Need suggestion on Robot to follow?

Hi folks,

I'm considering building my next generation of pull cart robots. The
first was controlled with a pic16876 and car door lock remote control.
Does anyone have a suggestion on methods and components to use for
getting the bot to follow me as I walk. I think the comercial versions
of this have a clip on device that transmits high frequency sound.
Any suggestions or good ways to approach this?
thanks
jim

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