This is a simple project that shows how to interface mbed to control the iRobot Create robot. The Create is basically a low cost Roomba without the vac parts designed for educational users. The Create has an internal microcontroller, a serial port, two reversible drive motors with feedback, and thirty internal sensors. Over the serial port you can send motor commands and read back sensor packets. The Create comes with a serial cable, but it uses RS-232 signal levels so that it can plug into a PC serial port. To control it using mbed, you need to connect to the serial port with TTL levels and provide power for the mbed module.

mbed Hardware Interface¶

Inside the Create robot cargo area is a DB25 connector that has all of these connections. With only four jumper wires you can connect everything needed. This is not a standard pinout for a serial connector, all of the pins have special uses for the Create. Pin assignments can be found in the Create User Manual.

Here are the four connections to the Create:

In the demo, a small breadboard with double sided tape was used to hold the mbed module in place. Jumper wires from the breadboard were inserted into the DB25 connector socket. Reliable connections are a good idea on anything that moves and shakes like a robot. If your jumper wires will not fit, get a male DB25 connector with solder pins and solder all of the connections.

If you have an RS232 level shifter breakout board for your mbed, as an alternative you can use the serial cable supplied with the Create, and run the robot demo code on a tether without having to mount and power the mbed on the robot. The Create serial cable has an LED display during serial data transfers. If the robot does not move running the demo code and the LEDs do not flash, you may also need a null modem adapter or cable (i.e, swaps TX and RX). The serial cable may be useful, but for initial testing only. The robot can only move a few feet before you will run out of cable. When using the serial cable, you will also need to turn on the robot first, and then reset the mbed.

mbed Create Demo Code¶

The demo code sends out serial commands to the Create. Serial commands can be found in the Create Open Interface Manual. Sensor values are also reported back over the serial port, but are not used in the short demo code. Here is a short video of the demo code running on the Create:

The demo code is shown below. Basically it waits for the Create to power up, sets the baud rate to 57600, and then starts sending a sequence of commands over the serial port with time delays between commands. The serial commands are sent in a binary format and are typically not printable ASCII characters. For robot safety, a start command must be sent before it will respond to motor commands. In normal mode, the robot motors will not run unless it is placed on the floor. IR cliff sensors on the bottom are designed to keep it from running off steps.

#include "mbed.h"

Serial device(p9, p10);  // tx, rx

// Definitions of iRobot Create OpenInterface Command Numbers
// See the Create OpenInterface manual for a complete list


//                 Create Command              // Arguments
const char         Start = 128;
const char         SafeMode = 131;
const char         FullMode = 132;
const char         Drive = 137;                // 4:   [Vel. Hi] [Vel Low] [Rad. Hi] [Rad. Low]
const char         DriveDirect = 145;          // 4:   [Right Hi] [Right Low] [Left Hi] [Left Low]
const char         Demo = 136;                 // 2:    Run Demo x
const char         Sensors = 142;              // 1:    Sensor Packet ID
const char         CoverandDock = 143;         // 1:    Return to Charger
const char         SensorStream = 148;               // x+1: [# of packets requested] IDs of requested packets to stream
const char         QueryList = 149;            // x+1: [# of packets requested] IDs of requested packets to stream
const char         StreamPause = 150;          // 1:    0 = stop stream, 1 = start stream
const char         PlaySong = 141;
const char         Song = 140;
                /* iRobot Create Sensor IDs */
const char         BumpsandDrops = 7;
const char         Distance = 19;
const char         Angle = 20;

int speed_left =  200;
int speed_right = 200;
void start();
void forward();
void reverse();
void left();
void right();
void stop();
void playsong();
void charger();

// Demo to move around using basic commands
int main() {
// wait for Create to power up to accept serial commands
    wait(5);
// set baud rate for Create factory default
    device.baud(57600);
// Start command mode and select sensor data to send back
    start();
    wait(.5);
// Move around with motor commands
    forward();
    wait(.5);
    stop();
    wait(.1);
    reverse();
    wait(.5);
    left();
    wait(1);
    stop();
    wait(.1);
    right();
    wait(1);
    stop();
    wait(.5);
// Play a song
    playsong();
    wait(10);
// Search for battery charger IR beacon
    charger();
}


