A guided tour of its working principles, capabilities, limitations
December 11, 2007
Detecting the presence of two sheets in a press feeding operation uses scientific principles, but the process is not an exact science. A typical press-feeding situation involves an air gap between the detector and the first sheet in the stack or an air gap between the top two sheets, and these air gaps reduce the detector's effectiveness. Understanding the capabilities of the detector and the limitations imposed by imperfect conditions can help stampers set up an effective double-sheet detector system.
In the sheet processing industry, it seems that nothing is so desired, yet so elusive, as a single-probe, gap-tolerant double-sheet-detection system. Nearly everyone would like to use a double-sheet sensor that acts like a regular proximity switch—that is, one that can detect double blanks reliably from some distance, perhaps up to 25 mm from the sheet surface.
Why is it so difficult to build one? One reason is that just a few detection methods are suitable for single-probe double-sheet detection: eddy current, ultrasonic, and magnetic. The problem with these three options is that none can achieve double-sheet detection that works with gap sizes that users want.
Eddy Current. This method has some ideal properties. In general, it does not need to contact the sheet. However, its behavior and its capabilities depend on the metal; it has one set of characteristics on nonferrous materials and a different set of characteristics on ferrous materials. While eddy current can penetrate several millimeters of nonferrous materials, it has only limited penetration capability on steel. In general, 0.5 mm is about the limit. The limited penetration capability restricts its use for double-sheet detection to tin-plated stock not much thicker than 0.25 mm. Therefore, it simply is not suitable for double-sheet detection of steel blanks as used in the automotive industry.
Ultrasonic. Ultrasonic waves penetrate much deeper into metal, well beyond the thickness of automotive sheet metal stock. However, this process requires a gel or some other coupling media to provide a connection between the probe and the material surface. This factor, and the generally slow detection process, makes ultrasonic impractical for stamping applications.
Magnetic. Magnetism, of course, is suitable only for ferromagnetic materials. Magnetic sensors for double-sheet-detection systems function both with permanent magnets and electromagnets. The functional mechanism is basically the same for both and is based on the conductivity of the magnetic flux of air and ferrous materials.
The magnetic conductivity in steel is called permeability, denoted by the Greek letter , and for simplicity is expressed in relative terms as r. The relative permeability (r) of a vacuum or air is defined as 1. In contrast, mild steel has a permeability of approximately 2,000. This means that steel is about 2,000 times more conductive to magnetic flux than is air. The permeabilities of some metal alloys exceed 5,000.
Flux lines flow from the north to the south pole or vice versa (see Figure 1a). The majority of the flux lines—also known as primary flux—travel through the air from pole to pole. The flux lines search for the shortest distance between the poles when traveling through air. A much smaller secondary flux takes a short cut and travels radially from pole to pole close to the sensor surface. This principle is the same for permanent magnets and electromagnets.
Figure 1b depicts a ferrous metal sheet attached to the sensor surface. The flux lines search for the path of least resistance by traveling through the steel sheet, which is much more conductive than air. If the magnetic sensor is strong and saturates the sheet with flux, some flux leaks out of the sheet and travels through the air. This is known as leaking flux.
The magnitudes of the primary and secondary flux have changed. The amount of primary flux has increased and the secondary flux has decreased (because more flux is traveling through the metal sheet). These changes in the secondary flux can be measured with magnetic devices and therefore differentiate between one and two sheets, as shown in Figure 1c. Adding a second sheet reduces the secondary flux even further.
Figures 1b and 1c represent ideal situations in which the sheets are firmly attached to the sensor and air gaps do not exist. For situations like these, it is relatively easy to develop double-sheet detectors that function reliably.
Stacked Sheets Without Air Gaps
It is difficult to develop reliable double-sheet detectors for the real world. In stamping operations, destacking blanks is not a simple operation. In the industrial environment, blanks seldom lie perfectly flat on top of each other. Many factors introduce gaps between the workpieces: Some sheets are not flat; some have stamping burrs; and some materials, such as tailor welded blanks, have two or more sheet thicknesses. The challenge, which is substantial, is to develop sensors that function reliably in such situations.
Figure 2a shows a gap between the sensor and the first sheet, also called a first air gap. In this situation, the system's performance degrades severely because a portion of the magnetic flux takes a short cut through the air and does not travel through the sheets. Thus, double-sheet detection is impaired if the sensor system is not sufficiently strong to bridge the air gap.
