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WARNING ! I developed this projects before following the C++ projects at 42. The standard and the style may shock :)

World's best coffee grinder

Hi there ! This is a preview of my coffee grinder project.

The aim is to drive my manual coffee grinder in order to get a precise amount of ground coffee.

Specifications :

  • error ~= 0.01 g ! ( ~=0.5g for average barista grade coffee grinders)
  • intuitive UI

This project covers following fields :

  • Electronics
  • Programming in C++
  • Mechanical design

Final solution :

Controller

Raspberry pi pico (this is a controller : no operating system)

  • cheap
  • amazing and very fast programmable I/O
  • support C++

Stepper motor

A stepper motor is selected to drive the grinder :

pros:

  • high torque, low RPM (no need of gearbox)
  • runs at precise
  • long lifetime
  • simpler mechanics
  • sounds like futuristic CNC machine, not like a cheap blender

cons:

  • challenging to drive (but very fun) => requires us to program our own driver !
  • expensive

specifications :

  • minimum torque (measured with a human scale) : 3 N.m
  • rotation speed : 150 RPM

choice

Nema 34 12N.m 6.0A DM860S (Cloudray)

solution

Our library :

  • Runs stepper pulse generator in background using PI/O (so we can do some stuff while the motor is running)
  • Guarantees continuous motor speed to avoid damages (constant acceleration)
  • Allows setting max acceleration and max speed

Scale

We choose a very light alumimium sensor, and a conventional HX 711 amplifier.

pros:

  • very accurate
  • very cheap

cons:

  • challenging to drive (but very fun) => requires us to program our own driver !

Solution

We use Pico's IO to read data output (and send gain choice) from HX711 according to supplier specifications : doc/hx711_english.pdf This was never done in C++, so we need to program our own library

Additional solutions required :

  • elastic coupling between motor and grinder
  • 80V to 5V power supply

Graph

  graph TD;
      A([PI Pico])
      B([Stepper motor])
      C([Stepper driver])
      D([80V Power supply])
      E([5V Power supply])
      F([220V House power])
      G([Coffee grinder])
      H([Amplifier])
      I([Aluminium scale])
      J([Screen])
      K([Push buttons])

      I-- "H bridge Analog signal (4 wires)" -->H
      H-- "Digital signal (2 wires)" -->A

      A-- "PI/O Pulse signal (2 wires)" -->C
      C-- "Pulses power (4 wires)" -->B
      B-- "Mechanical power" -->G

      A-- "Digital signal" -->J
      K-- "Digital signal" -->A

      F-- "220V" -->D
      D-- "80V" -->C
      D-- "80V" -->E
      E-- "5V" -->A
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Driver strategy

  1. sensor sensitivity calibration UI
  2. ask for mass target mt
  3. precise measurement of mass m0
  4. stripping (high acceleration short rotations)
  5. brew 50% at high speed w0 using live measurement (very inaccurate)
  6. precise measurement of mass m1
  7. measurement of time t1
  8. brew 20% at low speed w1 and high acceleration using live measurement (very inaccurate)
  9. measurement of time t2
  10. precise measurement of mass m2
  11. guess mass flow rate fr = (m2 - m1) / (t2 - t1)
  12. remaining mass mr1 = mt - m2
  13. measurement of time t1
  14. brew 1/5th of remaining mass during t3 = 1/5 * mr1 / fr
  15. measurement of time t2
  16. guess mass flow rate fr = (m2 - m1) / (t2 - t1)
  17. repeat 5 previous steps 4 times

Pictures

Videos upcomming !