Using Digi-Key Shaft Encoders
 

How would I go about programming one of these, and which one would be ideal for positioning a low speed (60 RPM) shaft? Any info is appriciated. Thanx.

Digi-Key Shaft Encoder Pages:
http://dkc3.digikey.com/PDF/T031/0830.pdf
http://dkc3.digikey.com/PDF/T031/0831.pdf
http://dkc3.digikey.com/PDF/T031/0832.pdf

At 60rpm, you can use just about anything as that's only 1 rps, meaning you could, in theory, sample down to 1/40th of a rotation.

As for programming, I'd recommend having a few lines of code to turn the input into a numeric identifier for the current position. Then, just compare that position to the one from the last loop to figure out how far you've rotated in the previous 26ms. The only non-trivial part is going to be dealing with loops (ie position was 4 last time, but now is 1--you need to recognize that in this case 1>4).

Positioning or speed?
 

Are you trying to measure the position (angle) of a shaft, or the speed of rotation. As Rob says, they will work for speeds like yours.

If you want position or the angle, like for a steering system, then the problem with these is the low resolution. Most have 16-32 "codes" per rotation. This means you can only get the position to +/- 1/32 of a rotation (~11 deg.). To get more resolution, you could connect one with a belt or gears so that the encoder rotated more than the shaft being measured, more complex.

We are trying to feedback angle from the shaft encoder, we are looking at the 256 ticks/rev so our resolution will be fine. We don't need it perfect, just somthing like <10 degrees.

In that case, you should be fine. 60rpm=1rps=9degrees/26ms. Therefore, in theory the Stamp could sample down to approx. 9 degree resolution. That's assuming, of course, that it hits the new position EXACTLY as the controller is sampling, but you should be able to get decent resolution none-the-less.

Yes it all should work fine.

In real life the rule of thumb is to sample at greater than 10 times the max output Frequency. That mean that the Max encoder frequency should be no greater than .26 sec.

.26 sec?
 

Don't you mean .026, That would be 26 milliSeconds.

26mSec is the update rate of the BS that we can't change so the max Freguency of our input is 260mSec or 3.8Hz. If your encoder signal is faster than this you will run the risk of missing pulses.

so if 26ms is your sampling time. this is about 38.4Hz

then 38.4 / 10 = 3.84 Hz. = 0.26 seconds or 260 ms.

your motors should not rotate faster then 3.84Hz.


Theoretically, you only need a two folds the sample time.
This is also called the nyquist frequency, or aliasing frequency.

sample rate for optical encoders
 

We are intending to use one of these, or something equivalent to measure steering angle. I've read through the posts above and I don't think the analysis above is correct.

The device generates 256 pulses per revolution. At 60 rpm, 1 rps, this is 4 milliseconds per pulse. Especially with last year's stamp, there's no chance of sampling fast enough to use this device.

With this year's processor, maybe it will work. Has anyone tried? Or measured how often an interrupt can be serviced under "normal" cpu loads? 4 ms is probably only a few hundred instructions, perhaps close to 1000. I'm a bit doubtful that you can do it without totally consuming the cpu.

If you can tolerate less angular resolution and use the 128 or 64 pulse per rev device, seems like you have a lot better chance of making this work.

Or am I missing something here?

Bill

To measure steering angle (Unless you want continous rotation) remember a potentiometer works great and is 'absolute'by nature so you don't need a running count or a calibration on power up.

Re: Using Digi-Key Shaft Encoders
 

If you're talking about using the mechanical encoders on these pages, make sure you use the gray scale version. The BCD will screw you up.

If you're talking about the optical encoders, then you have a couple of choices. I think you can rig the RC as a counter. One way would be to tie the encoder into a low priority interrupt and fire an interrupt on each pulse. This may become very band-width intensive though.

There are also chips available which do all the counting and demodulating for you. I haven't looked through digikey to find one, but I'm sure they sell them. I've use HPs (now Agilent's) chips, in particular the HCTL1100, which is far more expensive and powerful than you would need.

Or, you could rig up a counter circuit of your own. Depends on your electrical design skills.

If you need more resolution, you can always attach a measurement gear to your system. This has the added advantage of protecting your encoder from the shocks and abuse that your drive wheels will experience. If you do this, you need to remember to use a fine pitch gear set or an anti-backlash gear to minimize the errors introduced by backlash.

using potentiometer as shaft encoder
 

Piher makes a continuous rotation pot, the N15 series. Spec sheet here:
http://www.piher-nacesa.com/pdf/n15.pdf
Mouser sells them (mouser.com). For example, mouser part# 531-N15TV-100K. They only do 340 degrees, so you need two of them to get 360 coverage.

The advantage over the optical sensors is
- direct readout (no CPU load)
- easy to program
- cheap @ $3

The disadvantages are
- poor accuracy (3% = 11 degrees)

Bill

Re: using potentiometer as shaft encoder
 

Actually, there's another disadvantage of these Piher potentiometers - you can't get them from any of the permitted suppliers (newarkinone, future-active, digikey), so I guess you can't use them. Does anybody know of any "out" for this? Does every little $2 or $3 part I might buy at some local electronics store for the robot also have to be obtainable from the listed suppliers? What would seem reasonable to me is that there be a lower limit, maybe $5 or $10 dollars, under which you could buy electronics parts from anybody.

It's too bad if we can't use these Piher pots (for example). They are less than $3 each and make great continuous rotation sensors. I have not yet found anything similar from the listed suppliers.

Bill