Reference · For engineers, buyers and researchers
Motion Platform Glossary
Two suppliers can use the same word for different things. “3DOF” means pitch, roll and heave on a motion platform and three rotations in a VR headset. “Payload” may or may not include the fixture. “Bandwidth” may have been measured empty. Most project misunderstandings start with a term both sides thought they shared.
How to use this page: every term has a plain definition and a note on how it gets misread or why it matters when you buy. Each entry has its own permanent link, so you can cite the exact definition in a specification or an email.
Glossary · Maintained by the CSCmotion Motion Control Engineering Team · Updated October 5, 2026
Fig. 1 · One entry, three parts. Definition, caveat and a permanent link you can quote.
Start here if you only read one table
The Terms Buyers and Suppliers Read Differently
These eight cause more mismatched quotations than any others. Each row gives the common assumption, what the term actually means, and the one question that settles it.
The 8-Term Misreading Map Ask the last-column question of every supplier, including us.
| Term | Often assumed | What it actually means | The question that settles it |
|---|---|---|---|
| 3DOF | Any three axes | On a motion platform, pitch, roll and heave; in VR tracking, three rotations | "Which three axes, exactly?" |
| Payload | Everything the platform carries | Often only the item under test; gross moving load is the full figure | "Does the rating include fixture, cabin and cabling?" |
| Stroke | How far the platform moves | How far each actuator extends; platform travel is a separate, geometric result | "Is that actuator stroke or platform travel, and at which reference point?" |
| Travel (per axis) | All axes can reach their maximum together | Each maximum is measured with the other axes near neutral | "Can my combined trajectory be checked against the workspace?" |
| Accuracy | A measured property of the machine | Sometimes a design value derived from encoder resolution | "Measured or derived? With which instrument, at what load?" |
| Bandwidth | A fixed property of the platform | Depends on payload, axis and amplitude | "At what payload and amplitude was it measured?" |
| Cycle time | How fast the platform responds | Often the controller's interpolation period, not the loaded response | "Where does the measurement start and end?" |
| FAT | A demonstration before shipping | A test against agreed load, trajectory, reference point, instrument and tolerance | "Can I see the test items before I order?" |
Seen one of these in a quotation and not sure which meaning applies? Send the line, and an engineer tells you how to read it. Get a spec-sheet term checked →
Glossary A to Z
Motion Platform Terms and Definitions
Search, filter by category, or jump to a letter. Every entry links to itself — copy the link to cite a definition.
Acceleration (peak and sustained)#
Specification
Peak acceleration is what the actuators deliver briefly; sustained acceleration is what they hold without exceeding thermal limits. Electric actuators are thermally constrained, so the two can differ substantially.
Why it matters: An acceleration figure is meaningless without the gross moving load it was measured at, the axis, the pose and the duration.
ADRC (active disturbance rejection control)#
Control & software
A control strategy that estimates and cancels disturbances and unmodelled dynamics in real time, instead of relying on an exact model of the plant. Typically built from an extended state observer, a tracking differentiator and a feedback law.
Why it matters: Useful where payload and friction vary between runs, because the controller corrects for what the model leaves out.
Angular rate limiting#
Motion cueing
The cueing stage that caps the angular rate of the tilt-coordination output so it stays below the human detection threshold, keeping a tilt from being perceived as a rotation.
Why it matters: Set the limit too high and the occupant feels the platform tilting; too low and the sustained cue builds too slowly.
Attachment point#
Mechanism · Also called Joint centre, hinge point
One of the twelve points — six on the fixed base, six on the moving platform — at which the limbs connect. Their coordinates define the mechanism's kinematics completely.
Why it matters: The as-built positions always differ slightly from the drawing; identifying them is what parameter identification does.
Axis coupling#
Motion & axes · Also called Coupling
The property of a parallel mechanism whereby one limb contributes to several platform motions at once, and one platform motion needs several or all limbs to move together.
Why it matters: No actuator owns one degree of freedom. Even a pure yaw rotation changes all six limb lengths.
Backlash#
Specification · Also called Lost motion
Free play in a drive train that produces no output motion when travel reverses. In a servo electric cylinder it appears as a small dead band around each reversal.
