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[DPRG] RE: Wheel encoder theory
Subject: [DPRG] RE: Wheel encoder theory
From: Vimal Shankar
vimalsshankar at yahoo.com
Date: Mon Oct 9 14:02:00 CDT 2006
> There are a number of easy to use transmissive type
> optical encoders out
> there which are already amplified and will produce a
> digital output. Check
> Mouser or Digikey for these. They will have 3 wires
> on the receiver for
> power, ground, and output instead of 2 wires of a
> phototransistor. When
> selecting a device, make sure that the aperture is
> as small or smaller than
> the smallest encoder slot in your wheel.
I am biased against readymade solutions and definitely
would like something made from elementary parts like
LED/IR...
Makes the thing portable and easy to maintain.
> You don't need an absolute encoder for wheel
> encoding, because you can count
> the increments as they happen with the micro.
> Absolute encoders are also a
> lot more complex.
Any articles/writeup you have there ?
> Incidentally, I also built a differential drive
> robot with a dedicated micro
> for each motor control. It worked ok, but the
> problem was that a
> differential drive robot needs to constantly make
> adjustments between the
> two wheels in order to go in a straight line. Even
> if you have a PID
> algorithm, one motor will inevitably go faster than
> the other for a moment,
> and this will cause the robot to turn. (Not to
> mention the wheels not being
> the same circumference.) To correct for this, a good
> controller will need to
> track the difference between the motor speeds and
> compute a direction
> vector. Then it can compensate for the erroneous
> turning. This is
> accomplished much more easily with a single
> controller for 2 wheels, rather
> than 2 separate controllers.
AHA ! Gotcha ;)
Would you mind letting us ripoff your work ?
But seriously, a writeup would be just what the doc
ordered.
The DPRG website suffers from a lack of materials in
this subject.
You can definitely help with your experience.
If required, I will do the writeup for you - you just
supply me the data, schematics.
Promise.
> On a separate note, I have also used this technique
> of a separate controller
> for each motor in a 6 axis robotic arm and it works
> quite well. In this
> application though, there is not the critical need
> for correlation between
> the motor speeds.
I thinkI would like ONE microntroller PER differential
PAIR. I made a mistake if I said per motor.
Makes coding easier - I think.
> PS: If your encoders are connected directly to the
> wheels, you don't need to
> worry about gear slop in the motors, because it will
> not affect the encoder
> count.
Hmmm... better still. Maybe we should connect the
encoder to the 3rd free wheel, which is being driver
passively ?
> I would be interested to see this VB script for
> designing encoder wheels.
> Can you provide the link?
Do give me 32 hours.
> If you are interested in designing quadrature type
> encoders, I can make a
> quick tutorial. It's not as simple as it might seem.
I know.
That's why I asked !
This list rocks.
Vimal
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