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Engineer’s Angle: The ABCs of PLCs; It’s not witchcraft

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Engineer’s Angle: by JD Anson

As yacht builders strive to differentiate themselves, the boats are becoming more complex. Increasingly, push-button computer interfaces are being installed to operate everything from starting the engines to turning on lights. While these computers have become more reliable, they are not infallible. It is not unheard of to see a yacht running back to port with garage door open or the passerelle sticking out of the behind. This is the nautical equivalent of walking around with toilet paper stuck to your shoe. It’s just not proper yachting.

Programmable Logic Controllers are the most common type of computer control interface found on board. They normally are a matched set of DIN rail mounted modular devices consisting of a small computer and power supply coupled to input and output modules. Feedback from sensors give the computer a snapshot of the state of what it is controlling, and the program tells which output to activate.

In the above mentioned passerelle, a set of buttons, when pressed, send a signal to the PLC telling it what needs to happen. The computer then checks for certain parameters to be met by sensors before activating the pump and valves to create motion. It does this not with black magic, but via a program written in Boolean logic. This language reduces variables to a YES/NO simplicity.

For instance, it will see if the door is unlocked, open and the passerelle extended fully before activating the hydraulic valve to drive the gangway up. If any of these states read NO, then the valve will not actuate. This is a safety to avoid damage by slamming the gangway into the housing.

In Boolean architecture, the use of AND, NOT, OR, NOR and XOR makes multiple parameters possible. Thus, the example above must read “door unlocked AND open AND passerelle extended” before allowing the valve to move.

In order to give the information to the computer, a multitude of sensors are available to cover every conceivable type of condition – position, temperature, pressure and more. These can either send a digital YES/NO signal, or an analog variable, usually a 4-20mA. Regardless, the program will look for a condition to be satisfied before moving on to the next step. These signals, along with the outputs, are the many small wires seen on the modules. Each wire is connected to a specific component, and carries signals to and from that component, allowing the program to see the information it needs.

Most failures in PLC systems occur because of an input failure. A button not making contact will not send its input. A failed or out-of-adjustment proximity sensor will not give input to the module about whether the door is open. A bad pressure transducer will not send the correct mA that the computer needs to see.

Digital (YES/NO) sensors are easiest to troubleshoot. A proximity sensor is an inductive coil that moves when metal is close enough to it. This coil then closes a switch inside that completes the circuit. Placing a wrench up to its end should activate it. Many have a small LED to show when the switch is closed and will light when the wrench is against it. Check also the gap between it and the metal when it is to be activated. Some require less than 2mm gap to activate. Digital pressure sensors that switch at a preset point can easily be tested via a multimeter. If needed, the leads can be jumped to bypass a bad sensor.

Analog sensors are more difficult, and without special mA generators, there’s no way to fool the computer. Best to carry spares for these and replace if suspect.

The worst scenario is a corrupted or failed computer. This does happen, especially in aged systems. For example, how many WIN 95 computers are still in use? Yet many PLCs are of the same vintage. Spare computer modules are sometimes available. However, without the specific program for the application they are useless. Best practice is to always insist on a copy of the program being written to be kept on board. Many times a simple part swap will end up a full system replacement, costing many thousands of dollars for lack of a simple thumb drive. Also, the company that writes the program could disappear without notice, thus losing any chance of support.

This simple troubleshooting can allow emergency repairs enough to avoid ruining the boss’ vacation.

JD Anson has more than 20 years of experience as a chief engineer on megayachts. He is currently project manager at Fine Line Marine Electric (finelinemarineelectric.com) in Fort Lauderdale. Comments are welcome below.

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