Measuring Placement Accuracy to Compare Label Sensor Technologies

Optical, ultrasonic, and capacitive sensors are the three major kinds of label sensor technologies used for automated pressure-sensitive label applications. Capacitive sensors are of two types: single and differential. Each of these types of label sensor technologies has its own distinct pros and cons.

Although each new type of sensor can be applied to a diverse range of label designs, no single label sensor can be compatible with every label. However, newer sensor technologies can work with a wider range of label materials and designs compared to already existing technologies.

This article describes the experimental results of measuring placement accuracy with three different label sensor technologies. The performance of any label sensor is based on the range of acceptable materials, speed and registration accuracy. Here, we will rate each of these three parameters on a scale of 1 to 5.

Registration Accuracy Chart

Registration accuracy is determined by recording sensor outputs and then comparing the output timing with known label edge locations. The test is carried out using a specially designed machine with an encoder to track label positions.

The number of times the sensor output dropped into 0.1 mm error bands from the correct label edge position can be seen from the results. The results are then compiled by detecting 240 labels. Figure 1 illustrates registration error vs number of occurrences while measuring registration accuracy using ultrasonic sensors.

Figure 1. Graph illustrating registration error vs the number of occurrences while measuring registration accuracy using ultrasonic sensors

Performance of Sensor Technologies

Optical Sensors

Optical sensors are operated by placing an infrared light source beneath the web and a detector over the web for measuring the light brightness. The label edge can be detected by comparing the changes in the label opacity with the liner between labels.

The inability of the optical sensors to detect clear labels, irrespective of the liner material, is one of the major limitations of these sensors. In some cases, eyemarks are added to the liner between the labels. These eyemarks are more expensive than the sensor that can sense clear labels.

Optical sensors provide accurate results at low speeds. However, the accuracy degrades as the speed increases. The registration accuracy of optical sensors rating is 5, speed rating is 4, and a range of acceptable materials rating is 2. Figure 2 and 3 show the graphs illustrating the performance of optical sensors at 70 m/min and 220 m/min.

Figure 2. Graph illustrating the performance of optical sensors at 70 m/min

Figure 3. Graph illustrating the performance of optical sensors at 220 m/min

Capacitive Sensors

Capacitive sensors measure web thickness using electric fields. The sensor is triggered by the change in thickness between the labels and gaps. Capacitive sensors are of two types: differential and single-ended.

Capacitive sensors are very fast and precise at all speeds. They have a registration accuracy rating of 5, a speed rating of 5, and the range of acceptable materials rating of 3.

Differential capacitive sensors have a registration accuracy rating of 5, a speed rating of 5, and the range of acceptable materials rating of 3. They consist of two capacitive sensing elements that sense web thickness. The outputs of the sensing elements are subtracted from one other such that the sensor produces an output only when one sensor is over a label and the other is over a gap.

One advantage is that small changes in distance between the sensing elements and the baseplate, as a result of temperature shifts or vibration, do not affect the sensor. However, differential sensors cannot be used if metallic ink or materials are used on the label or liner. Figures 4 and 5 show the graphs illustrating the performance of differential capacitive sensors at 70 m/min and 220 m/min.

Figure 4. Graph illustrating the performance of differential capacitive sensors at 70 m/min

Figure 5. Graph illustrating the performance of differential capacitive sensors at 220 m/min

Single-ended capacitive sensors, however, consist of a single sensing element that measures the web thickness. The measurement is carried out by adjusting the liner thickness below the sensor’s trigger point. When the thicknesses of the label and liner exceed the trigger point, the sensor is triggered. The registration accuracy rating of these sensors is 5, their speed rating is 5 and the range of acceptable materials rating is 4.

The apparent thickness of the metal labels overwhelms the sensor, even at the gap, making it unable to detect the gap. Typical solid foil labels will work with single-ended capacitive sensors. However, it is necessary to maintain good web control against the backplate. Figures 6 and 7 show graphs illustrating the performance of single capacitive sensors at 70 m/min and 220 m/min.

Figure 6. Graph illustrating the performance of single capacitive sensors at 70m/min

Figure 7. Graph illustrating the performance of single capacitive sensors at 220 m/min

Ultrasonic Sensors

Ultrasonic sensors use high-frequency sound waves transmitted by a transducer beneath the web to a receiver above the web, to measure web thickness. A greater amount of sound energy passes through the web at the gap than during the label. These sensors are not sensitive to metallic materials and they have the ability to sense labels of any material.

Ultrasonic sensors can detect a wide range of materials. However, they are less accurate than the capacitive sensors at low speeds. The accuracy of the sensors is related to the web speed. The registration accuracy rating of these sensors is 3, their speed rating is 2 and the range of acceptable materials rating is 5. Figures 8 and 9 show graphs illustrating the performance of ultrasonic sensors at 70 m/min and 220 m/min.

Figure 8. Graph illustrating the performance of ultrasonic sensors at 70 m/min

Figure 9. Graph illustrating the performance of ultrasonic sensors at 220 m/min

Conclusion

This article describes the experimental results of placement accuracy measured using optical, capacitive, and ultrasonic label sensor technologies.

Capacitive sensors are the most accurate sensors followed by optical sensors. Ultrasonic sensors are related to speed and their accuracy decreases with the increase in speed.

This information has been sourced, reviewed, and adapted from materials provided by Lion Precision.

For more information on this source, please visit Lion Precision.

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