Limit Mask Test Limit Trapezoid Profile

Question

So to one of our proportional valves, we are going to use the gain scheduling set point profile to create a trapezoidal input current to the valve. This trapezoidal input represents one cycle for the rocker arm we are controlling (it is periodic and this profile is repeated for many cycles). I'm creating a fail-safe system and am wanting to create a limit profile that is essentially an offsetted trapezoid of +/- 1 mA. The largest and smallest trapezoids would be the constraints and the middle one would be the input to the system. I want it where if the input current exceeds this inner or outer trapezoid, the system goes into the safe state. I've tried using the Mask and Limit VI and defined my upper and lower limits by making 4 different points for each limit but when I give it my input signal and observe the tested signal, all I see is the input to the system with a horizontal line at y=0 which is the green colored limit. I can get the constant upper and lower limits to work but I'd really like a closer more defined constraint since we are controlling a very big structure. What am I doing wrong? I've attached a picture of the input to the system.
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trapezoid profile.jpg ‏57 KB

Answer 1

Hello,
I believe the issue is that you do not have enough datapoints defining your upper and lower limits.  You only placed coordinates for the vertices of each trapezoid.  The Mask and Limit Testing VI will not automatically generate datapoints between these points it needs a full set of datapoints defining the whole graph.
Your input data has much more datapoints than the actual boundary you've defined, that's why it appears as a line at y = 0.  It plots four points on a chart that probably that has much more data points on the input.  You should generate trapezoidal signals and wire those signals into the upper and lower limit inputs of the express VI.  That way you do not have to fill out all the datapoints manually.
Below I've attached an example LabVIEW project that shows how to do this.  I used this example to create a subVI to generate a trapezoidal signal.  It uses ramp functions to piece together an array of datapoints that looks like a trapezoid.  
I think you should use the "Generate Trapezoid" subVI I included in my project.  Generate the upper and lower trapezoids outside the while loop with the same amount of datapoints as you expect from your input data.  This should save processing so you don't have to generate the waveform each loop iteration.  It will look something like this:
Regards,
Isaac S.
Applications Engineer
National Instruments
Attachments:
TrapezoidalLimits 2010.zip ‏839 KB