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Motion platform Glossary
Eighty-seven terms defined in plain language, arranged alphabetically. Definitions are drawn from our own engineering documentation rather than a general reference — so the entries on parameter identification, washout channels and workspace analysis describe the methods actually used to build these machines. Where a term is commonly used two different ways, the entry says so.
87 terms
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Motion platform Glossary（87 terms）
Arranged alphabetically. Terms tagged by subject so you can see at a glance whether an entry is about the mechanism, the control system, a specification figure or the way motion is cued. Every heading is a permanent link.
Motion 16 Specification 15 Control 15 Motion cueing 13 Mechanism 10 Testing 8 Kinematics 7 Standards 1 Commercial 2
0–9
1 term
6-TPS parallel mechanism
Also called 6-TPS configuration
The mechanism CSCMotion builds: a moving platform and a fixed platform joined by six servo electric cylinders through Cardan joints. Each cylinder is a prismatic pair (P) and the piston-to-barrel connection adds one rotational freedom (R), giving the T-P-S chain the configuration is named for.
In practiceCounting the fixed platform there are 14 rigid bodies and 18 joints, which by the Kutzbach-Grübler formula gives 6(14−1) − 18×4 = 6 degrees of freedom.
Stewart platform design →
A
5 terms
Acceleration (peak and sustained)
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.
In practiceAn acceleration figure is meaningless without the gross moving load it was measured at and the pose it occurred in.
Sizing guide →
ADRC (active disturbance rejection control)
A control strategy that estimates and cancels disturbances and unmodelled dynamics in real time instead of relying on an exact plant model. Implemented here through the PID plus LESO plus differential-tracker structure.
Engineering method →
Angular rate limiting
The stage that caps the angular rate of the tilt-coordination output so it stays below the human vestibular detection threshold, keeping the tilt from being perceived as rotation.
Motion cueing →
Attachment point
Also called Joint centre, hinge point
One of the twelve points — six on the fixed base (B1–B6), six on the moving platform (P1–P6) — at which the limbs connect. Their coordinates define the mechanism’s kinematics completely.
Stewart platform design →
Axis coupling
Also called Coupling
The property of a parallel mechanism whereby one limb contributes to several platform motions at once, and one platform motion requires several or all limbs to move together.
In practiceNo actuator owns one degree of freedom. Treating the platform as six independent linear axes produces incorrect motion and unplanned joint loading.
Working principle →
B
1 term
Backlash
Also called Lost motion
Free play in a drive train that produces no output motion when travel direction reverses. In a servo electric cylinder it appears as a small dead band around each reversal.
C
5 terms
Cardan joint
Also called Universal joint, Hooke joint
A joint with two rotational degrees of freedom that constrains rotation about the limb axis. CSCMotion platforms use Cardan joints at both ends of each cylinder rather than the spherical joints assumed in most textbooks.
In practiceConstraining axial rotation gives a determinate load path and higher stiffness under alternating load, at the cost of a more restricted articulation range that must be checked at every pose.
Joint selection →
Centre of gravity (CoG)
Also called Centre of mass
The point at which the combined mass of the moving assembly acts. Its height above the moving platform surface is a moment arm.
In practiceCoG height frequently governs sizing more than mass does. Lowering it in the fixture design is often the cheapest saving available.
Sizing guide →
Closed-loop control
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 this operates at the actuator level every control cycle.
In practiceActuator-side feedback proves actuator position, not payload pose. Independent measurement is a separate evidence layer.
Working principle →
Commanded versus measured
A direct comparison of the motion requested and the motion an instrument recorded. This pairing is what distinguishes evidence from a demonstration.
Coordinate frame
Also called Reference frame
The axes and origin against which a pose is expressed. Base frame and platform frame are distinct.
In practicePositive directions and rotation order are not standardised between suppliers. Both belong in the interface control document, agreed before integration.
D
3 terms
Degrees of freedom (DOF)
Also called DOF
The independent ways a rigid body can move in space. Six is the complete set: three translations and three rotations.
In practiceOn a motion platform 3DOF conventionally means pitch, roll and heave. In VR headset tracking 3DOF means three rotations with no translation at all. The two are different axis sets, and specifying against the wrong one is a common and expensive error.
3DOF vs 6DOF →
Dexterous workspace
The smaller set of poses the reference point can reach from any orientation. A more demanding and more useful measure than reachable workspace.
