An accelerometer is a device that is used in taking the measurements of acceleration. Acceleration refers to the rate of change of velocity with time and is in most cases measured in meters per squared seconds or in G-forces (Baron, 2012). Accelerometers are important in the detection of vibrations that occur in systems. They are also useful in the orientation of applications. Accelerometers are devices, which are electromagnetic in nature and work by detecting either dynamic or static forces of acceleration. While dynamic forces include such forces as movement and vibrations, static forces include gravity. Accelerometers can take measurements of acceleration on one, two or three axes. The 3-axis units of measuring acceleration are gaining popularity due to their cost-effective nature.
Sensors work on a principle of a mechanical sensing element, which is composed of a proof mass that is linked to a mechanical suspension in relation to a reference frame. The proof mass deflects due to gravity or acceleration resulting from the force of inertia. This is according to Newton’s Second Law. The measurement of the acceleration is done electrically as per the physical changes in the displacement of the proof mass in relation to the reference frame. Piezoresistive and piezoelectric are the most commonly used types of accelerometers (Emilio, 2013).
In piezoelectric accelerometers, the applied acceleration causes the sensing element to bend thereby resulting in a displacement in the proof mass and an output voltage proportional to the acceleration applied is produced. These accelerometers do not respond to constant acceleration components (Amadi-Echendu, 2010). Piezoelectric materials are able to change part of the energy that comes with the internal mechanical strain into electrical energy that can be recovered, and the reverse is true.
This property of photoelectric materials is said to be electro-static coupling. Through photoelectric effect, photoelectric transducers change mechanical energy into electric signals, which is normally proportional to the mechanical strain of the piezoelectric material. The signals generated are proportional to the shock event or the system vibration. The piezoceramic is the active element in the piezoelectric accelerometer and has one of its side connected to the body of the accelerometer in a rigid manner while the other side has a mass added. A force is created and acts upon the piezoelectric element when the accelerometer is subjected to vibrations (Ceceri, 2012). A charge output is thus created that is of equivalence to the force exerted by the vibration.
In piezoresistive accelerometers, the sensing element is composed of a proof mass and a cantilever beam, which is formed through bulk micromachining. The piezoresistors in proof mass and cantilever are used in the detection of the motion that occurs in the proof mass due to acceleration. These resistors are responsive to DC and can be used in measuring constant acceleration for example gravity (Ceceri, 2012). Lower output levels and temperature sensitive drift in piezoresistors are the main drawbacks of this type of accelerometer sensors.
Piezoresistive accelerometers are designed for use in making measurements of high shock and high frequency. Contrary to piezoelectric accelerometers, piezoresistive accelerometers take advantage of the changes in the resistance of the piezoelectric material and convert the resistance to a mechanical strain to a DC output. Most of the designs of piezoresistive accelerometers are found to be either the type of a bonded strain gauge (fluid damped) or MEMS type, which are gas damped.
These accelerometers are most suitable and accurate for taking measurements when the g level and the frequency ranges are relatively high. Piezoresistive accelerometers find their applications mostly in the automobile safety testing that is composed of traction control system, anti-lock braking system as well as safety-air bags. Still, the accelerometers find their application in weapon testing and seismic measurements (Deodatis, 2014). Micromachined accelerometers are used in biomedical applications for taking measurements of extremely small dimensions as in the case of submillimeter piezoresistive accelerometers.
Accelerators have internal capacitive plates some of which are fixed in positions while others are attached to the miniscule springs that experience motion internally when acceleration forces act upon the sensors. The capacitance between the plates changes as they move in relation to each other and the changes in the capacitance is used in the determination of the acceleration of the moving plates (Baron, 2012).
Other accelerometers are centered on piezoelectric materials. Piezoelectric materials are tiny structures, which are able to output electric charge should they be subjected to mechanical stress such as acceleration. Low power consumption, swift response to the motion and low power consumption are among the advantages that come with differential capacitive accelerometers. As a result of the low levels of noise of capacitive detection, the better sensitivity of the accelerometers is also achieved.
