Controlling Lego Power Functions Servo Motor

 Idea

Control Lego Power Functions Servo motor to it's full functionality. The Servo motor moves to 90 degree on both sides assuming, center is at 0 and it can also do 7 steps to each side.

Implementation

Lego Power Functions uses PWM frequency of 1150 Hz. Using this frequency, Power Functions servo can do 7 steps using duty cycles, 26.6, 38.9, 51.2, 63.0, 75.0, 87.2 and 100. I confirmed my tinkering with oscilloscope, that it is giving correct frequency and duty cycles. 

Below are my findings so far

Microbit

Kitronik All in one Robotic board

This is the first board, I have tinkered with. It can be powered with Lego 8881 battery box. By setting PCA frequency to 1150, it can step through with correct duty cycles. 

The microbit Makecode is here. To set desired PWM frequency for this board, see line 132 in robotics.ts.  I also added a functionality to brake motor, see Line 303 robotics.ts

Dfrobot Driver expansion board

My tinkering failed with this board. After updating the code, stepping always tunring motor to all the way 90 degree. Oscilloscope reading was not showing correct frequency and duty cycle which I was using.  

I don't have electrical, electronics knowledge but my guess is it is due to the design of the board. This board has led which light up when motors are running. These leds are lighting up when move motors by hand.

Elecfreaks Motor:bit

This board just have a TB6612 based motor driver chip. PWM has to be set by Microbit using analogSetPeriod. See forward function here at Line 24

Testing of this is below in Slow motion

or

Youtube Link

Raspberry Pi

Adafruit DC motor Bonnet

The board is available here, Adafruit has detailed steps on how to use this board with Raspberry Pi, so just used those steps

By default, PWM frequency is set to 1600. After installing the library, I changed the code to accept pca_frequency when creating the class object. To do so

1. Edit /usr/local/lib/python3.7/dist-packages/adafruit_motorkit.py or locate adafruit_motorkit.py based on installation
2. Add default parameter, pca_frequence=1600 like this
1. 
   
   
   
   

Wiring

Lego PF servo uses all four wires. Side wires are to power and middle two wires are used to set duty cycle and direction. 

I use Lego PF Extension wires to connect Servo with custom Motor Driver boards. This way the motor can be reuse in Pure Lego based projects

This picture should give an idea about wiring and connections 

Semi-Autonomous AWD Transmission Vehicle using Lego Technic - Phase 1

Idea

Create a Lego Technic based vehicle with AWD transmission. The vehicle should avoid front, sides and back collisions using Sensors(Ultrasonic, Infrared etc). 

Control vehicle using a remote. The remote should offer functions like steering, speed, transmission. Remote should also show relevant information like sensor readings, steering, speed, transmission etc. 

Execution

Chassis

I used Formula off-roader 42037 as base and reused most of its parts. For front steering with differential, I took idea from The Unofficial LEGO Technic Builder's Guide, 2nd Edition book.

The chassis uses two l-motor-88003 motors in coupling mode, one servo for steering and one servo to shift gears. These motors are powered using AA battery box.

I used Extension Wires and attached Dupont Connectors to it. This way, motors can be re-used in pure Lego projects. I used this tool for crimping

AWD Transmission

I modified Sequential AWD 4-speed gearbox with V8 fake engine and fit it in the chassis. I also replaced two gear changing knobs with gears. I was hoping that gear would help in gear shift using servos but I failed to achieve what I wanted. Will talk about it later

Motor Driver Board

I used All-in-one Robotics Board for BBC micro:bit. It's nice board with ability to drive four motors. This board can get input from AAA or AA battery box due to it's high input voltage.

Remote

For now, I used Yahboom Gamepad Joystick Breakout Board for BBC Micro:bit. But my plan is to use elecfreaks-joystick-bit with iic-oled later.

Current remote is showing Power/Speed level, Gear shifts and steering using Micro:bit led screen.

To-Do

1. Fix steering angle, for now it's ratio is 1.67:1
2. The overall build is not sturdy. It's front wheels wobbles on steering
3. Gear slips when driving on slope or climbing high objects
4. Gear doesn't shift to 2 and 4 as i am not able to step servo between 0 and 180
5. Attach steering with servo   
6. Attach Sensors
7. Beautify the model

Demo

Pictures

Video

Driving

Gear Slip

Driving Slope

Climbing Object

Code

Remote

Remote Code Yahboom

Car

Car Code Kitronik

***Update-1***

Fixed clicking sound in gear system by putting a beam next to Differential

Videos

No click going up hill

No click on climbing object

***Update-2***

Fixed stepping through servo. Lego PF Servo responses to 1150Hz PWM and uses duty cycles to step through its 15 positions. I will post a bit details about it in separate blog.

I modified code for the shield to work with Lego PF. The code is available here