July 13, 2004
Almost everyone has experienced a double-sheet problem. Copiers, printers, and fax machines sometimes feed two sheets, which then jam the machines and have to be removed.
While two sheets of paper seldom cause major harm to office equipment, two sheets of metal can cause major damage to tools and dies and to the processing machinery itself. In addition, double sheets can result in lost productivity, especially important in the competitive automotive industry.
Four common strategies for removing double sheet include oscillation, bending, and dropping followed by lifting.
Because high productivity is a goal for all stampers, the prevention, detection, and removal of double sheet—automatically, if possible—are very important, but they also pose major challenges.
Prevention. The primary methods for preventing double sheet are fanning magnets, air knives, and mechanical sheet peeling.
Fanning with separator magnets is one of the oldest methods of preventing double sheet. Placing strong magnets on one or two sides of a stack of blanks causes the sheets to develop opposite magnetic poles, so they float on top of each other. While this method makes it easier for operators to pick up sheets in manual operations, it works only with magnetic material.
With the steadily increasing use of aluminum in automotive blank production, other methods for preventing double sheet are required. One common method is the air knife, which uses one or two nozzles to blow air into the stack to separate the sheets. Another method is to lift the sheet on one end while holding down the main body of the blank stack, thus peeling one sheet off another.
Detection. When a prevention method fails, a stamper needs to be able to detect the double sheet. Optics, lasers, vision systems, ultrasonics, and weight measurement techniques sometimes are used in sheet metal processing to detect double sheet, but they usually are confined to specialized applications with custom-designed machinery.
By far the most commonly used methods in the stamping industry are based on the magnetic, inductive, and eddy-current principles. Sensors can be single-sided or dual-head styles.
Removal. What can be done about double sheet after it's detected? Since double blanks generally are perfectly good sheets, the best thing to do is remove the second sheet and process it again. Figure 1 shows some common strategies. Another method is to use a destacker or robot to remove the double blank automatically from the process and move it to a double-sheet bin. This also can be done manually in simpler machines.
Double sheets detected by a dual-head sensor system (see Figure 2) sometimes can be transported backward for removal from the feeder. In other designs, the sheets are moved forward and then dropped into a double-sheet bin.
Double sheets detected by a dual-head sensor system sometimes can be transported backward for removal from the feeder.
The single-sided method performed by magnetic or eddy current sensors always involves contact measurement. The sensors should sit perpendicular on the sheets and fully touch the material.
Since the measurement principle is based on the law of magnetism, the sheet's magnetic properties and the environment influence measurement. Magnetic conductivity is expressed by the relative permeability r. Air has a r equal to 1, and regular steel has a r of 1,000.
Based on these physical properties, it is immediately apparent that undefined air gaps have to be avoided under all circumstances. All single-sided sensors can tolerate some air gaps, but the tolerance amount varies according to circumstances. Therefore, air gaps should not be a design parameter or an excuse for sloppy mechanics; when uncontrolled, they will reduce sensor performance.
Typical fault conditions often are overlooked because of the difficulty of observing these mechanical problems in a real production environment.
Covering the sensor face with Teflon® or similar nonmagnetic materials is a possible, but not always a practical, solution to the air gap problem. Depending on the thickness of the material, this cover can reduce the measurement range substantially and thereby reduce the maximum thickness of the sheets to be controlled.
Single-sided sensors can use either the permanent magnetic principle or the electromagnetic principle. Because of its pulsing action, an electromagnetic measurement system is more sensitive to air gaps than permanent magnetic measurement.
Permanent magnetic sensors typically are used for thin sheets. The magnetic force of the permanent magnet tends to move the sensor into a perpendicular position to the sheet, thereby reducing the probability of unwanted air gaps. Sensors with rare-earth permanent magnets allow small sensor bodies but have strong magnetic forces. They are suitable for applications such as hidden-parts detection in automotive welding and assembly operations.
For aluminum blanks, the eddy-current measurement principle for single-sided double-sheet control is not as sensitive to air gaps as magnetic types. Air gaps smaller than 1 mm between the sensor and first sheet generally can be tolerated, depending on the sheet thickness and the electrical conductivity of the material. Therefore, in some cases, testing may be necessary.
