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AmazingTrans

PLC read Potentiometer value?

23 posts in this topic

Hi there, I have a MicroLogix 1000, Currently I am testing out functions of PLC before I go on to a bigger project. I was wandering whethere can I input a Potentiometer in the PLC, and the PLC will read the Current of the potentiometer? After that, I would like to control a motor speed or either a light bulb brightness? Any idea gurus.? Thanks.

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I don't think you can read a current feedback from a potentiometor but you should be able to do a 0-10 voltage feedback. You will need a 10 VDC power supply. Hook it across the potentiometer and hook the wiper to your +analog. Hook your -analog to the common of your 10 VDC power supply

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Which ML1K do you have? You will need a model that has analog inputs.

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Yes the part number of the micrologix must be a 1761-L20AWA-5A, 1761-L20BWA-5A, or 1761-L20BWB-5A

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I have the 1761-L16BWA seriesB. Can CompactLogix L32E do the job if that doesn't? How? Thanks alot

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The Micrologix will not work without the 5A extension. You can do it with the CompactLogix using the 1769-IF4, 1769-IF8, or the 1769-IF4XOF2. The last one also would give you a 2 channel analog out to control your motor speed

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You will require RSLogix-5000 to program the compact logix PLC. RSLogix 500 which is used for the micro-logix will not program the compact logix.

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To get adjustable values in a Micrologix 1000, you can use an Optimate Setpoint Panel. They are cheap and a little crude in function, but they have been very reliable for me. This model is programmed to read, display, and optionally write to a series of six N7 addresses in the Micro. Optimate OP 413 You also need a programming cable, $30 software and power supply.

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Check these out http://www.miinet.com/products/sg_temperature.shtml

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I have RSlogix 5000 enterprise series and compactlogix 17690IF4XOF2. How do I program in ladder logic to read the analog input voltage? Thanks

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did you mean to say 1769-IF4XOF2 for your module type? ... if so, then when you configure the "New Module", you should get a series of new controller-scoped tags ... suppose that your module is located in Slot #1 ... then the tag "Local:1:I.Ch0Data" should contain the data from the first analog input channel ... note that the first channel is numbered #0 - not #1 ... you can use this address in your program ... or (better yet) set up an alias tag with an easier-to-use name ... if this isn't enough help to get you started, then post more details about your system ... especially about how the hardware is plugged together ... once we know the assembly order, we can give you more specific help with the addresses ... Edited by Ron Beaufort

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There are also some scaling issues that have to be dealt with in putting a potentiometer on the analog input. the minimum value should not be zero, and you must put some code in to make the values linear as you turn the potentiometer. When you first start turning the potentiometer the adjustment will be really fine. As you continue turning it, the adjustment will get much sharper. Just think about Ohms law. But its been a number crunching day for me and I don't think I have enough brain power to explain it. Maybe Ron or someone else can or I will try to tommorrow

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If you arrange the circuit correctly then it will be linear. The pot needs to be used as a potential divider, eg a 10k linear pot with a potential of ten volts across it will give a linear 0 - 10v at the wipper (the PLC input when set to volts can be assumed to be high enough to make negligable difference). A bit of ohms law and a series resistor will allow the use of a more common 24v supply. Offsetting the output voltage isn't strictly necessary unless you need to know the pot wiring/power has gone faulty, again ohms law will quickly give suitable values.

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Thats nice to know, how do you arrange circuit. It's been a while since I used a potentiometer, but we had problems with them being non linear. It was more of a curve. It started out gradually and got sharp at the end. But we were using a larger panel mount potentiometer and it seems that it was linear and should have been tapered or vise versa. Which type gives you the right reading on a Plc.

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I just remembered, we were using 24 VDC to power ours and had a resistor in series to drop the voltage. When you do this the output is non-linear. A straight potentiometer hook up does not have a problem Snerkel - Is there a way to make the potentiometer linear with the resistors in series through the wiring or must you do it through the program. I have a program sample somewhere I could post for making it linear but it would be much easier in the future if we could hardwire it.

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There are two kinds of pots. Linear taper and Audio taper. Linear taper pots are the most common and are (naturally! ) linear. Audiio taper pots are less common, but these pots are not linear. Edited by Alaric

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The input impedance of the analogue card in parallel with the 'bottom half' of the potentiometer is what introduces the apparent non-linearity. If the input impedance is large compared to the pot (>100x), then the effect is negligible. The effect is greatest near the maximum reading. The 1769-IF4XOF2 has fairly low input impedance of only 150K. If you use a 10K pot with this module, the effect will be quite noticeable.

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TWControl, is it possible that you can post a sample of your program? Why can't we start the minimum value from zero on the pot? Thanks everybody for all the explaination. I will try to run it.

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For most purposes the error will be negligable, probably within the error the power supply itself would cause. eg to give three examples 10% expected reading 0.1v actual 0.107v 50% expected reading 5v actual 5.189v 100% expected reading 10v actual 10.055v This is based on a 13k ohm resistor in series with the 10k pot. The assumed impedance of the PLC input 150k. Power supply 24v. I personally would stick 14k of resistance in series with the pot. It would be near enough linear for most situations (eg motor speed control) and would give 9.993v at full scale (ish)

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The only real reason to have a non-zero minimum would be if you wanted to detect failure of the power supply to the pot circuit, or the failure of the pot itself. Again in most situations the fact that a motor is running at its minimum speed or has stopped is indication enough that the pot has a problem. End of the day it depends on your application.

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The non zero on the scaler was a bad post. It was only true because we were hooking 24VDC to ours and using a resistor to cut it down below 10 volts. I'm trying to remember exactly what our resistor setup was. I might try to dig up the drawings. Its possible that we did not have our resistors setup correctly. But it was pretty much a CPT statement to cancel our the resistor that was in series.

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I have seen circuits that omit the connection from one leg of the pot to ground and use 4-20ma through the 250 ohm input impedence in MA mode. Very non linear. Somewhere along the line this post seemed to switch between ma input and voltage input. Perhaps that is also causing some confusion.

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