The sure way to avoid paper jams

Paper jams… … not just annoying and a waste of time, but also an unnecessary expense

Pepperl+Fuchs: We’ve all experienced the situation: you stand in front of the photocopier waiting patiently for the last sheet of paper only to discover that it has folded itself carefully between the rollers and is now blocking the path of all subsequent copies. Those who have the patience and time to consult the manual to find out what to do are fortunate.

But not all of us have the time or the desire to find out about the intricacies of the copier and its feed system.

While this may be a minor irritation in the consumer sector, such paper jams can cause enormous cost in industry due to stoppages and reject goods and often also result in high-cost repairs to the sensitive rollers of the printing presses or other parts of paper processing machinery.

Accordingly, the primary aim must be to detect these paper jams (caused by the feeding of doubled sheets of paper) before they happen and to prevent them. In recent years, ultrasonic technology has proven its effectiveness for this central task in a printing press and has been continuously further developed.

How do paper jams happen?

In many cases, the cause of a paper jam can be found in the automatic pickup of a sheet of paper from the paper stack. For various reasons, it can happen that two sheets of paper can be removed from the stack instead of one, whether because the machine is not correctly adjusted to the paper, the quality of the paper has changed in a new batch or the sheets of paper tend to adhere to one another for mechanical reasons. Electrostatics and moisture also play a key role. The consequence of such double sheet occurrences is usually a paper jam further along the paper path when the two sheets of paper detach themselves from one another.

If the two sheets do not separate as they pass through the printer, a problem will occur during subsequent automatic collation because there will be one blank page, leading to wastage. The folding machines further downstream can also be damaged because these are precisely adjusted to the paper thickness.

Ultrasonic double material sensors are used in most of these applications and are deployed in any situation where it is necessary to provide protection for high-grade machine parts and to avoid wastage.

The principle of double sheet detection

The principle of double sheet detection is based on the attenuation of an ultrasonic signal by one or two materials.

Two ultrasonic sensors are positioned opposite each other. One sensor is used as the transmitter, while another serves as the receiver.

The acoustic signal emitted by the transmitter meets the boundary layer of the first sheet of paper, which has a different wave resistance (Z) to air. Because of this incorrect acoustic adjustment, part of the sound energy will be reflected, while the other part will penetrate the paper material.

This process is repeated at every changing boundary layer from one medium to the other. The example below is based on air and paper.

After the sound has passed 4 boundary layers, its amplitude is attenuated to such an extent that the difference from the individual sheet of paper (2 boundary layers) can be evaluated with certainty.

Depending on the attenuation of the signal, it is possible to distinguish between no paper (air), a single sheet and a double sheet. It is not possible to differentiate between three or more sheets because the sound amplitude between sheet 1 and sheet 2 is already almost 0 due to multiple reflection. The principle of double sheet detection is therefore based on the existence of a cushion of air between two sheets of paper.

In most cases, the surface roughness of standard papers ensures a sufficient cushion of air between the sheets of paper. In some cases, laminated papers or films can adhere to one another to such an extent that the boundary layer is not sufficient.

Ultrasonic technology demonstrates its particular benefits when it comes to the question of material surfaces. Because of the physical principle, the sensor operates independently of the color or transparency of the object. It makes no difference whether the paper is black or white. It also makes no difference if it is printed or how it is printed. In addition, reflective or transparent surfaces behave in the same way for acoustic waves.

Optical double sheet sensors that evaluate light attenuation by the paper are at the limit of their capabilities here. However, it is not just transparent and reflective materials that pose a problem - paper dust that gathers on the lenses distorts the evaluation of the optical signal. Ultrasonic sensors are self-cleaning thanks to the mechanical vibration of the transducer surfaces, removing dirt and dust, making them perfect for use in paper processing machines.

Not every double sheet is the same

In addition to the many standard applications in which double sheet sensors have been successfully used for some time, there are special cases that need to be considered.

The question of which objects can be detected with ultrasonic technology depends primarily on the sound absorbency of the material to be tested. A first indicator of sound absorbency can be the weight of a sheet of paper. The weight indicates the area-related mass in g/m2.

