Step 8: Conclusions, Notes, Room for Improvement
Both axes have issues with not quite centering due to the potentiometers - causing the the digital servos to whine a lot at rest. For occasional use, it's probably fine. For TeleToyland, we used a separate servo powered switch to just turn them off when not in use. We may get a digital servo programmer to see if narrowing the dead band will help. Higher quality potentiometers may help, but we may also be reaching a practical resolution limit in this approach.
(June 2009) For the linear slides, we used drawer glides. These are working fine so far, but they do stick out when the XY table is in the home position. So, we are thinking about using 16mm Linear glides from www.vxb.com - that seems like the lowest cost ones around.
(June 2009) Note that for the Y axis (the one on top), the servo motor is working fine, but for the X axis, we have had some issues. So the current system is using a
Pololu 3A Motor Controller with Feedback and a 12VDC 250RPM DC Gearhead Motor. The board from Pololu works just like a servo control board, and we already have the external potentiometer. The motor is much more robust, and has been working well. One minor issue is that the PID algorithm sometimes overshoots a bit, but it's not too much, and the board allows you to set the PID constants - just need time for fiddling. :-) You can also use a servo board and adapt it for the same use - we'll look into that sometime too.
This is an interesting approach, and might make an excellent arm configuration for a mobile robot. The Leaf Project members are interested in arms, and this could be used for part of it. You could even add a counterweight on the belt opposite the end effector so that the arm would balance automatically - as the hand moves out, the counter-weight would move back. Adding a second linear system behind it would allow the weight of the object picked up to be balanced too.
The following sites provided some of the inspiration for this project:
Easy to Build Desk Top 3 Axis CNC Milling Machine Tom McWire
Improving Servo Positioning Accuracy David P. Anderson
ServoCity Servo Power Gearboxes
We didn't use this, but it looks interesting - a timing belt pulley for MXL belts (0.08" pitch) for Hitec servos
We found this after building ours - uses gears in a similar way, and the drawer glides are similar:
Autonomous Foosball Table.
You can drive this project live at TeleToyland