Need help starting a program daily at 8:00 AM

[mk42]

40 ppm is the same as 0.004 percent so it's actually not a lot of variation if we compare to many other things. But it adds up over time.

That's a fair point. Over the span of a year, losing only an hour or two is actually incredibly precise.

 

[Tharon]

These inaccuracies seem pretty high to me, a lot higher than I'm used to with the RTC on my relatively low cost C-More HMIs. I update my times twice per year (at DST) and usually see a max of five minutes off. I've never seen anywhere near an hour.

Now, maybe the C-more HMIs have a more accurate RTC? Seems odd that they would be that much more accurate when we are talking hours over a year. I couldn't find any specs on the RTC in the C-mores though.

Or are we talking compounding errors always one way? Like a worst case scenario? And the errors could shift it both ways (faster, and slower) and work out to be less than expected on average?

 

[Tim Ganz]

But my 10 dollar alarm clock is only about 4 minutes off from the NTP time site I set it to 2 years ago and it has a 9 volt battery for power outages and its 1 10 dollar Timex alarm clock.

The clock in my car never gets out more than 2-3 minutes every couple years.

 

[Pete.S.]

OK, I did some more research.

Watches/clocks uses something called inhibition compensation which means that at manufacturing the crystal is made to run too fast. Then at the factory the clock is compared to an accurate time source and calibrated in software to be accurate. According to wikipedia a typical quartz clock or wristwatch will gain or lose 15 seconds per month, so +/- 3 minutes per year.

High end quartz watches can be calibrated and certified by COSC and then they have to drift less than +/- 25 seconds per year.

Modern PLCs are microcontrollers and they have real time clock (RTC) components either on the chip or in an additional circuit. According to info from Atmel (one of the large manufacturers) more accurate RTCs have temperature compensation in software. If they are calibrated at room temperature and then temperature compensated in software an accuracy of +/- 10 ppm from 5C/40F to 65C/150F can be expected. +/- 10 ppm is +/- 5 minutes per year.

It looks like most PLC manufacturers don't specify the accuracy of their RTC but some do.

Micrologix 1100
+11 to -30 minutes per year (over entire operational temp)

Schneider Premium PLC
+11 to -53 minutes per year (over entire operational temp)


In general the crystal oscillators are very temperature dependent.
From the data above I can only draw the conclusion that PLCs have less sophisticated RTC circuits and probably no calibration, compared to more advanced circuits that are available and could be used. Even cheap watches have more advanced circuitry /algorithms. To some degree it makes sense because a watch, even a cheap one, is a time keeping device, while a PLC is an automation controller. This is also my personal experience - computers and PLCs are worse than a cheap watch.

Because of the temperature dependency it also makes sense that the drift could vary widely not only from unit to unit but also based on it's environmental conditions. And the same unit could be running to fast at same temperature and then to slow at another. As the crystal ages it's characteristics will change. Of course with some luck and stable temperature some system may not drift as much as others.

Quick rule of thumb is that 1 ppm equals 1/2 minute per year.


Some sources:
http://www.atmel.com/Images/Atmel-42251-RTC-Calibration-and-Compensation_AP-Note_AT03155.pdf
https://en.wikipedia.org/wiki/Quartz_clock#Accuracy
.

 

[padees]

I wonder how accurate the High Speed Clock in the ML1400 really is? Been playing around with that a bit because have to capture some reaction times. Was gonna ask a question last week, but figured it out. Narrowed it down to a couple of spots where you don't want to do anything with it, lest you fault the PLC out.

 

[Aabeck]

Two things I have discovered:

1 In SLC PLC's the clock loses time every time the second value is read in a scan. If you move it once in a scan to an integer the clock keeps time a lot better than reading it a few times each scan. This difference can mean an incredible difference in time loss, I had a 5/05 losing 3 to 5 minutes every hour by using the seconds value too much.

2. Running the exact same timer settings can be different between related PLC's. I had a routine I programmed in a SLC5/03 (so I could do online edits) and tweaked it to exactly the timing I wanted, then I transferred it to a Micrologic PLC and although the timers were set to the same presets they ran considerably faster in the ML, I had to change every timer preset up almost 15% to get the timing I had.