Let's Make Robots!

Outdoor navigation

Hi all,

Modest as I am, the decision has been made to build an outdoor autonomous robot which will mow my lawn. :-D

The environment:
The garden is 50*50 meter (150*150 feet) with the house somewhere in the middle, completely fenced off with a gate. 2/3rd covered by grass in different sections separated by driveway,paths/house/garage. Altitude difference is 6m (18 feet) between lowest and highest point. Maximum slope is 30 degrees (containing grass). Soil, no rocks.

I would like your input on the following thoughts regarding the autonomy and navigation of the robot.

1) bumpers
Allways good to have for situations where new or moving objects 'magically' appear. A gotta have.

2) map
I plan to let the robot build up a map of it's environment. It will use it to 'plan' a route (e.g. when it decides to do the driveway sides of the garden) as well as 'discover' it's world. It will also keep track of obstacles being found and storing these on the map. Reoccurrences imply static objects. Others might be movable objects (important for path planning)

3) odometer
Use dead-reckoning with the odometer and the map to move around

1-3) combined
give a reasonable system to move around but the disadvantage is the 'drift' which unavoidably slips into the system. Imagine the robot sliding a bit on wet grass when it is moving on a slope. Worst case the drift accumulates (it does NOT average out).

So I need to have a system which will enable the robot to sync it's location one way or the other
5) compass
I have a digital (cheap) compass lying around that just begs to be hacked. It 'might' improve the precision of the whole system a bit but it doesn't solve the problem. I am even thinking of using it to not randomly have the lawn mowed but more 'controlled', just another topic

6) gps
have an old serial gps lying around as well. Definately not precise enough but it will prevent the lawnbot to leave the premises go to the neighbours and declare love to the big-assed sit on lawnmower they own. It will allow the robot to more or less find it's way back to it's docking station (within a couple of meters) if it gets completely lost.

1-6) combined
will already give an interesting system as the dead reckoning with compass and gps will enable the mapping software to determine the probability of 'being' at a certain location.

7) But it would be better if that could be verified.
Now I could use proximity sensors (IR, Ultrasonic, ...) to sense the environment and 'see' if the navigator got it right and matches it with the reading of the sensors, but you run the risk it a) looses track of where it is b) somebody kicks a ball around in the garden.... and it will be utterly confused.

next idea was RFID tags buried in the garden at certain locations. It will help synchronise. Seems awfully complex to me.

9) beacons
nice idea but
9a) IR/laser/light beacon: do they work outside? at what range? my garden has quite some altitude differences and the accompanying slopes so it's not the lab environment where the IR beam is exactly at a certain hight relative to the robot.
9b) laser. seems complex as well. i have seen solutions with moving mirrors and such but in the climate here (alps) it gets darn cold in winter and hot in summer. just seems to fragile with me. another issue is safety. lasers are NOT stuff to mess with.
9c) RF beacons. Timing is too critical.
9c) ultrasonic beacon. range? drive my dog nuts?

10) electric fence buried in ground
good idea but a lot of work and we have a lot of rodents in the garden that seem to love eating cables. besides that I see an electronic fence as a 'reverse bumper'. it's good that it's there but if navigation and location are correct you should never be using it.

I am still fascinated by a beacon systems to synchronise, either by active beacons (transmitting constantly or at regular intervals (5-10 minutes) or 'passive' ones which will transmit when a signal is received from outside.

Another thought is to have the robot actively search for 'recogniseable' points (like the docking station) to return to regularly (or when in the area) to synchronise again. 

 Now my question is simple: what are your thoughts and experiences?


P.S. the mowing part I will build in last as I want to make sure that it is all properly working. My wife has added a rule to the 3 robotics laws of Asimow and calls it law 0: mowed flowers equals instant death.

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.

Servus! (I am dutch and living with my english wife in the alps in Germany)

No worries, that's exactly my plan. I am doing a lot of research myself as well before I build anything, but will definately report back on progress and achievements as well as failures.

I would not call it stealing. It is just incorporating the ideas of others. Let's make robots!


Preceeding outdoor robots :

Mower robots from Friendly Robotics use a buried "wire fence" around the perimeter of the area they are to mow. Think a small oscillation is on the wire, so that it is more easily detected by the mower, and it is recommended that a perimeter drive of the robot occurs before it is allowed to zigzag randomly across the remaining area. Bumper sensors and tilt sensors (to shut off blades if overturned).

Outdoor point drive robot contestants like jbot use a combination of odometry (which looses a little track) and IMU data to maintain fine (few inches) location, while GPS provides a more general location within perhaps a 10 foot area. 

A quick personal test on a GP2D12 sensor out pointed towards the sun, showed little if any reduction in measurement capabilites, though others claim they get smamped. Simply hook a sensor to a 4 AA battery pack, and measure output voltage with a meter outside for your own tests. GP2D12s are more short range, could try GP2D0Y02YK for more distance.