Why it matters: At the twelve joints of a hexapod, small amounts of play stack into visible jitter at the payload, and cannot be tuned out in software.
Cardan joint#
Mechanism · Also called Universal joint, Hooke joint
A joint with two rotational degrees of freedom that blocks rotation about the limb axis. Every CSCmotion 6DOF platform uses Cardan joints, rather than the spherical joints assumed in most textbooks.
Why it matters: Blocking axial rotation gives a determinate load path and higher stiffness under alternating load, at the cost of a narrower articulation range that must be checked along the trajectory.
Centre of gravity (CoG)#
Specification · Also called Centre of mass
The point at which the combined mass of the moving assembly acts. Its height above the moving platform surface acts as a moment arm on the actuators.
Why it matters: CoG height often governs sizing more than mass does: at the same mass and acceleration, raising it from 0.3 m to 1.2 m quadruples the moment.
Change order#
Commercial · Also called Variation order
A priced change to the agreed scope after the purchase order, typically for interface work, site work or acceptance items that were not in the original quotation.
Why it matters: The usual way a low quotation becomes an expensive project. Normalising scope before comparing prices prevents most of them.
Closed-loop control#
Control & software · Also called Feedback control
A scheme in which measured output is continuously compared with the commanded reference and the error corrected. On a motion platform it runs at actuator level every control cycle.
Why it matters: Actuator-side feedback proves actuator position, not payload pose. Independent measurement at the payload is a separate evidence layer.
Combined trajectory#
Motion & axes · Also called Representative trajectory
A realistic motion profile in which several axes move at the same time, as they will in service.
Why it matters: One combined trajectory tells a supplier more than six per-axis maxima, because it describes a motion that has to exist all at once.
Commanded versus measured#
Testing & standards
A direct comparison of the motion that was requested with the motion an instrument recorded. This pairing is what separates evidence from a demonstration.
Why it matters: A no-load video shows that a mechanism moves; it does not show that it moves as commanded under your load.
Coordinate frame#
Motion & axes · Also called Reference frame
The axes and origin against which a pose is expressed. The base frame and the platform frame are distinct.
Why it matters: Positive directions and rotation order differ between host systems. Both belong in the interface control document, agreed before integration.
Degrees of freedom (DOF)#
Motion & axes · Also called DOF
The independent ways a rigid body can move in space. Six is the complete set: three translations and three rotations.
Why it matters: On a motion platform, 3DOF conventionally means pitch, roll and heave. In VR headset tracking, 3DOF means three rotations and no translation at all. Specifying against the wrong set is a common and expensive error.
Design value#
Specification · Also called Nominal value
A performance figure derived from the design model or from component specifications, rather than measured on a finished platform.
Why it matters: A design value and a measured value can share the same digits. Ask which one you are reading, and under what conditions.
Dexterous workspace#
Motion & axes
The set of positions a reference point can reach in every orientation. Smaller and more demanding than the reachable workspace.
Why it matters: Useful when the payload must hold arbitrary orientations at any position, as in some positioning and test tasks.
Duty cycle#
Specification
The proportion and pattern of operating time — hours per day, continuous or intermittent. It governs the thermal rating of electric actuators, which is often the binding limit rather than peak force.
Why it matters: Two platforms with identical peak requirements can need different actuators if one runs a full shift.
Electric cylinder#
Mechanism · Also called Servo electric cylinder, electromechanical actuator
A linear actuator pairing a servo motor with a screw drive, forming the variable-length limb of an electric motion platform. CSCmotion assembles and single-axis tests its own.
Why it matters: Thermal rating, not peak force, is usually what limits it — which is why duty cycle belongs in every specification.
EtherCAT#
Control & software
A real-time industrial Ethernet fieldbus giving deterministic, low-latency communication between a motion controller and servo drives.
Why it matters: A fieldbus cycle time is not the platform's response time; the two are often confused on spec sheets.
Euler angles#
Motion & axes
Three angles describing the orientation of a rigid body as a sequence of rotations about successive axes.
Why it matters: The same three numbers in a different rotation order describe a different orientation. Rotation order must be stated in the interface document.
Extended state observer (ESO)#
Control & software · Also called LESO, linear extended state observer
An observer that estimates both the system state and the lumped disturbance acting on it, so the controller can cancel the disturbance rather than merely react to it. The linear form is called a LESO.
