I have a Mechanical Engineering Master Thesis about positioning control of a pneumatic system with Model Predictive Control. I am primarily an electrical engineer and i do not know much about electronics. I need to design an interface card in order to connect a valve (5/3 way), two pressure sensors, a linear scale to my NI-PCI-6010 data acquisition card.
I have a bunch of questions about them. I tried to find solutions about them but none of them is enough for me. I wish you can help me.
However proportional valve (http://xdki.festo.com/xdki/data/doc_ENGB/PDF/EN/MPYE_EN.PDF) needs to be driven with 4-20 mA, my PCI-6010 card (http://www.ni.com/pdf/manuals/371498a.pdf) doesn't support it as i understand. Its analog output have a range of -5 to +5 V, i need to convert it into 4-20 mA.
First of all, do I need to convert -+5V or can I use 0-5V instead? I have seen lots of voltage-to-current converters in other websites, but all of them is about converting 1-5 V to 4-20 mA, I think there is an offset problem for 0-5V. I have found a non-inverting summing amplifier that can do what I want, but when input voltage reaches 5V, output current reaches 20,496 mA and this is surely a problem, also i need linearity.
I have found an IC that can convert 0-5 V to 4-20 mA (XTR110, I think you all have a word to say about it), but I cannot wait to purchase it, because i couldn't find any distributor in my country. So that I want to design a PCB in order to get over this problem.
Also I don't want to use a passive voltage-to-current converter. Here is the circuit I designed.
V1 = 5 V, Vref = 1 V (for offset problem)
Also; when V1 = 0V, output current is 4,037 mA. But it is not a big problem. Other values are;
V1=1 V, Iout=7,329 mA
V1=2 V, Iout=10,62 mA
V1=2.5 V, Iout=12,267 mA
V1=3 V, Iout=13,912 mA
V1=4 V, Iout=17,204 mA
As you see, I cannot achieve a setpoint value for my valve too (It needs 12 mA for middle position).
Of course I don't want you to do my thesis for me, but I stuck on early stage of my thesis and if I cannot found a solution about it I absolutely have a real bad time.
Thanks in advance.