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Thread: help on stepper motor overdrive

  1. #1
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    Default help on stepper motor overdrive

    hello friends..........
    first of all let me make it clear that i have read almost all the topics on this forum . i found some problems similar to mine but the answer is not clear. let me tell my problem....

    i am building a micromouse using 89c51 and stepper motors
    # two stepper motors, from junk. so dont know specifications. it looks like the one on the link: http://www.vimms.org/stepper.jpg
    #it is 5 wire unipolar
    #resistance between comman and one phase --- 75 ohms
    # i have tested it on 12 v supply and it works fine
    #i am using uln2803 to drive the motors
    #sequence used for front motion is:
    1000 1000
    0100 0100
    0010 0010
    0001 0001
    #power is from 7805(for microcontroller) and 7812 (for motor)

    now, when i put the mouse resting on its back the tyres move freely and in correct sequence. but when i put the mouse on its tyres, it just vibrates. weight of mouse approx 1 kg. i know stepper is not good at torque and i need to overdrive it. but how. i cant afford those driver ics which costs about 250rs each.

    on one forum it is written to use pwm to limit current while keeping voltage high. now i want to know which signal is to be chopped. the one which is given as an input to uln2803 on pins 1-8 or the one given to the common terminal of motor. how can we do it in cost efficient manner.

    which stepping mode provides maximum torque?
    1) 1000
    0100
    0010
    0001

    2) 1100
    0110
    0011
    1001

    3) 1000
    1100
    0100
    0110
    0010
    0011........

    also tell me upto how much voltage can i apply to motor with and without overdrive. i read somewhere that we need to check motor for getting heated up. but with 12v and without any added resistance my motor doesnt even get warm even after hours of drive. does this means that its non overloading max voltage is higher??? how can i check it????

    currently i give 12v at the 10th pin of uln. if i increase motor drive voltage, do i also have to increase this voltage???

  2. #2
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    Default

    1) 1 Kg is a bit heavy. You will have to reduce this weight, to avoid slip.
    2) PWM is basically a square wave @ diff. duty cycle. - so any square wave generator can help.
    3) cheap = 555.
    4) If you want torque, use the sequence which activates 2 coils. theory is simple - you have to work more hard to escape from 2 people pulling you, than when one person is pulling you.
    5) You can give upto 18 - 20 V, depending upon the motor & PWM duty. In your case, you have to find this out by yourself, by trying out at different voltage and PWM duty.
    6) chose duty cycle of PWM close to Vnorm/Vsupply
    7) Reduce the diameter of the wheels
    8 ) Try by reducing the speed of the motor - Speed is invers. prop. to the torque.
    9) Use an AND gate to route the PWM.
    10) You may even use 89C51 for PWM. but after the micromouse algo, I dont think the controller will have any juice left or any timer left for a PWM.

    good luck.
    .... I didnt do it ! ....

  3. #3
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    Default

    thanx allbits for replying so early. would i be demanding too much if i ask for some links where i can learn overdriving and find its circuits. i was using large tyres . got to find smaller ones.

    a few more one liner querries:

    1)well, the weight of steppers only is around 800gms. can i make it any lighter?
    2) if during testing i give more voltage will the stepper be damaged at once or it will get hot and then be damged providing me time to swithct off the supply and saving it?

  4. #4
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    hello friends...
    i want to ask that if i use stepping sequence no. 2 mentioned above should i give double the volatage (as now the resistance is doubled) to achieve same torque?

  5. #5
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    Some suggestions out of own experience ...
    1) motors with 75Ohm winding res don't provide much torque I faced same problem when I had built my very first bot. For good torque u need motors having winding resistance between 15Ohm to 30Ohm. Resistance around 25 Ohm would be good. Normally u get 22Ohm,25Ohm,35Ohm etc... not perfectly sure just check in the market.

    2) As allbits said ... for 75Ohm one YOU HAVE TO REDUCE stepping speed ... start from 1 step @ 50ms and check if motor can handle robot's weight. If it ccan then go on reducing stepping time .

    3) You can overdrive motor upto 15V for small amount of time(5-10mins) . For motors with resitance less than 30Ohm, don't use overdriving unless there is no other option. Else it will heat up quickly.

    4)If you get these new less resistance motors, then use separate 7812 for each motor.

    5)Power supply also matters...if you are using those cheap 9V batteries, then ur motors will never run properly. You have to use NiCd/NiMH batteries.

    So you can do these things first ...
    - Check power supply of robot. Replace batteries if required.
    - Follow point number 2) mentioned above.
    - If it still doesn't work, overdrive motors upto 15V using more cells.
    - If that also doesn't work, you have to get new motors.


    For driving stepper expensive driver IC is not reqired as such, if you carefully do programming and keep all above things in mind.
    For every minute that is spent in organizing, one hour is earned !

    http://elecrom.wordpress.com/

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    Quote Originally Posted by ashish_agarwal123456
    hello friends...
    i want to ask that if i use stepping sequence no. 2 mentioned above should i give double the volatage (as now the resistance is doubled) to achieve same torque?
    >> Resistance is not getting doubled !! actually effective ressistance is getting halved. You are activating two nearby windings simultaneously.
    For every minute that is spent in organizing, one hour is earned !

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  7. #7
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    thanx elecrom..... i will try what u suggested.....
    i have got one spare stepper motor (identical to those i am using) and its helping me a lot in my testing (or u can say those crazy attempts).. i read on some site that on if u keep ur stepper on the rated maximum voltage in stationary state (i.e giving 12v supply to common and ground to any one phase) then stepper gets heated up upto 50 degree celcius in 2 min.....and its perfectly safe for motor upto 70 degree celcius.....so i thought of using this trick for getting the maximum voltage rating for my motor. i applied 24v directly (wihtout any resistance or chopping) and its working good. torque has increased and its not even getting warm after 5min........

    just for satisfying my crazriness i applied 29v and 36v also. on 36v after 2 min it became a little warmer (abt 30 degree )....i think i have got some very high voltage rated motor... 8O

    but one strange thing happened. on 12v torque was low. at 24v it was more. on 29v it became maximum. on 36v it was not even equal to 24v.....i think its because of the heat. i read somewhere (actually in a comic book) that heat destroys magnetic properties..... but still motor is working great for 29v..... but i am looking forward to keeping the voltage as low as possible as more voltage=more batteries= more weight=more torque requirement = even more voltage required=....=.... (and the cycle continues.

    i have never worked with batteries.. everyone suggested me ni/cd batteries..... how much they cost and weight.....
    i saw a micronouse running over 4 pencil cells (at least they look like that) were they ni/cd ???

  8. #8
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    [batteries ...]

    yes they look like pencil cells, one cell will cost you 50-70Rs depending on the capacity ... see this to get an idea of prices ...

    Battry and other component prices
    For every minute that is spent in organizing, one hour is earned !

    http://elecrom.wordpress.com/

  9. #9
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    Default hi

    hey thanks elecrom........

    can u suggest me any batteries?

    my requirements are:
    voltage- 12v, current- 1A , time- 10min (15 min to be on safe side), weight&cost -as low as possible

  10. #10
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    NiCd : 700mAh : 40Rs per cell

    you have to cascade 10 such cells to get 12V. Search on net regarding NiCd batteries and you will get lot of information. They are available at any decent hardware shop.
    For every minute that is spent in organizing, one hour is earned !

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