[Robotgroup] MOTORtHrEAD

Fri May 2 16:42:14 PDT 2008

Robert,

PWM is pulse width modulation. Basically it's a way to turn something on and
off quickly and have the resulting voltage average out somewhere between
fully off and fully on. A capacitor or inductor (in this case the inductance
of the motor winding) averages the voltage. So a 12V battery PMWed at 50%
gives you 6 Volts. Note that there are some losses in any system, so you
won't get exactly 6V at 50%, but it's close enough for an example. 25% PWM
would get you 3 volts with a 12V battery.

Transient currents are those that are changing and usually short lived.
Let's say you build a circuit of a battery, switch and capacitor in series.
When you turn on the switch for the first time, assuming the capacitor had
no charge in it, you will get a fairly large current. As the capacitor
charges, the current will drop. Eventually the current flow will be equal to
the leakage in the capacitor. The large initial current is the transient
current. It's also called in-rush current usually when dealing with
capacitors or inductors (including motors and lamp filaments).

Overclock means to run something faster than it was designed to go. Such as
putting a 20MHz crystal in the place of a 16 MHz crystal on a microprocessor
like the Basic Stamp. (This is most frequently used on PCs where people want
to see how much faster it will run.) This is not guaranteed to work, but
since manufacturers and designers are fairly conservative in designing
things, you can sometimes get a little more performance out of something.
The term may also be used when talking about turning the voltage up a bit on
something - like running servos at 6Volts when they are designed for 4.8V.
(Many modern servos can run at higher voltages, so this example may be
dated.)

Locked rotor amps or stall current is the current a motor will draw when it
can't turn. It takes more current to get a motor moving, than it does to
keep it moving. Even with no external load (ie motor shaft not connected to
anything), the start-up current is usually the same as locked rotor current.
(At the instant you first apply power to a motor, the shaft is not turning,
so it is a locked rotor.) This high current usually only exists for a short
time until the motor starts to turn and then decays/settles down to "no load
current". With a load on the motor (something attached to the shaft and
using mechanical power) the current will start at locked rotor current
(high) and settle down to something less, but not as low as no-load current.

I doubt I explained this at a 9-year old level, but it should be of some
use...

Paul

On Fri, May 2, 2008 at 6:02 PM, Robert Carter <rcarter at ischool.utexas.edu>
wrote:

> Hey y'all--
>
> I feel kinda proud of myself that I was able to follow most of the
> thread on RC motor controllers without being completely lost. While
> reading, I started keeping a list of terms/phrases I didn't
> understand. Here's the list; if anyone would like to fill in some of
> the blanks for me, that'd be great.
>
> ·      PWM (example: "… but you can drive 5A with PWM 80% duty cycle…")
> ·      Transient currents
> ·      Overclock ("…you can run it at 6V and run it faster and
> overclock it as well.")
> ·      Locked rotor amp ("…you pretty much need to have a motor
> controller rated for the locked motor amps of the motor you are going
> to use." )
>
> That last one, I think I understand what it means from the context,
> but when I tried to put it in my own words I couldn't come up with
> anything that makes sense, so I'm curious to hear how someone might
> explain it to an inquisitive 9-year old.
>
> I've got more questions specific to what I'm trying to do with my
> project, but let's start with these. BTW, has there ever been talk
> about creating a glossary page on the wiki for stuff like this? Or
> are there already a lot of good ones out there, that I just don't
> know about?
>
> Thanks--
>
> Robert
>
>
> On May 2, 2008, at 2:04 PM, brooksdesign wrote:
>
> >
> >  Allthough I think there are programs out there that I've heard of
> > you just made public one of the parts of a new video game I've be
> > contemplating. And in saying that and letting that cat out of the
> > bag, any of you game programmer guys want to come out and talk
> > about how to turn my scripts, cad files and ideas into something
> > real that I won't leave sitting on the shelf because I can't get
> > around to learning to write code? This project is waay more than a
> > game.
> > -brooks
> > -----Original Message-----
> >> From: Robert Carter <rcarter at ischool.utexas.edu>
> >> Sent: May 2, 2008 2:24 PM
> >> To: The Robot Group Mailing List <robotgroup at puremagic.com>
> >> Subject: [Robotgroup] MOTORtHrEAD
> >>
> >> <clip>
> >> The nice thing about electronics is that physics is the final
> >> judge of
> >> your choices.
> >>
> >> You can get people to "vote" for various approaches, but even if
> >> all of
> >> us voted for item "A" (placing a fuse in parallel with the motor
> >> controller) that wouldn't make one whit of difference to the circuit.
> >> </clip>
> >>
> >> Hell yeah, that's my way of learning: forge ahead with crazy ideas,
> >> and learn from the consequences. If nothing else, you'll get some
> >> good stories. Like when my brother and his best friends discovered
> >> the formula for gunpowder and started a fireworks shop in the attic
> >> over our garage. But, when it comes to electronics, I have to admit,
> >> it gets expensive and time-consuming to have to keep running back to
> >> the store or to the internet for replacement parts when something
> >> explodes/melts/starts to smoke.
> >>
> >> I keep hoping that someone will invent an interactive website for
> >> designing circuits, aimed for grade school students and/or middle-
> >> aged project addicts with more enthusiasm than sense (like yours
> >> truly). You could drag and drop motors of various amperage, LEDs,
> >> transistors, capacitors, etc. onto a virtual breadboard to create a
> >> circuit. If it works, you get the results in animation form: the
> >> motor spins, the lights blink, etc. Or conversely, if you design it
> >> badly, you can show the sparks and smoke as your components burn to a
> >> crisp. Fun, inexpensive, and no one gets hurt.
> >>
> >> Usually when I think of something and it seems like a really good
> >> idea, something like it already exists. Does anyone know?
> >>
> >>
> >>
> >> Robert Carter
> >> Visual Resources Collection
> >> School of Architecture
> >> The University of Texas at Austin
> >> rcarter at ischool.utexas.edu
> >>
> >>
> >>
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> >
> >
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> Robert Carter
> Visual Resources Collection
> School of Architecture
> The University of Texas at Austin
> rcarter at ischool.utexas.edu
>
>
>
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