// Start  - send start and safe mode, start streaming sensor data
void start() {
   // device.printf("%c%c", Start, SafeMode);
    device.putc(Start);
    device.putc(SafeMode);
    wait(.5);
  //  device.printf("%c%c%c", SensorStream, char(1), BumpsandDrops);
    device.putc(SensorStream);
    device.putc(1);
    device.putc(BumpsandDrops);
    wait(.5);
}
// Stop  - turn off drive motors
void stop() {
    device.printf("%c%c%c%c%c", DriveDirect, char(0),  char(0),  char(0),  char(0));
}
// Forward  - turn on drive motors
void forward() {
    device.printf("%c%c%c%c%c", DriveDirect, char((speed_right>>8)&0xFF),  char(speed_right&0xFF),  
    char((speed_left>>8)&0xFF),  char(speed_left&0xFF));

}
// Reverse - reverse drive motors
void reverse() {
    device.printf("%c%c%c%c%c", DriveDirect, char(((-speed_right)>>8)&0xFF),  char((-speed_right)&0xFF),  
    char(((-speed_left)>>8)&0xFF),  char((-speed_left)&0xFF));

}
// Left - drive motors set to rotate to left
void left() {
    device.printf("%c%c%c%c%c", DriveDirect, char((speed_right>>8)&0xFF),  char(speed_right&0xFF),  
    char(((-speed_left)>>8)&0xFF),  char((-speed_left)&0xFF));
}
// Right - drive motors set to rotate to right
void right() {
    device.printf("%c%c%c%c%c", DriveDirect, char(((-speed_right)>>8)&0xFF),  char((-speed_right)&0xFF),  
    char((speed_left>>8)&0xFF),  char(speed_left&0xFF));

}
// Charger - search and return to charger using IR beacons (if found)
void charger() {
    device.printf("%c%c", Demo, char(1));
}
// Play Song  - define and play a song
void playsong() { // Send out notes & duration to define song and then play song

    device.printf("%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c%c", 
                  Song, char(0), char(16), char(91), char(24), char(89), char(12), char(87), char(36), char(87),
                  char(24), char(89), char(12), char(91), char(24), char(91), char(12), char(91), char(12), char(89),
                  char(12),char(87), char(12), char(89), char(12), char(91), char(12), char(89), char(12), char(87),
                  char(24), char(86), char(12), char(87), char(48));

    wait(.2);
    device.printf("%c%c", PlaySong, char(0));
}

What is next?¶

Once you have the demo running, you can get started on more complex robotics projects. First, you would want to read the incoming serial sensor packets to use sensor data to modify the commands sent. Additional sensors could be added to the robot such as IR and Sonar to detect objects without hitting them and perhaps a wireless link. The Create is the pickup truck of small robot kits. It is not the fastest or smallest, but it can handle a lot of additional hardware in the cargo area.

Power¶

Once you decide to work on more projects a bit with the Create, it is a good idea to invest in the rechargeable battery. The low cost version comes without them and it needs 12 AA batteries. The robot can find the charger on its own, if you get the charger with the IR beacons.

With additional sensors, it is likely that you will need to provide a 5V regulated supply since the current is very limited on the internal 5V supply being used for mbed. It is already at the maximum level. This is possible without an additional battery by using the 18V unregulated battery voltage from the Create's battery. It is also available on the DB25 connector pins. Make sure that the 5V voltage regulator chip used can tolerate that high of an input voltage. The low cost linear regulator chips will overheat with this large of an input voltage. A special 5V 3A switching regulator board designed just for this purpose is available from Robotics Connection. The first version of this board had a LM 2596 switching regulator that could handle the input voltage from the battery, but there may have been some recent production changes. The data sheet on the web now says 16V max, so you may need to add a resistor or some diodes in series on the input to drop it down to those levels. Once you add a regulator, it would be a good idea to get a male DB25 connector and solder all of the connections.

A small 5V regulator board for extra power on the robot

Mounting Additional Hardware¶

It is common to mount a sheet of plastic over the cargo bay area using the 6-32 screw holes provided. Additional sensors and hardware can then be attached to the plastic sheet. Here are some examples of earlier non mbed projects using the Create.

One such project using a clear plastic sheet to mount a robot arm is seen below.

And here is another level used to mount a web camera




This one below has several Sonar and IR sensors facing in different directions. A servo rotates the IR distance sensor on top. The wireless link reports readings back to another computer that constructs a map of the area.

Here are a couple of videos of student projects showing even more hardware mounted on the Create.

Create with a windshield washer pump to put out fires.

Parts from an old printer on the Create use powder to print on the floor.