A Cable Robot is a parallel manipulator that positions an end-effector by pulling on multiple tensioned cables wound on motorised winches, with each cable acting like one leg of a parallel kinematic chain. Unlike a rigid-link robot arm, it replaces heavy beams and joints with cables, which slashes moving mass and lets the workspace scale to a stadium. The purpose is to move a payload — a camera, a spray head, a 3D-printer nozzle — fast and accurately over an area no rigid arm can reach. Skycam covers an entire NFL field at over 9 m/s using exactly this principle.
Cable Robot Interactive Calculator
Vary the frame size and end-effector position to see the four inverse-kinematic cable lengths update.
Equation Used
The calculator applies planar inverse kinematics for a four-cable rectangular cable robot. Each cable length is the straight-line distance from its corner anchor to the selected end-effector position.
- Four cable exit points are the rectangle corners.
- Target position is expressed as percent of frame width and height.
- Cable sag, stretch, pulley radius, and platform attachment offsets are ignored.
- Planar geometry only; all anchors and the end-effector lie in the same plane.
How the Cable Robot Works
A Cable Robot, also called a cable-driven parallel robot or CDPR, hangs an end-effector inside a frame from anywhere between 4 and 8 cables, each spooled by its own servo winch in a corner of the workspace. The control system reads where you want the platform to go, solves the inverse kinematics to get the required cable lengths, then commands each winch to pay out or reel in while monitoring tension on a load cell at every cable exit point. The cables can only pull, never push — so the platform stays controllable only when the combined wrench from all cables can balance gravity and any external load. That condition is called the wrench-feasible workspace, and it is the single most important concept in CDPR design.
Why build it this way? Cables are light, cheap, and stiff in tension. Replacing a 200 kg robot arm with 6 Dyneema cables and corner-mounted winches means the platform itself can be under 5 kg, so accelerations of 4 g or more are possible. The price you pay is that the system is underdetermined when any cable goes slack — even momentarily — and that is the most common failure mode you will see. Slack happens when a winch pays out faster than the platform falls, when a cable resonates and snaps tight again, or when payload shifts off-centre and one corner unloads. If your tension drops below roughly 50 N on a typical sport-camera build, you will see visible platform jitter on the next acceleration spike.
The other failure mode is cable creep. Synthetic ropes like Dyneema SK78 stretch under sustained load — typically 0.3 to 0.7% — which means after 30 minutes of operation your zero position has drifted several millimetres. Production CDPRs either use steel cable with a known modulus, run a vision-based homing routine every cycle, or both.
Key Components
- Servo Winch Drum: Each cable spools onto a grooved drum driven by a servo motor with absolute encoder feedback. The drum diameter is sized so one full revolution corresponds to a known cable length — typically 100 to 250 mm — and the groove pitch must match the cable diameter within ±0.1 mm to prevent overlap-spooling, which causes length errors of several centimetres on long pulls.
- Cable: Steel rope (1×19 or 7×19) or synthetic high-modulus fibre like Dyneema SK78 or Vectran. Diameter is typically 2 to 8 mm depending on payload. Working load limit must include a safety factor of 5× minimum because a single cable failure on a fully-suspended CDPR drops the payload.
- Pulley Exit Point: A guide pulley at each corner of the frame redirects the cable from the winch up to the platform. The pulley centre is the kinematic anchor — its position must be surveyed to within ±2 mm of nominal, because every millimetre of error there propagates directly into platform position error.
- End-Effector Platform: The moving body that carries the payload. Cables attach at known points on the platform; the geometry of those attachment points sets the rotational stiffness. A platform with cables attaching close to the centre of mass tilts easily — spread the attachments wide for stability.
- Tension Sensor: An inline load cell or a force-sensing pulley at each cable. Real-time tension feedback lets the controller redistribute load if any cable approaches slack or exceeds its working limit, which is essential in redundant (more than 6 cables for 6 DOF) configurations.
- Controller: Solves inverse kinematics, distributes wrench across redundant cables (a quadratic program in real time), and closes a position loop at typically 1 kHz. Skycam-class systems run on dedicated motion controllers; research builds use ROS with a custom CDPR plugin.
Real-World Applications of the Cable Robot
Cable robots show up wherever the workspace is too big for a rigid arm or the payload is too light to justify one. The defining trait is reach — a single CDPR can cover thousands of square metres, which no articulated robot can match without a gantry the size of a hangar. They also dominate when you need to keep heavy machinery out of the working volume, such as filming a live crowd or printing a building. The trade is geometric: you need anchor points at every corner, and those anchors have to handle the full cable tension, which can easily exceed 5 kN per corner on a sport-camera rig.
- Broadcast Sport: Skycam and Spidercam systems used at NFL, Champions League, and Olympic venues to fly broadcast cameras over the field at speeds up to 9 m/s using 4 cables driven by 5 kW winches in each stadium corner.
