This may also have some reference to the Previous Post "Pulse Counting"
I have plans for the programming of the closed control loop. My question
is..what is the best way to implement the shaft encoder hardware. I'm
sceptical about the strength and durability of just "sticking" an encoder
on. Well..any ideas on implementation..More of what my question is..what
encoder wheels should I use and are there any sensors already made for
I missed the other thread. What size of motors/shafts are these? Are the
(As an example, the Tamiya worm motors I like to use are double-shafted,
allowing you to connect a wheel to one end, and the other to a cheap
encoder wheel from a mouse. Not all motors are double-shafted, and on
those that are, the auxillary shaft is typically not gear-reduced, and
this requires much faster counter electronics.)
If you are planning a single-channel scheme, mounting a transmissive or
reflective disc is no big issue, except be sure stray light can't easily
get to to the phototransistor. If you are using a dual-channel encoder,
then in addition to the above, you need to maintain the proper
relationship between the two channels.
While there are ready-made shaft encoder solutions, even a low-cost
commercial product starts at $60. So most people make their own. I have
some 2" diameter encoder wheels I developed for Budget Robotics, but I'm
not yet ready to officially release them. If you'd like to test some
out, write me offline with your mailing address and I'll send you some.
You will need two slotted switches with a gap of no less than 0.2" --
All Electronics sells these for 2/$1.00. If you get four switches, you
can experiment with placing them for quadrature. (My e-mail address is
not obfuscated, but I do have anti-spam/virus software running, so avoid
any e-mails with attachments, HTML, or spam-like subject headers.)
Author: Constructing Robot Bases (Forthcoming)
Robot Builder's Sourcebook, Robot Builder's Bonanza
Rick Cassidy wrote:
: I have plans for the programming of the closed control loop. My question
: is..what is the best way to implement the shaft encoder hardware. I'm
How big is your wheel ?
I've had good luck with my set-up based on useing CD's as a reflective
surface on a 7" lawnmower wheel. See http://www.westnet.com/~chris/Robots /
Chris Candreva -- firstname.lastname@example.org -- (914) 967-7816
You might consider a Hall Effect Transducers. Every application is different
and this may not work for you but.
Mount a small magnetic (3 or 4 mm dia.) on a shaft or gear and locate a Hall
Effect Transducer nearby. Using a 'latched' sensor two magnets, a N and a S
mounted at 90 degrees, you can also get direction if you need it.
My application will use one magnet and two sensors mounted 180 degrees apart
per wheel. It won't tell direction, but I/m driving the motor so I should
know which way it's turning. I'll get one pulse for every 8 mm of ground
An interesting type of Hall effect sensor was designed by Austria Microsystems
I'm planning to do some prototyping with it.
Mounting might be a problem though. And then there's the issue of getting
magnets with the correct N/S orientation.
If I mount one on my motor shaft, with a 4:1 reduction, I'll get 256 counts
per wheel rotation. If I used a 45 and 8 gear to get a 5.625:1 reduction I'd
get 360 counts per revolution. Interesting.
I have a sample of a magnet with 3mm diameter, which is about the
same as the shaft diameter of cheap surplus gearmotors I bought. Shaft
doesn't extend beyond the back end of the motor, but I'm planning to
glue small plastic washer (less than 3mm diameter) to the back of it
and glue magnet on top of the washer. I will use a Surfboard (DigiKey
part no. 9081CA-ND) to solder the sensor to. I will then glue the
board to the motor, centered above the magnet, sitting on plastic
spacers. I will do centering before the glue sets - it has to be less
than 0.25mm off center. It should work in theory and seems to be
applicable to any motor as long as the back of the shaft is accesible,
which usually is the case.
Many of the small DC motors I've got have a raised area (a boss) around the
motor shaft on the endbells. A piece of brass tube could fit around this
boss. If you etched your own circuit board you could include a ring area
where the brass tube could be soldered.
A piece of 3mm ID bras tube would make mounting the magnet to the motor
shaft simple, if it sticks out far enough.
MicroMark has concentric metric brass tubing.
Do you mind if I ask about your source of magnets and sensors?
brass tube idea sounds interesting. There would be a problem with
PCB to magnet alignement though, unless you cut pieces of the tube out
to be able to see inside. The shaft on the motors I have doesn't stick
out at all, so the smaller tube would't work there.
I received magnet and sensor from a friend. If I have to buy or get
more I will post to the group how I did it.
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.