Servo - January 2014

To summarize what was decided so far:

Mikronauts "EZasPi" prototyping board specifically to make it very simple to build circuits that need to connect to servo connectors, headers, screw terminals, and 300/600 mil DIP chips. The short bus lines do most of the wiring for you, as long as you place the components carefully. First, I added the upper resistors in the voltage divider for all eight Pi GPIOs that I wanted to bring out to a servo header. The placement of the parts is critical in order to minimize the number of jumpers that will be needed.

Next, I added the 4.7K SIP resistor network, placing the common lead farthest from the edge of the prototyping board. The 3x8 servo header was next; I located it so that two rows of pins were placed into two of the power busses, and the third pin into the bus connecting to the corresponding resistor.

1. Use a voltage divider for 5V to 3.3V bi-directional interfacing. 2. Use continous rotation servos for the drive motors and a standard servo for panning. 3. Use a regulated 5V power bank to power the Pi and a 4.8V NiMH battery pack for the motors.

4. Use an Ardubot chassis.

5. Use a SeeedStudio ultrasonic range sensor.

Building the Simple Raspberry Pi Robot Controller

I will freely admit that I am not particularly fond of cutting and stripping a lot of wire, so I designed the

Photo 6: EZasPi prototyping board.

SPRITE Demonstration Code How to Program SPRITE

My demonstration program for SPRITE is unimaginatively called demo.c. After initializing the robot, it presents a simple text menu of actions: You can program SPRITE in a number of ways:

1. Local console using a USB keyboard and HDMI monitor.

2. Local desktop using a USB keyboard, mouse, and HDMI monitor.

3. Remote console using SSH.

4. Remote desktop using the RDP protocol.

The forward/backward/left/right commands take an optional single digit argument which specifices how many seconds to do the action for. If a digit is not specified, one second is assumed.

The sonar scan demonstrates using the panning head to look in different directions, and it shows the range to the closest object at each servo position it tries.

If you use a local interface, just plug in your keyboard, mouse, and monitor, and type away. This works, but you have to plug in a keyboard and monitor (and a mouse for a graphical desktop, so if you are using Wi-Fi you also need a powered hub), then shut down, unplug it all, run the test, re-plug ... rinse and repeat for debugging ... it’s very time consuming.

The random walk code keeps going forward until the range sensor finds something closer than 30 cm. Then, it turns left in place until it finds a direction with at least 50 cm to travel.

while (1) {
  while (last_range > 30) {
 ping();
 delay(100);
 servo(LMOTOR, 500);
 servo(RMOTOR, 3000-500);
  }
  puts("Obstacle detected");
  while (last_range < 50) {
 ping();
 delay(100);
 servo(LMOTOR, 500);
 servo(RMOTOR, 500);
  }
}

You will notice that if you go forward or reverse, the robot may not travel in a straight line. This happens due to variations in servo motors, and can largely be compensated for by adding a compensation factor to the code.

My personal preference is a remote console over SSH. I use the excellent free Pu TTY SSH client, and I connect to the dynamic IP address my router assigns to SPRITE. Fortunately, my router has a page that shows the IP address of attached devices, so I do not have to use a static IP address (and the more complicated Wi-Fi configuration it requires). You can get the latest version of Pu TTY at www.chiark.greenend.org.uk/ ~sgtatham/putty/ download.html. Android and iOS SSH apps are available in their respective markets, and Linux distributions tend to come with SSH clients pre-installed.

You can easily connect to a remote Raspbian desktop running on SPRITE. All you need to do is install XRDP as shown earlier, and use the remote desktop client supplied with Windows (or any other RDP client) or the free clients available for Android and iOS devices. You can then open a terminal on the remote desktop and away you go!