Control Matters SPLat Controls' aperiodic newsletter

22 Oct 2009 View online

A $69 PLC

I discovered the other day that one of the PLC companies has just released a small 14I/O PLC with a $69 price tag.

Question: Should I feel worried about this? After all, our lowest priced controller is the CC18, currently priced at $79 for the OEM version.

Answer: No, we are not worried. A PLC, at any price, is not competing with our unique offering, which is enabling original equipment manufacturers to secure custom designed, perfect-fit electronic controls for their products, economically and without risk. Our off the shelf products exist primarily for Proof of Concept and Rapid Prototyping.

That said, many companies with low volume production requirements still find our off the shelf products much better overall value that any competing solutions. Our controllers with built in user interface (MS120, MMi202) are exceptionally cost effective, and we have many useful "weird" peripheral functions. The CC18 has 2 analog inputs and the capacity to run sophisticated math algorithms as well as 32 independent tasks.

I have worked up a case study that shows that a custom SPLat can save you 68% or more compared to a PLC based solution, with a break-even on NRE of under 70 units. The example is real, the numbers are real, the customer is real (though not identified). Click here to learn how you could save 68% or more by using a custom SPLat.

David Stonier-Gibson

Driving relays and solenoids safely

Coils are inductors. It is in the nature of an inductor to oppose any change in current by generating an opposing voltage. In your car, when the ignition contacts interrupt the current through the ignition coil, the coil tries to keep the current flowing by generating a voltage so great that a big fat spark results. In a relay or solenoid coil the same thing may happen, and the resulting spark can trash the transistor that is driving it. With a coil driven off DC the fix is to place a diode across the coil with the cathode to the positive end of the coil.
 
AC coils are more complex. Inductance resists a change in current but with AC the current is changing all the time. Hence the more inductance the less AC current. Here's the kicker: The inductance of an AC relay coil depends on the position of the moving armature. When the relay is open the inductance is much lower than when closed. Hence, an open AC relay draws more AC current that a closed AC relay (or solenoid). Thus AC relays have an inrush current that must be allowed for in the driver design. AC relays may also generate a high voltage at turn-off, but you can't use a diode to suppress it. Instead you must use an RC snubber. Don't use a MOV - MOVs have a finite lifetime.
 
In AC relays the inductance limits the current. In DC relays the coil resistance limits the current. AC relay coils have very little resistance compared to DC relays of the same voltage rating. Hence, if you drive an AC coil with DC, the current will be many times more than expected; smoke will come out.

Out of control - Nothing much to do with controls, (but interesting)

The world's oldest hotel - nearly 1300 years old

I spotted this the other day: Hoshi Ryokan is a hotel and spa in Komatsu, Japan. It was established in 718AD, and has been in the same family for 46 generations. Its famous hot springs have been refreshing and revitalizing patrons for 1291 years. That's what I call history (we are proud of our "mere" 28 years in the controls business!). This got me to wondering about old companies, so I did a google. It turns out that the Japanese are way ahead of the field. Somewhat sadly, the very oldest company, Kongo Gumi, a builder of Buddist temples (sic), folded in 2006 due to overly optimistic financial dealings. 

Normal people believe that if it ain't broke, don't fix it.
Engineers believe that if it ain't broke, it doesn't have enough features yet.

Insanity: doing the same thing over and over again and expecting different results.   
Albert Einstein 

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