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Sensor density of strain gauges vs fiber optics sensing

Posted on by Pierrick Vulliez

Due to the time and resource intensive installation process, traditional electrical strain gauges are often deployed in limited numbers. Because of budgetary or schedule constraints, engineers are often forced to determine critical points throughout the structure where individual strain gauges will be installed. This lack of comprehensive coverage creates unforeseen risks that can translate into catastrophic failure. Ignorance is not bliss when it comes to testing. FOS technology enables engineers to capture significantly more data than they can practically with strain gauges and ensure that potential problematic areas on a structure are not missed.

One application amongst many where FOS has demonstrated its usefulness is in the structural testing of a large-scale sandwich composite cylinder (CTA8.1) at NASA Marshall Space Flight Center, shown below in Figure 1.

Figure 1: (a) CTA8.1 Test Assembly
(b) Finite element model of CTA8.1 Test Assembly.

The testing was completed in 2016 under NASA’s Shell Buckling Knockdown Factor (SBKF) Project. The goal of the project was to improve thin-walled shell buckling design guidelines widely used throughout the aerospace community that have not been updated since the 1960s. For modern launch vehicles, buckling is often the critical failure mode and the current guidelines have proven to be overly conservative in most cases. Under improved design guidelines, structural margins can be confidently reduced while maintaining safety of the structure, yielding significant weight savings. For modern launch vehicles, significant costs savings per launch are realized due to reduced material costs and increased payload capacity.

The composite cylinder (CTA8.1) was instrumented with 16 optic cables (40-foot long), each containing over 1,000 individual fiber optic strain sensors (0.50” gauge length). The 16 fibers were directly connected to a modified version of the Sensuron RTS125+ FOS system. The installation layout, shown in Figure 2 (a), included eight axial (vertical) runs and five circumferential (horizontal) runs. Additionally, 144 individual fiber optic strain rosettes were implemented near the top and bottom interface rings (Figure 2 (b)). Note that the installation is mirrored on both the outer mold line (OML) and inner mold line (IML) surfaces (not obvious in the Figure).

Figure 2: (a) FOS Instrumentation layout on the outer mold line
(b) FOS rosette layout.

The axial and circumferential fiber optic cables (Figure 8 (a)) are installed in order to capture the global strain distribution throughout the cylinder for the subcritical load cases as well as to identify the failure location for the critical load case. The FOS strain gauge rosettes are installed near the top and bottom attachment rings (Figure 8 (b)) to monitor the uniformity of the load introduction onto the composite cylinder. In addition to the 16,000 fiber optic strain sensors, traditional measurements were collected from 256 electric strain gauges, 28 displacement transducers, and 14 digital image correlation (DIC) photogrammetry systems. All 3 technologies used as part of this experiment were complementing one another.

The test article was initially subjected to a series of subcritical axial compression load cases (ranging from 20% to 50% ). At these subcritical load steps, the axial fiber runs confirmed that the axial strains were being distributed essentially uniformly. During the final load case, uniform axial compression was incrementally applied until buckling failure occurred. As the applied load approached , deformations throughout the structure began to produce a non-uniform internal load distribution due to stiffness changes.

As thin-walled shells are loaded critically in compression, the amount of bending present is a useful parameter to monitor as it is indicative of the amount of radial deformation occurring in each panel, leading to buckling. The FOS sensors were purposely installed on the OML and IML surfaces to characterize the bending strain in each panel. In the Figure below, the bending strain distribution throughout the cylinder is shown just prior to the failure.

Figure 3: (a) Distributed bending-strain measurements just prior to failure
(b) Bending-strain intensity color scale.

The bending strains shown in Figure 9 are calculated as one-half the difference between the IML and OML strain measurements. Since the FOS system was operated at a 0.50” spatial discretization, the IML and OML strain measurements were aligned within 0.50”. As shown in the Figure, the maximum bending strain occurred at approximately 10 inches above the mid-height of the cylinder at the 45° circumferential position. Pre-test buckling predictions often deviate from actual buckling, making it difficult to identify critical points for traditional strain gauges to be installed. The large spatial coverage provided by the 16,000 FOS sensors greatly increased the probability of capturing bending strain from the critical panel and location.

