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Payload, Stroke & Acceleration
How to Size a Motion Platform
Most motion platform enquiries arrive as three numbers: 500 kg, ±300 mm, 1 g. Each is a reasonable request. Together — with no centre of gravity, no reference point and no duty cycle — they describe a machine nobody can quote without guessing. The guess comes back as either the most expensive platform that meets every line, or a week of clarifying questions.
The short answer: payload, stroke and acceleration are one engineering problem, not three separate numbers. Fix any two and the third is decided by physics and by how much actuator you are willing to buy. Size against gross moving load, state the centre-of-gravity height, describe one real combined trajectory instead of six maxima, and tell the supplier which number cannot move.
Sizing guide · Reviewed by the CSCmotion Motion Control Engineering Team · Updated October 4, 2026 · Reading time ~16 min
Fig. 1 · Height is a multiplier, not a detail. The actuators do not just hold the mass; they control it through a lever equal to the centre-of-gravity height. Double h and the moment doubles at the same acceleration.
The one idea this page exists to convey
Payload, Stroke and Acceleration Are One Problem
Almost every enquiry treats the three as a shopping list. They are three faces of one problem. Force is roughly mass times acceleration, applied through a lever equal to the centre-of-gravity height. Stroke sets how large the actuators and the mechanism must be to deliver that force over the distance you need. There is no configuration that maximises all three.
Direct answer
Raise the payload and you either accept lower acceleration or buy larger actuators. Raise the stroke and the whole machine grows — longer cylinders, taller neutral height, more structure, a bigger footprint. Raise the acceleration and both force and heat go up. Fix any two, and the third is decided for you.
The Pick-Two Sizing Triangle Whatever you hold fixed, something else pays for it.
| If you fix… | …the third is decided by | What it means for your project |
|---|---|---|
| Payload and acceleration | How much actuator force and heat capacity fits inside the stroke you can accommodate | Travel becomes the variable; a larger envelope means a larger, costlier machine |
| Payload and stroke | Actuator force and thermal rating | Acceleration becomes the variable; more of it means bigger motors and screws |
| Stroke and acceleration | The force the mechanism can transmit across the workspace | Payload becomes the ceiling; GML and CoG height decide whether you are under it |
| All three, none negotiable | Budget and footprint | You receive the most expensive machine that meets every line, or a round of questions |
Here is the honest version: a specification that states all three at their ideal values, with no hint of which one can give, cannot be quoted accurately by anyone. Telling a supplier which number has the least room to move is the most useful line you can add — it changes the answer more than any other input.
Already know which number cannot move? Say so in the first message, and the first answer will be sized around it. Tell us your non-negotiable number →
Start here — it decides everything downstream
Payload: What Actually Counts as Gross Moving Load
The industry term is gross moving load, or GML. It is not the mass of the thing you are testing. It is the mass of everything the platform has to accelerate — and the gap between the two is routinely large. A 300 kg test article arrives on a 400 kg fixture. A cockpit shell outweighs the two people inside it.
- Device under test, test article or vehicle — the only line most enquiries include
- Fixture, adapter plate and mounting hardware — often as heavy as the article itself
- Cabin, cockpit shell or enclosure — usually the largest single mass in a simulator
- Occupants, seats and restraints — size for the heaviest realistic occupant
- Instrumentation, sensors and data acquisition — added late, rarely weighed
- Cable looms and hoses — their mass and their drag both cost capacity
- Ballast, counterweights and later additions — the line that appears after commissioning
Why we ask for GML, not payload: underestimating it is the most common single cause of an under-sized platform, and it is usually discovered at commissioning, when nothing can be fixed cheaply.
How a complete payload specification reads
| Gross moving load (GML) | 2,000 kg |
| GML CoG above moving platform | 1,000 mm |
| Ixx about payload CoG | 4,000 kg·m² |
| Iyy about payload CoG | 4,000 kg·m² |
| Izz about payload CoG | 4,000 kg·m² |
| Actuator stroke | 650 mm |
| Fixed height | 1,260 mm |
Illustrative, taken from a delivered configuration. The structure is the point: the CoG height is stated above the moving platform surface, and the inertias about the payload CoG. Both reference points must be explicit, or the numbers cannot be used.
