What Is The Accuracy Of The Correction Controller?

 About the accuracy of the corrector

As a supplier of coil correctors, we will be asked one of the most common questions: “What is the accuracy of your corrector?” If I give you a quick answer: eg “our accuracy is usually +/- How many millimeters or less. You should doubt my answer." Because there is not enough information in your question to determine the final accuracy of the corrector. Generally, the accuracy of the corrector is determined by three factors: the deviation of the coiled material, the accuracy of the correction system itself, and the accuracy of the installation of the corrector. Just asking the accuracy of the corrector based on the design of the correction system is like asking how fast the car can stop based on the design of the car and the tire. If we don't know the driving speed of the car and the road conditions (cement surface, gravel road, or Nisshin surface), we can't answer this question accurately. Further, the unique behavior of the coil: the positional offset of the coiled material, the lateral movement of the coil or the size of the oscillating weight are important factors in determining the accuracy of the final correction. Let us first ask a simple question: Why do we need a corrector? The answer is clear to the industry: we may have to align the edges or centerlines of the coil before the coating, printing, compounding, slitting, and winding processes. Otherwise, the lateral misalignment of the coil can result in waste or even downtime. This is why we use a corrector. Usually, the tracking method of the corrector is divided into three types: heel, heel, and heel. So how do we define the accuracy of the corrector? The corrector is usually installed upstream of the critical process and is as close as possible to the process, minimizing positional errors when entering critical processes. As a supplier of correctors, we can only focus on the position of the edge, line or center line when the coil is just coming out of the probe. Therefore, we recommend installing the corrector at the position closest to the key process, but if the end user installs what kind of machine between the corrector and the key process, or the accuracy or parallelism of the roller is bad, it affects the correction. Precision, we are beyond control. Therefore, from the perspective of the supplier of the corrector (also from the perspective of this article), the accuracy of the correcting system is defined as the positional accuracy of the web just after it exits the probe. As we all know, the driver of the corrector has a driving limit. All the correctors can correct the positional offset within a certain range, which 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 type of drive, if the position of the incoming coil is offset by more than 75 mm, the corrector will stop by moving to the extreme position and will not be able to teach the positional offset of the part exceeding 75 mm. How the lateral motion of the coil affects the accuracy of the correction is a more complicated problem. The lateral motion velocity (Vy) consists of three components:

(1) Size of lateral movement (S)

(2) Length of coil (L)

(3) Velocity of the coil (Vx) We can get the transverse movement speed (Vy) of the relationship of the other three variables:

Formula one

 Generally, the faster the lateral movement speed, the greater the difficulty of correcting the deviation. According to this formula, the length of time (tx) in which the lateral movement occurs is an important factor affecting the accuracy of the correction. When the lateral position shift occurs in a very short time, we call this position offset the instantaneous position offset. This bias is usually produced when the length of the web is short or the coil speed is high. This immediate positional offset can also be caused by material, equipment, or process variations (such as sudden changes in tension). For example, the lateral position offset due to the undesired web switching bond. Since this positional offset is instantaneous, the lateral velocity of this positional offset is infinite and thus the most challenging positional offset. The most challenging reason is that the corrector cannot have an infinite tracking speed, so the corrector has a correction delay for this instantaneous position offset. In order to improve the quality of the correction, we should try to avoid or reduce the instantaneous position offset when entering the roll. If the lateral offset (S) is always on one side of the centerline, we call it the steady state offset. This is a common bias in the unwinding process. The offset of the coil steady state is usually caused by the deviation between the web and the reel, the reel and the inflation shaft, the unwinding frame and the subsequent process centerline. In addition, during the transfer, the steady state offset may also result from non-parallel guide rolls, uneven diameter guide rolls, the bag-like nature of the web itself, or external forces such as air flow. The steady state offset has no lateral motion speed. Therefore, as long as the drive limit of the driver of the corrector is greater than the distance at which the coil is stably biased, the lateral bias of the steady state does not affect the accuracy of the correction. In addition to the instantaneous positional offset and steady state offset, the web also produces progressive lateral positional offsets for a variety of reasons: untidy or slanted edges of the web, pocketed edges The movement of the loose roller, the sliding or sticking of the coil on the roller will cause the coil to migrate; the change of working conditions of the machine or the process will also cause the progressive web position to shift. For example, changes in tension, speed, lubrication, or temperature can interfere with the mechanism of web transport, causing the web to gradually shift. In addition, the corrector may also cause the web to shift. If the control loop is not adjusted well, the blind area of the probe is too large, or the drive has a loose/bounce connection, the corrector system will cause the web to shift. There are several reasons for the loose/bounce connection of the drive: the fit of the drive connector to the frame is not tight, the axial movement of the roller bearing is slightly shifted, and the deformation of the corrector frame is made. Each type of corrector has several important mounting indicators, including: calibration width, coil winding angle, swing center position, and swing direction. If the installation requirements for these factors are ignored during installation and design, the corrector may cause coil deflection and unstable control. We will not discuss these indicators in detail here. The corrector uses a proportional feedback control loop. Obviously, the control loop consists of a coil, a probe, a controller, and a drive. The probe detects the offset of the web and sends an offset signal to the controller; the controller sends a calibration signal to the driver; the drive provides a calibration speed to push 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 typically have a higher gain (GAIN) setting for faster response times. The gain of the entire system is a function of the gain of each component. The gain of the probe (K1) is the current or voltage signal that varies with the deflection of the web; the gain of the driver (K3) is the drive rate (mm per second), and the drive rate varies with the input voltage; the gain of the controller ( K2) is to adjust the entire control loop and has the function of compensating for looseness/bounce of the connector and other non-ideal component errors. To achieve optimal system gain, the resolver supplier designs and calculates the gain of each component and system. The gain of the overall open system can be expressed in K5, K5 (system) = K1 * K2 * K3. The units of these gains are: mA/in, volts/mA, and inches/sec/volt, so we have the unit of system gain: inches/second/inch or 1/second (also known as reverse seconds). In fact, the entire open system can achieve as low as 4 reverse seconds or lower, as high as 40 reverse seconds or higher. The higher the gain of the entire system, the better the accuracy. Accuracy or calibrated offset can be obtained by dividing the positional offset velocity (Vy: obtained by Equation 1) by the system gain (Equation 2).

