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Payload, Stroke & Acceleration: How to Size a Motion Platform
These are the three numbers every enquiry starts with, and the three that are most often stated in a way nobody can quote against. They are also not independent — you can fix any two, and the third follows from the machine you are prepared to buy. This guide explains what each one actually means, and helps you write a specification that gets an accurate answer the first time.
Sizing guide · CSCMotion Engineering Team · Updated August 6, 2026 · Reading time ~14 min
Fig. 1 · What the actuators are actually working against
Height is a multiplier, not a detail. The actuators do not simply hold the mass — they control it through a lever equal to the centre-of-gravity height. Doubling h roughly doubles the moment at the same acceleration.
Gross moving load
Everything that moves
Everything that moves, not just the item being tested.
CoG height
A moment multiplier
Frequently governs sizing more than mass does.
Stroke ≠ travel
Two different quantities
Actuator length change is not platform displacement.
Pick any two
The third follows
Payload, stroke and acceleration are coupled, not independent.
Payload, Stroke and Acceleration Are Not Three Separate Numbers
Almost every enquiry we receive treats them as a shopping list: this much payload, this much travel, this much acceleration. But they are three faces of one engineering problem, and the moment you fix two of them the third is decided by physics and by how much actuator you are willing to pay for.
Direct answer. 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 required distance. Increase payload and you either accept lower acceleration or buy larger actuators; increase stroke and the whole machine grows. There is no configuration that maximises all three.
Variable 01
Payload
Determines the force required at a given acceleration, and — through centre-of-gravity height and inertia — the moment the platform must control. This is the input that is most often understated.
Variable 02
Stroke
Determines the physical size of the actuators and the mechanism. Longer stroke means longer cylinders, taller neutral height, more structure and a larger footprint — and it interacts with buckling and lateral stiffness.
Variable 03
Acceleration
Determines the force the actuators must deliver dynamically, and the thermal duty they must sustain. This is the variable most often over-specified, because peak figures get quoted where sustained figures are meant.
The practical consequence for your enquiry: a specification that states all three at their maximum desirable values, with no indication of which one is negotiable, cannot be answered accurately. It will either receive a quotation for the most expensive machine that satisfies every line, or a set of questions that delays the project by a week. Telling a supplier which of the three has the least room to move is the single most useful thing you can do.
Payload: What Actually Counts as Moving Load
The industry term is gross moving load, or GML, and the distinction matters. It is not the mass of the thing you are testing. It is the mass of everything the platform has to accelerate.
We ask for GML rather than payload because the gap between the two is routinely large. A test article of 300 kg arrives on a fixture of 400 kg. A cockpit shell weighs more than the two occupants inside it. Cable looms, instrumentation, seats, ballast and the mounting interface all move with the platform and all consume capacity.
Underestimating GML is the most common single cause of an under-sized platform — and it is usually discovered at commissioning, when nothing can be done about it cheaply.
- Device under test, test article or vehicle
- Fixture, adapter plate and mounting hardware
- Cabin, cockpit shell or enclosure
- Occupants, seats and restraints
- Instrumentation, sensors and data acquisition
- Cable looms, hoses and their drag
- Ballast, counterweights and anything added later
Two further properties matter as much as the total, and both are about how that mass is arranged rather than how much of it there is.
How a complete payload specification reads
This is the format we use internally and the format that lets us answer an enquiry without a round of questions. The figures below are illustrative — the structure is the point.
| Gross moving load (GML) | 2000 kg |
| GML CoG above moving platform | 1000 mm |
| Ixx about payload CoG | 4000 kg·m² |
| Iyy about payload CoG | 4000 kg·m² |
| Izz about payload CoG | 4000 kg·m² |
| Actuator stroke | 650 mm |
| Fixed height | 1260 mm |
Note that the centre of gravity is stated above the moving platform surface, and the inertias are stated relative to the payload centre of gravity. Both reference points must be explicit or the numbers cannot be used.
Same mass · easy
100 kg in a compact block
A dense payload sitting low on the platform, roughly 100 mm cube. The centre of gravity is close to the mounting surface, the moment arm is short and the inertia is small. Almost any platform rated for 100 kg will handle this comfortably.