Why it matters: The linear form has fewer parameters to tune, which makes it practical to commission on a real machine.
Factory acceptance test (FAT)#
Testing & standards
Verification at the manufacturer's facility before shipment, against an agreed test load, trajectory, reference point, instrument and tolerance. CSCmotion proposes FAT items before the order, and welcomes the customer or a third party such as SGS or BV to witness.
Why it matters: A no-load demonstration is not a FAT. Agree the items in writing before the order, not after delivery.
Finite element analysis (FEA)#
Testing & standards
Numerical stress and deflection analysis in which a structure is divided into elements, their stiffness and load matrices are assembled into a global equilibrium equation, and the result is solved under defined boundary conditions.
Why it matters: Only as good as the load case fed into it: on a motion platform, that comes from the dynamic solution of your trajectory.
Fixed base#
Mechanism · Also called Lower platform
The stationary frame that transfers reaction loads into the foundation. Its attachment radius sets limb inclination and force transmission.
Why it matters: A wider base improves tilt stiffness but costs floor space and rotation range.
Forward kinematics#
Kinematics · Also called Direct kinematics
The calculation from six measured limb lengths back to one platform pose. A general Stewart geometry has no closed-form solution, so it is solved numerically. CSCmotion production controllers seed the solver with a trained neural network and refine the estimate by numerical iteration.
Why it matters: The pose a controller reports is usually this calculated result — a model output, not a measurement.
Frequency response#
Specification · Also called Bandwidth
How output amplitude and phase vary with the frequency of the commanded input. Describes dynamic fidelity more completely than a single bandwidth number.
Why it matters: Ask at what payload, axis and amplitude a bandwidth figure was measured — all three change it.
Gross moving load (GML)#
Specification · Also called Total moving mass
The total mass the platform accelerates: device under test, fixture, cabin, occupants, instrumentation and cabling.
Why it matters: Not the payload alone. Underestimating GML is the most common cause of an under-sized platform, and it is usually found at commissioning.
Gross moving load margin#
Commercial · Also called Payload margin
Reserve capacity specified above the expected moving load, to absorb instrumentation, harnesses, seats and ballast added after commissioning.
Why it matters: Margin agreed at specification stage costs far less than a shortfall discovered after delivery.
Hardware-in-the-loop (HIL)#
Testing & standards · Also called HIL testing
A test arrangement in which real hardware — a sensor, a controller or a whole subsystem — rides on the platform while a simulation drives the motion around it.
Why it matters: Puts the update rate and the host interface at the centre of the specification, because the loop closes through the platform.
Heave#
Motion & axes · Also called Vertical translation
Translation along the vertical Z axis — up and down. Present on a conventional 3DOF platform.
Why it matters: Often the most useful single axis for road and vertical vibration input.
Hexapod#
Mechanism · Also called Six-leg platform
A common name for a Stewart-type parallel mechanism with six variable-length legs. In precision engineering it often means a small, stiff positioning device; in simulation, a motion base.
Why it matters: The same word covers machines several orders of magnitude apart in size; check payload and travel before assuming a match.
High-pass angular velocity channel#
Motion cueing
The cueing path that limits commanded angular velocity, applies a high-pass filter to keep only its fast component, and integrates it into an angular displacement added to the tilt-coordination output.
Why it matters: Reproduces the start of a rotation; the sustained part is washed out so the platform can return to neutral.
Hydraulic actuator#
Mechanism · Also called Hydraulic cylinder
A linear actuator driven by pressurised oil from a hydraulic power unit, controlled by servo or proportional valves.
Why it matters: Still wins at some extremes of force per leg and shock loading; brings a power unit, oil handling and more maintenance with it.
Interface control document (ICD)#
Commercial · Also called Interface document
The document defining the boundary between platform and host: command set, coordinate convention, units, update rate, state machine and fault behaviour. CSCmotion issues it before integration starts.
Why it matters: Getting it at delivery rather than before integration is the most common cause of first-power-on surprises.
Interpolation#
Control & software
The step that densifies a planned path into closely spaced intermediate points for the servo loop. Linear, circular and spline interpolation are used depending on the path.
