Shaft Encoders

I missed the other thread. What size of motors/shafts are these? Are the motors double-shafted?

(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.)

-- Gordon 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

Hi Rick,

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 motion.

Jay

An interesting type of Hall effect sensor was designed by Austria Microsystems . I'm planning to do some prototyping with it.

Paul.

Hi Paul,

Neat sensor.

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.

Jay

Hi Jay,

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.

Paul.

Thanks, that seems better to me than trying to mount a lightsource on one side of a disk and then the sensor on the other side

Rick-

Hi Paul,

Some ramblings.

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?

Jay

Hi Jay,

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.

Paul.