Robot Arm Tutorials

Friday Facts 19: Marlin for Robot Arms

Today I’m going to show you how to set up Marlin firmware – the code in the robot brain – for your board so it thinks it is a robot arm and I will be using the Sixi 3 as my example. When we’re done we’ll have a 6 axis machine ready to receive gcode from apps like Robot Overlord.

Building a custom robot arm is easier if one uses common hardware. Thanks to the popularity of 3D printers it is now super easy to get stepper motors, limit switches, and MCUs that drive five, six, or even more motors. Marlin takes away all the headache and lets anyone talk to a robot with gcode, the standard language of CNC machines like 3D printers, mills, and lathes.

The major steps are:

  1. Fork Marlin
  2. Customize it
  3. Flash your board
  4. Test it

Fork Marlin

To “fork” is to make a special copy. it’s special because it includes a feature to update your copy with changes to the original. When the Marlin developers improve something, you can press a button and get all their updates.

The code for Marlin is available at https://github.com/MarlinFirmware/. I have a fork for robot arm Sixi 3 already started. You can get that fork as well right here: https://github.com/MarginallyClever/Marlin/

Make sure that (1) the branch is set to sixi3, then (2) click the code button and then (3) Open with Github Desktop or Download Zip. If it’s a ZIP you’ll have to unpack it somewhere like Documents/Github/Marlin.

Customize Marlin

Here’s a list of lines in Configuration.h that I’ve changed. The bold parts are unchanged so you can use that to search the file. The stepper motors in Marlin are named – internally only – as X, Y, Z, I, J, K.

#define STRING_CONFIG_H_AUTHOR “(Marginally Clever, Sixi3)”

#define MOTHERBOARD BOARD_RUMBA

#define CUSTOM_MACHINE_NAME “Sixi 3 robot arm”

#define EXTRUDERS 0

Because it’s a RUMBA board I also had to redefine a few of the pin settings. Normally all supported boards are defined in Marlin/src/pins/*.

#define I_STEP_PIN                         23
#define I_DIR_PIN                          22
#define I_ENABLE_PIN                       24
#define J_STEP_PIN                         26
#define J_DIR_PIN                          25
#define J_ENABLE_PIN                       27
#define K_STEP_PIN                         29
#define K_DIR_PIN                          28
#define K_ENABLE_PIN                       39

#undef Y_MAX_PIN
#undef Z_MIN_PIN
#undef Z_MAX_PIN

#define I_MIN_PIN                           34
#define J_MIN_PIN                           33
#define K_MIN_PIN                           32

The type of driver and the external name of each motor is next.

#define X_DRIVER_TYPE  A4988
#define Y_DRIVER_TYPE  A4988
#define Z_DRIVER_TYPE  A4988
//#define X2_DRIVER_TYPE A4988
//#define Y2_DRIVER_TYPE A4988
//#define Z2_DRIVER_TYPE A4988
//#define Z3_DRIVER_TYPE A4988
//#define Z4_DRIVER_TYPE A4988
#define I_DRIVER_TYPE  A4988
#define J_DRIVER_TYPE  A4988
#define K_DRIVER_TYPE  A4988

...

#ifdef I_DRIVER_TYPE
  #define AXIS4_NAME 'U' // :['A', 'B', 'C', 'U', 'V', 'W']
  #define AXIS4_ROTATES
#endif
#ifdef J_DRIVER_TYPE
  #define AXIS5_NAME 'V' // :['B', 'C', 'U', 'V', 'W']
  #define AXIS5_ROTATES
#endif
#ifdef K_DRIVER_TYPE
  #define AXIS6_NAME 'W' // :['C', 'U', 'V', 'W']
  #define AXIS6_ROTATES
#endif

Limit switches are tricky because the original Sixi 3 still doesn’t have them. (The plan is a new PCB that has always-on sensors). For Sixi 3 only, I have to trick the sensor code. When the robot turns on it will believe it has already homed, no matter where it is. A better robot with switches would call G28 to find home, and then the invert would depend on the type of switch (normally open vs normally closed) and I don’t remember what plug does.

#define USE_XMIN_PLUG
#define USE_YMIN_PLUG
#define USE_ZMIN_PLUG
#define USE_IMIN_PLUG
#define USE_JMIN_PLUG
#define USE_KMIN_PLUG

...

#define X_MIN_ENDSTOP_INVERTING false 
#define Y_MIN_ENDSTOP_INVERTING false 
#define Z_MIN_ENDSTOP_INVERTING false 
#define I_MIN_ENDSTOP_INVERTING false 
#define J_MIN_ENDSTOP_INVERTING false 
#define K_MIN_ENDSTOP_INVERTING false 
#define X_MAX_ENDSTOP_INVERTING true 
#define Y_MAX_ENDSTOP_INVERTING true 
#define Z_MAX_ENDSTOP_INVERTING true 
#define I_MAX_ENDSTOP_INVERTING false 
#define J_MAX_ENDSTOP_INVERTING false 
#define K_MAX_ENDSTOP_INVERTING false 

Motors also need gearbox and speed settings. Sixi 3 has a 70:1 harmonic gearbox and then a further pulley reduction in each unit. Since each motor does 200 steps per turn, that makes 105 steps per degree!