- Construction & 3D Printing: The NIST RoboCrane and the IPAnema 3 from Fraunhofer IPA — large-format CDPRs that position lifting hooks, welding heads, or concrete-extrusion nozzles across building-scale workspaces of 10×10×5 m or larger.
- Rehabilitation Robotics: The String-Man and similar gait-training systems use 4 to 8 cables to support patient body weight and apply controlled forces during treadmill walking, replacing bulky overhead hoists.
- Radio Astronomy: The 500 m FAST telescope in Guizhou, China, uses 6 long cables to position a 30-tonne feed cabin to within 10 mm above the dish reflector — the largest CDPR ever built.
- Warehouse and Logistics: Cable-driven pick-and-place systems for parcel sorting, where a single platform serves a 20×20 m floor area at 3 m/s without any rails or gantry beams in the workspace.
- Wind Tunnel Testing: DLR's SUPRA cable suspension system holds aircraft scale models inside test sections with minimum aerodynamic interference, replacing rigid stings that disturb the flow field.
The Formula Behind the Cable Robot
The core sizing question on any cable robot is: how much tension does each cable need to hold the platform steady at a given pose? That answer comes from the static wrench-balance equation. At the centre of the workspace the cables share the load fairly evenly and tensions sit in the comfortable middle of their working range. Push the platform towards a corner and one cable goes nearly slack while the opposing cable spikes — that is where you find the edge of the wrench-feasible workspace. Move outside it entirely and the system loses controllability. The sweet spot for a 6-cable suspended CDPR is roughly the inner 60% of the convex hull of the anchor points.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Ti | Tension in cable i (must be ≥ 0, cables cannot push) | N | lbf |
| ûi | Unit vector from platform attachment point to anchor pulley for cable i | dimensionless | dimensionless |
| W | External wrench on platform — gravity plus payload plus any applied force | N (and N·m for moments) | lbf (and lbf·ft for moments) |
| n | Number of cables (must be ≥ DOF + 1 for full controllability) | count | count |
Worked Example: Cable Robot in a vineyard spray-canopy cable robot
A precision viticulture startup outside Mendoza, Argentina is building a 4-cable suspended cable robot to fly a 12 kg foliar-spray head over a 30 m × 30 m × 4 m vineyard block. They mounted winches on 4 m posts at the corners and need to know the cable tension at three operating positions: dead centre of the block, mid-edge, and 2 m from a corner. Total payload including the platform is 18 kg.
Given
- m = 18 kg
- g = 9.81 m/s²
- Workspace = 30 × 30 × 4 m
- Anchor height = 4 m
- n = 4 cables
- Operating height = 0.5 m above canopy
Solution
Step 1 — compute the total downward load the cables must support at any position:
Step 2 — at the nominal centre position (15, 15, 1) the platform sits 3 m below each anchor and 15 m horizontally from each. The cable elevation angle θ is roughly 11.3° above horizontal. Each of the 4 cables shares the load equally, but only the vertical component holds weight:
That is a comfortable working tension — well within the rating of any 4 mm Dyneema rope and easily handled by a 200 W servo winch.
Step 3 — at the low-load end of the workspace, the platform sits high (operating height 3 m, only 1 m below anchors). Cable angle steepens to roughly 45° and the geometry favours the cables:
Tensions this low are dangerously close to slack territory. Below about 50 N you start to see cable sag and platform sway under the slightest gust — so the centre-high zone is where wind disturbance shows up worst.
Step 4 — at the high-load end, near a corner at (28, 28, 1), the geometry goes asymmetric. The far cable does almost all the work while the near cable goes near-slack. A static analysis with the actual pulley positions gives:
900 N on a single cable means you must size the winch motor and the rope for that worst case, not the nominal. And 15 N is effectively slack — the platform is on the edge of the wrench-feasible workspace, which is exactly why production CDPRs limit travel to the inner 60% of the anchor footprint.
Result
Nominal cable tension at the centre of the vineyard block is 225 N per cable. That is the sweet spot — every winch is loaded equally, the rope is in its happy zone, and platform stiffness is high enough that a 5 m/s wind gust deflects the spray head by only a few centimetres. At the high-altitude centre the tension drops to 62 N, near the cable-sag threshold where wind disturbance dominates; near a corner one cable spikes to 900 N while the opposing cable falls to 15 N, which is effectively slack. If you measure tensions that disagree with these numbers, the usual suspects are: (1) anchor pulley positions surveyed wrong by more than ±20 mm, which throws the cable unit vectors off and skews the load distribution, (2) platform centre-of-mass offset from the geometric centre because the spray tank drains asymmetrically — fix this with baffles or a pressurised reservoir, or (3) Dyneema cable creep after the first hour of operation, which lengthens one cable more than the others and unloads it.
