Robot hand simulator: pose all 22 joints of the XH-22

What you’re looking at

The XH-22 is a human-size right hand made from 34 printable parts. Nothing inside the hand has a motor. The 22 MG90S micro servos sit on two decks behind the wrist, eleven rows of two, and each one pulls braided Dyneema line through a PTFE sheath to exactly one joint. No coupled fingers, no shared drive. That’s why the lab gives you 22 independent sliders instead of five.

Four fingers get four joints each. The thumb gets five. The last one is easy to miss: a pivot at the base of the little-finger metacarpal that cups the palm, the way your own palm hollows when thumb and little finger meet.

Source: XH-22 design files (parametric OpenSCAD, STEP export). 29 tendon strands in total.

Why the joints don’t fight each other

Most tendon hands have a coupling problem. Bend the knuckle and the tendons for the fingertip get dragged along with it, so the controller spends its life correcting. The XH-22 gets around most of that with geometry. Every joint pivots on split stub pins, which leaves the centre of the joint open, and the tendons for the joints further out pass straight through that centre. Move one joint and the others’ tendons barely change length. The leftover error is the radius of the tendon bundle itself – under a millimetre.

Flexors wrap a pulley plate that sits concentric with the pin, 4.7 to 8.5 mm in radius. Tendon travel is simply r·θ across the whole range. Linear, and easy to calibrate.

Side-to-side joints and the palm arch run on loops instead: one servo, two strands on a double capstan, driven both ways. The Bowden sheaths stop at the root of each finger module, so bending the wrist or a neighbouring finger leaves a tendon’s length alone, to first order. Friction still climbs with every bend in a sheath, so on a real build they want long, smooth arcs down the forearm.

What you can do in the lab

Start with the presets. There are 19, from the Humanoid Guide pose the lab opens in to the four thumb-to-fingertip pinches that came out of an opposition search over the thumb’s workspace. Keys 1 to 0 cover the everyday ones, G brings back the brand pose.

Then take a joint by hand. Each slider carries its PCA9685 board and channel, and hovering one lights up its tendon and capstan in the 3D view. Motion is rate-limited to MG90S speed, so the hand can’t move faster than the real servos would. The telemetry table shows every servo’s angle, pulse width in microseconds and tendon take-up, and the scope plots the last eight seconds.

• Sequences: a finger wave, counting to five, an opposition tour, piano taps, a power grasp, rock-paper-scissors and a full sweep of every actuator.
• Keyframe recorder that exports a 50 Hz CSV of pulse widths for all 22 channels, ready to stream to two PCA9685 boards.
• X-ray shells, true section cuts with solid caps, an exploded view and a left-hand mirror.
• Pad-gap gauges for thumb opposition and a collision meter that turns red the moment two links touch.

Two poses are hard limits of the geometry, not bugs. At a full fist the fingertips meet the palm cover. Drive two neighbouring fingers towards each other past about 10° of abduction and they collide, so spreading has to be coordinated in software. The collision meter will tell you when you’ve found the edge.