torque of a stepper motor - Electronic Circuits Projects Diagrams Free

bananasiong · 23rd April 2006, 08:23 AM

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:

**Nigel Goodwin** · 23rd April 2006, 08:30 AM

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.

spuffock · 23rd April 2006, 12:33 PM

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?

motion · 23rd April 2006, 01:02 PM

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.

**Nigel Goodwin** · 23rd April 2006, 03:00 PM

Quote:

Originally Posted by spuffock

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?

If it isn't broken, why try to fix it? 8)

spuffock · 23rd April 2006, 03:47 PM

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?

Mike, K8LH · 23rd April 2006, 09:59 PM

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

bananasiong · 24th April 2006, 01:45 AM

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.

bananasiong · 24th April 2006, 01:51 AM

Quote:

Originally Posted by Mike, K8LH

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

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.

motion · 24th April 2006, 05:15 AM

Quote:

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.

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.

bananasiong · 24th April 2006, 06:31 AM

Quote:

Originally Posted by motion

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.

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..

motion · 24th April 2006, 06:57 AM

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.

http://www.doc.ic.ac.uk/~ih/doc/step...html#half-step

bryan1 · 24th April 2006, 09:02 AM

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

bananasiong · 24th April 2006, 05:47 PM

Quote:

Originally Posted by motion

BTW, the motor is rated 12V. You're only applying 9V. You should apply at least 12V to get the rated torque.

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

or

bananasiong · 24th April 2006, 05:53 PM

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!!

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23rd April 2006, 08:23 AM	(permalink)
bananasiong	torque of a stepper motor 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:

23rd April 2006, 08:30 AM	(permalink)
Nigel Goodwin	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. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

23rd April 2006, 12:33 PM	(permalink)
spuffock	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?

23rd April 2006, 01:02 PM	(permalink)
motion	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. __________________ "Having to do with Motion Control"

23rd April 2006, 03:47 PM	(permalink)
spuffock	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?

24th April 2006, 01:45 AM	(permalink)
bananasiong	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. __________________ Superman returns..

24th April 2006, 06:57 AM	(permalink)
motion	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. http://www.doc.ic.ac.uk/~ih/doc/step...html#half-step __________________ "Having to do with Motion Control"

24th April 2006, 09:02 AM	(permalink)
bryan1	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 __________________ " The only way to avoid human error is to avoid the use of humans"

24th April 2006, 05:53 PM	(permalink)
bananasiong	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!! __________________ Superman returns..