The Design of Stretch: A Compact, Lightweight Mobile Manipulator for Indoor Human Environments
TL;DR The design behind the Stretch RE1, a home mobile manipulator that moves the end of its arm in x, y and z with just four actuators (a differential-drive base, a lift and a telescoping arm). It is 34 cm wide and weighs 23 kg. A static-stability model predicts its worst-case payload at full reach as 3.47 kg, against 3.46 kg measured.
What they do
The authors argue that smaller, lighter and cheaper mobile manipulators would see more use in homes. They present the design of Stretch (the Stretch RE1 from Hello Robot) and justify it with anthropometry, static-stability models and tasks in real homes. Stretch is 34 cm wide, weighs 23 kg and costs about $20,000. HSR, Fetch, TIAGo and PR2 are 43-67 cm wide and weigh 37-220 kg.
How it works
- Four main joints. A differential-drive base, a lift and a carbon-fiber telescoping arm move the end of the arm in y, z and x, so the Jacobian is the identity. Only the lift works against gravity.
- Two modes. For navigation, the arm retracts into the base footprint. For manipulation, the arm extends sideways and the base drives it left and right. A 330° wrist yaw stows the tool. The optional Dex Wrist adds pitch and roll.
- Matched to people. Reach (71 cm) and camera height (131 cm) come from human body measurements. The gripper is adapted from a commercial reacher-grabber. Low gear ratios let motor current detect contact.
- Stability models. Closed-form tipping limits give maximum payload as a function of reach, and maximum push or pull force as a function of arm height.
Results
- Worst-case payload at full reach is 3.46 kg measured, against 3.47 kg predicted.
- Measured pulling forces track the model. They cover assistive tasks from the literature, such as opening drawers (under 20 N) and lifting objects people with disabilities want retrieved (under 1.2 kg).
- Against a force gauge fixed to the floor, with force limits disabled, the robot pulled over 70 N and lifted over 60 N, so stability, not the actuators, is the limit.
- 19 tasks, 17 of them in homes: 13 teleoperated and 6 autonomous.
Limitations
The paper has no limitations section; these points come from its own analysis. Payload and force are capped by stability and shrink with reach and arm height. The stability model assumes the center of mass is midway between the drive wheels. Sideways motion of the end of the arm requires driving the base. Most tasks were teleoperated by an expert, and no success rates are reported.
Figures
Abstract
Mobile manipulators for indoor human environments can serve as versatile devices that perform a variety of tasks, yet adoption of this technology has been limited. Reducing size, weight, and cost could facilitate adoption, but risks restricting capabilities. We present a novel design that reduces size, weight, and cost, while supporting a variety of tasks. The core design consists of a two-wheeled differential-drive mobile base, a lift, and a telescoping arm configured to achieve Cartesian motion at the end of the arm. Design extensions include a 1 degree-of-freedom (DOF) wrist to stow a tool, a 2-DOF dexterous wrist to pitch and roll a tool, and a compliant gripper. We justify our design with anthropometry and mathematical models of static stability. We also provide empirical support from teleoperating and autonomously controlling a commercial robot based on our design (the Stretch RE1 from Hello Robot Inc.) to perform tasks in real homes.
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