They are suitable for post processing results at a particular time, e. Dytran models should be prepared in the same way as all other finite element models, using a graphical preprocessor. It must be available in the area from where the job is submitted. Termination time Output required Constraints and loading to be used Add Extra Bulk Data Data items that could not be defined using tuyorial modeling package must be added. They are identified by the Jobname of the analysis, a logical name used in the input to identify the file, and the timestep number when data was first written to it. Nastran displacement file to be used as input for a prestress analysis.
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An accelerometer is an electromechanical device that will measure acceleration forces. These forces may be static, like the constant force of gravity pulling at your feet, or they could be dynamic - caused by moving or vibrating the accelerometer. What are accelerometers useful for? By measuring the amount of static acceleration due to gravity, you can find out the angle the device is tilted at with respect to the earth.
By sensing the amount of dynamic acceleration, you can analyze the way the device is moving. However, engineers have come up with many ways to make really useful products with them. An accelerometer can help your project understand its surroundings better. Is it driving uphill? Is it going to fall over when it takes another step? Is it flying horizontally or is it dive bombing your professor?
A good programmer can write code to answer all of these questions using the data provided by an accelerometer. An accelerometer can help analyze problems in a car engine using vibration testing, or you could even use one to make a musical instrument. In the computing world, IBM and Apple have recently started using accelerometers in their laptops to protect hard drives from damage. In a similar fashion, high g accelerometers are the industry standard way of detecting car crashes and deploying airbags at just the right time.
How do accelerometers work? There are many different ways to make an accelerometer! Some accelerometers use the piezoelectric effect - they contain microscopic crystal structures that get stressed by accelerative forces, which causes a voltage to be generated.
Another way to do it is by sensing changes in capacitance. If you have two microstructures next to each other, they have a certain capacitance between them. If an accelerative force moves one of the structures, then the capacitance will change. Add some circuitry to convert from capacitance to voltage, and you will get an accelerometer. There are even more methods, including use of the piezoresistive effect, hot air bubbles, and light. What things should I consider when buying an accelerometer?
Analog vs digital - First and foremost, you must choose between an accelerometer with analog outputs or digital outputs. This will be determined by the hardware that you are interfacing the accelerometer with. Analog style accelerometers output a continuous voltage that is proportional to acceleration. Digital accelerometers usually use pulse width modulation PWM for their output. This means there will be a square wave of a certain frequency, and the amount of time the voltage is high will be proportional to the amount of acceleration.
If you are using a BASIC Stamp, or any other microcontroller with purely digital inputs, you will most likely need to go for a digital output accelerometer. The disadvantage here is that it requires you to use the timing resources of the microcontroller to measure the duty cycle, as well as performing a computationally intensive division operation.
Number of axes - For most projects, two is enough. However, if you want to attempt 3d positioning, you will need a 3 axis accelerometer, or two 2 axis ones mounted at right angles. Sensitivity - Generally speaking, the more sensitivity the better. This means that for a given change in acceleration, there will be a larger change in signal. Since larger signal changes are easier to measure, you will get more accurate readings.
Bandwidth - This means the amount of times per second you can take a reliable acceleration reading. For slow moving tilt sensing applications, a bandwidth of 50Hz will probably suffice. If you intend to do vibration measurement, or control a fast moving machine, you will want a bandwidth of several hundred Hz.
The solution to this is to use a low input offset rail to rail op amp as a buffer to lower the output impedance. Where can I find more information on accelerometers? Texas Instruments has a great accelerometer guide , including how to do some of the necessary math. The DE-ACCM datasheet has several examples of how you can use accelerometer readings to determine tilt and acceleration values.
If you really want to get into the hardcore low level details of accelerometers, and want to try soldering surface mount packages, Analog Devices has a huge selection of datasheets covering both analog and digital PWM style devices. All Rights Reserved.
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