Same 100 kg, two different problems Moment of inertia about the vertical axis, and the torque needed for 100°/s² of yaw acceleration.
| Payload arrangement | Mass | Inertia Izz | Torque at 100°/s² | What it means for your project |
|---|---|---|---|---|
| Compact block, about 100 mm cube | 100 kg | 0.17 kg·m² | 0.29 N·m | The spread payload needs about 1,350 times the rotational torque. Almost any 100 kg platform handles the block; the frame can defeat a platform rated for several times its mass. Mass was never the number that mattered. |
| Same mass as two 50 kg ends of a 3 m frame | 100 kg | 225 kg·m² | 393 N·m | The spread payload needs about 1,350 times the rotational torque. Almost any 100 kg platform handles the block; the frame can defeat a platform rated for several times its mass. Mass was never the number that mattered. |
I = m(a² + b²)/12 for the block · I = Σ m·r² for the frame, r = 1.5 m · torque = I · α
Have a CAD model of the moving assembly? Export its mass properties — total mass, CoG and inertia tensor — and we check them against the GML ledger. Send your mass-properties export →
The number most enquiries leave out
Centre-of-Gravity Height: The Number That Multiplies Everything
The actuators do not simply support the mass. They control it through a moment arm equal to the height of the combined centre of gravity above the moving platform. Run the arithmetic once and the reason we ask for CoG height before anything else becomes obvious.
The CoG Moment Multiplier One 800 kg moving load accelerated horizontally at 0.5 g: inertial force ≈ 3.9 kN in every row.
| CoG height above the platform | Inertial force | Moment the platform must control | What it means for your project |
|---|---|---|---|
| 0.3 m · low, dense test article | 3.9 kN | 1.2 kN·m | The baseline: most of the actuator capacity goes into moving the mass |
| 0.6 m · seated occupant, small cabin | 3.9 kN | 2.4 kN·m | Twice the moment from the same mass and the same acceleration |
| 1.2 m · tall cabin or raised fixture | 3.9 kN | 4.7 kN·m | Four times the baseline. The front and rear limbs see this as extra push and pull on every brake and launch. |
F = m · a = 800 kg × 4.9 m/s² ≈ 3.9 kN · M = F · h · gravity load of 7.8 kN acts in every row as well
That is why a tall, light payload can be harder to move than a low, heavy one, and why a quotation given against mass alone should be read as provisional. The moment also grows with every centimetre the CoG is allowed to move between configurations, so state the range as well as the nominal position.
One practical lever sits on your side of the interface: every millimetre you lower the fixture, the cabin floor or the heaviest instrument comes straight off the moment. Lowering the CoG is often cheaper than buying the larger actuators a tall payload needs.
Not sure where your CoG sits? Send overall dimensions and a mass breakdown; we estimate CoG height and inertia before sizing. Get a CoG and inertia estimate →
The most misquoted number in the industry
Stroke: Actuator Length Is Not Platform Travel
Actuator stroke is how much each limb can change in length. Platform travel is where the payload actually ends up. The two are related through the mechanism geometry and never equal, and confusing them produces specifications that cannot be met.
The Stroke-to-Travel Checklist Six distinctions, and what to write in your enquiry instead.