When to Use a Cable Robot and When Not To
Cable robots compete with rigid-arm robots and gantry systems. The choice comes down to workspace size, payload, accuracy requirement, and how much of the working volume must stay clear of structure.
| Property | Cable Robot (CDPR) | 6-Axis Articulated Arm | Cartesian Gantry |
|---|---|---|---|
| Workspace volume (typical) | 100 to 100,000 m³ | 1 to 30 m³ | 1 to 500 m³ |
| Position accuracy | ±2 to ±20 mm | ±0.05 to ±0.2 mm | ±0.05 to ±0.5 mm |
| Max tip speed | Up to 10 m/s | 2 to 4 m/s | 1 to 3 m/s |
| Payload capacity | 1 to 100 kg typical, up to 30 t (FAST) | 0.5 to 2,000 kg | 10 to 5,000 kg |
| Installed cost per m³ workspace | Low — roughly $5 to $50/m³ | High — $5,000+/m³ | Medium — $200 to $2,000/m³ |
| Workspace obstruction | Cables only — floor stays clear | Arm sweeps through the volume | Gantry beams overhead |
| Setup and calibration time | Hours to days — anchor survey is critical | Plug-and-play, ~1 hour | 1 to 3 days for rails |
| Stiffness (controllable) | Low to medium — depends on tension | Very high | High |
Frequently Asked Questions About Cable Robot
Horizontal acceleration commands change the cable angles, which couples directly into vertical tension changes. If your control loop only commands cable length and not tension, every pulse-acceleration excites the natural sag-mode of the cables — typically 1 to 3 Hz on a 20 m span. The platform bounces because each cable behaves like a soft spring with about 5 to 20 N/mm stiffness, far softer than a rigid linkage.
Fix it by adding a tension-feedforward term: command the controller to pre-tension all cables before the move starts, and add inline load cells if you don't already have them. On Skycam-class systems this is standard; on hobby builds it's the missing piece.
For 6 degrees of freedom you need a minimum of 7 cables to keep all of them in tension at every pose (n ≥ DOF + 1). 4 cables only give you 3 controllable DOF — fine for a Skycam-style camera platform where orientation is handled by a separate gimbal, but useless if you need to actively control roll, pitch, and yaw of the platform itself.
Going to 8 cables instead of 7 buys you redundancy: if one winch fails, the system can still hold the platform. It also lets the controller redistribute load to keep all tensions inside a target band. The extra cable doubles your real-time computation cost because the wrench-distribution problem becomes a quadratic program with inequality constraints, but on any modern controller that runs in well under 1 ms.
That is almost certainly cable creep. Synthetic ropes like Dyneema SK78 exhibit time-dependent elongation under sustained load — typically 0.3 to 0.7% over the first hour, then stabilising. On a 15 m cable at 200 N tension, 0.5% creep is 75 mm of rope length change, which translates to several millimetres of platform drift depending on geometry.
You have three options: switch to steel cable (modulus around 100 GPa versus Dyneema's effective 30 to 50 GPa, and almost no creep), pre-load the cables overnight to bleed out the primary creep before operation, or close an outer position loop with a vision system or laser tracker that homes the platform every cycle. Production CDPRs running synthetic rope all use option three.
For workspaces above roughly 50 m² it is genuinely cheaper, but only if you already have something to anchor to. A 20 × 20 × 4 m gantry needs 80 m of linear rail at $300 to $1,000 per metre plus the cross-beams — easily $40k in steel before motors. The equivalent CDPR needs 4 to 8 winches at $2k to $5k each, the cable, anchor brackets, and the controller — call it $25k to $40k all in.
What kills the cost saving is when you need to install dedicated anchor towers because the building corners aren't strong enough. Each anchor must take the full corner tension — on a 50 kg payload CDPR that can be 5 kN — and a code-compliant freestanding tower rated for that load can cost $10k each. So check your anchor situation before assuming cables win.
The wrench-feasible workspace is the set of poses where every cable can stay within its tension limits — minimum tension to avoid slack, maximum tension to avoid breaking. As payload increases, the lower bound is easy to satisfy (more weight means more tension everywhere) but the upper bound on the heavily-loaded cables hits the ceiling much sooner.
Geometrically, the workspace contracts inward from the corners first because that is where load distribution is most asymmetric. A common rule of thumb: doubling payload roughly halves the usable horizontal area at a given height. If you find your platform losing controllability near edges only when carrying tools, this is exactly why — and the fix is either bigger winches and stronger cable, or accept a smaller working envelope.
Yes, but with caveats. The platform stiffness in the cable-tension direction is decent — typically 5 to 50 N/mm depending on cable length and pre-tension — but stiffness perpendicular to a cable is essentially zero except what the other cables provide. So pushing a tool sideways against a workpiece works fine; pushing along a single cable direction is springy.
For polishing or light assembly the answer is to use an impedance controller that exploits redundancy: with 8 cables on a 6-DOF platform you can independently command pose and internal pre-tension, which sets effective stiffness. Research groups at Fraunhofer IPA have demonstrated drilling and milling with a CDPR doing exactly this. Don't expect rigid-arm levels of stiffness, though — sub-millimetre force-controlled tasks are still where articulated arms win.
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