View Full Version : help on stepper motor overdrive

05-30-2008, 07:29 PM
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

2) 1100

3) 1000

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???

05-30-2008, 09:31 PM
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.

05-30-2008, 11:32 PM
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?

06-01-2008, 02:12 PM
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?

06-01-2008, 06:35 PM
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.

06-01-2008, 06:45 PM
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.

06-02-2008, 11:39 AM
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 ???

06-03-2008, 08:30 AM
[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 (http://elecrom.wordpress.com/2008/03/20/component-prices-at-lamington-road-mumbai/)

06-03-2008, 11:17 AM
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

06-03-2008, 11:37 AM
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.

06-03-2008, 10:21 PM
Nickel Metal Hydride (NiMH) batteries are the best bet, inspite of the cost. They have a very good size to power ratio and are far superior to Ni-Cd cells, in all respects. While Ni-Cd cells are good for high discharge currents, they have to be charge-cycled intelligently, or they will collapse after a few charge-discharge cycles.
The cost of a AA Ni-MH 2700mAH is about Rs. 50/-

06-04-2008, 10:51 AM
thanx docel......
can u tell me what is the voltage rating of those ni-mh batteries??? actually, my motors are slightly overweighted and so my voltage requirements is high....its slipping a lot on 12v so i am thinking of using 24v..... in this case using 20 batteries in series will be problematic from the point of view of weight......

i am thinking of non-rechrgable batteries of 12v rating.... they cost about 15rs.... but i am not sure about its current ratings..... one of my friend say he used it for controlling a single stepper motor ( one like i a using) .... but he has to demonstrate the project only for 2 min and so he also do not know for how long it can drive..... also there was no load on stepper in his case ( stepper was resting on its back )...... so i am not sure if it will work for me...

06-04-2008, 12:10 PM

If motors are sleeping and not serving your purpose, then is not correct way to drive motors at 24V for prolonged time if they are rated for 12V. Motors may fail at any time when driven by almost double voltage. Also obviously you will need more number of batteries to get that much voltage which will add extra weight. There is no escape from this situation. Prefer changing the motors with the ones which have less winding resistance and you can drive them easily at 12V with limited batteries and also they will give sufficient torque.

As energy stored per kg increases , cost of the battery increases. Ni-Mh/Ni-Cd will be best as they can provide huge currents.

Non-rechargeable batteries are useless. During ur project development you will spend more on these batteries than purchasing new rechargeable ones !

There is already lot of info available in forum/web regarding Ni-Mh batteries. Search a bit.

06-04-2008, 05:33 PM
15 rupees batteries dont work for high torque motors.
The best is Nicd which have very low internal resistance. hence they will supply very high currents. I use 700mAh Nicd batteries for my 100A brushless motors. The motors run beautifully for 3 minutes. noone uses them nowadays for various reasons.

Nicds need to be charged very carefully ( and discharged/stored, too) for good life and current. If not, they will die within a few charge cycles due to the 'memory' effect.
It maybe Ok because it is cheap, but then, they become unreliable if care is not excercised.

The NiMh cells are fairly excusing and tolerate incomplete charge, etc., dont have the Memory effect and live longer. They are better for the cost/performance. I suggest NiMh simply because, it is easy to charge.

It is time you read a little more about Motors, Power sources , current draw and their proper usage, before you do anything else. It will prevent you from spending what you saved with the existing Motor !

Elecrom is quite right: change the Motors!!

06-09-2008, 11:25 AM
so, for the last 3 days i am searching whole jaipur for those high torque low resistance stepper motors but hard luck...... so i am back to my steppers... my project submission date is near so i am trying whatever i can to make my mouse work.......

first thing i did is to keep my mouse on dieting..... it has lost 350g in 1 day... how???? well i tried some unorthodox ways..... firtly i removed all those aluminium support for steppers -150g (used duct tape instead) ..... removed those readymade tyres -100g (used plastic bottle caps with electrical tape for grip) and also removed the castor wheel together with its wooden base -100g (used a simple round headed screw with thermacole).......

now its working fine with power supply unmounted....... well thats enough for my formal project but not enough to satisfy me.... so i am looking for more improvements.......

the first thing (nasty thing) which came in my mind was --- """"could i reduce the weight of steppers???""""" well once in mind, i had to try it. thanks to my friend for that spare stepper . i opened it up and after 5 hrs of investigation i figured out that the exta weight in stepper could only be removed if i get a sofisticated structure designed from lathe machine... but thats too much i could expect from my college....so i dropped that idea

so i am back with tose 400g steppers.... now i started thinking of using other means to increase torque..... i figured out some ways but i dont think any of them is going to help me.... i am a little disagreed about them.........

1) using gears : well this will give me more torque but at the same time make my mouse move like a snail..... its already running with 1cm/sec and i dont want to reduce the speed more....

2) using pwm : well this is something new for me- "pwm with steppers"..... after all my thoughts i think its not a good idea... i am alredy running my stepper on 24v continous supply ( those whow are worried abt the health of my stepper--- its working fine without even getting warm even after 30 min .so i am sure its maximum supply voltage is at leat 24v)..... now according to theory pwm can only reduce the current if i keep voltage at 24v so torque can only ne reduced... yes it may help if i increase the voltage further but i am not thinking over it for now....

now, i come to batteries.... well batteries can only be employed if my mouse can carry them.... they will weigh at least 500g..... now here comes my confusion.... one of my friend told me that ""even if i keep the voltage same then also more current will flow through batteries and so i will get more torque...."" ..... well i dont agree with him.. it is possible only if something in my circuit is limiting the current .... i am using a 12-0-12 transformer, half wave rectifier, 7824, and uln2803 for driving two steppers ( single power supply for both the steppers)....... i think everything here can handle upto 1A and uln upto 500mA per pin.... am i right????

after writing above statement a new confusion came in my mind... i am using sequence 1100,0110...... for driving. so at every instance 4 pins of uln are active.... now coul it provide 500 - 500 mA on four pins at the same time ????

another confusion: my calculations are
voltage- 24v
phase resistance- 75ohm
so current per phase - 320mA
if i am using above sequence will the single stepper draw double current??? i.e. 640mA for 1 stepper ,, means 1.28A for two steppers???? because if it is true my power supply cant give that much current and so i am not getting torque..... but at the same time i haven't burned any of my ics..... can u help me out????