Same mass · hard
100 kg spread over 3 metres
The same 100 kg distributed across a large frame. The centre of gravity may sit well above the platform, and the inertia is orders of magnitude higher. This case can challenge a platform rated many times the mass — because the number that matters was never the mass.
Why this matters. Two payloads of identical weight can impose completely different demands. This is why we ask for centre-of-gravity height and moment of inertia rather than accepting a mass figure alone — and why a quotation given against mass only should be treated as provisional.
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. These are different quantities, related by the mechanism geometry, and confusing them produces specifications that cannot be met.
Distinction 01
Stroke is an input, travel is a result
On a parallel platform, every axis of platform motion is produced by all six limbs acting together. Platform displacement is a geometric consequence of the six lengths and the attachment layout — it is calculated, not read off the actuator.
Distinction 02
Travel is not constant across the workspace
Available travel on one axis depends on where the platform currently is on the others. Near the edge of the envelope, remaining travel can be a fraction of the headline figure.
Distinction 03
Per-axis maxima cannot be added
Each published maximum is measured with the other axes near neutral. Using travel on surge removes it from heave and pitch. A combined pose at three simultaneous maxima does not exist.
Distinction 04
Travel needs a reference point
A stated translation applies to a specific point. Rotation about a pivot low in the mechanism moves a high payload much further than the same rotation about a pivot at the payload centre of gravity.
Distinction 05
Longer stroke grows the whole machine
More stroke means longer cylinders, greater neutral height, more structure, a larger footprint and more ceiling. It also affects buckling margin and lateral stiffness, so it is never a free parameter.
Distinction 06
Joint articulation can bind first
Particularly with Cardan joints, the angular range of the joints can become the limiting constraint before the actuators run out of stroke. Workspace checks must test joint angles, not just lengths.
What to state in an enquiry: the translation and rotation you need on each axis, the point on the payload those values apply to, and — most usefully — one representative combined trajectory rather than a list of individual maxima. A single realistic profile tells a supplier more than six maximum values, because it describes a motion that has to exist simultaneously. See Stewart platform design for how geometry sets the envelope.
Acceleration: The Number That Needs the Most Qualification
An acceleration figure on its own is close to meaningless. The same platform can be described as capable of 1 g or 0.3 g depending on payload, pose, whether the figure is peak or sustained, and which axis is being discussed.
Qualifier 01
Peak versus sustained
Peak acceleration is what the actuators can deliver briefly. Sustained acceleration is what they can hold without exceeding thermal limits. Electric actuators are thermally constrained, so the two figures can differ substantially and both belong in a specification.
Qualifier 02
At what payload
An acceleration figure is only meaningful paired with the GML it was measured at. A headline number quoted at low or zero payload tells you nothing about performance with your load on the platform.
Qualifier 03
Where in the workspace
Force transmission varies across the envelope. Acceleration available near the centre of the workspace is generally higher than at the edges, where geometry is less favourable.
Qualifier 04
Linear or angular
Linear acceleration in g and angular acceleration in degrees per second squared are separate specifications governed by different properties — mass for one, inertia for the other. State both if both matter.
Qualifier 05
Single axis or combined
Peak acceleration on one axis with the others quiet is a different demand from simultaneous acceleration on several. Combined manoeuvres share the same six actuators.
Qualifier 06
Duty cycle around it
Hours per day, continuous or intermittent, and the duty pattern all affect actuator selection. Two platforms with identical peak requirements can need different ratings if one runs a full shift.
