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Pro/ENGINEER Advanced Mechanica Takes Out the Stress
- by Jim Buchanan
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Ask an engineer and they'll tell you intelligent sensor technologies are all the rage in industry today. Computer-based sensors are increasingly popular data acquisition applications that range from oil exploration and mining to industrial and automotive control systems.
Intelligent sensors face transform information about their surroundings - analog information - into digital data. To do this, and to maintain reliability, they must be able to take the inevitable punishment that comes with their physical territory.
For engineers who design sensor subsystems, the challenge is to test these devices for what might be very pronounced and unpredictable stresses. For those designers who use Pro/ENGINEER, the obvious place to turn is to Pro/ENGINEER Mechanica and Advanced Mechanica.
Spike Graves is a mechanical engineer for EAC Design in Burnsville, Minnesota. At EAC Design, Spike works on a wide range of projects for EAC clients in a number of industries. EAC designs subsystems for everything from electronic game controllers to electronic motors for high-end industrial equipment, and one of Spike's jobs is designing sensors, particularly transducers, for demanding environments - the types found in modern aircraft.
With the increase in computer controls, the aircraft industry faces a growing demand for sensors and other measurement devices that can translate real-world events into digital information, and do so with ultra high reliability. Today's aircraft requires hundreds, sometimes thousands, of transducers and sensors, says Graves.
The challenging part of Graves' job is making sure these transducers work, day in and day out, flight after flight. If a transducer fails in flight, the result could be tragic, so they must be designed with a large safety margin, well able to withstand the forces associated with five-G turns and other such actions.
Think, for example, of slamming your laptop into a wall. That produces a force - a deceleration force - that's not far from what a plane might experience in a crash. And transducers are expected to be able to survive these types of shock loads.
This brings us to the second part of the challenge: many transducers are one-off designs, designated for a specific role, and a specific place, in a specific aircraft. That means each must be stress tested individually, which can be a time-consuming chore.
Making all of these chores easier for Graves is Pro/ENGINEER Mechanica and Advanced Mechanica.
"We use Pro/ENGINEER Mechanica for static stress testing - for finding weak spots or areas of high stress in the basic design, under aerodynamic and acceleration loading conditions," he says. "Then, when we have a general idea of where the stress concerns will be, we go to Advanced Mechanica for the dynamic testing."
"For transducers, two types of dynamic testing are critical. The first is to determine stresses from frequency dependent forces, usually vibration from the engines or from air turbulence."
"The second is for time-dependent forces, like in the force of impact," Graves says. "Resistance to force is particularly important at times of acceleration or deceleration, or in tight turns. What's tricky is that the forces can come from any direction, not just up and down, but from any angle, and all of that has to be taken into account in the testing."
For directional testing, Graves starts by testing for static stress on each of three axes - X, Y, and Z. This way he is able to determine which loading direction or directions create the worst-case stresses. Then, as he applies vibration and force loads, Mechanica makes it easy to adjust the angle of force by manipulating the values of the original X, Y, and Z axis tests.
"Normally, you'd have to set up your test along all three coordinates," he says. "But this way, by separating the data for each axis, I have the flexibility to change directions very easily."
Testing a virtual prototype. For his dynamic tests, Graves applies the appropriate conditions - materials, constraints, and, for an impact test, load - to his model. Pro/ENGINEER Advanced Mechanica then identifies those areas with the highest stress levels. Based on this evaluation, Graves makes changes to his model. He might thicken the walls at points of high stress; he might try a different material; he might even try a different type of transducer-to-aircraft attachment - a differently shaped bracket, for instance.
"I decide what change I want to make," he says. "If it's a material change, I go into Pro/ENGINEER Advanced Mechanica and make my changes - maybe increasing the strength of the material, for instance. Material information is kept in Advanced Mechanica for each component in the model, so it's just a matter of opening the Materials dialog box.
"To make a geometric change, I go to the model in Pro/ENGINEER, and do it there - maybe thickening the ribs, or adding another support. Then, when the model regenerates, the new information goes right to Advanced Mechanica for the next round of testing."
By using virtual prototypes for testing, Graves saves substantial time over what it would take to do the same testing with physical prototypes.
"I can change and analyze a virtual prototype in a few minutes," he says. "But to do that with physical prototypes - to make a material change, or a geometric change, and then test it again - could take weeks or even months. Plus, with the extreme loads I'm using for testing - I might apply a 10-G force, for example - it would be difficult, and expensive, just to find the physical test equipment."
A geometric mesh. Graves also likes Pro/ENGINEER Mechanica and Advanced Mechanica because they use geometric elements, rather than finite elements, to apply the testing mesh to the model.
With finite elements, the mesh can look like someone draped a fishing net over the model. Each intersection on the mesh forms a point that takes in information about the stress in that area. To increase test accuracy, the engineer must increase the number of points; to accommodate a change in model geometry, the engineer must create a new mesh, which must be adjusted again for accuracy.
By contrast, the Pro/ENGINEER Mechanica geometric mesh adjusts the placement of the points, and the shape of the connections between them, according to the needs of the model geometry. Because the geometric mesh employs algorithms that understand model geometry, it can decide which elements need a higher order equation to describe its shape, and adjust its mesh accordingly.
"The geometric mesh really helps me," Graves says. "If I change the model geometry, the mesh adapts to it automatically. I don't have to tweak it, rebuild it, or worry about its degree of accuracy. It saves me a lot of time, and that's important because these are frequently one-off products, so each has to be tested independently.
"It's nice not to have to worry about the mesh," says Graves. "Instead, I'm able to keep focused on solving the problem at hand."
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Click on images below for larger view
The geometric mesh created by Pro/ENGINEER Mechanica's AutoGEM function adapts to changes in the model automatically
Results from a Dynamic analysis are easily interpreted from fringe plots in Mechanica
Graph results give the ability to inspect the component's response to excitation at different times or frequencies
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