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Speed up a stepper motor

Discussion in 'Robotics & Mechatronics' started by mesamune80, Jul 14, 2006.

  1. phalanx

    phalanx Member

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    1140 RPM (19 rot/sec) is doable if you have a motor willing to cooperate but you will have so little torque that the slightest load may stall it.

    What is preventing you from using a speed controlled DC motor?
     
  2. mesamune80

    mesamune80 New Member

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    DC motor is fast but easy to spoil,so everytime the motor spoil i need to replace a new one,that's why i think of this stepper motor.is there any other way i can acheive 8 microstep/step if i use a bigger motor?and one thing is that if only DC motor itself there will not be any torque present.
     
    Last edited: Jul 20, 2006
  3. phalanx

    phalanx Member

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    DC motors are easy to spoil? A stepper motor is a DC motor so I'm not sure what you mean by that.

    In terms of microstepping, you can use 8 microstep/step on any stepper motor. That does not mean that each microstep will be 1/8 of a normal step. Tolerances in the manufacturing process as well as the driver's waveform results in different step sizes when moving between full steps. While not uniform in size, their sum will total a full step for the motor. This has little to do with the size of the motor but more so its build quality. More expensive motors tend to have a more uniform microsteps.

    What do you mean by "if only DC motor itself there will not be any torque present?" Fundamentally, a stepper motor has considerable holding torque which means it is very good at holding a stationary position. On the other hand their torque steadily reduces when their speed increases. Normal DC motors (even with a very good servo controller) have much less holding torque than a stepper. They do provide considerably more torque when rotating, especially at high speeds.
     
  4. dave

    Dave New Member

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  5. mesamune80

    mesamune80 New Member

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    what i mean here is that when turning at high speed the DC motor will hv 0 torque and this will cause the DC motor to easily got spoiled.Another question is that for this UCN8048B is it neccessary for me to "ONLY" use 6 wire stepper motor?because when i try 4 wire or 8 wire stepper motor with this IC it wont work.Could anybody tell me what had happened?
     
  6. phalanx

    phalanx Member

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    You are incorrect about DC motors. They do make plenty of torque at high RPMs. Stepper motors make less torque the faster they are spun.

    If you have an 8 wire stepper motor, you can make it behave like a 6 or 4 wire motor. There is no sense in writing everything twice so I suggest you read through this excellent stepper motor tutorial:

    http://www.cs.uiowa.edu/~jones/step/
     
  7. hjames

    hjames New Member

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    What is the maximum desired RPM, and how much torque do you need? I just finished tacking some pretty beefy steppers onto a milling machine, and they barely do 10rev/sec.

    The three main types of DC motors are: DC brushed motors - cheapest, easiest to get, steppers - high torque at zero velocity, low-medium speed only, and DC brushless - most expensive and semi-custom, capable of very high power density and extremely high speeds.

    As for stepper motor types - 5 or 6 wire ones are usually "unipolar" varieties. 4 wires are "bipolar" - they have higher performance than unipolar kinds, but require more complex drivers, and 8 wire'd ones are configurable to be either type. Keep in mind that the driver chip you've chosen can only drive unipolar steppers, or the 8 wire kind if you setup the coils properly.

    James
     
  8. mesamune80

    mesamune80 New Member

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    thanks james,i think i configured the wire wrongly ,but how to make sure my configuration of the motor of 8 wire (Bifilar motors) correct as unipolar motor,because i need to use the 8 wire motor as unipolar motors,i got 8 wires coming out from the motor yellow ,orange,black,red,red stripe,black stripe,orange stripe and yellow stripe.how to connect this 8 wire for me to use it as unipolar motor?Thanks.
     
    Last edited: Jul 23, 2006
  9. hjames

    hjames New Member

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    You should read section 1 of the link phalanx posted - it'll give you a general idea of how to do the testing, but it isn't going to be terribly easy... And going by colors is probably not useful at all.

    First grab an ohmmeter and figure out how the wires are paired. Then the way I'd proceed (since I have a DC power supply handy, and don't feel like trying to setup an AC power supply) is to find turn each coil on and try to "match" them up. When you apply power to a particular coil, it'll cause the stepper to snap to a certain position. There should be another coil that causes it to snap to the same position - obviously you'll need to test and label the coils and *polarities* as you go along. Once you've identified everything, just wire up each pair of coils in series, and presto, 6 wires.
     
