The art and science of reliability.

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.

[ILLUSTRATION OMITTED]

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.

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.

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.

[FIGURE 1 OMITTED]

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).

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 OMITTED]

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.