Rotary encoders

I have some micro metal gear motors I've soldered Pololu 3.3V optical rotary encoders onto micro metal gear motors. These have a direct analog output. I thought from a quick read of the instructions they might be tuned to work on digital inputs. If you read all the way to the bottom it suggests using comparators.

That's going to be a pain, because what you compare to is going to have to be adjustable. I can't get anywhere near the output voltage Pololu claim, and the two signals have quite different voltages on the one I've experimented with so far.

A rotary encoder generates two signals. Going forward, one signal will be 90 degrees forward of the other, and going in reverse, the opposite.

The rising edge of one signal can be used to trigger a capture of the other signal which can be used to determine the direction.

The capture needs to happen within 1/8th of the period of the encoder signal to be reliable. The motor could run up to 30K RPM, The encoders have 3 arms or 5 arms, I think this means the signal is 3x or 5x the frequency of the motor. This is (30K/60)*5 = 2.5KHz, so the period is 400us.

For better or worse, I've tried this using MBED in the first instance, using an STM32F303K8 Nucleo board.

400us/8 = 50us. I need to sample every 50us in order to be within 1/8th of the zero crossing. When I detect that, I sample the other channel.

MBED only offers one way of sampling - which is ask for a sample and wait until it's done, but the STM32F303K8 can complete a sample in 0.2us! Even if mbed is insanely inefficient, I should be able to do this without any additional hardware: I can use the ADC and signal processing.

The hardware could just sample both channels on a schedule, and interrupt the application on completion. If I used Chibios I could easily do this, however I'm wasting < 1% of the CPU doing it synchronously. On a board with slower ADC (and most are much slower than this) this could be a good idea.

The code could also be auto zeroing: just by tracking the minimum and maximum of the output signal and averaging the two. This means you don't need to use an oscilloscope to measure the signals and get things working. That's TBD.

Here's some code.



Popular posts from this blog

3D Printer ramblings

 I have a Prusa i3 mk2.5 in storage. I used to use it in my shed I used not to worry about it's harmful emissions: I just didn't share the space with it. I had, however, added an enclosure, and was planning to add a HEPA filter too it. Once it's in an enclosure, it's massive. Even if I had added the filter, there's no where to put it. I'd like a small printer that has an enclosure or is easy to enclose. I figure I could get by with 150x150x150 build area: I was rarely patient enough to wait for my i3 to print bigger models. 200x200x200 would probably actually replace my current printer, so that would be a bonus. Not many printers have HEPA filters, which is really a shame. It seems just to be the result of a misconception about HEPA filters, and about filtration in general. Read this . E.g. I was looking at a used Lulzbot mini, and there are ready made enclosure s for it, however . In this particular case, it's no big deal: opportunities to buy a Lulzbot min
Read more

Balance Bot V2

Image
My current balance bot sort of works, but it's surprising it does: it's centre of gravity is actually only about 60mm above the motor axis. The whole thing ways 320g. The 6 AAA batteries only weigh 70g, so despite being right at the top, the don't affect the centre of gravity much. 6 AA batteries would weigh about 180g. That would add a 34% to the overall weight, and move the centre of gravity much higher. Seems like a dramatic increase in weight: it would be nice to be a bit more incremental. More AAA batteries isn't really worth considering: they only weight 12g, vs 30g for an AA. 4 AA would deliver much less power/torque, so that's no good. There's no middle ground, in other words. I'm also going to make the whole thing a bit taller: I need more space to add more sensors in any case. Last time I made the frame, I needed an elaborate jig to get it square and it actually didn't work that well. This time, I decided to use some short bits of a
Read more

Robot arm

Image
I 3d printed this robot arm I original found on Thingiverse, but the Instructable is recommended. There's a MK2, and also a delta style arm, both of which look like good alternatives, but this will do for now. Here's a video of it moving: I've attached it to an STM32F4 Nucleo board, via a prototype shield: In the video, I'm sending commands from a python script that runs on my desktop, communicating via USB serial. I've written an implementation of canonical s-expressions in c++ that doesn't require memory allocation, as well as a python version to talk to it. It's the main reason I haven't posted for so long, and of course overkill for just controlling some servos, but should prove more generally useful. Here's the code . It uses platformio for the build, and mbed rtos.
Read more