Introduction
This
application note describes the use of vibration monitoring for an
Engine Test Cell application. The
application is implemented at an automobile engine manufacturing
facility. At the end of
the production line of 4-cylinder gasoline engines are six test
cells where the completed engines are rigged up and started-up for
the first time after manufacture.
Historically, the engines were allowed to warm up for a few
minutes and then inspected manually by the operator.
The operator would listen for unusual noises and check for
oil or water leaks using an ultraviolet lamp (the oil and water in
the engine contain dye that fluoresces under ultraviolet light).
The next step in the evolution of the test cells was to use a
portable data collector to acquire vibration data from several
points on the engine block while the engine was running.
From 5 to 10 analysis parameters were defined to aid in
diagnosis of internal problems in the engines.
Alarm limits were assigned to the parameters to provide a
pass-fail result for the test.
After successful demonstration of vibration analysis for the
engine test cell program, an automated vibration monitoring system
was designed to replace the manual vibration data collection
process.
Technology
Proof / Project Justification
Vibration
analysis of production engines was proven very successful in
reducing the number of customer returns to the engine production
facility. The concept of using vibration analysis was begun at the
facility under a services contract using a portable data collector.
Several months of testing showed a remarkable reduction in the pull
rate of engines (see graph).
For
the purposes of technology proof and project justification, the
manual method of data collection proved useful. On a long-term
scale, however, the manual mode was expensive and required a full
time person onsite. An automated method for data collection and
alarming on developed frequency ranges and alarm levels was desired.
A
Cooperative effort between the vibration services contractor and
Engine Test Engineering developed the following criteria for an
automated monitoring system.
·
Simple graphical user interface for the test units
installed in the test cells. Test
preparation after an engine entered the test cell should consist of
entering the engine model and serial number, and attaching the
sensor(s) to the engine.
·
Test process automated with no further intervention
required by the test cell operator.
·
Test results presented to the operator as a simple
pass or fail indication.
·
Ethernet network connection of the individual test
cell units to the PC server.
·
Master database which stores test configurations,
test results and statistics maintained on the server.
·
The individual test cell units should be capable of
continued operation if disconnected from the network.
·
Bar code readers on the test cell units to allow
the operator to input both the engine model number and the engine
serial number by simply scanning the traveler tag attached to the
engine.
·
All data from the tests (spectra, waveforms,
analysis parameter values) to be saved in the database.
·
Full compliment of vibration analysis capabilities
including order tracking, waveform parameters, spectral bands, etc.
The
system was successfully designed and implemented. All criteria were
met.
Automated
System Test Description
The
following section describes the steps that take place during a
typical engine test.
1.
An engine comes into a test cell off the assembly
line. The test cell
operator “rigs” the engine.
This consists of connecting the sensors, fuel, water and
electrical fittings to the engine as well as an exhaust manifold.
The operator then starts the engine to begin a warm-up
process.
2.
Next, the operator scans a traveler ticket attached
to the engine. The
traveler ticket has bar codes for the engine model number and the
engine serial number. The
engine model number is used by the test unit to select the
appropriate test setup form the database and the engine serial
number is used to identify the test results.
3.
The test unit monitors the tachometer for a target
RPM that is specified in the test setup.
An analog tachometer with the current RPM is displayed on the
screen of the test unit.
4.
Once the target RPM is reached, a timer in the test
unit begins a delay time (value of the delay time is specified in
the test setup).
5.
When the delay time is finished the test unit
begins acquiring data from the sensors.
All sampling parameters (sampling rate or f-max, number of
samples, number of averages, etc.) are contained in the test setup.
6.
After the data acquisition is complete, the test
unit analyzes the data and computes all the parameters specified in
the test setup.
7.
Next, the test unit compares the results of the
analysis parameters to the alarm values specified in the test setup.
If any of the alarm values are exceeded, the test screen
indicates that the engine failed.
Otherwise, the screen indicates the engine passed.
8.
All the data and test results are then transferred
over the network to the master database on the server.
If the network is not available, the data and results are
cached on the local hard disk of the test unit until the network is
available.
9.
If desired, the server can send a failure report to
a printer on the network with pertinent test information.
With
the automated vibration monitoring system in place, up to six tests
cells can now be monitored full time for about the same cost as one
man monitoring the engines for one month! The new system is
cost-effective and presents the operator with a minimum of new
responsibilities as outlined in the testing section above.
A
few specific examples of defects detected
Gear
defects in the engines proved to be the most easily identifiable
defects as shown below.
