As the summer semester raged on, I found myself complaining of ailments to fellow students, co-workers and my family. Now, it wasn’t apparent to me but I guess this was an everyday occurrence. I finally had to evaluate the situation when my wife was considering me to be a hypochondriac. I wasn’t doing anything out of the ordinary to create these annoying aches and pains that I was voicing to others as they rolled their eyes thinking “Oh boy, here we go again.” Finally as I entered my so called office or “man-cave” in my basement I took a second look at my computer area and was realizing the first cause of my nagging condition. My computer chair had a few cracked wheels and the backrest was basically non-existence since it was stuck in the way-back position that would cause me serious bodily damage if I were to lean that far back. My keyboard was completely dilapidated complete with missing numbers and lopsided causing it to wobble as I type. I had figured out where to start and hopefully get on the fast track to better office health. I remembered that the term used for body-friendly tools or equipment was ergonomics. I had a few bucks left over from my tax return that was on the brink of being spent frivolously. I figured I could leverage this to get some new office equipment and convert my ergonomic absent home-office into something comfortable to crank out lab reports and feasibility studies for my engineering courses. After much deliberation, I found my source of products that provided me a great range of

My New Chair

 choices at low enough prices that I could handle. It is an online source called www.kareproducts.com. I started by looking to replace my sorry excuse for an office chair with something from their selection of ergonomic office chairs and I found a low-back style chair, which I prefer, at a great price. Next, I was after a new keyboard to replace mine that looked as if I got it in 1987. In the computer ergonomics section I found a ton of things I wanted to add to my list,

"Beam Me Up, Scotty!"

especially with the amount of typing I have been doing this semester. I have two technical communications based courses that are going to give my carpal tunnel syndrome before I am done. I decided on a new keyboard that splits in the middle. It took me a little bit to get used to but it is unbelievably comfortable on the wrists and fingers. Besides that I feel like I am in control of the Starship Enterprise when I am using it. I also picked up a great new mouse and an anti-glare screen. I never realized how my eyes were straining to see past the glare of my basement lights. This equipment has finally got me back to feeling like a well-oiled machine again and my friends and family are much more obliged to communicate with me. Since I replaced my chair, I started thinking about saving up to get me a new desk as well. I checked out some adjustable computer desks and there were some really customizable desks that could adjust to fit my style. I like to sit low to the ground in my chair and these desks adjust right with me. The monitor and be adjusted to many different locations and heights as well. I am here to say that I can finally get back to work comfortably and if you are developing aches and pains that you can’t describe, check out your office equipment before you start annoying your friends like I did.

Until we meet again,

PEACE

Anatomy Of An Accelerometer

This document describes the physical mechanics of the ACCELEROMETER, a Micro Electro-Mechanical System (MEMS) used in popular applications commonly seen in most consumer’s technological tool and toy boxes. The category of device, MEMS, describes the initial physical feature implicitly with the use of the word micro. While nano-technology ( 1 x 10^-9 ) is a common buzzword used for current and future of electronic developments, micro-technology (1 x 10^-6 ) is evident around us and can actually be seen by the human eye. The additional term electro-mechanical is used to describe the device as having electronic as well as mechanical operating characteristics.

FUNCTION – The function of an accelerometer is to provide accurate response to movement in measurable increments small enough to be converted to useful signals. This is accomplished by converting physical measurement of movement from a mass into electronic impulses that can be interpreted by a separate electronic device and measured compared to gravity or 1 g-force as the basic unit. Movement direction and degree of force can be determined with an accelerometer and can be used a 3-axis (x, y, and z-planes) configuration to provide a 3-dimensional response to be measured. Common uses of this can be seen in the user feedback feature of the Nintendo Wii video-game controller, in which speed, rotation, and force is measured from the user and provided as input data to the game console to influence movements on the monitor or television screen. Another example commonly encountered is the ability of Personal Digital Devices (PDA’s) such as cell phones and Apples I-Pod line to automatically re-orientate the screen to always be readable. The application of strain gauges that measure the forces applied to a mechanical structure is another popular and advantageous use of accelerometers.

