Help creating a simulation

I am new to Solidworks, and am currently trying out the 2003 learning
edition. I am attempt to try out a design for a pneumaticly operated
motion base (like flight a simulator). Since the design is already
created in AutoCAD, I am really just trying to see if I can try out
the motions in Solidworks, and double check our control system because
there are redundant forces which will try to tear the frame apart if
the 4th actuator is not the correct length.

The geometry of the part that moves is a rectangle 8' x 10'. There
are 4 actuators, each with 20" of stroke, one on each corner. They
simply push up on the frame which is free to move. That moving frame
is prevented from falling over by an outside rail.

I've tried a few approaches, but am running into difficulties. My
first attempt was to just create a new part, then;

---
First, to create the base and actuators, then I plan on constraining
the lines representing the actuators, so that they stay upright, so I
won't need to created the outside rails;

-In a new sketch drew a rectangle to define the base (also 8'x10').

-Then I drew 3 lines to represent the actuators, intending to drive
their dimensions from a table. (The reason only 3 of the actuators
will be given dimensions, is that the 4th must be a driven dimension
since its position is determined by the other 3.)

-The 1st line is constrained so that one end is fixed to the base
(all the actuator lines have one end point fixed to the base). The
other end of line 1 can only lengthen. The line cannot change angle
in any plane.

-The 2nd line is constrained to be coincident to one plane, so that
it can lengthen or change angle in that plane.

-The 3rd line is not constrained to any plane, so its free to wiggle
around.

I now have my actuators.
---
To start to define the moving frame;
-I create a construction line between Line 1 and 2. Then create a
reference plane coincident with that construction line, and the end of
line 3.

-Then I create a new sketch on that reference plane. I draw a
rectangle 8'x10' to create the moving frame. The first corner of the
rectangle is constrained to the top of the 1 actuator line.

-My next intent is to go back and constrain the tops of the other
actuators to be conincident with the corners of the moving frame.
But, this is where I get stuck. If I edit the sketch which has one of
the lines, the moving frame disapears. If I just hightlight the two
endpoints, there doesn't seem to be any provision to add constraints.

---
So, I try another method. I create 6 individual parts, the first two
each contain a sketch of a rectangle, 8'x10' to represent the base and
moving frame. The next 4 consist of as sketch with a single line to
represent the actuators.

Then I create an assembly, and bring in each part. The base first, so
it is fixed in place. Then with each of the actuators, I attempt to
constrain their first point (the origin) to each of the corners of the
base. I can do this with the first actuator, by just mating both
origins. However, at the second actuator, I get stuck. I can select
the origin of that actuator, but I cannot select any of the corners of
the base but the base origin.

---
I am now going to try a 3rd method, by actually extruding shapes in
the parts, so I am not just working with sketch lines.

Any advice to offer? Am I going about this the wrong way?


Joe

 

I studied such structures in deep some years ago. Check
http://www.dynabits.com/delta/index.htm for a SW model of the fastest robot
in the world based on such a "parallel" structure.
From what I understood, your structure is hyperstatic ("3 of the actuators
will be given dimensions, the 4th is determined by the other 3"), which will
lead to an overconstrained design if you don't care.
The important thing is, SW assembly mates solver operates on parts that are
"rigid", or fully determined. You can't expect a "driven dimension" in a
part to be driven by the assembly.
I can see 2 ways of solving your problem:
1) remove the "Line" from the 4th actuator, and replace it by a driven
dimension between its 2 ends in assembly
2) model your actuators as an assembly where the piston can slide in the
cylinder, then drive your model with the 3 "distance" mates.
e-mail me your model if you need more help.

 

Yes, I was aware of that issue, and did take your approach (i.e.
only creating 3 actuators in Solidworks). My difficulty was just in
creating that geometry. My first effort to just do it using sketches
didn't work, either because of me being new to Solidworks, or because
that process is not workable. However, I did succeed by actually
creating solid parts and then each actuator had a design table, so
that I could choose the different lengths at my assembly.

In fact, my entire purpose was to do a reality check of our control
algorithm. I would enter the lengths of 3 of the actuators, and see if
the resulting distance for the 4th actuator agreed with what our
control system would do.

I think it would have been possible to create a spreadsheet at the
assembly where I could have typed in the various possible positions,
and got the results there. However, I couldn't figure out how to do
it. I ended up manually editing each actuator and assigning its
length.

As for the results; our simple control algorithm assumed each
actuator stayed perfectly vertical, which was incorrect... but I
wanted to determine if it was good enough. We are creating a Boat to
be used in a theatrical production. Our actuators are pneumatic and
have more than enough compliance to accept the positioning error I
measured in Solidworks.

Joe Dunfee