Optical Encoder Applications for Vibration Analysis

Optical Encoder Applications for Vibration Analysis Jack D. Peters Accelent Technology LLC 19 Olde Harbour Trail Rochester, New York, 14612 jack4accelent@aol.com Abstract: The application and use of an incremental optical encoder to measure torsional vibration or the instantaneous changes in machine speed will be discussed. Basic encoder operation, conversion from frequency to voltage, and engineering units will be explained. Illustrations and examples in the time waveform and FFT will provide additional understanding for this technology. Key Words: Time Waveform, FFT, Optical Encoder, Encoder Wheel, Frequency to Voltage Convertor, Web, Torsional Vibration, Instantaneous Change in Speed, Shaft, Rotor, Roll, Coupling, Carrier Frequency, Modulation, Demodulation, Angular Velocity, Angular Displacement Accelent Technology LLC, 2017 Page 1

Introduction: As vibration analysts we employ a wide range of sensors and instruments to measure and analyze vibrations in rotating machinery. Accelerometers are often used to measure radial (horizontal & vertical) and axial vibrations in machines with rolling element bearings. The accelerometer is capable of measuring the transmitted vibration as it passes from the rotating shaft, through the rolling element bearing, to the outside of the machine casing, and base of the accelerometer. If the machine utilizes fluid film bearings (sleeve bearings), then the sensor of choice is typically the proximity probe to measure the radial vibrations (X & Y). The proximity probe is a non-contact sensor that operates on an eddy current principle measuring the vibration of the shaft (AC), and location of the shaft centerline (DC). Both the accelerometer and proximity probe are well known to most vibration analysts, and their application and use are popular across many industries. Although the accelerometer and proximity probe are the work horses of measuring vibration on rotating machinery, they lack the ability to provide a direct measurement of torsional vibrations of a shaft, rotor, or roll, and are unable to detect the instantaneous changes in speed of a roll surface or web that is being conveyed over the surface of the roll. A web can be defined as any product like film, paper, steel, linoleum, wall paper, or polymer materials found in food packaging, that require uniform thickness, superior surface finish, or high quality printed text or images. The torsional vibrations in the shaft, rotor, or roll, and the instantaneous changes in speed of a roll surface or web can often be measured by using an incremental optical encoder. An incremental optical encoder, as shown in Figure 1, provides a specific number of pulses for each revolution. Typically these pulses will be in the form of a square wave, but some manufacturers also offer sine wave outputs. The incremental optical encoder provides a continuous output of pulses without interruption, and differs from an absolute optical encoder that Accelent Technology LLC, 2017 Figure 1: Optical Encoder (Image Courtesy of Dynapar) Page 2

provides binary counting for accurate positioning. The incremental optical encoder typically provides output channels A, B, and Z along with the inverse channels A not, B not, and Z not. Although these inputs are very useful as feedback signals for frequency drives and motor control, the vibration analyst will only require the A or B channel outputs for multiple pulses/revolution in measuring torsional vibration or instantaneous change in speed. The once/revolution Z channel pulse could be used as a once/revolution trigger for timing or phase related measurements. Figure 2 illustrates a typical encoder output for channels A, B, and Z. A B Z Figure 2: A, B, Z Encoder Outputs In Figure 2, the A & B pulses are generated by a rotating optical disk inside the encoder. The disk contains a very accurate mapping of +/- density (dark and light) segments around the circumference of the disk. LED light is focused on the disk, and as the dark and light density segments pass under the LED light, they block or transmit light to the photodiodes to create the square wave output. Figure 3 illustrates an optical disk, LED, and photodiodes. Although it is interesting to note that the A & B channels are 90 degrees out of phase, it is not useful in vibration analysis. We will simply choose either the A or B channel to work with, and begin relating pulses/revolution (PPR) to the operating speed of the shaft, rotor, or roll. Accelent Technology LLC, 2017 Page 3

