Help with Analog Computer circtuit? SOLVED

I am teaching a 300-level Electronics course this semester which is all online using CircuitLab instead of the usual hardware for practicing the theory. The culmination is building an Analog Computer simulation. The online source for the past 2 years of hardware practice has been based on the following site:

Our student results were quite remarkable and really stretched the physics students to compare to a mechanical oscillator.

I have arranged a circuit which is listed below as a link. My problem now is that I'm just not sure what or how to set up for sweeps of data. Or should I keep the input frequency constant and run a sweep of the potentiometer(s)? You'll notice in the Projects99 link above that the frequencies were held constant for each data collection of input versus output wave. I'm just getting flat lines, not even the input action of the sine/square/triangle waves.

Any ideas? This might be beyond what CircuitLab is designed for (or I might just be making a stupid mistake somewhere), but I'd really like to try to make it work if it's possible.

Any help is greatly appreciated, Carol Strong UAH

by strongc
August 04, 2020

Try a GND element on the top (+) end of V1. The Time Domain sim no longer flat-lines, but no idea if the shapes are correct...

by EF82
August 04, 2020

2 Answers

Answer by EF82


This comment has taken longer to appear than my previous one because a) its taken a while for me to remember decades ago some control systems theory, and b) we have here several questions "for the price of one" (!). I have used engineering intuition to identify and respond to a few "big" points.

1] Electronics view

Good news: CL is certainly capable of implementing what you need. For example, take a look at this complex, two-frequency (100mHz and 1kHz) non-linear feedback circuit "Incandescent Lamp" Try running the Time-Domain Simulation, you can see an under-damped response to a step input (power on). You will also see the Projects99 "Parallel-Feedback Integrator" (re)appear as a "leaky integrator" - cross-discipline nomenclature!

Not so good news: My electronic engineer response is that Project99's "Here is the circuit schematic for this block diagram" is a shorthand for the description in the block diagram above it "block diagram for the second circuit that we built". I think that the 3x pot input to Sigma (summation) would be better implemented as in "Summation (sketch)" below. In short, joining potentiometer sliders directly is a tricky thing. However, much depends on the balance of qualitative vs. quantitative results you need for teaching purposes.

"Summation (sketch)"

2] Input frequencies (I think you mean V1) The repetitive input of V1 is, I'm sure, a relic of previous mechanical and marginally stable valve (electron tube) or transistor circuits. The Projects99 "Conclusion" saying "a floating voltage source voltage that could quickly be disconnected" and "a challenge to zero out the DC offset" largely disappear in digital CL. (I'm not sure about the exact modeling of OA input offset currents in CL). I suggest that you try aligning the frequency of input V1 with the Time-Domain sim's duration, so that one V1 cycle covers the duration of the plot, for enhanced detail. The choice of frequency needs to be determined by the nature of the time-domain response - being able to see e.g. the effects of over- vs. under- damping.

3] Potentiometer sweeps I would be guided by physical-world significance here. Which pot(s) enhances/diminishes the effects of, say, tangible quantites such as force, acceleration, displacement, friction, etc? However, this just reflects my pragmatic way of learning, moving from the real to the abstract.

4] Answer - overshoots I don't fully understand the equations of Projects99 but the time response of an under-damped system is to overshoot. I think it might be possible, by subtle selection of individual pot positions, to produce over-damped or critically-damped responses. I would need a long close look at he equations and circuit values (particularly R5/C1, R6/C2).

These are possibly not the responses you want with fast approaching Fall classes (I know, a family member is a teacher) but hope this is useful clarification.

ACCEPTED +1 vote
by EF82
August 04, 2020

Whew, that gives me a lot to think about and a decision whether or not to try further to make it "work" or to scale down my ambition to something I'm sure I can handle.

Thanks so much for your serious look at the problem.

by strongc
August 05, 2020

Answer by strongc

That helped and something I didn't even think of. I now have data to look at, but not what I expected the response to the input square wave is completely out of phase. Also, as the square wave changes from rising to falling, the response of the circuit overshoots rather than following the square wave pattern.

Here's the updated sweep of data...

+1 vote
by strongc
August 04, 2020

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