The Art and Science of Reliability
by Roger W. Lockhart, DATAQ Instruments
John Deere is a major supplier of agriculture,
construction, forestry, consumer, and commercial equipment with
a reputation for quality and innovation. At the company’s
Construction and Forestry Division in Dubuque, IA, engineers are
deploying cutting-edge technology to ensure utmost product
performance and reliability.
Vital to this effort is the division’s data
acquisition program that allows actual field measurements to be
combined and compared with design models. This melding of theory
and fact leads to better insights and more efficient designs and
allows John Deere to reach the market faster with better
products.
John Deere’s program covers practically every
aspect of industrial data acquisition with measurements taken
from three primary equipment systems: structure, power train,
and hydraulics. Within these categories, the engineers measure a
vast array of parameters. They combine in-house expertise with
off-the-shelf products to create appropriate data acquisition
solutions.
Typical Cabling Implementation for a 96-Channel Application
To meet the data acquisition
hardware/software and signal-conditioning requirements, the
company uses products from DATAQ Instruments. John Deere
engineers integrate these products with transducers and
packaging suitable for the punishing environments where they
will be used. They also combine in-house setup and translation
programs with commercial analysis software to yield optimal
final results.
Signal Types
Many of the variables that must be acquired
and pooled to construct an accurate picture of a complex
mechanical system like an 11 ton backhoe or a 90 ton excavator
during operation are listed in Table 1. In a large
backhoe, for example, tension and compression stresses place an
enormous load on the structure, defining the need to measure
hydraulic pressures, flows, and temperatures. The heart of the
equipment—the engine and power train—demands accurate
measurements of torque, horsepower, temperatures, and pressures.
Depending upon the goal of a given test, the engineers could
measure as few as 16 or as many as 256 channels of these signal
types in any combination, but typical applications rarely exceed
a channel count of 128.
Table 1. Range of Signal Types Measured by John Deere Engineers
Signal Conditioning
The wide range of measured signal types and
levels requires signal conditioners that are flexible and
modular. Since as many as 256 channels can be acquired at a time
for one test, and John Deere maintains dozens of test systems
that can run in parallel, low cost also is important.
John Deere selected DATAQ’s 5B-style
amplifiers, described as rugged, hockey puck-like devices that
plug into a data acquisition backplane. Each of the nearly 100
amplifiers in this line measures a specific function over a
fixed range so one is used for each channel. The modules can be
mixed and matched in any combination to conform to the
requirements of a given application.
In addition, the amplifiers provide built-in
input-to-output isolation,
which is crucial when measurements are taken from disparate
sources on a massive vehicle. Ground in a situation like that
always is a relative term, with
a high probability of differences in ground potential.
The isolation barrier allows the front end of
an amplifier to float relative to those of other amplifiers. As
a result, the inevitable presence of common-mode voltages will
not damage the data acquisition system, and accurate
measurements will be reliably made in spite of them.
Ethernet Data Acquisition
The backplane with 32 channels of signal
conditioning is packaged in the same enclosure as the data
acquisition hardware. The front end features a delta-sigma ADC
per channel with anti-alias filtering and simultaneous
conversions to facilitate the eventual analysis of
structure-related stress and vibration waveforms.
Each enclosure provides a programmable total
sample throughput rate ranging from 200 Hz to 320,000 Hz.
Measurement resolution is 16 bits, and all acquired data is
delivered to an integral 100Base-T Ethernet interface, one per enclosure,
that allows multiple enclosures to be daisy-chained using
standard CAT-5 cable for higher channel counts.
Moreover, data acquired from all channels
across multiple enclosures is fully synchronous. For example,
four daisy-chained enclosures can yield a total data throughput
rate of as much as 1,280,000 S/s, and each sample period is
fully synchronized across 128 channels. Finally, all information
is delivered in real time to the Ethernet interface of a
cab-mounted laptop PC running WinDaq data acquisition software (Figure
1).
Figure 1. Cab-Mounted Off-the-Shelf Laptop PC Providing Real-Time Feedback for Acquired Signals
The John Deere engineers chose an Ethernet
approach that provides hardware synchronization via two unused
CAT-5 cable pairs. One pair carries a master 16-MHz clock and
the second a trigger signal. The clock is daisy-chained between
units, and each unit incorporates a phase-locked loop (PLL) that
provides failsafe operation and exactly reproduces the master
clock with zero phase delay (Figure 2).