The distributed data from the FOS measurements provided a high resolution strain map of the axial and circumferential strain occurring throughout the structure. Additionally, the mirrored FOS installation made it possible to monitor the bending strain occurring throughout each panel as the applied load approached Pcr.

Fiber Optics Sensing vs. Strain Gauges – Installation Effort/Complexity

Posted on by Pierrick Vulliez

Any experienced experimental stress analysis engineer would agree that the quality of a strain gauge installation greatly influences the accuracy of the measurement. Simply put, a strain gauge can only function as intended if the substrate strain is transferred to the gauge properly, which requires comprehensive expertise from the installer. As a matter of fact, the British Society of Strain Measurement (BSSM) has offered a formal strain gauge certification program since 1964 to formally qualify individuals who demonstrate the ability to competently install strain gauges. The most basic qualification (Level 1) offered for technicians or engineers requires 2 full days of training and typically 2 months of practical installation experience. In other words, proper strain gauge installations are not easy and precision is required during every step of the installation. Large aerospace organizations often provide technicians with lengthy self-inspection checklists to ensure each step of the process is correctly followed. A typical strain gauge and FOS installations are shown in Figure 1.

Figure 1: Typical strain gauge and FOS installations.

Due to the laborious nature of the installation process, strain gauges are often deployed in limited numbers at probable critical points throughout a structure. FOS sensors are installed using similar methods, but at a significantly faster pace. The reduced installation effort is an enormous advantage for large structures where hundreds or thousands of strain sensors are required. In the following subsections, a comparison of installation time and cost for both traditional strain gauges and fiber optic strain sensors is provided.

 

 Traditional Strain Gauges Installation Time

Install time for a single gauge varies greatly depending on the application (bridge configuration used, substrate material, difficulty of access, routing requirements, etc.). As a baseline reference, the time allotted (180 minutes) during the BSSM Level 1 certification assessment to install a general-purpose quarter-bridge strain gauge is used. The following assumption is made to determine the per sensor installation time:

An experienced strain gauge installer only requires 15% of the 180 minutes allotted during the BSSM Level 1 certification assessment. Thus, 27 minutes of installation time is required per strain gauge.

 

Required Installation Material Cost for Traditional Strain Gauges

Strain gauges are procured in hundreds of different configurations depending on the application (backing material, gage length, pattern type, resistance value, etc.). The price per strain gauge varies depending on the rarity of gauge and the ordered quantity. As a baseline reference, the price of a common general purpose axial strain gauge (CEA – XX – W250A-350) is used. This commercially available strain gauge is available for $13 when ordered in quantity (>250). The following assumptions are made to determine the material costs required to install a traditional strain gauge:

 

Cost Summary for Traditional Strain Gauges

Using the assumption that the labor rate for a skilled technician is $50/hour, the per sensor cost breakdown for traditional strain gauges is summarized in Table 1.

Table 1: Per sensor cost breakdown of traditional strain gauges

Material CostLabor Cost
Per individual sensor$231$22.52

 

Traditional strain gauges are discrete point sensors that are installed on a per sensor basis. Multiplexing multiple sensors together is not possible, therefore, there is no cost break when using multiple sensors.

 

Required Installation Time for Fiber Optics Sensing

A representative FOS installation is shown in Figure 2. The installation consists of 4’ of fiber bonded to the surface of a uniaxial carbon fiber beam. With the operational gauge length set to 0.0625”, the installation comprised of 798 individual strain sensors spaced uniformly along the length of the fiber.

Figure 2: Representative fiber installation using similar procedures to a strain gauge installation. Fiber is installed at the locations and directions where a strain measurement is desired. 72 minutes were required to install the 798 individual strain sensors.

72 minutes were required to complete the installation including fiber connectorization, material surface prep, fiber routing, and application of the adhesive. The following observations and assumptions are used to determine the per sensor installation time:

Based off these observations and assumptions, each individual fiber optic strain sensor within this installation required approximately 7 seconds1 of installation time.

