How to Use a Shift Register for Rejecting Items: Step-by-Step Guide for Beginners

If the conveyor suddenly slows down or there is a gap in the bottles, how can the system accurately track the bottles that have already been registered?

• 30-08-2024
• parky

Parky asked about how the conveyor system tracks bottles in case it slows down or encounters a gap. The system uses parameters such as time delays to ensure smooth movement of items. These time delays are adjusted whenever the conveyor speed changes, dictating how long it takes for items to move from one point to another. By setting values on the shift register and factoring in time delays, the system can accurately track the items. If no item is detected after the designated time, the system will move the value to the specified word address. If you need further clarification, please feel free to let me know.

• 31-08-2024
• RBB20

Instead of using a prox on a star wheel to match the conveyor speed, you are depending on timers, which may be a bit unreliable. However, if the distance between the printer and cylinder is short, this should not be an issue.

• 31-08-2024
• parky

Parky mentioned that instead of using a proximity sensor on a star wheel to sync with the conveyor speed, timers are being relied upon, which may be a little unreliable. However, since the distance between the printer and cylinder is short, it may not be a problem. The setup includes DI1 triggering if a label has a defect, DI2 detecting the bottle entering, the Printer, DI3 detecting the bottle exiting, and the Cylinder rejecting defective bottles with defect labels. Each distance between these components is short. Any suggestions for the programming I can implement for this setup? Thanks!

• 31-08-2024
• RBB20

When DI2 and DI3 detect bottles at the inspection and reject stations, a First-In, First Out (FIFO) array is preferred over a bit-shift register. For DI2, the current value of DI1 (1 for defective bottle labels, 0 for good labels) is pushed onto one end of the FIFO. When DI3 is triggered, the value is popped off the other end of the FIFO: if the value is 1, the cylinder is activated to reject the bottle; if the value is 0, the good bottle passes the reject station. Using a FIFO is more efficient despite the option to use bits, as memory is affordable and FIFO instructions are easier to work with. Initializing the FIFO array when the PLC is turned on can be challenging. The most reliable method involves ensuring no bottles are between DI2 and DI3, and setting the FIFO length to 0. To comprehend FIFO operations, refer to the flowcharts for FIFO-Load (FFL) and FIFO-Unload (FFU) for True input rung cases. Consider adding delay timers from the rising edge of DI2 and DI3 inputs to reading the DI1 input bit and writing the cylinder reject bit for accuracy. Although the provided links are not safe for all systems, the concept applies similarly to S7-1200 FIFOs.

• 31-08-2024
• drbitboy

Drbitboy suggested using a FIFO array instead of a bit-shift register when DI2 and DI3 detect bottles at inspection and reject stations. When DI2 is triggered, store the current value of DI1 (1 for defective labels, 0 for good labels) at one end of the FIFO. Upon triggering of DI3, retrieve the value from the other end of the FIFO: if the value is 1, activate the reject cylinder; if it is 0, allow the good bottle to pass through. Although it is possible to achieve this with bits, using a FIFO array is simpler and cost-effective. The challenge lies in initializing the FIFO array state when the PLC is powered on - ensure no bottles are between DI2 and DI3 and set the FIFO length to 0. Refer to flowcharts for FIFO-Load and FIFO-Unload operations in the manual for a better understanding. It may be necessary to introduce delay timers between the triggering of DI2 and DI3 and the corresponding actions, but with careful adjustment, accurate settings can be achieved. This solution is primarily applicable to Logix PLCs, but can also be adapted for S7-1200 FIFOs.

• 31-08-2024
• RBB20

Around five or six years ago, in a thread on this forum, @parky, @drbitboy, and a few others collaborated on coding examples of a selection/rejection program. This program utilized both the bit-shift method (tracking conveyor movement) and the FIFO approach (tracking individual items). If you search the forum using keywords like FIFO and bit-shift, you may be able to find the original thread discussing these coding examples.

