Not a PLC Question - Controls Related Though

Hi;
Thank you for reading this. I've got a simple problem that I need help with. I have a situation where I need to control a pump to maintain pressure. I'm using an Allen Bradley 160 variable speed drive and a pressure transducer. As the pressure drops, I need the pump to speed up.

The particular transducer we were using (we are on our last spare) outputs 10v with zero pressure, 0v with max pressure (30PSI). That matches up well with AB drive; low signal low speed - high signal high speed.

The real simple solution is to find a similar transducer. However I have access to transducers that output 0v at 0 pressure (or 4ma - 20ma at high pressure) and these are what I want to use.

It seems like there ought to be a simple electronic gizmo that can swap input - change 0 to 10 volts and 10 volts to 0 volts (or the 4-20ma equivilant). Or, is there a trick that I am missing with the AB 160? Can the 160 do this conversion?

I can be the hero at work if I come up with a solution.

Thank you for your time and help;
Frank

Check the manual, you should be able to set parameters 16, 59, 60, 74, 75, and 76 to make it work with any option either way.

By default I think it would have worked the other way but not sure. Check those parameters carefully, you should be able to reset them to workas needed.

Thanks for quick reply

Thank you for your help. Knowing that the answer is there will get me to look deeper.

Again, thank you;
Frank

Not to be insultive to rsdoran, but Parameter 16 is read only on AB160s. He is correct on the other parameters to look at. Use parameter 16 to check your zero offset if necessary.

I gather from your post you are going directly from the transducer to the 160?

Yes, we are connecting the transducer directly to the AB 160. I think we are even powering the tranducer off the AB 160.

Thank you for the help;
Frank

Another option, if you don't want to reconfigure the drive, is to stick in a signal conditioner that will convert the 4-20 mA to 10-0 VDC. APi and M-systems and Action Instruments and Acromag and many others have this kind of device in stock.

Simplest Conversion V-mA

Actually the simplest conversion of a 4-20mA signal is to put a 250ohm resistor across the I/O input terminals. By Ohm's Law the signal would be converted to 1-5V. If you need a higher voltage range, use a 500 ohms resistor and you'll get 2-10V. Standard instrumentation signals have "live" zero values e.g. 4mA or 1V to differentiate a zero signal from a broken wire. You may need to provide a 24Vdc supply to the transmitter though if your A-B module doesn't provide this.

If you are having problems with the durability of your existing transmitters, you should use a high-end gauge pressure transmitter from manufacturers like Honeywell or Fisher-Rosemount. These have higher accuracy, reliability, repeatability and burst limits. Most new transmitters are "smart" meaning they have intelligent microprocessor-based electronics. A handheld programmer can configure ranges, limits, including direct or reverse action of output to input. You could also configrure signal "damping" to eliminate cyclic fluctuations or surges typical for pump applications.

With regards to your PLC, I'm not very familiar with A-B hardware but I'm sure you could easily configure an analog input to have a live zero like 2-10V instead of 0-10V.

If your A-B drive has a PID function, I would rather leave the PID processing to the drive instead on the PLC. The PLC could just provide the setpoint to the drive. Some PLC's process PID relatively slower than others especially when the user logic is quite big and scan times become longer.

Be careful

This might lead to very unstable speed control if the pressure transducer is connected directly to the inverter (VSD) speed reference.

What you are proposing is a Proportional Control Loop, but the line pressure is not proportional to pump speed (check the pump curve). So either the pump will never run fast enough or it will overrun, causing the inverter to ramp up and down cyclically.

I really would suggest getting a cheap PID controller (although you probably only need PI control) or using an inverter with a PID function.

Rube and Rsdoran are correct. For a very simple way of change the input signal so that 0v is full speed and 10V is no speed, all you need to do is to adjust parameter 75 and 76. Parameter 75 is the analog input minimum and 76 is the analog input maximum. If paramter 76 is lower then 75 then the drive acts like the analog input is inverted. I believe this is just what you want. You do want to watch other items like unipolar and bipolar or min & max frequency or zero offset.

hope this is what you are looking for.

Thank you for the help

Thank you everyone for your help. Altering parameters 75 and 76 did the trick. All the other suggestions were very much appreciated as well.

This forum is amazing. Your knowledge and generosity is overwhelming.

I made the mistake of showing my boss this site. My chance of being a hero is greatly diminished. Now he knows where I get my ideas from and who the real heroes are.

Thank you again;
Frank

Hey Frank, tell your boss that any damn fool can log onto the site - it takes a highly skilled professional such a yourself to ask the proper question and implement the selected answer!

I agree with Tom. Anyone can read a book, but some people actually know what they read and what to do with the information. I've met several PLC "programmers" who couldn't start a program from scratch or troubleshoot problems. It just depends on your level of experience and intuition. Chances are you'll be a hero again.

Just a piece of advice--never let the boss know everything. Teach him everything he needs to know, just not everything you need to know.

Re: Thank you for the help

Philadelphia Frank said:

I made the mistake of showing my boss this site.

Click to expand...

My boss made the mistake of showing ME this site. Three jobs, and three years later he still can't get rid of me.

This board is a tremendous resource. You can come here to beg for help, or to fill your back of tricks and trivia. Just be sure to share when you have information of your own!

AK