Hello everyone! I thought I had everything figured out, but it turns out it wouldn't work. Here is the situation I'm dealing with: I am using a 1769-L30ER flow meter with pulse output (100:1 ratio) for a batching process. Thanks to some valuable information on this website, I was able to construct this process successfully. However, for a small program I created, I am now focusing on utilizing SFC or SQI SQO. My objective is simply to mix two ingredients, meeting a preset requirement. While accuracy is not crucial, I am aiming to monitor pulses. The meter will count pulses and once it reaches the desired amount, it will close the valve and make a batch correction to avoid overshooting by "X" gallons on the first run. I want the PLC to automatically adjust this for each new batch. Initially, I calculated that 100 gallons would require 10,000 pulses (based on the V/V% of calculated amounts per ingredient). I determined the formula as follows: (TargetPulses - TargetOS) = TargetConfirmed (the target at which the valve should close in advance). When the stored pulse count reaches the target, the valve will close. Given that the valve takes about 3 seconds to close, I set a timer for 5 seconds. To account for any overshoot, I subtracted the PulseCount from the TargetPulses. Then, I transferred that result to TargetOS. Ideally, if the result was zero, the system would correct itself for each batch. For example, if 1000 pulses were counted and the target was also 1000 pulses, the overshoot would be zero. This would signal a restart of the cycle, ensuring accuracy for subsequent batches. I am wondering if there is anything essential that I may be overlooking in my setup. I have searched for batch controller code or examples, but it seems that most rely on a PID loop with a control valve.
In the second image, is there any logic present to the left of the rung? If not, the RTO will need to be reset on every scan cycle. Additionally, the description of the process may need further clarification.
@drbitboy The batching program combines two liquids, referred to as ingredient 1 and ingredient 2, introduced at different stages of the process. Once the user selects the desired batch amount, ingredient 1 is batched first, followed by ingredient 2. The two ingredients are then mixed in a tank mixer for a specified time period. While the process may seem simple, there are some technical aspects to consider, such as the behavior of the RTO (Real Time Output) function in PLCs. In my experience, I encountered an issue where the RTO value would reset on every scan if no code was present. This was my first time working with PLCs, but I have a basic understanding of their functions. I initially misunderstood the RTO function, expecting it to retain its value when true without the need for the RES command. Furthermore, when the batch order is confirmed, a value of 1 is assigned to BatchSeq, triggering the batching process for ingredient one. It's worth noting that this approach may not be the most efficient, and I am actively exploring alternative methods. While some resources suggest this method is acceptable as long as it is concise and well-understood, I acknowledge the potential for complications.
I now understand what you were referring to! Indeed, the scan resets each time. Rest assured, I have the batchseq EQU before that specific line. I apologize for any confusion.
The RTO will only reset when the accumulator time reaches 3500ms or more, resetting the accumulator without the need for the ONS function. It may be more effective to use TON instead of RTO to avoid issues with aborting operations and not resetting the accumulator time. Instead of comparing the accumulator time to 3500ms, you can preset the timer for 3500ms and use the DN bit to detect when 3.5 seconds have elapsed. This approach involves counting flowmeter pulses for 3.5 seconds after initiating the valve closure command to estimate the overshoot delivery of material. This estimate is then used to adjust the timing of the valve closure on the next cycle to achieve the target amount accurately once the valve is closed. This process is also known as "In-Flight" adjustment to optimize flowmeter pulse counts.
When conducting multiple batching processes, one effective method is to install a butterfly valve in the line with a 20mm hole (referred to as the trickle valve). Here's how it works: The trickle valve remains closed initially, then both the main valve and trickle valve are opened. Count the pulses until the quantity reaches the desired setpoint minus a certain volume. At this point, close the trickle valve to slow down the process and get closer to the target setpoint. Additionally, monitor and adjust for over/under dosing levels by incorporating a portion of this into the setpoint, within specified limits. While adding a small bore valve in parallel with the main valve is another option, it wasn't feasible in our case due to physical constraints. A major challenge we encountered was fluctuating water pressure when other equipment was in use, causing variability during the process. Implementing the trickle valve solution proved to be the most effective way to address this issue.
It sounds like you've put a good amount of thought into your setup and I believe you're on the right track. One suggestion might be to ensure adequate safety margins for any inconsistencies during each cycle. These may include factors such as delays in solenoid response, or inconsistencies in flow rates. While your prediction formula seems logical, it’s always good to have redundancies in place for more precise control and risk mitigation. Also, regarding the use of a PID loop with a control valve, it really depends on your specific process requirement. A PID loop provides a more dynamic control strategy compared to the On/Off control method, but it might be more complex to implement and may require a precise control valve to achieve good control characteristics.
It sounds like you have put strong thought into your process and have addressed a lot of details. However, you might want to consider accounting for the inevitable wear and tear on your flow meter. Your calculations are based on the assumption that the precision of pulse generation will remain fixed across batches. But over time, wear could subtly alter the frequency of pulses, potentially throwing off your results. Implementing a regular system calibration routine may be beneficial to mitigate this. Also, if you are not already doing so, you may want to consider auditing your batches for quality assurance. Small discrepancies can accumulate over time, leading to larger errors. For example, a small consistent overshoot could cause significant volume changes over a significant number of cycles. As for PID options, they might not necessarily provide an advantage in your case, unless your flow rates drastically fluctuate, requiring more dynamic control. The logic setup you've built appears more than capable for operations running at relatively consistent conditions. Finally, remember that PLCs usually operate best when tasks are broken down to their simplest states, potentially making procedural code more beneficial in your case than SFC.
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Answer: 1. What is the purpose of using a flow meter with pulse output in a batching process? - Answer: The flow meter with pulse output is used to monitor pulses and control the flow of ingredients to meet preset requirements in a batching process.
Answer: - Answer: The PLC system adjusts for overshooting by calculating the difference between the target pulses and the actual pulse count, then making a batch correction to avoid overshooting by a specified amount.
Answer: - Answer: The valve control system closes the valve once the stored pulse count reaches the target amount, with a timer set to ensure proper valve closure time.
Answer: - Answer: The system is designed to self-correct for overshooting by adjusting the target pulse count based on the actual pulse count, ensuring accuracy for each new batch.
Answer: - Answer: The current setup focuses on utilizing SFC or SQI SQO for mixing two ingredients in a batching process, without the need for a PID loop with a control valve.
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