4-20Ma Signal Question

We have a scenario where we are using a 4-20Ma signal for two different processes. The first, to read tank levels and the second to read fluid flow rate.

When we connect the 4-20Ma signal to the analog card we get the kHz reading but it constantly bounces by about 20 kHz. Is there a simple way to capture the reading and use it in a steady state? Any other suggestions?


Is this "bouncing" a characteristic of an analog signal?

4-20Ma signal, get the kHz reading?

We used to convert 4-20Ma signal to flowrate, for example Liters/100 ms, then add them together to get HL/min.

When I set up the Analog Card on the PLC side the default range was 0-50000....I am assuming that this is Khz.

Is it typical to see the signal jump around instead of a steady reading?

Could be scaling and display units, or resolution, or ... To answer your question, we really need to know a bit more about your system.

What are your engineering units on that 4-20mA signal? (4mA = ??, 20mA=??)

What PLC? Input module?

Now that I go back and read it, bouncing by 20 kHz doesn't make a lot of sense. Please explain what you mean.

Also, using an analog for a flow rate is fine, but if your intent is to use it to totalize, that's usually very difficult to do accurately in a PLC. You're typically much better off to get a pulse from your flow meter, then count pulses.

I just figured out that the 50000 is a setting within the ladder logic. It is defaulted at 50000 so I am assuming I can change that number to whatever I want to consider to be teh 20mA signal reading? So if I have a 14,000 gallon tank I am assuimng that I would want me top value to be 14000?

I am using an IDEC pentra 16 PLC and IDEC 4ghg touchscreen to display the readings. the analog card is a j4cn1.

I am assuming that this is a 16 bit resolution is why i am seeing the "bouncing? effect because it is reading to tightly?

If I was going to use for flow rate would I just try to capture the min and max flow signal from the flow sensor?

lefeverj said:

When I set up the Analog Card on the PLC side the default range was 0-50000....I am assuming that this is Khz.

Click to expand...

I think you are confused with how the PLC translates the 4-20mA signal. The 0 - 50,000 is probably the number of counts assigned to the 4-20mA signal. This means the 4-20mA signal is assigned into 50,000 discrete (aka digital) values. For example, a count of 25,000 (halfway between 0 and 50,000) would correspond to a signal current of 12 mA (halfway between 4mA and 20mA). The higher your count range, the more resolution you get in your signal.

Its hard to say how you should set up your input cards without knowing what hardware you have. I know with compact logix cards i have worked with, i always set engineering units from 0 - 20,000 to operate between 0 - 20mA so that that it was easy to convert between mA and counts. I would then use a scale function to convert the mA signal to actual values. With SLC's and Micrologixs, i always just used a mA generator and set it to 4mA and 20mA and wrote down what value i got. I then used those values in my SCL block.

In summary, i think you are misunderstanding how the analog to discrete (digital) conversion works. It may benefit you to look up some information on generic A2D conversion.

Helliana-
This is making more sense. In thinking through my situation then I may want to use values such as 0-288. My objective is to display gallons for a cone bottom tank as well as a flat bottom tank. My approach has been to take wherever I am in the tank and convert with math functions within the PLC. So if I want to perform this math operation every inch then would it be good to set the 4-20 signal to 0-288 if I have a 24 high tank?

I have no experience with the hardware you are using, but the general concept is you have to scale it somehow. Usually, scaling depends on the sensor. Your 20mA value will occur at the highest range your sensor detects, your 4mA signal will be at your 0 range.

For example, lets say you have a sensor that converts 0 - 100 PSI to a 4 - 20 mA signal. According to the sensor, it will "send out" 4mA when the there is no pressure on the sensor. If i hook up a 100 PSI line to the sensor, the sensor will send out 20mA. Now, if you go put this on a machine where the pressure only reaches from 0 - 50 PSI, you still have to take into account that the 20mA signal is at 100 PSI (so you will only be using half the effective range of the sensor).

With all that being said, when you hook your sensor to your PLC, it then assigns a number to any analog value within a range. This is your counts. So if your PLC input card range is 0 to 20,000 for a 0 - 20mA signal, when your sensor sends out 4 mA you get a count of 4000. When it sends out 20mA you get 20000. When you get a count of 0, you know that a wire is broken on your sensor because you are receiving no signal (fault and alert the operator). You then go inside your program and use a scale function (or you can derive a formula and use math functions) to convert the range of 4000 - 20000 to the PSI range of 0 - 100.

I'm not sure if this is the method you have to use for your hardware, but this is how it works on most platforms i've had experience with.

First, inject a 4 ma signal into your input, from the instrument
location, not at the input card. If it still jumps you have grounding problems. If it is steady then see if the transmitter has a dampening setting.

Ignoring the KHz units;
IF 50000 is the span scaling value, AND
IF 20 is the amount the display bounced, THEN

20/50000 = 0.0004 = 0.04% of span bounce, which isn't much bounce.

0.04% is less than the repeatability spec for most level devices.

Some analog cards have build in filters for this, check the manual for your card.