Workspace analysis →
Duty cycle
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 constraint rather than peak force.
Sizing guide →
E
2 terms
EtherCAT
A real-time industrial Ethernet fieldbus giving deterministic, low-latency communication between a motion controller and servo drives. CSCMotion controllers include an EtherCAT master option.
Controller & API →
Euler angles
A set of three angles describing the orientation of a rigid body as a sequence of rotations about successive axes. CSCMotion platforms use a Z-Y-X sequence.
In practiceThe same three numbers in a different rotation order describe a different orientation. Rotation order must be stated explicitly in the interface document.
Working principle →
F
5 terms
Factory acceptance test (FAT)
Verification at the manufacturer’s facility before shipment, against an agreed test load, trajectory, reference point, instrument and tolerance.
In practiceA no-load demonstration is not a FAT. Agree the protocol in writing before the order, not after delivery.
Buying guide →
Finite element analysis (FEA)
Numerical stress and deflection analysis in which the structure is divided into elements, element stiffness and load matrices are assembled into a global equilibrium equation, and the result is solved under defined boundary conditions.
Engineering method →
Fixed base
Also called Lower platform
The stationary frame that transfers reaction loads into the foundation. Its attachment radius sets limb inclination and force transmission.
Forward kinematics
Also called Direct kinematics
The calculation from six measured limb lengths back to one platform pose. For a general Stewart geometry there is no direct equation, so it is solved numerically.
In practiceCSCMotion solves it by Newton-Raphson iteration to a numerical precision of 1×10⁻⁹, seeded by a fast estimate so the solver converges reliably in real time.
The forward problem →
Frequency response
Also called Bandwidth
How platform output amplitude and phase vary with the frequency of the commanded input. Characterises dynamic fidelity more completely than a single bandwidth number.
In practiceAsk at what payload and amplitude a bandwidth figure was measured — both change it.
G
3 terms
GJB9001C-2017
The Chinese defence-sector quality management standard, adding requirements beyond ISO 9001 on design control, traceability, configuration management and process documentation.
Certifications →
Gross moving load (GML)
Also called Total moving mass
The total mass the platform accelerates: device under test, fixture, cabin, occupants, instrumentation and cabling.
In practiceNot the payload alone. Underestimating GML is the single most common cause of an under-sized platform, and it is normally discovered at commissioning.
Sizing guide →
Gross moving load margin
Also called Payload margin
Reserve capacity specified above the anticipated moving load to absorb instrumentation, harnesses, seats and ballast added after commissioning.
In practiceMargin agreed at specification stage costs far less than discovering the shortfall after delivery.
Sizing guide →
H
2 terms
Heave
Also called Vertical translation
Translation along the vertical Z axis — up and down. Present on a conventional 3DOF platform, and usually the most useful single axis for road and wave input.
High-pass angular velocity channel
The cueing path that limits and transforms commanded angular velocity, applies a second-order high-pass filter to retain only the high-frequency component, and integrates it into an angular displacement that is summed with the tilt-coordination output.
Motion cueing →
I
3 terms
Interface control document (ICD)
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.
Controller & API →
Interpolation
The step that densifies a planned path into closely spaced intermediate points for the servo loop. Linear, circular and NURBS interpolation are used depending on the path type.
Inverse kinematics
Also called Position inverse solution
The calculation converting a commanded platform pose into the six limb lengths required to achieve it. Closed-form, one solution, solved independently for each limb, and it runs every control cycle.
Working principle →
J
2 terms
Jacobian matrix
The matrix of first-order partial derivatives relating limb velocities to platform velocities at a given pose. Its condition indicates how well the mechanism transmits force and motion, and it also underpins the parameter identification model.
Hexapod kinematics →
Jerk
The rate of change of acceleration. Relevant both to human perception and to trajectory planning, where jerk limits keep motion smooth and reduce structural excitation.
K
1 term
Kutzbach-Grübler formula
Also called Mobility formula
The general expression for the degrees of freedom of a spatial mechanism, counting rigid bodies, joints and the freedom each joint permits. It is how a mechanism’s mobility is derived rather than assumed.
Stewart platform design →
L
5 terms
Laser measurement
Optical measurement of platform pose using a laser measuring system, used to determine positioning accuracy and repeatability at defined poses under load.
Engineering method →
Latency
Also called Transport delay
The delay between a host issuing a command and the platform beginning to respond. Matters most in closed-loop simulation where the host reacts to platform state.
