How 3D printer works

How 3d printer works?

​ Hello, I am Arno. Today we are going to find out how 3d printer works by writing a firmware. code link

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Develop platform

Hardware

MCU

• AVR mega2560
• MCUAVR mega2560

Mother board

• MKS GENL V2.1
• Mother boardMKS GENL V2.1

Stepper driver

• X TMC2008
• Y TMC2008
• Z A4988
• E TMC2225
• Stepper driverX TMC2008Y TMC2008Z A4988E TMC2225

Mechanical structure

Big fish i3

• Big fish i3
• Mechanical structureBig fish i3

Software

Download vscode

• Download vscode

Install addon platformio

• Install addon platformio

Open platformio tag, create new project, choose Board as mega2560, choose Framework as Arduino.

• Open platformio tag, create new project, choose Board as mega2560, choose Framework as Arduino.

Now we write some test code in src/main.cpp

// src/main.cpp
#include <Arduino.h>

void setup() {
    Serial.begin(115200);
}

void loop() {
    Serial.println("Hello from Mega2560.");
    delay(1000);
}

• Now we write some test code in src/main.cpp

// src/main.cpp#include <Arduino.h>void setup() { Serial.begin(115200);}void loop() { Serial.println("Hello from Mega2560."); delay(1000);}

Connect 3D printing motherboard with compute, click the ➡ button at the bottom of vscode. Then platformio will compile and upload the test code above into 3D printing motherboard

• Connect 3D printing motherboard with compute, click the ➡ button at the bottom of vscode. Then platformio will compile and upload the test code above into 3D printing motherboard

Open any Serial assistant you prefer, connect the motherboard, if you found Hello from Mega2560. is printed, then you are all set.

• Open any Serial assistant you prefer, connect the motherboard, if you found Hello from Mega2560. is printed, then you are all set.

Parse gcode

In this section, we should implement the parse function of Gcode class which is able to convert a gcode string like G1 X2.4 Y5.6 into a gcode object.

Most used gcode

G1 F200 X2 Y4 ; move to (2,4,0) and set feedrate as 200mm/min
G28 X0 Y0 ; move X/Y to min endstops
M104 S200 ; set hotend temp as 200℃

• Most used gcodeG1 F200 X2 Y4 ; move to (2,4,0) and set feedrate as 200mm/min
G28 X0 Y0 ; move X/Y to min endstops
M104 S200 ; set hotend temp as 200℃


gcode object example

gcode.cmdtype = 'G';
gcode.cmdnum = 1;
gcode.X = 2.4;
gcode.Y = 5.6;
gcode.hasX = true;
gcode.hasY = true;

Notice that if you try to use '\n' to split string like me, you need be careful that some gcode string might end with "\r\n"

• gcode object examplegcode.cmdtype = 'G';gcode.cmdnum = 1;gcode.X = 2.4;gcode.Y = 5.6;gcode.hasX = true;gcode.hasY = true;Notice that if you try to use '\n' to split string like me, you need be careful that some gcode string might end with "\r\n"

SD card

In general, we need to configure SPI to use SD card. But we are using mega2560, we can use the SD Library for Arduino. The only thing we need to do is find which pin is the CS (chip select) pin in schematic and call SD.begin(csPin) function in setup function.

Temperature control

Call hotend.update() every 150ms, which will decide turn the heater on or off according to current temperature.

• 150ms interval was implemented using timer(Heater::init)
• Temperature control algorithm is PID(Heater::calculatePid)

Motor control

In this section, we should figure out how to control stepper motor through a stepper motor driver like A4988.

A4988

Steps per unit

Now we know how to control stepper motor through A4988. But how long does the hotend travel(mm) if motor advance one step?

Synchronized pulleys and belts

Take the 2GT 20-tooth synchronous wheel as an example. 2GT means that one tooth corresponds 2mm, then if the synchronous wheel has 20 teeth, each time it rotates 1 revolution, the belt will travels 40mm. And if we motor drive as 16 subdivision, then one revolution means 3200 pulses is needed.

stepPerUnit = 3200steps / 40mm

• Synchronized pulleys and beltsTake the 2GT 20-tooth synchronous wheel as an example. 2GT means that one tooth corresponds 2mm, then if the synchronous wheel has 20 teeth, each time it rotates 1 revolution, the belt will travels 40mm. And if we motor drive as 16 subdivision, then one revolution means 3200 pulses is needed.stepPerUnit = 3200steps / 40mm


Screw

Take a screw with a pitch of 2mm and a lead of 8mm as an example. The lead means the linear distance traveled by the screw for one revolution.

stepPerUnit = 3200steps / 8mm

• ScrewTake a screw with a pitch of 2mm and a lead of 8mm as an example. The lead means the linear distance traveled by the screw for one revolution.stepPerUnit = 3200steps / 8mm


Endstop

Endstop has 3 pins: normal open(NO), normal close(NC), circuit common(COM). If we the NC and COM then:

State

on or off

MCU read

not triggered

on

LOW

triggered

break

HIGH

Motion control

How to advance

Assuming that our starting point is (0, 0), target point is (6, 3). Then motorX shoud advance (6 x 80) steps, motorY should advance (3 x 80) steps. Of course, we can demand the motorX to advance first then motorY like Plan B. But there is a better way like Plan A.

This is Bresenham's algorithm. Specifically, since it takes 480 steps in the X direction and 240 steps in the Y direction, hotend should move 480 times, each time motorX advance one step, however motorY advance one step each tow times. Each time of motion is called a step event. The total number of times is called step event count, and its value is the greater of X and Y.

// module/Planner.cpp - planBufferLine
block.stepEventCount = getMax(block.steps);

// main.cpp - motion control isr
motorX.deltaError = -(curBlock->stepEventCount / 2);
motorY.deltaError = motorX.deltaError;

motorX.deltaError += curBlock->steps.x;
if (motorX.deltaError > 0) {
    motorX.moveOneStep();
    motorX.deltaError -= curBlock->stepEventCount;
}

motorY.deltaError += curBlock->steps.y;
if (motorY.deltaError > 0) {
    motorY.moveOneStep();
    motorY.posInSteps += curBlock->dir.y;
    motorY.deltaError -= curBlock->stepEventCount;
}

Do not use float to calculate the number of steps directly, because it might lead to step missing.

Multiple motion commands

We should push a block which contains how many steps every motor should advance in a queue, and take it out when needed.

block

block.dir.x = 1
block.steps.x = 37
...
block.stepEventCount = 37
block.accelerateUntil = 6
block.decelerateAfter = 37
block.entryRate = 1808
block.nominalRate = 2001
block.exitRate = 2001

• block

block.dir.x = 1block.steps.x = 37...block.stepEventCount = 37block.accelerateUntil = 6block.decelerateAfter = 37block.entryRate = 1808block.nominalRate = 2001block.exitRate = 2001

Speed control

For example, if we want set speed as 1000steps/s, then every step event takes 1ms, which means every 1ms an interrupt should be generated.

Trapezoidal acceleration

We can set speed as constant, but changing the speed can improve efficiency.

Specifically, a block can be divided into an acceleration segment, a constant speed segment and a deceleration segment. Noted that if the length of block is very short, the acceleration graph should becomes a triangle.

Connection speed

In order to prevent the speed and coherence between each block. We need to calculate the entry speed and exit speed of each block. The estimation method is given below. Noted that the arc in the figure is only used to estimate the connection speed. The hotend does not go through this arc in practice.

I hope this article is useful or at least interesting for you. If so, can you please consider donating me 1 dollar? so I can live a little longer.