robotic arm issue !! (only for experts please )

[Asterix101]

Hello there, I just want someone who has worked with robotic arms before....I have an essential mechanical question...

If i was going to implement Motors for the movement of the wrist and the fore arm, would it better be to put them both at the shoulder's position, and link each motor's axe to a gear, and a chaine the elbow so that I can move the wrist, and the other motor will use another chaine that will be normally shorter than the other, and which will be used to rotate the fore arm ??

2nd option:
I will put a motor on the junction between the fore arm and the shoulder, and another one at the elbow's location to move the wrist directly with no chaines or gears used...


My last question will be:

Would it be better to use servo motors, or Stepper motors ? cause I will be measuring the tilting angle of each body part.

 

[MikeMl]

Motors for actuators on robots suck big time. They impart very un-natural motions/resonances to the skelatal structure. Have you ever visited DisneyLand/World? Most of the animatronics there (originally developed in our lab at the Univ of Utah) employ pneumatics or hydraulics. Some employ tendons (like humans) to place the actuator a long way from the joint. Look at some of the movies in this link from Sarcos where I worked. **broken link removed** of exo-skeleton, hydraulic actuation; no motors.

 

[Asterix101]

The thing is that I don't really know how to work with pneumatic devices, or hydraulics, and I would like to keep it as simple as possible, with the most realistic and easy movement to conrol....so if u could help me with the choices that I have mentionned, concerning which one is better, and the way to implement them...

 

[Sceadwian]

What Mike is saying is that the mechanical gear selection is not going to be as simple as you think, and it will not be realistically at all without advanced servo control loops for each joint. That being said it's always better if the motor can directly control the join itself with as few gears or other connections as possible, as each one adds more variability. Unless you're exceptionally good at making rock solid mechanical robots I think you'll be disaapointed with the abilities of something you home make but it really depends on your goals.

 

[MikeMl]

If you want to use a motor, then use a compliant drive. A "torque motor drive" is much better than a position servo which utilizes a DC motor and gears; i.e. a "stiff" system with no compliance. A stepper is dreadful because it excites the mechanical resonances of the system at every step. The servo drive needs to control the torque at the joint to a first order, and worry less about the absolute position of the joint.

 

[Sceadwian]

wormgear drive to a standard gear? VERY high compliance. at least within mechancial limits.

 

[Asterix101]

Well Mike I got u...but I just need u to explain to me about this torque motor drive,or just show me an example with a clear picture that might be useful for my university project....