Rochester Institute of Technology

Kate Gleason College of Engineering

Multidisciplinary Senior Design Project

Project Number: P13453

Dresser-Rand Bearing Health Monitoring

Sean Deshaies (Mechanical Engineering)

Brandon Niescier (Mechanical Engineering)

Michael Gorevski (Electrical Engineering)

Erin Sullivan (Electrical Engineering)

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Abstract

The purpose of this project is to monitor the health of the main crank bearings on a reciprocating air compressor. This will

be done through vibration measurements via wireless sensor technology. Currently, bearing wear is identified through visual inspection or audible noise. Neither option is ideal; inspection is time consuming, and noise only becomes apparent

when the bearing is near failure or has already failed. Research was completed on accelerometers, encoders and wireless

technology to provide an adequate knowledge base and allow the team to choose the best possible solution. The solution

was accomplished by selecting and installing accelerometers at strategic locations on the compressor, selecting modules to transmit and receive vibration data wirelessly, and integrating the devices with an existing data acquisition system (DAQ).

To test the functionality and reliability of the system, as well as to mitigate risk, a test fixture was constructed to mimic

operating conditions and troubleshoot the solution before installation. Testing successfully verified the functionality and limitations of the accelerometers and wireless components before and after installation. The system is successfully

installed on the compressor, and interfaces with the preexisting DAQ. Data collected for vibration analysis shows

vibration frequency concentrated around 6 Hz, consistent with expected behavior. This system will allow the operator to

monitor and identify wear on the bearings, enabling preventative bearing replacement before failure begins to occur.

Project Requirements

In order to monitor the health of the crankshaft journal bearings, the customer requested the use of accelerometers to

measure vibrations in the system. Two sensors would be used; one sensor to be stationary and mounted on the bearing

housing inside of the compressor, the other sensor to be mounted on the rotating flywheel hub. In order to obtain data from the rotating sensor, a wireless transmission method was determined to be the best solution.

Analysis of vibration measurements can show when the bearings are approaching failure. Therefore, the customer requested that the sensors can continually operate, and provide accurate and reliable data. Due to the oil and high

temperatures inside of the compressor, the stationary sensor chosen had to be of a robust design. These requirements led

to the creation of specifications, which were utilized to select the appropriate technology. The main specifications are

listed below:

1. Acceleration range of the sensor – 50 g

2. Acceleration sensitivity – 100 mV/g 3. Range of wireless transmission – 10 ft

4. Maximum operation temperature – 200 °F

5. Size of the sensor and wireless technology – Minimized

Project Background

This project focused on a Dresser-Rand ESH-1 compressor used for research purposes by students and faculty at the Rochester Institute of Technology (RIT). Although the solution is being designed for a specific compressor, the long term

goal for this project is to develop a “plug-and-play” system to monitor the health of the crankshaft journal bearings on all

Dresser-Rand reciprocating compressors. The primary focus the project is the implementation and validation of the wireless transmission of sensor data. This project will interface with systems installed by previous senior design teams.

Past senior design projects involving the compressor are listed below

P12453: Compressor Health Monitoring system

P11452: Compressor Facility Installation & Validation

P09452: Compressor Part Fabrication and Compressor Revamp

To complete this project, research was conducted on accelerometers, encoders, and wireless technologies. Summaries of

research completed are included in the appendix.

Concept Selection

In pursuit of designing and developing the wireless instrumentation to monitor the health of the bearings, different

concept selection tools were utilized. Engineering specifications were developed from the project requirements, and a

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weighted house of quality was used to rank each engineering specification based on its importance in regards to fulfilling

customer needs. From there, the system was decomposed into various functions. A list of potential solutions was then generated for each function. Concept generation was accomplished through open-team forum with some of the generated

ideas are shown in Figure 1. Pugh charts were used to narrow down solution options. Similar to a house of quality with

engineering specifications, concepts were evaluated against criteria, but all concepts were ranked with a “plus”, “minus”

or “same” compared to a datum concept. An example of the Pugh method used is shown in Figure 2. The concepts with the best scores were selected as the solution for each particular function. Further refinement of the concepts was achieved

through holding design reviews with peers and specialists, and interviewing with knowledgeable people in related fields.

Implementing all of these results narrowed down the options for each particular application.