Why it matters: Coarse interpolation shows up as roughness at the payload even when every point lies on the right path.
Interpolation period#
Control & software · Also called Interpolation cycle
The time between successive set-points the controller sends to the servo drives.
Why it matters: Often printed as if it were the platform's response time. It is not: the closed-loop response of a loaded platform is a different, longer figure.
Inverse kinematics#
Kinematics · Also called Position inverse solution
The calculation that converts a commanded platform pose into the six limb lengths needed to reach it. Closed-form, one answer, solved for each limb on its own, every control cycle.
Why it matters: On an illustrative hexapod, 50 mm of heave needs only 40.6 mm of limb extension: stroke and travel are different quantities.
ISO 9001#
Testing & standards · Also called Quality management system
The international standard for quality management systems: documented, audited processes for design, production and inspection. CSCmotion's manufacturing entity is certified to ISO 9001, ISO 14001 and ISO 45001.
Why it matters: It certifies how a company works, not how a given platform performs. Performance is proven by the factory acceptance test.
Jacobian matrix#
Kinematics
The matrix of first-order partial derivatives relating limb velocities to platform velocities at a given pose. Its condition shows how well the mechanism transmits force and motion there.
Why it matters: As the Jacobian approaches singularity, small limb motions produce large, poorly controlled platform motions.
Jerk#
Motion cueing
The rate of change of acceleration. It matters to human perception and to trajectory planning, where jerk limits keep motion smooth and reduce structural excitation.
Why it matters: A profile with unlimited jerk can excite structural modes even when its accelerations are modest.
Kutzbach-Grübler formula#
Mechanism · Also called Mobility formula
The general expression for the degrees of freedom of a spatial mechanism: M = 6(n − 1) − Σ(6 − fᵢ), counting n rigid bodies and the freedom fᵢ each joint permits.
Why it matters: For a classic six-limb UPS layout — 14 bodies, 6 universal, 6 prismatic and 6 spherical joints — it gives 6 × 13 − 72 = 6 degrees of freedom.
Laser tracker#
Testing & standards · Also called Laser measuring system
An optical instrument that measures the 3D position of a target on the platform with high accuracy, used to verify positioning accuracy and repeatability at defined poses.
Why it matters: A measured figure should name its instrument; a figure without one is usually derived, not measured.
Latency#
Control & software · Also called Transport delay
The delay between a host issuing a command and the platform beginning to respond. It matters most in closed-loop simulation, where the host reacts to platform state.
Why it matters: A latency figure means little without the payload, the measurement start and end points, and the instrument.
Look-ahead control#
Control & software
A control function that inspects the upcoming trajectory for abrupt direction changes and decelerates in advance, because a parallel mechanism cannot change speed instantaneously.
Why it matters: Without it, a sharp change in a recorded time history forces either an overshoot or an abrupt stop.
Low-pass acceleration channel#
Motion cueing
The cueing path that extracts the low-frequency part of commanded translational acceleration and passes it to tilt coordination.
Why it matters: Longitudinal and lateral only: gravity cannot be used to simulate a sustained vertical acceleration.
Modal analysis#
Testing & standards
Computation or measurement of a structure's natural frequencies and mode shapes at representative poses, used to find weak links before the design is built.
Why it matters: Natural frequencies change with pose and payload, so one result at neutral does not cover the whole workspace.
Moment of inertia#
Specification · Also called Rotational inertia
How mass is distributed about the centre of gravity. It governs rotational sizing the way mass governs translation, and is stated as Ixx, Iyy and Izz about the payload CoG.
Why it matters: The same 100 kg as a compact block or spread over a 3 m frame differs in yaw inertia by a factor of about 1,350.
Monte Carlo workspace analysis#
Testing & standards
Mapping the workspace by sampling a large number of candidate poses and testing each against limb length, joint angle, clearance, force and conditioning limits.
Why it matters: Used because no closed-form expression describes the boundary of a coupled six-dimensional workspace under every constraint at once.
Motion cueing#
Motion cueing · Also called Motion drive algorithm
The application-layer algorithm that turns vehicle or flight-model dynamics into bounded platform motion, producing convincing cues inside a workspace far smaller than the motion represented.
Why it matters: Not every application needs it. Industrial test systems normally command a physical trajectory directly.
