Fast Fourier Transform and Operational Deflection Shape Analysis for Vibration on Handheld Tools

Transcription

1 Fast Fourier Transform and Operational Deflection Shape Analysis for Vibration on Handheld Tools Mohd Azli SALIM 1, Aminurrashid NOORDIN 2, Ahmad Naim ISMAIL 3, Mohd Firdaus HASSAN 4 Shafizal MAT 5, Muhd Ridzuan MANSOR 6, Mohd Afzanizam MOHD ROSLI 7 (1, 3, 5, 6, 7 Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka, Malaysia) (2 Faculty of Electrical Engineering, Universiti Teknikal Malaysia Melaka, Melaka, Hang Tuah Jaya, Durian Tunggal, Melaka, Malaysia) Abstract: (4 Faculty of Mechanical Engineering, Universiti Malaysia Pahang, Kuantan, Pahang, Malaysia) The used of handheld portable machines is very useful for construction work since it is easy to mobilize. This handheld tool produced noise and vibration during operated. The noise and vibration cause disease that involves circulatory disorders, sensory and motor disorders, and musculoskeletal disorders. The aim of this study is to determine the vibration occurs in handheld tools using Fast Fourier Transform and Operational Deflection Shape methods. In the experiment, vibration at few points of the handheld tips is obtained using PC data acquisition system, amplifier (4 channels), stabilize table, and rubber support. Analysis from the data shows that the vibration may occur higher at certain point of hand drill and in this study it appears highest at point-2 with the highest frequency of Hz for the first reading and Hz for the second reading. Key words: Handheld tool, vibration, Fast Fourier Transform 1 Introduction The issue on comfort in usage a hand powered portable machines has become a compulsory factor to indicate the quality of the tool. Handheld tool is a one of the example and a problem that create an uncomfortable situation for a user. The research in hand powered portable machines attempts to identify the techniques to reduce the level of unwanted vibration and becomes significant to focus on the mechanism of solving the problem in analytical, numerical or experimental approach. One of the effects of undesired vibration is the Hand-arm Vibration Syndrome (HAVS). HAVS is a disease that involves circulatory disorders, sensory and motor disorders, and musculoskeletal disorders, which may occur in workers who use vibrating handheld tools [1], [2], [3], [4], [5], [6]. This undesirable vibration needs to be controlled and suppressed, it because it has a high potential to produce side effect to user. In European, more than 17% workers had made a report that they have used this handheld tool at least half of their working hour and this statement has been supported by Spanish report where at least 22.8% workers had spent their work at the same condition [2]. The main of this study is to measure the high vibration produces by handheld tools, that reveals the effect of HAVS to human body. From the literature review, the different frequencies of vibration interfere with different body parts and systems where the whole body parts and systems is occurred from 1-30 Hz, while segmental vibration at Hz and for above 100 Hz threshold, the hand in particular is an affected [2], [7]. Two methods have a potential to tackle the vibration phenomenon, that are passive and active vibration control. Passive vibration control (PVC) is based on the damping using viscoelastic materials whereas, active vibration control (AVC) involves the use of active elements along the sensor and controller that produce an out-of-phase actuation to cancel the possible disturbances [1]. 1

2 In this study, the selected handheld tool was mounted at onto a special table where it will reduce an unbalances response for amplitude and to increase the stability speed of rotor shaft system during operation. The finding of this experiment shows that there has a high vibration and it can give HAVS effect for the workers during operation. 2. Research Methodology In this study, there are two steps to make the experimental successful, which are Fast Fourier Transform (FFT) and Operational Deflection Shape (ODS). 2.1 Fast Fourier Transform (FFT) The invention of the Fast Fourier transform (FFT) algorithm finally paved the way for rapid and prevalent application of experimental technique in structural dynamics. With Fast Fourier Transform (FFT), frequency responses of a structure can be computed from the measurement of given inputs and resultant responses [8]. Therefore by using FFT method, the vibration of handheld tool can be traced [9]. This method is used to find the natural frequency of the handheld during vibration. With this natural frequency, than the spring and damping coefficient can be calculated. From the plotted FFT graph, the equation (1) is used to find the value of fn. ω n =2π f n (1) where, ω n = natural frequency f = frequency n The value of spring coefficient can be determined from this equation, where, k = spring constant m = mass of tool f n = 1 2π k m (2) Damping coefficient equation, c = 2ξ km (3) where, c = damping coefficient ξ = damping ratio Using Half-Power Bandwidth equation, ξ = Q = 1 2Q f 2 f0 f 1 2 (4)

