Performance
Work Envelope
A work envelope is the full three-dimensional volume of space the robot's tool center point can physically reach given the arm geometry and joint travel limits.
What it is
The work envelope is the complete set of points the TCP can occupy. It's a solid volume: sweep every joint through its allowed range and the space the wrist tip can touch is the envelope.
Its shape follows the arm. For a six-axis articulated robot the envelope is roughly a lopsided sphere with pieces missing: there's a hollow column around the base the arm can't fold into, a floor the arm can't dip below, and an outer skin set by full reach. The boundary isn't smooth because each joint stops at a hard limit.
How it actually works
Every point in the envelope is reachable, though some points are reachable only in certain tool orientations and some are poorly behaved. The volume includes singular poses, arm configurations where two joint axes line up and the controller loses a degree of freedom to control cleanly. Near those poses a small Cartesian move demands very large joint speeds, so the robot slows or faults even though the point is geometrically inside the envelope.
The near-base hollow is the other practical trap. An articulated arm can't reach straight down its own mast, so a workpiece sitting directly under J1 may be unreachable even though it's well within the maximum reach radius. When you place a fixture, you're fitting the part into the good interior of the envelope, clear of the base column and clear of the poses where the arm goes singular.
How it differs
- Reach · Reach is a single scalar: the maximum straight-line distance from the base to the fully extended TCP. The work envelope is the whole 3D volume that maximum sweeps out, complete with the interior hollow and the poses reach alone doesn't reveal. Two arms can share a reach figure and have very differently shaped envelopes.
- Singularity · A singularity is a specific pose inside the envelope where joint axes align and control degrades. It's a property of arm configuration, a matter of where the arm sits. Points near a singularity are inside the envelope but hard to move through smoothly, which is why 'reachable' and 'usable' can differ.
Where you meet it in the field
- FANUC M-20iD/25 · An arm whose published reach and per-joint travel limits together define the shape and size of its work envelope.
- Arc welding · Cell layout has to seat the entire weldment inside the reachable volume, torch angle included, keeping joints clear of their limits mid-seam.
Common questions
- If a part is within the robot's reach spec, is it always reachable?
- No. Reach is just the outer radius. A part can sit inside that radius and still be unreachable if it lands in the hollow column near the base, sits below the arm's lowest sweep, or can only be approached in an orientation the wrist can't achieve there.
- Why does the robot fault in the middle of its envelope, nowhere near a limit?
- You're likely passing through or near a singularity. At those poses the arm needs extreme joint speeds to make a small Cartesian move, so the controller slows the path or throws a motion fault even though the TCP is well inside the reachable volume.
- Does a bigger work envelope mean a longer reach?
- Not necessarily. Envelope size depends on both reach and how far each joint can rotate. An arm with generous joint travel can wrap into a larger usable volume than a longer-reach arm with restricted joints. They describe different things.