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Thread: Slave stepper motor

  1. #13
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    Default Re: Slave stepper motor

    Regulated supplies are not ideal for stepper drivers. During motor deceleration, the stepper drivers dissipate the energy back into the power supply. With a regulated supply, they usually have no way of handling that, and depending on design can enter a fault mode and shut down, or if you pump enough energy back in, they simply go pop.

    Now when you add the additional load of moving an axis into the equation, you have even more energy being dumped, so the problem can become even bigger.

    There are a couple options. You could implement a reverse energy dump circuit. Gecko do have an application note with a circuit diagram - Returned Energy Dump | GeckoDrive

    Or swap the switched mode supplies for an unregulated supply. All you need is a suitable transformer, bridge rectifier, and capacitor. Again, Gecko have a basic guide in their stepper motor basics. Scroll down to Section 6, Power Supply Basics.

    Personally, I'd nearly always go for the unregulated option, as it's the simplest most robust option, but switch mode/regulated are usually more compact and lighter for a given output.



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    Quote Originally Posted by m_c View Post
    Regulated supplies are not ideal for stepper drivers. During motor deceleration, the stepper drivers dissipate the energy back into the power supply. With a regulated supply, they usually have no way of handling that, and depending on design can enter a fault mode and shut down, or if you pump enough energy back in, they simply go pop.

    Now when you add the additional load of moving an axis into the equation, you have even more energy being dumped, so the problem can become even bigger.

    There are a couple options. You could implement a reverse energy dump circuit. Gecko do have an application note with a circuit diagram - Returned Energy Dump | GeckoDrive

    Or swap the switched mode supplies for an unregulated supply. All you need is a suitable transformer, bridge rectifier, and capacitor. Again, Gecko have a basic guide in their stepper motor basics. Scroll down to Section 6, Power Supply Basics.

    Personally, I'd nearly always go for the unregulated option, as it's the simplest most robust option, but switch mode/regulated are usually more compact and lighter for a given output.
    I ordered the parts for the reverse energy dump circuit and will see how well it works with regulated power supplies, geckos website states the reverse dump circuit protects both the drive and the stepper motor, they also say that with a unregulated power supply the capacitors suck up the reverse voltage, I would think it's never a good idea to backfeed voltage spikes into a power supply and would now probably add the reverse dump circuit even if I was using an unregulated supply, parts are less than $10.

    When I get the reverse dump circuit parts and installed I'll let you know how it works on my regulated power supplies, good or bad.



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    Default Re: Slave stepper motor

    The benefit of an unregulated supply, is because they use a big capacitor for smoothing the supply, that same capacitor will absorb the energy. Yes, the voltage will rise, but provided there is enough margin in voltages, no damage will occur.

    Using a reverse energy dump, will also mean every time the motors decelerate, all that energy gets turned into heat, so you'll need to monitor the main resistor temperature, especially if doing lots of accelerating/decelerating.



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    Default Re: Slave stepper motor

    I would like to present a slightly different view of power supplies.

    When you are running a motor driver such as a Gecko (I only say 'Gecko' because I know them), then you are relying on some rather complex and very high speed digital and analog circuitry inside the box. This circuitry is EVERYTHING to the drive. Yes, I have designed such units myself.

    When you use a simple diode bridge and capcitor as the supply, you are giving the motor driver exactly what it was designed for: a nice quiet supply which can buffer any insults fed back into it. In general the motor driver does not care about the mains ripple it gets: it simply does not matter.

    But when you use a modern switched-mode PS you are feeding into the motor drive some extremely dirty, noisy power, full of RF noise, and that RF noise is quite capable of totally stuffing the system. It might not damage the internals of the controller, but it will sure as hell confuse it. If there is back-feed from the motor it may even damage the devices inside the PS box.

    The mfrs of these elegant modern Class-D H-mode motor drivers are not playing games when they specify a simple diode bridge & capacitor PS: they KNOW what they are doing. If I was a mfr I would simply refuse any warranty claims caused by using an SMPS instead of the simple diode/cap version. But what YOU do is up to YOU.

    Cheers
    Roger



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    Quote Originally Posted by RCaffin View Post
    I would like to present a slightly different view of power supplies.

    When you are running a motor driver such as a Gecko (I only say 'Gecko' because I know them), then you are relying on some rather complex and very high speed digital and analog circuitry inside the box. This circuitry is EVERYTHING to the drive. Yes, I have designed such units myself.

    When you use a simple diode bridge and capcitor as the supply, you are giving the motor driver exactly what it was designed for: a nice quiet supply which can buffer any insults fed back into it. In general the motor driver does not care about the mains ripple it gets: it simply does not matter.

    But when you use a modern switched-mode PS you are feeding into the motor drive some extremely dirty, noisy power, full of RF noise, and that RF noise is quite capable of totally stuffing the system. It might not damage the internals of the controller, but it will sure as hell confuse it. If there is back-feed from the motor it may even damage the devices inside the PS box.