Figure 1: Concept Selection Figure 2: Pugh Chart for Mounting Sensor

on Shaft

Test Fixture

The test fixture designed for this project was created for several reasons. Most importantly, it was designed for risk mitigation. With at least three different groups working on RIT’s compressor during the execution of this project, there

was a risk for conflict when scheduling time to work with the compressor for testing and installation. Due to this risk a

test fixture was designed to validate the system. Early design decisions included determining what needed to be replicated

from the compressor environment on the test fixture. The diameter of the rotating shaft and the speed at which the shaft rotated were determined to be critical in testing, and therefore were replicated as close as possible on the test fixture. The

common shaft diameter is 2.75 in. while the rotational speed is 6 revolutions per second. Major differences between the

test fixture and the compressor include the use of different types of bearings and a different mounting strategy for the wireless transmitter. These differences were the result of the desire to both minimize the cost and design complexity of

the test fixture. In addition, these features were deemed to be non-critical to the purpose of the test fixture.

Figure 3: CAD Assembly of Test Fixture Figure 4: Completed Test Fixture

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The CAD model shown in Figure 3 illustrated the design of the test fixture. Aside from the aforementioned shaft (orange),

components include the power supply (red box) which powers both the motor controller (black box) and the encoder (green cylinder at end of shaft). The motor controller manages the speed of the motor while the encoder measures the

speed at which the shaft rotates. At the end of the shaft opposite the encoder is the motor (black cylinder) which drives the

shaft and is connected with a flexible coupling (green cylinder) and a stub shaft (not visible). The flexible coupling takes

up any misalignment in the system between the shaft and the motor, allowing for looser tolerances. Holding the shaft up on either of its ends are bearing housings (blue) with ball bearings held in place through an interference fit inside them

(red). The seals on the bearings were removed to decrease the rolling resistance. The last significant part of the fixture is

the gray box on top of the shaft, containing the wireless transmitter. The translucent fixture around the shaft is the safety shield to prevent injury or damage in case anything mounted to the shaft should come loose while rotating.

Wireless System Installation

The wireless system consists of an accelerometer and an enclosure containing the wireless module. The flywheel hub was

selected as the mounting location for the accelerometer for both ease of installation and ease of maintenance. To provide a

suitable mounting surface, a flat was milled onto the hub roughly 0.01 in. deep. The mounting surface was cleaned with acetone and the accelerometer was attached with an adhesive compound.

For ease of installation, the battery pack was detached from the circuit board. Header pins were soldered onto both the board and the battery pack. The board was attached to the lid of the enclosure via standoffs while the battery pack was

glued to the enclosure body. A switch was installed on the side of the enclosure to easily turn the wireless circuitry “ON”

or “OFF”. The enclosure was mounted in the center of one of the six spokes on the flywheel (Figure 6). Two ¼ in.-20 holes were drilled and tapped 0.5 in. deep in the spoke for enclosure mounting. In order to balance the flywheel, a

counterweight was manufactured and installed on the spoke 180 degrees apart from the enclosure. A ¼ in.-20 hole was

drilled and tapped about 0.5 in. deep to allow the counterweight to be fastened to the spoke.

The receiver module is attached to the data acquisition computer via a USB cable.

Originally, the accelerometer was to be mounted within the keyway of the crankshaft. However, it was discovered that the height of the accelerometer was greater than the height of the keyway and thus would interfere with the removal of the

crankshaft. A Pugh chart was used to evaluate alternate solutions. Based on the results of the Pugh as well as time

constraints, it was determined that mounting the accelerometer on the flywheel hub would present the best option.

Figure 5: Wired System Figure 6: Wireless System

Wired System Installation

The wired accelerometer was installed in the housing of the flywheel-side journal bearing (Figure 5). This bearing

mounting location was selected by the customer, who wished to have the wired accelerometer as close to the wireless

accelerometer as possible for data comparison. The bearing housing is part of the compressor frame, consisting of a cylinder of approximately 0.75 in. of cast iron supported by interior gussets. The mounting surface was prepared by

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stripping the paint off and ground flat using a rotary tool. Using a right angle drill, a hole was drilled approximately 0.5 in.

deep and then tapped with a ¼ in.-28 thread roughly 0.375 in. deep. To pass the wire through the crankcase wall, a hole

was drilled and tapped with a ⁄ in. NPT thread through the rear of the crankcase. The accelerometer mounting stud was

screwed into the bearing housing and the accelerometer was fastened onto the stud. The cable was fed through the hole in

the crankcase and a cable gland was installed in the tapped hole, forming an oil-tight seal around the cable.