3 (5) Value of f 1, f 2 and f 0 can be found in FFT graph of damping measurement. where, Q = Q factor f 1 = lower 3dB frequency (Hz) f 2 = upper 3dB frequency (Hz) f = resonance frequency (Hz) Operational Deflection Shape (ODS) Operational deflection shape (ODS) is defined as the deflection of a structure at a particular frequency. However, an Operation Deflection Shape (ODS) can be defined more generally as any forced motion of two or more points on a structure. Specifying the motion of two or more points defines a shape. Stated differently, a shape is the motion of one point relative to all others. Motion is a vector quantity, which means that it has location and direction. This is called a Degree Of Freedom [9]. Measuring using ODS can give the information of: a. The acceleration of moving machine. b. The direction of machine. 3.0 Experiment Setup 3.1 Setup for FFT experiment The equipments used are accelerometer, impact hammer with green tip, analysis software, PC data acquisition system, amplifier (4 channels), stabilize table, and rubber support. The used of accelerometer is to sense or detect the vibration made by the hand drill. Fig. 1 shows the equipments for this experiment. In this experiment, the accelerometer is placed at the rear handle of the drill. Result from this experiment gives the information about vibration that occurs at the handle when the machine is running. This method will identify which area gives a high level of vibration. The specification of the drill is shown as below: 1. Power input = 550W 2. Impact rate = rpm 3. Weight = 1.5 kg 4. Electric motor, number of poles = 2 5. Variable speed power drill = 3000 rpm 6. Blower = 39 vanes 7. Bevel gear reduction N1 = 4/N2 = Rolling element bearings 9. Cylindrical roller on gear box, 12 rollers/pins 10. Impact ratchet = 16 serrations 3

4 Point 2 Point 1 Fig. 1 Experiment setup for Fast Fourier Transform (FFT) Analysis (a) Equipment setup (i) (ii) The hand drill was placed on the top of the stabilize table. A rubber support was used to make sure the drill is not moving after the switched on. After stabilize table is set-up, the hydraulic machine is opened and the rubber is placed to support the surface of the table. 4

5 (b) Fast Fourier Transform (FFT) Analyzer Setup (i) (ii) (iii) The amplifier channel was set-up, where channel 3 was set with accelerometer wire for point 1 and channel 4 with accelerometer wire for point 2. Then the accelerometers were attached perpendicular 90 degree to the drill surface at the rear handle of drill using thin layer of wax The constant power was supplied to the accelerometer after all the cables were connected. (c) Data acquisition (i) Once the drill was switched on, the measurement was taken by clicking the measure button as shown in Fig 2. Fig. 2 DEWEsoft software (ii) The stop button was then being clicked to store the result. The results were generated by clicking the Analyse button by open the file name for the experiment. The Fast Fourier Transform (FFT) graph was shown. 3.2 Setup for ODS experiment Operational Deflection Shape was another method beside FFT, used to measure the vibration level on the rear handle of the hand drill. In ODS experiment, six accelerometers were placed on the handheld tool. This experiment involved 3 important equipments: i. 6 units of 3-axial accelerometer (KISTLER) Collect the response data in X, Y and Z direction. ii. PAK Muller-BBM FFT Analyzer Signal from accelerometer was connected to the channel analyzer for process as required. iii. Post-processing modal software (ME scope VES). Process the data to identify the modal parameter, execute the vibration criteria, and animate the mode shapes, and Operational Deflection Shape (ODS). 5

6 The procedure of the experiment was: Fig. 3 Experiment setup for Operational Deflection Shape (ODS) a. All the major forcing frequencies for the synchronous forcing at x1, x2, x3 RPMs for all components such as gear mesh frequencies, bearing frequencies, ratchet impact frequencies and all components need to be calculated. b. The analyzer is set to a suitable maximum frequency based on the forcing frequencies. c. Then, the accelerometer was placed at the flat spot on the drill. The data of speed for the drill with zero loads condition during turned on is logged. The Fast Fourier Transform spectrum for the minimum 10 averages is measured for the no load baseline conditions. d. The Fast Fourier Transform spectrums on the analyzer were recorded to examine all major vibration peaks and amplitude. All major frequencies including the drill x1, x2 RPMs and the gear mesh frequencies that can be identified were recorded. 4.0 Results and Discussion 4.1 Result for Fast Fourier Transform Graph in Fig. 4 and Fig. 5 show the highest vibration level occurred at frequency Hz. Point 2 produce higher vibrations than point 1. Using this value, the spring coefficient can be calculated. 6

