FEATURE TEST & MEASUREMENT A
ccuracy, resolution, repeatability and linearity are essential factors in
the selection of precision measuring instruments such as displacement sensors. While the terminology can be confusing, however, it is critical when it comes to selecting the right measuring instruments for an application – especially for displacement and distance sensors. If not, engineers may over- or under-specify the product.
FACTORS TO CONSIDER Resolution: This is one of the most frequently misunderstood and poorly defined descriptions of performance. The resolution of a sensor is defined as the smallest possible change it can detect in the quantity that it is measuring. However, resolution is not accuracy – an inaccurate sensor could have high resolution, and a 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, the application and the 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 the linearity, resolution, temperature stability, long-term stability and a statistical error. Repeatability: 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
40 JULY/AUGUST 2016 | INSTRUMENTATION When selecting non-contact displacement measurement
sensors, engineers need to fully understand the terminology. Chris Jones, managing director at Micro-Epsilon UK, guides us through the most important factors to consider
Understanding precision displacement measurement
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 will improve, but this does not continue indefinitely. Signal-to-noise ratio: The quality of a
transmitted useful signal can be stated by its signal-to-noise ratio (SNR). The SNR often limits the accuracy with which some measurements can be performed. Noise arises with any data transmission, and 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. 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 percentage of the measuring range or percentage of full- scale output (% FSO). In many applications, the
sensor non-linearity will play a large part in determining the actual measurement accuracy. Often, the linearity figure will be 10 or 20 times greater than the resolution so, if incorrectly specified, the measurement sensor will dramatically under perform. Long-term stability: The stability of sensors or measurement systems can change over time, i.e. with unchanged input quantity and ambient conditions, the possible change of the output signal over a certain time period is acquired. This figure is typically stated in % FSO / month. Temperature stability: A supplier of
high performance laser sensors is more likely to state the temperature stability of a sensor on the datasheet. Active temperature compensation algorithms may also be provided for the sensor, reducing temperature stability to as low as 100ppm/K or more.
Measuring range: This describes the space of a sensor in which the object to be measured must be situated so that the specified technical data are satisfied. The extreme regions of this space are termed the start and end of the measuring range; and some sensors exhibit a free space between the front of the sensor and the measuring range and the sensor. With contact sensors, the measuring range is the distance between the mechanical minimum and maximum possible distance of the sensor mounting to the measurement object. Offset distance: This is defined
“While the terminology can be confusing, it is critical when it comes to selecting the right measuring instruments for an application – especially for displacement and distance sensors”
differently from supplier to supplier and from one sensor principle to another. The offset distance corresponds to the distance between the sensor edge and the centre of the measuring range or the start of the measuring range. Response time: The period from the
time of the event to the signal output. This is often deemed to be achieved when 90% of the signal output is achieved. The response time may vary depending on the position of the measurement object. For example, if the object is out of the measuring range and then moves into the measuring range, the response time can be significantly longer than the quoted measurement speed
or measurement frequency. Also, if the object is already in the
measuring range but moves rapidly over a large percentage of the measuring range, say greater than 50%, again the response time will be longer than the quoted measurement speed. Care must be taken in this instance, as this can cause problems, particularly in closed loop control applications or in an application where fast moving individual components are moving through the measuring range in, for example, a production process.
Micro-Epsilon
www.micro-epsilon.co.uk
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