Building a Raspberry PI Robot Car part1

In the recent few months I have been very focussed on a few topics like humanoid robotics and robot interaction. Recently I have had some extra time and decided to take the next step and really design a robot from scratch. I thought for my first from scratch robot it would be handy to start simple and go for a relatively simple wheel based robot.

I will write a series of blog posts about the robot and how I am taking next steps to design and hopefully perfect the robot. In this first post I will discuss the basic concept and shows how I am going to power up the servo’s and control unit.

The robot concept

Let’s start of with setting the design goal of the robot:
Design an open source wheel based robot that has a holonomic drive solution capable of detecting obstacles and recognising objects it encounters

Given this goal let’s first start off with some basic requirements for the robot and what it needs to consist of. I will design this robot based on principles I have used in previous modifications of robots, this has lead me to these requirements:
* It will be based on a Raspberry PI based with Wifi
* Entire robot should be powered by a single LiPo battery for simplicity
* Distance based sensors for obstacle detection
* Rotatable vision camera
* Holonomic drive system where I can use four individual wheel servos for multi-vector driving
* Arm / Gripper for interaction

Servos

For the servos in this project I will for the moment re-use my trustworthy Dynamixel AX-12A servos which can be used in continuous rotation mode and therefore act as wheel servos. However given the desire to open source this project and the costs of these servos they will be replaced in the future, however for the first iteration it is best to stick to what I know.

Powering the solution

One of the important principles for this robot design will be that it needs to be powered by a single power source. In previous robots I always used the combination of the Robotis Lipo battery with a separate battery solution for the Raspberry PI. This has caused in multiple projects issues, like balancing issues or simply nowhere to leave the batteries.

LiPo Battery
In this robot I will use a single LiPo battery, I have picked a Turnigy NanoTech 3S battery with a 2200mAh capacity. This should be plenty to power the Raspberry PI and the Servos for a estimated 30-60 minutes, and easy enough to increase capacity in the future.

Power conversion
The Raspberry PI accepts 5 volt as input and needs roughly 1-2 Amps of current. In order to use a single LiPo battery I need a power converter as the 3S Lipo has an output voltage of minimum ~11.1 Volt and Maximum ~12.8 Volt. For this I will use a simple UBEC (Universal Battery Elimination Circuit) from HobbyKing. This Ubec can convert input voltages ranging from 6 volt to 23 volt into a stable output voltage of 5.1 volt with a maximum current of 3 amps, which is perfect for the Raspberry PI.

For the Dynamixel Servos I will use the Dynamixel official power converter a SMPS2Dynamixel . This can take input voltage up to 20 volts so can be directly connected to the 3S Lipo. All we need is a small 2.1/5.5MM DC power jack, I have managed to source one with a screw terminal but you can find different types.

Power wire harnass
In order to connect both the UBEC and SMPS2Dynamixel to a single servo I have to create a small power harnass that splits the power output from the 3S Lipo to both power converters. For this I have custom built a harnass using XT60 power plugs and some cables I have soldered together and put a screw-cap on the end to protect the wire ends. All is in the end topped off with some electric insulation tape, this looks as following:

Combining it all
Next step is connecting all the electronics. In order to control the Dynamixel servo’s I will use my trusted USB2AX which allows controlling the servo’s via a Dynamixel serial protocol. What remains is wiring up the power with a servo and the Raspberry PI. What better way then to show this with a picture:

In order to connect the entire solution I have had to hook the UBEC directly onto the 5v/Gnd header connectors of the Raspberry PI. Do this with extreme care, any wrong polarity will directly blow up your Raspberry PI. Make sure to check the pinning layout properly RED = 5v Black = GND and they need to go to the respected pin header on the Raspberry PI

Look at this slightly more zoomed in picture for the polarity / Raspberry PI Ubec connection and click on it for full zoom:

Next Steps

In this post I zoomed into the big project plan and in particular laid out the power setup. In the next post I will start with the 3D design of the robot and will show how I use Fusion 360 to create the design of the robot.

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