PLC vs Arduino Show Down

Hackaday readers don’t need an introduction to the Arduino. But in industrial control applications, programmable logic controllers or PLCs are far more common. These are small rugged devices that can do simple things like monitor switches and control actuators. Being ruggedized, they are typically reasonably expensive, especially compared to an Arduino. [Doug Reneker] decided to evaluate an Arduino versus a PLC in a relatively simple industrial-style application.

The application is a simple closed-loop control of flow generated by a pump. A sensor measures flow for the Arduino, which adjusts a control valve actuator to maintain the specified setpoint. The software uses proportional and integral control (the PI part of a PID loop).

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Fail of the Week: Arachno∙fail∙ia

Going down the list (FCC, CE, UL, etc.) we can’t think of a regulating body that will test for this failure mode. Reportedly, a $1M irrigation system was taken down by a spider. And an itsy-bitsy spider at that.

This fail turned up as a quick image post over on /r/mildlyinteresting but I wasn’t the only electronics person attracted like a moth to a flame. Our friend [Sprite_TM] popped in to answer a question about conformal coating. Seems this board was sealed in a waterproof enclosure but was obviously not conformally coated.

fotw-spider-short-relay-diagram[Sprite_TM] also helped out with some armchair-engineering to guess at what happened. It’s not hard to tell that the footprint on the board looks like a set of mechanical relays all in a line. He looked up the most likely pinout for the relay.

We’ve superimposed that pinout on the board to help illustrate the failure. High voltage comes in on the pin shown with the red trace leading away from it. On either side of that pin are the connections for the low voltage coil which switches from normally closed (the pin in the upper right that is not connected to anything) to the normally open pin (which has the wide trace leading away from it).

So there sat the high voltage pin in between the coil pins when, along came a spider. It shorted the pins and presumably all the way back to the power supply for the low voltage rail. [Fugly_Turnip] (the OP) share some additional detail about the system and this failure; in addition to this card it fried the control module as well.

Another comment on the same thread shares a different story of two boards mounted next to each other with a bug shorting a 1/4″ air gap between two boards and causing similar carnage. Have you encountered Arachno-fail-ia of your own? Let us know below.


2013-09-05-Hackaday-Fail-tips-tileFail of the Week is a Hackaday column which celebrates failure as a learning tool. Help keep the fun rolling by writing about your own failures and sending us a link to the story — or sending in links to fail write ups you find in your Internet travels.

Building an Industrial Control Unit With an Industrial Control Unit

Back in the 70s, industrial control was done with either relays and ladder logic or new programmable logic controllers. These devices turned switches on and off, moved stuff around a factory, and kept the entire operation running smoothly. In the late 70s, Motorola came out with an Industrial Control Unit stuffed into a tiny chip. The chip – the MC14500 – fascinated [Nicola]. He finally got around to building an ICU out of this chip, and although this was the standard way of doing things 30 years ago, it’s still an interesting build.

[Nicola]’s ICU is extremely simple, just eight relays, eight inputs, the MC14500, a clock, and some ROM. After wiring up the circuit, [Nicola] wrote a compiler, although this chip is so simple manually writing opcodes to a ROM wouldn’t be out of the question.

To demonstrate his ICU, [Nicola] connected up an on/off switch, a start button, and a stop button. The outputs are a yellow, green, and red lamp. It’s a simple task for even a relay-based control scheme, but [Nicola]’s board does everything without a hitch.

If you’re looking for something a little more complex, we saw the MC14500 being used as an almost-CPU last year.

Video below.

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