Figure 2. Architecture and Typical Interconnect of Data Acquisition Devices Supporting Synchronous Ethernet Technology
The failsafe feature is a unique aspect of
the PLL that ensures the lock is maintained in the event of
momentary or even longer-term interruptions in the daisy-chained
master clock, subject only to thermal drift. By incorporating
PLLs into each data acquisition unit, they remain precisely
synced to the master clock in frequency as well as phase,
guaranteeing synchronous analog-to-digital conversions among
individual units and between individual channels. The addition
of a master trigger signal on the second CAT-5 cable pair
completes the picture to ensure that all units in the
distributed chain initiate sampling at the same instant.
The synchronous Ethernet interface solves many problems:
• It leverages a ubiquitous standard. The
Ethernet interface is found everywhere and precisely defines the
connectors and interconnecting cables that the standard uses.
• It supports modularity and expansion.
Enclosures can be easily swapped, added, or removed to increase
or decrease channel count as the application requires.
• Full channel synchronization is crucial for
subsequent frame stress analysis. For example, data points
acquired on channel one in enclosure one fall in precisely the
same time slot as data points acquired on channel 256 in
enclosure eight.
• Ethernet is a standard with inherent
isolation. Data communications reliability is bulletproof even
in the presence of common-mode voltages.
• The communications cable lengths are
virtually unlimited. Ethernet intrinsically allows distances as
great as 100 meters between switches and hubs. Since the data
acquisition system’s synchronous interface provides a built-in
Ethernet switch, the distance between individual enclosures and
between them and the PC can be as long as 100 meters, more than
enough to instrument even the largest John Deere equipment.
Data Acquisition and Analysis Software
John Deere engineers assembled
a complementary collection of off-the-shelf and custom software
to quickly move the measurement projects from initial setup to
final results. A maximum count of 256 channels means that an
efficient approach to swiftly configure and reconfigure tests
was mandatory. The setup program developed by John Deere accepts
inputs such as channel selection, engineering unit scaling
constants and tags, and sample rate and creates a setup file for
each data acquisition enclosure.
The file assumes the same filename and format
as the default configuration file used by WinDaq, enabling the
configuration defined by the test engineers upon power-up
without further manipulation. The software synchronously
acquires channel data across multiple enclosures, produces a
real-time graphical display in calibrated units on the display
of the cab-mounted PC’s display, and streams all the data to the
PC’s disk for a permanent record.
Following data acquisition, attention is
turned to analysis where engineers use a software suite from
nCode. This process is augmented by another in-house program
that seamlessly converts acquired data files into nCode’s format
and performs turnkey analysis at the same time.
The range of signals accumulated during data
acquisition is split into two analysis paths. Stress and
vibration signals from the equipment’s frame and structure are
applied to nCode’s nSoft suite of analysis software, which is
optimized for fatigue analysis.
From it, John Deere engineers derive
predictive failure analyses that feed their design qualification
assessments. This life testing aspect is a major component that
drives company quality programs and, ultimately, equipment
reliability and customer satisfaction.
Signals from the engine and power train take
a different analysis path through another nCode application
called Glyphworks. Raw data values are subjected to a suite of
advanced mathematical functions and reduced to min/max values
and histogram outputs for evaluation. These results merge with
frame and structure data during the design qualification process
to complete the analysis picture.
What’s Next?
The next step is to augment current power
train sensor-based measurements with CANbus information,
acquiring the latter synchronously along with analog sensor
data, according to Robert Wagner, senior engineer at the John
Deere facility in Dubuque. The company is collaborating with
DATAQ to design CANbus hardware that will feature the same
synchronous Ethernet interface as current analog data
acquisition systems.
The product that emerges from the effort will
support SAE J1939 and drop into the daisy-chained architecture
of existing analog products. Its inclusion will allow engineers
to synchronously correlate real-time analog sensor data with
digital engine control unit (ECU) information, providing yet
another perspective to evaluate product performance and
reliability.
Most intriguingly, John Deere engineers have
compiled a massive database of information through their
on-going field tests, essentially sensor-generated time
histories for force and position. These electronic fingerprints
precisely describe all mechanical systems of John Deere
equipment recorded while the machines execute both typical and
atypical maneuvers.
The ultimate goal is to fold this information
into an effort that virtually models and tests new designs
without the need to actually build a prototype. The implications
of this are obvious and will have a huge positive impact on
product development and marketing flexibility.
John Deere engineers have mastered both the
science and art of acquiring and meaningfully analyzing a vast
array of sensor data in a grueling environment. In so doing,
they have built a framework to evaluate and advance the
performance, reliability, and value of the massive machines
their customers depend upon both today and well into the future.
About the Author
Roger W. Lockhart is vice president of DATAQ
Instruments. DATAQ Instruments, 241 Springside Dr., Akron, OH
44333, 330-668-1444, e-mail: rlockhart@dataq.com