Direct drive
No gearbox, no belt, no ballscrew — what that removes from the load path, and what it asks of the motor instead.
Engineering · RSCV021
Six motors.Six encoders.Air carries
the weight.
The Portal Development Kit is a six-degree-of-freedom parallel robot built on patented adjustments to the Stewart platform (CA 3217174 C). Direct-drive motors move the legs, pneumatic cylinders carry the static weight, and high-resolution encoders close the loop fast enough that the body reads the result as one event.
Authority
What we claim, and how tightly.
Four figures. Each carries its source.
6
independent direct-drive actuators
2 to the power of 21
encoder counts per revolution
8kHz
internal control loop · 125 microseconds
<1ms
game telemetry to motion
Loop rates are internal control-loop figures. They are not the rate at which a game or PC receives telemetry from the platform.
Motion
Six degrees of freedom.
The platform moves along three axes and rotates about three. The PDK uses a right-handed coordinate system. Below, one render takes each axis on its own out to its nominal limit, through to the other and back — scroll to move it.
Every one of the six legs contributes to every one of the six motions. That coupling is what makes a parallel robot stiff and fast — and what makes its control mathematics interesting (see kinematics).
Rendered from the RSCV021 CAD at the single-axis limits in the envelope table. The sweeps are illustrative, not recorded telemetry.
01 · Translation
Surge
Front and back, along the longitudinal axis.
02 · Translation
Sway
Side to side, along the transverse axis.
03 · Translation
Heave
Up and down, along the vertical axis.
04 · Rotation
Roll
About the longitudinal axis.
05 · Rotation
Pitch
About the transverse axis.
06 · Rotation
Yaw
About the vertical axis.
Actuation
Direct drive.
Each leg is driven directly by a high-torque outrunner motor — six custom 132 mm brushless DC motors at 53.3 V, Kv 13.5 — through reinforced shafts and application-specific end plates. There is no belt to stretch, no gearbox to introduce backlash, nothing that wears into play over time. On the RSCV021 build each motor is a cast-aluminium bell on a steel drum with 30 magnet bars, SKF bearings and the 21-bit encoder on a flex PCB.
The consequence for motion fidelity is simple: when the controller asks for a small correction, the platform makes exactly that small correction, immediately, with nothing in between to absorb it.
Worked example
- A belt drive with 0.5 mm of accumulated slack must first take up that slack before the platform moves — a dead zone on every reversal.
- A direct-drive leg reverses the moment the current reverses. The only "slack" is the encoder's resolution, which is 0.00017°.
Feedback
21-bit position sensing.
Each motor carries a 21-bit magnetic encoder: 221 = 2,097,152 discrete positions per revolution, or 0.000171661° per count. Encoder data reaches the motor controllers over SPI; the controllers run field-oriented control with integrated PID loops and talk to the system over CAN bus.
Resolution this fine does two things. It lets the controllers correct errors long before they grow into anything you could see or feel, and it gives the platform a positioning repeatability that makes it useful as a research instrument, not only as a simulator.
Per revolution
2,097,152
discrete positions, every motor
0.000171661deg
per encoder count
SPI to the controllers · CAN to the system
Kinematics
Stewart platform mechanics.
A Stewart platform is a hexapod: six actuators between a fixed base and a moving top. Inverse kinematics — given a desired pose, what length should each leg be? — is straightforward and is solved every cycle to turn a six-dimensional trajectory stream into six joint targets.
Forward kinematics — given six leg positions, where is the platform? — has challenged roboticists for decades because there is no closed-form solution. ARK's solver makes it fast enough to use in the loop.
| Forward kinematics | Figure | Source |
|---|---|---|
| Random pose calculation | 207 µs average | ARK engineering record |
| Sequential pose processing | 109 µs (≈9,174 / s) | ARK engineering record |
| Arithmetic precision | double, 1 × 10−14 | ARK engineering record |
| GPU throughput (Nvidia Quadro M6000) | > 1,200,000 / s | ARK engineering record |
Fast forward kinematics opens the door to task-space control architectures — controlling the platform in the coordinates the rider feels, rather than only in leg lengths. Today the motion control runs in joint space: six independent linear controllers in a decentralised PID structure, each fed by its own high-resolution encoder, at an 8,000 Hz internal loop on two ARM Cortex-M7 processors at 600 MHz.
