Robotic Pointer: Accurate Pointing with Polar Coordinates and Stepper Motors

[benzun_1999]

help me! robotic ponter

hi all!
I am currnenty working on a robotic pointer that can point any point in a x, y coordinate plane. for this i am using a structure with two platforms one on another. the platform at the bottom is attached with the platform on top of it. while one platform rotates in the x-axis another rotates in the y axis. thus enabling me to get all points. the pointer must point using polar coordinates, that is using degrees. I am trying to mechanize and computerize this here i am suffering some problems.
1. how can i turn the platform by degrees accurately? (a small mistake and the point will not be marked properly) for trying to turn the platform should i use gears? if so what will be the best ratio to turn by .5 or 1 degrees?
2. can stepper motors help? My sir adviced me to use them but i have no proper idea how to implement them.
3. how can i computerize it? i am aware of using serial port for interface and have already gone through many circuits but without the above solved the rest is in vain.

Hope u will help and suggest me.
Thanks in advance for the help.

-Benzun

 

[berkeman]

You would normally use stepper motors to move your platforms. You can use gearing or "micro-steps" to get finer resolution than the simple stepper can give you. Here are a couple links:

http://www.netmotion.com/htm_files/mc_motor_dmicro.htm (vendor of stepper motors)

http://www.ams2000.com/stepping101.html (stepper motor basics)

http://motion-controls.globalspec.com/Industrial-Directory/micro_stepper_motor (search on micro stepper motors)

You could also use a servo motor technique to get finer resolution, but that is more tricky to design and get stable. You should be able to get an interface card for your PC to drive the stepper motors from National Instruments or B&B Electronics.

 

[berkeman]

...And here's a good pointer into a Photonics Spectra Magazine article and advertiser about controlling x-y tables:

http://www.photonics.com/spectra/newprods/XQ/ASP/newprodidp.7201/QX/read.htm

 

[benzun_1999]

thanks

you cleared a lot of doubt and helped me. i am thankful to u. i already got circuits that can control stepper motors from pc. Does stepper motor have enough troque to turn the disc? I am aware of noraml DC motor but these stepper motors are completely new to me (i am not a electronics student). Moreover i believe these stepper motor are also found in the computer electronic component i believe.

 

[benzun_1999]

even if i use stepper motor i believe i must use gears?

 

[berkeman]

even if i use stepper motor i believe i must use gears?

That depends on the resolution that you want to get out of your system. A regular stepper motor has pretty coarse steps -- I think only a dozen or two per revolution. You have two choices if you need better resolution: gearing down, or using microstepper motors. The advantage of microsteppers is that you won't have the backlash that you get with a geared system when you change directions. The disadvantage is usually the cost of the microstepper motors, I think.

One thing that has worked well for me in the past for linear positioning is to use a regular stepper motor and have it drive a long screw shaft with fine threads. Then you have a shaft coupler thing (I forget the name -- it has threads on the inside and you connect the outside to the moving stage -- it's kind of like a big precision nut) that is what drives the moving stage. With fine enough threads you don't get much backlash. And you can even calibrate the backlash out by measuring it and keeping track of it when you change direction. You'll also want limit switches at the ends of your linear travel, to static the stepper motor when its linear stage makes it to the end of its travel.

I think you were focusing mainly on rotational stages, not linear stages, and gearing them down is harder usually. Microstepper motors would probably be the way for you to go. Again, just check the specifications for how many steps per revolution they can acheive, and compare that to the resolution that you hope to achieve. Good luck, -Mike-

 

[benzun_1999]

thanks for ur help