APPEARANCE – The physical attributes of the accelerometer are replicable to the commonly seen Integrated Circuit (IC) design as seen in Figure 1 below. Slightly bigger than a dime,

source: www.directindustry.com

Figure 1 – Size Comparison and Physical Appearance

the overall dimensions are quite small and can be used in many applications. The outer casing is made of a silicon-type material that houses the internal MEMS. The manufacturer’s name and logistical markings are printed on the outer casing in white or black letters. The output is signaled via a wire bundle that exits the silicon portion and is held tight by a machine crimped steel coupling. The output wires are individually insulated as well as channeled through a separate insulation creating a one-wire transit for the data to be provided to the receiving device. There are many other designs styles and size ranges but the attributes described above are found in most common accelerometer applications.

            OPERATION – The accelerometer operates mechanically and then produces an electrical signal as an output. As previously stated in the functionality description, the force of movement measured by the accelerometer is accomplished mechanically by using a free floating mass and determining how hard and fast it moves relative to gravity. Early accelerometers used a large scale mechanism compared to the modern MEMS type accelerometers in which a mass was linked between two equally matched springs and the movement was measured relative to the resting position of the mass. In Figure 2 we can see the modern configuration of the accelerometer.

source: R.J. Noriega-Manez – Stanford University

Figure 2 – Physical Model of MEMS Accelerometer

v  Substrate – The material or portion of the MEMS that the movement of the sensing unit will measure against to determine the direction and magnitude of force applied to the system.

v  Anchor – The connecting element of the substrate to the rest of the sensing system to provide a rigid attachment point.

v  Resistor – Usually a piezo or pressure changing resistor is used to sense deflection and cause a change in resistance and subsequently a change in current. This can be received analogous or digitally to a sensing or measuring device.

v  Cantilever – This provides the physical means to allow deflection to be recorded based on its configuration of one side being connected rigidly and the other being unattached causing a spring-like effect to any movement or outside force encountered.

v  Test Mass – This calibrated to allow zero movement at rest and adds sensitivity to the system by allowing the slightest of force applied externally to the system to cause the cantilever to deflect and the resistor to change value and produce measureable results. This again can be communicated analogous or digitally to the sensing device and measured to provide input to a system or feedback to a measuring device (e.g. strain gauge). Depending on the sensitivity and complexity of the accelerometer system forces can be sensed and measured in very small increments to provide accurate feedback on forces that can’t be detected by humans.

The Process of Stopping A Bullet

Since the invention of gunpowder by the Chinese in 1232, humans have become a force to reckon with on the battlefield. The second major advancement in ranged warfare took place during the 15^th Century with the invention of the matchlock gun. This was the first mechanically fired weapon for personal use. Until this point in time personal body armor needed only to defeat swords and hand-made arrow tips fired from bows. This protection tended to be thick leather with a metallic outer shell to soften the blow and deflect a direct hit. This is not the case in modern day warfare and law enforcement realms. The threat to personal injury due to firearms has increased immensely due to technological advancements in ammunition and weaponry. The matchlocks fired a single .74 caliber (18.7mm) lead ball projectile with a muzzle velocity (MV) of about 800 fps and could fire up to 4 rounds per minute with and experienced soldier. The modern day military equivalent (U.S. M16A2) uses a .219 caliber (5.56mm) full-metal jacketed (FMJ) projectile and reaches a speed of 2800 fps and can fire at a rate of 800 rounds per minute with a 30 round capacity. It is evident by comparison that advancements in firearms would require a reciprocating level of advancement in body armor to match that threat in order to provide a chance of survival once hit.