Most sensors are designed with self-checking features, as well as features that signal unreliable operating conditions. The most important one is the detection of unwanted air gaps between the sensor and sheet surface.
For this purpose, the user should analyze the under-gauge switching threshold (UT) and set it larger than 80 percent. An air gap reduces the measured value. The same applies to partial gaps, such as in the cases of tilted sensors or bowed sheets. If measured value drops below the threshold, an under-gauge condition is signaled at the 0-Sheet output. When that happens, it is highly recommended to have a programmable logic controller issue a fault signal and stop the process. Only after elimination of this fault condition should the process continue.
Unfortunately, some programmable logic controller programmers want to save themselves this extra work and don't use this important feature. Thus, unreliable operating conditions may go undetected, causing double sheet to enter the machinery.
Figure 3 shows typical fault conditions that often are overlooked because of the difficulty of observing these mechanical problems in a real production environment. More than 90 percent of all failures are mechanical in nature. Because of the many self-checking features built into a sensor's electronic circuitry and software, electronic system components seldom are the cause of a failure to detect double sheet.
If single-sided measurement does not lead to satisfactory results, then the use of an additional dual-head pass-through system should be considered—especially at a mounting position right in front of the tooling.
In a dual-head sensor system, the transmitter generates an electromagnetic field, which in turn generates eddy currents in the sheet to be monitored, based on the sheet thickness and material type. The receiver measures the electromagnetic field generated in the material. The sensors are unshielded, so the electromagnetic field also is horizontally active.
Dual-head measurement can be made on-the-fly or stationary. The sheets can move freely within the sensor gap, and both steel and nonferrous sheets can be monitored with the same sensor system.
The most important parameters regarding the selection of appropriate dual-head sensors are:
Sensor diagrams describe a sensor pair's performance range. Areas in this diagram are denoted with Fe for ferrous material and Al for nonferrous material. Aluminum with a conductivity value of 25 millisiemens has been selected as being representative for nonferrous material.
Sensor diagrams (see Figure 4) describe a sensor pair's performance range. Areas in the diagram are denoted with Fe for ferrous material and Al for nonferrous material. Aluminum with a conductivity value of 25 millisiemens (mS) has been selected as being representative for nonferrous material. This type of aluminum is considered typical of automotive sheet metal.
However, the conductivity value of nonferrous material (as expressed in millisiemens) can vary over a wide range, as the following examples show:
As a rule, the permissible sensor gaps get smaller and controllable material thickness gets thinner if the value of electrical conductivity of the material increases.
Because of the many possible combinations of conductivity and thickness, testing a pair of sensors on the materials to be controlled may be required to determine the pair's performance value. Figure 4 shows the air gaps between the sensors through which the sheet can move vertically without creating a nuisance trip output signal.
Assume that 0.7-mm-thick steel has to be controlled for double sheet. The selected sensor pair has an outside diameter of 54 mm. In Figure 4, the vertical line cuts the field for ferrous material at a sensor distance of 10 mm on the minimum side and at 80 mm at the upper side. This means that the minimal distance between the sensors (Ax) can be 10 mm, up to a maximum of 80 mm.
Therefore, if properly calibrated, the chosen pair of sensors could provide reliable double-sheet control of material with a minimal size of three times the sensor diameter. This means that single or double sheet can be at any point in the sensor gap (flutter, vibrate) and still lead to reliable results.
For a sheet of 0.7-mm aluminum, the selection or sizing is done the same way, leading to a minimum between-sensor distance of about 30 mm and a maximum distance of about 105 mm.
Correct mounting is essential for these double-sheet sensors to function reliably.
Mounting a Single-sided Sensor. A flexible sensor bracket should be used for mounting instead of rigid assemblies. The bracket allows the sensor to sit perpendicular on the sheet, even if the assemblies are not completely aligned. However, horizontal cable pull should be avoided because it reduces the positive effect of springs.