MEMS Accelerometers
MEMS is an abbreviation for micro electro mechanical systems and is applicable to any sensors that have been manufactured using the techniques of microelectronic fabrication. In conjunction with microelectronic circuits, MEMS sensors are used in taking measurements of physical parameters including acceleration. The capacitive type is one of the most commonly used types of MEMS accelerometer due to its unique properties including high accuracy at high temperatures and high sensitivity (Dorf, 2010). These device does not alter values based on the nature of the base materials used but instead only depends on the capacitive value which exists due to the change in the distance between the plates.
MEMS Accelerometer
Shown in the diagram above is a typical MEMS accelerometer which can also be referred to as a simple one-axis accelerometer. 2 or 3-axis accelerometer can be made by having more capacitor sets kept at 90 degrees to each other. A MEMS transducer is made up of a microstructure that is movable or a proof mass that is linked to a mechanical suspension system and hence on to a reference frame (Bentsman, 2016). The mobile plates and the fixed outer plates serve as the capacitor plates such that should acceleration be applied, the proof mass would move accordingly thereby generating a capacitance between the fixed outer plates and the movable plates.
A displacement is experienced as X1 and X2 between the two plates upon the application of acceleration which turns out to be a function of the generated capacitance. The circuit used in the calculation of the acceleration is as shown below. The acceleration is a derived by finding the change of the distance between the capacitor plates.
Applications of MEMS Accelerometers
Accelerometers are sensors used in taking the measurements of the accelerations of moving objects along a reference axis. The measurement of physical activity is using accelerometer signals is preferred since acceleration is directly proportional to the external force producing it and hence can be used in the reflection of the frequency and intensity of human movement. From the accelerometer data gathered, information on the displacement and velocity can be derived through integrating the data with respect to time (Gamble, 2011).
Some accelerometers can respond to gravity and thereby provide tilt sensing in relation to the reference planes during the rotation of the accelerometers with objects. The resultant inclination data from the tilt can be used in the classification of the orientations of the body. As such, accelerometry and accelerometer signals can be used in the generation of enough information that can be used in the measurement of physical activity as well as a range of human activities (Jones, 2014).
Measurements based on sensors have been used to measure human activities and have provided for the assessment of physical activity. The use of accelerometry techniques allows continuous, automatic and long-term measurement of activities of subjects that interact with the free-living environment. Accelerometer signals allow for basic steps in counting and intensity, which are usable in the estimation of the energy spent during physical activity. This method is widely used in the management of weight and diet (Hoffman, 2014).
The relationship between 3-axis accelerometer and weightlifting techniques
One of the previous works that have a significant contribution to the success of this project was a study that was done to establish the validity and reliability of 3-axis accelerometer for measuring weightlifting movements. In this study, the comparison was made between 3-axis accelerometer and kinematic data derived from 3D videography (Knudson, 2013). The data used was obtained from 11 track and field throwers who did three trials with each of the trials having different loads in the power clean, jerk and power snatch. The study conducted the validity and reliability tests separately in which for the case of validity test, ANOVA analysis, the coefficient of variation of the method error Pearson product-moment correlation and method error were used in each of the phases of the study.
All the methods demonstrated a good correlation between the criterion measures and accelerometers since the analysis of the variance revealed insignificant difference (p>0.05). The findings from the study established a strong correlation between the 3-axis accelerometer measures and those derived from 3D videography data. Using these findings, it could be deduced that 3-axis accelerometers are very reliable and valid for the measurement on the z-axis on weightlifting movements. 3-axis accelerometer is thus a useful hand tool that is easy to use in the measurement of the acceleration for weightlifting performance and training sessions (Lee, 2011).
Ideas on intelligent weight training emerged as early as 1984 when Aerial presented ideas on various weight training machines. These machines operated on principles that mainly revolved around force, displacement as well as the time of movement for the cases of a framework that is controlled by a feedback. These ideas and technologies presented a variety from which the most suitable method of exercise was selected.
There are very many instruments that are used in taking measurements of weightlifting performance (Larence, 2011). Among these instruments include 2D/3D motion capture, video analysis, potentiometer/ encodes, the V-scope TM among other instruments. Despite the achievements that these devices have managed to offer in the field of measuring weightlifting performances, most of these devices if not all come with shortcomings that make their efficiency as instruments for measuring weightlifting performance greatly reduced.
