Flash ADC Motor Controller

Discussion in 'The Projects Forum' started by Nara Shikamaru, Jan 24, 2008.

  1. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    hey everybody!!!.....i have a problem here.....i need to design the following:

    i need to control the speed and direction of a DC motor using a potenciometer that goes to a Flash ADC (built with comparators and the 8 to 3 line encoder, not the integrated)....the you must design some sort of driver that manipulates those 3 bits to control a DC motor!!!...thats the thing.....
    i did a design but the teacher said that it should be Enhanced...or redesign!!!....

    and it should have now 4 output Bits and i dont know how to do that...and also the teacher said that it should drive a 3/4hp DC motor.....i send you here the eschematic of what i could came up....please tell me what must be changed so i can drive a 3/4hp DC motor and to agregate a 4 bit...thanx
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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    Can you use a 74LS147 to get the four bits?

    To drive a 3/4hp motor, you'll need to use your four bits to generate some kind of PWM. You can then drive the motor with a MOSFET.
     
  3. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    ok!!...i just saw it!!...thanks!!....so i just need to add 2 more comparators and a 74147 so i can get 4 Bits.....and those Bits how do i used them to generate a PWM??.....and the mosfet would go directly to the motor as on the schematic or would i need to do an H bridge???....
     
  4. beenthere

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    Apr 20, 2004
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    beenthere, the OP mentioned that his motor was DC. I'm afraid that the reference you pointed out won't be much help to them.

    Seems like what's needed is an H-bridge with a chopper circuit driven by the binary inputs.
     
  6. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    Hey Sgtwookie!!.....thats right!!....i need to drive a DC motor of 3/4hp to 1hp....so how would i change the circuit to do what you said???.....how that of and H bridge with chopper??....you inyect the digital signals directly??
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    An N-bit flash A/D requires (2^N)-1 comparators, so for 4 bits, you need 15 comparators.
    74LS147 is a BCD encoder. It won't handle 15 input lines, which is what you will get from your A/D. Furthermore, the output of the A/D is what's called "thermometer code", which is simple to convert to analog with 15 equal-valued resistors, all summed together. From there, you can easily make a PWM circuit. However, this would be a Rube Goldberg contraption, because you can easily convert the original input to PWM without the flash A/D. Maybe the instructor has something else in mind.
     
  8. beenthere

    Retired Moderator

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    I saw the 3/4 horse rating and missed the DC part - assumptions always get you in trouble.
     
  9. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    ok!!...jajaja.....Hey Ron H....so ok....so i add the rest of the comparators to get 15.....now...is there a priority encoder that gets those 15 inputs and outputs the 4 Bits??......the problem is that my teacher is a pain in the as! and he wont let us use a Flash ADC integrated....what about the digital driver for the motor?.......any ideas?....what about sgtwookie´s idea of the H bridge chopper?.....
     
  10. Ron H

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    Expanding on my comments in my last post, converting to PWM from the A/D output seems to require a D/A converter following the A/D, which makes the A/D redundant. You can easily go from the analog input directly to PWM. Maybe the instructor wants the student to go through the A/D - D/A process, just as an exercise.
    You might be able to convert the A/D output to PWM without the D/A by somehow using some decoding logic, a clock, and an up-down counter.:confused:
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You don't need an H-bridge unless you need to reverse the motor direction, or have dynamic braking. For unidirectional speed control, all you need to do is PWM a MOSFET, and have a big diode across the motor to absorb the flyback.
     
  12. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    I totally understand you!!.....but you were absolutly right about the rube golberg comparation!!!.....i think he´s his succesor!!!....

    creating the Flash ADC to obtain 4 Bits is a Must!!....that part has to go mandatory :confused: thats why he´s a pain in the as1....after that you must do some sort of driver to handle those Bits directly to the motor!!....check out at the beginning my attached file!!...thats the circuit i could design!!....but how would i improve or redisign that part now to handle 4 Bits and 1hp Dc motor??.......sorry for asking so much!!!
     
  13. Ron H

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    Your design has a D/A between the encoder and the output (I'm not sure you realize that's what you designed). It is simpler to create a D/A directly from the output of the A/D, as I mentioned above. PWM can be done by using a comparator with one input being the D/A output, and the other being a sawtooth or triangle wave (either will work). The output of the comparator drives your MOSFET.
    If you really want to use a priority encoder, followed by a binary-input D/A, the TI datasheet shows how to cascade 74HC148's to handle 4 bits of output. Keep in mind that your A/D only has 15 outputs, while the encoder will have 16 inputs.
     
  14. thingmaker3

    Retired Moderator

    May 16, 2005
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    I don't think anyone makes a 16-to-4 encoder chip. One can, however, cobble together a 16-to-4 encoder. IRRC it needs half a dozen 4-to-2 encoders with some clever tricks on the input wiring. I also found the following via Google: http://www.geocities.com/abe_karnik/encoder16to4.htm

    One possible PWM topology would be a four bit counter and a four bit magnitude comparator. The A<B output of the magnitude comparator would control the MOSFET. Input word A would be from the flash ADC and input word B would be from the counter. A 555 or other oscillator could feed the four bit counter.
     
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