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Mechanism Dynamics Option has a serious lack of functionality relative to Mechanica Motion


Mechanism Dynamics Option has a serious lack of functionality relative to Mechanica Motion

Mechanism Designers and PTC developers,

Mechanism Dynamics Option (MDO) has a serious lack of functionality
relative to Mechanica Motion: It does not have what used to be called
Gimbal joints, Bearing joints, or 6DOF joints. Before you reply that MDO
does in fact have Bearing and 6DOF joints, I'll say right now that the
ones in MDO are only a shadow of their predecessors. In MDO you can no
longer model friction in any joint that has 3 rotational degrees of
freedom such as ball joints. Nor can you measure rotation angles or
attach loads or drivers to these joints.

In the following I describe why this is so important to us and to anyone
who designs anything but the simplest mechanisms:

A joint used often in our mechanisms is the narrow footprint lug/clevis
pin joint. In these joints there is a primary rotational axis and in
addition 2 transverse rotational degrees of freedom of limited range
that result from an intentionally narrow lug in combination with the
pin-bore mechanical clearance. These joints are not intended to react
transverse moments. Manufacturing tolerances ensure the transverse
rotational limits are not exceeded. In Mechanica Motion a narrow
footprint lug/clevis pin joint is modeled using a three rotational
degree of freedom joint called a "Gimbal joint".

Side clearance in the lug/clevis if present, adds a translational degree
of freedom in the direction of the primary rotation axis. Again this is
of limited range and manufacturing tolerances ensure it isn't exceeded.
In Motion a narrow footprint lug/clevis pin joint with side clearance is
modeled using a joint with one translational and three rotational
degrees of freedom joint called a "Bearing joint".

Narrow footprint lug/clevis pin joint are frequently used in pairs to
create a hinge that can withstand transverse moments without binding. A
wide footprint lug/clevis pin joint falls into this category. In these
cases both joints have three (one primary and two limited) rotational
degrees of freedom and in addition one of the pair has side clearance
for a translational degree of freedom. In Motion a "Gimbal joint" and a
"Bearing joint" make up the pair as shown in this figure from the Motion
Help files.

This arrangement yields a statically determinate hinge connection with a
single resulting rotational degree of freedom and no redundant load
paths. When modeling a pair of such joints the separation between them
is set to the width (footprint) of the actual hinge. Transverse moments
are then resolved into the correct friction producing reaction forces
thus properly capturing frictional binding behavior. Note that a real
hinge with an insufficient footprint will bind (seize) due to friction
when subject to transverse moments of sufficient magnitude relative to
the direct moment. To enable such behavior to be captured in an
analytical model the above described functionality is required.

A single rotational degree of freedom joint that is not subject to
transverse moments is the only type that can be modeled with what is
called a "Pin joint". This type of joint has no defined width
(footprint) associated with it so while it does resist transverse
moments the resulting friction cannot be determined.

Ball/socket joints are modeled in Motion using the three rotational
degree of freedom "Gimbal joints". A three rotational degree of freedom
joint called a "Ball joint" in both Motion and MDO is not useful here
because in lacks defined axes of rotation at which to apply friction
loads (when using "Gimbal joints" in Motion to model ball/socket type
joints the three joints axis directions must be carefully chosen so as
to avoid gimbal lock throughout the range of motion of the mechanism).

In addition to the "Gimbal", "Bearing", and "Pin" joints a fourth type
of Motion joint called a "6dof joint" is used frequently. A "6dof joint"
has three rotational and three translational degrees of freedom. This
joint is used in conjunction with spring and damping loads on the
translational degrees of freedom to model compliance in the otherwise
rigid bodies representing the moving bodies in a real mechanism.

Eight years ago we modeled a deployment system which contained a torsion
bar spring set which had a unique attachment mechanism to the hinge it
was driving. In this system friction from transverse moments was present
but none of us thought it was of any consequence. The initial Motion
checkout model (no friction added yet) showed the system behaving as we
assumed. When we added friction the mechanism locked up. Any non-zero
friction, even a friction coefficient far below realistic levels caused
all motion to stop. Had we modeled this system in Mechanism Dynamics we
might have built hardware that didn't work and would have had to
redesign and rebuild.

PTC's workaround to obtain a 3 rotational degree of freedom joint is to
use 3 pin joints and two dummy parts to connect them to the two bodies.
We're in the process of testing this kludge. The mechanism we're
currently trying to model has 18 moving bodies and 38 joints all of
which are Gimbal, Bearing, or 6DOF. In implementing PTC's workaround
we'll end up with 94 bodies and 114 joints. This is ridiculous to say
the least.

Question to PTC: What were you thinking? When will you fix it?

I'm not holding my breath here. The last piece of missing functionality
relative to Mechanica Motion was the camera-on-body and it's been 6
years since I complained about that.

Thanks for listening,

Mike Foster

ATK Space

Goleta, CA

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