The motor gets larger
Direct drive is why the PDK uses six large-diameter, high-pole-count motors rather than six small ones behind reducers. Torque comes from radius and magnets, not from gearing.
Direct-drive 6DOF motion platform Nothing between the motor
and the load.
Direct-Drive 6DOF (DD6DOF) is a six-degree-of-freedom motion platform moved by six direct-drive motors, with no gearbox, belt or reduction stage anywhere in the load path: the architecture of ARK Dynamics' Portal Development Kit (PDK). In sim racing, “direct drive” usually names the steering wheelbase; in DD6DOF it names the six motors that move the platform. A transmission is a machine for trading precision against torque. The PDK does not make that trade.
What it removes
Every reduction stage is a place
the position goes soft.
Gear teeth have clearance, or they seize. Belts stretch under load and recover late. Ballscrews have a reversal band you can measure. None of these are defects — they are what those parts are. They only become a problem when the thing you are building is a machine whose whole job is to put a platform exactly where the solver said, and to do it again on the way back.
The boundary
Removing the gearbox moves the problem, it does not delete it.
A reduction stage multiplies torque. Take it away and the motor has to produce, at the shaft, all the torque the platform needs — and hold it while standing still, which is the case a motor likes least.
Holding still costs current
A worm drive holds a static load with no power at all. A direct-drive axis holds it with current, and current is heat.
The weight needs somewhere to go
So it does not sit on the motors. Pneumatic cylinders carry the static mass and leave the motors free to do the part only they can do — move it.
Patent CA 3217174 C covers the enhancements to the Stewart architecture that make this arrangement work. The patent is a fact; what it is worth is a judgement, and we do not make one here.
What replaces it
If there is no gearbox, what holds the position?
With nothing mechanical between command and load, the loop rate is the stiffness. These are the rates the platform actually runs at. None of them is a rate your PC or your game will see.
| Stage | Rate | What it does |
|---|---|---|
| Game telemetry | the title's own | Typically 60 Hz. Not ours, and not something we can raise. |
| Washout DSP | 20 kHz | Filters the game signal into a motion cue the platform can actually execute. |
| Game to motion | 1 kHz | The motion cue, a new pose for the housekeeper every millisecond. |
| Control loop | 3.3 kHz | One pose resolved into six leg targets. |
| Drive commands | 2.5 kHz or more | A new target for each drive over CAN at 2 Mbaud, 3.3 kHz preferred. |
| Drive loop, per motor | 8 kHz | 125 µs. Current and position closed on the encoder every single cycle. |
How we know this
- Drive loop 8 kHz / 125 µs — read off the drive firmware timer ARK drive firmware source
- Control loop 3.3 kHz ARK engineering record
- Game to motion 1 kHz; drive commands at 2.5 kHz or more per drive, 3.3 kHz preferred, the encoder returned per command rather than on a timer ARK encoder-path constraint · ARK engineering record
- Washout DSP 20 kHz ARK engineering record
Questions
What people ask about direct drive.
What is DD6DOF?
Direct-drive 6DOF: a six-degree-of-freedom motion platform moved by six direct-drive motors, with no gearbox, belt or reduction stage anywhere in the load path. It is how the PDK is built: each motor turns an arm, and a fixed-length rod carries the motion up to the top plate.
Is DD6DOF the same as a 6DOF rig with a direct-drive wheel?
No. A direct-drive wheelbase drives the steering wheel. DD6DOF means the motion platform itself is direct drive: six direct-drive motors move it. The PDK + ARK Ecosystem has both, a DD6DOF platform and the ARK direct-drive wheelbase.
What is direct drive?
The motor drives the leg without a reduction stage between them — no gearbox, no belt, no ballscrew, no coupling to wind up and give back. A transmission is a machine for trading precision against torque; direct drive declines that trade and asks the motor to produce the torque itself.
What are the disadvantages of direct drive?
Three, and taking the gearbox out moves each of them rather than deleting it. The motor has to produce all the torque at the shaft, so it is large-diameter and high-pole-count rather than small and sitting behind a reducer. Holding a static load costs current, and current is heat, so standing still is a thermal case rather than a rest state. And the standing weight has to go somewhere: pneumatic cylinders carry the static mass so that it is not sitting on the motors.
Do direct-drive motors get hot?
Holding a load still costs current, and current is heat — so standing still is a thermal case rather than a rest state. A worm drive holds a static load with no power at all; a direct-drive axis holds it with current. That is why the motors are not asked to carry the standing weight at all: pneumatic cylinders take the static mass and leave the motors free to do the part only they can do, which is move it.
Keep reading
Backlash
The specific defect direct drive exists to remove, and how it shows up in a motion platform rather than a machine tool.
21-bit encoders
Direct drive only pays off if you can measure the shaft finely enough to control it. This is the sensor that makes it possible.
The Ecosystem
What you actually receive, what it costs, and what you assemble yourself.
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).
Order a development kitAll four builds ›
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