The motors run cooler
A motor holding a static load draws current continuously and turns most of it into heat. Take the standing load off it and the thermal case becomes the motion itself, which is intermittent by nature.
Engineering · Load path
Motors move the load.
Air holds it up.
A direct-drive axis holds a static load by burning current, and current is heat. So on the Portal Development Kit (PDK) the standing weight does not sit on the motors at all — pneumatic cylinders take it, and the motors are left free to do the one thing only they can do.
The split
Holding still and moving
are different jobs.
Gravity changes only when the payload does — so it needs a spring, trimmed occasionally, not a fast control loop. Motion is the opposite: it changes continuously and needs a fast, precise, closed-loop actuator. Asking one component to do both means sizing an expensive, hot, high-bandwidth motor around a static load it never has to think about. So the PDK does not ask.
Two timescales, and the word passive belongs to only one of them. Between motion events the compensation is active: the Housekeeper reads the current the motors are drawing, and the compressor and valve manifold trim cylinder pressure until that current falls to near zero — the behaviour patent CA 3217174 C describes as detecting the current drawn by the positioning actuators and pressurising the cylinders to minimise their load. During motion nothing is being regulated: the valves are closed and the cylinders behave as air springs, which is why the legs below are called passive during motion rather than passive outright. Both halves matter — a spring alone would be wrong for every payload but one, and a fast pressure loop would put valve latency in the motion path.
Maglev
Maglev makes it feel weightless.
Maglev is our name for the technology that makes the platform feel weightless. The drive firmware's maglev maps cancel friction and gravity across the travel: a measured map of 28,800 values (3,600 positions × 4 bands × 2 directions). On the wheel, the friction left over is 0.00435 N·m against a 0.005 N·m pass mark.
What it buys
What does carrying the weight with air actually buy?
The torque is spent on motion
Whatever torque is not committed to holding the platform up is available to move it. The headroom goes into the part you can actually feel.
On power loss, designed to settle
A pneumatic cylinder does not need electricity to keep pushing. Remove power from a platform whose weight is carried by air and it is designed to settle rather than drop — a property of the load path, not of a safety routine that has to run correctly.
Payload is published as up to 250 kg.
The arrangement
Nine legs. Six of them move you.
The load path is easier to picture once you count the legs. Six are motor-driven and produce every degree of freedom; three are pneumatic and do nothing but hold the platform up. They are arranged together, not bolted on afterwards.
| Group | Count | Role |
|---|---|---|
| Servomotor legs | 6 | Directly coupled rotary actuators. All active positioning and every degree of freedom come from these. |
| Pneumatic legs | 3 | Part of the automatic weight-compensation system. Passive during motion — they carry, they do not steer. |
| Total elements | 9 | Arranged axially symmetric, in three clusters of three connection points. |
How we know this
- Six actuated legs is what makes it a Stewart platform — six legs, six degrees of freedom the architecture itself · /technology/stewart-platform/
- Three pneumatic legs, nine elements in three clusters, components tilted off vertical to widen ball-joint range and fit a 1.00 × 1.00 m platform footprint ARK architecture write-up
Questions
What people ask about carrying the weight.
Why doesn't the platform just use stronger motors to hold the weight?
Because a motor holding a static load draws current continuously and turns most of it into heat, so a motor sized around a load it never has to move is an expensive way to build a warm machine. Gravity changes only when the payload does; motion changes continuously. They are different jobs, so they are given to different parts — pneumatic cylinders take the standing weight, and the motors are left with the one thing only they can do.
How many legs does the PDK have?
Nine. Six are motor-driven and produce every degree of freedom; three are pneumatic and do nothing but hold the platform up.
What happens if the power fails?
A pneumatic cylinder does not need electricity to keep pushing. Remove power from a platform whose weight is carried by air and it is designed to settle rather than drop — a property of the load path, not of a safety routine that has to run correctly.
Is it a gravity-compensation system?
It is a pneumatic gravity-compensation arrangement, which the wider field also calls a pneumatic counterbalance. The term covers several mechanisms — springs, counterweights and cams among them — and this one uses air.
Keep reading
Direct drive
Why holding a load with current is a problem worth designing around in the first place.
Field-oriented control
What the motors do with the torque this arrangement frees up.
Patents
Patent CA 3217174 C and what it actually covers.
Every development kit starts with this platform. The PDK + ARK Ecosystem development kit finishes it — racing chassis, wheelbase, wheel, load-cell pedals and button box, $8,499.99 (development-kit price).
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Built in sequence for qualified recipients: ready to ship an estimated 3–6 months from order confirmation; delays of up to 12 months are possible