Motion
Joint Interpolation
Joint interpolation (motion format J) moves a robot to a taught endpoint by driving all axes so they start, accelerate, and stop together, with no control over the path shape or tool attitude in between.
What it is
Joint interpolation is the basic motion format on a FANUC controller, taught as J in a motion instruction. You give it an endpoint, and the robot figures out how to get every axis there at the same time. It doesn't control the shape of the path the TCP traces through space, and it doesn't control tool attitude along the way.
Because it only cares about the endpoint, the path the tool tip actually follows is normally non-linear. That's the trade you accept for the fastest, least constrained move between two points.
How it actually works
When a J instruction runs, all axes accelerate together, travel at the commanded rate, then decelerate and come to rest simultaneously. The feed rate for a joint move is a percentage of the axis maximum, specified from 1 to 100%. It can also be specified in time, from 1 to 3200 sec or 1 to 32000 msec.
Since each axis is just commanded toward its own endpoint angle, the TCP path is usually non-linear. Tool attitude is whatever falls out of that coordinated joint motion and is left uncontrolled. Compare that to a linear move, where feed rate is given in mm/sec (1 to 2000) and the controller actively distinguishes start and end attitude to hold the tool tip on a straight line.
How it differs
- Linear interpolation · Linear motion (L) controls the TCP path from start to end so the tool tip travels a straight line, and it controls attitude by blending the start and end orientations. Joint motion (J) only guarantees all axes arrive together, so the tip follows a non-linear path and attitude is uncontrolled. Use J when you just need to get somewhere fast, L when the tip has to follow a defined line.
- Circular interpolation · Circular motion (C) drives the TCP through a taught passing point to an endpoint along a controlled arc, with attitude interpolated between start and target. Joint motion has no passing point and no path guarantee, so it can't be used where the arc geometry matters.
Where you meet it in the field
- Setting the reference position · Jogging and moving to a reference position is typically done with joint moves, where you only care about reaching the pose.
- Arc welding · Joint moves reposition the torch between weld passes, where speed matters and the path between passes is free space.
Common questions
- When should I use J instead of L?
- Use J when you only care about arriving at a point and the path between doesn't matter, which is the faster move because no axis is held back to keep the tip on a line. Use L when the tool tip has to follow a straight line, such as tracking a seam or approaching a fixture along a defined vector.
- Why does the TCP swing out in a curve when I command a joint move?
- Because a joint move only coordinates the axes to reach their endpoint angles together. The tip path isn't enforced, so the combination of rotating axes drives the tool tip along a non-linear path. That's expected behavior.
- Can I set a joint move by time instead of speed?
- Yes. Joint feed rate is normally a percentage of the axis maximum (1 to 100%), but it can also be specified in sec (1 to 3200) or msec (1 to 32000).
- Does joint motion control the tool orientation during the move?
- No. Tool attitude is left uncontrolled during a joint move and is whatever results from the coordinated axis motion. If you need attitude blended between start and end, that's what linear and circular moves do.