[dhitchco]

Here's a way to weigh your robot with stuff you have in the shop right now. I'll describe how to make a "beam scale" which is pretty accurate, but not exact.
*** DISCLAIMER: Be very careful in the process of putting a "lifting sling" on your robot lab ceiling as well as in the process of lifting the robot off the floor.

Steps:
1) Get three sections of PVC pipe you were using for practice goals (the longer tetra pieces). They should be 60" long.

2) Tape them together with lots of duct tape

3) Drill one hole through all three pipe sections about one inch from the end. Thread a nylon cord through these three holes, tie tight and make a hanging loop

4) Measure EXACTLY TWO INCHES AWAY from these three holes and repeat the three hole process with a new nylon loop.

5) Attach the "inner" nylon cord to the STURDY, RIGID ceiling beams of your shop area. Do NOT attach the the suspended ceiling tile grid, etc. It MUST be able to support 150 lbs.

6) Now, go and fill two 2-liter soda pop bottles (the big ones) FULLY with water.

7) Attach one bottle to the END of the PVC where you had the first nylon loop.

8) Attach the 2nd one-liter bottle onto the other end of the beam, and SLIDE it towards the middle until the beam balances.

9) Measure the distance between where the sliding bottle balanced and the center hanging point.

10) Mark this SAME spacing (ever 2") along the PVC pipe out to well beyond the 55" mark. Make marking IN-BETWEEN these marks; namely EVERY inch for the full length of the PVC.

10) Now, attach your robot under the "short" end of the beam, tighten the harness until the PVC pipes are angled upwards, and place the adjustable water bottle onto the PVC pipe bundle until it balances. do NOT lift your robot too far off the floor; only enough so it hangs freely.

*** The number of inches away from the centerline is your robot's weight (approximately) in KILOGRAMS.

The robot must weigh less than 54.43Kg (120 lb.) WITHOUT the battery.

Good luck; remember that this is just a testing method.