# Overconstrained mechanism

#### Dantes548

##### Newbie
Hello,
I'm completely new to CATIA and I'm trying to build a robot for my class. This is the robot I need to simulate (ABB IRB 6660)

and this is my attempt at it:

I made all the constraints and now I'm trying to make joints. I fixed the first part and made revolute joint between it and the second part. But as I tried to make second joint, message: "the creation of the joint is impossible because the mechanism is over constrained" shows. If I try to make some other joint (for example on axis 6 on that picture), than DOF becomes 6 and I can't simulate it anymore.
I would really appreciate your help (it is entirely possible I'm making some dumb mistake). If you need any further information, just say.

#### Dantes548

##### Newbie
Do you have any idea what to look for, why particular joints make mechanism overconstrained?

#### MrCATIA

##### Super Moderator
CATIA DMU Kinematics is intended to be used for mechanisms with 1 DOF. (one input resulting in one orientation)

You're working on a robot, which is not a mechanism. Your robot has multiple inputs with multiple results, and multiple DOF.

Since you have multiple DOF, I wouldn't worry about the message. Just follow the video, especially at the end of the video where there are multiple input commands.

----

You might want to do this in steps, starting with the fixed base and the first part (Axis 1). Define this Joint and then simulate it. Then add the next joint (Axis 2) and simulate the 2 DOF with 2 commands. Keep building from there adding one Axis at at time.

#### Dantes548

##### Newbie
CATIA DMU Kinematics is intended to be used for mechanisms with 1 DOF. (one input resulting in one orientation)

You're working on a robot, which is not a mechanism. Your robot has multiple inputs with multiple results, and multiple DOF.

Since you have multiple DOF, I wouldn't worry about the message. Just follow the video, especially at the end of the video where there are multiple input commands.

----

You might want to do this in steps, starting with the fixed base and the first part (Axis 1). Define this Joint and then simulate it. Then add the next joint (Axis 2) and simulate the 2 DOF with 2 commands. Keep building from there adding one Axis at at time.
Yes, I tried to do it that way but second axis is disabled and that message appears. I can't ignore it because it simply disables making of that joint. I don't know. Robot from that video I made without any problems.

#### MrCATIA

##### Super Moderator
I just noticed you have assembly constraints - delete all of them. The joints will act as constraints.

#### Dantes548

##### Newbie
I just noticed you have assembly constraints - delete all of them. The joints will act as constraints.
Ok, I did that and managed to make all of the joints. But now I have a problem where I get a message about having too many commands, although I have 6 - which is required as you can see in the first picture. Do you know what could be problem there?

*also one more question - how come these assembly constraints were a problem and they weren't in that video that you recommended?

#### MrCATIA

##### Super Moderator
Look carefully at your joints to find the ones you don't need. In the second picture, it looks like you have 11 parts - maybe you used some of the wrong ones?

The assembly constraints and kinematic joints are basically the same thing. So having both types of constraints was redundant.

#### Dantes548

##### Newbie
I used the parts equivalent to this: https://grabcad.com/library/irb-6660-130-310-1
If I try to remove some commands (by making joints not-angle driven), than DOF increases, so again I'm not able to simulate mechanism. And to me, joints seem logical, these are the 6 angle driven revolute joints that I initially made:

And these are all of them:

Look carefully at your joints to find the ones you don't need. In the second picture, it looks like you have 11 parts - maybe you used some of the wrong ones?

The assembly constraints and kinematic joints are basically the same thing. So having both types of constraints was redundant.

#### MrCATIA

##### Super Moderator
Dantes, thanks for the extra information and for coloring the parts to help identify each part.

1. Please Hide these parts: 3, cetvrti,, sesti, and osmi

2, Make a new Mechanism

2a. part 1 (yellow) is fixed

2b. add Revolute joints with Angle Driven between the following pairs of parts:
1 - 2
2 - 5
5 - 7
7 - 9
9 - 10
10 - 11

3. simulate the mechanism using 6 input commands

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#### Dantes548

##### Newbie
Dentes, thanks for the extra information and for coloring the parts to help identify each part.

1. Please Hide these parts: 3, cetvrti,, sesti, and osmi

2, Make a new Mechanism

2a. part 1 (yellow) is fixed

2b. add Revolute joints with Angle Driven between the following pairs of parts:
1 - 2
2 - 5
5 - 7
7 - 9
9 - 10
10 - 11

3. simulate the mechanism using 6 input commands
Ok, I did it and it works well so far.

#### MrCATIA

##### Super Moderator
Good - you did that pretty fast!

The next step is to bring back the hidden parts. These parts make up a "4-bar mechanism" which replaces the Command 3 with a piston made up of parts 3 and 4 (cetvrti).

But I'm a little confused about part 6 (sesti/green) and how it's attached to the other parts. Could you hide part 2 (orange) and send me a picture, please

#### Dantes548

##### Newbie
So, maybe it's hard to notice, but 6 and 5 aren't connected

#### MrCATIA

##### Super Moderator
Dantes,

thanks for the pictures and information about the green part sesti. I'm still not sure how this part mates with the other parts, but I think I might have a possible solution, since there appears to be a motor that drives the green part.

I"d like to make another simple version of the robot with a different set of parts

1. Please Hide these parts: 1 (yellow), 9 (deveti), 10 (deset) and 11 (jedanes)

2, Make a another new Mechanism

2a. part 2 (orange) is fixed this time

2b. add Joints between the following parts as follows (not all are driven, and one is not a Revolute):
Revolute between parts 2 & 4
Prismatic between parts 4 & 3 (Length driven)
Revolute between parts 3 & 5
Revolute between parts 5 & 2
Revolute between parts 2 & 6 (angle driven)
Revolute between parts 6 & 8
Revolute between parts 8 & 7
Revolute between parts 7 & 5

3. simulate the mechanism using 2 commands (one length, and one angle)

#### Dantes548

##### Newbie
Thanks for your help, but it's too late now. I got the solution.
Bye.

#### MrCATIA

##### Super Moderator
Glad to hear you were able to figure it out!

I would like to see your solution.