# dc motor position control

Discussion in 'Homework Help' started by ayodele234, Sep 1, 2011.

1. ### ayodele234 Thread Starter New Member

Apr 26, 2011
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the attached diagram is used to control dc motor direction of rotation. if the two
potentiometer is equal in value the motor will stop rotating.
My problem is this, if the pots values is below 80% say pot 1=70 and pot 2=70. i expect at this value that the motor should not rotate, but the motor still rotate in both forward and reverse direction.
Please, anybody with better circuit or who have an ideal on how i can make the two ics ua741 not to give output when the two pots values is the same.

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2. ### praondevou AAC Fanatic!

Jul 9, 2011
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What do you mean by "both forward and backward"? Does it oscillate?

3. ### ayodele234 Thread Starter New Member

Apr 26, 2011
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yes it oscillate.

4. ### praondevou AAC Fanatic!

Jul 9, 2011
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Yes, use a H-bridge with two upper p-FETs and two lower n-FETs and one switch on each leg.

What are the potentiometer values?
Where did you find this circuit?
Why do they use upper n-FETs ?

I redrew the circuit without the LEDs and a resistor as load.

• ###### h-n-bridge dc motor.PNG
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Last edited: Sep 1, 2011
5. ### ayodele234 Thread Starter New Member

Apr 26, 2011
20
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The value of the potentiometer could be any value provided they are equal in value.

on the net.

I believe it will work or any better one i can use.

Thank you very much for ur response.

6. ### ayodele234 Thread Starter New Member

Apr 26, 2011
20
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thank you very much, i had simulated the circuit and it worked.
How can i include LED because it as and indicator when the motor is out of sight.
Once again thank you.:cool

7. ### praondevou AAC Fanatic!

Jul 9, 2011
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Wait a second. I didn't do anything, just redrew the circuit. It's the same circuit. Did you build your initial circuit physically or just simulated it?

8. ### strantor AAC Fanatic!

Oct 3, 2010
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all hail praondevou & his magical redraw skills!

praondevou likes this.
9. ### ayodele234 Thread Starter New Member

Apr 26, 2011
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Sir, kindly the particular upper P-FETs and lower n-FET, i should use.
Also sir i should i connect it. Thanks

10. ### praondevou AAC Fanatic!

Jul 9, 2011
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Here you go. Whatever H-bridge you use I still don't know why this should work properly. The Opamps have no feedback, so depending on the voltage difference at the input their outputs go either low or high, i.e. one of the outputs is always high. That means the motor is always powered, in one or the other direction.

For the H-bridge with p-FETs you can't use the 741 because it's output doesn't go to the power supply voltage, and the p-FET needs it to turn off, so you'd need an Opamp with a rail-to-rail voltage output.

You still didn't post the reference. Where did you get it from? Can you post a link?

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11. ### ayodele234 Thread Starter New Member

Apr 26, 2011
20
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sorry for not post it earlier. http://www.nelnick.com/motorposition.html. the entire i deal of this project is to detect the position. just like controlling the angle of rudder or something like that. Sir to be sincere i am a beginner in electronic.

12. ### praondevou AAC Fanatic!

Jul 9, 2011
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The link brings me to a site with a lot of ads... Is that the right link?

13. ### ayodele234 Thread Starter New Member

Apr 26, 2011
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file:///C:/Users/comparq/Desktop/elect/desktop/power%20supply/DC%20Motor%20Reversing%20Circuit_files/Controlled%20electric%20shock.htm.
Kindly check the above link. thank

Jul 9, 2011
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15. ### ayodele234 Thread Starter New Member

Apr 26, 2011
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this is the material it explain what i want to achieve. kindly read through.

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16. ### praondevou AAC Fanatic!

Jul 9, 2011
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Apparently you don't have the feedback described in the pdf you posted. P1 is somehow mecanically coupled to the motor, so whenever you adjust P2, the motor will turn until P1 equals P2.
I wonder what potentiometer this is, as it has to be one that can make a full turn, otherwise if something goes wrong and the motor continues turning it could damage the potentiometer...

17. ### ayodele234 Thread Starter New Member

Apr 26, 2011
20
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please if you have another circuit that can achieve this purpose kindly send and guild me through. I PRAY I WILL BE AN ELECTRONIC GURU LIKE YOU ONE DAY.

18. ### ayodele234 Thread Starter New Member

Apr 26, 2011
20
0
i had installed a limit switch that will protect the potentiometer. The mechanical unit will have to travel long distance before it make even half of the potentiometer. So no confusion.

19. ### ayodele234 Thread Starter New Member

Apr 26, 2011
20
0
hello everybody, assist if you can pls, i still need response to this thread.

20. ### praondevou AAC Fanatic!

Jul 9, 2011
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Just to make sure, IS one of the potentiometers mecanically coupled to the motor shaft?