Motion platform#
Mechanism · Also called Motion base, motion system
A mechatronic system that moves a payload in controlled translations and rotations: mechanism, actuators, controller, software, limits and an acceptance method.
Why it matters: Buy the system, not the mechanism. The arrow diagram is the least informative part of a motion platform.
Motion spectrum#
Motion cueing · Also called Time history
A recorded or synthesised time history used to drive the platform — road, wave, flight or seismic. Replaying one reproduces a real environment rather than a synthetic profile.
Why it matters: A spectrum is a demanding combined trajectory; check it against the workspace before it reaches a quotation.
Moving platform#
Mechanism · Also called Upper platform, top frame
The frame that carries the payload and is moved by the six limbs. Its attachment geometry sets the moment arm through which the limbs control orientation.
Why it matters: Everything mounted on it counts toward gross moving load.
Multibody dynamics#
Testing & standards
Simulation of the coupled motion and forces of connected rigid bodies. It returns the peak thrust each cylinder must deliver along the commanded motion, which sizes the actuator and becomes the structural load case.
Why it matters: A supplier who sizes from static payload alone has skipped this step.
Natural cut-off frequency#
Motion cueing
The tuning parameter of each washout filter. Raising it increases platform excursion but risks the platform not returning to neutral before the next cue; lowering it shortens response and travel.
Why it matters: There is no universally correct value; it is tuned to the motion content of the application.
Natural frequency#
Specification · Also called Resonant frequency
A frequency at which the structure resonates. Platform natural frequencies must sit clear of the commanded motion content, and they vary with pose and payload.
Why it matters: Ask at which pose and payload a natural frequency was computed or measured.
Newton-Euler method#
Kinematics
A recursive formulation of rigid-body dynamics used to compute the forces and torques in a mechanism, including a parallel platform.
Why it matters: It is how peak limb forces are found for a trajectory, before any actuator is chosen.
Newton-Raphson iteration#
Kinematics · Also called Newton's method
A numerical method that linearises a non-linear system about the current estimate and iterates toward the solution. Commonly used for forward kinematics, which has no closed form.
Why it matters: Fast when started close to the answer, unreliable when not — which is why a good initial estimate matters.
NURBS interpolation#
Control & software · Also called Non-uniform rational B-spline
Interpolation along a smooth spline curve instead of straight or circular segments, used where a path must be followed without corner discontinuities.
Why it matters: Smoother paths mean lower jerk and less structural excitation for the same route.
Onset cue#
Motion cueing · Also called Motion onset
The initial transient at the start of an acceleration, which the vestibular system detects as a change rather than a steady state. Reproducing it needs genuine horizontal translation.
Why it matters: This is the capability a 3DOF platform lacks: tilt cannot create an onset without being felt as rotation.
Parallel kinematic machine (PKM)#
Mechanism · Also called Parallel manipulator, parallel robot
A mechanism in which every limb connects the base directly to the moving platform, so load is shared across all paths instead of passing through a serial chain.
Why it matters: Errors average rather than accumulate, so parallel machines are stiff and repeatable for their mass — and have a small workspace for their footprint.
Parameter identification#
Control & software · Also called Kinematic calibration
Measuring the as-built geometry of a platform and compensating the difference from the design values inside the controller.
Why it matters: A common model identifies seven parameters per limb — the coordinates of both attachment points plus the limb zero length — 42 in total. It is the step that turns a correct equation into an accurate machine.
Payload#
Specification
The item the platform carries. In specification work the more useful figure is gross moving load, which adds the fixture, cabin and everything else that moves.
Why it matters: A payload rating that does not say whether it includes the fixture cannot be compared between suppliers.
Perception threshold#
Motion cueing · Also called Detection threshold
The smallest motion a person reliably detects. Cueing algorithms keep washout and tilt rates below it so corrective motion is not felt.
Why it matters: Thresholds vary between people and axes, so tuning is checked with real occupants, not only in simulation.
Pitch#
Motion & axes
Rotation about the lateral Y axis — nose up and nose down.
Why it matters: State whether positive pitch means nose-up or nose-down; host systems disagree.
Pose#
Motion & axes · Also called Position and orientation
The complete placement of a rigid body in space: three translations and three rotations, six numbers in total.