• 31-08-2024
• drbitboy

If you are looking for information on tracking rejected items on a conveyor belt using FIFO and bit-shift techniques, you can easily find it by searching the forum. Click to expand for more details. One helpful tip is to check out examples of the code provided in the PDFs, as they are likely to be the most informative. In a discussion thread, @parky shared a method using a bit-shift register controlled by a timer, which works effectively as long as the conveyor speed remains constant. In my own approach, I converted the items on the conveyor into encoder-like "pulses" to trigger the FIFO, as explained in post #11. I may have opted for a circular buffer over using the FIFO instructions (FFL/FFU) to keep track of items in the array, but the underlying principle remains unchanged.

• 31-08-2024
• drbitboy

The process described here is completely untested and may require some adjustments for optimal functioning. While there are only three rungs in OB1, the majority of the work happens in the FIFO FB1. I consolidated FFL, FFU, and RST into a single instruction, which I believe aligns better with the TIA model. The decision to set the array size at 1024 may not have been the most optimal choice. Although there is a FIFO available in the Siemens "Library of General Functions," I personally prefer the A-B model. The logic controlling the rejection output to the cylinder is somewhat theoretical, possibly utilizing a one-shot process driving a TOF. The final diagram represents my interpretation of the overall process.

• 31-08-2024
• drbitboy

drbitboy pointed out that the process described is completely untested and may require some adjustments for optimal performance. While only three rungs are used in OB1, the majority of the work is carried out in the FIFO FB1 block. By combining FFL, FFU, and RST into one instruction, it aligns better with the TIA model. The fixed array size of 1024 may not be the most optimal choice. While a FIFO function is available in the Siemens Library of General Functions, the A-B model is preferred. The logic controlling the DO reject output is questionable and could possibly be improved with a one-shot driving a TOF. The final image provided is an interpretation of the entire process. Thank you for sharing the pdf for further study.

• 31-08-2024
• RBB20

drbitboy mentioned the need for testing before implementation. While the initial setup may need adjustments, the main work is in the FIFO FB1. The incorporation of FFL, FFU, and RST into one instruction is aligned with the TIA model. The array size of 1024 could be optimized further. Siemens offers a FIFO in the "Library of General Functions," but the A-B model is preferred. The logic for rejecting defective items may need refining, such as using a one-shot driving a TOF. The provided image illustrates the process, but the question remains: does the program automatically adjust to reject specific defective items?

• 31-08-2024
• RBB20

RBB20 inquired about the automatic adjustment capability of a program. Can it accurately reject a defective bottle out of a batch that passes through DI1 and DI2? The program should be able to do so if no bottles are present between DI2 and DI3. However, there may be timing challenges if certain components are too far apart. While this feature remains untested, there is potential to simulate the process in the future.

• 31-08-2024
• drbitboy

RBB20 inquired about the program's auto-adjusting feature. For example, if only three bottles or items pass through DI1 and DI2, and one of them is defective, will the program still be able to reject that specific bottle? The FIFO logic breakdown below explains how the program operates: - The Load logic ensures that when a bottle passes the DI2 station, it is added to the FIFO array. This means that the newest element's value is initially set to 0, indicating a good label. - The Unload logic ensures that when a bottle passes the DI3 station, it is removed from the FIFO array. The bottle's pass or fail status is taken from the first and oldest element of the FIFO array, while the remaining bottles' values are shifted back in the array. - The Detect failed label logic overwrites the most recently added bottle's status with a value of 1 to signify a bad label. Please note that if the number of bottles between DI2 and DI3 stations is m.POS, then the status of the most recently added bottle can be found in Arr[m.POS-1]. Load logic Unload logic Detect failed label logic.

