Hi Lacy,
It's a combination of two sweeps: a parametric and a dc sweep. I swept what I thought was the potentiometer's wiper and found the dc point with each change of the pot's wiper, but Multisim produced inaccurate results.
See attached pot_sweep circuit.
Understand that the line I'm calling Vwiper is actually the voltage at the wiper with a resistor in series. To get the actual voltage at the wiper itself, the series resistance can be made very small. What I'm trying to do is I'm trying play with the linearity of the pot, examine how each resistor affects the overall slope (the linearity), the character of the Excel plot.
Through doing it the hard way, using the voltmeter and making one change at a time, I've been able to demonstrate my hypothesis to myself, finally, but it'd be so much nicer, much more presentable, if I could make changes on-the-fly and then port the results to Excel. I just haven't been able to pull off what I can do manually using Multisim's sweep option.
What I've found is that the sweep only works when the pot stands alone (plus an impossibly high shunt resistance, so as to name the line of the output variable) and is driven by a current source that produces the desired max voltage. However, when shunting real-valued resistances are added, the current source also feeds the parallel path, of course, so the numbers are wrong again. Hence, I went to a voltage source again, but the numbers are still wrong. Thus, in order to get the results I was expecting, I could only manually perform the sweep, by using a series of manual pot changes, and with each change, I manually transfered the voltmeter's number to Excel.
It seems I've set up the sweep correctly, because, with only the current source, the pot's wiper seemed to change, based on the results. Likewise, in the resistance sweep (parametric sweep), it shows the current resistance value for the variable selected in the drop down box; you can change the wiper manually and then go back and note that the current value changes, reflecting your manual change, proving you're looking at the wiper and not the resistance from end-to-end.
I even tried a current source (which it liked alone) into the pot and a voltage source across the shunt resistances (which it also likes,) but the sweep still produced bad numbers. I found that what could be done manually just couldn't be done automatically. The only other thing I can think of is maybe I'm, despite the evidence of the wiper change, still not using the appropriate variable, as, in truth, I really don't understand what it means by "r1:xr1," while I couldn't seem to find any explanations in the manual for the various esoteric expressions available in that drop down list.
But I'd sure like to figure this thing out, as this would be a wonderful way to demonstrate Multisim's ease of use and capability. It takes time to manually tweak the pot and then transfer the new voltage drop number to Excel, time that, for the impatient people I work with, I don't have. However, were I to copy a new column of numbers into a second Excel sheet with each sweep, well that would be quite acceptable, as it'd make for an on-the-fly cause and effect demonstration.
I hope you can help Lacy.
Incidentally, assuming you have Excel, I'm also attaching an Excel output that demonstated the pot sweep worked when the pot stood alone and was driven by the appropriate current source. This plot will also show you the plot I'm trying to get when I add the other resistors to the circuit. (By hanging the "_none" suffix on the filename, I'm showing this data was gathered from only the stand-alone pot, with no other resistors, aside from the impossibly large shunt resistor explained earlier.)