Patented
Passive load compensation.
Holding a person and a platform up against gravity is constant work. If the motors do it, they spend most of their thermal budget standing still. The PDK instead carries the static load on air: three pneumatic cylinders (50 mm bore, 140 mm stroke, up to 6.0 bar) with dual dead-volume tanks per cylinder, an integrated compressor and a 10-station valve manifold, and automatic load monitoring that re-balances as the payload changes.
With gravity negated, the platform floats. The motors then handle only the difference between where the platform is and where the trajectory wants it — which is why a 15 W static hold and a 30 kW burst can live in the same one-square-metre machine.
15W
static hold, platform up
30kW
burst, on demand
6.0bar
maximum cylinder pressure
Three coupled energy systems
- Pneumatic — air springs carry the static pose force. Passive during motion; the compressor trims pressure between events, not while the platform is moving.
- Electromagnetic — a capacitor bank (3 × 0.47 F at 63 V = 1.41 F) sits behind the drives. Deceleration pushes kinetic energy back into the bank through the motors; the next acceleration draws it out. Energy recirculates instead of being dumped as heat.
- Mechanical — the platform mass and leg compliance store strain energy through each stroke.
- Framing: ARK engineering record ("gravity-negated floating mass in a recirculating electromagnetic energy field").
Comparison
Compared with conventional platforms.
Belt, gear and actuator rigs against the PDK, property by property.
| Property | Belt / gear / actuator platforms | PDK |
|---|---|---|
| Transmission | Belts, gears or ball screws — backlash and wear | Direct drive — no intermediate transmission |
| Static load | Carried by the actuators (heat, derating) | Carried pneumatically (15 W hold) |
| Position feedback | Typically 14 bit | 21 bit (2,097,152 counts) |
| Motion bandwidth | 20–100 Hz typical | 1 kHz. Vibration up to 629 Hz, measured with up to 250 kg on board · up to 2,000 Hz on each independent motor. See direct drive. |
| Footprint | Varies; often > 2 m² | 1.00 × 1.00 m; 1.57 m in diameter with the extension feet |
| Energy path | Braking energy dumped as heat | Capacitor bank recirculates braking energy |
The "conventional" column describes typical consumer and prosumer motion rigs and is illustrative, not a claim about any named product.
Envelope
Motion envelope.
One envelope is published. The nominal envelope is the single-axis limit with every other axis at neutral: the axes trade against each other, so no axis reaches its limit while the others are working. A combined-axis extended envelope is larger and real, but it is not published — see the note below the table.
The gauges show the nominal envelope on one shared scale; the table beneath carries it axis by axis, with the source for every figure.
Nominal envelope · single axis
- Surge−75.49 … 82.74 mm
- Sway± 71.92 mm
- Heave± 58.68 mm
- Pitch−7.98 … 8.09°
- Roll± 8.68°
- Yaw± 9.80°
| Axis | Range | Total span | Source |
|---|---|---|---|
| Surge | −75.49 … +82.74 mm | 158.23 mm | T1 boot banner |
| Sway | ± 71.92 mm | 143.85 mm | T1 boot banner |
| Heave | ± 58.68 mm | 117.35 mm | T1 boot banner |
| Pitch | −7.98° … +8.09° | 16.07° | T1 boot banner |
| Roll | ± 8.68° | 17.36° | T1 boot banner |
| Yaw | ± 9.80° | 19.60° | T1 boot banner |
Where these numbers come from
- Read from the T1 firmware boot banner, 2026-07-07 capture — the firmware carries no envelope literals and computes its limits at boot from the platform geometry ARK firmware source · transcribed into the RSCV021 live viewer
- Single-axis limits. The axes trade against each other, so no axis reaches its maximum while the others are working parallel kinematics
- The firmware refuses an out-of-envelope pose rather than clamping it — the inverse kinematics returns an error and no motion happens ARK firmware return codes
All six figures are read from the T1 firmware boot banner, 2026-07-07 capture. The firmware carries no envelope literals: it computes its limits at boot from the platform geometry and refuses an out-of-envelope pose. A combined-axis “extended” envelope is larger, and it is published when it is measured.
