Accuride Corp. runs an aggressive research and development program at its Corporate Engineering Center in Henderson, Kentucky. Accuride engineers at this facility test and analyze contemporary designs, and conduct tests to verify the quality of products coming off the production lines. Some of the work involves accelerated fatigue testing in which wheels are subjected to very high loads for extended periods of time to determine where fatigue cracks might start and how they would propagate. Loaded and ready, an Accuride wheel has sensors in place for a test run. Information from the sensors is transmitted to a stationary receiver in the cab of the truck. One approach that Accuride employed to collect data from wheel strain gauges was to mount a data acquisition device directly to the wheel. This device obviated the need to transmit the data from the rotating wheel to a stationary receiver. Digital rotor telemetry offers several advantages for the testing team at Accuride Corp. Because the data are digitized before transmission, they are virtually immune to electrical noise. This means the signal is much clearer and cleaner than an analog system.
Wherever there's a wheel, on a half-ton pickup carrying the family for errands on Saturday morning or on an 80,000-pound gross weight tractor-trailer hauling goods across the country, a manufacturer's main concern is product integrity: The wheel has to hold up regardless of operating conditions. That's why a company continually tests designs before they go into production and tests products as they come off the line.
Engineers may run computer simulations, then compare those results with information from physical tests. They mount sensors on wheels, which are put through their paces. They look for ways to catch the data from those rapidly spinning gauges.
Accuride Corp. manufactures a wide range of steel and aluminum wheels for vehicles ranging from pickup trucks to class 8 tractor-trailers, cement mixers, and dump trucks. With plants in the United States, Canada, and Mexico, the company supplies forged aluminum wheels for both original equipment and aftermarket applications, and is the standard steel wheel supplier for every major truck manufacturer in North America.
So Accuride has a decided interest in the safety not only of that pickup doing the weekend errands, but also of the 18-wheeler on the long haul. That's why the company runs an aggressive research and development program at its Corporate Engineering Center in Henderson, Ky.
Accuride engineers at this facility test and analyze new designs, and conduct tests to verify the quality of products coming off the production lines. Some of the work involves accelerated fatigue testing in which wheels are subjected to very high loads for extended periods of time to determine where fatigue cracks might start and how they would propagate.
A great deal of the testing involves stress and strain analysis. For these testing investigations, technicians apply strain gauges to critical locations on the wheel and then operate the wheel under various loading conditions. Actual strain values are compared with the theoretical strains calculated through finite element analysis.
To do an adequate job of strain testing, anywhere from 30 to 100 strain gauges are applied to a wheel. The engineers want to capture the data from each of the gauges as the wheel is rotating. And that 's where the problem begins: It is difficult to get data off rotating equipment.
Slip Rings and Collection Devices
One approach used by Accuride was slip rings with brushes that make an electrical connection between the rotating sensors and the data collection unit. Unfortunately, slip rings present several problems. First, the sliding brushes make unwanted electrical noise that can corrupt the data being collected. It is possible to reduce this noise by filtering, but the filtering is a nuisance, an inconvenience, and not always very effective.
Second, slip rings are limited in the number of simultaneous channels of data that can be collected. As more channels are added, the slip ring becomes bigger until, at around 30 channels, the entire unit becomes very unwieldy. If engineers want to collect data from a larger number of strain gauges dynamically, they have to perform two or more test runs.
Finally, slip rings are generally heavy. That limits their usefulness, particularly if engineers want to measure strain dynamically as the wheel is going down the ro ad. A 30-channel slip ring is a heavy, bulky device that adds substantially to the unsprung weight during the test period.
Another approach that Accuride employed to collect data from wheel strain gauges was to mount a data acquisition device directly to the wheel. This device obviated the need to transmit the data from the rotating wheel to a stationary receiver. Instead , the data was stored in the module that rotated with the wheel until the test was done. Later, when the wheel was stopped, data was uploaded from the collection device to a computer for analysis.
However, there are significant limitations to the use of this data acquisition module. Size and power consumption constraints limit it to about seven simultaneous strain gauges. As a result, the same test had to be repeated several times to collect data from all strain gauges. But this approach proved to be unsatisfactory. In successive test runs, the wheel position and consequently the angular position of the strain gauges would be different during certain events, such as hard bumps or potholes.
Further, the memory capacity of the module limited the total number of data points that could be stored. This forced Accuride engineers to either accept a very limited test duration or to reduce the data sample rate to extend the test duration . With reduced sample rates , however, criti cal information could be missed, such as the peak strain levels as the vehicle hit a large bump in the roadway.
