Push, don’t pull

There are two basic ways to move a finger from somewhere else. You can pull it with a tendon, the way human forearm muscles do and the way Tesla’s Optimus and the Shadow Hand do. Or you can push it with a rigid link. The linkage route puts every motor inside the hand itself: actuators sit in the palm, and stiff bars carry the motion across the knuckles.

The trick that keeps the motor count down is coupling. In a typical linkage finger, a four-bar mechanism ties the middle and tip joints to the base joint, so one actuator curls the whole finger along a fixed path, much like a person closing a fist without thinking about each knuckle. That’s how Inspire’s RH56DFX gets 12 joints out of 6 actuators: one per finger, plus two for the thumb’s bend and swing. You lose independent fingertip control. You gain a hand that’s light, cheap enough to sell in volume and simple to bolt onto any arm, because nothing has to run through the wrist.

Inside a micro linear servo

Open up one of the palm actuators and you’ll find four parts in a row. Inspire describes its LAF series as a coreless motor, a precision planetary reducer, a precision screw mechanism and a sensor, all wrapped in its own closed-loop controller. A built-in absolute position sensor means the actuator knows where it is the moment power comes back.

Each part earns its place. The coreless motor has no iron in its rotor, which makes it light, low in inertia and easy to build in very small diameters – Zhaowei and ARC Advisory both single out fast response and compact size as why hand designers pick it. The planetary stage trades speed for torque. Then the lead screw converts rotation into a straight push with a big mechanical advantage, so a motor the size of a lipstick can press a fingertip with several kilograms of force.

The screw has a second job nobody puts on the box. A fine-pitch lead screw generally can’t be turned backwards by pushing on the nut: friction in the thread is higher than the torque the load can create. So when an object presses against the fingers, it can’t spin the motor. Inspire lists “power off self-locking” on the RH56DFX, and its quiescent draw is just 0.09–0.1 A at 24 V against a 2 A peak. A hand holding a tool all shift barely sips power once it has closed.

What the numbers look like

Linkage hands come in two broad flavours. The cost-down industrial kind uses five or six actuators and coupled fingers. The high-DoF kind packs an actuator behind almost every joint. Unitree’s Dex5-1 sits apart: it puts hollow-cup (coreless) motors and micro gear transmissions into the joints themselves rather than pushing rods across them.

Ten newtons at a fingertip is about one kilogram of force. That’s enough to pinch a phone or a part off a tray, not enough to wrench a stuck valve. It’s also a choice, not a limit of the linkage idea. The ILDA hand, published in Nature Communications in 2021, packs 15 DC motors with ball screws into a 1.1 kg hand with no forearm module and reaches 34 N at the fingertip in a bent pose.

Three routes, and the supply chain behind them

ARC Advisory sorts Chinese hand makers into three camps. Coreless motors with micro reducers and tendons give the most human-looking fingers and the finest control, but limited continuous torque and higher cost – Linker Hand is its example. Micro servo electric cylinders, often with iron-core motors and screws, give more grip, better durability and easier mass production at the price of slimmer fingers – InTimes Robotics. Hybrid tendon designs, with motors in the palm or forearm, are simple to build but have cables that stretch and wear, which DH-Robotics manages with tension compensation.

In practice the lines blur. The China Humanoid Robotics Tracker notes that Jiangsu Leili runs linkage, tendon and servo-cylinder hands side by side, that LinkerBot sells all three routes, and that hybrid designs – tendons plus rigid structure – are becoming the industry consensus. Component makers lean toward linkages because they’re stiff, precise and easier to cost down.

Whatever the route, the hand is a micro-motor market. 36Kr estimates a dexterous hand needs about 20 coreless motors and puts coreless motors at roughly 12% of a $20,000 humanoid bill of materials. Maxon and Faulhaber are the long-standing benchmarks. Chinese makers are pricing hard against them: the Tracker cites Moons’ coreless motors at ¥1,200–2,300 against ¥4,000 and up for foreign parts, and Aoyi developing its own motors at around ¥100 a unit. Zhaowei sells complete drive modules that integrate micro motor, gearbox and lead screw. At the finished-hand level, LinkerBot’s six-DoF O6 lists at ¥6,666 – roughly $920.

What to ask a hand supplier

• How many actuators drive how many joints. “12 joints” and “6 DoF” can describe the same hand.
• Fingertip force and grip force separately, with the finger pose they were measured in.
• Whether fingers self-lock with power off, and what happens to the screw when the hand takes a side impact.
• Rated grip cycles. Leisai quotes more than one million for its DH2015; many suppliers quote nothing.
• Where the micro motors and screws come from, and whether a second source exists.

FAQ

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