The accelerometer cable was fitted with a BNC connector, and fed into the preexisting National Instruments (NI) DAQ.

Wireless Transmission

For this application, the TelosB wireless module was selected for its low power sensor network capabilities. This module

combines a low power microprocessor along with a radio and an integrated on board antenna. The wireless transmission

of the signal occurs between the two wireless modules. One of the modules is responsible for taking in the data from the sensor into an analog to digital converter, sampling that data, and transmitting it over the air. The other module is

responsible for receiving the data and decoding it so it can be analyzed. These modules are configured through code

written in the TinyOS environment. The transmission occurs at 2.4 GHz and the modules are programmed to operate on a

channel to avoid interference with other RF networks in the surrounding area.

Test Plans

Verification of Accelerometers

Both the PCB Swiveler (wired accelerometer) and PCB 660 (wireless accelerometer) were placed on a shaker which already had a known good accelerometer (B&K 4393) on it. One at a time the purchased accelerometers were compared

to the known good accelerometer. They were mounted to a shaker with beeswax due to its high vibration transmissibility.

This involved running a chirp signal through the shaker and reading the outputs of the accelerometers in SigLab. A transfer function and phase difference analysis was run to see how well the project’s accelerometers compared to the

B&K 4393. Additionally, the B&K 4393 was validated by comparing it to a B&K tri-axial accelerometer (4326A)

Verification of Wireless Signal Transmission

In order to verify the consistency of the wireless transmission, a test was created and executed. This test utilized a

function generator, the TelosB wireless modules, and LabView’s Signal Express software. The function generator was

used to produce a sine wave, with the signal split through a T-connector. One end of the T-connector was fed to a computer through a USB connection. The sine wave was then monitored using the Signal Express software using the same

sampling frequency as the TelosB modules. This data was recorded and exported to Microsoft Excel. The other end of the

T-connector was wired to an ADC input of the TelosB transmitter. The data was received by another TelosB module and fed to the computer via a USB connection. The data was captured into a text file and brought into Microsoft Excel. The

two sets of data were lined up as closely as possible and plotted on a graph. This process was repeated for five different

frequencies as per request of the customer. After data collection was complete, the wireless data was brought into MATLAB and an FFT function was used to analyze and verify that the power content was centralized around the selected

frequency.

Summary of Results

The B&K 4393 accelerometer selected as a reference performed well, with a magnitude near unity and a phase difference

of approximately 0 degrees throughout the testing range when compared to the B&K tri-axial (4326A) accelerometer. Hence, the B&K 4393 is a valid reference. When the PCB Swiveler is compared to the B&K 4393 (Figure 6), the

magnitude is close to one up to about 1800 Hz. The phase difference starts near zero and gradually increases with

frequency. Above 1800 Hz, the magnitude and phase begin to exhibit some significant deviation. When the PCB 660 series is compared to the B&K 4393 (Figure 7), similar behavior is exhibited. In each of the tests, at approximately 175

Hz, there was some fluctuation in both the magnitude and phase difference. Since this anomaly occurred in all three tests,

it is most likely due to an issue with the shaker system rather than an issue with the accelerometers. For each of the tests,

it is assumed that the results are valid over the entire range of the accelerometers (i.e. over a range of amplitudes). Each

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test was conducted over a frequency range from 0 to 2500 Hz. It was observed that for an input frequency above 2500 Hz,

the resulting magnitude and phase began to vary widely in an inconsistent and unexpected manner. This is most likely due to the beeswax behaving as a spring and affecting the dynamic response of the system.

Figure 6: PCB Swiveler Transfer Function Plot Figure 7: PCB 660 Transfer Function Plot

For the wireless system tests, the data acquired for each frequency showed a similar trend, as seen in the plot for 50 Hz in

Figure 8. The wireless data was slightly offset from the generated data due to difficulties in synchronizing the datasets. In addition, it was discovered that the raw wireless data had a scale factor of about 1.19. Taking this scale into consideration,

the wireless data was almost identical to the generated data, disregarding the offset. However, at 500 Hz and 1000 Hz,

some clipping of the wireless waveform occurs. This clipping is due to the limitations of the wireless system to transmit

high frequency data.

The FFT plots for each frequency were as expected. Most of the signal power is concentrated at the respective test

frequency with minimal distortion observed, indicating that the wireless system properly transmitted and recreated the signal from the function generator. Figure 9 displays the FFT plot for 50 Hz.