| Distinction | What goes wrong when it is ignored | What to state instead |
|---|---|---|
| Stroke is an input; travel is a result | Quoting actuator stroke as platform travel overstates the envelope | Translation and rotation required on each platform axis |
| Travel is not constant across the workspace | Near the edge of the envelope, the travel left on one axis can be a fraction of the headline figure | The pose each motion starts from, if it is not neutral |
| Per-axis maxima cannot be added | A pose at maximum surge, heave and pitch at once does not exist | One representative combined trajectory |
| Travel needs a reference point | Rotation about a low pivot moves a tall payload much further than the same rotation about its CoG | The point on the payload the values apply to |
| Longer stroke grows the whole machine | Longer cylinders, higher neutral height, more structure, larger footprint, less buckling margin | Ceiling height, floor space and access you actually have |
| Joint articulation can bind first | With Cardan joints — which every CSCmotion 6DOF platform uses — joint angle can run out before stroke does | Any large combined tilt, stated explicitly |
The numbers behind the distinction. In the illustrative hexapod worked through in How a 6DOF Motion Platform Works, 50 mm of heave needed only 40.6 mm of limb extension, while a surge-with-pitch move took 42.3 mm from two limbs that neither motion alone pushed past 26.0 mm. Stroke runs out on whichever limbs the combined motion loads — which is why one realistic trajectory tells a supplier more than six maximum values.
Holding a table of per-axis maxima? Replace it with one combined trajectory, and we tell you which limbs and joints it actually loads. Send a combined trajectory instead of six maxima →
Peak, sustained, and where in the envelope
Acceleration: The Number That Needs the Most Qualification
On its own, an acceleration figure is close to meaningless. The same platform can honestly be described as a 1 g machine or a 0.3 g machine depending on payload, pose, axis, and whether the figure is a peak or something it can hold.
The 6-Qualifier Acceleration Decoder Attach all six to every acceleration figure you send or receive.
| Qualifier | Question to answer | Why a bare figure misleads |
|---|---|---|
| 01 · Peak or sustained | How long must the acceleration be held? | Electric actuators are thermally limited; brief peaks and sustained demand can need different ratings |
| 02 · At what payload | What GML and CoG height was the figure measured with? | A figure measured empty says little about your load on the platform |
| 03 · Where in the workspace | Near neutral, or near the edge of travel? | Force transmission falls toward the edges, where the geometry is less favourable |
| 04 · Linear or angular | g for translation, °/s² for rotation — which do you need? | Linear acceleration is governed by mass; angular by inertia |
| 05 · Single axis or combined | Does the manoeuvre use one axis, or several at once? | Combined manoeuvres share the same six actuators |
| 06 · Duty cycle | Hours per day, continuous or intermittent? | Two platforms with identical peaks can need different actuators if one runs a full shift |
Why we do not print one acceleration figure per platform family: without the GML, the axis, the pose and the duration attached, it is a brochure number. We would rather give you a figure for your load case, with its conditions stated, than a headline you cannot hold us to.
Comparing acceleration figures across quotes? Send them over, and an engineer marks which of the six qualifiers each one is missing. Get the qualifiers checked on your quotes →
From platforms we have built and delivered
What Delivered Projects Show About Payload and Acceleration<
The bands below summarise motion platforms we have engineered and delivered. They are shown as bands rather than models, because each platform was built to an individual load case rather than picked from a catalogue.
The Delivered-Range Ledger Ranges across delivered CSCmotion platforms — not a product specification, and not a performance commitment for any particular configuration.
| Gross moving load | Linear acceleration delivered | Translation range delivered | What it means for your project |
|---|---|---|---|
| Under 100 kg | 0.3 to 0.6 g | ±50 to 380 mm | Precision positioning and laboratory work, where accuracy usually matters more than acceleration |
| 100 to 500 kg | 0.5 to 0.6 g | ±150 to 400 mm | Single-seat simulators, compact test rigs and research platforms |
| 500 to 2,000 kg | 0.3 to 0.6 g | ±180 to 380 mm | Full cabins, driving and flight simulators, industrial test rigs |
| 2,000 to 5,000 kg | 0.4 to 1.0 g | ±250 to 900 mm | Vehicle-scale rigs — and the widest spread of all. Read the note below. |
Heavier load cases are engineered and quoted individually.
Read the 2,000 to 5,000 kg row carefully
Within one payload band we have delivered platforms reaching anywhere from 0.4 g to about 1.0 g. The high-acceleration machines are physically much larger, with around ±900 mm of travel and correspondingly larger actuators. Acceleration is not a function of payload. It is a function of how much actuator capability was specified relative to the load — budget and footprint, in other words. A 3-tonne platform can reach 1 g; it simply costs and occupies more than a 3-tonne platform built for 0.4 g.