Cost, extensive engineering expertise, extensive scientific expertise as well as a considerable delay in the time between collection and return of data are among the shortcomings of most of these devices. This makes it important for further study and research to be done so as to establish devices that can overcome these among other challenges. By overcoming these challenges, the efficiency of the instruments for measuring weightlifting performance is increased (Lu, 2011). This literature review is thus important as does an in-depth analysis of the various many instruments that are used in taking measurements of weightlifting performance, their advantages, and shortcomings. The literature is then used as a basis for establishing a research gap on what can be done in order to improve the efficiency of weightlifting performance.
Following the findings of the study, it will be established to whether advancements can be done on the existing instruments or otherwise a total overhaul would be deemed fit. By total overhaul, it means coming up with completely new devices which do not bear the shortcomings of the existing devices. The existing devices are extensively discussed below.
Potentiometer
A potentiometer is a resistor with three terminals having a sliding or rotating contact that forms a voltage divider that can be adjusted. The potentiometer acts as a rheostat or a variable resistor in case only two of the three terminals are put in use. In essence, a potentiometer is a voltage divider that is used in taking a measurement of electric potential i.e. voltages and are commonly used in control devices among them those that control volumes on audio equipment. The output voltage of the potentiometer is determined by the position of the wiper (Margolis, 2011).
The potentiometer can be treated as two distinct resistors that have been connected in a series in which the position of the wiper is the determinant of the ratio of the resistance of the first resistor to the second resistor. The rotary potentiometer is the most commonly used form of the potentiometer. Thus potentiometer is in most cases applied in the control of audio volume besides numerous other applications (Miller, 2013). Various materials are used in the construction of potentiometers among them cermet, metal film, conductive plastic, wire wound and carbon compositions.
Measurement of the unknown voltage in the potentiometer is done by determining a position in the sliding contact at which the reading of the galvanometer is zero. By indicating a zero reading on the galvanometer it means no current flows through the path hence there is a potential drip in the sliding contact. The voltage drop is determined using the expression
E1=working current*resistance of the segment of the wire
Determining this requires the determination of the operating current of the potentiometer as well as the resistance of the segment of the wire used when the reading of the galvanometer is zero. It is assumed that the resistance of the wire is uniform since the wire has a uniform cross-sectional area (McComb, 2011). Assuming that the resistance of the whole length of the wire is R, then the resistance of the chosen segment will be determined by R*length of segment/total length of the wire
From this calculation, it is possible to adjust the working current using the variable rheostat that is chosen by the process of standardization. During the standardization process, a cell of a standard voltage of 1.0186 V is picked and the witch connected to calibrate. The sliding contact is positioned at 101.86 cm from the starting end of the wire. Adjustments are made to the rheostat Rh so as to achieve a zero galvanization at the point G. At S, the voltage across the 101.86 cm of wire remains 1.0186 V (Baron, 2012). Assuming that the total length of the wire is 200 cm and that the resistance of the wire is 200 ohm, then the resistance of the 101.86 cm length of the ire segment will be 101.86 ohms. The working current is thus determined from the equation I=V/R=1.0186/101.86=0.01A as the working current.
Potentiometers are classified using various categories and properties. One of such ways of classifying potentiometers is based on whether they are adjustable or preset. Preset potentiometers are mostly used when there is need to set a value within a circuit especially within the production set-up and test stage when it is being manufactured. Some of the presents are made up of a single turn adjustment hence can be coursed in cases where the accurate setting is needed (Murthy, 2011). Adjustable potentiometers, on the other hand, have a spindle and can be used with the help of a knob. This type of potentiometer is in most cases used for applications such as control of tones and volumes on radios. They can also be used in cases where there is need to set a value by the user.
The relationship between the potentiometer and weightlifting techniques
A potentiometer was used in assessing weightlifting performance in a study that involved selected characteristics of kinematics during a bench press in which disabled powerlifting athletes were involved. In the study, the potentiometer was used in taking records of the time of movement as well as distance in which it was established that there was no significant deviation between empiric distributions of the analyzed characteristics from the normal distribution.
