Author Topic: stepper motor questions  (Read 2101 times)

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Offline benjiTopic starter

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stepper motor questions
« on: April 14, 2008, 09:42:25 AM »
i got a 6 pins stepper ,one step == 1.8 degrees for an ir scanner as i dont trust the servo to rotate equal degrees,,,
so i have to turn it 100 steps to cover 180 degrees front view.
anyways i believe that 2 pins should be connected together and to +Vcc (12 volts) and im using a driver ic (darlington pairs)
ULN2003A .

1/isnt it better to connect a resistor between vcc and motor? (i dont need much torque as its only a scanner)

2/can i power it with less than 12volts? what would happen if i power it with 6v ?

3/how much delay should i include between steps?

good ol' BeNNy

Offline paulstreats

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Re: stepper motor questions
« Reply #1 on: April 14, 2008, 01:31:01 PM »
Just a quick note about using a stepper for this.

When your robot first starts, how will you know what angle the stepper is at? (most devices that use steppers use some kind of optical switch such as on a inkjet printer, when the printer first starts it moves the printhead until it hits a switch or breaks a beam, that way the printer knows that the head is in the correct start position). without this type of system you will eventually run into problems.

any way, for your questions:-

1) you should really calculate how much power the motor requires to run in order to determine wether you need a resistor or not. If you used the l298 motor drivers, you would use resistors or even better a potentiometer and power transistor pair so you could alter it depending on the expected load. You can also use the sense pins to help determine stalls etc in the mcu.

2)You can power it with less than 12 volts but it wont be quite as strong, you will have to use trial and error to see wether 6 volts is enough for what you want. The difference is that there will be a trade off between speed/torque meaning that it wont be as strong when moving fast

3)The delay is also likely based on trial and error, you can probably run it without any delay or at least start off slow and build up to no delay. The important thing is getting the switching poles in the correct order. You will probably get a very satisfying high pitch "ring" sound when using a stepper at a high speed. It is worth setting it up for continuous motion first just to hear it ring for a bit.

Also remember that stepper motors by their design will get hot through continuous use.

Offline Ro-Bot-X

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Re: stepper motor questions
« Reply #2 on: April 14, 2008, 01:44:44 PM »
To add to what Paul allready said, you can use some gears (from a printer for example) to have less than a degree precision per step. Since you need just to rotate a sensor, a 12v printer stepper will work on 6v no problem. Steppers hold their position when powered, but as Paul said, they get warm. The delay between steps is dictated by the A/D conversion time for the Sharp IR sensor. You may set up an interrupt routine to move the stepper one step when the conversion is done.
You may also use a stepper from a floppy drive (bipolar, you can drive them with a L293 H-bridge, they run on 5V) but I'm not sure how many steps per rotation they have.
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Offline benjiTopic starter

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Re: stepper motor questions
« Reply #3 on: April 14, 2008, 02:19:16 PM »
well, its good to hear that its possible to power it with 6 volts, i dont need strong movement as i said it only carries a sharp ir on its top
but its extremely important that it keeps its step precision (1.8) i need to keep this precise so bad, would the 6 volts cause a problem with this?

the ir reading delay is nearly equal but no idea which is longer, i read that a stepper needs around 50 ms,,should be very close to the ir one.

its written on the motor that it ll drag 500 mA , 12v
how can i estimate the current needed to turn my ir, i think its not close to 500mA, much less, any idea ?

the bad news is about where does it start,,,,,,so when powering it should search for its initial position right?
im thinking of a  photo coupler with a pinched piece of carton on the shaft , any easier solution? i dont wanna open no printers
,,ready to open up a floppy driver for such a photocoupler  ;D 
good ol' BeNNy

Offline ed1380

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Re: stepper motor questions
« Reply #4 on: April 14, 2008, 02:54:31 PM »
maybe make put a sheet of paper with a black line behind the sensors normal scanning range, so at startup it'll find the line and then rotate 180 and know that its pointing straight ahead
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Offline benjiTopic starter

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Re: stepper motor questions
« Reply #5 on: April 14, 2008, 03:33:27 PM »
behinde the minimum normal range would be 20 cm, thats far to put stuff there as it would be outside the robot body
good ol' BeNNy

Offline paulstreats

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Re: stepper motor questions
« Reply #6 on: April 14, 2008, 05:06:23 PM »
You would also want your stepper motor to break the photo interrupter at set intervals so you can keep track of possible error.

maybe just a regular ir emitter/detector pair such as these:

http://www.sparkfun.com/commerce/product_info.php?products_id=246
These are my favourites at the minute, they are such a neat package.

The sharp ir should stick out from the servo slightly at the sides when its pointed forwards, all you need to do is point the emitter detector pair straight up and it will register whenever the sharp ir is pointing forwards.

If you want to know how much power it consumes, it might be that 500ma is set for normal operation, meaning that with a load or without a load it will consume 500ma, this is due to the nature of steppers once again. There should be a rating for stall torque aswell even though this wouldnt apply for your application. I would just stick with them consuming 500ma with or without a load. The thing with steppers is when they are used properly is that 1 coil is puching and another is pulling at the same time, so their strength or torque remains constant regardless of how light the load is.

Dont forget to hear it ring (and make sure you get the coils energised in the right patterns)

Offline ed1380

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Re: stepper motor questions
« Reply #7 on: April 14, 2008, 07:48:22 PM »
behinde the minimum normal range would be 20 cm, thats far to put stuff there as it would be outside the robot body
LOL. i meant the black line being behind the sensor. like if the sensor is all the way up front have the line be at the back of the robot, so it find the line and then turns 180 to face forward.

geez sounds like i just resaid my last post
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Offline benjiTopic starter

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Re: stepper motor questions
« Reply #8 on: April 15, 2008, 03:28:42 AM »
Quote
like if the sensor is all the way up front have the line be at the back of the robot, so it find the line and then turns 180 to face forward.

in order to find the line it should be inside the view range which is 20cm-150cm
so it should be 20 cm at least far from the sensor's back lets say
i actually considered putting a tail instead of the black line ,,,but also i would want to get away from this approach cuz it takes a time to read the adc at each step

Quote
The sharp ir should stick out from the servo slightly at the sides when its pointed forwards, all you need to do is point the emitter detector pair straight up and it will register whenever the sharp ir is pointing forwards.
do u mean the pair with a pinched disc rotating with the shaft? if yes im afraide of that the hole would let light pass even if the motor didnt get to the step i want it to ,(a step behind may let kight pass through the hole? !! )
good ol' BeNNy

Offline Admin

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Re: stepper motor questions
« Reply #9 on: April 20, 2008, 03:57:44 PM »
Quote
1/isnt it better to connect a resistor between vcc and motor? (i dont need much torque as its only a scanner)

bad idea!!!

Quote
2/can i power it with less than 12volts? what would happen if i power it with 6v ?

3/how much delay should i include between steps?

Its all in the datasheets, man :P
http://www.societyofrobots.com/robot_faq.shtml#read_datasheet

 


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