As the supplier of the web corrector we will be asked one of the most common questions: "What is the accuracy of your corrector?" If I give you a quick answer: "Our precision is usually in the range of +/- How many millimeters or less. "You should doubt my answer was drawn. Because there is not enough information available in your topic to determine the ultimate accuracy of the corrective device.

　In general, the accuracy of the deflector is determined by three factors: the deflection of the web, the accuracy of the system itself, and the accuracy of the corrector. Just as the corrective system is designed, ask the correctness of the corrector as if it were only for a short period of time, depending on the design of the car and the tire. If we do not know the speed of the car to stop the road ahead and road conditions (cement surface, gravel road, or Nepal clear surface), we are not able to answer this question correctly. Further, the unique behavior of the web: the positional bias of the web, the lateral movement of the web, or the magnitude of the wobble are important factors in determining the accuracy of the final correction.

　Let us first ask a simple question: Why do we need corrector? The answer is clear to the industry: we may have to align the edges or centerlines of the web before coating, printing, laminating, slitting, and winding, otherwise the lateral misalignment of the web will cause waste or even downtime. This is why we use the corrective device. There are three ways to follow the corrective device: follow side, center line and line.

　So how do we define the accuracy of the corrective device? The corrective device is usually installed upstream of the critical process, and the closer to the process the better, so as to minimize the key process into the position error. As a supplier of correctors, we can only focus on the position of the edge of the web as it comes out of the probe. Therefore, it is recommended that the corrective device be installed at the position closest to the critical process. However, if the end user installs any machines between the corrective device and the critical process, or if the accuracy or parallelism of the rollers is not good, Accuracy, we can not control. Therefore, from the point of view of the corrector supplier (and also from the standpoint of this article), the accuracy of the correction system is defined as the positional accuracy of the web as it emerges from the probe.

　As we all know, corrective drive is a driver of the limit. All correctors can correct for a range of positional deviations, and this range must be less than the drive limit of the drive. The limits of the drive can be adjusted according to the needs of the user. Most drives have a drive limit of plus or minus 75 mm. For this drive, if the position of the web is more than 75 mm, the deflector will stop by moving to the limit position, and the position offset beyond 75 mm can not be taught.

　How the lateral movement of the coil affects the accuracy of rectification is a more complex subject. The transverse velocity (Vy) consists of three components: (1) the size of the traverse (S) (2) the length of the web (L) (3) the velocity of the web (Vx) ) The relationship between the other three variables: Formula One.

　In general, the faster the lateral movement is, the more difficult it is to rectify the deviation. According to this formula, the time that lateral movement occurs is an important factor affecting the accuracy of rectification.

　This positional offset is referred to as the instantaneous position offset when the lateral position shift occurs in a very short time. This bias is usually generated when the length of the web is short or the web speed is high. This instantaneous positional bias can also be caused by material, equipment, or process variations such as abrupt changes in tension. Such as the lateral positional offset that occurs due to the undesired roll-to-roll bonding. Since this positional offset is immediate, the lateral velocity of such positional displacements is infinite, and thus the most challenging positional offset. The most challenging is that the corrective device can not have an infinite tracking speed, and thus, for this real-time positional bias, the corrective device will have a correction lag. In order to improve the quality of correction, we must try to avoid or reduce the volume when the real-time position bias.

　If the lateral offset (S) is always on one side of the center line, we call it the steady-state bias. This is a common offset during unwinding. The steady state deflection of the coil is usually caused by the deviation of the web from the reel, the platen and the plenum, the unwinding frame and the subsequent process centerline. In addition, the steady-state bias during transmission can also be caused by non-parallel guide rollers, guide rolls of uneven diameter, bag-like characteristics of the web itself, or external forces such as air flow. The steady-state bias has no lateral velocity. Therefore, the lateral bias in the steady state does not affect the accuracy of rectification as long as the drive limit of the actuator of the deflector is greater than the spacing of the stable bias of the web.

　In addition to the offset between the immediate position and the steady state, the web also has a progressive lateral position offset that can occur for a variety of reasons: the unclean or slanted edges of the web, , The movement of loose rollers, the slippage or cohesion of the web on the rollers can cause the web to wander; the machine or process operating conditions can also cause progressive roll position deviations. Changes in tension, speed, lubrication, or temperature, for example, can interfere with the transport mechanism of the web, causing gradual deflection of the web.

　In addition, the deflector may also cause web deflection. If the control loop is not adjusted properly, the probe's dead zone is too large, or the driver has a loose connection / rebound phenomenon, then the deflection system will cause the deflection of the coil. One of the reasons for the drive connection loose / rebound for a variety of reasons: the driver connector and the rack with the tight, roller bearing axial micro-shift, corrector frame deformation and so on. Each corrector has a number of important installation indicators, including: calibration width, coil winding angle, the location of the swing center and the direction of swing and so on. If the installation and design of these factors ignored the installation requirements, corrective device may cause the coil offset and unstable control. Here we do not discuss these indicators.

