Trim pots on the Micrologix 1100 and an analog question

Holescherlj

Member
Join Date
Apr 2002
Location
Minneapolis,MN
Posts
4
Hello, I am looking into using the Micrologix 1100 to control our facilities compressors in a lead/lag configuration and to keep them off when there is no demand. I was reading the user manuals and came across a section for trimpots.

What are they and how can they be used. The manual had a very brief decription but didn't make any sense. Could someone clue me in?

My second question involves the analog inputs. The built in inputs are 0-10VDC.I want to use a pressure transducer installed on the piping in our production floor to monitor the supply pressure to the floor. The controller is going to be installed in a compressor room 5 feet away from a 40HP motor and 15 feet from a 25HP motor. Needless to say there will be alot of noise.

I know 0-10VDC signals are more suceptible to noise then 4-20ma signals. I found this application note (A16562) In AB's knowledgebase on how to convert a 4-20ma signal to a 2-10V signal. I guess I'm wondering if this method would help with the noise or should I just use a 0-10VDC transmitter and deal with it.

I realize being this close to the compressor's motor isn't ideal but it would be a major pain to move the current enclosure.

Your thoughts and suggestions would be appreciated,
Luc
 
On the 4-20 mA front you could use a 4-20mA transmitter and at the plc use a signal convertor to get your 0-10vdc
 
The trim pots are accessed through the function files. Unlike the 1200, these are not litteral pots but are set through the LCD and keypad on the front of the ML. In the RSL500 project tree you can find the function files under the controller. Click the LCD tab. In software it would be LDD:0.POT0.

The ML1100 and ML1200 are inexpensive and are just fine for this application. In fact, I believe that it is applications such as this that AB had in mind when they released the ML1100. The LCD and keypad make it ideal for OEM applications where a field tech might need to change parameters but normally would not need access to the software. I expect to see more of them in compressors and chillers.

Use a good shileded cable such as Belden 8760 and noise shouldn't be a problem. Those two analog inputs are only 10 bit resolution (1 part out of 1024), but that should still be more than adequate for compressors. Check the manual, even though they are 10 bit values, they might still be left justified into 16 bits like analog IO on the ML1200 is.
 
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You can have the best of both worlds on the analog signal very inexpensively. Put a 500 Ohm resistor across the analog input terminals and you will get 2-10 VDC (remember Ohm's law). Then you don't have to buy expensive signal conditioners or worry about noise in a long signal line.
 
Geniusintraining,

I haven't seen them, but it looks to be close to the same thing I am going to replace. The compressors we have already have these features built in, they just need to have a relay close to tell them when to unload and start.

I'm not sure how much one of those will cost but I would imagine it would be close to what I will spend on this project and have less flexibility.

We are replacing a custom control box built by our supplier that has been confusing to the production staff, and has been the blame for more than a few hours of plantwide downtime since it's installation. The timers inside lose time on a regular basis and it's been a headache. The current system also uses a pressure switch located right next to the compressor that is usually 10 to 15 psi higher than what is seem on our production floor.

By locating the transducer on the floor we will get a more accurate reading of what the machines are seeing and we will be able to react quicker to changes in pressure.

I also want the operator interface to be as user friendly as possible.
Ideally if the compressors are not already running (on the weekend for example) I want a single push button for the operators to use to turn the system on for half hour increments, and I would like the display to show the operator what time the compressors will shut down based on how many times they pushed the button.

Thanks for the suggestion,
Luc
 
Alaric,

So the pots are "virtual" pots that can be used in the program to adjust values?

I must confess I have been waitng for a reason to use one of these, and this project fit the bill. With our upcoming machine network it will be nice to be able to connect to this from home in the event of a problem.

I am using a 0-200psig transducer. I think the resoultion will be fine I don't believe I will need anymore than +or- 1psi.

Thanks for the reply,
Luc
 
Tom,
That's what the application note said to do I was just wondering if this method would still be as susceptible to the noise, since in the end we are still using a 2-10VDC signal.
Trying to find the precsion resistor AB reccomends has been fun.

Thanks,
Luc
 
Because the resistor is close coupled to the input the effect of EMI/RFI noise is negligible.

You don't necessarily need the precision resistor - we don't use them. You just use a simulator to put 4 mA in, read the data, use a simulator to put 20 mA in, read the data, and plug those values into your scaling function for converting to engineerinig units.
 
I think the best thing to try in this case with the pressure transducer/transmitter is to use a signal conditioner to power up and convert the signal to voltage at teh PLC end. A company called PRelectronics makes a device called the 4114 that will do this for around $150USD.
 
No, Tom is correct, as usual. All you need is a 500 Ohm resistor AT the PLC. Bring the signal in as 4-20 mA from the machine floor, so there will be no signal loss or noise issues (with shielded cable). the resistor automatically converts the signal to 0-10 volts, and Bob's your Uncle.

If you can't find a 500 Ohm resistor, then I have used two different values that total near 500. Just solder two of the leads together, then loop the other ends around the Input screw terminals.
 

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