solenoid control from a 0-5 analog signal

[canuckflyboy]

I'm building a control system that actuates a bank of 19 solenoids from an input analog signal of 0-5 volts.  At first I was thinking about using a PLC and programming it with ladder logic, but soon figured out that PLC's have much to slow a reaction time.  I need the system to have a response time of at least 10ms.  From what I have read on the NI site I have figured there are three options:

-PCI card + Host PC

-FieldPoint Stand-alone

-CompactRIO FPGA

I think the last two options are overkill, but I'm looking at all the options, and there are tonnes to think about.  What would you feel the most streamline solution for this design problem be?

[AnalogKid2DigitalMan]

How does the analog 0 to 5 V signal control the solenoids? Does the level of the signal determine which or how many solenoids are activated or do they all fire at once at a given threshold?

Depending on the above, it may be possible to do the task all in pure hardware.

In my past experience, PLC's processed scan rates processed in the order of milliseconds, but never have worked with one with an analog input module.

A host PC may not give the determinism you need since Windows can hog resources away from your application at times.

Have you thought about your solenoid driver circuit? Poor optimization here can really slow down how fast the plunger actuates and retracts.

[canuckflyboy]

Firstly, each voltage step is associated with a particular solenoid and each solenoid will fire once at time given an associated voltage.  The Allen Bradley PLC’s that are fast enough to operate the system are slightly higher in price then even the CompactRio system.  As well, time based events may be introduced into the system thus the use of labview is critical.

My bigger question is the overall difference between the FieldPoint system and the CompactRio system.  They both claim realtime performance but I find it hard to find a processor speed for the FieldPoint.  What are the main trade off’s with FieldPoint and CompactRio. 

[xseadog]

Hi

maybe I miss understand your explanation:  You have 19 solenoids that will be individually enabled as the voltage varies from 0Volts to 5 Volts. Assuming a logical step of 0.25V   then your supply must : 1) be stable  with no ripple
                                                    2) able to cater for the glitch as your solenoids energise.

I would use a solution that enable relays to energise the solenoids eliminate the voltage transients.

Compact RIO can be built to be  a stand alone system. Your application, first impression, it would be overkill with compact RIO.

Just my 2 cents worth

xseadog

[canuckflyboy]

I've always had it in the back of my head that solid state relays could control the solenoids.  However, I need something with programmable logic as this system will be used in a laboratory setting thus using labview is a big benefit.  I'm wondering what the best two options for a stand-along and PC hosted NI control system would be.

[Jochen]

Hi,

as there are some Real-Time requirements in your application you should consider using LabVIEW RT for your task. Since LabVIEW 7.1 it is possible to run a Desktop PC under LabVIEW Real-Time ETS. Here you can find more information about this topic. If you are looking for a more rugged solution you also could use a PXI-system. Depending on the processing power of the system, these systems can run analog control loops with up to 20 kHz.

cFP may provide enough performance for the application that you have outlined. Please have a look at these benchmarks. Still the computing power of a compact FieldPoint is limited, so much cleaner programming is required on such a system compared to a PC to get good performance.

On a cRIO system analog control tasks can run with up to 200 kHz and the performance doesn't decrease when adding more control tasks, as on the FPGA these tasks are exectued truely in parallel.

Summary:
Probably the easiest solution is a desktop or PXI system running LabVIEW Real-Timewith one or several I/O boards. If you are looking for a standalone system, cFP might be an option but performance could be an issue. cRIO provides definitely enough performance but as LV Real-Time and LV FPGA programming is required, I agree that it might be a bit oversized.

Best regards,

Jochen Klier
National Instruments Germany