Motion Studies (and How to Do Them)
Since the early days of CAD, engineers have been able to use software to transform their ideas from sketches and calculations on paper into virtual models to assist with assembly work, as well as for exporting files for manufacturing in various CNC machines.
But that’s fairly elementary. Even the most basic CAD packages allow for this functionality. It’s all very useful if you wish to create singular objects such as brackets or plastic molds or items of that nature, but what if you are building dynamic components that move in relation to each other? What if you want to build a machine? Can CAD help?
Of course, it can. And that’s where SOLIDWORKS motion studies come into play.
The motion features of the software can assist with a wide range of motion study depending on how complicated your requirements are, and they can be divided into three categories:
- Animation: If you simply wish to create some nice visuals for presentation or marketing without consideration of mass and gravity effects, then animation is for you.
- Basic Motion: For an extra layer of complexity that takes into consideration the effects of mass, springs, gravity and physical collision detection, then a Basic Motion study is more suited for your requirements.
- Motion Analysis: This is the top tier of motion study and takes into account a wider range of physical interactions such as impact effects, damping, force, momentum, etc.
In this article, we will take a look at the Animation and also the Motion Analysis categories. I will provide some links to Basic Motion videos at the end of the article.
You can perform the most basic level of animation at a part level. For our purposes here, we are going to skip that level and go straight on to the assembly mode, because we are interested in seeing how components visibly interact with each other.
All of the motion studies begin in the same way. You open up an assembly, load up the motion add-in, and then you can see the Model tab and the Motion Study tab at the bottom of the screen. Click the Motion Study tab to bring up the Motion Manager timeline view.
This has invoked the motion study manager, and you are now ready to select your type of study, be it Animation, Basic Motion or Motion Analysis.
For each example, I will be using the assembly models found in the software’s tutorials. Each type of motion study will use a different model, and you can access these models yourself via Resources>Tutorials.
For the animation in this example, we will use a model of a plunger. I have no idea what it is a plunger of—or what it is meant to plunge—but you can find the model in the following path:
The first step is to define the starting point/position.
Go into the MotionManager timeline and move the time indicator to the 0 seconds position. You can align the camera to your required starting position, too. For this example, I will just select an isometric view.
Next, we have to decide which part we want to animate and for how long we wish the movement to last.
In this example, we want the orange arm to move to a vertical position relative to the horizontal base.
We find the part labeled “arm left” in the list of parts within the MotionManager, and left-click the selection to highlight it. Then we drag the time bar from 0 seconds over to 5 seconds. This will define how long we want the movement to last. We then click the “add key” icon (shown with the red arrow in the image below), which will cause a horizontal bar to appear in the Motion Manager, in the “arm left” row, running from 0 seconds to 5 seconds. A keypoint (the diamond symbol) will appear at the end of 5 seconds in the “arm left” row. Make note of this little diamond as we will be using it later.
This has defined the part that we wish to move, and for how long we wish to move it.
Next, we will go into the main window and locate the actual part (the orange arm). We will then physically move it to the required position (upright). Once the final position is determined, we can press the “calculate” icon above the MotionManager tree (pictured below), and all of the horizontal rows will fill up with yellow lines, and the part will come to life in the main window.
If we want the component to end in the same place that it started, then we can do this by selecting the little keypoint diamond at 5 seconds, and pressing CTRL+C. Then, while still holding CTRL key down, we can drag the keypoint back to 2.5 seconds. We can then release the mouse button/pointer before releasing the CTRL key. If this step has been completed correctly, you will notice that a gap in the green changebar will appear between 2.5 seconds and 5 seconds.
Now we need to right-click the keypoint diamond at 0 seconds, and paste that to the 9 seconds point in the timeline. This will effectively copy and paste the start position later on in the timeline, which will let the animation know that it should return to its default position at the end of our defined time frame (9 seconds). Pressing the calculate icon again will recalculate the motion and display the animation in the main window, which will make gaps (representing a pause in the animation) appear for all components in the 2.5 seconds to 5 seconds region. The green changebar will extend to 9 seconds, indicating the end of this part of the animation. If this step has been performed correctly, then the plunger arm should move upward, pause, and then return to the default position.
The animated plunger will look like this:
You can change the position of components and also their appearance within the MotionManager. Simply drag the time bar to the point where you require the appearance change to take place, and change it there. For example, you may wish to hide a part, to change the texture, or make it translucent to show components beneath it. The tutorial will show this process in more depth.
And, of course, if you wish to render your animation for a more realistic appearance, you can do so, and I have explained how to do this in another tutorial.