What these numbers don’t promise: each band is the outcome of specific load cases and trajectories. Your achievable envelope comes from running your GML, CoG and trajectory through the dynamic solution — the next section — not from finding your mass in a table.
Wondering where your load case lands? Send GML, CoG height and the acceleration you need, and we tell you which side of the band it sits on and why. Ask where your load case sits →
What happens after you send it
How We Size the Platform After You Send the Specification
Your specification becomes the input to a fixed sequence. Each step produces the number the next step needs — which is why a missing mass property stalls the whole chain at step two.
01
Build the load model
GML, CoG position and inertias become a mass model, including the range the CoG may occupy across configurations.
02
Map the workspace
Candidate geometry is sampled with Monte Carlo sampling and boundary search, then checked for limb length, joint angle, clearance and singular poses.
03
Solve the dynamics
Your trajectory runs through the mechanism with the real mass model to find peak limb force, velocity and the worst-case pose — rarely the one people expect.
04
Select motor and screw
Total torque is split into motor inertia, screw inertia and payload gravity contributions; motor speed and screw lead are checked against the velocity profile.
05
Verify the structure
The same peak forces become load cases for stiffness, buckling, modal analysis, transient shock and fatigue on the frame and joints.
06
Optimise against cost
The selection is judged as performance against cost. A stated priority — which number is least negotiable — changes the answer here more than anywhere else.
Your GML and trajectory then become the factory acceptance test: run on the assembled platform before it ships, with your own team or a third party such as SGS or BV welcome to witness.
What no web form can tell you: actuator force, model choice and achievable performance come out of steps 03 to 05, run per project. Any online tool claiming to size a platform from four fields is guessing, and a guess is not something you can build a procurement on.
Writing the acceptance criteria now? We propose the FAT test items your specification would generate, before you place the order. Request proposed FAT test items →
Turn this page into an enquiry
Write a Quotable Motion Platform Specification
Fill in whatever you know. Blanks are fine — a stated unknown is more useful than an optimistic placeholder. The builder formats your inputs into one structured block you can paste into an email or a tender document. It runs entirely in your browser; nothing leaves the page until you choose to send it.
Your specification block
Fill in the form and press Generate Specification. Blank fields are marked [ to be confirmed ], so the engineer reading it knows what still has to be established.
Or paste it into the enquiry form and attach your CAD mass properties or trajectory file. The builder formats; it does not size. Force, model choice and achievable performance come from the dynamic solution we run per project.
Six recurring specification errors
Common Motion Platform Sizing Mistakes
Every one of these costs nothing to avoid, and every one of them arrives in our inbox regularly.
The 6 Sizing Errors We See in Tender Documents Each fix is one line in your specification.
| Mistake | Why it fails | The one-line fix |
|---|---|---|
| Quoting payload, not GML | Fixtures, cabins and cabling often weigh as much as the item carried; the platform accelerates all of it | State gross moving load, itemised |
| Omitting CoG height | Height multiplies the moment; mass alone cannot reveal how hard the load is to move | State CoG height above the moving platform, plus its range |
| Confusing stroke with travel | Stroke is an input to the geometry, not a statement of platform displacement | Specify platform translation and rotation, with a reference point |
| Adding per-axis maxima | The workspace is coupled; three maxima describe three poses, not one | Send one representative combined trajectory |
| Peak without duty | Electric actuators are thermally limited; a peak alone cannot set a rating | State sustained acceleration and hours per day |
| Sizing with no margin | Instrumentation, harnesses and ballast always arrive after commissioning | Agree a GML margin now, while it is cheap |
Reviewing a tender document before it goes out? Send the draft, and an engineer marks which of the six errors it contains. Have your tender specification reviewed →
From numbers to a platform family
Where Your Numbers Land
Axis count is settled before sizing; sizing then decides the actuators and the geometry. Each family below is built to the load case, not picked from a fixed catalogue.