LESO (linear extended state observer)
Also called 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. CSCMotion combines a LESO with PID and a differential tracker.
In practiceCompared with a non-linear observer it has fewer parameters to tune, which makes it practical to commission on a real machine.
Look-ahead control
A control function that inspects the upcoming trajectory for abrupt direction changes and decelerates in advance, because a parallel mechanism cannot change speed instantaneously.
In practiceWithout look-ahead, a sharp change in a recorded spectrum forces either an overshoot or an abrupt stop.
Low-pass acceleration channel
The cueing path that extracts the low-frequency part of commanded translational acceleration through a second-order low-pass filter, and passes it to tilt coordination.
In practiceLongitudinal and lateral only — gravity cannot be used to simulate a vertical acceleration component.
Motion cueing →
M
7 terms
Modal analysis
Computation or measurement of a structure’s natural frequencies and mode shapes at representative poses, used to find the weak links in a design before it is built.
In practiceCSCMotion computes modal results twice on independent foundations — ANSYS by finite element and variational principles, ADAMS by multibody dynamics and the Euler-Lagrange formulation — and treats agreement between them as the release condition.
Engineering method →
Moment of inertia
Also called Rotational inertia
How mass is distributed about the centre of gravity. It governs rotational sizing in the same way mass governs translational sizing, and is stated as Ixx, Iyy and Izz relative to the payload CoG.
In practiceTwo payloads of identical mass, one compact and one spread out, impose very different rotational demands.
Sizing guide →
Monte Carlo workspace analysis
Mapping the workspace by sampling a large number of candidate poses and testing each against limb length, joint angle, clearance, force and conditioning limits.
In practiceUsed because there is no closed-form expression for the boundary of a coupled six-dimensional workspace under all constraints at once.
Workspace analysis →
Motion cueing
Also called Motion drive algorithm
The application-layer algorithm that converts vehicle or flight model dynamics into bounded platform motion, producing convincing perceptual cues inside a workspace far smaller than the motion being represented.
In practiceNot required by every application. Industrial test systems normally command a defined physical trajectory directly, with no cueing layer at all.
Motion cueing →
Motion spectrum
A recorded or synthesised time history used to drive the platform — road spectrum, wave spectrum, flight spectrum or seismic spectrum. Replaying one reproduces a real environment rather than a synthetic profile.
Moving platform
Also called Upper platform, top frame
The plate that carries the payload and is moved by the six limbs. Its attachment geometry sets the moment arm through which the limbs control orientation.
Multibody dynamics
Simulation of the coupled motion and forces of connected rigid bodies. Returns the peak thrust each cylinder must deliver across the commanded envelope, which then sizes the actuator and becomes the load case for structural analysis.
Sizing guide →
N
5 terms
Natural cut-off frequency
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 time and travel.
In practiceThere is no universally correct value — it is tuned to the motion content of the specific application.
Motion cueing →
Natural frequency
Also called Resonant frequency
A frequency at which the structure resonates. Platform natural frequencies must sit clear of the commanded motion spectrum, and they vary with pose and payload.
Engineering method →
Newton-Euler method
A recursive formulation of rigid-body dynamics used to compute the forces and torques in a parallel mechanism. CSCMotion uses it for the dynamics module that sizes the cylinders.
Newton-Raphson iteration
Also called Newton’s method
A numerical method that linearises a non-linear system about a current estimate and iterates toward the solution. Used for the forward kinematics problem, which has no closed form.
NURBS interpolation
Also called Non-uniform rational B-spline
Interpolation along a smooth spline curve rather than straight or circular segments, used where a path must be followed continuously without corner discontinuities.
O
1 term
Onset cue
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 requires genuine horizontal translation.
In practiceThis is the specific capability a 3DOF platform lacks — tilt cannot generate an onset without being felt as rotation.
3DOF vs 6DOF →
P
7 terms
Parallel kinematic machine (PKM)
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 six paths rather than passing through a serial chain.
In practiceErrors average rather than accumulate, which is why parallel mechanisms are stiff and repeatable relative to their mass — and why their workspace is small relative to footprint.
Stewart platform design →
Parameter identification
Also called Kinematic calibration
Measuring the as-built geometry of a platform and compensating the difference from design values inside the controller.
In practiceFor a 6-TPS platform there are 42 parameters to identify — seven per limb, being the three coordinates of each platform and base attachment point plus the limb zero length. This is the step that turns a correct calculation into an accurate machine.
