So my NXT-G is probably a bit rusty (not coaching this year) but the use of two threads to control motors looks odd, especially since both the top thread and the bottom thread are issuing commands to motor "C". There could be some timing or synchronization issue here (threads are tricky that way!). I've only used parallel threads when controlling two different things (e.g. a sound and a display). What if you used a "motor" command (not the "move" command you've used) to turn on motor "C"? the "motor" command is under the "action" palette (icon is a single gear). put it in series with the next box (which turns on motor "B"). make sure that the "wait for completion" box is NOT checked on the first motor command (so that it starts the "C" motor, then immediately moves to the next step, which will be to start the "B" motor in the opposite direction). Then you wouldn't have to use two threads. That feels like the source of your problem. - Glenn From: Bob and Ann Henshaw [mailto:[log in to unmask]] Sent: Tuesday, October 23, 2012 9:12 PM To: 'Glenn Roberts'; [log in to unmask] Subject: RE: [VADCFLL-L] looking for help with erratic robot behavior Here is a simple program where the robot moves, stops on a line, executes a 2 motor 90 degree spin, and moves forward. At the end, it suddenly lurches to the right, despite the stop block. Thanks for looking at it! Bob From: Glenn Roberts [mailto:[log in to unmask]] Sent: Tuesday, October 23, 2012 7:54 PM To: 'Bob and Ann Henshaw'; [log in to unmask] Subject: RE: [VADCFLL-L] looking for help with erratic robot behavior My gut says there's a subtle bug in your software. Not sure, is there any way to share an NXT program with this email group so others can look at it? From: First Lego League in Virginia and DC [mailto:[log in to unmask]] On Behalf Of Bob and Ann Henshaw Sent: Tuesday, October 23, 2012 7:16 PM To: [log in to unmask] Subject: [VADCFLL-L] looking for help with erratic robot behavior I am a 5 year veteran home based coach with a 2nd year team of 10 year olds. We are working with a Mindstorms NXT system that we purchased new last year, with a fully recharged battery. We are experiencing several intermittent erratic behaviors (listed below) that is driving our team crazy. Please let us know if you have any debugging suggestions. Observed behaviors: 1. After completing a planned series of instructions, with the last instruction having 2 wheels turning equally (straight line), the robot suddenly makes an additional turn (one motor, rotating robot about 30 - 40degrees). This occurs even after deleting the program, recompiling and redownloading it, and even after adding an all motor stop block to the program. Our team is referring to this as a ghost and are convinced that the robot is now haunted, just to get us in the Halloween mood. 2. The robot, using a two wheeled turn (one motor going forward, one going backward) intermittently turns too far or too short. Programming is being done in rotations for both motors. Because both motors are running together, I can't tell if it is related to the problem above. This occurs about 1 in 5 runs. I do not think that this is a cable issue, as we are not seeing anything beyond the usual minor navigation errors due to lineup, wrinkles in the mat, etc. That leaves the motors and the NXT itself. Could this be a bad servo motor and if so, is there any way to test them? Appreciate your suggestions! Sincerely, Bob Henshaw _____ To UNSUBSCRIBE or CHANGE your settings, please visit https://listserv.jmu.edu/archives/vadcfll-l.html and select "Join or leave the list". VADCFLL administrative announcements are sent via VADCFLL-ANNOUNCEMENTS-L. Visit https://listserv.jmu.edu/archives/vadcfll-announcements-l.html to subscribe. -- To UNSUBSCRIBE or CHANGE your settings, please visit https://listserv.jmu.edu/archives/vadcfll-l.html and select "Join or leave the list". -- VADCFLL administrative announcements are sent via VADCFLL-ANNOUNCEMENTS-L. Visit https://listserv.jmu.edu/archives/vadcfll-ANNOUNCEMENTS-l.html to subscribe.