Six coupled axes
6DOF Motion Platform
For load cases that need surge, sway or yaw, or a full rigid-body pose reproduced as one motion.
- Fits research labs, industrial motion testing, simulator integrators
- Not for tasks that only ever tilt and heave
Three axes, simpler sizing
3DOF Motion Platform
Pitch, roll and heave with three actuators — fewer parts to size, integrate and maintain.
- Fits tilt and heave testing, compact simulators
- Not for anything that needs surge, sway or yaw
Stiffness first
Stewart Platform
The hexapod sized around stiffness, joint layout and positioning duty rather than large motion.
- Fits precision positioning, test fixtures, calibration benches
- Not for large-amplitude motion where workspace dominates
Load case outside every family? Custom builds start from your GML, CoG and trajectory. See custom motion platform engineering →
Frequently asked
Motion Platform Sizing — FAQ
Short answers to the questions that come up before an enquiry — including the sceptical ones.
What counts as payload on a motion platform?
Everything the platform moves: the device under test, its fixture, any cabin or cockpit, occupants, seats, instrumentation, cabling and ballast. The industry term is gross moving load, or GML. Quoting only the primary payload is the most common cause of an under-sized platform.
Why does centre-of-gravity height matter so much?
Because it acts as a lever. The actuators control the load through a moment arm equal to the CoG height above the moving platform. For 800 kg at 0.5 g, raising the CoG from 0.3 m to 1.2 m raises the moment from about 1.2 kN·m to 4.7 kN·m — four times as much, with the same mass and acceleration.
Is actuator stroke the same as platform travel?
No. Stroke is the length change available in each limb; travel is the geometric result of all six limbs acting together, and it varies across the workspace. Quoting stroke where travel is meant overstates what the platform can do. See How a 6DOF Motion Platform Works.
Does a heavier platform always mean lower acceleration?
No. Acceleration is set by how much actuator capability is specified relative to the load. In our delivered 2,000 to 5,000 kg band, platforms range from 0.4 g to about 1.0 g, because they were specified for different jobs. The pairing decides the machine, not the mass.
Do I need to provide moment of inertia?
If your CAD model can export it, yes: Ixx, Iyy and Izz about the payload CoG. Inertia governs rotational sizing the way mass governs translation — the same 100 kg can need about 1,350 times the yaw torque when it is spread across a 3 m frame. If you cannot export it, send overall dimensions and a mass breakdown and we estimate it.
Why won't you quote a price from payload alone?
Because the same GML can need machines several times apart in actuator capability, depending on CoG height, travel, acceleration and duty. A price from mass alone would be a guess we would later have to revise. Price and lead time are confirmed at quotation, against your full specification.
Can an online calculator size the platform for me?
No, and we do not offer one that pretends to. Actuator force and achievable performance come from running your trajectory through the mechanism with your real mass model. The Specification Builder on this page formats your inputs so that analysis can start immediately.
Why do you ask for duty cycle?
Because electric actuators are thermally limited. Short intermittent manoeuvres and a full shift of continuous motion can need different actuator ratings for the same peak force. Hours per day and the ratio of peak to sustained acceleration both change the selection.
Before or after this decision
Related Motion Platform Engineering Guides
Sizing sits between choosing an architecture and writing the interface specification.
Principle
How a 6DOF Motion Platform Works
Why six actuators do not equal six axes.
Alongside
Electric vs Hydraulic
Drive technology interacts with duty cycle and payload.
Goes deeper
Stewart Platform Design
How geometry sets the envelope your travel must fit.
Ready when you are
Send the Specification, Get a Sized Configuration
With a complete set of inputs, an engineer can normally come back with a platform configuration, an achievable performance envelope and a quotation, without a round of clarifying questions. If something is still unknown, say so — we will tell you what it changes.
An engineer, not a sales inbox, replies within 24 hours — weekends included. Lead time is confirmed at quotation. Every platform carries a 12-month warranty. cscmotionglobal@gmail.com · +86 152 5530 5272