torque of a stepper motor

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bananasiong

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hi all,

it's me again.. i've a problem of the torque of the stepper motor with the circuit as shown below:


:cry:
previously, i thought that the torque is high enough.. but... when the wheel is conected (i used a modified custer wheel), i found that the torque is not high enough to drive it.

i'm using a uni polar stepper motor, 2 center tapped. drive with around 9V using battery.

can anyone help me with this?? i will appreciate it.. thanks a lot.. :wink:
 
Stepper motors don't have very high torque, but to make the best of it you can feed the motor from a far higher voltage with current limiting resistors - wire wound high wattage ones. This gives improved torque, and higher speed.

If you google for "stepper motor tutorial" you can find details on calculating them.
 
Big, hot resistors? Oh, come on Nigel, I would have expected you to suggest using a PIC, umpteen times the stepper's rated voltage, and starting off with a pulse long enough to reach the rated current, followed by a mark-space ratio to sustain it. And maybe suggestions about microstepping.
You haven't reverted to old technology,have you?
 
First, check if there is enough torque when the motor is stationary and the current is still flowing through the coils. This is the motor's holding torque.

Next, apply the sequence of pulses to move the motor. Slowly at first and then gradually accelerate the step rate. Find out the speed at which the motors lose all torque. Post the speed here so we can tell if there is room for improvement. Post also the full motor specs (voltage, current, torque rating, etc.)

Try removing diodes D5, D6, D7 and D8. They slow down the decay rate of the current through the coils and act as brakes to the motor motion. Check if the maximum speed before the motor loses torque increases with this modification.

BTW, 1N4148 diodes are signal type and not good for serious use in this application. Replace them with fast recovery higher current rated diodes.
 

If it isn't broken, why try to fix it? 8)
 
But he wants more torque, and the resistors will probably cost more than the PIC will.
Besides, he'll need to learn microcontrollers sometime, won't he?
 
The "Torque Step" table I use where two coils are energized each full step (reference: Jones on Stepper Motors) and the old "normal step" table...

Hope this helps... Regards, Mike

Code: 
;
;  torgue step, full steps (2 coils energized)
;
STBL    dt      b'00001001'     ; B2 & A1
        dt      b'00001010'     ; B2 & A2
        dt      b'00000110'     ; B1 & A2
        dt      b'00000101'     ; B1 & A1
;
;  normal step, full steps (1 coil energized)
;
;STBL	dt      b'00001000'     ; B2
;       dt      b'00000010'     ; A2
;       dt      b'00000100'     ; B1
;       dt      b'00000001'     ; A1
 
oh.. about the spec.. i bought it from the component shop and they don't even know how to drive a stepper motor.. what i know is, it is a 12V stepper motor, 15 degree per step, 6 wires (2 center tapped). that's it. mayb i should post the photo as soon as possible.

i'm not following the circuit exactly. because i use a 68HC11 micro controller to control it with 5v, but i supplied the stepper with around 9 to 10V. so the J1 switch doesn't exit in my circuit.

is it better if i remove D5 to D8? what is the purpose to include them in? i just follow it but don't know what about that.

thanks for all your helping.
 

hi Mike,
can i energize 2 coils at the same time? i didn't know that!!! and i will try it. is the code given here is following the sequense? i think it is. anyway, thanks! i will tell u the result.
 
is it better if i remove D5 to D8? what is the purpose to include them in? i just follow it but don't know what about that.
Click to expand...

You're the one who presented this circuit. I suppose you ought to know why you placed them there. However, I think people find it a habit to put freewheeling diodes whenever they see coil as load of a transistor.

Let us examine closely what is really happening in the case of a unipolar connected stepping motor. Following your diagram, L1 and L2 are not really two independent coils but bifilar-wound coils. That is, they are two coils of a 1:1 turns ratio transformer.

If you turn ON one coil to move the rotor one step, it induces an equal but opposite voltage on it's other pair. The freewheeling diode shorts out this voltage and causes an unwanted current to flow. The current build-up on the opposite coil acts as a drag to reduce the torque.

Without the coil, the equal and opposite voltage will still build-up but it will rise to about double the supply voltage (w.r.t. ground).

Of course this model ignores any leakage inductance which can generate some voltage spikes. But these should be suppressed through a different way like a RC snubber circuit from the collector of the transistor to ground.
 

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motion said:
You're the one who presented this circuit. I suppose you ought to know why you placed them there. However, I think people find it a habit to put freewheeling diodes whenever they see coil as load of a transistor.
Click to expand...

no, i get this circuit from this forum at other page. so, i will try to remove the D5 to D8 and try the coding given by Mike. see whether the torque will increase or not.

thanks ya..
 
BTW, the motor is rated 12V. You're only applying 9V. You should apply at least 12V to get the rated torque. Not only are you getting lower torque, but also, the torque falls more rapidly with increasing speed. You're only getting half the power with this.

Also with a large 15 degree per step motor, you ought to consider half-step drive. This is a combination of one-coil excitation and two-coil excitation.

**broken link removed**

**broken link removed**
 
I've done a fair bit of research on steppers as I'm making up a cnc machine in my limited spare time. One thing I found was when a stepper bogs down with torque required the current drops and the voltage rises.(off memory I could be wrong) What is needed is a constant current source and Oatleys here in Oz sell a kit which in my opnion is much better than a chopper drive or ballast resistors. For example the first axis I made, for test purposes I put a 5 volt bipolar stepper rated at 1 amp as a drive. Using a h-bridge circuit and driving it thru a simple pic program on a 16f628 and was powered from a 6 volt 4 amp hour battery, I found when I put some tension on the table the stepper stalled but when I power the stepper from the constant current kit I couldn't stop the stepper. I recommend looking at the datasheet Oatleys have on controlling steppers as it's a good resource for infomation but it does devote most of the info to their kits.

Cheers Bryan
 
motion said:
BTW, the motor is rated 12V. You're only applying 9V. You should apply at least 12V to get the rated torque.
Click to expand...

yup.. i know that.. but i'm using 8 pcs of rechargeable AA batteries. so i have limited the voltage to drive the stepper motor.

and.. if i use the half step drive, is it only 7.5 degree per step for my stepper motor? so which one is more suitable for me to use? i think i will try both



**broken link removed** or **broken link removed**
 
and.. shall i remove D5 to D8 and connect Vcc only to the + of the stepper motor and the collector to the coil of stepper motor??

thanks!!
 
yes, i've already tried the coding. the step is decreased. and i have removed the D5 to D8 already. but i still need higher torque.. can anyone help me with this?? thanks a lot..
 
bananasiong said:
yes, i've already tried the coding. the step is decreased. and i have removed the D5 to D8 already. but i still need higher torque.. can anyone help me with this?? thanks a lot..
Click to expand...

You need a higher voltage! - did removing the diodes help at all?.
 
Nigel Goodwin said:
You need a higher voltage! - did removing the diodes help at all?.
Click to expand...

i don't think it helps.. but i just remove them from my circuit...
and.. is there any other way? because i cannot increase the voltage.. maximum 9 or 10 volts only..

shall i use the half step? or connecting 2 coils gives more torque??

thanks..
 

You may be able to get very slight increases?, which isn't any help to you - you NEED more voltage, particularly as you're running the motors well below their rated voltage at the moment.

If you can't increase the battery voltage?, then I suggest a switch-mode inverter to give you a higher supply rail?.
 
Nigel Goodwin said:
If you can't increase the battery voltage?, then I suggest a switch-mode inverter to give you a higher supply rail?.
Click to expand...

oh.. then i will think abou that... to increase the voltage... and what is a " switch-mode inverter"??

thanks!!
 
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