Re: regarding Constraints of my arm configuration

This type of robot is not used for completely general robot tasks, but for 
task that only need 5 d.o.f. e.g. tasks with a rotation symmetric tool (like 
a grinding disk).

Kinematics for this are not provided in Orocos.

There are two problems to solve :
(1) solving the inverse kinematics.  
	-For your type of robot there still exists a closed form solution.:
		- out of the specified orientation you can determine the POSITION of the 
intersection between axes 4 and 5.
		- This position determines the first three axes.
		- Once the first three axes are determined, you can determine the two last.
(2) Specification : 
	you have to specify your task in 5 dof task space.
	I know of two ways to do this :
		- using a 6 dof frame specification, and ignore a degree of freedom (e.g. 
rotation around X-axis).
		- using a 5-d.o.f. specification, like in Numerical Control (NC) milling 
machines :
			* position (3)
			* two direction cosinuses A and B,
 		   i.e. your 2 dof orientation can be specified with a normalized vector.  
The projection of this vector in some plane are A and B.
	This choice of specification is important because it will determine your 
ability to cooperate with other components.  

Best regards,
Erwin
		

On Thursday 15 June 2006 07:28, H.S. Randhawa wrote:
> I mailed yesterday regarding the configuration which is littlebit different
> from Serial 321 configuration.
> In this configuration the following spec. along with motor rotation are
> there:
>
> LINK                 LENGTH           MOTOR (rotation angle)
> Rotation axis(according to OROCOS)
>                                                   [ Counter-Clockwise +ve ]
>
> BASE              3"                       0-180 Degree
>       Z
>
> SHOULDER     4.75"                  0-180 Degree                          
> X
>
> ELBOW           4.75"                  -150  to  +30 Degree
> X
>
> WRIST            5.75"                  -90   to
> 90                             parallel to Shoulder and elbow
>
> WRIST ROTATION -----             -90  to 90
> Perpendicular to Wrist.
>
> GRIPPER                               opening 2" max.
> Perpendicular to both wrist and wristRotation axes.
>
>
>
> The serial family has six *revolute* joints: a "wrist" with *three*
> revolute joints with intersecting axes; an "arm" with *two* parallel axes;
> and a "shoulder" with *one* revolute joint whose axis is orthogonal to the
> axes of the "arm" set. but my arm is having three axes parallel (2nd, 3rd
> and 4th) and more over  last three are  not intersecting  at a point. Only
> 4th and 5th are intersecting. Sixth is at some offset of 2.2" from that
> intersecting point.
>
> As this is not exactly a 6 DOF arm Its a 5 + gripper but still if we ignore
> the last joint and want to move it through desire points, it should move
> accordingly.
> For some given point which is already in its workspace, Orocos is
> calculating joint positions (radians) which are contradicting with motor
> constraints so robot can't move to that point. eg  -ve value for base, or
> some other value which is out of range for that particular joint motor.
> How it can be resolved?
>
> Thanks
> H.S.Randhawa



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