Figure 3: Encoder Optical Disk, LED, Photodiodes (Image Courtesy of Dynapar) Encoder Mounting by Direct Coupling: If our primary interest is in the torsional vibration of the shaft, rotor, or roll, then a directly coupled encoder on the end of the shaft, rotor, or roll will provide a measure of torsional vibration. In many applications, an encoder may be found mounted to the non-drive end of a motor providing feedback and control for the electrical drive in the motor control center (MCC). If this is the case, it may be possible to use this existing signal for analysis purposes. This will require discussion and planning with the engineers and technicians in charge of the MCC, and experimentation and care must be taken not to alter the motor drive signal with the added application of torsional vibration analysis. If the A channel or B channel pulse train are to be borrowed from an existing encoder mounted on the non-drive end of the motor, then consideration should be given to the couplings that connect the encoder/motor/rotor/roll under test. In order to measure the torsional vibration of the rotor or roll, the couplings connecting the encoder to the motor, and motor to the rotor/roll must be extremely stiff so that the torsional vibration will transfer back through the couplings to the encoder. In the configuration shown in Figure 4, there will be two couplings that may or may not interact with the torsional measurement, and measurement results will depend on the torsional stiffness of both couplings. Accelent Technology LLC, 2017 Page 4

Encoder Motor Rotor/Roll being tested for torsional vibration Encoder Coupling Shaft Coupling Figure 4: Existing Encoder on Motor Driven Rotor/Roll A much more direct measurement of the rotor/roll torsional vibration could be made by coupling the encoder directly to the rotor/roll as shown in Figure 5. This application will only require torsional stiffness in the encoder coupling and may in fact not only identify a torsional vibration in the rotor/roll but also a torsional weakness in the shaft coupling. Motor Rotor/Roll being tested for torsional vibration Encoder Shaft Coupling Encoder Coupling Figure 5: Encoder Directly Coupled to Rotor/Roll In each of the applications shown in Figures 4 and 5, it is critical that the encoder coupling has extremely high torsional stiffness so that it does not affect the measurement. Alignment of the encoder shaft to the motor shaft (Figure 4) or rotor/roll shaft (Figure 5) will be critical to reduce the influence of operating speed frequency (1X) caused by angular misalignment, or two times operating speed frequency (2X) caused by offset misalignment. Figure 6 shows examples of encoder couplings that provide torsional stiffness and will improve signal quality and measurement of torsional vibrations in the rotor/roll. Accelent Technology LLC, 2017 Page 5

Figure 6: Encoder Couplings (Images Courtesy of Ruland Manufacturing Co. Inc.) Encoder Mounting by Measuring Wheel: In some applications there is no possibility of direct coupling, or the primary interest is the surface speed of the roll where an instantaneous change in speed of the roll or web may cause product damage in the forms of scratches, thickness variations, or smeared printing. Tension variations, torsional vibration, or mismatched speeds between roll and web are typical suspects, and are difficult to measure with accelerometers or proximity probes. In these applications, a wheel may be mounted on the encoder, and then the wheel can be run on the surface of the shaft, roll, or web. Figure 7: Encoder Wheels (White Rubber & Dual O-ring) (Images Courtesy of Dynapar) Accelent Technology LLC, 2017 Page 6

Two examples of encoder wheels are shown in Figure 7. The author s personal preference is the Dual O-ring wheel because the O-rings can be changed for chemical and temperature compatibility with the machine process. The diameter of the measuring wheel should not be equal to or a multiple of the shaft, rotor, or roll diameter as it will be difficult to separate the wheel frequency from the operating speed or orders of operating speed. If a measuring wheel is used, the encoder should be mechanically mounted using a pivoting bracket that allows the wheel to run on the surface of the shaft, roll or web. Normally the weight of the encoder, wheel, and bracket is sufficient to maintain wheel traction, but if the total mass interferes with the machine process, the pivoting bracket may need to be locked in place. The wheel should maintain just enough contact to continuously turn without adding pressure to the shaft, roll, or web as shown in Figure 8. Web Encoder with Wheel Encoder Bracket Drive Roll Idle Rolls Figure 8: Bracket Mounted Encoder with Measuring Wheel It should be noted in Figure 8, that the speed of the roll surface and the speed of the web surface should be the same. There will only be one surface speed if the encoder wheel is placed on the web provided that there are no variations in tension or drive roll speed. In Figure 9, the operating speed of the web surface will be faster than the operating speed of the roll surface due to the differences in radii between the surface of the roll and surface of the web. High resolution analysis in this application would normally indicate two operating speeds. Accelent Technology LLC, 2017 Page 7