Why it matters: A stated pose is either commanded, calculated or measured. Only the last is independent of the model.
Positioning accuracy#
Specification · Also called Absolute accuracy
How closely the platform reaches a commanded pose, measured at a stated reference point, under a stated load, with a stated instrument.
Why it matters: A figure derived from encoder resolution and gear ratio is printed the same way as a measured one. Ask which you are reading.
Quaternion#
Kinematics · Also called Euler parameters
A four-parameter representation of orientation that avoids the gimbal-lock and ordering ambiguities of Euler angles.
Why it matters: Hosts that send quaternions avoid rotation-order mistakes, but the convention still has to be agreed.
Reachable workspace#
Motion & axes
The set of all positions a reference point can reach, regardless of the orientation it holds there.
Why it matters: Larger than the dexterous workspace; a reachable position may not be reachable in the orientation you need.
Reference point#
Motion & axes · Also called Datum point
The point on the platform or payload to which quoted travel, angle and accuracy figures apply.
Why it matters: Table centre and payload CoG are different points. Without a stated reference point, motion figures cannot be compared.
Repeatability#
Specification
How consistently the platform returns to the same pose on repeated commands. Usually tighter than absolute accuracy, and often more relevant for testing.
Why it matters: Ask whether a repeatability figure is a design value or a measured result, and which instrument measured it.
Residual function#
Control & software
The difference between the theoretical limb extension from the inverse solution and the extension actually measured at a given pose. Parameter identification minimises the sum of the squared residuals.
Why it matters: The quality of the identification depends on how well the measured poses cover the workspace.
RFQ (request for quotation)#
Commercial · Also called Request for quote
The document asking suppliers to price a defined scope. For a motion platform it should carry gross moving load, CoG, one combined trajectory, duty cycle, scope boundary, host interface, acceptance expectation and site constraints.
Why it matters: An RFQ that omits the scope boundary produces quotations that cannot be compared.
Roll#
Motion & axes
Rotation about the longitudinal X axis — banking left and right.
Why it matters: The sign of a positive roll is a convention, not a law; agree it in writing.
S-curve acceleration#
Control & software · Also called S-type profile
An acceleration profile that ramps jerk instead of stepping acceleration, giving smooth transitions between speed changes and less structural excitation.
Why it matters: Costs a little time per move compared with a trapezoidal profile, in exchange for smoother motion.
Safety factor#
Specification · Also called Design factor
The ratio between the load a component is designed to withstand and its rated working load.
Why it matters: A safety factor without the load case it applies to says little; ask which loads and which components it covers.
Scope boundary#
Commercial · Also called Scope of supply
The written line between what the quotation includes and what it does not: controller, software, documentation, acceptance, freight, installation, commissioning, training, spares and site works.
Why it matters: The largest single source of non-comparable quotations. CSCmotion quotations include the controller, control software, API documentation and the interface document.
Screw drive#
Mechanism · Also called Ball screw, roller screw
The screw-and-nut mechanism inside a servo electric cylinder that turns motor rotation into linear limb motion. Ball screws suit most duty; planetary roller screws carry higher force per leg.
Why it matters: Screw lead trades speed against force: a longer lead moves faster at the same motor speed but needs more torque for the same thrust.
Servo drive#
Control & software · Also called Amplifier
The power electronics that turn controller commands into motor current, closing the current and velocity loops beneath the position loop.
Why it matters: Drive tuning is part of commissioning, which is why who owns the controller matters after delivery.
Singularity#
Kinematics · Also called Degenerate configuration
A pose at which the mechanism loses the ability to control or resist motion in one or more directions. Force transmission degrades sharply nearby.
Why it matters: No forward-kinematics solver behaves well near a singularity; software limits keep the platform away from these poses.
Site acceptance test (SAT)#
Testing & standards
Verification repeated at the customer's site after installation, confirming the platform performs as accepted at the factory once foundation, power and host are connected.
Why it matters: Only meaningful if the FAT was written first; the SAT repeats agreed items, it does not invent new ones.
Specific force#
Motion cueing
The non-gravitational acceleration acting on a body — what the vestibular system actually senses. Motion cueing reproduces specific force rather than acceleration itself.
