Jumbo Servo

Over the weekend I’ve made some progress on the jumbo servo.

So that I can tell the position of the servo I’ve made a gear to fit on to the pot shaft and connect to the output gear of the servo. I tried various sizes until one fitted. I then used a negative “gear” to put splines onto a hole so it would fit tightly onto the potentiometer shaft. Note that due to the ratio of gearing of the output gear and the pot gear it’s possible to go over 360° before reaching the limits of the pot. I’ll likely limit this more in software so it does not cause any problems. I have some microswitches so limit switches could be added with some kind of cam.

I also designed an output spindle to look like the spindle on a smaller servo.

After one false start, a good quality gear was printed in Nylon12 from Rigidink using the Cel Robox PA12 settings. I needed to craft my own eject script in GCode as the inbuilt on was too cool and did not work. I did that by tracking down the commands in the Automaker C:\Program Files\CEL\Common\Macros folder and increasing the two temperature parameters.

;Eject Sequence
M104 S210 ;Set & heat nozzle to eject temp
M109 ;Wait for nozzle to get to temp.
G0 E-50 ;Create a small ‘neck’ in the filament which can be snapped easily
M104 S175 ;Go to snap temperature
M109 ;Wait for nozzle to get to temp.
G0 E-1500 ;Eject Filament

The gearbox seemed to cause some problems when first fitted but I added some silicone spray and it soon got loosened up. I could run the motor down to 3v and it still turned fine.

So the next step is to wire up the electronics which will be an L298 H-Bridge driver module and an Arduino. I’m using an UNO for the prototyping but will swap that with the new Itsybitsy board recently released by AdaFruit.

I also did some research into protocols used by digital servos, looking for an I2C one. There was the “OpenServo” project but that seems to be abandoned. I did find some code for that on GitHub but it looked rather over engineered. But I’ve tried to borrow some ideas from it. I’ve also borrowed ideas from the Arduino tutorials that provide a reader and write slave. One key thing to ensure is that when you respond the data recieved event is to read all the data available. If you don’t do that I’ve discovered that the event does not fire again.

2 thoughts on “Jumbo Servo

  1. Dennis says:

    Your progress on the Jumbo Servo can serve as great inspiration to all of us who want to succeed at difficult projects. I recently started a position in IT, and I lack the confidence it takes to commit to projects. Even with all of my online coding classes, I need inspiration from others to get me going in my field. I appreciate that you’re willing to admit that you do plenty of research during your projects.

    • Cheers Dennis, yes I’m quite rare in the maker world with regard the amount of research I do. I’ve been reading some quite varied details on servos such as PID control, inertia measurements. It’s through that research I get the confidence, I can see others who’ve succeeded and also know the pitfalls I’m likely to come across. You also need to be wary not to fall into the “Facebook world” trap, it might all seem rosy on the outside but the person documenting might have skipped some of the problems they’ve found along the way. So it looks like they built something more quickly and easily than it looks. I do try to incorporate some of my errors where it might help others but not everything gets included. For confidence find a small problem or project so you can get it to a stage of completion. In the end all big and difficult projects are made up of many smaller and easier steps.

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