775pro encoders

[Munchskull]

My one of my mentors wants to know if they can handle 19000rpm

[Ether]

Originally Posted by Munchskull

My one of my mentors wants to know if they can handle 19000rpm

Well, it was designed specifically for the 775Pro whose free speed is 18730 so I would hope the sensor can mechanically and electrically handle that speed. Andrew, can you post an oscope trace at full speed?

As for the FPGA, the new higher FPGA sampling speed should handle it, even with the quadrature errors shown on the oscope trace posted earlier in this thread:

19000	rpm
316.7	revs/sec
3	cycles per rev (assuming 6-pole magnet w Hall sensor)
4	edges per cycle (quadrature)
12	edges per rev (quadrature)
3800	edges/sec (quadrature)
40.7	quadrature error (degrees)
182.6%	quadrature error (percent)
6937	edges/sec (with quadrature error)

If you are going to use this for speed control of a shooter wheel, follow the advice Richard gave in his earlier post and use rising edges only in one channel only, to give the cleanest (jitter-free) speed signal (especially if using bang-bang control).

[Andrew Duerner]

Hi everyone,

Ether, here is a screen shot that we took at around 13 or 14 volts.

philso, I don't know if the spikes are from the scope or the encoder. I would have to borrow the scope again to play with more settings.

As for shooter wheel design, from my experiences in 2012, having the wheel be designed to mechanically impart enough energy into the ball can lead to a very accurate shot. Our encoder was used for ensuring that a very accurate rotational velocity was achieved so that the wheel would impart the same energy each time. We added closed loop PID control helped bring the wheel back up to speed in a short amount of time for the next shot. Which in 2012 was important as you could hold 3 balls. This year the cycle times will be much longer.

[Ether]

Originally Posted by Andrew Duerner

Ether, here is a screen shot that we took at around 13 or 14 volts.

Yeah, looks to be spinning about 22000 rpm:

0.0009	seconds per cycle
3	cycles per rev
0.00270	seconds/rev
370.37	revs/sec
22,222	rpm

[GeeTwo]

These certainly aren't the prettiest traces I've seen. However, given that these traces are directly on the motor, teams willing to invest a bit of time and a few cents on tiny capacitors will blow through these rough spots like they don't exist.

[Stephen.Yanczura]

We ordered a few of these and mechanically, they seem to be a very nice solution. Kudos on the design!

@Geetwo -
How would you utilize some baby capacitors to address the issues mentioned? What value and technology caps would I buy and how should I put them in the system? Thanks!

[Ether]

Originally Posted by Ether

Yeah, looks to be spinning about 22000 rpm

BTW, this also shows that there are 3 cycles per rev, aka:

- 3 rising edges per rev on each channel

- 6 edge transitions (rising+falling) per rev on each channel

- 12 edge transitions per rev in quadrature