Why it matters: This is why tilt works: tilting the platform redirects gravity into a specific force the occupant reads as sustained acceleration.
Spherical joint#
Mechanism · Also called Ball joint
A joint with three rotational degrees of freedom, including free rotation about the limb axis. The textbook assumption in Stewart platform literature.
Why it matters: Free axial rotation means the limb cannot resist torsion, so torsional load must be carried by the arrangement of the limbs together.
Stewart platform#
Mechanism · Also called Gough-Stewart platform
A parallel mechanism joining a moving platform to a fixed base through six variable-length limbs. Built by Gough in 1949 for tyre testing and published by Stewart in 1965 for flight simulation.
Why it matters: It names an architecture, not an axis count. Not every 6DOF mechanism is a Stewart platform, and not every Stewart platform is a simulator.
Stiffness#
Specification · Also called Rigidity
Resistance to deflection under load. Parallel mechanisms achieve high stiffness for their mass because load is shared across the limbs.
Why it matters: Longer stroke usually lowers lateral stiffness and buckling margin; stroke is never a free parameter.
Stroke#
Specification · Also called Actuator stroke
The change in length available in each actuator. An input to the geometry, not a statement of platform displacement.
Why it matters: Stroke is not travel: platform motion is the geometric result of all six limb lengths acting together.
Surge#
Motion & axes · Also called Longitudinal translation
Translation along the longitudinal X axis — forward and backward. One of the three axes a conventional 3DOF platform does not provide.
Why it matters: Braking and acceleration onsets need real surge; tilt can only imitate the sustained part.
Sway#
Motion & axes · Also called Lateral translation
Translation along the lateral Y axis — side to side. One of the three axes a conventional 3DOF platform does not provide.
Why it matters: Cornering onsets need real sway for the same reason surge is needed for braking.
Third-party witnessing#
Testing & standards · Also called Witnessed test
Attendance by an independent inspector — such as SGS or BV — at a factory acceptance test, to observe and sign the record.
Why it matters: Welcome at CSCmotion FATs. Agree in advance which items are witnessed and what the inspector signs.
Tilt coordination#
Motion cueing
The cueing module that turns the low-frequency part of a commanded translational acceleration into a small, slow tilt, so gravity supplies a sustained specific force within a finite workspace.
Why it matters: Reproduces the plateau of an acceleration, never its onset — which needs genuine translation.
Trajectory#
Motion & axes · Also called Motion profile
A time-parameterised sequence of poses the platform is commanded to follow.
Why it matters: One representative combined trajectory is more useful to a supplier than a list of per-axis maxima.
Trajectory planning#
Control & software · Also called Velocity planning
The stage that turns target positions into an executable, time-parameterised motion that respects velocity, acceleration and jerk limits.
Why it matters: Skipping it lets the platform pass through poses nobody commanded on the way to the ones they did.
Travel#
Specification · Also called Platform travel
The displacement the platform achieves at a stated reference point. A geometric result of the six limb lengths, and it varies across the workspace.
Why it matters: Per-axis maxima are each measured with the other axes near neutral and cannot be added together.
Update rate#
Control & software · Also called Command rate
How often the host sends new pose commands and the controller accepts them, in hertz. It affects the smoothness of host-driven motion and perceived latency.
Why it matters: Agree it in the interface document; a mismatch shows up as stutter, not as an error message.
Vestibular system#
Motion cueing · Also called Inner ear balance organ
The balance organ that senses angular rate and specific force. It responds to changes in motion rather than steady states — the property every cueing algorithm relies on.
Why it matters: Explains why a short real translation plus a slow tilt can feel like a long acceleration.
Washout filter#
Motion cueing · Also called Washout algorithm
The filtering inside a cueing algorithm that returns the platform toward neutral after a manoeuvre, freeing workspace for the next input. Classical versions combine high-pass and low-pass filters on specific force and angular velocity.
Why it matters: The return must stay below perception thresholds, or the occupant feels the washout itself as a false cue.
Workspace envelope#
Motion & axes · Also called Motion envelope
The coupled six-dimensional set of poses a platform can reach and use in practice, after limb length, joint angle, clearance, force and conditioning limits are applied.
Why it matters: Not a rectangular box. Usable workspace is not the sum of per-axis maxima.
