[MBDyn-users] Torque control in MBDyn

Pierangelo Masarati pierangelo.masarati at polimi.it
Thu Jun 26 08:12:42 CEST 2014

On 06/24/2014 11:34 PM, Tingnan Zhang wrote:
> Hi All:
> Anybody know if there is a torque control mechanism in MBDyn? For
> example, suppose we impose the rotation angle of a revolute pin by a
> file driver, we then would like to monitor the torque at the pin. If the
> torque exceeds the maximal value (allowed by the motor in real world),
> the rotation will stop until the file drive reverse the direction of
> rotation or the torque decreased due to other reason.
> Is it currently possible in MBDyn? If it is not, what alternative
> solution can we use?

There is no such model that can be used in a straightforward manner.

The solution would be to have the imposed motion as

     x - x_ref = slack

where slack is 0 when the reaction torque is within the bounds, and 
becomes a variable when the torque saturates.

At the same time, the reaction torque produced by the motor would be 
saturation(lambda), i.e. lambda when between the bounds, and the bound 
value otherwise.

The unknown would be either lambda, when between bounds, or slack otherwise.

A similar, but smooth behavior could be obtained by regularization of an 
imposed motion joint, where the imposed motion equation is something like

     x - x_ref = c * lambda

and the corresponding reaction torque is lambda filtered by a saturated 
ramp (possibly approximated with some regular function, like tanh(lambda)).

The regularized joint is already implemented (see the "joint 
regularization" element), which allows the constraint to be violated by 
an amount that is proportional to the entity of the constraint reaction, 
thus mimicking, for example, the presence of a deformable constraint, 
like a bushing.

What needs to be implemented is the saturation of the reaction force. 
I'm not 100% sure it works, because the problem is actually becoming 
non-smooth, but it is probably worth giving it a try.

Cheers, p.

Pierangelo Masarati
Associate Professor
Dipartimento di Scienze e Tecnologie Aerospaziali
Politecnico di Milano

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