Group 11 Kinematics for a Trebuchet
Transcript of Group 11 Kinematics for a Trebuchet
Group 11
Kinematics for a Trebuchet By: Alex Miller, Sun Chun Cheng, and Meagan Hughes
Introduction: We will be going over how to simulate the movement of a trebuchet within the
DMU Kinematics toolset in Catia. We will discuss how to make a mechanism capable
of being simulated. At the beginning of the tutorial you will be given a fully constrained
assembly which you will then edit or create joints in Catia and remove the degrees of
freedom that prevent the simulation. We will explain the concept of auto created joints
but we will not force you to go over the constraints of the trebuchet due to the potential
of an incorrect constraint that would make this tutorial inaccurate.
Objectives: To understand basic of Auto Joint Creations
To understand some of the many joints available in the
DMU Kinematics Workbench
To Simulate the movement of an assembly with a large
amount of parts
A brief introduction to the simulation with laws.
A working kinematics of a trebuchet
Open Folder Final, Open trebuchetassembly_final.CATProduct file.
Once the fully constrained assembly is open, start the DMU Kinematics. To do this go to start
Digital Mockup -> Kinematics.
Click on Assembly Constraints Conversion.
Click New Mechanism - Mechanism One
Click Auto Create
When you Auto Create joints the system takes the constraints that exist between your parts and
creates relationships between these parts. With a well-designed constraint system you can
remove degrees of freedom before going into the joint manipulation. I recommend designing
your constraints in such a way that it reduces the editing of the joints post creation.
The Unresolved Pairs should now read: 0/20.
These joints are determined by the degrees of freedom that exist between your parts due to
their constraints. So, one thing that one can do is restrict these degrees of freedom. Create
relationships between nonmoving joints and remove any ability for them to move. The
processes that we go through in this tutorial are removing these degrees of freedoms.
Later in this tutorial you will also create commands. Commands are driving movements so they
do not remove the degree of freedom but they will allow for the movement of these parts and
allow for simulation.
Moving forward in this tutorial it is important to understand the references underneath
the titles of joints are more important to which you should edit. When using a feature called
Auto Create Joints it will sometimes create the joints in a slightly different order.
Open Mechanism One DOF 7 in the tree. This is under Applications and then Mechanisms.
Open Joints in the tree underneath mechanisms.
The Parts that the joints are between is more important than the number of the joint.
Double click on Cylindrical.2 (MainAxis, Arm). Check Angle Driven
Double click on Cylindrical.2 (MainAxis, Arm) and make -277.2 lower limit and 0 upper limit.
Right click Cylindrical.4 (Weight Axis, Arm). Delete. Do not delete children
Select Rigid Joint from the toolbar.
Select Weight Axis and Arm for Part 1 and Part 1. This will replace Cylindrical 4.
Again it is more important you have the two parts that are part of the relationship then the
number.
Delete Cylindrical.6 (Axis sleeve, Part2.1). Do not delete children
Create a Rigid Joint between axis sleeve one and main axis
Delete cylindrical.11 (cap.1, Part2.1). Do not delete children
Delete Revolute 14. Do not delete children
Create a Rigid Joint between cap.2 and MainAxis.
Create Rigid joint for cap.1 and main axis.
Delete cylindrical 16 (leg right and mainaxis).
Create rigid joint between Leg right and main axis.
Double click on Cylindrical.19 (pin, Trough Base). Check angle driven and length driven
Double click on Cylindrical.19 (pin, Trough Base).
Make degree 0 and 180 and length 0 and 6(if it goes through instead of out change it to -6 to 0)
Set revolute.20 to angle driven. Again ensure that it is the joint between Weight and Weight
Axis.
Revolute.20 (Weight, Weight Axis) 0 to 277.2
The mechanism should now be capable of being simulated. What we have done is
removed the ability for any parts, outside of the parts that are desired to move, capable of
moving. Once this is complete Catia is capable of simulating the movement. The number of
commands that you create will be the number of parts capable of movement. When simulating
our trebuchet you will move the arm, weight and pin. Although there is a command for turning
the pin this movement is not necessary to our simulation although it is intended in the design.
Click on Simulation located in the toolbar.
Select Mechanism.1
Move Command.4 to opposite of end of starting position. Click Insert
Move Command.1 and 3 to opposite ends of starting position
Click Insert
Click Jump to Start
Change time step to .04
Play (ok)
Click on the Formulas button. We are now going to add formulas that control the movement of
the trebuchet. With correct formulas and understanding of the math behind the movements you
can simulate the movement of any object accurately and then take other measurements from
these movements.
Select Mechanism.1 DOF=0 in the Tree.
Click Mechanism.1\Commands\Command.1\Angle
Add Formula
Enter this as your formula =(-277.2deg)/(1s)*(Mechanism.1\KINTime)
What does this mean? This is the distance that the arm is going to travel per second over the
course of one run time. Mechanism.1\KINtime is one circuit of simulation. With that in mind one
is capable of applying mathematical measurements into the system and simulates accurate
movements.
As you can see the Active Column in the table now says yes in the row for
Mechanism.1\Commands\Command.1\Angle
Bear in mind that the number of the Command is determined by when they are created. What is
important to identify is that there are three Angle commands two of them belong to cylindrical
and so these are the only possible commands to be confused by. Check what joint each
command is related to. The only command that is unused is the Cylindrical Angle related to the
Pin.
Click Mechanism.1\Commands\Command.3\Angle
Add Formula
Enter this as your formula =(277.2deg)/(1s)*(Mechanism.1\KINTime)
Click Mechanism.1\Commands\Command.4\length
Add Formula
Enter this as your formula =(-3.2in)*(Mechanism.1\KINTime)/(.01s)
Under The Column Active in the three Rows that you have now added formulas too, it should
say Active. If it does not then the formulas have been inputted incorrectly.
Click on Speed and Acceleration for Reference Product Select Main Axis.
For Point Selection in Speed and Acceleration choose Point.1 in the part NewHook.
Click Simulation with Laws
Select Activate Sensors checkbox
Activate
Speed-Acceleration.1\X_Point1
Speed-Acceleration.1\Z_Point1
Speed-Acceleration.1\Y_Point1
Speed-Acceleration.2\Linear Acceleration
These will be what your simulation is measuring.
This is a representation of the movement of the point through time. In this scenario it is
simply a measure of the location of the point as it travels during the simulation. There are many
different options that you are capable of measuring and if your simulation is running according
to the math that you have tested you can find many other complex values through the
kinematics simulation instead of having to do math on every process.
Summary: Through this tutorial you have been given a basic understanding of the
kinematics simulation process. We touched on constraints and how they influence the
creation of joints. We demonstrated how to use joints to create the degrees of freedom
that we are looking for and remove the degrees of freedom that are not intended. We
went over how simulation with laws works and how to apply it with real mathematical
equations but it can be done with approximations or “random” numbers to give a
simulation.