Stepper Motor - Limit Switch for Position

Question

I'm following this guide https://howtomechatronics.com/tutorials/arduino/diy-motorized-camera-slider-pan-tilt-head-project/

My question is with the following piece of code used to calibrate the position values.

 // Move the slider to the initial position - homing
 while (digitalRead(limitSwitch) != 0) {
 stepper1.setSpeed(3000);
 stepper1.runSpeed();
 stepper1.setCurrentPosition(0); // When limit switch pressed set position to 0 steps
 }
 delay(20);
 // Move 200 steps back from the limit switch
 while (stepper1.currentPosition() != -200) {
 stepper1.setSpeed(-3000);
 stepper1.run();
 }
}

I understand the first while loop is to to get a zeroth position for calibration. But why move it 200 steps back and then set speed to -3000?[Q1] I don't understand that part.

And then later on in loop(), there is

// Limiting the movement - Do nothing if limit switch pressed or distance traveled in other direction greater then 80cm
 while (digitalRead(limitSwitch) == 0 || stepper1.currentPosition() < -64800) {}

I understand the first condition but why is second one there? Is it because 80cm is the max length of the slider/track and you don't want the stepper to move more than that? [Q2] Also how does 64800 = 80cm? [Q3]

Full Code: [Not mine - from link above]

/*
 DIY Camera Slider with Pan and Tilt Head
 by Dejan Nedelkovski
 www.HowToMechatronics.com
 Library - AccelStepper by Mike McCauley:
 http://www.airspayce.com/mikem/arduino/AccelStepper/index.html
*/
#include <AccelStepper.h>
#include <MultiStepper.h>
#define JoyX A0 // Joystick X pin
#define JoyY A1 // Joystick Y pin
#define slider A2 // Slider potentiometer
#define inOutPot A3 // In and Out speed potentiometer
#define JoySwitch 10 // Joystick switch connected
#define InOutSet 12 // Set Button
#define limitSwitch 11
#define inLED 8
#define outLED 9
// Define the stepper motors and the pins the will use
AccelStepper stepper1(1, 7, 6); // (Type:driver, STEP, DIR)
AccelStepper stepper2(1, 5, 4);
AccelStepper stepper3(1, 3, 2);
MultiStepper StepperControl; // Create instance of MultiStepper
long gotoposition[3]; // An array to store the In or Out position for each stepper motor
int JoyXPos = 0;
int JoyYPos = 0;
int sliderPos = 0;
int currentSpeed = 100;
int inOutSpeed = 100;
int XInPoint = 0;
int YInPoint = 0;
int ZInPoint = 0;
int XOutPoint = 0;
int YOutPoint = 0;
int ZOutPoint = 0;
int InandOut = 0;
void setup() {
 // Set initial seed values for the steppers
 stepper1.setMaxSpeed(3000);
 stepper1.setSpeed(200);
 stepper2.setMaxSpeed(3000);
 stepper2.setSpeed(200);
 stepper3.setMaxSpeed(3000);
 stepper3.setSpeed(200);
 pinMode(JoySwitch, INPUT_PULLUP);
 pinMode(InOutSet, INPUT_PULLUP);
 pinMode(limitSwitch, INPUT_PULLUP);
 pinMode(inLED, OUTPUT);
 pinMode(outLED, OUTPUT);
 // Create instances for MultiStepper - Adding the 3 steppers to the StepperControl instance for multi control
 StepperControl.addStepper(stepper1);
 StepperControl.addStepper(stepper2);
 StepperControl.addStepper(stepper3);
 // Move the slider to the initial position - homing
 while (digitalRead(limitSwitch) != 0) {
 stepper1.setSpeed(3000);
 stepper1.runSpeed();
 stepper1.setCurrentPosition(0); // When limit switch pressed set position to 0 steps
 }
 delay(20);
 // Move 200 steps back from the limit switch
 while (stepper1.currentPosition() != -200) {
 stepper1.setSpeed(-3000);
 stepper1.run();
 }
}
void loop() {
 // Limiting the movement - Do nothing if limit switch pressed or distance traveled in other direction greater then 80cm
 while (digitalRead(limitSwitch) == 0 || stepper1.currentPosition() < -64800) {}
 // If Joystick pressed increase the Pan and Tilt speeds
 if (digitalRead(JoySwitch) == 0) {
 currentSpeed = currentSpeed + 50;
 delay(200);
 }
 // If Set button is pressed - toggle between the switch cases
 if (digitalRead(InOutSet) == 0) {
 delay(500);
 // If we hold set button pressed longer then half a second, reset the in and out positions
 if (digitalRead(InOutSet) == 0) {
 InandOut = 4;
 }
 switch (InandOut) { 
 case 0: // Set IN position
 InandOut = 1;
 XInPoint = stepper1.currentPosition(); // Set the IN position for steppers 1
 YInPoint = stepper2.currentPosition(); // Set the IN position for steppers 2
 ZInPoint = stepper3.currentPosition(); // Set the IN position for steppers 3
 digitalWrite(inLED, HIGH); // Light up inLed
 break;
 case 1: // Set OUT position
 InandOut = 2;
 XOutPoint = stepper1.currentPosition(); // Set the OUT Points for both steppers
 YOutPoint = stepper2.currentPosition();
 ZOutPoint = stepper3.currentPosition();
 digitalWrite(outLED, HIGH);
 break;
 case 2: // Move to IN position / go to case 3
 InandOut = 3;
 inOutSpeed = analogRead(inOutPot); // Auto speed potentiometer
 // Place the IN position into the Array
 gotoposition[0] = XInPoint;
 gotoposition[1] = YInPoint;
 gotoposition[2] = ZInPoint;
 stepper1.setMaxSpeed(inOutSpeed);
 stepper2.setMaxSpeed(inOutSpeed);
 stepper3.setMaxSpeed(inOutSpeed);
 StepperControl.moveTo(gotoposition); // Calculates the required speed for all motors
 StepperControl.runSpeedToPosition(); // Blocks until all steppers are in position
 delay(200);
 break;
 case 3: // Move to OUT position / go back to case 2
 InandOut = 2;
 inOutSpeed = analogRead(inOutPot);
 // Place the OUT position into the Array
 gotoposition[0] = XOutPoint;
 gotoposition[1] = YOutPoint;
 gotoposition[2] = ZOutPoint;
 stepper1.setMaxSpeed(inOutSpeed);
 stepper2.setMaxSpeed(inOutSpeed);
 stepper3.setMaxSpeed(inOutSpeed);
 StepperControl.moveTo(gotoposition); // Calculates the required speed for all motors
 StepperControl.runSpeedToPosition(); // Blocks until all are in position
 delay(200);
 break;
 case 4: // If Set button is held longer then half a second go back to case 0
 InandOut = 0;
 digitalWrite(inLED, LOW);
 digitalWrite(outLED, LOW);
 delay(1000);
 break;
 }
 }
 // Joystick X - Pan movement
 JoyXPos = analogRead(JoyX);
 // if Joystick is moved left, move stepper 2 or pan to left
 if (JoyXPos > 600) {
 stepper2.setSpeed(currentSpeed);
 }
 // if Joystick is moved right, move stepper 2 or pan to right
 else if (JoyXPos < 400) {
 stepper2.setSpeed(-currentSpeed);
 }
 // if Joystick stays in middle, no movement
 else {
 stepper2.setSpeed(0);
 }
 //Joystick Y - Tilt movement
 JoyYPos = analogRead(JoyY);
 if (JoyYPos > 600) {
 stepper3.setSpeed(currentSpeed);
 }
 else if (JoyYPos < 400) {
 stepper3.setSpeed(-currentSpeed);
 }
 else {
 stepper3.setSpeed(0);
 }
 // Slider potentiometer
 sliderPos = analogRead(slider);
 // If potentiometer is turned left, move slider left
 if (sliderPos > 600) {
 sliderPos = map(sliderPos, 600, 1024, 0, 3000);
 stepper1.setSpeed(sliderPos); // Increase speed as turning
 }
 // If potentiometer is turned right, move slider right
 else if (sliderPos < 400 ) {
 sliderPos = map(sliderPos, 400, 0, 0, 3000);
 stepper1.setSpeed(-sliderPos); // Increase speed as turning
 }
 // If potentiometer in middle, no movement
 else {
 stepper1.setSpeed(0);
 }
 // Execute the above commands - run the stepper motors
 stepper1.runSpeed();
 stepper2.runSpeed();
 stepper3.runSpeed();
}

