Optimizing Proportional Valve Control in Aluminum Saw Setup for Smooth Feed Operation

Question:

Hello everyone! I am currently facing a challenge with a proportional valve on a PID loop in an aluminum saw setup. The Process Variable (PV) in this case represents the amps at the saw blade, while the Controlled Variable (CV) is the proportional valve that regulates the feed rate. The issue arises during the initial movement of the blade, where the amp draw is minimal before it makes contact with the metal. This results in the feed operating at maximum output, set at a limit of 50% within the PID. Once the blade hits the metal, it accelerates at full speed, causing a sudden surge in current and damaging the saw blades. To address this problem, I have implemented a ramp down timer to gradually control the valve output, starting quickly and slowing down as it approaches the metal. Once the current surpasses the idle level, the PID takes over the control. However, a new challenge has emerged where the PID's output remains at 50% when it transitions, leading to a sudden increase in blade load before readjusting. Is there a way to configure the PID to start at a lower output, such as 20%, and then gradually increase if necessary to meet the cycle time and blade load requirements? This adjustment aims to prevent the initial spike in current when the PID assumes control. Please refer to the attached screenshot for a visual representation of the issue.

Top Replies

What is the target amperage value setpoint? Are there any other significant events captured on the trend, such as when the saw first makes contact with the material? What is the frequency at which the amperage is sampled? What is the loop update time configured for the PID controller? How is it ensured that the PID controller runs at the specified loop update time? Which brand and model of PLC is being used in this scenario? What specific PID instruction is being utilized in this setup?

Are you anticipating a linear correlation between feed speed and amperage in the system gain?

Thank you, Dr. Bitboy, for your assistance. The equipment is controlled by an Allen Bradley PLC5/60 processor utilizing a general PID control block. The target amp value is set at 2375. The only indication of the saw making contact with metal is the increase in amps, as there is no position feedback from the saw stroke. The amp measurement is sampled every 12.5 milliseconds, with the PID loop updating every 0.01 seconds. It is important to ensure that the PID executes at the loop update time, but there is currently no check in place for this. The PID control block runs continuously with no interruptions, but without a verification mechanism. Additionally, it is expected that there will be a linear relationship between speed and amperage, with amperage increasing as speed increases.

During each scan, the SP value is checked against the SP_limit. If SP is less than SP_limit, the SP value is incremented by ramp_gain multiplied by scan_time. I impatiently await the doctor's responses. It seems possible that by not saving or displaying data frequently enough, we may be missing out on valuable information. I must admit, I struggle with PID instructions. Could it be that there is integrator oversaturation, as I suspect?

Taking into consideration the possibility that the original poster may not possess extensive qualifications, I recommend the following: If the current equals the no-load current, set the setpoint to 0; if not, set it to 50.

It seems like you're on the right track with using a ramp down timer, but I understand how the transition to the PID can become an issue. I recommend you look into implementing a cascade control strategy. The primary controller would still be your PID, configured as you need it. However, you'd also provide a secondary controller (set to start at a lower output like the 20% you suggested) which guides the valve closer to the desired state before passing control to the PID. This way, the PID wouldn't have to correct such a large error resulting in a high step change; instead, it would work from a more manageable state, reducing that initial current spike you're facing.

Hey there! It sounds like quite a tricky situation you've landed in. A possible solution could be "bumpless transfer" - it's a feature that many PID controllers have. Normally when the PID controller switches from manual to auto mode, the output jumps suddenly, like in your case. Bumpless transfer ensures the output changes gradually, potentially solving your issue. Also, you could consider designing a better tuning algorithm for your PID controller. Good luck!

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Frequently Asked Questions (FAQ)

FAQ: 1. How can I prevent sudden surges in current and damage to saw blades in an aluminum saw setup using a proportional valve on a PID loop?

Answer: Answer: One approach to prevent sudden surges in current and blade damage is to implement a ramp down timer to gradually control the valve output, starting quickly and slowing down as it approaches the metal. Additionally, adjusting the PID settings to start at a lower output, such as 20%, and gradually increase if necessary can help mitigate the initial spike in current when the PID assumes control.

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