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Servo/ Stepper?

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lil'gal

New Member
Hi,

I am a newbie in robotics and am clueless abt lotz of things :oops:

Have some enquries:

1) Suppose I'm building a robotic gripper which can grip and hold objects of about 320g, would it be better to use stepper/ servo motors?
I've heard that DC motors can do the job with some gears (is it true?).. In this case which motor would be the most ideal?
(Note: suppose the robot is small-sized, abt 20x20cm)

2) What's the significance of servo having limited rotation and the stepper having full rotation? In this case, suppose i'm using servo motor, how do i modify the motor such that it can have a complete rotation?

3) Is there any ingenius idea that can allow a square base to rotate on the ground without any translation? juz pure rotation abt an axis.. ie.. what kind of wheel configuration should i have?

4) Finally, any tips on building a robot? :)

Deeply grateful for any responses :)

[/i]
 

bmcculla

New Member
1) A DC gear motor or an RC car servo would be my choice. The only real advantage to stepper motors is that their design gives them built in position control. The down side is that if something external causes them to turn you lose track of their position. I'd use a simple DC Gear motor and monitor the current to tell the amount of force aplied to the gripper (motor torque is proportional to motor current).

2) Depending on your aplication it might be better to make your own servo motor. The word servo is usualy used to discribe a motor that is controlled by a feedback loop (it can be a verb too - to use feedback to control a motor or other device). RC Car servo motors have a pot in them to do position feedback. You can modify a RC servo for continuous rotation. Its also fairly easy to build an optical encoder and put it on the motor of you choice (this is good for drive wheels because you can chose a high torque motor and easily do velocity control)

3) if you put two wheels on the centerline of the robot so that when they turn in different directions the robot spins around its center and have a caster wheel to keep the robot stable you should be able to turn without translation.

4) Make sure you have a well thought out plan before you build anything. Make sure you think about how you will rout the wires between circuit boards. Use good, easy to detach connectors - its easy to wire wrap or solder but a nice connector will make things much easier to work with. Be sure of your failure states - if your controler does something unexpected make sure the robot fails gracefully (e.g. motors don't go to full speed when the controller resets.). This sounds like it might be a waste of time and effort but it may keep your robot from suddenly destroying itself. I know this one from experience - my RC Car Robot spontaniously reset and went to full speed. It hit the wall hard enough to pop components off the circuit boards and shear off some nylon standoffs. Thats all I can come up with right now for advice.

Brent
 

subhasis

New Member
i have a beautiful idea for making ur robot rotate about an axis only.... u need to use only 1 motor for that....
use a motor in the center and use one gear to transmit the motion to one wheel and 2 gears of the same size with the other wheel....
so one wheel will move clockwise and the other anticlockwice..... with the same speed.... this will make the system rotate about the center.... u have to use 2 caster wheels.....
if u don t want to use wheels.... u can put the whole system on a bearing and use one motor to rotate the base only. this wud be easier to construct. u can easily get a bearing of a bicycle.
well, i would suggest u to use a stepper motor, if you are interfacing the system with computer. it is easy to control 3 stepper motors simultaneously n independently from the parallel port of the computer.....
u can easily calculate the torque in stepper motors....
u can plan n control the motion better with stepper motors.
i hope this helps....
Subhasis
 

Gundam82

New Member
hey... i doing this before.... at first i usnig servo motor as my gripper but... due to the weight.. i change it to dc gear motor... !! its not heavy as servo motor.... !! cos if i use servo motor as my griiper.. my arm motor cannot support anymore....!!! overload..... u can refer to pic below... u just modify it by place the dc gear box to one of the gear shown ....!!!!
then it will work perfectly.!!!!!

hope can help u~~!!!! :wink:
 

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