# Fanuc J2 & J3 relationship

• Hello,

I'm am new to the robot world so if I say something naive please forgive me.

I have an used Fanuc R2000iB 100P with a R-30iA controller. I have been working through some different issues and have found some useful info already on this forum. Currently I am trying to understand how the relationship between J2 and J3 works. I mastered and calibrated the system but when I jog J2, J3 moves as well but the position screen does not update the position of J3, only J2 updates. How does the robot know where J3 is ? As I jog the robot around I am setting both physical and software limits to the axis. I found that there is a magical number of -79 degrees for the combined values of J2 & J3. This doesn't allow me to move the head of the robot close to the base. Also I found that to transport the robot you need to move the J2 axis up to its max of -120 degrees and it shows the J3 axis down as far as it can go, which should be a large - degree but the table associated with the diagram says +41 degrees (J2+J3 = -79 degrees). What am I missing, do you master J3 to offset by 180 degrees ? If anyone can enlighten me on this J2 + J3 relationship I would greatly appreciate it.

Thanks,

Mike Kirchmeier

• What you are seeing is normal operation on Fanuc arms.

J3's angle is relative to the base. This is a throwback to the old 'Whale-tail' design, where J3 was controlled by a linkage next to J2's motor (still the case on some arms). On those robots, if you jogged J2 only, J3 would stay at the same angle relative to the base casting, due to the four bar linkage system used. I guess Fanuc wanted to keep their jogging style consistent across the entire product line, so they made it so on non-whale-tail arms that J3 runs in the opposite direction when J2 is jogged.

If you want J3's real angle (i.e. relative to the J2 casting), add J2 to J3's current angle. This is useful for when you need to attach hardstops to J3. On newer controllers (R30iB) the joint pos screen will show J3's 'independent' angle.

In terms of where to master J3, do it at the witness marks. In this particuar arm, when the robot is at 0 degrees on all axes, J2 will be straight out from the base casting, and J3 will be pointing at the floor.

Reference, page 4 : http://www.fanucamerica.com/cm…R-2000iB%20Series_177.pdf

Check out the Fanuc position converter I wrote here!

• Also, because of the linked motion, when performing a zero position master, be sure to jog j2 to the zero witness mark before jogging j3 to the zero witness mark.

If you do it backwards, j3 will be off the mark when you set j2.

• I guess every manufacturer makes it differently to avoid paying royalties for using already patented methods. Is just matter of conventions: In fanuc's J2 is measured from the vertical and J3 from the horizontal. What's why when you master/calibrate link2 is vertical and link3 is horizontal. The mathematical model requires an additional operation when calculating the position of the tool from the encoder values (direct kinematic problem)

• Does the robot look like this when you run to those numbers?

If it doesn't you may have a mastering issue.

Check out the Fanuc position converter I wrote here!

• I am sorry I mislead you, it is because the robot is rack mounted model and the zero position is different: For R2000iB 100P the zero position J2 is horizontal and J3 vertical down. J2 angles are measured from the horizontal and j3 is now measured from the vertical (down). That's is why in the transportation position J2 is -120 and J3 is +41.
I don't know were the marks are located in this robot. If you move the robot to marks and J2 is vertical up and J3 horizontal, then you should use single axis masterization and enter -90 for j2 and 90 for j3.

Edited once, last by robotero ().

• I would probably try a single-axis master for J3 - compensating for the difference in J2 actual position. Right now, old gray matter can't tell whether to add or subtract the difference...

• Hi, I checked the robot posture in roboguide. At zero position link 2 is horizontal & link 3 is vertical down. If moving the robot to marks is not posible then use single axis master: link 2 vertical up and link 3 horizontal then use the -90 for J2 and 90 for J3 as master position