Standard paper as used in magazines, newspapers or photocopying is in the range of approx. 40…200 g/m2. However, thicker and heavier materials, such as plastic or metal foils and cardboard up to 2000g/m2 can be detected with ease.

However, if such papers are laminated or coated, for example, the weight can only be used as the sole criterion for detectability to a limited extent. In some cases, a laminated layer may be detected as a second thin sheet of paper, which may be equated with a double sheet in physical terms.

Open-pored corrugated cardboard in which air is enclosed by definition is already a double sheet by its construction.

However, in the case of fine corrugated cardboard it is quite possible that the sound will be transmitted from one side of the cardboard to the other with sufficient amplitude through the positioning and stability of the webs. In such cases, it is also possible to distinguish between a single sheet and a double sheet.

Sound damping is not always proportional to the thickness of the material.

Thus, for example, a thin plastic foil can absorb sound to a greater extent than a metal foil of the same thickness.

Even what seems like a thin sheet of paper can absorb sound to a greater extent than a heavier thick sheet of paper.

How well a material conducts or absorbs the sound depends not just on its physical characteristics, but also on the frequency at which the sound meets the material.

Variations in the material, such as perforations, holes, extreme variations in paper thickness, etc. which can cause the sound to be absorbed differently, must be taken into account when using double sheet detection.

For these reasons it is vital to know the object to be detected precisely so as to exclude special cases, thereby avoiding malfunctions.

Other areas of application for sonic scanning processes

The principle of evaluating sound absorbency can also be used in special cases to detect adhesive strips on continuous paper and for detecting labels on labeling machines.

The basis for such an evaluation is always a sufficiently high amplitude difference between the two events, paper and adhesive strip or label and backing material.

In the case of a connection point or adhesive strip between two layers of paper, there is no detectable cushion of air in the overlapping area of the sheets of paper due to the adhesive. The adhesive strip causes the sheets to stick together directly. In this case, the sound only penetrates and rebounds from the medium once. The attenuation difference from a single sheet of paper may therefore be very small. For this reason, it is helpful to teach the sensor with regard to the backing material to enable minor variations to be detected.

Also in the case of label detection, the label adheres directly to the backing material as with an adhesive strip. The change in amplitude between the backing material and the label is similar to that of an adhesive strip. However, suitable electronics and special signal evaluation can be combined for reliable detection.

These applications differ substantially from the classic double sheet detection described above and thus cannot be compared directly. With adhesive strips, it is mostly necessary to learn the base material due to the smaller differences in amplitude.

Areas of application for double sheet detection

• Photocopiers
• Printers
• Document scanners
• Sheet counting machines
• Sheet printing machines
• Continuous paper printing machines
• Collating and binding machines
• Packaging machines
• ...

Advantages

• Color-independent object detection
• Reliable operation with transparent films and glossy surfaces
• Different materials can be detected
• All types of paper are reliably detected, even in mixed combinations
• Immune to dirt

Summary

The technology of double sheet detection using ultrasonic signals has more than proven itself in the printing machinery and packaging sectors and has become indispensable in this field. Color-independent object detection and the robustness of ultrasonic technology in dusty, harsh environments are advantages that eclipse other optical, capacitive or mechanical/tactile processes.

This means that printing machine stoppages and high repair costs due to double sheet events are a thing of the past.

Ultra compact... the 18 mm ultrasonic double sheet sensor from Pepperl+Fuchs.

• Detection of no sheet, single sheets and double-sheet materials
• No “teach-in” process required for the material
• Weights from 10 g/m2 to over 2000 g/m2 can be detected
• Insensitive to printing, colors and reflective surfaces.
• Signal output via 3 PNP outputs with reverse polarity protection and short-circuit protection

The evaluation electronics are integrated in the compact receiver housing. For this reason, installation is restricted to just two elements, the transmitter (d=18 mm, l= 22 mm) and the receiver (d=18 mm, l=53 mm).

Because double sheet sensors are used in all situations where it is necessary to provide protection for high-grade machine parts and to avoid wastage, the compact UDC-18GM sensor series from Pepperl+Fuchs offers high performance at low cost.