wemakerobots
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basicBra!nz — Pathfinder

Pathfinder — a robot modelled on the Mars rover, with a holonomic drive. Four-wheel drive, 6V geared motors. The red one is also where I'm testing an encoder.

Pathfinder — a robot modelled on the Mars rover, with a holonomic drive. It has four-wheel drive on 6V motors with built-in gear reduction. As a bonus, the red robot is where I'm testing an encoder :)

About the build

The obvious inspiration here was NASA's Sojourner and Pathfinder — both built to explore the surface of Mars. They reached the planet in 1997 and sent back over 550 photos.

My build is suitably simplified — it isn't made for sand and rocks, and the radio range is decidedly shorter :)

Key features:

  • full holonomy — the robot can move in any direction at any time
  • 360o field of view from the distance sensors (mapping)
  • low power use; camera, light etc. can be turned off remotely
  • Bluetooth, RS232, Serial TTL communication
  • ADC, SPI, I2C and an extra servo connector exposed
  • wireless colour camera
  • two interrupt inputs (INT0, INT1)
  • encoder built around a reflective sensor

Holonomy here is a modified synchro-drive (wheels run at the same speed but aren't mechanically linked). Servo movement isn't coupled — each wheel can independently turn through 180o. There are convenience commands like "right" and "left" that turn the drive train by a given angle. Communication runs over Bluetooth, which makes the robot pleasant to interact with. The on-board camera is 1.2GHz on purpose — so it doesn't step on the 2.4GHz radio link.

Camera

The cameras stream MJPEG, which means no special software is needed to view the feed.

Communication

The robots use Bluetooth — to connect, you need a computer with a working Bluetooth stack and adapter. Scan for devices, then pair with "basicBRA!NZ" or "sentinel". On Windows the pairing password in both cases is "default".

Serial parameters:

  • Baud 115200
  • 8 Bit
  • no parity
  • 1 stop bit
  • no CTS/RTS

Commands

To bring the robot into a known state at the start of a session, run:

  • reboot
  • reboot
  • init

Every command must be terminated with /n (ASCII 13).

After a successful "init" the robot replies "Hello at basicBRA!NZ." From that point you can issue commands:

  • "dir": flip wheel rotation direction
  • "reset": reset servo and speed settings, stop the robot
  • "go": drive at the configured speed
  • "stop": stop
  • "back": drive backwards
  • "ff": drive forwards
  • "cam": turn on the camera
  • "ir": turn on the LED
  • "left": turn wheels left by N degrees (1-90)
  • "right": turn wheels right by N degrees (1-90)
  • "llturn": set wheels to drive in a left arc (1-24)
  • "rlturn": set wheels to drive in a right arc (1-24)
  • "zero": reset servos to driving position
  • "scan": scan surroundings with the Sharp sensor
  • "reboot": reboot
  • "x": stop robot
  • "sh": move servo and read Sharp (30-120)
  • "speed": set speed (1000 → slow, 1 → fast)
  • "ssize": set number of scans
  • "dir": change driving or turning direction
  • "adcdump": stream ADC0-3 values every 50ms until a new command is sent

An invalid command returns "ERR".

sonarView

sonarView is a "remote presence" app — you can drive every behaviour of the robot from it.

Features:

  • turn camera on/off
  • turn the light on/off
  • visualise readings from the Sharp distance sensor
  • support for Ackermann-like, differential, and "rotate-then-drive" steering modes
  • send manual commands to the robot
  • visualise the camera feed

To download the program, click the menu on the right. SonarView needs .NET 3.0 installed.

Nothing stops you from using a different framework like Pyro, Player etc.

Talking to the robot

sonarView is a .NET app, but you can equally write your own client in Python or Java.

The Python example needs the following installed (Linux skips pywin32):

After installing, here's a starter snippet that initialises the robot:

import serial
import time
 
ser = serial.Serial('com5',115200, timeout=1)  #change COM5 to actual port
#where Pathfinder is located
#creating ser object automatically opens the port
 
while 1:
    ser.write('reboot%s' % (chr(13))) #reboot robot
    ser.write('init%s' % (chr(13)))    #initialize and sync robot
    print ">>> Robot >>> %s" % (ser.readline()) #get response
    time.sleep(1)
    ser.write('reboot%s' % (chr(13)))
    time.sleep(1)

Pathfinder and swbStick

For testing, you can use swbStick with the firmware that emulates Pathfinder's behaviour.

Flash the HEX file using MCS Electronics Bootloader.

Downloads: