Test & measurement
times? Speed changes in the line can induce upward or downward movement. Is the setup able to capture these events if needed? Ideally, the sensor ranges should overlap and cover the full range of material movement or at least control the movement.
LINEARITY
The accuracy of a sensor is often referred to as its linearity. The linearity value describes the deviation from the ideal, straight characteristic curve. Each measurement sensor has its own measurement uncertainty, or non-linearity. This means that at any given point in the measuring range the actual reading from a sensor can vary by a percentage of its measuring range. So taking just two sensors without any additional processing means that both sensors’ uncertainties need to be considered with respect to the accuracy looking to be achieved. For example, without adjustment, just moving a target 200 micron up or down can result in errors of 8 micron. The position in the range also has potential to affect the true value. To solve this, the sensors must be calibrated together as a whole.
with Interferometer or confocal technology. The use of light refraction creates ‘edges’ or return signals that indicate the transition between the air and the material. Knowing the refractive index of a material enables the accurate measurement of the material thickness provided it stays within the measurement or working range of the sensor.
MEASURING FROM BOTH SIDES If single-sided measurement is not suitable or the challenges cannot be overcome, in many cases customers want to know the true material thickness by measuring the material in ‘free space’, which requires having space on both sides of the material so that a measurement can be taken from both surfaces.
When you consider this set up, there are a number of challenges that must be considered and either overcome or accepted.
SENSOR ALIGNMENT
The sensors must be positioned so that the measuring spots coincide ‘through’ the full measurement range of the sensor. There should be no offset, tilt or inclination of the sensors in relation to the measurement object. For example, with a sensor offset of 1mm and inclination of 2°, the effective error equals 35µm, and at 10 mm target thickness increases to 41µm. Particularly with laser triangulation sensors, the location of the beam spot to the sensor
Instrumentation Monthly June 2024
housing should be noted. It should not be assumed that two apparently identical sensors will position the spot in the same place. Standard sensor housings are not usually precise enough for precision thickness measurements unless time is taken to align them correctly. To help customers make their own thickness set ups, the ILD1900 sensor from Micro-Epsilon uses an innovative sleeve mounting arrangement to tighten the spot spacing from housing to housing. When you have two sensors looking at the same target, you must consider that each sensor has its own cycle time. If your target is vibrating or moving in the gap between the sensors then it is very easy to introduce an error. Consider a target oscillating up and down by 1mm at 20Hz (times a second). A difference in capture time of 1ms between your sensors would equate to an error of 125µm.
POSITIONING OF THE SENSORS/MEASURING RANGE The next challenge is the relative position of the sensors and their measurement ranges. Depending on how the sensors are arranged, the position of the target edges must stay within the measuring field. If the sensors are set so that the measuring zones do not overlap, then situations can arise where one sensor may not see the target. Consideration should also be given to process ‘start’ and ‘stop’ conditions, for example, is the material held in tension at all
EFFECTS OF THERMAL CHANGES Even when the sensors have been aligned and synchronised, there is still a further challenge that can affect everything – thermal changes. When measuring a target thickness in free space, the gap between the sensor and the opposite sensor is critical, as this is the constant on which the differential measurement is based. Taking a mechanical frame with a pair of sensors and cycling the temperature shows that the effective change with just a 5°C swing is up to 20 micron.
SYSTEM CAPABILITY
The final challenge – if the previous ones have been overcome or the errors associated have been accepted – is to prove the capability. Exactly how do you check or confirm the performance of your solution? There are two factors to consider here: the system repeatability, i.e. how much variability in the measurement system is caused by the measurement device, and reproducibility – how much variability is caused by different operators.
Micro-Epsilon
www.micro-epsilon.co.uk
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