Current Monitor

Hi Guys,
I am hoping to combine all your favourite topics in one thread cos I have a question about a Car wash that I am refurbishing for a HNC final project in the UK. This really is a real job that I have to do!

Its a rollover Wagon Washer and the pressure applied by the horozontal brush is controlled by monitoring its motor current. This was done by three seperate relay devices but I am going to fit a Pheonix Contact current monitor (MCR-S-10/50-UI-DCI) and connect it to an analogue input on a mitsubishi FX48MR.

The conversion is 12bit so a value of 4096 in a data register will represent Max current and I will use comparision instructions against preset values to detect various levels.

Will I need to do some kind of smoothing to the register value (averaging?) before using it. I am concerned it will jump around a lot.
Should I introduce some hysteresis by using different latch and unlatch levels?

I have not programmed this type of thing before any hints would be useful.
Cheers ian

Hi Ian

I use a lot Phoenix Contact relays and doing what you did when it come to one analog input or output.
Keep in mind that you need to connect the converter to a same input word.If you have 16 bit input word and you going to use the other 4 input you might have problem.
You will have to scale your inputs 0 bits=0A 4096 bis =xA.
When you do comparison use timer of 3 sec or so to make sure you dont compare pick or noise.

All the best

ian.smith7,

I would say that you should average the curernt input before trying to use it in a comparison block. In the real world, analog inputs are often jumping around. This is most often seen with motor current, air pressure in a duct, flow rate in a pipe, and many other types of analog inputs.

Your average function can be simple or complex. My favorite is:

Use a timer set to read your analog input periodically, say once a second, or whatever period makes sense for the input. Then when the timer is done (times out), perform the following calculations:

New Average Value = (New Value + 3 X Old Average Value) / 4

Old Average Value = New Average Value

Reset Timer to 0, and repeat cycle.

If this is not "smooth enough" then you can increase the 3 multipler/ 4 divider until it is smooth or you exceed your PLC maximum number limit.

The easist way is to introduce deadband (hysterisis) in your limits to avoid constant cycling of the bit.

There are also a number of digital filtering algorithms that you can use, similar to the one proposed by Lancie. If you search this site you should be able to find them. However, bear in mind that filtering also slows your response and may introduce a little offset into the analog value.

Thankyou for the replies
i favoured the hysteresis solution because i thought the lag introduced by averaging would be detrimental to the application. Is there a good way of programming deadband apart from the obvious high latch & low unlatch or is it really that straightforward. What would be a reasonable amount of deadband to start with, 2% to 5% would seem ok to me, would 10% be too much?
But I see from reading your replies that using this method on its own I could still trigger a response from an unrepresentitive current spike so maybe a combination of filter and deadband is the way to go.

ArikBY
because I have not looked in detail at the programming for this I am not sure I fully understand your comments yet, But I only need 1 of the 4 available analogue inputs so I do not think i will have a problem with bit limits.


Cheers Ian

Ian

What I mean is if you have input card of 16 inputs for the PLC it is 1 word OK.
Now you connect your converter to I0-I11 if you will use I12-I15 it might impact on your reading.That what you have to prevent.
I am trying not to use this inputs, but if you must you have to isolate
this bits from your anlog word.
For example you read now 4096 now I12 go on now you will read 8191.
you get it?

Hi Arik
Yes now I see what you are getting at, even though the conversion is 12bit the Input is read as 16 bits and use of the 'spare' bits will give me an incorrect value.

thanks for the tip. I will read the manual at work today.

thanks ian