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Changing circuitry of analog computer *During* simulations?

  1. Nov 9, 2016 #1
    I am wondering whether it would be possible for the analog circuitry itself to change DURING the time an analog computer does computations.

    Over and above the above mentioned question, is it possible that the circuitry itself can change DURING simulations, while this change in the circuitry is decided by the result of the calculations during the previous step (while solving a problem).

    I am asking the above question because I have not come across any example where an analog computer changes its circuitry (connections) DURING simulations. Or, the circuitry (connections) is constructed (wired) before the computations (simulations) start, and once a computation starts, the circuitry cannot change during that particular run.

    Of course, I know that a circuitry can be re-wired to carry out some other computation.

    And of course, the terms "circuitry", "connections", and "analogue computer" above may be interpreted in very general sense. I am aware that "re-wiring" need not necessarily involve manually re-wiring the circuitry ("re-wiring" may be accomplished using software tools).

    I believe that if at all it is possible for the circuitry itself to change DURING simulations, it may be advantages to use analog computers instead of digital computers while solving certain problems.

    Hope I am clear and looking for an answer.

    Thanks and best regards,
    Kirana Kumara P
     
  2. jcsd
  3. Nov 9, 2016 #2

    LvW

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    Of course, you can modify any component value (amplifier, integrator,..) DURING a simulation with an ANALOG computer. This is not a problem because an anlaog computer is nothing else than an electronic analog circuitry consisting of amplifiers, analog summing circuits, integrators, voltage dividers,..).
    Furthermore, there are block-based DIGITAL simulation packages (e.g. VISSIM) which also allow parameter changes during simulations in the time domain.
     
  4. Nov 9, 2016 #3

    anorlunda

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    I agreed with @LvW , of course they can.

    Even ancient analog computers from the 50s and 60s (programmed with plug cords) could have switches and relays that switch the equations being solved.
     
  5. Nov 9, 2016 #4
    Thank you for your reply. I wish to know answers to two more points: 1) Whether the "connections" can also change DURING a simulation 2) Whether the change in the component value and the change in the "connections" could be automatically calculated (decided) depending on a result that is already computed (but this result computed DURING the SAME simulation).
     
  6. Nov 9, 2016 #5

    LvW

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    (1) Yes - why not? However, in the time slot between both connection states you have undefined conditions, of course. More than that, this seems meaningful only in case you have a continuous input signal (and not a step).
    (2) This means that you will have an additional control loop which connects the output (decision maker) with one part of the circuit. In such a case, you must be careful in order to avoid self-excitement (stability problems).
     
  7. Nov 9, 2016 #6

    jim hardy

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    In the early sixties i was a wireman on an assembly line building hybrid computers, analog computer controlled by a programmed digital one. They were destined for Cape Canaveral .
    One of the assemblies i made was an array of multiturn potentiometers with little servomotors to turn the knobs. Of course today you'd use digital potentiometers.. That'd do part of what you propose.

    Digital computers soon afterward got fast enough to replace most analog .

    Doesn't a simple diode or analog comparator do what you suggest ?
    What's your application ?
     
  8. Nov 9, 2016 #7
    Thank you for your reply. In fact I am a mechanical engineer and do not know much about electrical circuits. I knew that an analog computer can be programmed using a digital computer (thus making it a hybrid computer in fact). But I was under the impression that this "programming/"building" the circuitry" is possible only before a simulation starts; I thought that once a simulation starts (in other words, once an analog computer starts solving one particular problem), neither the analog "circuitry" (by "circuitry" I mean "connections") nor any of the parameters of the components in the circuitry can be changed. But as per the above replies from LuW, one can change the connections as well as parameters DURING a particular simulation.

    My (speculative) application is in the area of the real-time simulation of biological organs (here very complicated calculations need to be completed within a very small fraction of a second). As of now, digital computers are incapable of meeting the requirement of real-time performance.
     
  9. Nov 9, 2016 #8
    Thank you once again for your replies. It would be helpful if you could answer the following questions also:

    1) Why do we need to have a continuous input signal (why not a step)?
    2) Is the self-excitation problem avoidable by a good design (at least to some extent so that it would not pose practical difficulties)?

    (Just for information, I am a mechanical engineer and do not know much about electrical circuits.)
     
  10. Nov 10, 2016 #9
    Could you provide some quantitative idea about the time needed to change the connections and/or parameters when compared to the time required for the "simulations" (Of course, in an analog computer the time required for the "simulations" is negligibly small).

    This is because I am thinking on the possibility of achieving real-time simulation of biological organs by building a suitable analog computer. I am thinking about analog computers because it is incredibly fast. I should be able to change the connections/parameters DURING the simulations because the geometry of biological organs can change DURING the simulations. Of course, for the time being I am not bothered about what would happen during the time the connections/parameters are changed. Hence I would get the solution using an analog computer for a particular set of connections/parameters; the solution can be obtained in real-time since I am using an analog computer. Next, if there is a change in the geometry of biological organs (because of a surgical cut, say), I would change the connections and parameters, and then I would again get the solution in real-time. But since I am interested to simulate the surgical cut in real-time (which means that I should be able to complete the solution within a very small fraction of a second), using analog computer will not solve my problem if changing the connections/parameters several times (which corresponds to cutting incrementally) cannot be completed within a very small fraction of a second.

    Hence I wish to know whether the whole simulation (including the task of changing the connections/parameters) can be completed within a very small fraction of a second if one can build a suitable analog computer. (Of course, individual simulations can be completed within a very small fraction of a second if one goes for an analog computer. The term "individual simulations" here means solving for a particular set of connections and parameters.)
     