Specifications
The platform, line by line.
| Architecture | Stewart platform · six independent direct-drive actuators · patented adjustments (CA 3217174 C) |
|---|---|
| Control | Joint-space decentralised PID · 8,000 Hz internal loop · 2 × ARM Cortex-M7 @ 600 MHz |
| Feedback | 6 × 21-bit magnetic encoders · SPI to controllers · CAN bus system link |
| Drives | 3 × dual-channel BLDC controllers · 53.3 V · 56 A per motor channel to the ArkDrive · field-oriented control |
| Load compensation | 3 × pneumatic cylinders 50 mm × 140 mm · 6.0 bar max · automatic load balancing |
| Power | Universal auto-switching PSU, 100–290 V AC: 3,000 W on 200–290 V, 1,400 W on 100–130 V · 3 × 63 V 0.47 F capacitor bank · >30 kW burst · 15 W static hold |
| Safety | 2 × mmWave presence radars standard · up to 6 for 360° coverage · emergency stop |
| Footprint | 1.00 × 1.00 m platform · 1.57 m in diameter with the extension feet · retracted height 0.50 m |
Questions
Frequently asked.
Is the platform really "magnetically levitated"?
No, and we no longer describe it that way. The static load is carried by pneumatic cylinders; the motors are electromagnetic, but they move the platform rather than hold it up. The older wording is being retired.
Does an internal 1 kHz or 8 kHz loop mean my game gets telemetry that fast?
No. Those are internal control-loop rates inside the platform. What your game or PC exchanges with the platform depends on the software path (for example FlyPT Mover, a third-party tool) and is a separate, lower figure.
Which envelope should I design my cockpit around?
Around every pose the platform can reach. The single-axis figures on the technology page are the nominal envelope, and combined moves reach further, so leave clearance beyond them.
Why does forward kinematics matter for a simulator?
Because it lets the controller reason in the rider's coordinates — surge, sway, heave and the rotations — instead of only in leg lengths. That is the foundation for task-space control and for reporting where the platform actually is.
Provenance
References.
-
Patent CA 3217174 C — Canadian Intellectual Property Office. Granted 11 February 2025; the grant and the official register entry are linked from Patents.
-
Encoder arithmetic: 221 = 2,097,152; 360° ÷ 2,097,152 = 0.000171661°. Verified by calculation.
-
Latency, bandwidth, envelope, forward-kinematics timings and power figures: ARK engineering record, as published on ark-dynamics.com/technology and /development-kits.
-
Energy-recirculation framing: ARK engineering record.
Deeper reading
Ten pages, one subject each.
This page is the overview. Each of these takes one part of the architecture and states what it does, what it costs, and where the number comes from.
Backlash
The dead zone at every reversal, and why a platform meets it far more often than a machine tool does.
21-bit encoders
2,097,152 positions per revolution, absolute, read on every drive cycle.
Field-oriented control
Smooth torque at any shaft angle, recomputed 8,000 times a second on every motor.
Closed-loop control
Three nested loops at three rates, and a clear account of what closing a loop cannot fix.
The Stewart platform
Six legs in parallel rather than six axes stacked, and what that geometry costs.
Passive weight compensation
Air carries the standing load so the motors do not — plus one published claim we will not repeat.
Motion cueing
Metres of travel, kilometres of road. Where a motion platform is really won or lost.
Software integration
What talks to the platform, how, and a clear statement of whose software each tool actually is.
Latency
Two endpoints or it is not a measurement. Under 1 ms, game telemetry in, motion out, with both points stated.
Every kit starts with this platform.
The PDK + ARK Ecosystem finishes it: racing chassis, ARK direct-drive wheelbase, wheel, load-cell pedals and button box, matched and tuned as one system. You add the seat, PC, screen and games.
Made to order · Batch One: ready to ship an estimated 3–6 months from order confirmation