Ampere Hour Totalisation/Timer Accuracy

[pauly]

We have a contract part of which involves totalising ampere hours and switching dosing pumps on at a certain value of A/H. I need to log the value of current every 6 seconds and generate an A/H total. I am proposing to do it with a one shot every six seconds according to this simple formula: 6*Amps Value/3600. I can remember seeing somwhere on this site a posting about timer accuracy, I think that was something to do with totalising as well but I can't seem to find it now. Does any one see anything wrong with totalising like this(the specification talks about logging once every six seconds)and any thoughts on timer accuracy? This is a PLC5

 

[Steve Bailey]

There are two issues to consider. First is the accuracy of the six second timer. The timer's DN bit will be true in the first program scan in which the ACC value equals or exceeds the PRE. That can be off by as much as the time for one scan. Also, when you reset the timer you lose the time in the scan in which you perform the reset as well as losing the residual time in the ACC register.

The second issue is the amount of variance in the current. When you sample once evry six seconds, you are making a tacit assumption that the sampled value of the current is the same as the actual current over the entire six second interval.

There are techniques you can use to minimize the impact of both effects. For the timer, instead of resetting at the end of six seconds, subtract the PRE from the ACC. When you sample a new value, keep track of the previous sample value. Each time you sample, calculate the average between the current sampled value and the previous and use the average to add to the amp-hour accumulator.

 

[pauly]

Timer accuracy

Thanks for the reply,do I not need to subtract Acc from Pre?Does the timer get reset from a comparison of these? And won't the same problem exist with scan times?

 

[Steve Bailey]

The status on DN is based on a comparison of ACC to PRE. Reset drives ACC to zero. Let's say that your PLC has a scan time of 25 milliseconds. The ACC value in the scan in which the DN bit is true could be as high as 6.024 seconds. If you reset the counter when the DN bit is true, you start the next scan with an ACC value of zero. If you subtract PRE (6.000 seconds) from ACC, you start the next scan with an ACC value of 0.024 seconds.

 

[Gerry M]

As an alternative, you may want to look into trying a watthour meter. They usually put out a pulse for every kw or some portion of a kw. Then count the pulses for a total.

 

[Andybr]

Pauly

Since you are measuring AH I assume that it is a DC application and the dosing pumps suggest some kind of plating. Steve's suggestion re the timing sounds ideal(I might use it myself someday) but I think that the simple averaging of 2 values is unlikely to be good enough to be accurate. I would think that the amp values could change quite significantly and very quickly and taking a reading every 6 seconds will only give you a snapshot at that particular time. A better solution might be to look at the value every scan, add all of the values over a six second period and count the number of scans so that you can calculate a truer average. This will still not be perfect as the sample time of your A-D card comes into play but it will be quite close.
Andybr

 

[averytc]

With a PLC 5 I would use the STI function. I normally set the STI to 500ms, which should give adequate resolution for totalization. The total can then be logged every six seconds to a db.

 

[pauly]

Your right Andy, it is a plating application. The six seconds comes from the company supplying the dosing equipment.

 

[pauly]

STI Function

How do you access the STI function in logic?

 

[averytc]

Go to Processor Status - STI tab. Choose the Setpoint ms (S:30), and the routine to be used (S:31). I lable the routine 'STI' so that I remember which one it is.