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Friday Facts 12: How to use Marlin in a Robot Arm

Building a robot arm is one thing, but what about writing the code to make it run? Some people want to learn the fine points of precision stepper motor control, forward and inverse kinematics, and then debug all that stuff. For the rest, working together gets the job done faster. For those people the Marlin 3D printer firmware is a great option. Today I’m going to show how I tweaked it to run in the Sixi 3 robot arm. Please share your experience with us so we can improve this post.

Marlin?

Marlin 3D printer firmware is the code in the brain of a very large number of printers. It is very flexible with a few changes. Most people might think of printers as having four motors – one for each direction and one for the extruder. But recent changes mean that Marlin can run up to six motors. That’s great for us, because most robot arms are 6 or less.

With Marlin installed you’ll be able to control the angle of each motor by sending gcode commands and even drive them simultaneously. With Marlin’s homing routines you could locate position, and new options coming in the near future will give real time feed back (more on that later)

What needs to be tweaked

Pour yourself a drink and settle in. This list will touch at least two files and take some time… OR you can use the sixi3 branch I maintain and adjust it for your speeds and gear ratios.

I keep trying new ways to make this list less dry. What do you think?

/Marlin/Configuration.h

Old valueNew Value
#define STRING_CONFIG_H_AUTHOR “(none, default config)”#define STRING_CONFIG_H_AUTHOR “(Sixi3, Marginally Clever Robots)”
#define MOTHERBOARD BOARD_RAMPS_14_EFB#define MOTHERBOARD BOARD_RUMBA
//#define CUSTOM_MACHINE_NAME “3D Printer”#define CUSTOM_MACHINE_NAME “Robot Arm”
//#define LINEAR_AXES 3#define LINEAR_AXES 6
#define AXIS4_NAME ‘A’#define AXIS4_NAME ‘U’
#define AXIS5_NAME ‘B’#define AXIS5_NAME ‘V’
#define AXIS6_NAME ‘C’#define AXIS6_NAME ‘W’
#define EXTRUDERS 1define EXTRUDERS 0
#define USE_XMIN_PLUG
#define USE_YMIN_PLUG
#define USE_ZMIN_PLUG
//#define USE_XMIN_PLUG
//#define USE_YMIN_PLUG
//#define USE_ZMIN_PLUG
//#define I_DRIVER_TYPE A4988
//#define J_DRIVER_TYPE A4988
//#define K_DRIVER_TYPE A4988
#define E0_DRIVER_TYPE A4988
#define I_DRIVER_TYPE A4988
#define J_DRIVER_TYPE A4988
#define K_DRIVER_TYPE A4988
//#define E0_DRIVER_TYPE A4988
#define DEFAULT_AXIS_STEPS_PER_UNIT { 80, 80, 400, 500 }#define DEFAULT_AXIS_STEPS_PER_UNIT { 105, 105, 105, 105, 105, 105 }
#define DEFAULT_MAX_FEEDRATE { 300, 300, 5, 25 }#define DEFAULT_MAX_FEEDRATE { 5, 5, 5, 5, 5, 5 }
#define DEFAULT_MAX_ACCELERATION { 3000, 3000, 100, 10000 }#define DEFAULT_MAX_ACCELERATION { 10, 10, 10, 10, 10, 10 }
//#define CLASSIC_JERK#define CLASSIC_JERK
//#define S_CURVE_ACCELERATION#define S_CURVE_ACCELERATION
#define E_ENABLE_ON 0 // For all extruders
//#define I_ENABLE_ON 0
//#define J_ENABLE_ON 0
//#define K_ENABLE_ON 0
//#define E_ENABLE_ON 0 // For all extruders
#define I_ENABLE_ON 0
#define J_ENABLE_ON 0
#define K_ENABLE_ON 0
#define INVERT_Y_DIR true#define INVERT_Y_DIR false
//#define INVERT_I_DIR false
//#define INVERT_J_DIR false
//#define INVERT_K_DIR false
#define INVERT_I_DIR false
#define INVERT_J_DIR false
#define INVERT_K_DIR false
//#define I_HOME_DIR -1
//#define J_HOME_DIR -1
//#define K_HOME_DIR -1
#define I_HOME_DIR -1
#define J_HOME_DIR -1
#define K_HOME_DIR -1
define X_BED_SIZE 200
define Y_BED_SIZE 200
//#define X_BED_SIZE 200
//#define Y_BED_SIZE 200
#define X_MIN_POS 0
#define Y_MIN_POS 0
#define Z_MIN_POS 0
#define X_MAX_POS X_BED_SIZE
#define Y_MAX_POS Y_BED_SIZE
#define X_MIN_POS -360
#define Y_MIN_POS 360
#define Z_MIN_POS -360
#define X_MAX_POS 360
#define Y_MAX_POS -360
//#define I_MIN_POS 0
//#define I_MAX_POS 50
//#define J_MIN_POS 0
//#define J_MAX_POS 50
//#define K_MIN_POS 0
//#define K_MAX_POS 50
#define I_MIN_POS -360
#define I_MAX_POS 360
#define J_MIN_POS -360
#define J_MAX_POS 360
#define K_MIN_POS -360
#define K_MAX_POS 360
#define HOMING_FEEDRATE_MM_M { (50*60), (50*60), (4*60) }#define HOMING_FEEDRATE_MM_M { (4*60), (4*60), (4*60), (4*60), (4*60), (4*60) }
//#define EEPROM_SETTINGS#define EEPROM_SETTINGS
//#define SDSUPPORT#define SDSUPPORT
//#define REPRAP_DISCOUNT_SMART_CONTROLLER#define REPRAP_DISCOUNT_SMART_CONTROLLER