The study aimed at exploring and gaining knowledge on the impacts of the weight of the barbell on the behavior of some dimensions of kinematics. It was concluded from the study that an increase in the bar load does not result in any significant changes in the kinematic parameters in cases of upward movements (McGuigan, 2017). The use of a potentiometer as a tool for measuring the time of movement and distance proved effective for this study and gave accurate results that were in support of the hypothesis.
Digital Rotary Encoders
A rotary encoder also referred to as a shaft encoder is a device that changes the angular position or motion of an axle or a shaft to a digital or analog signal. Digital rotary encoder comes in two distinct types: absolute and incremental. The absolute encoders show the current position of the shaft hence making them angle transducers while the output of incremental encodes give information on the motion of the shaft. The information provided is further processed at another location into information such as position, speed as well as distance (Mindedal, 2014).
Digital rotary encoders are applied in areas that need an unlimited rotation of the precise shaft. Among such app-location included in photographic lenses meant for special purposes, industrial controls, robotics, platforms for rotating radars as well as computer input devices among other applications. There are numerous types of rotary encoders and the classification is based on whether they are output signal or sensing technology as illustrated in the diagram below.
An incremental encoder works by providing pulse outputs A and B that do not bear any useful count information and instead the counting is done through the external electronics. The position of the counter in the external electronic determines the position where counting starts as opposed to the position of the encoder (Dorf, 2010). It is recommended that the position of the position of the encoder be correctly references to the device to which it is attached in order to get useful information from the measurements taken. Incremental encoders are known for their capability to report an incremental variation in the position of the encoder in relation to the counting electronics.
An absolute encoder, on the other hand, keeps the position of information when power is withdrawn from the system and hence it is availed as soon as power is applied. The relationship between the value of the encoder and the physical position of the machinery is normally set at assembly thereby the system is not required to return to the calibration point in order to maintain the accuracy of the position (Lee, 2011).
Operation of the rotary encoder
Pins A and B start making contact with the common pin as soon as the disc starts rotating. The result is the generation of two square wave output signals. The rotated position of the discs can be determined by any of the two outputs just taking the counts of the pulses of the signal. In the determination of the direction of the rotation, both of the signals must be considered simultaneously. From the diagram, it can be noticed that the two output signals are undergoing displacements that are at 90 degrees out of phase from each other. This means that the encoder should be rotating in the clockwise direction, the output A will be counted to be ahead of output B (Weintrit, 2013).
The time the two signals have opposite values can be determined by counting the steps every time there is a change in the signals. The steps can be counted in, either way, be it from high to low or from low to high signal changes. For measurements on the efficiency of this device in weightlifting performance to be done the following considerations are made:
Advantages of Digital rotary encoder
The major advantage of encoders is their digital property. The rotary encoders can easily interface with the modern control systems in which they can send quality signals back to the computer. This eliminates the need for an engineer to take part in the integration and wiring of the signal electronics (Amadi-Echendu, 2010).
Disadvantages of digital rotary encoders
A major disadvantage of the encoders is their complexity and delicate components. These properties make digital rotary encoders intolerant to mechanical abuse creates a limit on the allowable temperature. Most of the optical encoders cannot operate at temperatures beyond 120?C.
A previous study on the application of Digital Rotary Encoders in measuring weightlifting performance was an experiment that was done to determine the validity and reliability of the use of a rotary digital encode for measurement of kinematics during ballistic movements. In this experiment, the encoder system was used in measuring the acceleration of due to gravity (Bentsman, 2016). A load of 20 kg was dropped under conditions of free fall for a distance of about 2 m and ten trails completed on day one in which the rotary recorder was used in recording the encoder pulses and then further converted to displacement.
The findings from the study confirmed that the encoder system is reliable in measuring the displacement over the two distances that were tested during the experiment meant as was noticed from the mean distances that were measured and were found to be about 1.5 mm and 8 mm variation from the actual distances that were 0.700 m and 1.610 m respectively. Digital Rotary Encoders were also found to be having high accuracy levels with the worst value of error being 0.5% and most of the measures having an error of 0.2% or less. The experiment illustrated that the encoder systems have a very high level of reliability when it comes to measuring the two test distances.
A rotary encoder has been used in measuring the displacement of a bar during different pull movements that were used in Olympic weightlifting. From this measurement velocity power output and work were calculated and the system was found to offer advantages by providing real-time data on velocity, displacement, force and acceleration among other kinematic measurements (Murthy, 2011).