 

Required Installation Material Cost for Fiber Optics Sensing

In contrast to strain gauges, FOS sensors are procured in only a few different varieties because the sensor gauge length is software selectable and not physically inherent to the fiber. Sensors are available in a few different diameters and coating options. As a baseline reference, 195  ORMOCER coated fiber is used which is procured at $30/ft. The following observations and assumptions are made to determine material costs required to install a fiber optic strain sensor:

Cost Summary for Fiber Optics Sensing

FOS technology allows over 2000 individual strain sensors to be multiplexed onto the same fiber, thus providing some significant savings. The costs for individual sensors and multiplexed sensors are summarized in Table 2 using the following assumptions and observations:

Table 2: Per sensor cost breakdown of FOS strain sensors

Material CostLabor Cost
First sensor$20$22.5
Per additional multiplexed sensor$0.16$0.42

 

Cost Comparison between strain gauges and Fiber Optics Sensing

As shown in the Figure 3, the per sensor cost for traditional and fiber optic strain gauges are comparable for a single sensor.

Figure 3: Installation costs associated with a single traditional strain gauge compared to a single fiber optic strain sensor.

 

However, significant savings are realized when using multiple sensors due to the cost benefits associated with multiplexing several FOS sensors. For the same amount of money required to install two quarter bridge strain gauges, approximately 85 FOS strain gauges can be installed (11.25” of fiber discretized at 0.0625” gauge length). As illustrated in Figure 4, significantly more spatial coverage is accomplished via FOS as a result.

Figure 4: 85 FOS sensors can be installed for the same costs associated with 2 quarter bridge strain gauges, providing increased spatial coverage and significantly more insight into the behavior of the test article.

The increased spatial coverage is invaluable for complex structures, especially in areas where high strain gradients exist. Thom Rollins, Sr. Principal Engineer at Northrop Grumman said it best: “A single fiber allows us to replace thousands of strain gauges, saving us significant man-hours of effort on a single project and providing us with new insight we would not have gotten by using legacy sensing technology.”

For applications that require or can benefit from the use of several strain sensors, FOS technology is clearly attractive due to cost effectiveness, shown graphically in Figure 5. However, the reduced installation time is also advantageous for keeping projects on schedule.

Figure 5: Cost effectiveness of FOS strain sensors.

Fiber Optics Sensing vs. Strain Gauges – Episode 1

Posted on by Pierrick Vulliez

The medium used for strain gauges is typically a metal alloy connected to a few copper wires. The same type of technology enabled the telephone network to interconnect the world starting in the second half of the 19th century. At the advent of the internet, copper became a bottleneck to transmit data (as those of you who are familiar with dial up internet will remember). Fiber optics made it possible to start streaming information across significantly longer distances at bandwidth levels orders of magnitude higher than any copper transmission line. Because of the sheer amount of data that can be sent using fiber optics, an entire new set of applications and services became available to users, such as high speed video streaming.

Similarly, Sensuron’s fiber optics sensing technology enables a paradigm shift to take place in the area of structural testing, specifically for strain and temperature sensing. Spatially continuous measurements along the length of thin, flexible, and virtually weightless optical fiber make it possible to develop a new set of tools that provide deeper insights into how materials react to both strain and temperature. One great example of this is the structural testing of full-scale aerospace structures. Thousands of fiber optic strain gauges can be installed on an aircraft in a fraction of the time required to install traditional strain gauges, making it economically feasible to collect strain measurements at thousands of sensing points. Another one is the structural health monitoring of weight sensitive structures, such as a lightweight UAV, due to the remarkable reduction in cabling required for FOS sensors. The contrast in cabling requirements is illustrated in Figure 1.

Figure 1: Instrumentation comparison of strain gauge and fiber optic sensing technology

 

Trade studies performed at NASA Armstrong Flight Research Center (NASA AFRC) show that a typical FOS installation is 0.1% to 1% the weight of a traditional foil strain gauge installation [2]. While FOS sensors have created a paradigm shift in the structural testing arena, their operation is still analogous to that of an electric strain gauge. Instead of monitoring changes in electrical resistance, changes in reflected optical wavelength are monitored and correlated to mechanical strain via a gauge factor:

Strain gauge technology was developed nearly a century ago and has been considered the gold standard for performing experimental strain measurements for the past several decades. However, the use of FOS technology continues to become more prevalent as engineers increasingly take advantage of the advanced testing technologies available in the 21st century. The primary advantages of FOS technology will be discussed in subsequent blog posts.