Engineering method →
Payload
The item the platform carries. In specification work the more useful figure is gross moving load, which includes the fixture, cabin and everything else that moves.
In practiceA payload rating quoted without stating whether it includes the fixture is not comparable between suppliers.
Sizing guide →
Perception threshold
Also called Detection threshold
The smallest motion a person reliably detects. Cueing algorithms keep washout and tilt rates below it so corrective platform motion is not felt.
Pitch
Rotation about the lateral Y axis — nose up and down.
Pose
Also called Position and orientation
The complete placement of a rigid body in space: three translations and three rotations, six numbers in total.
Positioning accuracy
How closely the platform reaches a commanded pose, measured at a stated reference point under a stated load with a stated instrument.
In practiceA figure derived from encoder resolution and gear ratio is not the same as a measured figure, and both are printed the same way. Ask which one you are reading.
Q
1 term
Quaternion
Also called Euler parameters
A four-parameter representation of orientation that avoids the gimbal-lock and ordering ambiguities of Euler angles. One of three common representations alongside direction cosine matrices and Euler angles.
R
5 terms
Reachable workspace
The set of all poses a reference point on the platform can reach, without regard to the orientation it holds there.
Workspace analysis →
Reference point
Also called Datum point
The point on the platform or payload to which quoted travel, angle and accuracy figures apply.
In practiceTable centre and payload centre of gravity are not the same point. Without a stated reference point, motion figures cannot be compared between suppliers.
Repeatability
How consistently the platform returns to the same pose on repeated commands. Usually a tighter figure than absolute accuracy, and often the more relevant one for testing.
Residual function
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 these residuals squared.
Roll
Rotation about the longitudinal X axis — banking left and right.
S
13 terms
S-curve acceleration
Also called S-type acceleration
An acceleration profile that ramps jerk rather than stepping acceleration, giving smooth transitions between speed changes and reducing structural excitation.
Safety factor
The margin between the design load a component is engineered to withstand and its rated working load. CSCMotion designs mechanical components to more than three times rated load.
Servo drive
Also called Amplifier
The power electronics unit converting controller commands into motor current, closing the current and velocity loops beneath the position loop.
Servo electric cylinder
Also called Electromechanical actuator, electric cylinder
A linear actuator pairing a servo motor with a screw drive, forming the variable-length limb of the platform. CSCMotion designs and manufactures its own.
Platform range →
Singularity
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 severely nearby.
In practiceUsable workspace excludes poses close to singularity, not only those that are geometrically unreachable.
Hexapod kinematics →
Site acceptance test (SAT)
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.
Specific force
The non-gravitational acceleration acting on a body, which is what the vestibular system actually senses. Motion cueing works by reproducing specific force rather than reproducing acceleration directly.
In practiceThis is why tilt works at all: tilting the platform redirects gravity to produce a specific force the occupant reads as sustained acceleration.
Motion cueing →
Spherical joint
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.
In practiceFree axial rotation means the limb cannot resist torsion, so torsional load must be carried by the arrangement of the six limbs together.
Joint selection →
Stewart platform
Also called Gough-Stewart platform, hexapod
A parallel kinematic mechanism joining a moving platform to a fixed base through six variable-length limbs. Developed by Gough in 1949 for tyre testing and publicised by Stewart in 1965 for flight simulation.
In practiceDescribes an architecture, not an axis count. Not every 6DOF mechanism is a Stewart platform, and not every Stewart platform is used for simulation.
Stewart platforms →
Stiffness
Also called Rigidity
Resistance to deflection under load. Parallel mechanisms achieve high stiffness relative to their mass because load is shared across six limbs.
Stroke
Also called Actuator stroke
The change in length available in each actuator. An input to the geometry calculation, not a statement of platform displacement.
In practiceStroke is not travel. Platform motion is a geometric result of all six limb lengths acting together.
Sizing guide →
Surge
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.
Sway
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.
T
4 terms
Tilt coordination
The cueing module that converts the low-frequency component of commanded translational acceleration into a small angular displacement, so gravity supplies a sustained specific force within a finite workspace.
In practiceReproduces the plateau of an acceleration but never its onset, which requires genuine translation.
3DOF vs 6DOF →
Trajectory
Also called Motion profile
A time-parameterised sequence of poses the platform is commanded to follow.
In practiceOne representative combined trajectory is far more useful to a supplier than a list of per-axis maxima.
