
    A DC servo is A DC motor with a special design low-inertia rotor (usually the windings in air, bound with epoxy or something similar) with all the iron stationary to allow maximum acceleration.
    Many of them have current limits which if exceeded will at least partially demagnetize the permanent magnets making the field in the motors. (Only the newest ones with rare-earth magnets, or those with a DC field winding.)
    The benefit of the tach feedback is the ability to run at a very stable and very slow speed -- so you can move one axis very slowly while the other moves at a proper cutting speed to generate a very shallow angle ramp.
    Enjoy,         DoN.
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    I'm not trying to make things hard for him -- I'm trying to make sure that ne knows the possible options for *his* motors and amps. He will learn quickly enough -- he is good at that.

    Again -- which amps? I note that at least some of the AMC amps use tach feedback.
    Get the model number, and check against the data here:
        http://www.a-m-c.com/searchtool /
Of course -- it could be using intervals between pulses of power to check generated voltage using the same brushes as drive and tach. But this means that there must be dead periods during which there is no drive. More common with PWM than with a DC servo amp in my experience.
    Enjoy,         DoN.
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Karl Townsend wrote:

If you are using a velocity servo amp, then you WILL want to use the tach with your servo amps. If you use the tachs with a servo amp in velocity mode, you will get much better response. EMC will be perfectly happy with this arrangement. I am using velocity servo amps with tachs on my Bridgeport, with EMC.
Jon
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Iggy, All tachometers are unregulated voltage dc generators that provide a linear output where the output voltage rises as a function of speed. A simple test can identify this functionality. They are typically used in analog systems and are used to back off the output gain of an op amp, based on velocity or to apply dynamic braking. The cost of these devices directly relate to their linearity. They have been replaced by digital electronics because of cost. Very cool technology. Steve

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    It looks it to me.
    Some motors have a smaller diameter tach on the free end.
    Others simply have a second set of brushes at right angles to the ones which drive the motor. It looks as though you have a lot of life left in those brushes.
    Hmm ... that red dot at about 4:00 on the brush mounting plate looks as though it may be close to a similar mark on the inside of the case (not visible form this angle) to assure reinstallation in the proper orientation, given those slots for the mounting screws.
    The real test is to drive the motor by another source, and look for DC voltage on the extra pins. (Look at the connector for heavier and lighter gauge wires if there is only the one connector, exclusive of the encoder.
    And the tach feedback pair may be shielded, unlike the power *to* the motor.
    If these are SEM motors, then they almost certainly have tach outputs. IIRC, mine had a label inside the cover of the wiring box which the connector was mounted to. There were more pins than were needed.
    Enjoy,         DoN.
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Ignoramus8975 wrote:

Yes, the extreme thinness of the commutator indicates it must be a tach. I haven't run into many 4-pole tachs, but if it is smaller than the main motor, it has to be the tach.
Jon
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I don't think so. If it was a tach, I believe you would only see two brushes. That is based on the limited number of servo motors I've had a part. I could be wrong.
Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
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