Figure 8: 50 Hz Wireless Signal Comparison Figure 9: 50 Hz FFT Plot

Vibration data from the compressor was acquired following installation of both the wired and wireless systems. FFT plots

of both data sets can be seen in Figure 10 and 11. Both display expected behavior, with most of the signal power

concentrated at the shaft rotational speed of 6 Hz. A comparison of the two data sets can be viewed in Figure 12.

Differences in the measured data can be attributed to the different mounting locations of each accelerometer: one being stationary, the other rotating with the shaft. In addition, the wireless accelerometer experiences a centrifugal acceleration

of approximately 0.375 g, which would undoubtedly skew the resulting vibration measurement. The compressor

experiences lateral motion during operation, which further impacts vibration measurements.

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Figure 10: Wired FFT Plot Figure 11: Wireless FFT Plot

Figure 12: Data Comparison Plot

Conclusion

The installation of the accelerometers on the compressor proved successful. As expected, the accelerometers picked up

similar, though not exactly the same, vibrations due to different mounting locations.

The largest challenge for the project was the wireless transmission of the data from the flywheel hub and from inside the

compressor. Due to the effects of the Faraday cage, it was determined to not pursue wireless transmission from inside the

compressor and only focus on the wireless transmission from the hub. This may be worth pursuing in the future.

As far as the wireless from the flywheel hub, the most difficult part was programing the TelosB modules. With little

experience on the team, outside experts were brought in to help. Without their help the completion of the project would have been in doubt.

Going forward, integration of the wireless data into LabVIEW will make tracking the changes in vibrations easier. The

current method of taking raw data from LabVIEW for the wired accelerometer and from a text file for the wireless accelerometer makes data analysis cumbersome.

Overall all the project requirements were satisfied and a functional product was development and implemented. The outcome of this project is therefore considered to be a success.

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Further Information

Further information can be found at http://edge.rit.edu/edge/P13453/public/Home

Appendix

Accelerometer Research

The original plan for this senior design project was to have three accelerometers placed on the compressor. Two of these

accelerometers would be placed inside the crankcase on the bearing housing. One would transmit data wirelessly and the other via a wired connection. The third accelerometer was to be placed on the crankshaft in the keyway for the attached

flywheel. Due to the rotation of the shaft, this accelerometer would also have to transmit its data wirelessly.

The placement of these accelerometers added to the design challenge. Originally, the only design requirement necessary for the accelerometers was a measurement range of ±20 g. Concerns came up when ensuring that the two accelerometers

inside the crankcase could withstand both the higher temperature (~200 °F) and exposure to oil. Design changes

eliminated the need for two accelerometers on the bearing housing. After contacting numerous vendors, PCB’s 607A11 “Swiveler” accelerometer was chosen.

This accelerometer has a measurement range of ±50 g, well above the requirement of ±20 g. In addition to being able to withstand the harsh environment inside the crankcase, this accelerometer is stud mounted, which is preferred over other

mounting methods, such as epoxy, which have increased distortion of the vibration measurement comparatively.

The exterior accelerometer mounted on the shaft proved to be a greater problem. Due to it being on a rotating shaft, it

would be impossible to power the accelerometer from a traditional power supply without using a slip ring. A slip ring

was decided to be too difficult to work with, so the decision was made to power the accelerometer by battery. Due to the

restrictions of battery output voltages and relative size, finding a low powered accelerometer became a necessity. The vast majority of accelerometers required over 10 VDC, which would have required over six standard AA 1.5 V batteries.

This was problematic as there are space and weight restriction for mounting on the rotating shaft since any addition to the

system may induce vibrations in the system, skewing the data collected. Upon further research, the PCB Series 660 was discovered. It comes in various configurations, including a low power version which requires only 3 to 5 VDC to operate.

As the board chosen for wireless transmission is powered with 3 VDC from two AA batteries, this option proved to be the

best fit. A downside to this accelerometer is that it has to be mounted with glue instead of a stud mount. Precautions were

made in choosing a glue that would least interfere with vibration transmission.

How an Accelerometer Works

Accelerometers are devices used to measure proper acceleration. This means they only measure dynamic acceleration

relative to a free-fall and do not take into account the earth’s gravitational pull. In our system, piezoelectric

accelerometers are used.

