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Join Date: Jan 2013
Posts: 41

Help with Selecting a Stepper Motor Controller

08/21/2013 5:00 PM

Hi

I am doing a small project where i require to unload pieces of cards 50x30mm (one by one) from a stack (i need to just get them out of the block holding the stack). The mechanism i am going with is; a motor will be fitted underneath the stack with a roller sort of thing mounted on its shaft, the roller will have a raised face arranged such that when ever it comes in contact with the cards a card will be pushed out. This will happen twice per one revolution. i need to take out approx 40 cards per minute(this can vary).

Now i was going with a simple DC motor and controlling its speed manually using a 555timer and half bridge circuit. But suddenly the requirements changed and now i have to make a system with auto speed control as serial data will be transmitted to my system from some other machine. This naturally requires a microcontroller and a simple geared DC motor would not do.

So i decided to go for a stepper motor. I found a bipolar stepper meeting my torque and rpm requirements but the problem is selecting a motor drive circuit. My programming skills arent that good and i can not spend time making my own motor control circuit. I happen to find a few ready to use circuits but here is the problem:

My motor has the following specs

  • Motor Type: Bipolar StepperStep Angle: 0.018º
  • Step Accuracy: 5 %
  • Holding Torque: 48 kg·cm
  • Rated Torque: 48 kg·cm
  • Recommended Voltage: 12 V
  • DCRated Current: 1.7 A
  • Coil Resistance: 1.7 O

I found a motor control driver which is fairly easy to use but i am just not sure on one thing whether it will be able to give me my desired rpm. As per my calculation with the given step angle i need to generate a pulsed output of 14.6KHz [(48/60*360)/.018] from my microcontroller to the motor controller input. The question is will my motor controller chip be able to detect these pulses?

The model for the stepper motor driver board is Allegro's A4988.

Its not that i haven't tried. from the photo attached i perceived that the chip can detect a pulse of frequency of 500KHz but i just am not sure.

and is there any other parameter of the motor controller that i need to check and be careful about??

The reason for writing in so much detail is i am doing this sort of a project for the first time and looking forward to learn from it a lot. So in addition to the questions if you guys can give me some advice,tips, warnings or point out a mistake in what i am trying to do here it would be much appreciated.

thanks

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Power-User

Join Date: Feb 2011
Location: New Zealand
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#1

Re: Help with Selecting a Stepper Motor Controller

08/22/2013 10:48 PM

Check out the Animatics Smart Servo Motors. They have all the electronics built in. They have their own internal encoders for position feed back so you don't need steppers. They have excellent position, speed, torque and acceleration control and are easy to program.

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Power-User

Join Date: Mar 2011
Posts: 117
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#2

Re: Help with Selecting a Stepper Motor Controller

08/22/2013 11:53 PM

What is the thickness of the cards? The old computer cards with the square holes were about 7 thousandths and could be picked at 1000 per minute or any number up to that. They did not use the spinning picker as you describe. There are several ways to pick or grab a card but the thickness and flexibility of card stock comes into the solution. Some pickers were vacuum fingers that sucked the card against a spinning drum to start the card moving. Others used a blunt knife edge thinner than the card stock and sliding on a rail to allow knife to card contact to be complete, square and remain in contact until the card has been moved far enough to be grasped by the next control pinch roller or belts what ever.

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#4
In reply to #2

Re: Help with Selecting a Stepper Motor Controller

08/23/2013 3:32 PM

the cards will be 2-3mm thick.

i saw a video for the vacuum fingers, i am not sure if it is cheap and simple enough to be used in my case?

can you give me links or names of machines that use different types of mechanisms for picking the cards.. the generic search on google isn't helping a lot.

i was myself thinking of implementing the paper feeding method used in printers, where instead of the bottom the top most paper is sent into the machine for printing...lets see if i can find a detailed video for that

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#6
In reply to #4

Re: Help with Selecting a Stepper Motor Controller

08/23/2013 11:09 PM

I have searched for some photos or drawings and I cannot find anything. The equipment I am referencing was made by computer vendors for handling what was called IBM cards or TAB cards. They had 80 columns that would have square or round holes (some had 90 columns) later in the tab card life they used a downsized card with 96 columns of very small round holes. There were machines that could read the larger and the smaller cards by installing a guide fence in the input hopper.

Things you will have to address will be the picking or selecting only one card and rejecting or stopping the next in line card from following because of "sticktion". Adjoining cards tend to slide together; they often are cut with residual edge curl that matched the card next to it. The weight of the stack in the input causes the pressure on the next card to follow. Many of the machines had a throat that the card would have to pass through that was only a thousandth or two larger than the card. Consider the problem of having two cards fed through your process and the corrective steps needed if it happens to help determine how much effort you put into the card pick and select accuracy. If no harm done by having two selected occasionally, don't worry about it as the card supply is probably pretty cheap. Now if the individual cards are getting sequential numbering or other unique information, you will want to spend lots of time making sure only one card is selected or that you have a downstream reject or second selection. There are check processing machines that pass 2000 checks per minute with minimal erring and they have reject hoppers for second chance items.

The next concern will be stopping, positioning the card for your processing step or maybe you will process while moving but it still needs to be square and level in some fashion.

Finally exit and final stacking, packaging, sorting for the user.

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#9
In reply to #4

Re: Help with Selecting a Stepper Motor Controller

08/28/2013 4:50 PM

Check out computer card reader designs from the 60's 70's and 80's. They used solenoids to push the cards out from under the stack, and they were often then grabbed by belts to transport them away. the solenoid was pulling a lever which operated a sliding plate and the card when ejected was forced into contact with the belt.

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Commentator

Join Date: Jul 2013
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#3

Re: Help with Selecting a Stepper Motor Controller

08/23/2013 3:17 PM

Automation Direct makes a small PLC DL05 that has pulse output/dir for around $120 and software is free. They also include examples that work for basic stepper controls. Great way to learn and cheap

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#5

Re: Help with Selecting a Stepper Motor Controller

08/23/2013 3:41 PM

thanks for the options but i couldnt find a motor which suits my requirements besides even if i come down on the motor torque(i have calculated it for 2.5 times my actual requirement) i think these will be quiet expensive as they have there software and all

i personally am thinking for a solution that keeps my electronics including the motor under 100$.

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Guru

Join Date: Nov 2012
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#7

Re: Help with Selecting a Stepper Motor Controller

08/24/2013 3:17 PM

One of the problems I've had using stepper motors at high speed is 'stalling', where the motor stops and the step rate needs to be reduced to get it to pick up again...I'd recommend you check out the 'arduino'(about $40 ( atmel based micro) with open source software examples for motor control and a motor driver chip on a 'shield' that piggybacks on the arduino for pwm or step control( about $15) ...available online or at Radio Shack. Lots of examples available online.

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#8
In reply to #7

Re: Help with Selecting a Stepper Motor Controller

08/24/2013 3:40 PM

are u sure about the stalling issue. i am using a 44rpm motor and will be using it below that speed, shall i still worry about the stalling issue?

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