Kung Pao Chicken Little
Some of those that know me know that I like to cook. When the weather starts to get cooler, I like to delve into more spicy dishes and one of my favorites is Kung Pao Chicken. The recipe I use calls for real Szechuan peppercorns and Ancho chillies - I splurge for the real things and it does make a difference. Anyway, the cooler weather also reminds me that I have a hot tub on my deck. I ignore the tub during the summer, but when the cold shows up, there is not a much better activity when one is chilled than to make that brisk semi-nude trot from the backdoor to that bubbling oasis of warmth and pretend to be a lobster.
Anyway, on a recent weekend, I prepared my Kung Pao Chicken up to the step where it needs to sit in its magic marinade for 30 minutes or so give or take an hour (it’s not a precise science). Being a bit chilled because the wife refuses to turn on the furnace until she is sure the November tundra has arrived and unpacked its bags; I decide that being a lobster in the hot tub would be an excellent way to endure the chicken’s marinade time. First order of business is to pour a healthy measure of Knob Creek into a tumbler with one of those tennis ball size ice cubes. Next was the dialing in of Pink Floyd’s Wish You Were Here album on the bluetooth speaker. Then towel, then flip flops, and off to the tub I went.
It turns out that if I listen to the whole album but skip “Have a Cigar'', the marinade timing works out just right. So when I get into that heavenly space about half way through my Knob Creek and half way through “Shine On You Crazy Diamond”......POW!!! Something hits me (hard) on the head. I mean not something massive, but I was seeing stars just like Wile E. Coyote after he gets whapped on the head by the Roadrunner. I mean when it hit my head it sounded like an empty gourd. After the initial shock, I surmised that it must have been an acorn (as in GIGANTIC acorn) that fell from the large pin oak that towers over my deck where the hot tub is.
Thinking back to childhood lore, I could see why Chicken Little thought the sky was falling when an acorn hit his (or her?) head. Another 2 or 3 Knob Creeks coupled with another falling acorn on my noggin like that, and my neighbors may get an in person door to door “the sky is falling” news alert from yours truly.
As I often say, the astute followers of this column are patient and understand that I will eventually get to a relevant point here regardless of the elliptical trajectory. Thinking back to my conveyor PLC control days; it was typical to spend say 20% of your programming time making things work under “normal” conditions and 80% of the programming time dealing with faults and anomalies. “Faults” and “anomalies”, of course, are often like beauty: they reside in the eye of the beholder, so to speak. Traditionally in the conveyor motor control world, terms like “overload”, “overheat”, “stalled”, etc. conjure up various levels of “faultness” depending on who is doing the conjuring.
For big AC motors, overload usually meant the motor circuit protector had tripped and the contactor opened up and the motor stopped (or the W42 heater opened up for you really old guys out there who remember the clunky NEMA size starters). This of course was all unbeknownst to the PLC. Usually a wire from this tripped device was connected to a PLC input and your program would make sure that the logic telling the motor run would de-energize as well. Conveyor PLC guys usually treat this situation as a “sky is falling” condition and will commence to letting everyone know that something has to be done. They would program logic to turn on flashing lights and sound a loud horn so that someone would come and physically reset the motor starter and restart the conveyor.
With PULSEROLLER controls in particular, there are many conditions that are monitored and some of them could be interpreted as having “a sky is falling” name associated with them. So, it can happen that traditional conveyor PLC guys monitor PULSEROLLER motor conditions with names like “overload”, “overcurrent”, and “stalled” and treat these as “the sky is falling” and upon their detection immediately proceed to turn on alarms and such.
For PULSEROLLER controls, these three conditions in particular (overload, overcurrent, and stalled) are not necessarily faults and can briefly turn on and then off under normal operation. In many applications with heavier loads that are close to the rated capacity of the MDR, these conditions can flash on and then off for nearly every stop/start of the MDR. Only when these items turn on and stay on for an extended period (several seconds?) should they be treated as real faults that have to be dealt with.
I mentioned this because we recently had a PLC programmer at a site who was treating these conditions as “the sky is falling” and stopping the MDR in their program. But when the fault went away (sometimes milliseconds later) they were immediately restarting the MDR. As one may expect, doing this herky jerky stop/start to any electro-mechanical machine is not a good idea. At this site, they were eventually damaging the MDRs to the point where they actually failed. So, when you have a failed MDR; it has to be replaced, a spare has to be purchased, there is a little downtime to make the change, etc. This of course means it costs money, which makes the company’s bean counters run and truly shout that the “sky is falling”.
So my sage advice this time around kids is to think of momentary blips of these three conditions purely as status events that you can record to analyze over time to see if you actually have an issue such as perhaps an undersized MDR, or a problem with the mechanical drive train. However, keep in mind that for PULSEROLLER, the “overheat” and “short circuit” status actually internally stop the MDR regardless of your PLC program logic (just like tripping the old W42 heater, eh sonny?). So, when these happen, feel free to Chicken Little as you see fit.
Now, if one of those acorns drops into my Knob Creek, I may have to cut that oak tree down.