In test equipment, it isn’t enough to provide accurate measurements and results. Those results have to be consistent every time the machine is used. Variability between test data indicates that the achieved results are potentially flawed and, possibly, useless. That inconsistency not only damages the validity of the tests – it costs the operator money and time by forcing them to run more samples to achieve a quantifiably significant mean. Instead of 25 or 50 tests on one component, 100 may be needed.
Before discussing the different technologies, it is important to understand how the industry measures reliability and consistency. Historically, efforts to quantify these factors have met with varying levels of success. In 2005, however, a new standard, ASTM E2309, was published to normalize methodologies and metrics for linear displacement in testing equipment. Following the requirements outlined in ASTM E2309, the consistency and confidence of data captured via linear position sensors can be measured and compared between technologies.
ASTM E2309 has four levels of accuracy clarification:
Class A: +/- 0.5% of Reading or ± 0.001 Inch (0.025 mm)
Class B: +/- 1.0% of Reading or ± 0.003 Inch (0.075 mm)
Class C: +/- 2.0% of Reading or ± 0.005 Inch (0.125 mm)
Class D: +/- 3.0% of Reading or ± 0.010 Inch (0.250 mm)
As illustrated above, within each classification, there are two accuracy specifications – Relative Error, which refers to the percentage of the reading and the Fixed Error, which refers to the actual measurement error itself. There is a third specification associated with measurement resolution, which is insignificant to the points outlined here.
To determine an ASTM E3209 rating, two runs of data are required. The variances between these runs are then used to illustrate the level of confidence in measurement repeatability.
These classifications are important for manufacturers because they allow companies to select the test equipment that meets the exact parameters of their applications, as well as industry expectations and standards, while still considering other factors such as operating cost, ease of installation and environmental conditions.
One of the most common methods used to measure linear displacement in test equipment today is through the use of a linear variable displacement transformer (LVDT). LVDTs work by measuring an electric current along a cylindrical ferromagnetic core. A metallic object travels along the core and generates a signal, which is in turn measured by three coils placed along a tube. This technology offers several benefits, including compatibility with most industrial equipment, easy installation and fast start up (no recalibration required).
A second technology, magnetostriction, works by inducing a sonic strain pulse in a specially designed magnetostrictive waveguide by the momentary interaction of two magnetic fields. One field comes from a movable permanent magnet which passes along the outside of the sensor tube; the other field comes from a current pulse or interrogation pulse applied along the waveguide. This interaction produces a strain pulse, which travels at sonic speed along the waveguide until the pulse is detected at the head of the sensor.
The magnet’s position is determined with high precision by measuring the elapsed time between the application of the interrogation pulse and the arrival of the resulting strain pulse. Consequently, accurate, non-contact position feedback is achieved with absolutely no wear to the sensing components. Magnetostriction also requires no recalibration and can be easily installed in most industrial settings.
Factors Affecting Accuracy
In perfect laboratory settings (controlled temperature, electrical/magnetic interference, shock, vibration, etc.), most sensors will provide fairly consistent results. The true indicator is how the product behaves in real world environments.
LVDTs, particularly, are susceptible to environmental influences. By design, LVDTs are variable AC transformers, making them prone to errors from cable capacitance, impedance, demodulator phase variations, and minor variability in the physical winding of the transformer and its matched core. Additionally, since the core and the transformer remain in concentric and angular alignment, the element can degrade over time, especially when combined with grit or other contamination.
How they Measure Up – The Advantages of Magnetostriction
While LVDTs still have a strong position in the market, through direct comparisons using ASTM E2309, the advantages of magnetostriction are obvious. Where LVDTs struggle to comply with Class C specifications (± 2.0% of Reading or ± 0.005 Inch) over the physical range at the >25cm lengths using typical linearization algorithms, most magnetostrictive linear positioning sensors easily maintain a Class A (± 0.5% of Reading or ± 0.001 Inch) rating, especially when measured in actual field conditions. This advantage becomes even more prevalent in applications where very long stroke lengths (>100 cm) or high speed readings (50 cm/second) are required.
Magnetostrictive sensors are also less susceptible to environmental factors, such as EM inference, shock and vibration, when compared to LVDTs. Additionally, since magnetostrictive sensors utilize no moving parts, wear and tear due to continual use is not an issue. These sensors can operate indefinitely with little or no maintenance required. They can be mounted in virtually any space without impeding performance.
Magnetostrictive sensors are compatible with many different electronic protocols, including EtherNet/IP™, EtherCAT®, Profibus, DeviceNet, CANbus, SSI, Analog and general Ethernet. MTS Systems Corp., Sensor Division, recently introduced a model incorporating detached electronics. By detaching the electronics, manufacturers can further remove sensitive equipment from harsh environments where they may be damaged by temperature, shock, vibration or other conditions, without impacting the performance of the sensor.
While LVDTs still offer value in lower end performance applications, magnetostriction provide tangible benefits in equipment where long term, reliable data is essential. In testing equipment, magnetostrictive linear positioning sensors will provide more consistent results with less wear and tear and allow for considerably higher ASTM E2309 classification, enabling manufacturers to better meet application specifications, customer expectations and industry regulations.