PWM

[Al Skierkiewicz]

I kind of like the water wheel analogy and it could lend itself to PWM discussion very easily.

Good thing I don't fit your "non engineer" requirement, Kevin, I have my own drawer full of stuff.

[Kevin Watson]

Originally posted by Al Skierkiewicz
I kind of like the water wheel analogy and it could lend itself to PWM discussion very easily.

Good thing I don't fit your "non engineer" requirement, Kevin, I have my own drawer full of stuff.

Hi Al,

Yeah, the water wheel has great possibilities. Okay, maybe another hint? Suppose we take that same water wheel, but all we have to drive it is a very high pressure (which is analogous to high voltage in a circuit) fire hose that has a fast valve at the nozzle. Given the above, how do we get a continuously variable angular velocity to do our work? Explain how it works too.

Maybe you can throw in some of your desk cruft to sweeten the pot . Okay, like I said above, I'll throw in one of the cheezy items found here for a really good explanation of PWM using the above example or one of your own choosing.

-Kevin

[Justin Stiltner]

Well I think of it like this,
Remember when you were young you turned your bicycle upside down and slapped the wheel to make it spin, well if you slapped it really fast... it went really fast, I don’t mean the speed of your arm, but the number of times per second that you slap the wheel.

If this was PWM, you would be slapping the wheel as hard as you could every slap, but say if you wanted the wheel to go slow.. You slap, and wait a few seconds, slap again, if you wanted it to turn a little faster... you slap a little more often and so on.

PWM is doing basically the same thing

or you could think of it as a huge force. We all know that end speed will equal the amount of force multiplied by the time that force is applied.... take for example you shoving say... a trash can on wheels, well you shove it and it goes a few feet... well what would happen if you shoved it one time every second... it would continually move at approximately the same speed all the time, well if you shoved it every half a second... it would move faster wouldn’t it? Because that huge force (you shoving) would be acting on the can for a longer period of time per second.

Same thing with a motor, the power is the force and the speed controller is basically flipping the power switch really fast on and off.

[Al Skierkiewicz]

Justin,
You are getting closer. Kevin will need to intervene here since he posed the original question. I have a hint running around in the back of my head for your bike tire analogy but I will wait for Kevin to rule on hints...

[Gadget470]

Each victor has it's own power and is like a volume knob controlled via the PWM's signal.
When you want a stereo louder, you turn the knob to a certain number higher than current state, 0 - 10. 0 volume being off, 10 volume being loudest possible.
For quieter, you turn the number lower than current state.

The PWM/Vicotor combination work the same way. The position of the "volume knob" is determined by the signal percentage. (See the image attached in Jnadke's post)

[Kevin Watson]

Originally posted by Al Skierkiewicz
Justin,
You are getting closer. Kevin will need to intervene here since he posed the original question. I have a hint running around in the back of my head for your bike tire analogy but I will wait for Kevin to rule on hints...

Justin,

Yes, you are getting closer. The whole hitting-the-tire-of-an-upside-down-bike-with-the-palm-of-your-hand routine is something many kids do and therefore something that many people can relate to in the physical sense. This kind of explanation is the best kind of explanation because it takes advantage of the physical intuition that we start refining from an early age.

If I'm reading Al's mind correctly (it's scarey in there, BTW ), an Engineer would say your approach to spinning the tire (or any spinning object with mass) has a bit of a flaw. That flaw is that the error bars on the angular rate are really big. Getting beyond the Engineer-ese, this means that your RPMs aren't very close to being constant. Your approach involves hitting the tire, which causes a big jump in RPMs, then you wait a period of time while the RPMs drop to some level and then you hit it again. Imagine being in a car that had four wheels behaving like that bicycle tire -- it wouldn't be a fun way to travel.

So you need to refine your approach to spinning that tire using PWM (and what you've described *is* PWM). Remember you have two parameters to work with: frequency and duty cycle (percentage of the time the force is applied). Amplitude (amount of force, voltage, etc.) is another parameter, but it's a fixed value here.

Al: If this isn't the hint you were thinking of, feel free to jump in.

-Kevin

[Al Skierkiewicz]

Kevin,
You must have slipped in there when I was asleep. I keep it scary cause it cuts down on the number of visitors.
I was thinking of a hint regarding "how does the size of your hand affect the speed?" Assuming you can only contact the tire at fixed time intervals, (say beginning of contact at once a second)how do you control speed?
I still like the water wheel, on/off fire hose better so shouldn't we stay with that.
As to the desk drawer fluff, if I give you some of mine and you give me some of yours, we would still know where to find it when we need it.
(it's going to get worse before it gets better!)