Accuracy Equation 2 For example, if the gain of the system is 20 reverse seconds (20/sec) and the lateral offset is 12 mm per second, the actual accuracy will be 0.6 mm. If the system gain is increased to 40 seconds, the accuracy will be 0.3 mm. When the lateral offset is 4 mm per second and the system gain is 40 inverse seconds, the correction accuracy can reach 0.1 mm. Here we assume that the system does not have any loose/bounce connections. But usually the system will have a certain degree of Song Dong / rebound. The loose/bounce of the connector creates two problems. First, it directly increases the output error of the corrector. Second, destroying the stability of the control loop forces it to reduce the gain, further reducing the accuracy of the system. For example, with a lateral offset rate of 2.5 mm per second and a system gain of 40 inverse seconds, the accuracy will be 0.0625 mm. However, if the system has loose/bounce connectors, the accuracy of the system will be greatly reduced, even exceeding 0.3 mm. Therefore, the installation of the system connector must be compact to avoid any loosening and rebound. Another notable misconception is that it is not enough speed for the drive to have a fast enough response. It also needs to have enough initial thrust to overcome the large drag when starting and pushing in the opposite direction. For a two-ton unwinding mechanism, the initial thrust required by the drive is much greater than the initial thrust of the drive of the inter-vehicle correction system. The auger-type electromechanical drive pushes loads of up to 50 tons and speeds of up to 40 mm per second, which is unmatched by hydraulic systems. The correction system also has a built-in response speed (frequency bandwidth) that can react to high frequency offsets. The frequency bandwidth of the system can be obtained by dividing the gain of the open control loop by 2p. A correction system with a gain of 40 seconds 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 size of the error in the output of the instantaneous position offset after the correction. The higher the frequency bandwidth, the faster the response to the instantaneous position offset and the smaller the offset after the calibration of the output. Now, let's go back to the original question: "What is the accuracy of your corrector?" Be confident and answer: "less than +/- 0.1 mm". Your corrector and coil need to have the following conditions:

1. The input web offset is a steady state offset.

2. The entered web offset is a graded web offset within the drive's limits.

3. The loosening/bounce of the corrector connector or the dead zone of the probe should be small enough.

4. The starting thrust of the drive should be large enough.

In addition, you have to confirm that your correction system has the following characteristics:

1. 40 gain in reverse seconds

2. The maximum lateral position shift rate is less than 4 mm/sec (the lateral offset angle is less than 6 degrees at 25 m/min line speed)

3. The frequency of the lateral cycle of any rack or coil is less than 6.

4 Hz. (The 25-degree line per minute speed, the periodic swing wavelength is greater than 76 mm) When increasing the line speed of the coil to 250 meters per minute, to achieve an accuracy of 0.1 mm, the lateral offset angle is less than 0.6 degrees, cycle The wavelength of the swing is greater than 760 mm. These conditions may sound harsh, but to achieve an accuracy of 0.1 mm or less, the coils, the corrector, and the equipment must be sufficient. A precision of 0.3 mm is usually sufficient for most crimping applications, which is what most correctors can achieve with conventional settings. This precision also greatly reduces the quality and equipment requirements of the incoming coil. In summary, if you want to buy a corrector and ask it to achieve a certain accuracy, you should not only consider the corrective ability of the corrector, but also take into account the characteristics of your coil and its on your equipment. Sports behavior. If higher precision can make your production line produce more competitive products and reduce your production waste, then you need to buy a system with a larger system gain, faster drive response speed, A correction system that may have little or no loose connectors and drive bounce.