Sensuron enables real-time structural health monitoring onboard X-56 NASA’s flexible wing aircraft

Posted on by Pierrick Vulliez

In contrast to the stiff, rigid wings found on most commercial aircraft, flexible wing technology is considered essential to next generation, fuel efficient aircraft. However, flexible wings are susceptible to “flutter,” or highly destructive aeroelastic instability. The Lockheed Martin X-56A multi-utility technology testbed (MUTT) unmanned aerial vehicle (UAV) – a modular high-altitude, long-endurance (HALE) technology demonstrator – was specifically designed with long, flexible, very high-aspect ratio wings in order to investigate and test active flutter suppression technology.

To better understand and mitigate flutter, engineers at NASA’s Armstrong Flight Research Center (AFRC) equipped the X-56A UAV, or “drone,” with fiber optic sensing (FOS) technology.

Learn more about flutter

For the last decade, AFRC has utilized FOS technology to perform distributed strain sensing and real time structural health monitoring during flight. Compared to traditional sensors, FOS technology provides an unprecedented level of insight into the behavior of a structure: a single hair-like optical fiber spanning up to 40 feet can act similarly to 2,000 or more strain gauges without the cumbersome and weight prohibitive instrumentation wire. In addition to strain, FOS can be used to measure temperature, deflection, stress, load, stiffness, and various other critical engineering parameters.

 

A repackaged version of the RTS125+ FOS interrogator flew onboard X-56 to acquire the FOS data. The overall weight of the interrogator (orange chassis) was reduced from 13 pounds to 6.7 pounds. (Image courtesy: Sensuron)

Engineers installed a two-line fiber system on both the top and bottom wing surfaces of the X-56A to simultaneously monitor the bending and cross-sectional rotations of each wing. The distributed strain data was acquired along the wingspan and used to derive the oscillation velocity of the wings which is indicative of flutter onset. As each fiber line is comprised of hundreds of sensing points, the FOS setup provided significantly more data and greater insight than traditional sensors.

Learn more about finite element analysis

Over the course of testing, the largest deflections occurred at the wing tips and the largest bending strains occur a few feet outboard the wing root. Historically, obtaining this level of insight into the overall strain distribution was only possible computationally through finite element models (FEM). Using FOS, finite element-like data is acquired experimentally on the actual structure. This allows critical areas to be confidently identified instead of just predicted, as well as serving as thorough FEM validation.

A histogram of bending strain and deflection data acquired through the leading-edge fibers over the duration of the FOS test flight. The x-axis corresponds to the sensor number corresponding to a discrete location along the leading-edge fiber on the. The color indicates frequency of that level in the data, where pink is the mean over time and blue is less frequent. (Image courtesy: Sensuron)

A repackaged version of Sensuron LLC’s RTS125+ real-time FOS interrogator flew onboard the X-56A to acquire the strain and deflection data. Considering the specific operating environment, the RTS125+ system’s size and weight was modified – while still offering a level of ruggedization – to ensure optimal performance throughout the mission.

 

Both the top and bottom of the X-56A wing surfaces received fiber applications. (Image courtesy: Sensuron)

Sensuron’s RTS125+ acquires continuous strain and temperature distributions on up to 320 feet of small, virtually weightless, and long-lasting fiber optic cable. Optimized for flight applications at AFRC, this technology provides test and instrumentation engineers a new level of insight into the strain distributions, fatigue life, and in-operation performance of aerospace structures.

In addition to real-time, in-flight structural health monitoring, there are a multitude of applications within the aerospace industry where FOS technology is being utilized, including the subcomponent level through full-scale structural testing, crack detection, composites embedment and cure monitoring, finite element validation, flight loads monitoring, and many more. For applications that necessitate the use of hundreds or even thousands of electric strain gauges, a significant reduction in installation time, weight, and complexity is realized by switching to distributed FOS technology.

For smaller scale applications, significantly more sensors can be installed in the same amount of time that it takes to install only a few traditional strain gauges. Regardless of the application, fiber optic sensing technology enables engineers to capture significantly more data than they can practically with traditional sensors, ensuring they literally cover all angles.

How Fiber Optics Sensing works – Let’s take a look under the hood

Posted on by Pierrick Vulliez

New technologies can often be intimidating to implement for a number of reasons, but one of the most common hurdles is a lack of understanding how the technology works or how it can be applied. Fiber optic sensing (FOS) is such a technology, but it’s not that hard to understand the basics. A look inside FOS can help open your imagination to the possibilities.

