Arbitrary Quadrature Encoder

[Bneufeld235]

Team 585, the Cyber Penguins have launched a new Kickstarter project, the Arbitrary Quadrature Encoder. We designed this circuit out of frustration with the high precision encoders that are currently available. For our uses, we were looking for a source of feedback that didn't have to be so precise but can plug straight into the Jaguar. We wanted to be able to control a window motor or a linear actuator with some ability to measure, but not so much that it was a burden to calibrate or keep from getting damaged.

After two years of development, we are now at the point where we are ready to create a printed and assembled circuit for our encoder so that more than just our team can benefit from our research and design. We explain how it works and have a demonstration video on the Kickstarter page. Please let me know what you think.

http://kck.st/147OyqI

[Levansic]

Brooke was able to gather some feedback at CMP, and should be posting some of that relatively soon.

In the meantime, we got some helpful feedback, both public and private, from some Kickstarter commenters. One was a FIRST alum from our team, who graciously offered some help with producing the prototype SMD boards much faster than we were able to get quoted elsewhere.

As a quick update, here is our current schematic. The schematic matches out wire-wrap prototype. We will be adding reverse-polarity and short-to-ground protection, very soon.

This is the current board design:


It looks big on the screen, but it is quite small when printed at actual size.

-- Len

[Bneufeld235]

We received a lot of great feed-back from talking to teams at Championships. Most questions fell under two categories: maximum performances and testing.

We have done testing at relatively low speeds so we are unable at this time to give a maximum speed. That being said, we calculated theoretical maximums based on the slew rate of our op-amps and are convinced we will never be able to test the maximum rate.

We realize that is a bold claim so here are some numbers. We have designed this to trigger at TTL levels but our worst case scenario is making every max voltage go from zero to maximum supply voltage to zero 111,000 transitions or 55,500 cycles per second. The default striped resolution is 0.2" (5mm) has 20 stripes around a 2.5" diameter hub. This would correspond to a speed of 166,000 rpm.

Absolute max performance is a signal that just trips the hysteresis with a much higher 500,000 transitions which is an approximate 750,000 rpm. This is theoretical in a perfect world and will not be any where close to that.

It should be noted that this is at default low resolution. The arbitrary part of the encoder is the resolution as well as the count per revolution. Higher resolution can be achieved by skewing the encoder. We will produce a table of stripe size to skew angle and post this once we verify performance.

We have posted this to receive feed-back from other FIRST teams so if you have any questions please ask.

[Joe Ross]

Originally Posted by Bneufeld235

We have done testing at relatively low speeds so we are unable at this time to give a maximum speed. That being said, we calculated theoretical maximums based on the slew rate of our op-amps and are convinced we will never be able to test the maximum rate.

I suspect your limiting factor will be the phototransistors, rather then the op-amp.

[cgmv123]

Originally Posted by Joe Ross

I suspect your limiting factor will be the phototransistors, rather then the op-amp.

Based on those numbers, I think the FPGA's limit of 39,000 pulses per second could be the limiting factor, a limit which hopefully NI will increase in the new controller.

[Bneufeld235]

Yes, the limiting factor would be the phototransistors. It is about three times slower compared to the op-amp. The phototransistor only needs to trigger the hysteresis, which can be slightly reduced. The theoretical peak of the phototransistor would then match the capabilities of the op-amp.

Theoretically, the encoder can go faster than the cRIO can handle but it does not even come close to what the jaguar can handle. For the Jaguar case the op-amp and the phototransistor are the limiting components.

[Ether]

Originally Posted by Bneufeld235

Theoretically, the encoder can go faster than the cRIO can handle but it does not even come close to what the jaguar can handle.

What can the Jaguar handle?

[FrankJ]

A 166,000 rpm shooter wheel? I think you are going to need a guard on that.

Amazing stuff.

[Bneufeld235]

Originally Posted by Ether

What can the Jaguar handle?

The Jaguar can handle one million transitions a second.

[Ether]

Originally Posted by Bneufeld235

The Jaguar can handle one million transitions a second.

For the record, would you please give your source for that information.

[Gdeaver]

The jag does have a hardware quadrature block that is fast but you have to service that hardware in a timely manor and handle many other time critical events. That may be a little high.

[Joe Ross]

Originally Posted by Ether

For the record, would you please give your source for that information.

http://www.chiefdelphi.com/forums/sh...mp;postcount=8 ;-)

[Ether]

Originally Posted by Joe Ross

http://www.chiefdelphi.com/forums/sh...mp;postcount=8 ;-)

Facepalm.

[Bneufeld235]

Originally Posted by Ether

For the record, would you please give your source for that information.

Texas Instruments, Board Data Sheet (page 6)
Stellaris Brushes DC Motor Control Module with CAN (MDL-BDC24)
It was downloaded last year when Texas Intruments still made the Jaguars and assuming they haven't changed.

[Bneufeld235]

We have now ordered the first round of circuit boards. Everything should arrive around May 14. These are for test and verification of the design, before we commit to making a bunch. I and several other students will be soldering the boards.

With the circuits we have ordered resistors with different values so we can do some testing and fine tune our encoder.The resistors that will most likely need fiddled with are involved with the voltage divider feeding into the first op-amp. We just don't know how sensitive the SMT phototransistors will be, compared to the through-hole version on the wire-wrapped prototype.