Not only can double sheet control be used in printing machinery to avoid costly machinery downtime, but also in the processing of foils and counting of labels or receipts, where it is important to spot paper or foil sheets which may, unintentionally, lie on top of each other.
Capacitive or optical systems, which are currently used for this purpose, cannot always guarantee the required functional reliability.
Capacitive sensors, for example, are sensitive to changes in ambient conditions such as humidity and temperature. They are, on principle, unsuitable for metal-coated foils. Optical methods, on the other hand, are easily confused by glossy or transparent surfaces and printed materials because, depending on the print pattern, translucency at the scanning point can vary considerable.
Reliable reference values cannot be preset even for different batches of the same paper type. What is more, optical systems are susceptible to dirt and dust contamination. Air gap as treacherous ‘sound barrier’.
Visolux Elektronik has now developed a double sheet control which uses ultrasound and which, it claims, works much more reliably than other non-contact methods. The double sheet control is based on the use of a transmitter, receiver and analyser unit.
The ultrasound signal goes through the object under test and the system measures the attenuation of the signal. If two sheets of paper lie on top of each other, the sound is reflected back and forth several times in the air gap between the sheets and then deteriorates.
The signal damping is therefore greater with double sheets than with a single sheet so that reliable identification is ensured. Paper types of 30g/m2 right up to card strengths of 1200g/m2 can be processed using the technique.
A transmitter and receiver are perfectly tuned to each other. They work on a narrow band range at frequencies of 180 kHz; the opening angle is 10°. A microprocessor controlled analyser unit calculates the result within a response time of 5ms and generates a corresponding switching signal. The short-circuit resistant PNP switching output switches currents up to 200mA.
If ambient conditions change, the automatic switching threshold tracking makes re-adjustment unnecessary, a feature which other systems do not have. Even where space is restricted, a suitable place can always be found to install the analyser unit, which is only 64mm wide, 98mm high and 37mm deep. LEDs signal the current switching conditions.
Transmitters and receivers are housed in cylindrical casings, size M18. In order to avoid mis-evaluations due to stationary waves, transmitter and receiver are mounted at an angle of 20…45° to the midperpendicular of the paper surface. The system complies with protection standard IP65, the working temperature ranges from 0…50°C.
Apart from the general detection of air, single and double sheets in paper processing, other applications include any process where automatic distinction of single and double sheets is required in order to minimise machinery downtime or to avoid waste, e.g. in web offset printing, packaging, folding, labelling, billing and envelope inserting machines.