FEATURE INSPECTION, TEST & MEASUREMENT FIRST CHECK THE SPEC
Chris Jones, managing director at Micro-Epsilon UK outlines the 10 most important factors to consider before purchasing a non-contact displacement measurement sensor
REPEATABILITY Repeatability is a quantitative specification of the deviation of mutually independent measurements which are determined under the same conditions. It defines how good the electrical output is for the same input if tried again and again under the same conditions. In terms of displacement sensors, repeatability is a measure of the sensor’s stability over time. Typically, sample-to-sample repeatability will be lower for very fast sample rates since less time is used to average the measurement. As the sample rate is lowered
Repeatability defines how good the electrical output is for the same input if tried repeatedly under the same conditions
D
espite frequent use, terms such as accuracy, resolution, repeatability and
linearity are often misunderstood. As critical factors in the selection of a displacement sensor as well as in many other precision measuring instruments, engineers must ensure they fully understand the terminology before making a purchasing decision. Unfortunately, not all displacement sensor specifications are presented in a way that allows direct comparisons to be made. The terminology applied to sensors can
be confusing but is critical when it comes to selecting the right measuring instruments for an application, especially for displacement and distance sensors. If engineers get this part wrong they could end up paying more than they need to for over-specified sensors. Conversely, a control system or product
may lack critical performance if the displacement sensor does not meet the required specification.
RESOLUTION One of the most frequently misunderstood and poorly defined descriptions of performance, a simple resolution statement in a technical datasheet rarely provides sufficient information for a fully informed sensor selection. The resolution of a sensor is defined as the smallest possible change it can detect in the quantity that it is measuring. Resolution is not accuracy. An inaccurate sensor could have high resolution and a
14 NOVEMBER 2016 | FACTORY EQUIPMENT
low resolution sensor may be accurate in some applications. In practice, the resolution is determined by the signal-to- noise ratio, taking into account the acquired frequency range. Often in a digital display, the least
significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made. The electrical noise in a sensor’s output is the primary factor limiting its smallest possible measurement. For example, a measurement of a 5µm
displacement will be lost if the sensor has a 10µm of noise in the output. It is therefore essential that the resolution of the selected sensor is significantly lower than the smallest measurement that is required. Best practice will require a resolution of at least 10 times greater than the required measurement accuracy. In addition, resolution is only meaningful
within the context of the system bandwidth, unit of measure, application and measurement method used by the sensor manufacturer.
ACCURACY
The accuracy of a displacement sensor describes the maximum measuring error taking into account all the factors that affect the real measurement value. These include linearity, resolution,
temperature stability, long-term stability and a statistical error (which can be removed by calculation).
repeatability will improve but this does not continue indefinitely. Beyond some slower sample rate repeatability will start to worsen as long term drift in the components and temperature changes cause changes in the sensor’s output.
SIGNAL-TO-NOISE RATIO The quality of a transmitted useful signal can be stated by its signal-to-noise ratio (SNR) which often limits the accuracy with which some measurements can be performed. Noise arises with any data transmission. The higher the separation between noise and useful signal, the more stable the transmitted data can be reconstructed from the signal. If, during digital sampling, the noise power and the useful signal power become too close, an incorrect value may be detected and the information corrupted. The SNR is calculated by dividing the mean useful power by the Mean Noise power. SNR is generally understood to be the ratio of the detected powers (not amplitudes) and often expressed in decibels. Usually, the definition refers to electrical powers in the output of some kind of sensor or detector. In optical measurements a common situation is that some light beam impinges a photodetector such as a photodiode which produces a photocurrent in proportion to the optical power with some electronic noise added. Depending on the situation, the SNR may be limited either by optical noise influences or by noise generated by the sensor electronics.
LINEARITY/NON-LINEARITY The maximum deviation between an ideal straight-line characteristic and the real characteristic is known as the non- linearity or linearity of the sensor. The figure is normally provided as a
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