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I recently decided to update the Polar Coaster project. The primary reason was to update the controller to use Grbl_ESP32 firmware. I also thought I could make it smaller, lighter and remove a little cost.
Controller
The old controller was not custom made and just sort of tacked onto the back. This increased the size and didn’t look very good. It had a Bluetooth module, but you still had to stream the gcode. You could use an Android app, but that was still a little awkward.
Firmware
The controller runs Grbl_ESP32. This was recently updated to include pen machine features. This allows precise control and calibration of the pen servo. You can control the speed, timing and endpoints of the servo travel.
WebUI
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The controller has a web server that serves a web app. This allows complete control of the machine. Web browser based control is ideal, because it is compatible with anything that can use a browser. We hope to add additional web apps for generating gcode from images and SVGs.
SD Card
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This is a great feature. If you have files on the SD card, they can easily be plotted. The WebUI lets you upload files and then click play.
Stepper motors
Both versions use NEMA 14 motors. The original version used 34mm deep motors. I used these because I had some. They are overkill, because there is very little load on the motors. I bought some 20mm deep motors for the new version. These are even a little overkill. I am only running about 0.3A of current into each one. There are not too many options for standard stepper motors that are smaller and lower cost than these.
Gear Driven Rotational Axis
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I decided to change from a belt driven rational axis to a gear driven one. Loop belts are a pain, because you have to order them from expensive places like SDP-SI and if you want to tweak anything, you might need to order a different size. I decided to use helix gears this time. These are 15° gears with a modulus of 1. They print and work great with my printer, even on a coarse layer height. The motor is mounted on slots and it is easy to get virtually no backlash. The helix keeps them quiet.
Rotational Platter
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The platter rotates around a central bearing. This pulls (3) smaller bearings, that act like wheels, into contact with the PCB. This causes the platter to stay very flat, without wobble. I created a copper ring, plated with solder on the PCB to make it extra smooth and wear resistant. It also looks really cool and shiny on the PCB!
Buttons
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Since there was a little room left on he board, I added three buttons. I have not added anything to the firmware yet. I think one will definitely be a homing button. I am not sure what to use the others for. I thought it might be handy to have a repeat button to repeat the last plot without having to fire up the phone app.
Battery Power
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I made a battery case in the base of the machine to hold an RC style LiPo battery. I am using a 3 cell (11.1V) 2200 mah battery. The idle current on the machine with the WiFi running is about 80ma. When plotting it peaks at about 800ma. I think I should get about 2 hours of continuous plotting.
Future
- Put kinematics into the firmware. currently files need to be pre-processed with the python script.
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