• 31-08-2024
• drbitboy

In the analysis provided by drbitboy, the FIFO logic functions are thoroughly explained. It is crucial to grasp that the Load logic in this system guarantees that each bottle passing the DI2 station is added to the FIFO array. This addition involves incrementing the number of active elements in the array, known as POSmem, by 1. Furthermore, the newly added bottle starts with a value of 0, symbolizing a good label upon entering the array. Any label modifications to 1 are handled separately. On the other hand, the Unload logic is responsible for removing bottles from the FIFO array as they pass the DI3 station. Removal leads to a decrement in the number of active elements and involves extracting the status (0 for pass, 1 for fail) of the first bottle in the array (Arr[0]). The remaining bottles between DI2 and DI3 are then shifted accordingly within the array. Additionally, the Detect failed label logic updates the status of the most recent bottle (0 from Load logic) to 1, indicating a faulty label. It is important to note that the element representing the latest added bottle's status can be found using Arr[m.POS-1] when the bottle count between DI2 and DI3 is m.POS. Feel free to refer to the Load logic, Unload logic, and Detect failed label logic documents provided for further details. Thank you for your assistance, and I have thoroughly reviewed all the materials you shared.

• 31-08-2024
• RBB20

RBB20 mentioned that they made modifications and adjustments to ensure everything works correctly. They will be presenting it next week and appreciate the help. It is crucial to study all the provided files as the process was not tested and their specific process is unknown. Keep us updated on the progress.

• 31-08-2024
• drbitboy

drbitboy mentioned that since the process was never tested and the actual process is unknown, it would be helpful to receive updates on its progress. I haven't had a chance to verify it yet, but I am curious about what could happen if the camera were positioned before the sensor 1. What if the camera detects a rejected label, but the array position remains at -1, causing the trigger to not register or be inputted correctly? Let's stay informed on the outcome.

• 31-08-2024
• RBB20

In the scenario presented by RBB20, the concern arises about the possibility of the camera scanning a rejected label before the sensor 1 is triggered. This raises questions about the timing and synchronization of events in the process. However, it is important to understand that the FIFO FB mechanism is triggered by the rising edge of DI2, while the writing action uses the rising edge of DI1. This means that the camera will always write a value of 1 to the most recent element in the array, which is determined by the values of m.POSm1 and m.FIFOpos. The value of m.POSm1 is calculated as (m.FIFOpos - 1), with m.FIFOpos representing the position offset from the starting point of the array where the next bottle's value will be added. This value also indicates the number of elements currently in the array that have been detected by DI2 but not yet by DI3. It is assumed that the rising edge of DI2 (indicating the detection of a new bottle by the upstream sensor) occurs before the rising edge of DI1 (indicating a bad label detected by the camera). This sequence of events is crucial for the proper functioning of the system.

• 31-08-2024
• drbitboy

In the implementation of the FIFO FB, the rising edge of DI2 is utilized for writing a value of 1, while the rising edge of DI1 is involved in the process as well. The latest element in the array, located at m.FIFOarray[m.POSm1], will always receive the value of 1. The position value m.POSm1 is determined by subtracting 1 from m.FIFOpos. m.FIFOpos represents the number of elements offset from m.FIFOarray[0] where the next value will be added to the array. This value also corresponds to the number of active elements in the array, indicating the items detected at DI2 but not yet at DI3. An underlying assumption is made that the upstream proximity sensor's rising edge (DI2 detects a new bottle) occurs prior to the rising edge of DI1 (camera detects a faulty label). It is now clear, thank you. The upcoming week will involve final testing procedures.

• 31-08-2024
• RBB20

RBB20 mentioned, "I understand now, thank you." This week will involve final testing. It is crucial to grasp when events occur rather than just what the events are. The rising edge logic ensures that necessary actions like writing a 1 or shifting data in the FIFO array happen only during the sensor's transition from 0 to 1 in one scan cycle. It's important to be aware of the possibility of a "bouncing" or noisy input, where multiple rising edges may be detected due to vibrations or rapid scan cycles. To address this, a debounce can be implemented either in the hardware circuit or within the PLC logic. Understanding the specific system and conducting empirical testing will help fine-tune the debounce logic. Consider whether a TON (Timer On Delay) or a TOF (Timer Off Delay) is more suitable for the process, as this will impact the timing of the edge detection.

• 31-08-2024
• drbitboy