Analog computation, Part 1: What and why
analogictips.com
analogictips.com
It's much easier, faster, maintainable and usually cheaper to add a cheap controller and peripherals than to find an electrical engineer to do the math and design a circuit.
Analogue systems have a lot of non-ideal characteristics such as drift, thermal sensitivity, parasitic capacitance and inductance, impedance matching problems, insane cumulative errors and manual tuning requirements that don't exist in the digital domain. They are also more expensive to modify, and the price goes up significantly with complexity and speed. Not to mention small and fast is really difficult in the analogue domain thanks to everything being a transmission line.
Any sufficiently advanced analog computer is indistinguishable from a noise generator.
Modern autopilots are all sensors, DACs, ADCs, and various digital computing resources. Cheaper, faster, more featured, and mystifying in their failure modes.
Although I am a big digital person, even for photography, audio and other seemingly analog things - I admire and appreciate the one major analog computer in my life.
Alas, after 7 years, it finally needs a repair. Sigh.
The altitude is “held” using a “static” pressure sensor and pitch is adjusted in a control and feedback loop to attempt to keep that signal constant. When it works, it is very smooth.
The system is not as a whole redundant, but there are many failsafes. You can put it into a mode where the system does not fly the plane but tells you what it thinks you should do. This is called a flight director. You fly it almost like a video game, get the pointers aligned.
Each component has some redundancy. It has a self test, an annunciator panel saying what mode it is in, a mode control panel, several controls on the yoke including a kill switch, several circuit breakers (one, the electric trim, which has a big red flange on it - see the recent 737 crash artickes), etc.
The workflow of using the autopilot is always command a change, check that the change is reflected on the annunciators and that the plane is adjusting attitude and such appropriately, then resume the normal scan. If at any time anything looks off my instant reaction is always to hit the big red button and take the plane’s control back. When you do this you need to be wary for an out of trim condition and other sudden forces on the yoke and rudder.
Not obvious to work with, but a great solution if you want to multiply two signals.
>"Q: What are some of the issues associated with analog computation?
A: They are similar to the ones that are of concern whenever analog functions (op amps and more) are used for signal processing or signal conditioning. These include linearity, offset, temperature-related drift, aging, and bandwidth."
I understand bandwidth but could someone elaborate in lay terms what these other "concerns" are in analog circuits? Specifically what are linearity and drift?
Drift: temperature changes, behavior of component changes (e.g. resistors change slightly in resistance, circuits to generate precise voltages produce slightly different voltages, oscillators change in frequency). E.g. precision frequency sources often have a heating element to keep their core oscillator at a stable frequency, audio devices thermally couple two components of which the ratio is important, so the changes cancel out as much as possible, ...
Drift: component values are not fixed! Most vary slightly with temperature, and capacitors change value over time too. Can be seen in any kind of old equipment which needs to "warm up".
One follow up - is it accurate to say that linearity and drift are always related then?
Offset: When something is supposed to read a specific value (usually voltage), what is it actually reading? Most op-amps (configured as buffers) won't actually read 0V if 0V is applied. Usually this offset is super small (10s of mV, or even way less) in modern op-amps. Sometimes it's offset current. Many op-amps specify both current and voltage offset.
Temperature-related drift: This is component value change specifically related to temperature. Multi-layer ceramic capacitors are especially susceptible to this. A 'good' MLCC might 'only' drift +/-20% across temperature
Aging: As a component ages, how does its value change? Electrolytic capacitors have this problem. If you've ever heard of people talking about replacing capacitors on old TVs or what-not, it's almost always those caps. The paste in the electrolytic caps dries out and affects the value of the capacitor. Once this reaches a certain point, the circuit won't work anymore.
Note I provided specified examples for each of those problems, but they exist across pretty much every type of component to various degrees. For example, linearity can be a big issue on ceramic capacitors. Their values can change as much as 80% (yes, 80%) with applied voltage which can cause some really weird non-linear behavior to some circuits. Most modern silicon probably doesn't have many problems with aging unless it's driven to the ragged edge of its performance or there's an actual defect, since typically chips are qualified to run for a minimum of 100,000 hours. That being said, eventually it WILL fail if it's running constantly. Electromigration affects all silicon. By comparison, usually electrolytic capacitors you'll be lucky to get 10,000 hours out of them. Less the hotter they get and the more the voltage applied is.