Figure 2b shows a different situation: a gap between the two sheets, also known as a second air gap. The path of the primary flux lines (and the corresponding reduction in secondary flux) between the first and second sheet indicates degraded performance in some unpredictable manner.
The worst situation has two air gaps (see Figure 2c). This situation contains too many variables to be reliable. If not rectified immediately, this situation, in all likelihood, will result in a catastrophic double-sheet loading.
Stacked Sheets With Air Gaps
No existing technology can monitor the second air gap reliably. However, with regard to the first air gap, some of today's sensors can determine its presence and measure its size, expressing the gap size as a percentage of the sheet's thickness. If the sensor's switching threshold is set to 80 percent, or better yet 90 percent, and the actual value drops below this threshold, this unit generates an undergauge signal and sends it to the programmable logic controller PLC. Unfortunately, some machine builders and system integrators strive to conserve programming efforts and neglect to monitor for undergauge material. Others have found that this extra effort pays for itself many times over in making the double-sheet detector more reliable.
First Air Gap
Advances in sensor materials, coil technology, and digital signal analysis have improved the performance of magnetic, single-probe double-sheet-detection systems, especially in the area of gap tolerance. Figures 3a and 3b show the gap-detection capabilities of three sensor sizes. Don't let this information be misleading: Gap tolerance is not a design parameter.
Second Air Gap
Figure 3a corresponds to Figure 2a, which depicts the first air gap between the sensor and sheet surface. It shows that sensor size has considerable influence on the tolerable first gap. The graph shows that when this 42-mm sensor is processing 2-mm sheet thickness, a double-sheet threshold of 120 percent results in a gap tolerance of 0.5 mm. Note that while the biggest sensor has the biggest air gap tolerance, it also has the disadvantage of being heaviest and slowest, corresponding to the generally thicker material that it is used for.
Figure 3b corresponds to Figure 2b and shows that the second air gap exhibits behavior similar to that in Figure 3a. In this example, a 42-mm sensor, 2-mm sheet thickness, and a double-sheet threshold of 120 percent results in a gap tolerance of up to 3 mm. In this case, similar to the previous case, a larger sensor provides more gap tolerance.
Of course, the air gaps in Figures 3a and 3b cannot be added. Figure 4 shows a combined gap resulting from the first and secondary gap for three sensor sizes. Only two sensors, the 42-mm and 75-mm sizes, provide meaningful output.
Figure 4. Just two sensor sizes, 42 mm and 75 mm sizes, provide meaningful output.
Just two sensor sizes, 42 mm and 75 mm, provide meaningful output.
Gap tolerance is that extra amount of sensor performance that should prevent catastrophic events if destacking operations are not optimal.
Substituting new sensors and control units can cut detection time by 50 percent.
Automotive stamping press builders continuously strive to increase press speed and sheet processing capability. Large transfer presses can run at more than 25 strokes per minute when processing automotive outer body panels. These transfer presses are capable of processing not just one blank at a time, but up to four in parallel. Double-sheet-detector systems have to follow this trend. The control units must connect multiple sensors and reduce the systems' reaction time to detect a double sheet so it can do so within the cycle time of each stroke.
Figure 5 shows the reduction in system reaction time after changing from an old 42-mm sensor connected to an old control unit (old system) to a new 42-mm sensor and a new control unit (new system). This change cuts the reaction time by 50 percent. In addition, instead of connecting one sensor to one control unit, it is now possible to connect up to four sensors directly to the same control unit and up to six sensors via a sensor switch box. Two operating modes are available: individual mode or sequencer mode. The sequencer mode initiates a measurement by the first sensor and a sequential measurement of the other connected sensors in rapid succession. The total measurement time is short enough that it fits into the cycle time of most applications.
Figure 5 shows the reaction time reduction of both the 42-mm-diameter and 75-mm-diameter sensors.
The reduction of the system's reaction time for the 75-mm sensor is of specific significance for thicker material. Blanks of up to 6.5 mm can be measured in 450 milliseconds (ms), reducing the cycle times for these thick blanks.
Sound mechanical design of sensor brackets and vacuum cups is a prerequisite for reliable double-sheet detection. Statistical evidence reveals that the majority of problems that occur with double-sheet-detection systems are due to worn-out mechanics or inferior mechanical design.