Pulse Width Modulation Modulation... not a typo

Discussion in 'The Projects Forum' started by Hpfiend, Jan 25, 2013.

  1. Hpfiend

    Thread Starter New Member

    Jan 11, 2013
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    Ok- let me begin by saying I know very little about electronics but am very intrigued by them.

    Here is what I know at this point...

    I have a variable pwm signal with a 2-3 amp maximum output to control dc motor speed. It is able to switch polarity on command from a switch to reverse the dc motor and then vary speed in the opposite direction. I would like to modify the motor speed further after the pwm signal. The motor has a resistance of about 3 ohms if this is relevant.

    My first approach was simply to start over from scratch as I know little about how the existing pwm signal is generated and build my own switching polarity pwm signal using a hex inverter with a schmitt trigger, and a resistor and transistor for the high output as http://www.robotroom.com/PWM.html elegantly describes.

    Now that I have ordered the materials and am trying to assemble the circuit (breadboard soldering beyond the experimentor board is not my expertise), wouldn't it be a lot easier to just use some sort of adjustable rc filter or maybe just a potentiometer to modify the signal I have? For example if I have a 50% duty cycle 5 volts which creates an effective 2.5 volts pwm to the motor, can I add a variable resistor to the pwm output before the motor to make each pulse say 4.8 or 4 volts to an effective 2.4 or 2 volts to the motor?

    I think I understand the reason to use pwm over a simple potentiometer is to not stall the motor and for less power consumption/heat generation- so I am reversing all of these benefits by adding the pot after? I read about rc filters one pole, two pole etc but got a bit confused on those as well- they seemed right for my application but I didn't see how to make one variable.

    If I were to run a high amp pwm output into a hex inverter schmitt trigger pwm afterwards to modulate the modulation I would fry it correct?

    I apologize if I have confused you as the reader as I believe I have now confused myself further!

    Thanks!
     
    Last edited: Jan 25, 2013
  2. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    The purpose of PWM is to provide speed control, so I can't quite follow what you want to do. You want reversible direction and speed control, right? What's wrong with what you have already?
     
  3. Hpfiend

    Thread Starter New Member

    Jan 11, 2013
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    0
    Correct reversible direction and speed control! I want the motor to respond differently than the existing PWM is commanding. IE the current PWM driver is resulting in 1000 rpm and I only want it to turn 500 rpm. There is no manual adjustment in the circuit I can find that I would understand (rotary or linear potentiometer/dip switch/etc) - which is why I was originally going to start over from scratch... Then I started reading about RC filters and thought I may be able to add a "knob" that way and then found this forum...
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    That's the minimum rpm? What is controlling the PWM signal? A schematic would be worth a thousand words here. ;)
     
  5. Hpfiend

    Thread Starter New Member

    Jan 11, 2013
    5
    0
    No that is not the minimum rpm, that was just a hypothetical example 1000 rpm to 500 rpm, 50% reduction by manual interface into signal.

    I do not have a schematic- I wish I did. Generating circuit is combined with other circuits from what I can tell. I do know that the pwm signal is being controlled by a pressure sensor. Negative amps are applied to motor, when turned on and 0 pressure, with increasing pressure amps are reduced negatively (-2, -1.5, -1.0, as hypotheticals) until threshold pressure at which it allows zero current, then as pressure increases beyond threshold it reverses polarity, applies increasing positive current to a 3 amp maximum.
     
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