Trajectory planning
Also called Velocity planning
The stage that converts target positions into an executable, time-parameterised motion respecting velocity, acceleration and jerk limits.
Travel
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.
In practicePer-axis maxima are each measured with the other axes near neutral and cannot be added.
Sizing guide →
U
1 term
Update rate
Also called Command rate
How frequently the host issues new pose commands and the controller accepts them, in hertz. Affects smoothness of host-driven motion and perceived latency.
Controller & API →
V
1 term
Vestibular system
Also called Inner ear
The human balance organ that senses angular rate and specific force. It responds to changes in motion rather than to steady states, which is the property every motion cueing algorithm exploits.
W
2 terms
Washout filter
Also called Washout algorithm
The filtering structure inside a cueing algorithm that returns the platform toward neutral after a manoeuvre, freeing workspace for the next input. Classical implementations combine high-pass and low-pass filters on specific force and angular velocity.
In practiceThe return to neutral must stay below human perception thresholds, or the occupant feels the washout itself as a false cue.
Motion cueing →
Workspace envelope
Also called Motion envelope
The coupled six-dimensional region of poses a platform can reach and use in practice, after limb length, joint articulation, clearance, force and conditioning limits are applied.
In practiceNot a rectangular box. Per-axis maxima are each measured with the other axes near neutral and cannot be added together.
Workspace analysis →
Y
1 term
Yaw
Also called heading
Rotation about the vertical Z axis — heading change. Not available on a conventional 3DOF platform.
Z
1 term
Zero calibration
Also called Home position
Establishing the reference zero of each actuator so commanded and actual positions agree at start-up. Performed before any accuracy figure has meaning.
Three things each entry does that a dictionary definition does not
Terminology in this industry is not standardised. The same word appears in two quotations meaning two different things, and the difference is usually discovered late. These entries are written to prevent that.
The first sentence stands alone
Every definition opens with a complete, self-contained statement, so it can be quoted in a specification or lifted by a search engine without losing its meaning. The elaboration comes afterwards, never before.
The “In practice” line
Thirty-eight entries carry one. It is not extra detail — it states where the term causes trouble in a real project. Most often that is where two suppliers use the same word for different quantities.
The synonyms are listed
Fifty-two entries name the alternative terms in circulation — hexapod and Stewart platform, gross moving load and total moving mass. You should be able to find an entry using whichever word your supplier used.
| Term | Where it goes wrong |
|---|---|
| Degrees of freedom | A motion platform's 3DOF is pitch, roll and heave. A VR headset's 3DOF is three rotations with no translation. Different axis sets, same label. |
| Payload | Whether the figure includes the cabin, the fixtures and the occupants varies by supplier. Ask what is inside the number before comparing two of them. |
| Stroke | Actuator stroke is not platform travel. Platform motion is the geometric result of six leg lengths acting together. |
| Positioning accuracy | A figure derived from encoder resolution and a figure measured under load with a laser system are different quantities. Ask which one you are reading. |
| Reference point | Platform centre and payload centre of gravity are not the same place. Without a stated reference point, travel figures cannot be compared between suppliers. |
Five terms that are routinely used two different ways, and where the difference bites.
Found an entry you disagree with? Terminology disputes in this field are usually productive — they surface a real ambiguity rather than a mistake. If a definition here conflicts with the one your organisation uses, tell us and we will either correct it or add the alternative usage to the entry. Send a correction.
From a definition to a decision
Sizing
Payload, stroke & acceleration
How the three constrain each other, and why an acceleration figure means nothing without the load it was measured at.
Principles
How a 6DOF platform works
Six actuators, one rigid body: the kinematics, the joints and the control loop, explained from first principles.
Comparison
3DOF vs 6DOF
Which axes each configuration gives you, what the third and sixth degrees of freedom actually buy, and when three is enough.
Evidence
Delivered project records
Where these terms appear in real projects — requirement, engineering decision and measured result for each.
Four words worth checking in a quotation
These four are the ones most often used differently by different suppliers, and the difference is rarely visible until acceptance. Each links to its definition here. Ask every supplier the same question about each — including us.
The four
Short questions, and the answers tell you a lot about who you are dealing with.
Payload. Does the figure include the cabin, the fixtures and the occupants?
Stroke. Is that actuator stroke or platform travel?
Accuracy. Measured or derived — and at what load?
3DOF. Which three axes, specifically?