Tag Archives: Wikipedia

3050 Session Five: Finishing Project One; Intro to Project Two; 9/11/14

On Tap Today:

  • Submission protocols for Project One reviewed (again)
  • Ranking message for Project One (explained, with example)
  • Common problems with Project One (for you to avoid)
  • Super-Secret Sneak Peek at Project Two (for the ambitious students)

https://i2.wp.com/jerryrushing.net/wp-content/uploads/2011/10/deadline.jpg

Project One  is due by 11:59 pm Tuesday, 9/16/14:  Post to Wikipedia (or other knowledgebase) and provide a copy of the text with a link to the Wikipedia entry (see an example here). Provide “before” and “after” screenshots as linked files with your references on your team blog page.

Every team member will be responsible for sending me an email by 11:59 pm Thursday that includes the name of your team and a numbered ranking of your teammates. Numbers are based on the number of people in your team excluding yourself (you will not rank your own contributions). The highest number should be assigned to the person who you felt turned in the best performance while working on this project (e.g., if your team has five members, in this ranking the highest number will be four). Here’s an example:

ranking email

 

The teammate with the “highest” score will receive a bonus point. Likewise, the teammate with the lowest score will lose a point from their grade for this project.

http://rorytrotter.files.wordpress.com/2013/07/rank-and-yank-hr.gif

Common Wikipedia Problems with Project One Executions (and common strategies for avoiding them)

1. Entry does not serve knowledge base 
  • Expand a stub, make an entry for a section of an existing article, or choose a “wanted” or “most wanted” entry 
  • Enter your contribution into Wikipedia in advance of the due date (to allow time to gauge readers/editors responses)
  • Familiarize yourself with what kinds of entries are cut (and why)
2. Entry does not follow formatting/style guidelines of knowledge base
3. Entry contains inaccuracies, unverified claims, and/or grammar/punctuation mistakes
  • Research your topic thoroughly
  • Be sure to cite sources appropriately using Wikipedia’s guidelines
  • Perform common editing/spell check functions, including importing your text into a word processor

Common Student Problems with Project One (and common strategies to avoid them):

The Procrastination Situation:
  • You decide to wait until Tuesday to familiarize yourself with Wikipedia’s markup language and editing procedures, only to discover that they are a bit more complicate than you anticipated and are thus forced to watch, teary-eyed, as the midnight deadline rolls past you like a giant boulder crushing your dreams of academic success.
    • https://i0.wp.com/upload.wikimedia.org/wikipedia/commons/thumb/a/af/TK_sandbox_icon.svg/1024px-TK_sandbox_icon.svg.pngPro-tip: Play with Wikipedia. Start today in the sandbox.  A link to your personal sandbox will appear in the upper right corner when you are logged in to your Wikipedia account.
The Citation Situation:
  • You compile your facts and evidence behind your entry from websites that you forgot to document and/or things your mother or some drunk guy told you, only to discover that Wikipedia, despite what its various critics (and possibly your mother or some drunk guy back when, as mentioned, they were acting as your primary research source) have suggested, actually has fairly formal standards for documenting evidence and listing citations.
The Illustration Situation:

Rough Draft Review

Hand a copy of your rough draft to a different team.  As a team, review your peers’ work. Complete the following tasks:

  1. Can you identify a sentence definition? Is it conveniently placed? If one is missing, would it help the readability?
  2. Is there a clear organization of information? How could it be improved?
  3. What questions remain unanswered?
  4. Identify the primary readership of this article.  Be as specific as possible.

Project Two will begin in earnest on Tuesday.  be sure to read chapter 24 in Anderson if you have not already done so.

3050 Project One Sample

Piezoelectric accelerometer

A piezoelectric accelerometer is an accelerometer that utilizes the piezoelectric effect of certain materials to measure dynamic changes in mechanical variables. (e.g. acceleration, vibration, and mechanical shock)

As with all transducers, piezoelectric accelerometers convert one form of energy into another and provide an electrical signal in response to a quantity, property, or condition that is being measured. Using the general sensing method upon which all accelerometers are based, acceleration acts upon a seismic mass that is restrained by a spring or suspended on a cantilever beam, and converts a physical force into an electrical signal. Before the acceleration can be converted into an electrical quantity it must first be converted into either a force or displacement. This conversion is done via the mass spring system shown in the figure to the right.

 

Introduction

The word piezoelectric finds its roots in the Greek word piezein, which means to squeeze or press. When a physical force is exerted on the accelerometer, the seismic mass loads the piezoelectric element according to Newton’s second law of motion (F=ma). The force exerted on the piezoelectric material can be observed in the change in the electrostatic force or voltage generated by the piezoelectric material. This differs from a piezoresistive effect in that piezoresistive materials experience a change in the resistance of the material rather than a change in charge or voltage. Physical force exerted on the piezoelectric can be classified as one of two types; bending or compression. Stress of the compression type can be understood as a force exerted to one side of the piezoelectric while the opposing side rests against a fixed surface, while bending involves a force being exerted on the piezoelectric from both sides.

Piezoelectric materials used for the purpose of accelerometers can also fall into two categories. The first, and more widely used, is single-crystal materials (usually quartz). Though these materials do offer a long life span in terms of sensitivity, their disadvantage is that they are generally less sensitive than some piezoelectric ceramics. In addition to having a higher piezoelectric constant (sensitivity) than single-crystal materials, ceramics are more inexpensive to produce. The other category is ceramic material. That uses barium titanate, lead-zirconate-lead-titanate, lead metaniobate, and other materials whose composition is considered proprietary by the company responsible for their development. The disadvantage to piezoelectric ceramics, however, is that their sensitivity degrades with time making the longevity of the device less than that of single-crystal materials.