#define DEFAULT_AXIS_STEPS_PER_UNIT   { 105, 105, 105, 105, 105, 105 }

105 steps per degree * 5 deg/s = 525 steps per second. Impressive for such tiny NEMA17 motors. It might not be fast but it works and it’s affordable. Cheap, fast, good… pick two.

#define DEFAULT_MAX_FEEDRATE          { 5,5,5,5,5,5 }

#define CLASSIC_JERK // uncommented this to turn it on

#define S_CURVE_ACCELERATION // uncommented this to turn it on

I make sure to leave motors on so the arm doesn’t suddenly “go limp” at the worst time.

#define DISABLE_X false
#define DISABLE_Y false
#define DISABLE_Z false
#define DISABLE_I false
#define DISABLE_J false
#define DISABLE_K false

Range of motion is important, Marlin won’t let you go outside the limits. Remember this code was written for square box 3D printers, so some of the terms are a little silly for our needs.

// The size of the printable area
#define X_BED_SIZE 360
#define Y_BED_SIZE 360

// Travel limits (linear=mm, rotational=°) after homing, corresponding to endstop positions.
#define X_MIN_POS 0
#define X_MAX_POS 360
#define Y_MIN_POS 0
#define Y_MAX_POS 360
#define Z_MIN_POS 0
#define Z_MAX_POS 360
#define I_MIN_POS 0
#define I_MAX_POS 360
#define J_MIN_POS 0
#define J_MAX_POS 360
#define K_MIN_POS 0
#define K_MAX_POS 360

#define MIN_SOFTWARE_ENDSTOPS  // but no sub-values like MIN_SOFTWARE_ENDSTOP_X
#define MAX_SOFTWARE_ENDSTOPS  // but no sub-values like MAX_SOFTWARE_ENDSTOP_X

#define EEPROM_SETTINGS // save important data to EEPROM

#define SDSUPPORT

#define REPRAP_DISCOUNT_SMART_CONTROLLER // or your favorite flavor here

#define NUM_SERVOS 1 // for the gripper

Flash your board

Press the Compile button (1) to check for errors. Press the Upload button (2) to send it to your connected device. Press the Connect button (3) to open a serial monitor and check that your device says it is now a Marlin device. If all goes well, you’re ready to rock!

Test your new device

Even before your board is installed in an arm you should be able to home it with G28 and move it with G0/G1. Remember: every bug is just a test you didn’t run!

Final thoughts

Now that you have a 3D printer board setup to run Marlin, it should be able to receive angle values as gcode. Each set is one forward kinematic pose of the arm. Moving between two poses will send the arm in curving arcs. Lots of poses close together will create the look of straight lines. Planning all those poses is easy for apps like Robot Overlord. That’s why I wrote it!

Got more questions? Something out of date in this post? Join us on Discord.

News

How to draw a rectangle in Gcode

So you’ve played with the Makelangelo‘s generated Gcode and now you want to experiment on your own. Cool! Here’s a simple rectangle to get you started.

Now let’s give some values to these parameters:

A20
B40
C10
D30

My method is to lift the pen, drive to a corner, put the pen down, and then trace the rectangle. That means going to five points because we have to come back to the start.

G0 Z90; # pen up
G0 X-10 Y20;  # p1 
G0 Z40; # pen down 
G0 X30 Y20;  # p2
G0 X30 Y-40;  # p3 
G0 X-10 Y-40;  # p4 
G0 X-10 Y20;  # p1 again
G0 Z90; # pen up

And voila! I’ve deliberately done this with the origin away from the center so the numbers are more obvious. I hope you see that in a cartesian system the origin could be anywhere. For example, if it were below and to the left of P4 all the X and Y values would be positive.

Tutorials

Creating my own Processing arc method

The Processing arc() command isn’t robust enough for my tastes so I wrote a new one that gives me more control over the things I care about. Along the way it lets me validate my arc code for the Makelangelo’s support of gcode commands G02 and G03. If people are learning Gcode then their arc() command would be helpful if it looked the same as a gcode command. Don’t you agree?

(more…)

Tutorials

How to build an 2-axis Arduino CNC Gcode Interpreter

Computer model of where the 3-axis CNC router will move to cut the shape

Purpose

3D printers, laser cutters, water jet cutters, robot arms, delta robots, stewart platforms, the Makelangelo: all of these are examples of computer numerical control (CNC) machines. CNC machines need to move accurately and on command. Stepper motors are a great way to move accurately – they move a predictable amount and then stay where you put them. To command the stepper motors we need a way to easily turn our human desires into machine instructions into stepper motor steps. In effect we need our robot brain to be an interpreter. I’m going to show you a simple interpreter written for Arduino that lets you move stepper motors for your robots.

read the explanation and see a video