7 Zoom area Fig. 4 Vibration level at Point 1 7

8 Zoom area Fig. 5 Vibration level at Point Verification the data from Fast Fourier Transform (FFT) Fig. 6 shows the reference data (theoretical data) for this experiment. Accepted value for natural frequency is 450 Hz. Percentage of errors for this experiment was calculated to verify the data from the experiment. ω n 8

9 Fig. 6 Theoretical data from Fast Fourier Transform (FFT) Percentage of errors = Experiment data theoretical data x 100% Theoretical data = x 100% = 5.7% Fig. 7 Damping measurement Table 3 Value of damping measurement No Description Frequency (Hz) 1 Resonance frequency, Upper 3 db frequency, Lower 3 db frequency, Results for Operational Deflection Shape The acceleration of the six nodal points of the rear handle of the hand drill has been successfully measured and plotted. 9

10 Fig. 8 Acceleration of the rear handle plotted in MATLAB The acceleration of six nodal points of the rear handle of the hand drill has been measured and plotted in MATLAB. The graphs show that the highest vibration level occurred at point-2. Compared with other graphs, point-2 produces the highest acceleration, which mean the higher vibration level occurred at this point. High acceleration will give more vibration to the drill. This point is actually the main area where, the user grips the handle while operating the hand drill. This area must have the safety precaution in order to give more safety to users and also to give longer lifetimes to the tool. 5.0 Conclusion The experimental results show the point, which occur higher vibration level. Fast Fourier Transform (FFT) and Operational Deflection Shape (ODS) experiments show that the vibration at the rear handle is not the same for each point. But between all points, there is one point shows the highest level of vibration. Result from both experiments, shows that point-2 produces the higher vibration during the hand drill was turned on as referred in Fig. 5 and Fig. 8. In Fast Fourier Transform (FFT) experiment, the reading was taken twice in order to get the accurate measurement. The result shows, point-2 gives the highest amplitude. Frequency for the highest amplitude is Hz for the first reading and Hz for the second reading. This frequency is the natural frequency obtained from the experiment. Percentage of errors from the experiment is 5.7% and 6.9%. The experiment data can be accepted. 10

11 6.0 Acknowledgment The author would like to thank to Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka (UTeM) and Mr. Hairul Nizam for the assist and cooperation to make this study possible. References [1] M.F. Hassan, M. Mailah, M.A. Salim, A. Juliawati. VibrationS of a Handheld Tool using Active Force Control with Crude Approximation Method. International Conference on Man-Machine Systems (ICoMMS) October Page 4A1-1 4A1-6. [2] E. Greenslade and T.J. Larsson, Reducing Vibration Exposure From Hand-held Grinding, Sanding and Polishing Powertools by Improvements in Equipment and Industrial Processes. Journal of Safety Science. Vol. 25(1-3). Page [3] M.A.Salim. Design a Control System to Dampen the Vibration in a Building like Structure. Universiti Teknikal Malaysia Melaka. B.Eng Thesis [4] M.A.Salim, M.K.M.Nor, M.F.Hassan. Analysis of Absorption the Level of Vibration Energy in a Building Structure using PIDC. ICORAFSS. Page [5] R.Oddo, T.Loyau, P.E.Boileu, Y.Champoux. Design of a Suspended Handle to Attenuate Rock Drill Hand-arm Vibration: Model Development and Validation. Journal of Sound and Vibration. Vol 275(35). Page [6] J.S.Taylor. Vibration Syndrome in Industry: Dermatological Viewpoint. American Journal of Industrial Medicine. Vol 8(45). Page [7] S. Snook. The Practical Application of Ergonomics Principles. Journal of Occupational Health and Safely - Australia and New Zealand, Vol. 9(6). Page [8] Brian J. Schwarz, Mark H. Richardson. Introduction to Operating Deflection Shapes [9] M.A.Salim, A.Noordin, W.M.W.M.Farid, S.Mat, M.A.M.Rosli. Vibration in Handheld Tool: A Methodology to Determine Damping Coefficient Using Fast Fourier Transform Technique. International Conference on Design and Concurrent Engineering September