  10. mesamune80

    mesamune80 New Member

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    Thanks for the way to test out the polarities of the wiring,but wont all the stepper motor manufacture hv to follow the colour code for the wiring?would they have certain standard for this?Anyway i'll try out the method you just told me,this can this be told by just using an ohm meter and measure out the resistance of any pair of wires?
     
  11. hjames

    hjames New Member

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    If you can find the particular manufacturer, they should have documentation on it, and chances that they'll keep a certain wiring color scheme among their line of products. In general, I doubt that there is a standard wiring code for something like this.

    You'll need to figure out the polarity of the coils, as well as how they're matched up against each other. You won't be able to do this with a meter. At the very least, you'll need some current-limited DC power supply - a couple batteries might be enough to just test out each coil - but without knowing anything else, that's the best I can recommend.

    James
     
  12. mesamune80

    mesamune80 New Member

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    Last edited: Jul 23, 2006
  13. hjames

    hjames New Member

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    The main limit that you need to observe when dealing with steppers is the max current limit -but I think they calculate the voltage based the coil resistance, so it should be okay. For testing purposes it's probably easier to use some fraction of the rated current/voltage though - you just need to see which way a coil pulls the rotor.
     
  14. hjames

    hjames New Member

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    Erm, so if you hook up the stepper according to the "Series connection" diagram on the lower left, that's all you need to do... And that's a 2Vdc stepper motor you have there...
     
    Last edited: Jul 23, 2006
  15. mesamune80

    mesamune80 New Member

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    2Vdc?not 24Vdc?ya i am facing a problem here,when i hook up the power soure for the dc motor with 12vdc(same common with main board) the motor move but with 24vcd(diffferent common with main board) the motor wont move,do need to equalize the common for the driver and for the stepper motor?for ur knowledge i am using 12vdc as my main board power source,it then goes through a regulator so i got my 5v for my microcontroller and 5v for my stepper driver.what is the reason behind this?i am still scratching my head on this >_<"
     
    Last edited: Jul 24, 2006
  16. hjames

    hjames New Member

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    If you are using this particular motor with that driver chip using an L/R drive, then you must choose R so that the maximum current is 3Amps. This stepper motor is meant to be driven with a "chopper" or current limited stepper driver. If you put 12 or 24Volts across a coil in this motor it *WILL* be damaged, What happens is that the rotor gets demagnetized if the current exceeds the rating by a significant amount.

    As for your circuit, I can't understand what you are describing - if you are asking about ground connections - everything should be grounded to the same point.
     
  17. mesamune80

    mesamune80 New Member

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    Ic, nowonder my driver suddently become hot when i connect 12vdc to the motor,but the current is just 500mA
    how come the driver can become hot?or i really need a special circuit to drive this kind of motor?Thanks.
     
  18. hjames

    hjames New Member

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    Well, the motor(if hooked up correctly) will have have a limit of 3Amps/phase. R = V / I = 12 / 3 = 4Ohms. Since the stepper itself is .76Ohms, the resistor needs to be atleast 4 - .76 to avoid toasting the motor....

    However the chip you've chosen can only drive 1.25A, so chances are that your motor is "safe" only because the driver chip is getting exceedingly hot.

    You'll need to post your schematic (along with how you've connected the motor) for any useful response...

    Incidentally, the chip I'm using for my stepper motors is the A3977 (by Allegro, the same manufacturer). This chip has current chopping so it regulates the current properly. There's a number of other chips that would also work, the A3977 was just easier to use.

    James
     
  19. phalanx

    phalanx Member

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    I can attest to the quality of Allegro parts. I've used their A3984 in several commercial and personal products. Fantastic little chip that frees up quite a bit of processor time, especially when controlling 3 axes with microstepping.
     
  20. mesamune80

    mesamune80 New Member

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    ic,that's why my driver burn out =.=" one more thing,how to increase the torque of one motor?or the maximum torque is already fixed by each motor?
     
  21. hjames

    hjames New Member

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    Torque depends on magnetic flux, which is limited by the magnets in the motor. Exceed the maximum currents => exceed the flux => toast the magnets, and you'll have a dead motor. I think steppers, in particular, depend heavily on the magnets, so yeah, whatever torque is listed on the data sheet is the maximum "safe" torque you can get out of the stepper.

    James
     

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