Inverted Pendulum

Wanna build an inverted pendulum ? Like in this video :

you are in the right place. All what you need to known to get an incredible pendulum that jumps and balances is available here. I describe:

• how to make the electronic board
• how to make the shell
• how to get the other components
• how to program the firmware
• how to assemble the pendulum
• how to connect to the Control Station software
• how to calibrate the sensors

If you understand french, the long way explanation can be found on my blog. Otherwise, keep reading.

How to make the electronic board

Get the BOM

First, you need all the electronic components on this BOM list. This kind of components can be found on different sites like:

• Farnell
• RS Components
• DigiKey

Make the PCB

You need to make the PCB whose plans are given by the different files in this directory. (To be continued)

Soldering of the components

Refer to this implementation plan to solder the different components on the both faces. With enough dexterity and patience, all the components could be soldered by hand with a soldering iron. There are two exceptions:

• U400 and U401 are only available in LGA package, which means: no weldable legs !

You have two options for those ones:

• either you are confident enough to use solder paste and an oven ;
• or you do it like me and you ask your PCB manufacturer to do it for you. This option is ofen available and you can save money by asking them to only solder those two components.

How to make the shell

The shell of the pendulum is composed of three pieces, whose plans are given by the three STEP files in this directory

• Back of the shell, holding the PCB and the battery
• Front of the shell, holding the motor
• The hub of the flywheel

They have to be 3D printed. You can do it yourself in case you have access to a 3D printer, or just use an online service like Sculpteo. You need to upload the three STEP files, then to chose this material: Nylon PA12 (Plastic). Don't go to other materials, as it could change the weight of the pendulum ; weight is important!

How to get the other components

Some additional parts are still needed before getting a functional pendulum.

• The metallic outer ring of the flywheel. It can be ordered on the Misumi web site. Use the reference AWSM-D-D55-V48-T10 in the search box to land on the right page.
• The Maxon brushless motor whose reference is EC45 Flat 200142, available on RS.
• The Lipo battery. The model is not so important, as long as the physical dimensions and the output voltage are respectively: 5x3x1cm and 7.5V. A possible choice can be found here.
• Seven screws to atttach the PCB in the shell and to tie the two parts of the shell together. Check this Assembly BOM.
• Three screws to attache the motor in the shell. Check this Assembly BOM.
• The cable to connect the PCB to the battery. You have to assemble it yourself. Check this Cables BOM.
• The cable to connect the pendulum to the PIC microcontroller programmer. You have to assemble it yourself. Check this Cables BOM.
• A Lipo charger. Personnaly, I'm very pleased with the Graupner UltraMat 14 plus but there are many other ones.
• A Microchip programmation device like the PICkit 3 or the ICD 3, availabe on Farnell, for example.
• (optional) The cable to connect the pendulum to a serial cable. You have to assemble it yourself. Check this Cables BOM.
• (optional) a USB - Serial cable

How to program the firmware

The brain of the pendulum is a small Microchip 16bits µC. You must load the right firmware inside to give life to the pendulum.

• Download and install Microchip MPLAB X
• Connect your chip programmer (like the PICkit 3 or the ICD 3) to the electonic board P200 connector with the dedicated cable
• Connect your chip programmer to your computer
• Charge the Lipo battery and connect it to the electronic board
• Power up the electronic board by switching the power button: the green L301 LED should turn on
• Launch the application MPLAB IPE
• (1) In the device family field, select 16-bit DSCs (dsPIC33)
• (2) In the device field, select dsPIC33EP128MC202
• (3) Ensure the tool field displays the name of your chip programmer correctly
• (4) In the source field, browse your computer to select the iPendulum.hex firmware, available here
• (5) Click on Connect
• (6) Click on Program to flash the microcontroller
• (7) Click on Verify to check the microcontroller content

Test your flashed electronic board

You can easily check your PCB is fully functional by proceeding as follows:

• Connect the motor to the PCB by inserting its electrical flat ribbon inside the J100 socket, connectors facing down (see picture below)
• Connect the charged Lipo to the PCB
• Hold the motor in your hand firmly, but it must be able to turn freely
• Switch the power button B300 on

If everything is OK, the motor will turn briefly and stop. The red error LED L200 should NOT turn on or blink. The meaning of this LED is as follows:

• Simple blink: Lipo state of charge too low
• Double blink: Motor not detected
• Tripple blink: Could not intialize the gyro device
• Quadruple blink: Could not initalize the accelerometers device

How to assemble the pendulum

• Plug the power cable to the J300 connector on the PCB
• Use four plastic screws to mount the PCB into the back of the shell
• Use the three M3 screws to mount the motor into the front of the shell. Pay attention to the orientation of the flat ribbon cable.

• Mount the metallic ring around the plastic flywheel hub. It should be reasonably tight. I had to use a clamping vice.
• Drill a 4mm hole through the axis of rotation of the flywheel hub.
  • Test your drill bit in a stuck of wood before.
  • Use a drill column for the hole to be straight
• Plug the flywheel on the motor.
  • when you push on the flywheel to get it on the axis, be sure to apply a vertical force so as not to bend the motor
  • The flywheel should not be able to slip around the axis: use a bit of strong glue if needed.
• Insert the motor electrical ribbon cable in the PCB connector, connectors facing down.

• Assemble the two parts of the shell together and fix them with the three remaining screws.
• Put the Lipo battery in place and plug it to the power connector.

How to connect to the Control Station software

The Control Station is a small program written in C# (tested on Windows only) enabling to connect to the pendulum through a serial connection.

• Connect a USB-Serial cable to your computer
• Connect the adapter cable to the USB-Serial cable and to the pendulum
• Launche the Control Station Software
• Select the menu Tools->Connect to iPendulum
• Selecting the COM port corresponding to your USB-Serial cable
• Power on the pendulum

You should get to something like on this picture:

By clicking the Connect button, the software testes the communication by sending a request to the pendulum and check the response. If valid, the screen will be displayed in color and the drawing of the pendulum will follow the real pendulum movement.

The panel on the left let you see all the different variables of the running firmware, being related to the OS or to the pendulum sensors.

A thumb in the bottom of the screen makes the view switch to different graphs showing all the telemetries. They correspond to the different sensors measured on the pendulum.

How to calibrate the sensors

Calibration of the pendulum is saved in the flash memory of the microcontroller where it survives even when the pendulum is switech off. The Control Station software helps you to calibrate:

• The measured voltage of the Lipo: Tools->Calibration->Battery
• The measured angle of the pendulum Tools->Calibration->IMU

Battery voltage calibration only ask you to enter the real voltage of the Lipo, measured with a voltmeter. For the IMU, the software will ask you to put the pendulum in a succession of different positions. Just follow the instructions until the end of the procedure.

You can check the result of this calibration by posing the pendulum on its right of left side, on a flat table, and by checking in the interface that the angle is close to +45° and -45°. This measure can be seen on State estimation field of the left panel.