Hello, I'm currently working with the AB MPL-B540K in conjunction with the Kinetix 6200 and the 1756-L72 and 1756-M03SE Sercos interface, specifically for a flying shear application that is designed to cut metal sheets with a setpoint of 2400 mm. We are using a high-resolution line encoder with a rating of 20,000 PPR and RS422, connected to a measuring wheel with a circumference of 500 mm. This line encoder interfaces with the auxiliary encoder input on the K6200. We have performed auto-tuning on the servo system using a damping factor of 0.8, and currently, the machine operates with cutting accuracy ranging between 1.2 mm and 2 mm. However, our customer is anticipating achieving a cutting accuracy of less than 1 mm. The high-speed operation is set for a cycle time of 2 ms with a priority level of 5, and the Sercos connection is configured for 250 microseconds. Is there anyone who can provide insights or solutions to improve our cutting accuracy to meet customer expectations? Your expertise would be greatly appreciated!
To uncover the reasons behind variability in cut length, it's essential to analyze a trend chart that displays crucial process and machine metrics. Begin by examining the reference/master speed (line encoder), cutting assembly tracking speed, cutting assembly cut speed, and the cut trigger. A question regarding the design of the application is whether electronic gearing is employed between the line reference and the cutting assembly's tracking mechanism. Alternatively, do you rely on position and velocity monitoring alongside logic-based motion commands? When troubleshooting, consider the following inquiries to gain deeper insights: Is the reference speed (line encoder) consistent or does it exhibit noise? Does the cutting assembly tracking speed reach the reference speed prior to the cut trigger? Are the cutting assembly's tracking speed and reference speed aligned, indicating effective control? Is the cut fully executed before the cutting assembly attains the end of its travel? Additionally, is the cutting assembly at its "home" position before the next cut trigger is activated? These probing questions serve as a framework for examining the nuances of your application to help clarify the cut length variability you are experiencing.
Insightful points, Mispeld, but isn't the main issue that RSLogix trends fail to update quickly enough? How rapidly can RSLogix trends refresh? I suspect that the line speed of the encoder isn’t the source of the noise. Typically, the challenges arise from insufficient resolution or quantization errors. With higher resolution, you can significantly reduce velocity and acceleration discrepancies. The motion controller ought to accurately estimate the reference or master’s position, velocity, and acceleration. While positioning is straightforward, accurately gauging velocity and acceleration presents more complexity. Relying solely on gearing for position often leads to following errors. The cutting assembly needs to synchronize with both the reference speed and position; simple gearing strategies are inadequate. The concept of returning to 'home' is outdated—it shouldn't affect performance. The primary objective is for the master and slave devices to achieve synchronization at precise positions while matching speeds. Stopping at the home position disrupts momentum and wastes time as you decelerate and then reaccelerate. It’s more efficient to maintain acceleration as you pass through zero velocity. This improved method reduces the stroke distance, saving both time and energy. The cutter typically operates within a limited travel area to perform cuts, so it’s crucial to always execute cuts within this designated range.
I'm not an expert in the detailed mechanics of motion control systems or the complexities of motion controllers that analyze every variable. However, I want to share some insights that might be valuable. Let's break down the mechanical aspects: we're observing a variation of 0.047 inches to 0.078 inches. If we consider these measurements as your upper and lower limits, then your average cut length measures at 0.063 inches. On the surface, this puts you 0.008 inches above the upper tolerance limit. A statistical analysis, such as a bell curve chart based on 100 cut lengths, would provide a clearer picture of where you truly stand. Moreover, are there any gearboxes in your motion drive system? Have the actual gear ratios been taken into account during your setup? Just because a gearbox is labeled as a 10:1 ratio doesn't necessarily reflect its real-world performance. It could be closer to a ratio of 10.25522577:1—hypothetically speaking, of course. Additionally, it’s crucial to consider whether the gearboxes are filled with the appropriate lubricant. Are you dealing with stiction issues, possibly due to oil problems? Is this a brand new installation, or is it a retrofit or upgrade? If it's the latter, then this could explain any inconsistencies you're facing. Perhaps the customer's equipment was underperforming, and they believed that adding a servo would resolve the issue. While these observations may not apply directly to your situation, they represent the first thoughts that crossed my mind regarding motion control systems.
To effectively tackle challenges of this nature, a multidisciplinary technical approach is essential. It's insufficient to focus solely on servo control. Based on my experience, the issue may stem from the measuring wheel slipping against the sheet metal. A deviation of 2mm over a 2400mm length corresponds to approximately 0.1%, which is minimal when you consider measurement precision. If the mechanical design is robust, servo accuracy should be adequate to manage this discrepancy. Instead of relying on gearboxes, implementing servos that directly drive ball screws would be a more optimal solution.
Osias1970: Please take a moment to double-check the accuracy of your measuring wheel. It should be precisely 500 mm - not even 500.02 mm or close variations. Make sure to use a diameter measurement and verify the roundness of the wheel. Are you utilizing a pinch roll that presses down against the metal sheet with an encoder attached? This method will help prevent any slippage of the tracking wheel, ensuring accurate measurements.
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