Yaw#
Motion & axes · Also called Heading
Rotation about the vertical Z axis — a heading change. Not available on a conventional 3DOF platform.
Why it matters: Every limb moves for a pure yaw; there is no single yaw actuator.
Zero calibration#
Control & software · Also called Homing
Establishing the reference zero of each actuator so commanded and actual positions agree at start-up. It comes before any accuracy figure has meaning.
Why it matters: An accuracy figure measured without a defined homing procedure cannot be repeated on site.
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Put the definitions to work
How to Use These Definitions in an RFQ
A glossary is most useful when it becomes part of the contract. Three lines in your RFQ remove most of the ambiguity this page describes.
The 3-Line Definitions Clause Copy the wording; replace the links with your own glossary if you prefer.
| Line | Suggested wording | What it prevents |
|---|---|---|
| Define the load | "Payload figures refer to gross moving load, including fixture, cabin, occupants and cabling, with CoG height stated above the moving platform." | Platforms sized against the test article alone |
| Define the motion | "Travel and rotation refer to platform motion at the stated reference point, not actuator stroke; per-axis values are not assumed to be simultaneous." | Envelopes that exist only one axis at a time |
| Define the evidence | "Every performance figure states whether it is a design value or a measured result, with payload, reference point and instrument." | Derived numbers presented as measurements |
The definitions we work to are the ones on this page. When a supplier — including us — uses a term differently, ask which definition applies before comparing figures. The 10-Line RFQ Ledger covers the rest of the RFQ.
Drafting an RFQ now? Send the draft, and we mark every term a supplier could read two ways. Have your RFQ wording checked →
Frequently asked
Motion Platform Terminology — FAQ
The term questions we answer most often, in one or two paragraphs each.
What is the difference between payload and gross moving load?
Payload is usually the item the platform carries. Gross moving load is everything the platform accelerates: the item, its fixture, any cabin, occupants, instrumentation and cabling. Size against gross moving load; quoting payload alone is the most common cause of an under-sized platform. See gross moving load.
What do surge, sway, heave, roll, pitch and yaw mean?
They are the six degrees of freedom of a rigid body. Surge, sway and heave are translations along the longitudinal, lateral and vertical axes; roll, pitch and yaw are rotations about those same axes. Signs and rotation order are conventions that must be agreed with the host system.
Is a Stewart platform the same as a hexapod or a 6DOF motion platform?
Not exactly. A Stewart platform, or hexapod, is a mechanism: six variable-length legs between a base and a moving platform. 6DOF describes the motion, not the mechanism — serial robots also reach six axes. And in precision engineering "hexapod" often means a small positioning device rather than a motion base.
What is the difference between actuator stroke and platform travel?
Stroke is how far each actuator can extend. Travel is how far the platform moves at a stated reference point, which is the geometric result of all six limbs together. On an illustrative hexapod, 50 mm of heave needs only 40.6 mm of limb extension — the two are never the same number.
What is a washout filter in motion cueing?
The part of a cueing algorithm that returns the platform toward neutral after a manoeuvre, below the threshold the occupant can feel, so there is workspace left for the next cue. Combined with tilt coordination, it lets a small platform suggest a long acceleration.
What is the difference between accuracy and repeatability?
Accuracy is how closely the platform reaches a commanded pose; repeatability is how consistently it returns to the same pose. Repeatability is usually the tighter figure and often the more relevant for testing. For either, ask whether it is a design value or a measured result, with which instrument and at what load.
Can't find a term?
Send Us the Word That Isn't Here
If a supplier, a standard or a colleague used a term you can’t find on this page, send it with the sentence it appeared in. An engineer explains it — and terms people ask about become new entries.
An engineer replies within 24 hours — weekends included. cscmotionglobal@gmail.com · +86 152 5530 5272
Where these terms do real work
- How a 6DOF Motion Platform Works — kinematics, coupling, evidence
- Payload, Stroke & Acceleration — GML, CoG, stroke, sizing
- 3DOF vs 6DOF — axes, onset cues, tilt coordination
- Electric vs Hydraulic — actuators and duty
- How to Choose a Motion Platform — RFQ, scope, FAT
Commercial questions live in the FAQ.