Trying something new with 4-20ma

I have a joystick that will provied a 4-20ma output based on its position. That joystick will in turn control a hydraulic valve.

Customer has requested a potentiometer that would limit max output from joystick. In other words, if the joystick normally sends 4 to 20 ma from center to full on position, a setting of a separate potentiometer would limit the output. If the potentiometer where to be set to limit max output to 50%, the full range of the joystick would be 4 to 12ma.

I'm thinking that I could just insert adjustable resister in series with loop to control current. However, I know that we must be concerned with loads on analog current loops. I seems the additional resistance could burn up joystick circuitry.

Any ideas? Anyone done something similar? This aplication does not need extreme percision. I am open to a separate analog coversion module if one exist to do exactly what I need.

I'll throw this in as well. The joystick is bi-directional. It will be 4-20 ma (+) one way and reverse polarity the opposite direction 4-20 ma (-). I don't want this information to bog down ideas though. If desired control is acheived one way, the other solution will be close by.

Thanks in advance for any ideas..

Does the hydraulic valve have on board electronics so you can connect your 4-20mA signal directly to it? If not you will need some form of power amlifier and most of them has a possibility to limit max solenoidcurrent.

Yes the hydraulic valve is loop powered. The joystick and valve are same brand and directly compatable.

When you say loop powered do you mean that it is completely powered by the 4-20mA signal(2 wire) with no seperate 24V DC suply to the valve? I find it difficult for a proportional valve to be powered by a 4-20mA signal. What brand/type of valve and joystick are you using?

Yes, loop powered and having only two wires. The brand of both are Saur Danfoss. I don't have the exact part numbers. The way these valves work is they are held in a center position. This way a slight shift via 4-20 ma command is all that is needed then the pressure of the hydraulic system helps to shift the valve position. The valve does not have to fight the flow.

Any ideas on limiting an existing 4-20 ma command?

Andy, I'm not sure about how functional that would be. What do you think of the adjustable resister in series?

Is the joystick just a pot in itself or does it have a active (semiconductor) output?

It has ICB. It takes 24 vdc input. It gives 4-20 ma output bi-directional and is matched to same vendors valve. The bottom line is I have a 4-20ma loop. Can I put anything in that loop to control maximum current?

Thanks for your input..

The device is going to attempt to drive the required current at its output. If the load resistance is increased the output is still going to attempt to drive the same current by raising the voltage.

My reason for asking is most of the 4-20 output devices I have dealt with have close to a zero minumum load resistance but do have a max resistance and if you exceed that max the output will become unstable.

By putting the pot in parallel you are shunting some part of the output current, and the output device does not know the difference.

Sorry if I am off base here,
Andy

A potentiometer to shunt some of the current is a bad idea. Let's say you install a pot that shunts 40% of the current, so that at 20 mA you actually have 12 mA going to the valve. The same potentiometer will only allow 2.4 mA to the valve when the current is 4 mA. This is obviously below the range the valve will accept.

You need a signal isolator with the zero still set at 4 mA but the span set to 12 mA.

This might help

http://www.sauer-danfoss.com/Literatur/
Danfoss has a hydraulic specialist, but I can`t find his card here at home. I`ll check at the office tomorrow and see if i can find his number. He can tell how to do this if possible. He helped us with a project last summer and was a life saver. Can`t believe i`ve lost his card!

Well rytko has part of the solution with the signal isolator. That will give a correct current signal - at one setting. I believe that g. Robert needs it to be adjustable, like with a pot. How do we get that for him? Using off the shelf stuff, how abut TWO signal "isolators" (conditioners). The first one (current-to-voltage) translates the 4-20 mA signal into 0-10 vdc. Then add a pot to adjust that voltage on-the-fly, wherever it needs to be. Then send the modified voltage into the SECOND signal converter (voltage-to-current), which changes it back to a current signal, now at a reduced level. Then connect the current to the hydraulic valve.

Another flash-in-the-pan possible method: Put a big pot in series with the 24 VDC power supply to the joystick (or get an adjustable power supply). Unless the joystick has a built-in voltage regulator, then by Ohm's Law, I=E/R, so if you reduce E, you should reduce loop current also (within certain limits).

I work with electronics for Sauer-Danfoss valves almost every day and has not yet seen a valve from them with 4-20mA signal. Is this any sort of PVG32 valve? Normally these are controlled by a 25%-75% of supply voltage signal and that makes this task easy by connecting serial resistors to the joystick and therefore reducing the percentage of the signal.

Robert,
do everybody a favor and establish the exact type number of that valve ! It cannot be so difficult, just walk over and read it off the type plate.

If it really is 4-20 mA, and there is no PLC to do the "limiting", then PR Electronics have modules than can do all kind of things with 4-20 mA - including adding, subtracting, multiplying and limiting.

edit:
Just noticed that the Joystick has -20..-4ma & +4 .. +20mA range.
I have never seen anything like THAT before !
I doubt that the PR modules can be used with negative 4 .. 20 mA signals. I cannot think of anything that can interface with a signal like that !

The resistor in series won't work with a 4-20 mA device - the current through all elements in the loop must be the same (Kirkhoff's Law?).

Put your pot in parallel with the input on the valve. If the valve impedence is 250 Ohm and the pot is set to 250 Ohm you will get 50% of the current through each. If the Pot is at zero, you will get no current through the valve. If the pot is at 1000 Ohm, well you figure it out. It is a simple parallel resistor circuit.

Don't guess on the permitted joystick load - that should be in the published specs for it. Then calculate the load for the combined pot and valve resistance with the pot at both ends of the range. You should be OK.