Using multiple channels on NI-9474 to control a electromagnetic valve

[RuedigerSmooth]

Hello, 
I´m using a c-daq 9178 and a NI 9474 card to control an electromagnetic valve. To fully open the valve, you need at least 2.5 A. I need to operate the valve in the full range, so I must be able to set a current between 0 – 2.5 A. Since one channel of the NI 9474 can only deliver 1 A max, I need to use three channels of the NI 9474 card. So, this is where my problems start. If I use three times the “DAQmx Create Channel.vi”(ini_valve_output_control.PNG) the 3 channels start at different times (tek00024) and because of the Duty Cycle behavior I don’t get the current I need (tek00023).

After analyzing this behavior I implemented a delay of 2 ms and 4 ms in the first two “DAQmx Create Channel.vis”. This solved my issue at the beginning, but when I operated the valve for some time, it looked like the the different currents from each channel shifted and I got the same problem again that I don’t have the desired current.

Problem here is, that you can’t predict the shifting. Since the valve opening angle is set by a corresponding current. Means no current -> Valve closed. 2.5 A -> Valve completely open. Furthermore, the valve is used to set the system pressure. Therefore, because of shifting in the current I have a different system pressure. 

[RuedigerSmooth]

Attached you can find the VI

[GerdW]

Hi Ruediger,

somehow I gave the same advice someone else some days ago:

Why don't you use a MOSFET as an amplifier (or a SSR or any other circuit capable of driving 2.5A) and use just one CTR output? No hazzle to synchronize 3 outputs…

Btw. what is the purpose of that WHILE loop? Use a feedback node for the DAQmx reference and remove the loop…

[RuedigerSmooth]

 Hi GerdW,
thanks for the fast response. Yeah that was probably my supervisor. 
For me it's about understanding how it works, why in this case it doesn't work. If I have a look into the manual I can see that increasing current works easily by wiring the channels in parallel. Having done that I can say, that this is not the case. How would I have to continue if I want to make work, using just the NI 9474?

Thanks for your help! 

[GerdW]

Hi Rüdiger,

setup a task containing 3 DO channels (plain DO, no CTR).

Switch those DO each 100ms and check with your scope how synchronous they switch…

[RuedigerSmooth]

Hi GerdW,

thanks again for your reply!
I tried this one, and the current looks way better. But now I have the problem, that the current is oscillating on a bigger time constant, this causes my system pressure to oscillate as well. So I think Counter is the better way to go. You have any other idea how I can approach this topic just using the NI-9474?
Is it in general possible to ensure that the 3 counter work simultaneously for the whole time? Because in my first post I stated, that in the beginning I had the 3 counters working simultaneously but over time they lost synchronisation.. 

Thanks again for your help   

[GerdW]

Hi Rüdiger,

using an external amplifier with just one CTR channel will solve all your problems...

An oscillating current is no problem of the NI9474: it basically is operating as a switch. The current is determined by the power supply and your valve.