There is a nice video at the following link explaining the animation process in more detail:
For this section, we will use a cam and follower system (similar to a valve lifter in a car engine), and we are going to examine the contact forces between the components as they change over time. We will then plot the results graphically.
This is the most advanced kind of motion study, and we have moved from the realm of mere animation into that of simulation.
First, open up the assembly model found in the following folder: <install_dir>\samples\tutorial\MotionStudies\Valve_Cam.sldasm.
In the MotionManager, click the tab labeled “1200,” then select “Motion Analysis” as the type of study. Press the “Calculate” icon, which will bring the assembly to life as the motion is calculated.
Next, we will define the contact faces that we wish to analyze. Select Isometric view in the main window, and then click the “results and plots” icon at the top of the MotionManager. This will open up a new window labeled “Results Property Manager,” where you will see several drop-down menu boxes.
In the first menu box (category), select “Forces”; in the second menu box (subcategory), select “Contact Force”; and for the third menu box (result component), select “Magnitude.” You can take a few minutes to look at these different options after the tutorial. This is where your type of analysis is determined.
Finally, there is a component selection field. Click on that, and then go into the main graphic window and select the two faces that are in contact and which we will be analyzing. These are the faces of the rocker and the camshaft (pictured below).
For good measure, I have also included a displacement plot that shows the displacement cycle in relation to the reaction forces.
What we can see from these plots is that the reaction force increases just as the cam begins to raise the follower. This is to be expected as the spring is starting to undergo compression. There is also a second greater peak occurring as the rocker passes the cam and begins to lower. This is due to the acceleration caused by the spring relaxing.
Because a picture is worth a thousand words and a video (with plots) is worth about ten thousand, I have recorded the motion and the plots in a video below, so you can see exactly how the force and displacement relates to the physical movement of the model.
OK, so that’s all very nice. We have some plots, but what if we wish to change the parameters of the components and see the effects of those changes?
First, go to the “1200” tab at the bottom of MotionManager and right-click it, then press Duplicate.
Rename the new duplicated tab as “2000” because we are going to step up the speed of the motor to 2000 RPM and see what happens.
Now drag the time bar back to the 0 seconds point, then go into the component tree in MotionManager and find the RotaryMotor2 component. Right-click it, and select Edit Feature. Now change the motion from 1200 RPM to 2000 PRM, and press the green tick icon to close that window. Press Calculate to recalculate with the new parameters.
Now you can see in the animation that as the cam passes, lifting the rocker, the rocker bounces back onto the cam. (A video of the bouncing effect can be seen here: https://youtu.be/2jVQVtZ9HGg.) The spring is actually losing contact with the cam, and as it bounces the spring is compressed again, and it releases that energy back into the rocker, causing the rocker to bounce. The bounce is visible in the displacement plot as a second peak each cycle.
This is caused by the faster rotation. The contact force is actually at zero when the rocker leaves the cam, so we need to ensure that the cam does not lose contact with the rocker. We can do this by altering the spring constant. Increasing the spring constant should allow us to maintain the motion.
To do this, go into the component tree in the MotionManager, right-click “LinearSpring2” and press Edit Feature.
Here the spring properties are visible. Currently, the value for k (spring constant) is set at 0.1 N/mm. We will increase this value 100fold to 10.00 N/mm. Now click the green tick to close the window and press Calculate again.
Now you can see in the new plots that the bounce has been eliminated in the displacement plot and that there is also an increased force from the spring that ensures that the rocker remains in contact with the cam, regardless of the new speed.
And there you have it. You are well on your way to creating pretty animations and also complex motion studies.
We have deliberately skipped over the Basic Motion section—purely because the skills required for Basic Motion lay somewhere in between Animation and Motion Analysis. If you can do both Animation and a full analysis, then doing the Basic Motion study should be very easy for you. And besides, where is the fun in being shown everything! One of the reasons I love doing these articles is because it forces me to sit there and relearn features that I may have not used for a while. It blows the cobwebs away.
There are lots of cool videos on YouTube showing the Basic Motion in more detail if you wish to look for yourself.
Here is one of my favorite videos involving a Geneva Wheel mechanism. Part One shows the creation of the wheel from scratch, whereas Part Two (linked below) shows how to perform the Basic Motion study on the mechanism.
Of course, the examples in this article can be found in the SOLIDWORKS tutorial section of the software. They provide a good foundation of the skills needed for motion simulation and animation.
I definitely recommend searching on YouTube for other videos on this subject, and trying to replicate the video examples for yourself. YouTube is a great (free) resource for this type of thing, and every video will add to your growing skill set as an engineer.
Until next time!