  11. Nov 10, 2016 #10

    LvW

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    Please note that this option is not available (as far as I know) for SPICE-based circuit simulation programs.
    In this context, I have mentioned BLOCK-oriented programs only (like VISSIM).

    1) The step response consists of a transient starting at t=0. If - during the response time - the system is changed by switchung between two states the transient (which you are interested in) is destroyed because intial conditions are altered.
    2.) Yes - of course. However, you need to be familiar with feedback theory and stability criteria.
     
  12. Nov 10, 2016 #11

    f95toli

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    But not as fast as a digital computer. The Strong Church Thesis tells us that any analog computer can be efficiently simulated using a digital computer. Since digital circuits are much, much faster than analog circuits it follows that there is nothing to be gained by this method in terms of speed.

    Note that the speed of analog computers is limited by the same factors that limits the speed of any other analog circuits; there will always be some time associated with transferring information around a circuit and we do not have efficient memories or buffers to make this process easier.
     
  13. Nov 10, 2016 #12

    Baluncore

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    This topic is all a bit hypothetical. Maybe you could give us some idea of the form of the equations that need to be solved. Many of us have years of electronic computation experience and know ways of doing quite complex things very simply and quickly.

    It would be a pity to attach your project to analogue computing if there was a more flexible digital solution available. I would be quite surprised if we could not digitally out-compute an analogue computer with an array of digital RISC or signal processors.

    On the other hand, if there were simple analogue solutions, we would probably recognise them quite quickly.
     
  14. Nov 10, 2016 #13
    My problem is to solve a set of coupled nonlinear partial differential equations over an arbitrary region (solution region). I may also use numerical techniques like the finite difference method or the finite element method to get the solutions. Do you think it is impossible to obtain faster solutions using analog computers when compared to digital computers, when the set of equations, boundary conditions, and the solution region are specified?
     
  15. Nov 10, 2016 #14

    anorlunda

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    You must put numbers on it before we can answer. We have no idea what you mean by fast.
     
  16. Nov 10, 2016 #15

    Baluncore

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    It might be that your non-linear equations perfectly fit some electronic analogue. Without seeing the form of the equations, and the non-linearity, it is impossible to tell.

    An analogue computer can get trapped in a dead end as easily as a digital computer. With the digital computer you can repeat the failure exactly and analyse the problem. Because of analogue noise, an analogue computer will not always take the same path to a destination so it is difficult to repeat a failure for analysis.

    I believe the digital simulation of an analogue computer has for some time now been faster and more accurate than the analogue computer itself.
     
  17. Nov 10, 2016 #16

    jim hardy

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    The idea that animal tissue can outrun a computer just doesn't ring true for me.
    Nerve impulses move only around 400 ft/sec i was taught. A physical cut proceeds only as fast as a scalpel can move . Or are you simulating something more kinematic like a high power laser ?


    As fascinating as it'd be to do this analog
    i recommend you write a program with one of your finite element solutions , have it access a timer and report how many microseconds elapsed during execution.

    I did something similar on an embedded microcontroller running interpreted Basic which is really slow . It was for a crane weigh cell that put out an ASCII string every 0.3 second representing the weight on the hook, some tens of tons. That ASCII number had to be converted to analog voltage with a DAC and handed to a monitor that compared tension to strain gages looking for unexpected deformation..
    I had it set one output line high at routine start and set it back low at when finished. Watching that line with a 'scope i could see how long it took.
    Wow did i learn about streamlining a program with that one ! Cut my execution time by 75% with common sense things like eliminating loops and unnecessary calculations..

    So my point is
    If your criterion for "Extremely Fast" is what you can perceive with your senses, i think you need to familiarize yourself with just what the digital guys can do nowadays.
    Search on DSP IC" .

    old jim
     
  18. Nov 10, 2016 #17
    By "fast" I mean that I should be able to complete the calculation within 30 milliseconds. It would be even better if I could complete the calculations within 1 millisecond.
     
  19. Nov 10, 2016 #18

    anorlunda

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    Huh? Your definitions must not match mine. Two electrons experiencing Coulomb force are an analog computer that runs instantaneously. A resistor with voltage applied instantaneously solves Ohm's Law (or Maxwell's Equations if you prefer.). How could digital be faster than that?
     
  20. Nov 10, 2016 #19

    anorlunda

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    Do you mean that to simulate the process in real time, you would like to calculate 1 ms of elapsed time in 1 ms of computer time?

    Most of today's CPUs can accomplish very complicated things in 1ms.
     
  21. Nov 10, 2016 #20

    f95toli

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    I guess it depends on what you mean by a computer. This is far outside my expertise, all I know about this comes from reading about e.g. adiabatic quantum computing and whether the D-Wave computer (which if it is classical is analog) is faster than a digital computer.
    However, I think the point is that the time required for a digital computer to solve a given problem is bounded by a polynomial function of the resources used by the analog computer.
    According to one of my review articles one standard reference where my statement is discussed in more details (although it mainly deals with the NP completeness etc)

    Vergis, Anastasios, Kenneth Steiglitz, and Bradley Dickinson. "The complexity of analog computation." Mathematics and computers in simulation 28.2 (1986): 91-113.

    You can find a PDF of the article online.

    Also, the following is somewhat more readable
    https://www.cs.princeton.edu/courses/archive/fall06/cos576/papers/yao_acm03.pdf

    (see the bit about the ECT)

    One interesting consequence of this is that physical systems of any kind can not solve NP complete problems.

    Also, on somewhat related note, I recently saw a talk about work on using superconducting electronics to simulate neurons; although the circuit was still digital.
     
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