In applications when low sensitivity piezoelectrics are used, two or more crystals can be connected together for output multiplication. The proper material can be chosen for particular applications based on the sensitivity, frequency response, bulk-resistivity, and thermal response. Due to the low output signal and high output impedance that piezoelectric accelerometers possess, there is a need for amplification and impedance conversion of the signal produced. In the past this problem has been solved using a separate (external) amplifier/impedance converter. This method, however, is generally impractical due to the noise that is introduced as well as the physical and environmental constraints posed on the system as a result. Today IC amplifiers/impedance converters are commercially available and are generally packaged within the case of the accelerometer itself.

History

Behind the mystery of the operation of the piezoelectric accelerometer lie some very fundamental concepts governing the behavior of crystallographic structures. In 1880, Pierre and Jacques Curie published an experimental demonstration connecting mechanical stress and surface charge on a crystal. This phenomenon became known as the piezoelectric effect. Closely related to this phenomenon is the Curie point, named for the physicist Pierre Curie, which is the temperature above which it loses spontaneous polarization of its atoms.

The development of the commercial piezoelectric accelerometer came about through a number of attempts to find the most effective method to measure the vibration on large structures such as bridges and on vehicles in motion such as aircraft. One attempt involved using the resistance strain gage as a device to build an accelerometer. Incidentally, it was Hans J. Meier who, through his work at MIT, is given credit as the first to construct a commercial strain gage accelerometer (circa 1938)(Patrick). However, the strain gage accelerometers were fragile and could only produce low resonant frequencies and they also exhibited a low frequency response. These limitations in dynamic range made it unsuitable for testing naval aircraft structures. On the other hand, the piezoelectric sensor was proven to be a much better choice over the strain gage in designing an accelerometer. The high modulus of elasticity of piezoelectric materials made the piezoelectric sensor a more viable solution to the problems identified with the strain gage accelerometer.

Simply stated, the inherent properties of the piezoelectric accelerometers made it a much better alternative to the strain gage types because of its high frequency response and its ability to generate high resonant frequencies. The piezoelectric accelerometer allowed for a reduction in its physical size at the manufacturing level and it also provided for a higher g (standard gravity) capability relative to the strain gage type. By comparison, the strain gage type exhibited a flat frequency response above 200 Hz while the piezoelectric type provided a flat response up to 10,000 Hz (Patrick). These improvements made it possible for measuring the high frequency vibrations associated with the quick movements and short duration shocks of aircrafts which before was not possible with the strain gage types. Before long, the technological benefits of the piezoelectric accelerometer became apparent and in the late 1940’s and in 1950 large scale production of piezoelectric accelerometers began. Today, piezoelectric accelerometers are used for instrumentation in the fields of engineering, health and medicine, aeronautics and many other different industries.

Manufacturing

There are two common methods used to manufacture accelerometers. One is based upon the principals of piezoresistance and the other is based on the principals of piezoelectricity. Both methods ensure that unwanted orthogonal acceleration vectors are excluded from detection.

Manufacturing an accelerometer that uses piezoresistance first starts with a semiconductor layer that is attached to a handle wafer by a thick oxide layer. The semiconductor layer is then patterned to the accelerometer’s geometry. This semiconductor layer has one or more apertures so that the underlying mass will have the corresponding apertures. Next the semiconductor layer is than used as a mask to etch out a cavity in the underlying thick oxide. A mass in the cavity is supported in cantilever fashion by the piezoresistant arms of the semiconductor layer. Directly below the accelerometer’s geometry is a flex cavity that allows the mass in the cavity to flex or move in direction that is orthogonal to the surface of the accelerometer.

Accelerometers that based upon piezoelectricity are constructed with two piezoelectric transducers. The unit consists of a hollow tube that is sealed by a piezoelectric transducer on each end. The transducers are oppositely polarized and are selected to have a specific series capacitance. The tube is than partially filled with a heavy liquid and the accelerometer is excited. While exited the total output voltage is continuously measured and the volume of the heavy liquid is microadjusted until the desired output voltage is obtained. Finally the outputs of the individual transducers are measured, the residual voltage difference is tabulated, and the dominate transducer is identified.

Applications of piezoelectric accelerometers

Piezoelectric accelerometers are used in many different industries, environments and applications. Piezoelectric measuring devices are widely used today in the laboratory, on the production floor, and as original equipment for measuring and recording dynamic changes in mechanical variables including shock and vibration.

AMETEK is one of many companies that manufacture piezoelectric accelerometers. Their piezoelectric accelerometers are used in aircraft engines, helicopters, land gas turbines, compressors, gas generators, launch vehicles, missiles and marine vehicles.

Another company, Endevco, also manufactures piezoelectric accelerometers. Their products include pressure transducers, microphones, electronic instruments and calibrations systems. Companies in the aerospace, automotive, defense, medical, industrial and marine industries tend to be buyers of Endevco’s products. With respect to the defense industry, accelerometers are used in a wide range of applications because of the availability of smaller and cheaper accelerometers with a greater operating range, higher resonance frequency, lower amplitude range, and integrated electronics.

References

  • Norton, Harry N.(1989). Handbook of Transducers. Prentice Hall PTR]. ISBN 013382599X
  • Patrick, Walter L. The History of the Accelerometer 1920’s-1996 Prologue and Epilogue. 2006.

External links

*‘Piezoelectric Tranducers’

*‘Piezoelectric Sensors’

*‘The Principles of Piezoelectric Accelerometers’