Web Encoder with Wheel Idle Rolls Drive Roll Encoder Bracket Figure 9: Bracket Mounted Encoder with Measuring Wheel Never hold the encoder by hand as this is not a safe practice and may lead to personal injury or machine damage! Utilize a mounting bracket similar to the designs shown in Figure 10. Figure 10 Brackets for Encoder Mounting Conditioning the Encoder Output: The encoder is going to provide a pulse train based on the pulses/revolution (PPR) of the encoder design and the speed of the shaft, rotor, roll, or web. The output of the encoder will be treated as a carrier frequency, and the modulations of the carrier frequency will be related to the torsional vibration frequencies or instantaneous changes in speed. If the encoder output is PPR, and the shaft, rotor, or roll is operating at Accelent Technology LLC, 2017 Page 8

1,800 RPM, the carrier frequency will be 180,000 CPM or 3,000 Hz. If the encoder output is 0 PPR, and the shaft, rotor, or roll is operating at 1,800 RPM, the carrier frequency will be 1,800,000 CPM or 30,000 Hz. A Frequency/Voltage (F/V) Convertor can be used to convert the carrier frequency to a DC voltage output. The variations in the DC voltage created by the frequency modulations will be the torsional vibration frequencies or instantaneous changes in speed. Basically, the F/V is providing demodulation of the carrier frequency. Two examples of Frequency to Voltage Convertors are shown in Figure 11. Figure 11: Frequency to Voltage (F/V) Convertors Images Courtesy of Validyne Engineering (FC62) and Dynapar (FV3) Typical of the F/V convertors shown in Figure 11, there should be selectable ranges for the frequency input, and selectable filter ranges for the DC voltage output. Since the variation of the DC output voltage will be a measure of the torsional vibration or instantaneous change in speed, it should not be significantly filtered. Typically, a 200 Hz low pass filter will provide good signal transfer and analysis of the variation in the DC output voltage. Torsional vibrations or instantaneous changes in speed greater than 200 Hz would be extremely rare as the rotating mass of the shaft, rotor, or roll is acting as a flywheel and reduces the response at higher frequencies. In Figure 12, a square wave without modulation is shown. This would be typical of the nominal speed of a shaft, rotor, or roll that has no torsional vibration or instantaneous changes in speed. In Figure 13, a square wave with frequency modulation is shown. This would be typical of the nominal speed of a shaft, rotor, or roll Accelent Technology LLC, 2017 Page 9

that has torsional vibration or instantaneous changes in speed. The F/V convertor will be used to demodulate the signal shown in Figure 13. Figure 12: Square Wave, No Modulation Figure 13: Square Wave, With Modulation The Demodulated Result: The waveforms of Figures 12 & 13 are the equivalent of an optical encoder providing PPR at 1800 RPM. This will generate a frequency input for the F/V of 180,000 CPM or 3,000 Hz. Using the modulated waveform of Figure 13, as the input to the F/V, the output of the F/V was adjusted for 1 VDC using a digital voltmeter. Some fluctuation in the amplitude was noted because of the frequency modulation. Figure 14 illustrates the demodulated Time Waveform and FFT output of the F/V, and clearly indicates a modulating frequency of 27.81 Hz with a Peak amplitude of 0.14 volts. + 0.14 Volts Peak - 0.14 Volts Peak 27.81 Hz 0.14 Volts Peak Figure 14: Demodulated Output with Amplitude Measured in Volts The measured results in Figure 14 indicate a vibration at 27.81 Hz which can be easily compared to the operating frequency of the shaft, rotor, or roll, but provides Accelent Technology LLC, 2017 Page 10

amplitudes in voltage only. To have a meaningful machine relationship for the voltage amplitudes, divide the vibration amplitude at 0.14 volts Peak by the nominal DC voltage output of 1 volt and then multiply the result by %. The result is a variation in speed of 14% Peak or 28% Peak to Peak. If this same technique had been used prior to the measurement, then the nominal DC voltage output of 1 volt (0 mv) could have been divided by % to provide an engineering unit of 10 mv/%. If this engineering unit had been used in the setup of the measurement, then the amplitude output could have been expressed directly in % as shown in Figure 15. + 14% Peak - 14% Peak 27.81 Hz 14% Peak Figure 15: Demodulated Output with Amplitude Measured in % In Figure 16, the engineering unit is calculated for the nominal DC voltage output of 1 volt (0 mv) divided by 1,800 RPM to provide an engineering unit of 0.556 mv/rpm. Meaningful amplitudes could be realized by generating engineering units relative to %, RPM, Feet/Minute, or Meters/Minute. If the F/V convertor does not allow adjustment of the DC voltage, just measure the nominal DC voltage output at running speed and divide by the unit value to achieve acceptable and meaningful engineering units. Accelent Technology LLC, 2017 Page 11