Motion Capture Systems
Motion Capturing (MoCap) system in the context of animation defines the movement of an actor and then applying the information collected in the animation of the character models in either 2D or 3D computer animation. This technique offers numerous opportunities to the animators among them timesaving and simplification of the animation process (Platt, 2012). Only three among the numerous Motion Capturing techniques are applied in the animation industry. Among the most important aspects of Motion Capturing systems include:
The animation in real-time: This is the ability of the computer to track the movements of the actor with the shortest delay time. This parameter makes it possible for the actor to observe the response of the character to the movement simultaneously. From such an observation, the actor has the opportunity to try out with different movements without necessarily waiting for too long for the analysis of the data to be analyzed by the computer.
Accuracy: This is in line with any errors that might arise from taking measurements of the movement of the actor. Such errors result in both unrealistic and undesirable animated characters
Freedom of movement: Motion Capturing techniques are able to follow the complicated movements that the actor can make. Free movement of the actor is very important in animation as it tends to be enriched as well as make the animation process interesting.
Motion Capturing systems are classified into three main basic systems:
Motion capturing systems with magnetic field
In this system, the sensors are put on the body of the actor to takes measurements of the magnetic fields which are of low frequency and are generated from the transmitter source. The sensors collect information on the position and rotation of the actor. This system is relatively cheap besides offering data of high levels of accuracy. The information provided is of 6-degree freedom and the markers an independent form (McComb, 2011).
This means that interruption is virtually not possible in this system. The vulnerability of the system to metallic objects and magnetic fields forms the main disadvantage of this system. The advantages of this system include no occlusion, low prices, high levels of accuracy, identification of the sensors as well as its capability to give the position and orientation of data. The disadvantages include noise, low speed of recording, eternal interferences and limit freedom of movement.
Motion capturing systems with markers
In these systems, markers are fixed on the actor’s face and body and shooting g of the actor is done by a camera. The software is then used to analyze the position of each of the markers in every frame. The movement of each of the body parts is then calculated and the determined data sent to animation software. The shapes and positioning of the markers may differ depending on the type of software used for the analysis as well as the algorithm that is to be used in the analysis of the movement (Mindedal, 2014).
The markers come in different colors to enhance easy identification and for the purposes of easy identification, materials that reflect fluorescent lights are used in making them. Applicability in numerous applications, high accuracy, high recording speed, popularity, recording of facial expression and freedom of movement of the actor are among the advantages of this system. High prices, interruptions of recordings, complications of the software and immobility are the disadvantages of Motion Capturing systems markers
Motion capturing systems with accelerometers
Accelerometers are the objects that are fixed on the body of the actor. Motion capturing systems with accelerometers have gained popularity as a result of the development of better accelerometer parameters and the lowered prices in the market (Bhattacharyya, 2013). Among the advantages that come with this system include portability, high accuracy levels, identification of sensors, high recording speeds, the simplicity of the software, 6 degrees of freedom and affordability. The main disadvantage of the system is that it has no position data (McComb, 2011).
Motion Capturing (MoCap) systems are used in sports for tracking the function of the body in every perspective and extract joint angles. The data collected by the Motion Capturing (MoCap) system is used by the coach in studying the movement of the athletes, and the same data is used by the medical specialists to establish the reason for the athletes getting hurt first. Gait analysis of the most commonly used motion capture system in sports as every activity that is based on the land will always incorporate some bit of running motion (Minsoo, 2015). The system watches an athlete perform an athletic motion, breaks downs the motion into various components and then make a comparison with the way in the same movement is performed by a beginner.
V-ScopeTM Method
The V-scope is a tracing system used in monitoring the motion of a three-dimensional multi-body. The principle of operation of the V-scope is based on an active tracing of buttons that operate a small battery which is attached to the bodies whose movements and motions are to be traced. The buttons are capable of emitting ultrasonic pulses upon being triggered by infrared pulses. The emitted short ultrasonic pulses are detected using three receivers. A continuous trace of the motion of the three-dimensional view can be achieved by the possibility of repeating the sampling up to 100 times per second (Weintrit, 2013).
