Setting Zero Initial Angular Velocity

[Gian Alvarez]

Hi,
I'm doing a bipedal robot mechanism in pro E and I have a hard time
setting the initial (i.e. at time=0) angular velocity of the right thigh to
zero. I defined a servomotor with Position and time as the input (is this
the right thing to do? or is it better to have velocity/acceleration as the
input?). So at a certain time, the right thigh is oriented at a certain
angular position. In my case, the initial position of the right thigh is 35
degrees with respect to the torso oriented vertically. It can only rotate
until -10 degrees (in which, the angular velocity is desired to be zero). In
a sense, the right thigh is oscillating in a 45 degree angle. I also want the
link to go back to the original 35 degree angle and have that zero initial
condition again. The cycle time is 1.6s. Please help me to do this. I need
to learn it as soon as possible. Thanks!

 

[atalet]

I think you should define it in "initial conditions" by setting the respective joint's "motion-axis velocity" to zero.
Furthermore you can control the angular speed by defining a servo motor with a "table" option. you can either use time or a parameter as variable.

 

[Gian Alvarez]

I think you should define it in "initial conditions" by setting
the respective joint's "motion-axis velocity" to zero.Furthermore you can
control the angular speed by defining a servo motor with a "table" option.
you can either use time or a parameter as variable.

Thanks. but i have a hard time simulating the motion in an oscillating
pattern. Just imagine the thigh while walking. Does defining a table for
position solves it?

 

[atalet]

The oscillating motion can be defined by a sin or cos function. If you want to simulate the action with acceleration then you can use SCCA (sinus-contant cosine acceleration). (i have not used it before

)

 

[Gian Alvarez]

really? great! so using SCCA would give me an oscillating motion?

 

[Gian Alvarez]

The oscillating motion can be defined by a sin or cos
function. If you want to simulate the action with acceleration then you can
use SCCA (sinus-contant cosine acceleration). (i have not used it before

)

 

[Gian Alvarez]

would it still be possible for me to have oscillating motion if the input is
constant velocity?

 

[atalet]

would it still be possible for me to have oscillating motion if the input is

constant velocity?

If we are talking about the oscillation of position then it is possible to have constant velocity. However for the oscillating velocity, naturally our velocity will not be constant, but oscillating

For the oscillating position, you can set a zero initial angular velocity and use a constant velocity. For the oscillating velocity, you can again set the initial velocity to zero, but you should define the SCCA function variables properly. Unfortunately I do not have experience with the SCCA function.

p.s: sorry for the late answer. I am not available in weekends.

 

[Gian Alvarez]

would it still be possible for me to have oscillating
motion if the input is
constant velocity?

If we are talking about the oscillation of
position then it is possible to have constant velocity. However for the
oscillating velocity, naturally our velocity will not be constant, but
oscillating

For the oscillating position, you can set a zero initial angular
velocity and use a constant velocity. For the oscillating velocity, you can
again set the initial velocity to zero, but you should define the SCCA
function variables properly. Unfortunately I do not have experience with
the SCCA function.p.s: sorry for the late answer. I am not available in
weekends.

Thank you for your kind response. You've been really of very great help.
I'm trying to use the "initial condition" analysis right now since I'm not
also familiar with the SCCA function. I'm setting the velocity of the joint
axis to zero since I want the initial velocity of the thigh to be zero. same
with the shank. but whenever I try to run the analysis, it fails. the thigh
doesn't move anymore. It gives a message that the analysis failed. How
can I fix this? Hope you can help me with this because I'm rushing my
thesis. Thanks a lot.

 

[atalet]

I think the reason of the failure is the inappropriate definition of the servo motor. If you set the initial velocity of a joint connection to zero, then you can not define the servo motor with a constant velocity for the respective joint. You can try the zero inital velocity with an acceleration servo motor. I hope this was the problem.

 

[Gian Alvarez]

I think the reason of the failure is the inappropriate
definition of the servo motor. If you set the initial velocity of a joint
connection to zero, then you can not define the servo motor with a
constant velocity for the respective joint. You can try the zero inital
velocity with an acceleration servo motor. I hope this was the problem.

I see. I'll try to do that thing. thanks a lot. But I have another problem.
The results I'm getting (Vel, acceleration, and torque) don't seem to
compensate the effect of the shank attached to the thigh by a pin
connection. It's position at a certain time is also defined by a servomotor.
My design and simulation (i think) just computes or measures the
parameters in the thigh alone though the shank is attached to it. I was
able to set the material properties of the thigh but it still doesn't seem to
work. I also include gravity in my analysis. What might be the problem
with this?

 

[atalet]

You should create connection reaction measures to see the reactions at the shank also.

analysis>measure>new-->connection reaction. you should make the proper definitions for "expressed in" and "exerted on" areas.

 

[Gian Alvarez]

You should create connection reaction measures to see the
reactions at the shank also.analysis>measure>new-->connection reaction.
you should make the proper definitions for "expressed in" and "exerted on"
areas.

Thanks for our response. I already did that thing. But my professor doesn't
seem to believe the results that Pro E is giving. Does it mean that if it gives
certain amount of connection reactions, the weight of the shank is taken into
account during the analysis?

 

[atalet]

if you have assigned the proper material and you activated the gravitation in the analysis definition box then the weight of the shank should be taken into account.i do not think friction will have a remarkable effect on the results. either pro/e or your prof is missing something

 

[Gian Alvarez]

if you have assigned the proper material and you activated
the gravitation in the analysis definition box then the weight of the shank
should be taken into account.i do not think friction will have a remarkable
effect on the results. either pro/e or your prof is missing something

Yeah! I think so too. But how will I explain it to him?

 

[Gian Alvarez]

You should create connection reaction measures to see the
reactions at the shank also.analysis>measure>new-->connection reaction.
you should make the proper definitions for "expressed in" and "exerted on"
areas.

Would it be possible that I'm not using the right constraints?

 

[Gian Alvarez]

if you have assigned the proper material and you activated
the gravitation in the analysis definition box then the weight of the shank
should be taken into account.i do not think friction will have a remarkable
effect on the results. either pro/e or your prof is missing something

Hi!
I'm measuring the torque produced on the hip joint of the robot but i get
weird erratic values. I defined it as an axial moment at the pin connection.
but when I consider the thigh alone, (that is, removing the shanks) it
measures a more realistic torque and generates a smooth curve. How can I
correct this?

 

[atalet]

but when I consider the thigh alone, (that is, removing the shanks) it

measures a more realistic torque and generates a smooth curve.

you should check "expressed in" and "exerted on" areas if you have such a situation. if the reaction forces are realistic then there should not be a weird result. I can not think of another reason...