+ 252 RPM Peak - 252 RPM Peak 27.81 Hz 252 RPM Peak Figure 16: Demodulated Output with Amplitude Measured in RPM A secondary conversion from RPM to angular degrees could also be calculated. If 1 RPM equals 360 degrees/minute, then there are 6 degrees/second. In Figure 16 there is a change of 1,512 degrees/second Peak representing the angular velocity. Angular Velocity = 252 Revolutions Peak x Minute 360 Revolution x 1 Minute 60 Seconds = 1,512 Second Peak It would then be possible to calculate the angular displacement by dividing the angular velocity by 2πf. Angular Displacement = 2π Cycle 1,512 Second Peak x 27.81 Cycles Second = 8.66 Peak x 2 = 17.3 Peak Peak Some Frequency to Voltage (F/V) convertors may have a direct output in degrees. In the case of the Validyne FC62 or Dynapar FV3 discussed in this article, the above calculations will be necessary. Accelent Technology LLC, 2017 Page 12

A significant amount of modulation was placed in the waveform of Figure 13 so that it would be visible for this article. This has created some very large Peak or Peak to Peak variations in Figures 14, 15, & 16. Normal variations in torsional vibration or instantaneous speed variation would be much smaller. A +/- 1% variation would be significant in many applications where product quality is a concern. Applications: In the example of Figure 17, there is one 200 mm drive roll, one 75 mm idle roll, and four mm idle rolls. The encoder measuring wheel is running on the surface of the web. The frequencies measured in the FFT indicate two web speed variations. The web speed variation associated with the 75 mm idle roll is easy to pinpoint, but the web speed variation associated with the mm idle rolls will require additional investigation. Relocate the encoder and measuring wheel to each of the mm idle rolls for evaluation. Typical mechanical faults will be out of round idle rolls or unbalanced idle rolls causing them to whip and change the speed of the web. Encoder with Wheel on Web Surface 75 200 Figure 17: FFT Results of Speed Variation on Web Surface The simulated time waveform in Figure 18 has the encoder measuring wheel running on the surface of the web. In this case, there may not be any repetitive frequencies to measure, but there could be transients in the time waveform due to variations in web speed. Transients like this are often caused by the web instantaneously changing speed as a result of variations in web tension. The source of these transients can be very difficult to identify, but mismatched drive speeds or rolls Accelent Technology LLC, 2017 Page 13

that are not maintaining a constant speed are likely sources. Don t be surprised to see scratches or deformities in the backside of the web, or excitation of natural frequencies due to the snapping of the web as the tension changes. Encoder with Wheel on Web Surface 75 200 Figure 18: Simulated Time Waveform Results of Speed Variation on Web Surface Summary: The optical encoder can be combined with the F/V to make a very good vibration sensor for measuring torsional vibration or instantaneous change in the speed of shafts, rotors, rolls, or webs. It can be directly coupled to the shaft, or a measuring wheel can be used on the surface of the shaft, rotor, roll, or web. Amplitude units can be developed by dividing the DC output of the F/V by the desired unit value. Never be afraid to try an alternate technology or sensor to achieve better results in vibration analysis. The typical accelerometers and proximity probes in use every day don t offer a direct measure of torsional vibration. When analyzing torsional vibration or instantaneous changes in speed, always look at the FFT and the Time Waveform. Work safely and never hand hold an encoder with a measuring wheel. Accelent Technology LLC, 2017 Page 14

References: BEI Industrial Encoders for Dummies by Colleen Totz Diamond, Wiley Publishing Inc. ISBN: 978-0-470-17736-5 Dynapar Series HA25 Industrial Encoder Data Sheet Dynapar Measuring Wheels Data Sheet Dynapar Series FV3 Frequency to Voltage Convertor Data Sheet Ruland Manufacturing Co. Inc. - Coupling Selection for Machine Vision and Optical Inspection Systems Servometer Flexible Shaft Couplings for Motion Control Validyne Engineering FC62 Frequency to Voltage Convertor Data Sheet Validyne Engineering MC1-3 Signal Conditioning Modules Data Sheet Accelent Technology LLC, 2017 Page 15