BTW Luciano TY very very much for posting up the documents, because of the new information I found in there I am going to make a few design adjustments to my circuits and MCU program.
|
BTW Luciano TY very very much for posting up the documents, because of the new information I found in there I am going to make a few design adjustments to my circuits and MCU program.
|
================================================== ======
My previous post:
The PDF was not modified. You will have to draw the two op-amp's
and their connections on page 4. The 100 ohm resistor on the output
of the op-amp "Pump current control" is present in the Tech Edge circuit.
================================================== ======
The CJ120 also measures the Nernst-cell internal resistance (Ri) which
is used to determine the temperature of the sensor. The internal resistance
of the Nernst-cell is 80 ohm when the sensor ceramic temperature is approx. 750°C.
By knowing the (Ri) you can control the heater supply voltage so that the
temperature of the sensor is kept at a nominal temperature of approx. 750°C
which is needed for *precise* measurements.
The Nernst-cell internal resistance is measured by pulsing the Nernst-cell via a
capacitor and a resistor.The pulsing must be done between 1kHz and 4kHz.(CJ120 3kHz).
On the Tech Edge Pty. Ltd. schematic you can see that they measure the Nernst-cell
internal resistance and also measure the effective heater voltage and current.
On the CJ120 block diagram you can see that the effective heater voltage and
current are not measured. The document http://wbo2.com/lsu/Y258K01005e03mar21eng.pdf
describes the maximum permissible heat up rate when the heater is switched on.
When you know the Nernst-cell internal resistance then you can control the heater
without measuring its voltage and current.
Luciano
|
I have read the datasheet on the sensors many times before.
My original circuit had PID control of heater to maintain the 80 ohm resistence on the nermest Cell with a 3 khz ac signal, I made a mistake in my previous post about the 80 ohms in the pump cell when it should of been the nermest cell.
|
I just looked at the tech edge diagrams in detail, everythin looks fine a dandy except I did not notice a coupling capacitor to couple the 3 khz AC signal to sense the Nermest resistance, the "VsDrive" connection connects through a resistance (910 ohms) to pin 11 of the atmel8 AVR MCU, if you are coupling an AC signal of 0v to 5v then for the bosch lsu you would need something in the 10k range and also a capacitor. Also Pin 11 of the atmel8 AVR is not a PWM pin, if he is ouputing an AC signal he must be doing it via software.
I am thinking that the TE unit actually powers off the sensor and does DC sensing of the the resistence in the nermest cell, which would make the unit super slow.
I am jsut puzzled because no where on the schematic are any hallmarks of generating an ac signal, coupling an ac signal into the nermest cell, or low pass filtering to target a signal in the 3khz range. The only filtering around the VS port are low pass filters with a 3bd freq of about 10khz.
|
Hi,
See attached BMP file.
Nernst-cell internal resistance (Ri) measurement.
The circuit measures the resistance of the Nernst-cell and
also the resistance of the RiCall 82.5 ohm. Both measurements
are done with the same I/O, so we measure with the same voltage
when we source (+4.xx V) and when we sink (0.xx V).
The PWM is not used.
The AC current is ≤ 250 µA.
(See 250 µA current direction when the I/O sink or source).
The differential amplifier will output a voltage when
Ri and RiCall have different values.The output voltage is
measured with the A/D of the MCU. With the resistors you
can adjust the gain of the amplifier.
* * *
The Tech Edge does almost the same with the I/O but
they use a 910 ohm resistor so the current is more
than 250 µA. (???). The capacitor is the 100nF C201.
They use 2V as the virtual ground.
(See page 6 of the PDF for Max. current load of the Nernst-cell).
http://wbo2.com/lsu/Y258K01005e03mar21eng.pdf
* * *
This is just a theoretical circuit.
Never been tested.
Luciano
|
910 ohms would mean they are violating 250ua parameter by more than 10 fold with a 0-5v signal, c210 looks like it is just part of a lowpass filter to remove noise rather than to couple an ac signal into the nermest cell.
It looks to me that they really are powering off the sensor and doing a DC measurement of Ri. I also have no idea why they use an external DAC rather than the avaible PWM ports on the Atmega8 instead.
Luciano are you affiliated with Tech Edge? are you also building you own Wideband reader? Why are you using RiCal of 82.5? I believe it is a reference resistance for the nermest cell, shouldn't it be 80 ohms?
The way I am doing the Ri measurement is different as I try to use as little OP amps and resistors as possible and use a bit of digital signal processing instead.
|
I see a problem with the circuit of my last post. (BMP file).
When we are not measuring the Ri of the Nernst-cell, the output voltage of the cell +2.95V (+2.5V +0.45V) goes through the two 10K resistors and the 82.5 ohm resistor back to the 2.5V of the virtual ground. (We have 20082.5 ohm connected in parallel with the cell).
The circuit will need analog switches (like the 4066) in order to insulate the Nernst-cell.
Two years ago I helped my cousin on a similar project.
He married, his wife sold the car, the project never started.
Luciano
Members who have read this thread : 0You do not have permission to view the list of names.
Posting Permissions
Bookmarks