V-scope for Windows
This is the newest version of the V-scope methods. It is a comprehensive microcomputer-based lab system which provides for an additional sensor that can be used simultaneously with the tracing of the three-dimensional position. After revision of the software as per the evaluation study that was performed by students and teachers, the new version of the V-scope was established that had features among them:
A screen showing a relay from real ballistics experiment
The figure illustrates time graphs of the acceleration (az), vertical position components (Z), the projection of motion on the XZ plane and velocity (Vz).
A real-time presentation from a ballistics experiment example
On the left are energy graphs as the user-defined functions while on the right is the XZ plane having to mark of the velocity of the components as the different chosen sampling points.
V-ScopeTM Method application in weightlifting performance was established in a study that was conducted for a bilateral comparison of barbell kinetics and kinematics during a competition for weightlifting. The study aimed at analyzing both ends of a barbell simultaneously using 19 weightlifters as the participants (Wenner, 2016). The study also aimed at making a comparison to barbell-trajectory classification between the right and the left sides of the barbell.
The finding of the experiment was that there was no significant difference in the classification of trajectory between either side of the barbell for any of the lifts. From this study, the conclusion was arrived at that the V-ScopeTM system tends to enhance analysis of barbell kinetics, kinematics as well as trajectories in the weightlifting competition. This, the V-ScopeTM system, achieves regardless of the side of the barbell that is being analyzed.
The Miniature electronic inclinometer method
As illustrated in the diagram below, the new Miniature electronic inclinometer detector was the epicenter of a set up that was deployed in measuring the kinematics of torso curvature. This detector is a potentiometer that has a one axis electrolytic resistance. Miniature electronic inclinometer detector setup needs an alternating current excitation signal. It provided a relatively proportional output of voltage when the unit is tilted in relation to the vector if the vertical gravity (Platt, 2012). This property was a specific design by the manufacturer in order to take measurements of the gravity vector when the device was subjected to various vibrations and motions.
The Miniature electronic inclinometer is a very minute glass enclosure that is one piece of nature and about one centimeter in diameter. To prevent leakage of the electrolyte, the internal platinum contacts and the exterior terminals are completely sealed into the one-piece glass. The design of the internal geometry is such that a bubble is generated to maintain direct surface contact with the six of the eight walls when the chamber is partly filled with an electrolyte. Going by the explanations from the manufacturer, containing the bubble enhances stability to the performance of the detector when it is being rotated (McGuigan, 2017).
The design and construction of the instrumentation required for the utilization of the miniature electronic inclinometer were specially done. This design and construction included among them the appropriate amplification circuits, power supply, detection unit as well as the provision for analog outputs of the appropriate magnitude that is to be used for an input to the analog-to-digital converter (Weintrit, 2013). The analog-to-digital converter is normally connected to a Hewlett-Packard Series 1000 computer which permits the data from the new inclinometer to be safely stored through synchronization with the data obtained from the Selspot system.
The light emitting diodes of the Selspot system are attached to the new inclinometers to the bottom of a plastic spot which is supported to the body by the use of adhesive pads. Such a fixture allows for simultaneous placement of the inclinometers and the light emitting diodes at the same locations of the chosen body. A video camera is used for recording the composed procedure of the lifting tests while a video monitor is used for monitoring. Simultaneous superimposition of each of the information on the assigned parameters of the tests with the actual motions of lifting on the monitor takes place (Margolis, 2011). Such an online video control setup allows for the relation of each of the sets of geometrical measurement data (x, y, and angular values) to the operation of the task when undergoing its motion picture.
The setup for video control proved more useful and of utmost help in most cases when odd values are being evaluated when observations are made on the files of data output as well as during the process of analyzing the various phase of the act of lifting. Reflective photographic labels are attached to the posts that have been used in order to establish the exact location of the references points as recorded on the video monitor (Bentsman, 2016). The reflective photographic labels are seen on the video monitor as tiny bright light sources that can easily be followed. The figure below illustrates a generalized view of the subject image, the site and the job assignments as can be seen through the screen of the video control.
There are no studies or experiments that have been done to establish the use of Miniature electronic inclinometer method in weightlifting performance analysis or as a technique of weightlifting.
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