DIY-EGOR ideas (was: No Subject)

[Jörgen Karlsson]

I think that the simm100 is a good unit, mostly because everything can be
made available through ribbon cables. The fact that it is available as a
finnished and tested unit is good, if nothing else it feels safer and you
cannot forget to buy anything...

> Good stuff for experimenting but watch the temperature rating of the
> components if you want to go "live" with it.

I am sure that the guys as Lawicel can use components with a larger
temperature span if you ask them, or you can get parts with the right rating
and an empty board.

> > We cannot pwm anything across the pump cell, it has to be a
> > constant flow there, I think that the way to go is to filter the
> > pwm output to get a stable current. If the sensor ground is put at
>
> I haven't been able to find a reference about not being able to PWM
> the pump current. Are you able to tell us where you've found that
> information? What sort of load (RLC) does the cell present?

Yup, I guessed...;)

The addition of a rc network wouldn't hurt, would it. As I see it the filter
makes it harder to damage the cell. We can also use a higher maximum current
then if we only limit current by using a resistor. We can of course use
other types of current limiters but they are more complex.

I know that these sensors can measure afr's in the 9:1 range, maybe lower.
IF WE ASSUME, that anything above 6mA kills the pump cell we have to limit
the maximum current to 6mA (THIS NUMBER IS TAKEN OUT OF THE AIR!!!!) if we
don't have a filter we will pulse the cell at it maximum current all the
time! If we use the filter the cell only see the needed current...

> > 2.5v above the circuits ground the processor can drive the pump
> > cell and filter circuit directly.
>
> Depends if the cell is floating or tied to ground somehow. If it's
> floating, then a bridge circuit can be used to reverse current flow.
> It's quite common in DC motor drives.

The pump cell and the sensor cell has a common ground, but this ground is
not common with the heater ground... At least that is what i find when I
look at the schematic that Bruce posted. The sensor Ground is clamped to 2.5
volts there.

> What sort of filtering is necessary?

I would use a rc network.

> > We know that Bruce and the guys used up towards 1.1A heater
> > current, lets assume that is max. Let someone else do the damage
> > ;) First of all we filter the pwm output from the mcu and then we
> > use the filtered voltage to drive the power transistor.
>
> > We also need to measure the current across the sensor, we only put
> > a series resistor and measure the voltage drop. By using a
>
> That's why there's a current-sensing resistor in the ground-lead of
> the H-bridge in the circuit described earlier.

I haven't looked much on that schematic, I haven't printed it and it is not
fun to watch it when it is verticaly oriented on the screen...

> > We also need the voltage across the heater element it self, if we
> > have the current and the voltage across the heater we know the
> > resistance, that we can translate to temperature. I think that
> > aiming for a set resistance here is the way to control the heater
> > correctly, If the resistance goes up we must lower the heater
>
> Why not measure the voltage drop "directly" using an ADC channel in
> the AVR? As in my discussion circuit where there's a current sensing
> resistor, and a voltage-divider to scale the nominal voltage to
> below AREF. That reduces the number of external components.

That's a good one, I feel stupid...

> > current and the other way around. I think that we can be pretty
> > lay back while doing this, I think that we will use one voltage
> > across the sensor for initial fast heating, then at some
> > resistance we drop the voltage to prevent cracking, the correct
> > values can probably be found in the base shematic. After that we
> > let the raising resistance of the sensor take care of the
> > adjustment, we just put two more regulation points, one that
> > lowers the voltage a bit when the sensor is close to maximum temp
> > and one that almost completely shuts of the current when it gets
> > even hotter.
>
> Sounds very complicated. If you're controlling the current through
> the heater, then you're controlling its power output.

I am sure that many of the sensors will see a lot higher exhaust temperature
then 600ºC, when that happens the heater will have to be put on idle until
the temp drops within operating temp of the sensor again. Someone mentioned
that the sensor must be heated slowly at high temp. I don't think that we
can use any simpler temperature correction, at least not if the sensor will
se hot exhaust gases.

As I see it we can manage without the H bridge for the pump cell on your
circuit and use your current sensing on the heater element.

I think that the next problem to address is if we need to measure the
current across the pump cell or if we can use the voltage across it instead?

Jörgen Karlsson
Gothenburg, Sweden

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