Although FOS technologies have existed for a couple of decades, advancements in the past five years have propelled the technology into new markets by expanding addressable applications. Historically, FOS technologies have been developed for specific solutions in niche markets. Today, there are a handful of FOS technologies that are being developed that can be deployed in a myriad of applications ranging from monitoring how composite materials cure to determining the deflection of an aircraft wing in real time.

Specifically, FOS systems that use fiber Bragg gratings (FBGs) have undergone tremendous advancements in the past few years. Two notable innovations are the transition from point sensing solutions to spatially continuous solutions as well as the addition of real-time multi-parameter sensing functionality. Here, we’ll take a look at the operating principles behind optical frequency domain reflectometry (OFDR), one of the types of FBG-based technologies that make distributed multi-sensing possible.

The core of most FOS technologies, whether they use FBGs or not, is interferometry. Simply put, interferometry is a family of techniques in which waves are superimposed to extract information about the waves. In FBG-based systems, light reflected back to the interrogator (light source) gets superimposed with other reflected signals, and the resulting interference signals get translated into strain or temperature data. To understand the operating principles behind OFDR, a better understanding of optical fiber and FBGs is necessary.

Fiber Optic Cables and FBG Manufacturing

Fiber optic cables are composed of an outermost protective coating, and then two layers of glass. The outer layer of glass is referred to as the cladding, and the very small inner portion is called the core. The core is where the light travels and where the FBGs are inscribed. Fiber is typically produced on a draw tower, which is a standard fiber manufacturing installation that heat up glass tubes (preforms) and draws them out, creating the thin, multi-layer optical fiber.

During the manufacturing process, the core is made to be reactive to UV radiation so that a UV laser can inscribe the gratings. There are several techniques to execute this process of writing gratings. One manufacturing process inscribes the FBGs on a completed fiber. As a result, inscribing the gratings requires the fiber coating be stripped, the gratings written, and the fiber recoated. This process leads to reduced mechanical strength and can limit how the fiber is used in the field.

A more cost-effective method inscribes the gratings during initial fiber creation. A UV laser installed on the draw tower writes the gratings as the fiber is being drawn. The fiber with gratings is then coated and spooled. This process allows for separate or continuous gratings to be written and produces fiber with much higher mechanical strength.

Fiber Bragg Gratings—The Sensing Element

The fiber Bragg grating (FBG) forms the sensing element. FBGs are essentially microscopic, wavelength-selective mirrors, meaning they reflect a single, specific wavelength and transmit the rest of the optical signal that the interrogator generated. One way to think about this is with white light. White light consists of the entire color spectrum, or in other words, many different wavelengths. If white light was sent down a fiber incorporating a FBG, one would see a single color reflected, while everything else is transmitted.

To use the FBG together with OFDR, gratings are written continuously throughout the fiber. This means that reflections are being sent back to the interrogator from every point along a fiber’s length. The reflected wavelength at each location is referred to as the Bragg wavelength. When a fiber (hence, the grating) is stretched, compressed, or undergoes thermal expansion and contraction, the Bragg, or reflected wavelength, changes. The interrogator then uses a demodulation technique to observe the change in wavelength and translate this into strain and temperature measurements. The relationship between mechanical strain and the Bragg wavelength is described in the figure below.

Fiber bragg grating working principle

Figure 1 – Diagram of the working principles behind fiber Bragg gratings and the relationship between the Bragg wavelength, strain and temperature.

The technique used to interpret the superimposed wave created by the optical signals the FBGs have reflected largely determines a FOS system’s capabilities. Two of the most prevalent interpretation techniques are wavelength division multiplexing (WDM) and, as stated before, OFDR. The biggest difference to note between the two techniques is that OFDR allows for fully distributed sensing while WDM offers tens of discrete sensors per fiber.

The demodulation technique being used has a significant impact on the performance that is possible with a FOS system. The demodulation technique largely determines system’s refresh rate, sensing length, number of sensors, spatial resolution, and the interplay these parameters. Certainly, there are other variables that impact the performance of a FOS system, but understanding the capabilities and limitations of demodulation techniques is essential when evaluating a FOS system for an application.

Wavelength Division Multiplexing (WDM)

For WDM systems, each grating must be written at a different Bragg wavelength, which means only tens of FBGs can be multiplexed on a single channel. Most WDM systems in the market have twenty to thirty FBGs on each channel. While WDM systems typically have more than one channel, they can generally only interrogate one channel at a time.