What the Payload–Acceleration Relationship Actually Looks Like
The table below summarises ranges across motion platforms we have built and delivered. It is deliberately shown as bands rather than models, because these were engineered to individual requirements rather than selected from a catalogue.
| Gross moving load | Linear acceleration seen | Translation range | Typical character |
|---|---|---|---|
| Under 100 kg | 0.3 – 0.6 g | ±50 – 380 mm | Precision positioning and laboratory work; accuracy usually matters more than acceleration. |
| 100 – 500 kg | 0.5 – 0.6 g | ±150 – 400 mm | Single-seat simulators, compact test rigs, research platforms. |
| 500 – 2000 kg | 0.3 – 0.6 g | ±180 – 380 mm | Full cabins, driving and flight simulators, ship motion work. |
| 2000 – 5000 kg | 0.4 – 1.0 g | ±250 – 900 mm | Large cabins and vehicle-scale rigs. The widest spread of all — see the note below. |
| Above 5000 kg | 0.1 – 0.3 g at short stroke | ±100 mm upward | Heavy structures and large assemblies; acceleration is normally traded away for capacity. |
Indicative ranges from delivered CSCMotion platforms. Values are illustrative of what has been built, not a product specification or a performance commitment for any particular configuration.
Read the 2000–5000 kg row carefully, because it contains the lesson. Within one payload band we have delivered platforms achieving anywhere from 0.4 g to about 1.0 g. The high-acceleration examples are physically much larger machines, with roughly ±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 — which is another way of saying it is a function of budget and footprint. A 3-tonne platform can reach 1 g; it simply costs and occupies more than a 3-tonne platform that reaches 0.4 g.
Specification Builder
Fill in whatever you know — blanks are fine and are more useful than guesses. The tool assembles a structured specification block you can copy straight into an email or a tender document. It runs entirely in your browser and nothing is transmitted anywhere.
Your specification block
Copy this into an email to allcontroller@motionplat.com, or paste it into the enquiry form. Fields left blank are marked so we know what still has to be established.
What this tool does and does not do. It formats your inputs into a complete, unambiguous specification. It deliberately does not estimate actuator force, recommend a model or predict achievable performance — those come from the dynamic solution, which is run per project against your actual trajectory and mass properties. Any web tool claiming to size a platform from four fields is guessing, and a guess is not something you can build a procurement on.
How We Actually Size the Platform
Your specification becomes the input to a defined sequence. Each step produces the number the next step needs, which is why the inputs above matter — a missing mass property stalls the chain at step two.
01
Build the load model
Gross moving load, centre-of-gravity position and moments of inertia are assembled into a mass model, including the offset range the centre of gravity may occupy across configurations.
02
Map the workspace
Candidate geometry is sampled — we use Monte Carlo sampling with boundary search — and checked against limb length, joint articulation, clearance and singular configurations. Your trajectory must fit inside the usable volume with margin.
03
Solve the dynamics
The trajectory is run through the mechanism with the real mass model to find peak limb force and velocity, and to identify the worst-case pose — which is rarely the one people expect.
04
Select motor and screw
Total torque is decomposed into its contributors — motor inertia, screw inertia and payload gravity torque among them — then motor speed and screw lead are checked against the required velocity profile.
05
Verify the structure
The same peak forces become the load case for stiffness checks, buckling stability, modal analysis, transient shock response and fatigue life on the structure and joints.
06
Optimise against cost
The selection is evaluated as performance against cost rather than performance alone. This is where a stated priority from your side — which of the three is least negotiable — changes the answer materially.
3×
Mechanical components are designed to more than three times rated load
12 h
Continuous operation with actuator position drift held within 0.25 mm
Measured
Positioning and repeatability verified on a laser measuring system before shipment
Common Sizing Mistakes
Every one of these is avoidable at no cost, and every one of them we see regularly in incoming enquiries and tender documents.
Mistake 01
Quoting payload, not GML
Why it fails: fixtures, cabins and cabling routinely weigh as much as the item being carried. The platform accelerates all of it.
Mistake 02
Omitting centre-of-gravity height
Why it fails: height is a moment multiplier. A tall light payload can be more demanding than a low heavy one, and mass alone cannot reveal that.
Mistake 03
Confusing stroke with travel
Why it fails: actuator stroke is an input to the geometry, not a statement of platform displacement. The two are related but never equal.
Mistake 04
Adding per-axis maxima
Why it fails: the workspace is coupled. Maximum surge, heave and pitch describe three separate poses, not one combined pose that exists.
