Read all twelve Doosan cobots at once and one rule appears: precision is set by arm length, not by payload

Doosan Robotics sells twelve collaborative arms, and its own line-up page lists each of them with exactly three numbers: payload, reach and repeatability. Put side by side, those thirty-six numbers say something the product pages never state — the third number depends only on the second. Every Doosan arm with a 900 mm reach is quoted at 0.03 mm repeatability. That holds for the A0509 lifting 5 kg, for the force-sensing A0509S, and equally for the M1509, which lifts fifteen kilograms — three times the load, same reach, same precision. The arms that reach 1,200 or 1,300 mm are all quoted at 0.05 mm: the 9 kg A0912, its S and F variants, and the 10 kg M1013. And every long arm is quoted at 0.1 mm: the 20 kg H2017 at 1,700 mm, the 25 kg H2515 at 1,500 mm, and the new P3020, which carries 30 kg to 2,030 mm. The intuitive expectation is the opposite. A buyer asked which of two cobots will be more accurate usually reaches for the lighter one, on the assumption that heavy payloads bend things. Doosan's own catalogue says the load is not what costs you accuracy — length is. A small angular error at the shoulder becomes a large error at the tool, and it grows with the lever arm. Payload is handled by stiffer joints and stronger motors; leverage cannot be engineered away. There is exactly one exception in the line-up, and it is instructive. The E0509 also reaches 900 mm, but repeats to 0.05 mm rather than 0.03 mm. It is the sealed food-industry model, rated IP66 so it can be washed down without a protective jacket — and, as this catalogue noted earlier, sealing the joints is what costs it those two hundredths of a millimetre. It breaks the rule for a reason that has nothing to do with geometry. Seen through this lens, the P3020 that Doosan unveiled at Automate 2024 is not a machine that compromised on precision. It is the least precise arm the company sells, at 0.1 mm, because it is the longest — and it is aimed at stacking cartons on pallets, where a tenth of a millimetre is far below anything the job can notice. The same page shows why the shortest arms are the precise ones and why no amount of paying more will buy 0.03 mm at two metres of reach. None of this is a criticism of the manufacturer's disclosure: the numbers are published openly and consistently, which is what makes the pattern readable at all. It is a reminder that a specification table read one row at a time answers a different question than the same table read as a whole.