Kenneth Lerman wrote: > A simple solution is to use a dual drive. > > Use a course mechanism with long travel for course positioning and a > fine mechanism with limited travel for fine positioning. The fine > mechanism has a large mechanical advantage and can use a smaller motor > and driver. Of course, some sort of clutch mechanism could be used to > allow switching between the course and fine speeds.
You might be able to avoid the clutch as well. One possibility would be to stack a very high resolution slow stage on top of a fast coarse one. You could also do things like having the fast motor turn the screw, and the slow motor turn the nut with a worm gear to increase the resolution and decrease the speed. In this latter case, the stiffness of the fast motor will be an issue, even though it isn't turning. > You haven't answered the question of what type of mechanism you are > planning to use to provide smooth linear and rotary motions at this slow > speed. I think Ken has hit the nail on the head here. At very low speeds, it is very hard to get smooth motion. You are more likely get stick-slip behavior, where the motor turns a little but the table doesn't move. The screw and other parts deflect until they build up enough force to overcome the static friction and start it moving. As soon as it moves, the dynamic friction is much lower than the static friction, and it moves farther than you wanted it to, then stops. And the whole cycle repeats... The individual movements are tiny, but at the extremely slow speeds you are talking about, stick-slip is more likely to be the limiting factor than your encoder resolution. Gearing down does not help - the issue is the flexibility and friction of even the most "rigid" screw/nut/bearing combination. At these scales, you almost have to think of the metal parts as if they were hard rubber. Sliding ways are usually the worst choice for stick-slip. Rolling element Linear bearings are better, but they are not very happy in a grinding environment with abrasive dust. Depending on the loads, you might want to consider air bearings. They do not suffer from stick-slip, and to some degree are self-cleaning in a dusty environment. One of the regulars on IRC, who uses the name "toastydeath", works (or worked) for a company that makes air bearings, and may have some suggestions. One thing to consider might be a coarse drive using slide ways, with a short travel air bearing stage stacked on it. The coarse drive moves to the approximate location and stops. Once stopped, its relatively high static friction is actually an advantage - it ensures that the coarse stage doesn't move while the fine stage on top does the actual grinding. Regards, John Kasunich ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