Mistake 05
Specifying peak without duty
Why it fails: electric actuators are thermally limited. Peak force with no duty cycle stated cannot determine a rating.
Mistake 06
Sizing with no margin
Why it fails: instrumentation, harnesses and ballast always get added after commissioning. Margin agreed at specification time costs far less than discovering the shortfall later.
Sizing FAQ
Short answers to the questions that come up most often before an enquiry.
What counts as payload on a motion platform?
Everything the platform carries and moves — the device under test, its fixture, any cabin or cockpit, occupants, cabling and secondary equipment. The industry term is gross moving load, or GML. Quoting only the primary payload is the single most common cause of an under-sized platform, because fixtures and cabins are routinely heavier than the item they hold.
Why does centre of gravity height matter so much?
Because it acts as a lever. The actuators do not merely support the mass, they control it through a moment arm equal to the height of the combined centre of gravity above the moving platform. Doubling that height roughly doubles the moment the platform must overcome at the same acceleration, so a tall light payload can be harder to move than a low heavy one.
Is actuator stroke the same as platform travel?
No. Actuator stroke is the length change available in each limb. Platform travel is the geometric result of all six limb lengths acting together, and it varies across the workspace. A specification that quotes actuator stroke where platform travel is meant will overstate what the platform can do. See How a 6DOF Motion Platform Works.
Can I get maximum travel on every axis at once?
No. The workspace is a coupled volume, so using travel on one axis removes it from the others. Published per-axis maxima are each measured with the remaining axes near neutral. This is why one representative combined trajectory is more useful to a supplier than a table of individual maximum values.
Does a heavier platform always mean lower acceleration?
Not directly. Acceleration is set by how much actuator capability was specified relative to the moving load, not by the load alone. Across delivered projects we have built multi-tonne platforms achieving around 1 g and much lighter platforms limited to 0.3 g, because the two were specified for different jobs. Payload does not determine acceleration; the pairing of the two determines the machine.
What is moment of inertia and do I need to provide it?
Moment of inertia describes how mass is distributed about the centre of gravity, and it governs rotational sizing in the same way mass governs translational sizing. Two payloads of identical mass, one compact and one spread out, impose very different rotational demands. Provide Ixx, Iyy and Izz relative to the payload centre of gravity if your CAD model can produce them; if not, overall dimensions and a mass breakdown let us estimate them.
How much payload margin should I specify?
Size against the load you will actually have in service, including everything added after commissioning — instrumentation, harnesses, seats, ballast. Specifying at the exact anticipated figure leaves nothing for the additions that always appear. Deliberate margin agreed at specification time is far cheaper than discovering the shortfall after delivery.
Why do you ask for duty cycle when sizing?
Because electric actuators are thermally limited. A platform performing short intermittent manoeuvres and one running continuously for a full shift can require different actuator ratings for identical peak force. Hours per day, whether motion is continuous or intermittent, and the ratio of peak to sustained acceleration all affect the selection.
Before or After This Decision
Sizing sits between choosing an architecture and writing an interface specification. These are the pages either side of it.
Comes first
3DOF vs 6DOF
Axis count is settled before sizing. It determines which travel figures you even need to specify.
Comes first
Electric vs Hydraulic
Drive technology interacts with duty cycle and payload ceiling — worth settling alongside sizing.
Goes deeper
Stewart Platform Design
How geometry sets the workspace envelope your travel figures have to fit inside.
Product
Motion Platforms
All-electric 3DOF, 6DOF and Stewart platforms, servo electric cylinders and control systems.
Including when the answer is no
Ask Us Whether Electric Actually Reaches Your Requirement
This is the question worth asking, and it has a real answer rather than a sales answer. Send the payload, trajectory and duty cycle, and we will tell you whether an electric platform meets them — including when it does not, in which case you will at least have a documented reason for choosing hydraulic.
What we need to answer it
With these, a feasibility answer usually takes one exchange:
Total moving mass and centre of gravity
Required travel per axis
Peak and sustained acceleration
Duty cycle and hours per day
Any shock or impact content
Room size, ceiling and HVAC capacity
Existing platform, if replacing one
Host interface and control requirements