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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Historical question: negative feedback and the op amp
References: <3DD25F72.5793B8D5@webaccess.net> <6PuA9.5701$XF5.firstname.lastname@example.org> <3DD27F3D.667A4D75@webaccess.net> <3DD2FD36.8877972F@webaccess.net> <3DD3A4A5.688DDDDD@webaccess.net> <3DD4635C.3E820E6C@webaccess.net>
Date: Fri, 15 Nov 2002 14:08:18 GMT
NNTP-Posting-Date: Fri, 15 Nov 2002 06:08:18 PST
> "Chuck Simmons" schrieb im Newsbeitrag
> > This part of the thread was inspired by ideas set forth that negative
> > feedback in particular can somehow be replaced by digital systems of
> > arbitray precision and speed. In other words, the idea of the
> > operational amplifier is dead or soon will be.
> you expressed exactly what I wanted to say.
> Kevin brought up the application of an audio amp. Here latency doesn't
> disturb at all, so all kinds of filters can be used.
But you have taken my words out of context because my further point was
that I considered the given problem and proposals for digital control
from a standpoint of damage control. That is, I had to consider what I
could do with opamps and what I could do with a digital system and find
ways to keep the sacrifices required for a digital approach from being
too great. Even then, the digital approach closes a negative feedback
> It is also not much different from a normal control, just that all 16 or
> 24bit output states have to be reproduced continuously. There is the amp and
> the motor(speaker).
> But now the feedback signal is taken from the output voltage! Also it should
> be possible to connect all different kinds of motors with the amp staying
But this is not like a control system at all. In most control systems I
work with, I cannot compute a command that will cause the motor to go to
a position or a speed because the motor does not work that way. I can
command an acceleration. This is why I need negative feedback. In fact,
if you look at a block diagram of a digital control system, you often
can't tell it is digital because there is an output to an actuator and
an input from a sensor. The block between can be either analog or
digital. An analog block will often have better performance.
> And when measurements are taken, instead of the motor a fixed resistor is
> connected and now the amp produces 0.001% distortion and has an output
> impedance of 0.01 ohm.
> These measurements are absolutely irrelevant. The feedback signal is a sum
> of the counter voltage and output voltage.
> And look at the motor, or lets say 3 motors in a box. They have a crossover
> with 4th order filters, what can be said more.
> And when you measure the distortion of the original signal with 3 generators
> at different (fixed) frequencies connected to the output as well, that would
> be more appropriate. I do not want to see the result...
But this is not feedback control where the controller is given a goal
for a mechanical motion (for example) and the controller must achieve
it. The goal is to make the motion match the goal based on a mechanical
(or other) measurement. You are describing an open loop system designed
to achieve an arbitrary goal without direct measurement.
> Now analog this couldn't be done, but digitally we have the chance to
> calibrate the whole system with 2 mikes in the ear channel or whatever.
> We can even shoot a few times to create a pulse response of the room and
> mike, so the "feedback" is conserved, because it will be always similar and
> with the reverse filter many deficiencies can be already eliminated. With
> more DSP power even unlinearities at higher levels can be compensated for.
But how does this relate to feedback control? Remember, if the system to
be controlled is not well behaved, your open loop method fails while the
closed loop system works well whether digital or analog.
> Now sit in front of your expensive stereo system and put a good classical
> chamber music record. Why doesn't it sound like "live". Why can you exactly
> point to each loudspeaker and have problems to locate the virtual location
> of the sound sources?
> This is where I'm working at since a couple of years and when you visit me
> and listen to my sound system you will be surprised!
But what you have said is unrelated to feedback control systems. I
recall one customer who called me on the phone from half way around the
world because he observed that I had adjusted my servo power amplifier
(a power opamp - LM1877, often used in audio) so that my output DAC had
only the capability to position open loop to about 5 microns. I agreed
that I had done that precisely because I had to retain dynamic range and
I only had 10 bits. He then asked how I expected to meet the requirement
of positioning to 0.2 microns given I only had positioning capability 25
times worse. At this point, I did not know whether to try to teach
elementary control theory by international phone call or simply say
Think of it this way. In what you are doing, you are trying to reproduce
an exactly proportional voltage to a voltage measurement you are given.
With cost no object, that can be done. Control systems are trying to
keep a sensor on target given that both the target and the sensed
parameter can change in arbitrary ways. This is the feedback idea. This
was the idea in the steam engine governor, the feedback amplifier, the
opamp and the servo of whatever type. Remarkably little changes when you
substitute digital methods. In digital terms, feedback is a method for
real time correction of errors from unpredictable sources.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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