　The corrective control uses a proportional feedback control loop. Obviously, the control loop consists of a coil, a probe, a controller, and a driver. The probe detects the deflection of the web and sends an offset signal to the controller. The controller then sends a calibration signal to the driver. The driver provides a calibration speed to move the web in the opposite direction. The speed at which the drive moves will be proportional to the offset signal detected by the probe.

　Sensitive correctors usually have a high gain (GAIN) setting, which has a faster response time. The gain of the overall system is a function of the gain of each component.

　The gain (K1) of the probe is a current or voltage signal that varies with the web offset; the gain (K3) of the driver is the drive rate (milliseconds per second), the drive rate varies with the input voltage; (K2) adjusts the entire control loop and compensates for loosening / rebounding of the connector and other non-ideal components. In order to optimize the system gain, corrector suppliers to design and calculation of each component and the system gain.

　The overall open system gain can be expressed as Ks, Ks (system) = K1 * K2 * K3. These gains are in milliamps per inch, volts per milliampere, and inches per second per volt, so that we have the system gain in inches / sec / inch or 1 / second (also called inverse seconds). In fact, the entire open system can be as low as 4 seconds or less, up to 40 seconds or more. The higher the gain of the overall system, the better the accuracy. Accuracy or offset after calibration can be obtained by dividing the position offset speed (Vy: obtained from Equation 1) by the system gain (Equation 2).

　For example, if the gain of the system is 20 inverse seconds (20 / sec) and the lateral offset is 12 mm per second, the actual accuracy would be 0.6 mm. If the system gain increased to 40 seconds, the accuracy will be 0.3 mm. When the lateral offset rate of 4 mm per second, the system gain of 40 inverse seconds, the correction accuracy can reach 0.1 mm.

　Here we assume that the system does not have any connector loose / rebound. But usually the system will have a certain degree of Song Dong / rebound. The connector loose / rebound will have two problems, first, directly increase the corrector output error. Second, destroying the control loop stability, forcing it to reduce the gain, further reducing the system's correctness. For example, at a lateral offset rate of 2.5 mm per second and a system gain of 40 ns, the accuracy would be 0.0625 mm. However, if the system has a loose connector / rebound, the accuracy of the system will be greatly reduced, even more than 0.3 mm. Therefore, the installation of the system connection must be compact, to avoid any loosening and rebound.

　Another noteworthy error is that it is not enough for the drive to have a fast enough response and that it needs to have a large enough starting force to overcome the large drag at the start and reverse pulls. For a two-ton rewinding mechanism, the initial drive force required by the drive is much greater than the initial driving force of the actuator in the travel rectification system. Spiral push rod electromechanical actuators can push loads up to 50 tons, and 40 mm per second, which is unmatched by hydraulic systems.

　The correction system also has a built-in response speed (frequency bandwidth) that is capable of reacting to high frequency offsets. The frequency bandwidth of the system can be obtained by dividing the gain of the open control loop by 2p. An error correction system with a gain of 40 secs has a bandwidth of approximately 6.4 Hz, so the system can correct progressive web offsets below 6.4 Hz. The frequency bandwidth also determines the magnitude of the error in the output after the offset has been corrected. The greater the frequency bandwidth, the faster the response to the instantaneous position offset and the smaller the offset after the output calibration.

　Now, let's go back to the original question: "What is the accuracy of your corrector?" To be able to confidently answer: "less than +/- 0.1 mm." Your corrector and coil need to have the following conditions:
　1) The input web deflection is the offset of the steady state.
　2) The entered web offset is a gradual web offset within the drive limits.
　3) The loose / rebound of the deflector connector or the blind spot of the probe should be small enough.
　4) The initial driving force of the drive should be large enough.

　In addition, you have to make sure your correction system has the following characteristics:
　1) 40 inverse seconds gain
　2) The maximum lateral position offset rate is less than 4 mm / s (25 m per minute linear velocity, the lateral deviation angle is less than 6 degrees)
　3) The frequency of the lateral cyclical oscillation of any rack or coil is less than 6.4 Hz. (25 meters per minute speed, the cycle of oscillation wavelength is greater than 76 mm)
　When the linear speed of the web is increased to 250 m / min, to achieve a precision of 0.1 mm, the lateral deflection angle is less than 0.6 ° and the wavelength of the periodic oscillation is greater than 760 mm.

　These conditions may sound harsh, but to achieve accuracy of 0.1 mm or less, the web, the deflector, the device must meet the above requirements. Typically 0.3 mm accuracy will satisfy most of the curling applications, which is the most corrective device in the conventional settings can be achieved in the case of accuracy. The accuracy of the roll on the quality and equipment requirements are also greatly reduced.

　To sum up, if you want to buy a corrective device and ask it to achieve a certain degree of correctness, you must not only take into account corrective corrective ability, but also to take into account the characteristics of your coil and your device Sports behavior. If the higher precision can make your production line to produce more competitive products, while reducing your production waste, then you need to buy a system with a larger gain, faster response speed of the drive May be small or no connector loosening and drive rebound correction system.