About Redundancies and DOF

[DavidKhan]

a simple case

If i have a gear and a a ground body.the gear rotates through a motor.then

DOF=6M-N-O

in this case M is the bodies except ground which is 1

N is the total number of contraints imposed by joints.in my cas it is pin so it restricts 5 constraints

O is the driver which is 1.

Putting these in the Grubler equation gives

DOf=6*1-1*5-1=0

but i know that it should be one degree of freedom.ie as it rotates

the system types measures of DOF also gives 0.

What is the reason it gives 0 instead of 1.

[346gnu]

David,

The driver 'controls' the rotation, (the DOF you have in your mind). Because it is driven, it is not a DOF.

Regards

Charles

[DavidKhan]

thanks Charles

if i rotate the gear with a torque then DOF will be 1 but if i rotate the gear as mentioned with a motor the DOF will be 0.Could you explain it a little further?

[ChrisKaswer]

Servo motors directly control the DOF of the joint axis you apply it to, so that DOF is taken away and rendered "fixed", or "prescribed". Any single force or torque does not control the absolute motion or displacement of the joint, so it is available to move in all its "free" directions based on other loads, moments, velocities and accelerations acting on it.

[346gnu]

yup. Distinguish between force and servo motors.

A force motor : a torque will rotate the gear about it's axis DOF, the rotation can be influenced by other forces/inertia of the gear.

A servo motor : these PUT the axis at a position or enforce a velocity, acceleration or other profile and cannot be overcome by other forces/inertia.