• Gerben: Q1: my guess is that it's just to move the slider a bit away from the limit switch, so it no longer reads LOW. Q2 I think you are correct. The original project you linked to only has a rail of around 80cm. And no limit switch on the other end. So he uses software to limit the travel. An alternative would be to add another limit switch, or simply change the value to reflect the size of rails you are using.

Answer 1

Q1: It is a common practice to move the stepper a bit away from the limit switch after a calibration, so that it can again be activated, if the stepper is moved into that direction. In most cases the limit switch marks a point very near or at a physical limitation, where the stepper cannot proceed and will hit some kind of wall. The limit switch can turn the motor off, before that happens. And your causal chain is incorrect. The code doesn't move to -200 and THEN set speed to -3000. Instead it checks in a while loop, if the position -200 is reached, and when not will set the speed and run the motor. When the position is finally reached due to the motor being run, the while loops exits.

Q2: The 80cm are used as a soft limit, meaning that the slider at this point will not hit a physical limit or a limit switch, only the software stops the slider to go further and finally hit a wall. This is often used to save limit switches in a physically confined situation and also not hit any physical walls, since a collusion is mostly undesirable (due to possible harm to people and the machine). The 80cm are known here (because the rail for the slider is no longer, than that).

Q3: The Stepper does not count in length (cm), but in the number of steps, that he has taken. Depending on the used stepper motor, 1 steps corresponds to a specific angle, that the motor will spin. And the mechanics determine, what angle of rotation corresponds to what distance on the rail. Most likely the author of the guide had calculated this for his specific build. You can calculated this yourself, so that you can use the correct conversion from steps to distance:

distance for 1 step = circumfence of the driving wheel / number of steps per revolution of the driving wheel

Or shorter:

d1 = C / N

Then you can calculate the steps for a specific distance d by

Nd = d / d1