In addition, the mounting should ensure that the springs cannot get stuck. Otherwise, the sensor may be presented tilted to the sheet. Ideally, the flexible sensor bracket should be mounted in such a way that the sensor is presented with pre-tension to the sheet. This ensures stable positioning of the sensor on the sheet even under vibrating conditions.
A flat suction cup can be used with the flexible sensor bracket, which results in a tilt moment to ensure the sensor is presented perpendicular to the sheet. However, a sufficient vacuum must be created with the suction cup; Teflon tape on the thread of the sensor can aid air tightness for proper functioning.
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The suction cup should not be used as a lifting device. Otherwise, the rubber lips may separate from the sheet, causing a gap between the sensor and sheet. In addition, the sensor should be mounted into the suction cup so that the sensor surface is aligned properly to the inside rubber pad with the nipples.
The mechanical presentation of the sensor to the sheet is the most important factor for reliable double-sheet control. In case of a malfunction, the user should analyze the mechanical system first. Comparing the actual situation with the typical faults shown in Figure 3 can be a good starting point, as usually one or more probably are present when a malfunction occurs.
Sometimes it is not apparent that the sensor does not touch the sheet because the sensor is mounted in a vacuum cup and the air gap is not immediately visible. The check list in Figure 5 gives additional hints regarding possible malfunctions.
If the machine cycle time and the machine design allow it, redundant measurements on the same part at various places should be considered. Some double-sheet detector systems allow the direct connection of up to four sensors, or more with a sensor switch box. This allows monitoring with several sensors in various places within a production cycle.
When larger sensors are used, the permissible size of air gaps increases.
Another measure to improve reliability is the use of a bigger sensor. Figure 6 shows the permissible air gaps in relation to sensor size and sheet thickness. Bigger sensors are in a position to bridge bigger air gaps between the first and second sheet.
Mounting a Dual-head Sensor. Sensors should be mounted flush in sturdy plastic brackets. If the brackets are made of material with high conductivity, such as aluminum, the sensor bracket absorbs a lot of energy from the sensors, substantially reducing the sensors' performance.
For monitoring of steel, the sensors should stick out of the bracket at least 10 mm. Additional protection shields may be required to prevent the sensors from being damaged by transferred sheets. Recessed mounting in the brackets is not recommended, because chips and dirt can accumulate in the cavities and reduce the system's performance.
Sensors should be mounted so that the measurement target is at least three times the sensor diameter. If this is not possible, then the sheet should be placed repeatably into the same position for double-sheet control. During the teach-in process, the sheet should be in the identical position horizontally and vertically as in the actual production process.
For magnetic conveyor use, mounting of the sensors in this way generally results in reliable double-sheet control.
In magnetic conveyors, the holding magnets generate primary and secondary flux through the sheet. The magnetized sheets generate an induction voltage in the receiver when the sheet moves in between the sensors. This blinds the system for about 60 milliseconds until the induction voltage decreases to 0.
To combat this problem, special sensors have been developed for use in magnetic conveyors. They show an increased immunity to the specific noise generated by the conveyors, although the noise sometimes can be too strong for the use of these sensors alone. In that case, it's especially important to meet requirements for having the sheets cover the sensor fully.
The magnetic field can cause a large voltage spike when the material enters and exits the sensor gap. This can generate a double-sheet signal even if no double sheet is present. In this case, the programmable logic controller should issue a "measurement start" signal when the sheet already is within the sensor gap. Because the controller cannot exactly locate the sheet during transport, it is best to use entry and exit proximity switches for the precise determination of the sheet position.
Another solution for mounting in magnetic conveyors is to use specific double-sheet detector software. For instance, some new dual-head control units have a special correction function for magnetic noise generators.
Mounting of the sensors according to Figure 7 generally results in reliable double-sheet control. The holding magnets should be mounted so that no secondary magnetic flux results between the conveyor belts, either through the air (and through the sensor) or through the sheet between the sensors.
The sensors should not be mounted close to the magnets but in the largest possible distance between the rails. The brackets should be made of plastic or steel to help ensure the more effective sensor performance.
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