Low-cost dead reckoning scheme for autonomous lawn mower

Hi, I am working on an autonomous lawn mower with image processing (via RPI) and ultrasonic sensor to detect the boundaries and obstacles, so there is no need for laying down ground wire like conventional robot mowers. One issue I am trying to figure out is the mapping of the lawn to make it more efficient. I understand that pure accelerator/gyroscope based dead reckoning is not very accurate. What about with the addition of the ultrasonic sensor and wheel rotation? There can also be a home charging station as a fixed reference point (although it is not necessary because eventually I want to recharge it via solar). GPS and LIDAR are beyond the budget of this project. Any advice on a low-cost dead reckoning scheme that is somewhat accurate is much appreciated.

pure accelerator/gyroscope based dead reckoning is not very accurate

Won't work at all, in fact. You can read about why not here, but the problem is much, much worse in an uneven, noisy, vibrating environment.

Wheel encoders are helpful at measuring distance traveled, but if the wheels slip, forget it.

The "mapping" is quite a difficult problem, so if you are serious about that aspect of the project, solve it before worrying much about the other, much easier aspects.

If there was a 'low cost' way of driving a lawn mower (or other similar device) by some form of dead reckoning you might think it would have been implemented by now.

Why would people go to the time and expense of putting wires into a lawn if there was some other 'low cost' way of avoiding this ?

I know little about this but when you already have a RPi and camera onboard the optical recognition of some known objects (either natural like trees or some added for this purpose) may be possible.

srnet:
If there was a 'low cost' way of driving a lawn mower (or other similar device) by some form of dead reckoning you might think it would have been implemented by now.

Why would people go to the time and expense of putting wires into a lawn if there was some other 'low cost' way of avoiding this?

Got it in one!

+1

Perhaps you could put some light beacons (equivalent of light-houses at sea) at a few locations around the edges of the lawn and derive a position from them. It's not dead reckoning but it could be cheap and simple to implement.

...R

Robin2's idea worked for me. using encoders ,webcam, gyro and light beacon my mower can
mow an open field.

YouTube Mower video