Re: High speed encoder with slotted / flat end... can't find one.
 

what do you mean by "the equivalent of"? what part number were you using?

at 4000rpm, a variation of +/-0.13 degree in the absolute location of the edges¹ could produce the +/-200 counts/sec jitter you saw when using GetRate.


¹ equivalent to +/- 0.26 degree with respect to adjacent edges

Re: High speed encoder with slotted / flat end... can't find one.
 

We were using a 128 count bourns encoder, however it was geared down: we were taking readings from the chain with a sprocket on the encoder.

Re: High speed encoder with slotted / flat end... can't find one.
 

Using GetRate to read a chain-driven high-count encoder has the potential for producing a lot of jitter, even with a high-quality encoder. Any angular free play at the encoder can show up as jitter, especially in the high-vibration environment of FRC robotics.

Not trying to be a pest, but what do you mean by "we were taking readings from the chain with a sprocket on the encoder"? Could you post a picture?

Re: High speed encoder with slotted / flat end... can't find one.
 

Not sure I understand the distinction, but OK.

So does a simple low-pass filter of any type.

Re: High speed encoder with slotted / flat end... can't find one.
 

Thanks for sharing this, as we have a chain driven encoder as well and it is not geared down at all. Could a chain-driven encoder explain the extremly noisy graph that I showed on page 2? I can say that our shooter had a lot of vibrations at higher speeds, probably more than what it should have had.

All:
What are teams doing as a better alternative to a chain-driven encoder for the shooter?

Re: High speed encoder with slotted / flat end... can't find one.
 

Is the encoder separately chain-driven? Or is the encoder mounted directly to the wheel (or wheel axle) which is chain-driven?

Makes a difference.

Re: High speed encoder with slotted / flat end... can't find one.
 

Encoder's require very little torque to spin (usually). In the past I've used rubber bands with no issue to transfer one shafts rotation to an encoders.

Our particular shooter wheel has the end of the shooter wheel shaft drilled out. We then just wrapped the encoder shaft in a layer or two of Teflon tape and inserted the encoder into the shooter shaft. The encoder was mounted to a simple rectangular plate and some standoffs space the encoder off the shooter wheel's mounting plate.

-Brando

Re: High speed encoder with slotted / flat end... can't find one.
 

Direct-driving the encoder off the shooter, via several methods:

-Using a live axle shooter and connecting the encoder directly to it
-Using a live or dead axle shooter with a code wheel and optical sensor (Banner and Rockwell sensors seem to be the most popular)

In addition to the vibrations of a chain, I assume the constant modification of output power by the control loop would cause additional jitter if the chain was slightly (or more than slightly) slack, probably more than any other single source of jitter.

In our shooter system, I can hear the difference between running the control loop and applying fixed power, especially at low speeds where we're approaching the Victor deadband. The motor is pulsing at a low frequency, and I can hear the gears in the gearbox make nasty noises as they're being driven by the motors, then the motors back off and the inertia of the system drives the output gear faster than the motors, then the motors pick up. It's much less noticeable at high speeds.

Re: High speed encoder with slotted / flat end... can't find one.
 

Separately driven. It's a short 1:1 #25 chain off of the main shooter shaft.

-Parker

Re: High speed encoder with slotted / flat end... can't find one.
 

If you're using GetRate() with a separately-chain-driven 128 CPR encoder, that could be the source of a lot of your noise.

Re: High speed encoder with slotted / flat end... can't find one.
 

I've managed to find a rate compare dump that may be of interest... this shows GetRate() (magenta) vs. The GetDistance()/time (cyan)... where the yellow shows a minor change in time of 10-11 ms. I believe GetRate() has better results in this test. Setting that aside, notice how the faster speeds have more linear patterns at certain points. This is measuments in RPS from 0 to 35... where x is 10-11 ms iterations.

Re: High speed encoder with slotted / flat end... can't find one.
 

Two things:

1) You might want to consider converting your large (1+ megabytes) BMP file attachments to a compressed format (PNG would be nice) before uploading. Attached PNG file is 380 times smaller (only 3 kilobytes) than the BMP you posted, with no noticeable decrease in quality

2) 35 revs/sec @ 64 counts/rev = 2240 counts/sec = 22.4 counts/(10 ms), so +/- 1 count should give jitter of approx +/-1/22.4 = +/-4.5%. So I can't draw any conclusion from your graph except that something is not right.

Re: High speed encoder with slotted / flat end... can't find one.
 

With regards to the questions about how we were running our encoder in the past: we had a sprocket on the encoder shaft riding against the chain that drove the shooter. The shooter sprocket was smaller than the encoder sprocket by a ratio of 2:1, resulting in 2 turns of the shooter for 1 turn of the encoder.

Since then, in our quest to lower noise, we now have the encoder coupled directly to the output shaft with a very short piece of surgical tubing. That has worked very well (it puts less stress on the encoder shaft as well).

Re: High speed encoder with slotted / flat end... can't find one.
 

Like this?

That explains a lot of the noise:)

Makes sense.

Re: High speed encoder with slotted / flat end... can't find one.
 

No... our encoder and sprocket was on top of the chain, not under it..... ;)

Yes, exactly like that.