Pulse Width Modulation Modulation... not a typo

Thread Starter

Hpfiend

Joined Jan 11, 2013
5
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:

wayneh

Joined Sep 9, 2010
17,496
I would like to modify the motor speed further after the pwm signal.
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?
 

Thread Starter

Hpfiend

Joined Jan 11, 2013
5
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...
 

Thread Starter

Hpfiend

Joined Jan 11, 2013
5
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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