To overcome the limitations of slip rings or directly attached data collection devices, Accuride began using a custom-made AT-7000 Digital Rotor Telemetry System from Accumetrics Associates Inc. of Schenectady, N.Y. The modular system consists of a small cylindrical assembly 171 mm in diameter by 155 mm long that is supported by a steel bracket at the wheel hub. This assembly contains sealed electronic modules that acquire data from 88 sensor channels. All sensor data is amplified , multiplexed, and digitized into a single high-speed data stream for transmission off the rotating structure.
The cylindrical assembly, or rotor, has a bearing that supports a nonrotating end plate, a stator. The end plate is prevented from rotating by a flexible steel strap that extends to a mounting base on the vehicle's fender, suspension, or other nonrotating structure.
Using Rotary Transformers
An induction coil in the rotor and an adjacent coil in the stator work together as a radio frequency rotary transformer. This transformer has two functions. First, a radio frequency power signal generated in the system receiver electronics unit is conveyed through this transformer, providing energy to power the rotating electronics . Second, the transformer provides a path for a radio frequency carrier, modulated with the digital data stream, to be transmitted off the rotor.
The system acquires data from 56 quarter-bridge strain gauges, eight full -bridge strain gauges and 24 thermocouples. Eight strain gauge modules in the rotor are configured so that they can conveniently accept either 120-ohm or 350-ohm gauge inputs, an arrangement that adds flexibility for various testing operations. Each module accepts seven quarter bridges and one full bridge.
In addition to the strain gauge modules, the three temperature modules acquire signals from a total of24 thermocouple sensors and contain cold junction temperature sensors. All sensor signals are then amplified and digitized for transmission from the rotor to the receiver, where software linearizes and compensates for cold j unction effects for the thermocouples.
Digital Bus Connection
All of the data acquisition modules within the rotor system are connected to a master control module through a digital bus. The control module directs the sequential acquisition of data from each sensor channel. The information is then passed over the bus and arranged into a sequential data stream. Finally, this stream is radio frequency modulated for transmission through the induction coils to stationary electronic equipment.
The sampling of the strain gauges is at a rate of approximately 800 samples per second, giving sufficient bandwidth to reproduce data waveforms as the wheel is dynamically rotated. Sampling of the thermocouples does not need to be as high, since temperatures change much more slowly. Temperature data is transmitted off the rotor at a rate of about 30 samples per second. This rate is subsequently reduced even further in the receiving and data processing equipment.
In addition, the system is equipped with a rotational "once per rev" pulse generator. A Hall Effect sensor on the stator plate generates a narrow pulse each time a permanent magnet mounted on the rotating structure passes. his pulse provides both a wheel angular position reference and a pulse rate frequency that indicates wheel speed.
The nonrotating stator plate is connected by a coaxial cable to a receiver that may be located in the cab of the vehicle. This unit recovers the digital data stream for subsequent processing. Strain gauge data are converted to high-level analog voltages that are interfaced with Accuride's standard data acquisition equipment and stored on a personal computer. Thermocouple data are compensated, linearized, converted to engineering units, and then transmitted directly to the PC via an RS-232 serial interface, where the information may be displayed and stored.
Digital rotor telemetry offers a number of advantages for the testing team at Accuride Corp Because the data are digitized before transmission, they are virtually immune to electrical noise. This means the signal is much clearer and cleaner than an analog system. In addition, the large number of channels means that more data can be collected more quickly than through other techniques.
In the past, strain was frequently measured statically in a laboratory at very slow rotational speeds, and repeated tests were required to gather all the necessary data. A complete static test could take more than an hour to complete. Now, with digital telemetry, a dynamic test can be run and all of the data gathered in less than a minute.
Collecting Data Over the Road
The digital telemetry system permits collection of strain data while traveling over the roads. On vehicle tests, the problem of different wheel rotation positions on multiple runs was eliminated. Data also can be collected dynamically for longer periods of time. In addition, the test engineer can sit in the cab of the truck and watch the data on a lap top computer as it is being generated. This is a powerful tool for correlating test results with the real-world environment.
Another advantage of the digital telemetry system is its compact size. In laboratory testing with overinflated tires at extremely high loads, there are safety concerns. With the AT-7000, putting the necessary safety protection equipment in place is much easier.
The digital rotor telemetry makes data collection off rotating wheels faster, easier, and safer, while helping Accuride meet its research and quality assurance goals .