Neutralising ‘zebras’ by means of True DC sensor technology
Stuart Grantham explains how Hildebrand Technology’s Ioncontrol Fusion incorporating True DC sensor technology eliminates the zebra effect on materials
As the demand for more efficient machinery running at higher process speeds increases, the Achilles heel of conventional static neutralising equipment will start to become more apparent. The faster a machine runs, the more material passes under a static neutralising bar therefore the operating efficiency of the static neutralising equipment needs to increase to compensate for the surface charge moving past at a higher speed. Until now customers have primarily had a choice between two different types of neutralising technology: AC and Pulsed DC. In our experience the use of radioactive isotope bars has considerably declined over the past few years although they are still in limited use with some companies.
Both AC and Pulsed DC static neutralising systems operate at a given Hz frequency. AC is usually 50 or 60 Hz; DC can be adjusted but 20 Hz is popular with several manufacturers in the UK market. When visualised AC is a sine wave and DC is an instant sharp pulse. A Pulsed DC system operating at 20 Hz has 20 positive and 20 negative outputs per second.
If the material has a positive charge it will only require negative ions to neutralise it (the opposite applies for a negatively charged substrate). When the static neutralising bar emits positive ions they will be repelled. This means only 50% of the systems output is of use to neutralise the target substrate.
Due to AC being a sine wave a 50 Hz AC static neutralising system takes longer to build to peak voltage output. AC still produces 50 positive and 50 negative outputs per second but between each output the system has to cross the initial ionising/corona voltage points and a zero volt pass point. You also have the same scenario as for Pulsed DC where only 50% of the output is of use to neutralise the target material.
For example, we have a machine with an operating speed of 750m/min. Per second there are 12.5 metres or 12500mm of material passing under the bar (750 (m/min) divided by 60 (seconds in a minute) = 12.5m/12500mm).
If the static neutralising system is operating at 20 Hz Pulsed DC and the surface charge is negative the material will require positive ions to neutralise it. There will be a 312.5mm section of material travel past the bar between the end of the first positive output pulse where neutralisation of charge can take place and the start of the next positive output pulse (12500mm divided by 40 outputs (20 + & 20 -) = 312.5mm). This repeat will be constant throughout the whole of the reel while running at this speed.
The ‘zebra’ effect
This characteristic is commonly known as the ‘zebra’ effect because the material will have striped sections where the static charge has been neutralised and where it has not. The faster the process speed the worse the problem becomes. At 1000m/min 20Hz output frequency would see stripes of 416mm of material remaining charged, 1500m/min would see 625mm of material remaining charged. Leading slitter rewinder manufacturers would consider 1500m/min to be classed as a ‘normal’ operating speed for a primary winder today. A 1000m/min operating speed can be reached by some of the latest secondary slitter rewinders.
Our example speed for this article has been 750m/min however the Zebra effect is present at both higher and lower speeds. The same machine utilising 20Hz Pulsed DC static neutralising equipment running at 375m/min would have 156mm wide stripes.
Hildebrand Technology manufactures Ionstream Fusion which is a Pulsed DC based system operating at up to 100Hz frequency. Excellent neutralisation can be achieved at both low and high process speeds but even when operating at 100Hz you will still reach a stage when even a high pulse frequency is not enough and the zebra effect starts to become an issue again.
True DC sensor technology
To solve the ‘zebra’ effect Hildebrand Technology developed Ioncontrol Fusion (patent pending) which incorporates True DC sensor technology. Available in the UK from Webtek UK, the static neutralising bar incorporates three rows of tungsten carbide pins. The first is the sensor, the second and third are positive and negative output pins.
When Ioncontrol Fusion is first switched on it initially runs in 100Hz pulsed DC mode so static charge neutralisation is instant from the start of the run.
During this initial set-up period the True DC sensor technology looks at the difference in current between the original electrode current curve stored in its memory and the actual current curve when sampling the electrostatic field on the target substrate.
This current and its saturation behaviour is the key to knowing which polarity and value the charge on the substrate has. The system will then operate in the optimum mode for static neutralisation to be achieved. This can either be Pulsed DC, DCDC or at higher speeds and greater static charge levels, True DC. When operating in True DC mode the system switches off the same polarity as the target substrate and switches on the opposite polarity.
The effect is the same as turning on a water tap. The ions are generated in a constant stream without interruption. There is no zero volt pass point, pulse pause or same polarity output as the material – all the disadvantages present with any AC or Pulsed DC systems; 100% neutralisation of the target substrate is achieved.
If a polarity change is sensed when operating in True DC mode the system without operator intervention switches back to 100 Hz Pulsed DC to determine again the best mode for substrate neutralisation.
Ioncontrol Fusion and Ionstream Fusion can be fully integrated into production machines. Control and visualisation of the neutralising process and individual neutraliser’s performance can be viewed via our touch screen operator interface or our gateway for inclusion into the customer’s digital network. Electrode contamination and emitter pin status in addition to neutralising efficiency are continuously monitored and visualised in real time.
All fusion static neutralising and static charging products have their high voltage generator and firmware embedded within the static bar profile. Fusion products only require a 24V DC power supply and have no external high voltage components.
Key advantages of Ioncontrol Fusion include: embedded microcontroller system; 24V DC power supply; high voltage generator encapsulated within the neutralising bar profile; embedded expert firmware; communication with most common industrial fieldbuses via one gateway; visualisation via 10” touchscreen display; and TQM with data logging of individual production runs.
T: +44 (0)1278 765877
T: +41 (0)41 783 8999