PSN-L Email List Message
Subject: Re: 'soft force feedback'
From: "GMV" gmvoeth@...........
Date: Sun, 22 Feb 2009 07:38:24 -0700
Hello Randall Peters;
Have you ever used an industrial control PID loop
kind of thing with a Hall Effect sensor ?
I would imagine it may work quite well to keep a mass
locked in one position then look at the energy expended
to keep the mass locked when anything tries to move it.
You talk capacitive sensor but that is an active device
purring out all kinds of RFI in an already saturated RFI world.
I am interested in all forms of passive or baseband
devices that do not need artificially applied AC to make them work.
PID meaning proportional-integral-derivative.
Possibly more complex.
I have seen PID loops in auto cruise controls and
in food service industry to control flow rates
and temps. I imagine PID loops along with 20ma
circuits can do about any kind of control.
----- Original Message -----
From: "Randall Peters"
Sent: Sunday, February 22, 2009 7:15 AM
Subject: 'soft force feedback'
What you have indicated is indeed what I have used with a fully differential capacitive sensor monitoring the displacement of my
modified Sprengnether (zero-length, Lacoste) vertical seismometer. The output from the sensor goes to an opamp integrator, whose
output is a very weak correction signal (fed in turn to the original coil/magnet sensor, now acting as an actuator) to keep the
system from 'going to the rails' of my capacitive sensor.
As I have noted previously, to operate with a PID feedback and then use the (so called 'velocity' (really 'jerk' below the
corner frequency) output only-destroys low frequency response. This 'pulls out the frequency multiplier term' by the chain rule of
differentiation, causing the response to go to zero as the frequency goes to zero. I teach my students to recognize the important
differences between differentiation and integration when it comes to electronic signals containing noise. The former is a 'noise
enhancer' and the latter a 'noise reducer', as is well known to anybody who has looked at their differences using an oscilloscope.
About the differences between 'force balance' and 'soft feedback'. Force balance is 'hard' in the sense that ideally there is no
motion of the seismic mass whatsoever. The feedback signal is so strong that it allows one to monitor the 'error' value required to
eliminate motion-as representative of what the mass would do if allowed to move in an ideal Hooke's law oscillator.
Unfortunately, there are no Hooke's law oscillators. It has taken me a long time for the scientific community to begin finally
accepting my claims concerning mesoanelastic complexity. There are two types of anharmonicity, (i) elastic and (ii) damping. Many
of you know about (i) since a big, close earthquake will cause anomalous response from any seismometer, because it is afflicted
(large motions) with a restoring feature that is not perfectly harmonic. When seismic disturbances are 'low and slow', meaning low
frequency as well as small amplitude, the 'corrugation-like' features of the restoration potential come into play. Engineers know
about 'dithering' as a means to combat friction effects. In effect, that is what I recommend. It is advantageous to let the system
'skate' over the metastabilties of internal friction type, some of which can cause the system to be effectively 'latched' against
being able to see the low/slow signals.
For my Sprengnether, the time constant of the ompamp integrator was set at several hundred seconds, so as you say, to integrate
in a lower range than the one of interest. My approach to this is not the first. Erhard Wielandt mentioned at the IRIS Broadband
Conference that a German seismology team did effectively the same thing about a hundred years ago. They used water (probably
hundreds of gallons) in a feedback scheme to alter the tilt of their seismic platform to keep the instrument from going to the rail
because of the adversities of (i) buoyancy of air pressure changes associated with moving fronts, and (ii) temperature changes
altering the modulus of the spring.
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