DDL - faster Nernst cell sensing

[bcroe at juno.com]

OK, So I'm somewhat ignorant of the development.  

On Thu, 15 Nov 2001 10:11:48 +0800 (WST) Bernd Felsche
<bernie at innovative.iinet.net.au> writes:
> bcroe at juno.com tapped away at the keyboard with:

> I think that's where communications is breaking down. Direct Digital
> Lamda does not attempt to _emulate_ an analogue operation.


> > I thought you were trying to avoid the A to D frequency 
> > limitations by going to a simple fast comparator.
> 
> That's correct. The comparator has nanosecond delays.

> > Sure it gets simpler.  The analog  version uses a 
> > Proportional/Integral/Differential servo loop to develop
> > the output signal.  With a simple comparator (bang-bang
> > control) you have no idea of the rate of change, so the
> > D term goes away.  With no indication of the size of the 
> > error, the P term goes away.  So you are left with an
> > Integrator, and the penalty is much less optimum 
> > frequency response.
> 
> It's the "P" term that's not *immediately* available. It can be
> measured though. But there _may_ be little point in doing so if the
> Nernst cell voltage is constantly jitter about the 0.45V under
> the action of the controller. 


> Note that the pump cell is always operated at "full" current and
> alternating voltage pulses. A nett-zero current is achieved by short
> alternating pulses of equal magnitude. With the mixture leaner or
> richer, one of the pulses is lengthened; the effective pump current
> is the (signed) sum of the magnitudes (essentially the duration).
> That will cause the Nernst cell voltage to jitter about 0.45V at all
> mixture levels.

Right, I visualize this as replacing the linear drive with
class D.  This could allow a direct digital drive.  But this 
does not define your feedback algorithm.  With a A/D 
converter on the input you can implement the full PID
control digitally.  With just a comparator, I can think of a 
couple ways to go.

If you reverse the drive every time the comparator crosses
over, the drive frequency will be determined by the sensor
delay, probably in the low milliseconds.  Your output will
have a huge signal at that frequency which must be 
filtered out to prevent obliterating the desired info. 

If you reverse the drive at a much higher frequency, the
sensor may average it out well.  But now you are back 
to which feedback algorithm to use again, to set the 
drive duty cycle.  I don't see how a comparator input 
will allow more than a digital Integrator loop, which 
means giving up a lot of frequency response.  Do you 
have a different plan?


> As the sensor's response is unknown, 

> > The sensors' response is not unknown at all; take a look
> > at the PID response of the analog units and build on that.
> 
> Where is that documented? I haven't seen any sensor-specific
> information of ion migration velocities or Nernst time response as a
> function of voltage, temperature, pressure or partial pressure.

Do you really want to know ion migration velocities?  How do
those fit into your calculations?  Maybe the time delay and 
gain between when you change the drive and the Nernst 
output shift from .45 volt would be more useful.  Some of us 
have actually done these experiments to get the feedback
loop stabilized.  The results are obvious in the analog 
designs; BobR and I both concluded an integrator time
constant around 60 mil seconds was a conservitive number
over a wide range of situations.  We might be willing to do
some more specific numbers to help your project along,
but once you are operating, you could do your own tuning.

I don't see how pressure fits into the equation; do you have
a way to measure it?  Temperature I would think is a rather 
secondary effect in balancing the .45 volt, although
stabilizing temperature ought to improve accuracy at 
the DC level.


> More importantly; the digital pulses will not all be identical. And
> they do require measurement to ascertain cell resistance (hence
> temperature), as well as detecting sensor "age" effects.

If I understand correctly you are referring to an increase
in the resistance to the Ip drive current.  I have "curved" a
couple of these and they are something like 50 ohms.  
And pretty linear over the operating range.  So you would
note this value (in non volatile memory) and then 
periodically check for changes?  Or just compare to a
standard value?  Bruce Roe

> Bosch's use of the LSU 4.2 sensor is apparently on a virtual ground
> basis. The latest information made available indicates that it's
> highly likely that the Bosch circuit is quite similar to DDL.
> 
> -- 
> Bernd Felsche - Innovative Reckoning, Perth, Western Australia

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