INSPECTION, TEST & MEASUREMENT FEATURE


percentage of the measuring range or of full-scale output (% FSO). In many applications the sensor non-linearity will play a large part in determining the actual measurement accuracy. It is very common for users to use the resolution value of a device when actually the linearity figure is required. Quite often the linearity figure will be 10 or 20 times greater than the resolution. Therefore, if incorrectly specified the measurement sensor will dramatically under perform.


LONG-TERM STABILITY Despite the use of high quality components, the stability of sensors or measurement systems can change over the course of time, for example, 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 Check out the technical datasheet and you may find that most suppliers of low cost laser sensors do not state the ‘temperature stability’ of their sensors. So how do you know the actual measurement error or how to correct your results to account for this? Typically, measurement errors can be as high as 400ppm/K which can significantly


affect the measurement accuracy. On the other hand, a supplier of high performance laser sensors is much more likely to state the temperature stability of a sensor on the datasheet. In addition, active temperature compensation algorithms may be provided for the sensor, reducing temperature stability to 100ppm/K or more.


MEASURING RANGE The measuring range 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. Some sensors exhibit a free space between the front of the sensor and 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 The offset distance of a sensor is defined differently from one supplier to another and sensor principle to sensor principle. 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 is often deemed to be achieved when 90% of the signal output is achieved. Many sensor specifications do not state response time and often it is assumed this is equal to the stated measurement speed or measurement frequency. This is incorrect. Quite often the response time will 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. If the object is already in the measuring range but moves rapidly over a large percentage of the measuring range eg 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 where fast moving individual components are moving through the measuring range in, for example, a production process.


Micro-Epsilon UK T: 0151 355 6070 E: info@micro-epsilon.co.uk www.micro-epsilon.co.uk


The Rotronic Universal Monitoring System – RMS


Highest product quality in production and during storage requires an integrated monitoring system. The modular Rotronic Monitoring System – RMS is the perfect solution. It provides installation flexibility and full data availability, anywhere, and on a variety of devices. Rotronic can meet your requirements, incorporating multiple sensors for all parameters on a secure network. We can service the entire system. www.rotronic.co.uk/rms


ROTRONIC Instruments (UK) Ltd, Crompton Fields, Crompton Way, Crawley, West Sussex RH10 9EE T: 01293 571000, F: 01293 571008, E: instruments@rotronic.co.uk  FACTORY EQUIPMENT | NOVEMBER 2016 15


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