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https://spectrum.ieee.org/automaton/robotics/home-robots/hello-robots-stretch-mobile-manipulator

It looks so simple but it took an enormous amount of research and development to build such a useful robot that was so inexpensive at only $18,000USD per unit, lightweight, and portable. Compare that to the famous but aging PR2 which has been the mainstay of researchers for the past decade but which cost $400,000USD and weighed 225 kilograms.

Stretch has been relentlessly optimized to be the absolutely minimum robot to do mobile manipulation in a home or workplace environment. In practice, this meant figuring out exactly what it was absolutely necessary for Stretch to be able to do. With an emphasis on manipulation, that meant defining the workspace of the robot, or what areas it’s able to usefully reach. “That was one thing we really had to push hard on,” says Edsinger. “Reachability.” He explains that reachability and a small mobile base tend not to go together, because robot arms (which tend to weigh a lot) can cause a small base to tip, especially if they’re moving while holding a payload. At the same time, Stretch needed to be able to access both countertops and the floor, while being able to reach out far enough to hand people things without having to be right next to them. To come up with something that could meet all those requirements, Edsinger and Kemp set out to reinvent the robot arm.