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

• Framework
• Arduino (should install vscode and platformio first)
• MCU
• AVR mega2560
• Mother board
• MKS GENL V2.1
• Stepper driver
• X TMC2008
• Y TMC2008
• Z A4988
• E TMC2225
• Mechanical structure
• Big fish i3

G1 X2.4 Y5.6 => Gcode::parse() => gcode object

In this section, we should implement the parse function of Gcode class. If you try to use '\n' to split string like me, you need be careful that some gcode string might end with "\r\n"

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)

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

1. VMOT

8V~35V DC, A 100uf capacitor needs to be placed between VMOT and GND to quickly respond to the motor's power demand. Beware, this is the only pin that connect high voltage, if you connect other pins with 24V DC, this module will burn in seconds.

1. 1A 1B

Connect to coil 1 of the stepper motor

1. 2A 2B

Connect to coil 2 of the stepper motor

1. VDD

MCU power

1. DIR

Connect to MCU, high and low each represent a different direction.

1. STEP

Each time this pin receives a pulse, A4988 will dive the stepper motor advance one step. Normal stepper motor used in 3d printer takes 200 steps to complete a revolution, which means 200 pulses.

1. MS1 MS2 MS3

But in most conditions, to drive stepper motor advance one revolution, 3200 pulses should be feed into A4988. That's because one step is divided into 16 micro steps.

1. ENABLE

LOW: enable module

HIGH: disable module

1. SLEEP

LOW: sleep

HIGH: active

1. RESET

LOW: reset module

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

• 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

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

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.

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.

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

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.

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.