The number of sensors per channel is limited because the reflected wavelengths cannot be written close to each other. If the Bragg wavelengths are too similar, a grating under strain could shift to reflect the same wavelength as another grating, making the data useless. While WDM fiber sensors can have long lead lengths, only points of information can be obtained. If an event occurs between sensors, the user will miss critical data.

WDM systems can have high refresh rates, but each additional sensor added to a channel will significantly reduce its capabilities. Another limiting factor for WDM systems is that the user must specify exactly where the gratings need to be written on the sensing fiber. This means that each fiber must be customized for each project, a cumbersome process. The process of identifying where the gratings need to be inscribed, as well as lead times for custom fibers, can be lengthy.

Applications well suited for using WDM systems are those that only require a handful of sensing points but sense at very high speeds. For example, WDM systems can be used to measure strain on crash test dummies during the moments of impact or used to monitor detonation velocity in an explosion.

WDM working principle

Figure 2 – Diagram of a basic configuration for a WDM fiber optic sensing system.

 

Optical Frequency Domain Reflectometry (OFDR)

The other demodulation technique, OFDR or swept laser interferometry, can be used to determine both what and where events are occurring all along an optical fiber. There are two separate things that are being interrogated with OFDR: what wavelength of light is being reflected at any given point along a fiber’s length, and at what distance along the fiber that particular wavelength is being reflected from.

OFDR interrogation starts with a narrow linewidth, continuously swept light source. By narrow linewidth we mean that the light source emits roughly a single wavelength at any given instant in time. Continuously swept means that the light source sweeps its output across a given wavelength range. One full sweep across the wavelength range corresponds to one full acquisition of the FBG sensor array along the length of the fiber.

When the interrogation signal launches, the light travels down the fiber and at every point along its length, a single wavelength within the sweeping range gets reflected. The strain or temperature at each point along the fiber, because they have altered the spacing of the grating lines, determines the wavelength that gets reflected at that point. Therefore, where reflections are seen during the sweep of the laser the wavelength being reflected reveals details about the strain or temperature.

One full sweep of the laser yields reflected wavelengths coming from each of the gratings along the entire fiber – so interferometry comes into play to separate the multitude of signals. On its way back to the system, all of the reflected light is allowed to interfere with a reference signal and the result goes to an optical detector. The interference between the reference optical wave and each of the reflected waves results in frequency modulated signal information from the sensor. Reflections coming from early parts of the fiber (nearer the interrogator) modulate, or beat, at low frequencies. Reflections from far down the fiber modulate at high frequencies. Spectral analysis then reveals which reflections are returning from what point along the fiber.

Because the FBG sensing is occurring in a spatially continuous manner down the fiber’s length, the OFDR system enables the acquisition of fully distributed strain and temperature profiles along the fiber. This information can then be processed to reveal further detail. Other measurements that can be derived from these profiles include deflection, 3D shape, liquid level, pressure, and magnetic fields.

OFDR working principle diagram

Figure 3 – Diagram of the operating principle behind OFDR laid out from the laser pulse to the signal for analysis.

Because the gratings have been continuously written, OFDR technology offers significantly higher spatial resolution and exponentially more sensors than other sensing technologies in the market. Such spatially continuous information provides a number of advantages to engineers across multiple industries. For example, distributed data from OFDR sensing systems equips engineers with the data necessary to confidently validate their thermal, vibration, or strain models of a design and avoid costly failures after production has begun. Sensuron’s OFDR FOS systems, for instance, have been used in a number of applications including locating and tracking crack propagation in fatigued wind energy blades, monitoring folding and wrinkling of composite materials during manufacturing, and even determining the degree of unforeseen plastic deformation in critical load-bearing aircraft components.

Another benefit of OFDR FOS systems is that they can have multi-sensing capabilities, or the ability to monitor multiple parameters on different channels simultaneously. For example, the technology has been used to simultaneously monitor strain, deflection, temperature, and load of an aircraft wing in real time during flight testing.

The multi-sensing abilities and distributed data breakthrough that FOS sensing has made in the last several years will empower engineers to solve the design problems they face today and innovate beyond the problems of tomorrow. Now that you understand how it works, there is no longer a need to be intimidated by the technology.