Hi torque 3d printed cheap robot actuator

Leg prototype

Key updates:

• Single arduino can control 2 servo (my plan for 4 per arduino) - 0.7 ms per update now
• Leg Kinematic seems like ok
• Python test app

Leg kinematic prototype

Upper and lower links are 150mm each

Update:

I switched from L298N to TMC2130 SPI stepper drivers.

TMC2130 biggest advantages:

• Size
• SPI mode to control coils directly via XDIRECT
• Less wires - now i can control 4+ motors/servos with single arduino nano

My current setup:

• Arduino Nano
• 16CH A/D MUX HP4067 - to get values for up to 16 hall sensors (up to 8 motors 2 halls per motor)
• TCA9548A - to get data from 8 absolute encoders (AS5600 i2c devices)
• TMC2130 SPI drivers

Experimental PCB's

Left PCB for 4 motors drivers, right PCB is a base for motor with hall sensors and mounting holes

Auto calibration finalized, calibration results saved to EEPROM

To rewire motors(very boring process if done by hands) i made a simple machine.

Details:

• Marlin+Ramps 1.6 as electronics
• RepitierHost as software
• Python g code generator - https://github.com/pavlog/RobotUtils

Final result:

2 Weeks spent for Gearbox V 1.1

Whats new:

• Dowel pins
• Herringbone gears used
• Ratio 1:30.8148

More updates coming soon...

General

I decided to develop gearbox with ratio from 1:25 to 1:35 to archive 180 degrees per second with full torque (i hope).

Small backlash is acceptable (at least for robot leg actuator)

All designs will be created in Openscad.

Design

I  had a few experiments in the past: So my favorite designs are: 

1) Spur Pinion  double gears organized in 3 gears per stage (each stage is a 3 spur-pinion gears at triangle corners) with ring gear as output: Openscad drawings and visualization will be uploaded soon.

This type of gearbox have 1:33 ratio (can vary, but not limited at max)

2) Cycloidal - i experimented in past, I got 1:25 ratio (strong limit for 70x70 cm footprint), but my past experiments had some Issues (mostly because of missing bearing at critical places)

Best ideas for Cycloidal you can see here: 

https://hackaday.io/project/165653-3d-printed-robot-joint-with-active-compliance

https://hackaday.io/project/167855-simple-cycloidal-robot-leg-for-quadruped

Gearbox exploded view concept

Key features: 

• Ratio: 1:33.1852 (1st stage 1:2.66, 2nd 1:2.666 output is a 1:4.66)
• Output gears module can be bigger then stage modules
• Gear height can be different for each stage
• Place to embed as5600 i2C absolute rotary sensor (2$ on aliexpress)
• Extra stage can be added without problems (1 extra stage and you can get 1:88 ratio)
• Only output stage is rotate (all double gears are rotates on the static common shafts)
• Each stage have 3 points of contact (less backlash and more torque)

Gearbox disassembly video

Assembled gearbox

Actuator test without absolute encoder

Gearbox test with absolute encoder

Motor theory and practice

I use brushless 5010 motor rewired as stepper motor with 28 steps per revolution, controlled by L298N module, arduino and 2 hall effect sensors

Image bellow is a 3 phase brushless motor (5010 model) with 14 magnet poles and 12 stator poles, this motor need to be rewired as stepper motor with 28 steps per revolution

Motor wiring diagram (14 magnet poles, 12 stator poles) converted to stepper motor (top view)

NOTE: Arrows lines is always on top of stator lines

Motor phases animation with wiring above visualized in video bellow

Disassembled motor ready for rewire

I use small radius PTFE tube as a helper for rewiring

100 turns per pole by 28 AWG wire (very challenging process) 

Final resistance 3.7 Ohm per phase

Assembled motor (wires covered by nail polish (pole 1 marked by blue)

Motor base plate 3d Printed (small holes for hall effects sensors mounting)

2 Hall sensors ready to be mount on base plate (49E 9148G sensor)

Sensors mounted to base

Motor mounted to base plate 1st hall sensor look at stator pole 12 2nd to 9 (it should be 90 degrees)

Sensorless motor test run

Sensored test run

Motor torque test 

• 160 PWM - 70g/10cm or 700g/cm
• 255 PWM - 160g/10cm or 1600g/cm