DDL - faster Nernst cell sensing

[Bernd Felsche]

The following was published on the DDL mailing list about a week ago.

>From bernie Thu Nov  1 15:14:06 2001
>Subject: DDL - faster Nernst cell sensing
>Date: Thu, 1 Nov 2001 15:14:06 +0800 (WST)

[snip]

>One of the previous limits in DDL was the ADC delay for determining
>the Nernst Cell voltage. It came to mind during that discussion that
>the AVR has a perfectly-adequate analogue comparator on-chip and
>that this could be used to detect a swing from stoichiometric
>(0.45V) almost instantaneously. The deviation from the nominal
>voltage, and its direction could be used to determine the pump
>current direction required to re-balance the Nernst Cell.

>Circuit complications arise in requiring that the nominal reference
>voltage float above the cell's virtual ground as that pin has to
>be raised above the "signal" level when measuring the cell's
>resistance for temperature feedback (and hence heater control).
>The virtual ground voltage also rises to pump ions "out"; the pump
>and Nernst cells share one connection.

>At first these complications appeared onerous.

[snip]

See http://bernd.felsche.org/tech/EFI/DDL/DDL.html for the few
circuit changes required to implement the fast switching detection,
largely eliminating the bandwidth limitations imposed by ADC and
subsequent numerical comparisons. Response bandwidth for the
controller now approaches 1 MHz compared to the 15kHz or so limit of
the ADC strategy for Nernst cell voltage detection.

Furthermore, the detection method lends itself to a simpler control
strategy for applying pump pulses (e.g. bang-bang control relying on
sensor hysteresis), as well as the ability to measure intrinsic
sensor characteristics such as ion migration delays. Cell
contamination and other age determinants can thus be quantified.

-- 
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