To explain the process of defeating this threat it is first beneficial to analyze the projectile being fired. Since there are multitudes of rounds to choose from to examine closer a common military round used by countries all over the world should prove to be sufficient for this purpose. The AK-47 round, which is a .30 caliber (7.62mm) and 39mm in length has the MV of approximately 2700 fps. This round has a full-metal jacket (FMJ) projectile which means that it is comprised of a lead core surrounded, or jacketed, by a copper shell. The projectile is manufactured into a cylinder with a cone-shaped leading edge. The rifle in which this projectile, or bullet, is fired has spiraled grooves, called rifling, that span the length of the barrel causing the projectile to spin. This produces more accuracy since the projectile travels on a truer line. Once the target is struck with the bullet, it deforms causing the tip to “mushroom”. This slows the bullet and spreads out the energy creating the maximum amount of damage. This would explain why the entrance hole, or wound, will be considerably smaller that the exit, if there is one. The bullet can ricochet around the skeletal system in a human or animal, for example, in which the bullet could come to rest inside the intended target.

Now that there is a general understanding of the mechanics of a bullet, the process of defeating it can be addressed. This can be separated in the following four major steps: 1) Bullet Strikes Armor, 2) Bullet is Caught, 3) Impact Energy is Dispersed, 4) Bullet is Stopped.

Step one takes place when the bullet makes the first contact with the body armor. The armor’s first objective is to cause the bullet not to strike the intended target. The simple way to do this is by covering center of body mass which is where vital organs are located. There are several designs to accomplish this but most have the same basic components. The main vest is either pulled over the head or wrapped around the chest and secured in the front by Velcro or snaps. This provides the majority of the coverage area. Additional coverage components can be attached to cover the vulnerable areas such as under the arms and the arms themselves. Once the arm is lifted there is substantial vulnerability and a separate section attaches to the front and rear sections of the vest to ensure maximum coverage in this area. The shoulder and upper arm can also be shielded from impact by flexible pieces of armor that strap to the vest at the shoulder area and then around the arm. This provides a restricted mobility in exchange for protection of the upper arms.

Step two involves the body armor’s attempt to catch the bullet. Modern day body armor is constructed of a high tensile strength material that is woven in a tight mesh. The most commonly known material used is Kevlar 129, by DuPont. Kevlar is an organic fiber known as an aramid because it is a member of the aromatic polyamide family. These fibers have high strength, high modulus, toughness and thermal stability. The tensile strength of these fibers is 525,000 psi which is 4.7 times that of steel alloy. This mesh has the capability to overcome the penetrating energy of the bullet causing it slow in speed. As the bullet slow down its energy start to spread causing the force to start dissipating lowering the psi factor and restricting its destruction.

 The third step in this process is to deform the bullet and disperse the energy to an even wider span than in step two. As the bullet “mushrooms” it can break apart. This is unfortunate to happen inside the target because of the damage provided by multiple projectiles as the bullet fragments, but because this can happen inside the armor it becomes beneficial. The fragments are easily caught in the woven mesh of the aramid fibers like Kevlar. Additionally the deformed bullet is no longer spinning and has lost its penetrating tip eliminating the chances of making it through the armor.

The fourth and final step for this process is to continue dispersing of the energy into subsequent layers of the Kevlar-type material or into a ceramic plate for higher protection levels. This will eliminate the bullet’s penetrating properties causing it to stop in the armor and not penetrate into the intended target. This step in successfully in stopping the penetration from a bullet fired by a rifle does have limits. The first is that all body armor is rated to a certain threat level based on the type and size of the round that is the most powerful in nature and must comply with standard issued by the U.S. Department of Justice. For instance, a Level III type armor would be required to defeat the AK-47 round that is mentioned previously. An additional aspect of the body armor’s stopping capabilities is that energy is conserved based on Newton’s Conservation of Energy Law. This states that energy cannot dissipate but can only change forms. With that being said, the energy of the bullet which can reach upwards of 3000 ft. lbs. of energy needs to be absorbed by more than just the armor. This extra energy is absorbed by the target and can cause blunt force trauma in human targets. This is a small price to pay compared to being struck by the bullet itself but it is still significant enough to mention. There have been cases in which the victim still sustained enough blunt force trauma to result in death even though the bullet was stopped.