Machine Design

How to Question motion models

Dynamic analyses can tell a lot about a design without building physical models.

By Ian Hogg
Senior Technical Specialist
Dynamics Inc.
Ann Arbor, Mich.

Edited by Paul Dvorak

The four-cylinder engine is modeled in Catia V5 and examined for several kinematic and dynamic conditions with Kinematic Simulator and Dynamic Designer from Mechanical Dynamics Inc. To check that components are properly constrained, the user turns the crankshaft with the cursor. The model might not behave as expected. For example, the pistons and connecting rods might fall away from the cylinders, indicating missing constraints. The inset menu shows other allowable manipulations.

To drive the engine at a constant rate, and to extract more information from the analysis, users have assigned what simulation software calls a law — a formula in Catia kinematic nomenclature that defines motion. In this case, 360deg*Mechanism.1\KINTime drives the crankshaft at a constant rate. Mechanism.1 is the first part defined. In this case, it's the crankshaft

To graph a value of interest, such as the length from a piston to the crankshaft, assign a sensor to track the distance. Stepping through the recorded frames let users spot the minimum distance.

The plot shows displacement of the joint connecting the piston to the housing.

For a dynamic analysis, the engine is turned by applying torque to the crankshaft for half a second.

The digital model makes it possible to generate dozens of revealing plots. This one, for instance, shows the relation of a piston's acceleration to the reaction force in the joint connecting the piston rod and crankshaft. These forces are useful in stress, durability, and fatigue studies.

Determining whether or not a mechanism will work before building physical prototypes can be time consuming and challenging, especially for new designs. At one time, only physical prototypes made it possible to check for part interferences, determine a range of motion, verify working envelopes, set speeds and feeds, size motors and bearings, and predict power consumption.

Today, software that simulates mechanism dynamics provides another way to settle these issues before prototyping. Simulating the operation of a four-cylinder engine demonstrates the functions and features of kinematic versus dynamic motion simulations. The one used here, Dynamic Designer, works with Catia V5, as well as SolidWorks, Solid Edge, and Autodesk Inventor.

In a nutshell, the software builds the model in a 3D CAD system and then uses kinematic features in the modeler to check dimensions and proper element or link sizes. A third step uses dynamic-analysis capabilities to gauge and plot dozens of other parameters such as forces, torques, and accelerations.

After creating system components in CAD, Catia V5 in this example, an assembly-design function constrains parts within the assembly. It quickly determines if parts fit together. Users can then interact with the assembly to visualize a range of motion.

A manipulator panel lets users alter geometry to verify that connected parts move appropriately. For this engine model, turning the crankshaft should show pistons and connecting rods moving as expected. If not, constraints might have to be added. For example, adding a coincident constraint aligns the axis of a piston with the axis of its cylinder bore. This constraint aligns the two axis, but they are free to slide along each other, or rotate about a defined axis. This is known as a cylindrical joint in the dynamic model.

Kinematic simulations allows studying the motion of the assembly either through direct manipulation or a time-based simulation. It also lets users see motion and plot kinematic data such as a motion envelope to refine design issues such as packaging, part interference, range of motion, and minimum distances.

A digital mock up (DMU) option in Catia V5, for instance, lets users do some of these. The kinematic feature uses constraints created in the assembly modeler. The analysis option can examine two or more parts to see if they interfere, or come within a certain distance of each other. The analysis tool also provides reports pinpointing problems and offending components.

All model motion must be fully defined for a kinematic simulation to work. For example, solid models contain no data on constraints, so if the engine model is to move realistically, constraints must keep pistons in cylinders and attached to connection rods.

In the example, the crankshaft's axial rotation is not fixed, therefore users must create a command to control its angular motion. This requires checking a box to turn on the feature and assigning a rotational amount, such as 360°. After typing in the command, users can turn the crankshaft a full 360° by "grabbing" it with the cursor and turning it. It can also be moved as a function of time.

The latter option calls for creating a law, in software parlance, with a formula that acts on the previously created command. For instance, in Catia V5 a time function could read: 360 deg* Mechanism.1\KINTime, which means change the variable Mechanism.1 (the crankshaft rotation) from 0 to 360° in the period defined by KINTime. Another field defines KINTime as 10 sec. Compiling a simulation drives the mechanism through the defined motion or at a fixed velocity, generates a sequence of assembly positions, and creates what's called a replay object. VCR-style buttons control the action.

With the model moving, users can look for interference or minimum distances using analysis tools. To plot geometric data, activate one or more sensors in the simulation. For example, a user could select length from a list of parameters for cylindrical joints constraining pistons. Other parameters available for plotting include linear and angular displacements. Press the play button and a graphics button, and a plot of simulation results pops up.

The limitation of a kinematic simulation is that it does not allow the application of torque, friction, or contacts to predict reaction loads. Progressing into dynamic analyses lets the designer determine whether or not it will work under the influence of combustion, gravity, friction, inertia, and other physical forces.

Parts and constraints defined in the assembly can be automatically converted to a dynamic mechanism. The conversion is similar to language translations. Catia V5 constraints are written in a Catia modeling format which must be translated into a format the dynamic-motion solver understands. This automatic translation prevents users from having to repeat work. It can be painful to build an assembly only to find you have to start from scratch to build a dynamic mechanism. Automatic conversion avoids this. Dynamic motion joints will appear on the model in the graphics window and on the treeview. In the engine example, the dynamic-mechanism-structure simulation lets users verify that the assembly has been accurately converted.

The engine now needs an input drive, such as torque on the crankshaft. In our example, a Torque dialogue box allows applying torque of 1,000 N-mm for 0.5 sec. Hitting the replay button shows that the applied torque turned the crank with enough force to overcome gravity and continue turning. Mass properties came from the Catia parts, after defining their materials. Mass and velocity then dictate dynamic loads. Forces include those that overcome gravity and the inertia of parts about to move.

To plot dynamic-motion data, select the newly created analysis object — the CAD model with dynamic joints. Similar to the replay object, the analysis option places an analysis object in Catia's treeview. After a dynamic analysis, the objects contain calculations for part positions, velocities, accelerations, constraint forces, and moments.

A plot could show, for instance, the relation of a piston's acceleration to the reaction force in the joint connecting the piston rod to the crankshaft. Forces calculated in the dynamic simulation are often the accurate inputs for finite-element analyses, where loads have often been guesses. Users can plot other results including power consumption, momentum, acceleration, and potential and kinetic energy.

The motion simulator calculates these terms in its results. Power consumption, for example, is a matter of how much energy is expended over a finite time.

While CAD-based assembly-design features and the kinematic simulator are not physics based, they still provide valuable design information, especially for envelope studies. Understanding the influence of realistic forces acting on a design in the physical world calls for extending the capability of CAD modelers with simulation software that models dynamic mechanisms. The right mix of design and system simulation software can help design engineers predict how products will function under real-world conditions.

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