Seeking answers about how best to adapt his company’s packaging machinery to PLC control, Madhusudan Sikri, owner & CEO of Sikri Packaging Corporation and a member of Automation World’s LinkedIn group (http://bit.ly/automationworld), put his question to the Automation World group.
“We want to convert our packaging machinery [so that it can be] PLC controlled and [we] need help to learn programming,” posted Sikri.
Sikri Packaging manufactures basic vertical form-fill-seal machines in India and recently converted its machines from electro-mechanical to electronic, meaning that the machines are now controlled by sensors and relays. Sikri noted that this change increased the complexity of their machines’ wiring and, as a result, the company has called in help from several PLC suppliers to help them make the conversion to PLC control.
The problem, says Sikri, is that the suppliers are “all looking to plug maximum use of their own brands of switches, drives, motors and controllers rather than offer us an optimal solution.”
To help set Sikri on the right path, Faisal Mirza, development engineer, posted that there are two points Sikri must first be clear about before seeking a solution. The first of those points is to have a machine design strategy or method.
The principal factors behind the design strategy include having “a clear layout of the number of inputs/outputs (I/Os) needed,” said Mirza. “Out of those I/Os, [you have to determine] how many are going to be digital (switching between 0 or 24 V dc) and how many are going to be analog (4-20 mA signals, 0-10 V dc or some other analog variable). Also, you must consider if there are other quality control options required with specialized sensors or even machine vision systems. Such systems should integrate well with the main PLC, Mirza adds.
Another component of the machine design strategy involves motion control requirements. If this is required, you must determine the right kind of drives (variable frequency drive or simple motor starter) and, of course, motors, noted Mirza. “Then you will need to calculate the cycle time to realize the final solution in a deterministic fashion. You will need to have a clear overview of the complete process steps that the machine covers in each cycle,” he said.
The second component to correct PLC determination, according to Mirza, is zeroing in on the type of PLC hardware that can handle the requirements of the I/O, sensors and motion control components of your machine design strategy.
“It is quite natural for different vendors to suggest their own brand of control components,” said Mirza, “so it depends on the end user to have everything clearly mentioned in an end user specifications document. This document details everything about the process, including what the user requires from a vendor. You can then select from different components according to your own experiences or inventory preferences.”
When it comes to making a final selection, Mirza suggest opting not for the vendor that may be most popular in your area, but for the one which can provide the best service.
Having worked with a food industry OEM for five years, Brad Hart, now an automation specialist at C&E Sales, says that a “small, micro PLC is more than capable of doing the job. These machines have a small footprint, so you really do not need a distributed I/O network.”
Hart recommends the Siemens S7-1200 PLC series, the supporting Step 7 software for which can be programmed using either Ladder Logic or Function Block Diagram.
The most recent nugget of advice posted for Sikri, came from Ravi Abraham, an electrical and control systems engineer at the Australian Nuclear Science and Technology Organization, who suggests “that it is better to stay with one brand of PLC that your site is comfortable with” rather than seeking multiple types based on the best deal available. Abraham suggests the one PLC provider approach may not be such a bad idea when considered in light of future system needs like communication or reporting.
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