watlow pid

[DeclanKillilea]

I have attached a drawing of what i am hoping to do connectionwise with this project.I have been rooting through some discussion forums and i am beginning to undertand i need static i/o vis for my digital stuff,please have a lookk and advise.I done undeerstand exactly how to set up the digital i/o as i am unsure of what u8,u16 etc max task i should assign to it or are these the wrong type entirely and where should i look for a different type as both the inputs and outputs will just be changing from on to off and wont be containing any other digital data

Thanks all Declan

[J POWER]

Hi Declan,

                 Just to confirm you are struggling with the set un p of the DAQ task in MAX?

With in MAX you can set the device, channel and parameters of your DAQ devic. this is done here so that you dont have to add the channel configeration into the code.

What is the exact area tyou are getting stuck on?

Regards

[DeclanKillilea]

Hi I seem to have it working pretty ok now as i have made a good few changes .however i have one or two problems.When running the vi with the solid state relay unconnected to the output terminal of the usb if it calls for heater on i get 5 volts and 0 volts when its off,however when i connect the ss relay i only get 1.26 volts across the terminal of the solid state relay so my voltage is being dragged down somewhere,i need a good investigationas to why..

The other thing i would like to be able to do is when saving the vi i would like it to remember which port is used by which read / write vi as at the moment i have to reselect the lines everytime i close and open the vi.I am sure it is something quite simple.

I am using the for loops to add a type of pwm output to the ss relay should it work?

Have a look if you get a chance

Thanks

Declan.

[J POWER]

Hi Declan,

                 The way to initialize the ports, if you set the 'Physical Channel' you are using then right click on the white of the duologue box a list appears. Select the 'Data Operations' then select 'Make current value default'. Now save the VI and every time you open it these will be the ports configured.  (see attachment )

now looking at your block diagram I have noticed that you have placed the channel control, create channel and clear DAQmx task inside your loop. This is not the best way to code as LabVIEW will create a DAQ channel every time the loop iterates, this take time and oppressor power. Now if you place the channel control and the create channel in the outside of the loop (to the left) then pull the task wire into the loop and connect it to the read/write block. Then place the clear DAQmx task outside the loop (on the right) [see second attachment]

the relay is a current driven device and the USB-6008 digital line is a voltage signal with a small current, you will see the voltage drop when energies the relay and it is trying to pull more current than there is available.

one way you can get round this is by parallel connecting multiple channels to allow the current to be supplied at the required voltage.

(visit the link http://www.ni.com/pdf/manuals/371303l.pdf  and see page 21) 

[DeclanKillilea]

Thanks Jordan for the tips,

these should help alot.I measured the current needed to turn on the solid state relay and i got a reading of 8mA so I was thinking my NI unit should be able to deliver that for me.Last night however i connected just the solid state relay directly to the output with nothing else connected(to rule out any wiring issues i may have) and i was still coming out with poor voltage.I probably need to connect a resistor to bring up the current on the output.

I am just afraid that the volt drop could be caused by trying to use pwm on the output?could this happen?

I will experimant a bit more tonight with a true/false on off and see how i get on and i will upload the latest version of the vi after i have made the approriate changes to it to improve preformance of the vi

Thanks again for your help and i am looking forward to any more suggestions you may have

Declan.

[J POWER]

Hi Declan,

The use of PWM will not effect the current or volatge on a line as PWM is just turning on and off the output.. If your system is still not operating as you want it to, you could try some external circuitry with optoisolaters or transisters to switch a higher current.

Hope the project goes well.

Regards.