Figure 12: Accelerometer Diagram (PCB Piezotronics)

Figure 12 shows a simplified version of a piezoelectric accelerometer like the ones used in our system. As a force is

applied to the mass, the distribution of positive and negative ions in the piezoelectric material is changed. This change

induces a charge in the system related to the force induced by the accelerometer. The charge is then conditioned either

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internally or by a signal conditioner, depending on the type of accelerometer. Once this signal is conditioned, it is ready

for viewing, analysis, etc.

Encoder Research

An encoder is utilized to measure the speed of the rotating shaft within the test fixture. Rotary optical encoders can be broken down into two main types: incremental and absolute. Both function in a similar manner: an LED light is passed

through a transparent disk and is then detected by a photodiode. An incremental encoder contains a disk with equally

spaced opaque sections, which renders an output of equally spaced pulses measured in pulses per revolution (Encoder

Guide). The output signal takes the form of a square wave and can have either one or two channels. A single channel output is useful for systems requiring simple velocity and position information in only one direction, the encoder

functions as a tachometer. Two channel, or quadrature output consists of two signals phased 90° apart. This phase

relationship is used to determine the direction of rotation. In addition, incremental encoders can have an index signal which consists of a single pulse per revolution that corresponds to a fixed mechanical point of shaft rotation. The index

signal is separate from the output signal(s) and is utilized for precise positioning in motion control applications (Optical

Encoder Guide). An absolute encoder contains a disk with concentric opaque sections, generating an output of binary numbers that correspond to each defined shaft position. Since each position has its own unique code, shaft position is

always known. For both incremental and absolute encoders, the resolution is described in cycles per revolution. For

incremental, it is the number of increments on the disk. For absolute, it is the number of defined positions (Encoder

Guide).

Wireless Research

Wireless data transmission was a customer request given for the proposed system. Most common wireless technologies are a form of Radio Frequency (RF) transmissions that communicate data from a source (transmitter) to a destination

(receiver). For the purposes of this project the following wireless methods were researched- Bluetooth, Wi-Fi, ZigBee and

proprietary RF networks. Each option is a form of RF communication that is defined by different protocols defined by the Institute of Electrical and Electronics Engineers (IEEE). 802 protocols include families of various types of wireless

networks. There are several different standards under the 802 category, each with separate standards for data transmission

methods. Wi-Fi networks uses 802.11 standards and operates in the 2.4 and 5 GHz radio bands. (Discover and Learn) Bluetooth falls under IEEE protocol 802.15.1 and was specifically designed to transmit data over short distances using the

2.4 to 2.485GHz frequencies with a through-rate of 1Mbps. (What is Bluetooth?) ZigBee was built around IEEE protocol

802.15.4 and has a limited through-rate of 250 kbps operating at 2.4 GHz. (What is ZigBee?)

Each option, Wi-Fi, Bluetooth and ZigBee, have benefits and down falls, but overall each follows a protocol that limits

the data transmission and its functionality in some way. Each of the wireless methods discussed can operate at 2.4 GHz,

but they must operate under a set of IEEE restrictions. A proprietary RF network can also operate at 2.4 GHz but has the freedom to operate without the restrictions of any IEEE standards. (Comparison of 2.4-GHz proprietary RF and Bluetooth

4.0 for HID applications) With a proprietary RF network the range of transmission, through-rate and power consumption

can all be determined by the manufacturer or customer. This gives greater flexibility for customization and control to any customer seeking wireless solutions.

Works Cited

Encoder Products Company. (2006, Fall). Retrieved December 27, 2012, from Optical Encoders Guide.

Anaheim Automation. (n.d.). Retrieved December 27, 2012, from Encoder Guide:

http://www.anaheimautomation.com/manuals/forms/encoder-guide.php

Comparison of 2.4-GHz proprietary RF and Bluetooth 4.0 for HID applications. (n.d.). Retrieved January 3,

2013, from Cypress: http://www.cypress.com/?docID=26882

Discover and Learn. (n.d.). Retrieved January 3, 2013, from Wi-Fi Alliance: http://www.wi-fi.org/discover-and-

learn

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PCB Piezotronics. (n.d.). Technical Support Home. Retrieved January 11, 2013, from Introduction to

Piezoelectric Accelerometers: http://www.pcb.com/techsupport/tech_accel.php

What is Bluetooth? (n.d.). Retrieved January 3, 2013, from Tech Radar:

http://www.techradar.com/us/news/phone-and-communications/mobile-phones/what-is-bluetooth-

1063913

What is ZigBee? (n.d.). Retrieved January 3, 2013, from Wise Geek: http://www.wisegeek.com/what-is-

zigbee.htm