/Marlin/Configuration_adv.h

define AXIS_RELATIVE_MODES { false, false, false, false }#define AXIS_RELATIVE_MODES { false, false, false, false, false, false }
#define HOMING_BUMP_MM      { 5, 5, 2 }
#define HOMING_BUMP_DIVISOR { 2, 2, 4 }
#define HOMING_BUMP_MM      { 5, 5, 5, 5, 5, 5 }
#define HOMING_BUMP_DIVISOR { 2, 2, 2, 2, 2, 2 }

Notes

  • MOTHERBOARD is your choice of brain board. Anything Mariln supports AND has 6 axies will work.
  • DEFAULT_AXIS_STEPS_PER_UNIT is the gear ratio at the given joint. For all sixi3 gearboxes the ratio is 70:1 (harmonic) * 54:20 (timing belt) * 200/360 (for 1.8 degree stepper motors at full step) = 105.
  • Because the gear ratio is so high the motors are not physically able to exceed the DEFAULT_MAX_FEEDRATE. If you use faster motors or a faster brain board you may be able to improve on these numbers.
  • EEPROM_SETTINGS, SDSUPPORT, and REPRAP_DISCOUNT_SMART_CONTROLLER are not required. I use these to tweak settings for testing, run programs from the SD card, and to have an LCD panel on my robot.
  • Every other change is to adjust from 3 axies to 6.

Homing and Real time feedback

There are some exciting new features coming to Marlin that should make real time feedback possible. This means we’ll know the robot position without having to guess or to home. It also means we can tell when the actual position deviates from the expected position too much that a collision has occurred and that can save a lot of trouble! The new configuration options to explore are:

  • REALTIME_REPORTING_COMMANDS adds some “quick commands” that get processed before anything else in the gcode buffer of the robot. Great for emergency breaking and for requesting position information (Gcode “S000”)
  • M114_REALTIME adds “M114 R” which reports the real-time position of the robot instead of the projected position at the end of the planned moves.
  • I2C_POSITION_ENCODERS is a first pass at adding real time sensors. This will no doubt be expanded later to include other types and features.

Further Reading

The Marlin Configuration guide online

News

Friday Facts 9: Daisy Stepper Driver 1.0

Daisy Stepper Driver is a closed-loop stepper controller that can be daisy-chained.

A stepper controller is a dual h-bridge circuit for controlling a stepper motor. You may be familiar with the A4988 stepper driver common in 2020s 3D printers. This board includes a more advanced model of the same. It can drive any one stepper motor at 24V up to 2A.

Closed-loop means that the board has a brain chip (MCU) that can read the motor position as well as direct. This way it can tell if it missed a step, bumped into something, is being driven, etc.

Daisy-chained means that they can be hooked sequentially. Sequential wires are much shorter. Shorter wires make for easier construction and repair. They are designed for up to 6 Daisies in a row. The Daisies talk to each other via the CANopen protocol.

The MCU is an STM32F103. In speaks CANopen natively while driving the stepper and listening to the sensor.

The CANopen master at the base of the robot can be any MCU that speaks CANopen. It synchronizes each motor and provides a single USB or bluetooth serial connection to the world.

What does this have to do with robots?

This board is designed to fit inside each gearbox of the Sixi 3 robot arm.

The old way a wire from every part had to run all the way through the arm and out the base. To replace the second joint from the bottom you would have to dismantle the entire arm. In the daisy chain model you only need to remove the elbow and disconnect the two cables to the next link in the chain.

Here’s a video of the arm moving IRL. Note the large cable hanging off the side – this version does not have the board in yet. It is the “outside” version of the wiring. I live in constant anxiety that they will get caught on something while moving.

Editor’s note – in this video one motor misbehaves. It was later found that the motor was negative 400-steps per turn instead of the expected 200-steps-per-turn.

Why isn’t the board in already?!

To test the board I have to be able to program it. To program the STM32F103 MCU I need to flash it with a bootloader via the programming pin J6. I have an STLINK V3 MINI to flash the bootloader BUT the MINI has male pins at 1.25mm pitch and my board’s J6 is 2.5mm pitch.

Editor’s note – I believe the larger connector is also a reversible connector.

Do you want to see this board get done? Say something nice on Discord.

What comes next?

Assuming this board is good and can be programmed… the next tasks are:

  • mount the board in the actuator
  • attach the disc to the output of the gearbox
  • run wires from one to the next and test the daisy chain
  • get an appropriate brain board and drive everything synchronously
  • make more boards

At the last stage it will be time to go to Kickstarter to scale production and bring the price down.

Did you design this board yourself?

No, and I’m proud of it! I have played with KiCAD in the past and done a few but this was beyond my skill. I found a great EE on Fiverr, gave him my specs, and he routed everything in a couple of days. Truly blown away by his skill. He even had a PDF step-by-step guide for ordering from PCBWay so I didn’t have to assemble the board or nothing.

Sadly, he closed his account and I have no way to reach him any more. Electroniikka! If you’re out there, know that I want to work with you again. I love working with talented people, it’s fun, easy, and it helps me scale up to the speed I long for.