A deep dive into FANUC Position Registers (PR), User Frames (UF), and Tool Frames (UT). Learn how to use PR offsets for flexible palletizing, pallet stacking, and multi-fixture cells.
Position Registers (PR) are one of the most powerful features in FANUC TP — and one of the most underused. While P[] positions are taught and fixed, PR[] positions can be computed, modified, and offset at runtime. Mastering PR turns a rigid pick-and-place into a flexible multi-recipe cell.
P[] vs PR[] — Key Differences
Comparison:
- P[n] — taught position, fixed in the program, visible only in that program
- PR[n] — global position register, readable/writable from any program, modifiable at runtime
- PR[n,1..9] — access individual elements: X, Y, Z, W, P, R, E1, E2, E3
- PR[] can be computed from other PR[], from R[] numeric registers, or from Karel
Stack Height Calculation for Palletizing
The most common PR use case: calculating the Z offset for each layer of a pallet stack. Each layer adds one product height to the base pick position.
1: PR[10]=P[3:PICK_BASE] ; ! Copy base pick to PR
2: R[1]=R[5:LAYER]*R[9:HEIGHT] ; ! Compute Z offset
3: PR[10,3]=R[1] ; ! Set Z element of PR
4: L PR[10] 200mm/sec FINE ; ! Move to computed position
5: DO[1:GRIPPER]=ON ;User Frames (UF) for Multi-Fixture Cells
A User Frame defines a coordinate system relative to a fixture or pallet in the world. Instead of teaching absolute positions per fixture, you teach positions relative to UF[1] for fixture 1, then switch to UF[2] for fixture 2 — same taught positions, different physical location.
1: UFRAME_NUM=1 ; ! Switch to User Frame 1
2: L P[10:PICK] 300mm/s FINE ;
3: UFRAME_NUM=2 ; ! Switch to User Frame 2
4: L P[10:PICK] 300mm/s FINE ; ! Same P[10], different UFAlways verify your User Frame calibration with a 3-point or 4-point calibration before production. A 1mm error in the UF origin becomes a 1mm position error on every motion using that frame.
Tool Frame (UT) Setup
The Tool Frame defines where the TCP (Tool Center Point) is relative to the robot flange. For a gripper, this is the center of the gripper fingers at close. Incorrect UT causes rotational motion to pivot around the wrong point, introducing position error on reorientation moves.