    The mfrs of these elegant modern Class-D H-mode motor drivers are not playing games when they specify a simple diode bridge & capacitor PS: they KNOW what they are doing. If I was a mfr I would simply refuse any warranty claims caused by using an SMPS instead of the simple diode/cap version. But what YOU do is up to YOU.

    Cheers
    Roger
    Roger I hear what you are saying and I will take your advice and use a diode/cap power supply, the gecko is rated for 80 volts, gecko recommends a power supply rated no less than 4 times motor voltage and no more than 20 times motor voltage. The motors I am using are rated 5.7 volts so I plan on using a 70 volt supply does that seem proper, it is my understanding the higher the voltage the more high speed torque so I will be changing the voltage from 48 volts (the original switching power supplies) to 70 volts (the new diode/cap power supply). Also I plan on making and using the gecko design energy dump circuit to further protect the gecko drives from back feed spike of motor regeneration or is this a mistake.



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    Default Re: Slave stepper motor

    Hi Mirocha

    I am running my Gecko servos at about 55 VDC, and they go just fine without missing input steps. So 70 VDC should be fine. I would not go higher.

    I have not put in any energy dump system as the 55 VDC is far enough down that I don't need to worry. But I have not seen any surges on the rail while machining, and I do have a voltmeter on the rail. Actually, the system draws only low current for most moves, except when drilling.

    Cheers
    Roger



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    Default Re: Slave stepper motor

    I built the gecko "return energy dump circuit". My problem was with the 48 VDC "Mean Well" regulated switching power supplies my Y axis has two stepper motors and the Y axis would fail after the first deceleration. The X and Z axis single motors had no problem. I installed the gecko "return energy dump circuit" one each on the two Y axis stepper motors and it has not fail after numerous tests. I am going to change the power supply to a 70 VDC diode/cap unregulated power supply and will be using the return energy dump circuit. I know the circuit is working, it solved my problem for the regulated power supply. I would like to add an LED to the circuit to light when the circuit is shunting current to ground, I could keep an eye on it to see that it is still working. If any one with electronic knowledge could help me and add to the circuit attached what I would need to add a LED if it would work.
    Slave stepper motor-return-energy-dump-jpg



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    Default Re: Slave stepper motor

    You could add an LED to let you know when the dump has been activated, but you will probably only get very brief flashes. Fractions of a second is all one would expect.

    The circuit you are showing looks wrong to me. You have the current from the supply going through a 1kohm resistor - that would never work at any power level! If you changed the 1k resistor to 1 ohm it might work - maybe.

    It is a bit hard to say how to add an LED without more details. Assuming that we are talking about a +70 VDC supply, that means the collector of the transistor may rise to about +70 V. In that case you could put a LED in series with a resistor across the 33 ohm resistor. The anode of the LED goes to the collector, and the resistor should be about 70 V / 0.01 A = 7,000 ohm - say 6.8 kohm ( to ground).

    Cheers
    Roger



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    Quote Originally Posted by RCaffin View Post
    You could add an LED to let you know when the dump has been activated, but you will probably only get very brief flashes. Fractions of a second is all one would expect.

    The circuit you are showing looks wrong to me. You have the current from the supply going through a 1kohm resistor - that would never work at any power level! If you changed the 1k resistor to 1 ohm it might work - maybe.

    It is a bit hard to say how to add an LED without more details. Assuming that we are talking about a +70 VDC supply, that means the collector of the transistor may rise to about +70 V. In that case you could put a LED in series with a resistor across the 33 ohm resistor. The anode of the LED goes to the collector, and the resistor should be about 70 V / 0.01 A = 7,000 ohm - say 6.8 kohm ( to ground).

    Cheers
    Roger
    The circuit is off the gecko website and it does work, the current goes through the diode to the stepper motor gecko drive.



  10. #22
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    Default Re: Slave stepper motor

    Gaarrk !
    I overlooked that. Sorry. Frustrating morning.
    The bit about the LED remains.

    Cheers
    Roger



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    Default Re: Slave stepper motor

    Built the Returned energy dump and added an LED to let me know it is working. I added a diode, capacitor, 6.8 K resistor and a LED to the original circuit. When the there is returned energy the capacitor charges, it lights the LED and the LED gradually dims as the capacitor discharges, if you have returned energy again while the LED is dimming the LED comes back to full brilliance and starts dimming again. I built the circuit to mount above my Gecko drives, I will be adding one to each drive, also have the LED attached to a terminal strip, I may wire it to LED's on control pane cover. Protects the Gecko drives and eliminates voltage spikes to the power supply. Slave stepper motor-img026-jpgSlave stepper motor-img_0353-jpgSlave stepper motor-img_0354-jpgSlave stepper motor-img_0356-jpg



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    Default Re: Slave stepper motor

    You should of added the diode to the opposite side of the 33ohm resistor. The operation of the original circuit, is the capacitor absorbs the return energy quickly, and it's then dissipated through the 33ohm resistor.
    With your circuit, the capacitor still absorbs the energy, however it's now only draining that charge via the LED and 6k8 resistor, as the 33ohm resistor is now behind a diode. If you get lots of sustained energy dumps, there is a real possibility the drive is going to go overvoltage due the energy not getting bled of quick enough.



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