Choosing a robot arm for VLA research in 2026: X5, PiPER, WidowX AI and SO-101 compared
Short answer: for imitation learning and VLA work, pick an arm by five things in this order: whether a leader–follower setup exists for it, the SDK and control rate, payload and reach for your tasks, whether other labs publish with it, and the price per complete station. On published figures, ARX X5 has the highest payload of the four arms below (2 kg rated, 3 kg peak), AgileX PiPER is the cheapest finished arm, Trossen WidowX AI reaches furthest, and SO-101 is the low-cost DIY option.
Published figures
Figures are from each maker's own page on October 10, 2026. Check them again before you buy.
| Arm | DoF | Payload | Reach | Arm weight | Repeatability | Software (as published) | US list price |
|---|---|---|---|---|---|---|---|
| ARX X5 | 6 | 2 kg rated, 3 kg peak | 620 mm working radius | about 3.3 kg with gripper | Not published | Python and C++ (arx5-sdk), ROS, MoveIt | Quoted; bimanual platform $5,999 for the first 100 units |
| AgileX PiPER | 6 | 1.5 kg | 626 mm | 4.2 kg | 0.1 mm | Python API, ROS 1 and ROS 2 | $1,999 |
| Trossen WidowX AI | 6 | 1.5 kg | 700 mm | 4 kg | 1 mm | LeRobot, ROS 2, MuJoCo, OpenPi | $4,545.95 base; $4,995.95 follower with RealSense D405 |
| SO-101 | 5 joints + gripper | Not published | Not published | Not published | Not published | LeRobot | Parts list in the SO-ARM100 repository |
"Not published" means the figure was not on the maker's page we checked, not that it is poor.
How to choose
- Start from the station, not the arm. Imitation learning needs follower arms, leader arms and cameras. Price the complete station. ARX sells a complete bimanual platform with leader arms and cameras; Trossen sells leader and follower versions; SO-101 is built from a parts list.
- Check the control interface you will actually use. If you train with your own stack, a Python SDK without ROS matters. X5 has Stanford's arx5-sdk (
pip install arx5-interface) with a 500 Hz joint loop. - Size payload and reach to your tasks. A wrist camera plus gripper plus the object all count against payload. If your tasks lift tools or containers, payload margin matters more than a few centimetres of reach.
- Look at who publishes with it. Comparing results is easier on hardware other labs used. ARX arms appear in π0's training mixture, in RoboChallenge's physical evaluation platforms and in 25 public projects.
- Ask about lead time and support for your country. A cheaper arm that arrives months later can cost a semester.
When each arm fits
- ARX X5: labs that want force control, a 2 kg working payload and an arm already used in published bimanual and mobile work.
- AgileX PiPER: the lowest list price for a finished 6-DoF arm.
- Trossen WidowX AI: teams that want the longest reach of the four and the software Trossen lists.
- SO-101: teaching, hobby projects and very tight budgets, with LeRobot as the software path.
