Servo - January 2014

The blue and white jumper wires connect to the eight Raspberry Pi GPIOs that are not used for I2C or SPI. Note the hole where I cut the power bus.

Are we there yet???

Almost! First, I used a Dremel to drill out one of the holes on the power bus. This isolated the battery powered section. I then used the long red jumper to connect the Pi's 5V to the non-battery power bus.

Adding a 3x4 servo header and connecting it with the right-most four battery powered signals allows me to add things like SeedStudio’s range sensor that need a regulated power supply on the smaller header.

Yes! We are finally there!

Note: I deliberately avoided adding jumpers to the bottom of the board — in fact, there is only one jumper down there; the bent lead of the LED's resistor is tack-soldered to the ground terminal on the header above it.

Just look at all the space we have left for additional circuitry — or a even small breadboard! Okay, now that the hardware is ready, it's time to get the Raspberry Pi loaded with all the needed software and the demo program.

Controlling Servos and Reading Digital Inputs

There are several excellent GPIO libraries for the Pi. Initially, I was going to use ServoBlaster, but as it often requires having to build a kernel module (pre-compiled modules are not available for up-to-date Raspberry Pi kernels), I changed my mind.

WiringPi looked great, however, its current PWM implementation would not have given me the level of control I wanted over the servos I am using for the robot.

I ended up using "pigpio" — an excellent, easy to use library for C / C++ found at http://abyz.co.uk/rpi/pigpio.

Controlling the servos and reading digital inputs is a piece of Pi with pigpio. (You thought I'd say a piece of cake, didn't you?) There was only one minor gotcha with pigpio: the way it measures a pulse length. The handler has to NOT measure the length of the pulse trigger!

Fortunately, simply adding a guard variable around the echo measurement code solved that problem. Once you have your Raspberry Pi running, installing pigpio is easy: Type the following lines into the console: wget http://abyz.co.uk/rpi/pigpio/pigpio.tar tar xvf pigpio.tar cd PIGPIO make make install

You can find SPRITE related material including the demo source code at http://mikronauts.com/robot-zoo/sprite/. Click on sprite-demo.c to download the code. After installing pigpio, compiling the SPRITE demonstration software is very easy:

Type "cc -o sprite-demo sprite-demo.c -lpigpio -lrt - lpthread"

To execute the demo: Type "sudo ./sprite-demo"

Wrapping Up

I hope you enjoyed this article and that you construct your own Raspberry Pi based robot. I really enjoyed building SPRITE, and I will continue to improve it over time.

In the near future, I will add:

1. An analog-to-digital converter so I can use sensors that output an analog signal.

2. A digital compass so SPRITE can use absolute and relative compass headings.

3. A higher capacity battery pack for the motors.

4. A simple web browser based interface to the robot. 5. More demonstration code.

And much, much more!

In case you want a SPRITE of your own but prefer not to build it on a prototyping board, Mikronauts should have a SPRITE printed circuit board available — no need to cut and solder jumpers.