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Old 07-22-2003, 12:58 PM   #24
Scooby Newbie
Member#: 40416
Join Date: Jul 2003
Location: CA
68 Porsche VW Bus


Hi guys,

Rob pointed me to this forum about our LM-1. Because I know this thing well I will try to answer some of the questions:

1. Patent application:
We filed for a provisional patent in January. The full patent application is in process. Provisional patents are not listed in the patent data base. Within the next few months the full patent application should be in the database (government speed).

2. Accuracy:
The accuracy of the meter was determined by using lab-grade calibrated gases usually used for calibrating smog-check equipment. By testing about 50 sensors we found an accuracy of < +- 0.1 AFR for the worst case sensor. The problem is repeatability for even the same sensor. Depending on ambient air pressure, humidity and (in case of the Bosch sensor) sensor housing tenperature causes variations can occur.
The unit DOES NOT use the built-in calibration resistor of the sensors.

3. Maximum operating temp and temp control
The heater system in the LM-1 operates off the impedance of the pump cell, not the sensor cell or the heater resistance. During warmup the LM-1 continuasly monitors the resistance of the heater element (positive TK) until it reaches about 85% of the nominal heater resistance value. Then it switches to regulate temperature of the pump cell using a PID algorithm. When a sensor is powered up the first time it warms up by heater resistance first, then regulates the temperature by the impedance of the measurement cell (Nernst cell) to the nominal resistance of the Nernst cell as per Bosch specs. After a settling time of 20 seconds with constant Nernst impedance it measures
and stores the dynamic pump cell impedance. Sensor manufacturers don't specify the pump cell impedance because it cannot be easily measured, specially at stoich. At stoich the pump current is 0 in conventional designs and you cannot measure impedance with 0 current. Our design works differently and therefore allows to measure it. Using the pump cell impedance was severall advantages:
a. Oxygen pump factors are dependent on pump-cell temperature, not Nernst temperature. Measuring pump cell impedance allows more precise temperature control.
b. Pump impedance has an even steeper temperature coeff. than the Nernst cell impedance.
c. Injecting a high frequency signal into the Nernst cell for impedance measurement requires removal of that signal through filters. These filters increase the response time of the system.
Placing a WB sensor in front of a turbo means that the maximum operating temperatures of the sensor can easily be exceeded (even with the heater off). Because the Bosch sensors are much more temperature dependent than for example the NTK the accuracy will greatly suffer. The NTK on the other hand is less temperature sensitive but much more pressure sensitive than the Bosch, so it will suffer more from pressure induced inaccuracies.
The turbo itself also acts as a great mixing device. Exhaust gas exiting the headers are anything but uniformly mixed, specially on the rich side. This causes noise in the output simply because richer and leaner gas pockets pass by the sensor (if the response of the system is fast enough).

3. Analog output resolution
The analog outputs are two 10 bit DACs with a range of 0-5V leading to an output resolution of 4.88mV. The lambda signal is first converted to a range of 0.5 lambda to 1.523 lambda (0.001 lambda resolution at 1024 steps). The result is then mapped to the specified curve for each output. Because of the 10 bit resolution of the DACs a 0..1V output would have a resolution of 200 steps.
The numeric display (lambda and AFR) is the lambda value averaged over 0.3 seconds. Any faster and the last digits would just blur in the real world. For logging the lambda data is averaged over 1/12 second (the logging data rate). The bar-graph on the display and the analog outputs are derived from instant lambda with a sample rate of 50..150 Hz depending on sensor response speed and lambda. The minimum lambda is sensor dependent. In case of the Bosch it's minimum is about 0.65 lambda.

4. Display size
To add a bigger display for AFR you could just use a digital voltmeter module (0..1.99V) available from several manufacturers. You can also use a round instrument as for example supplied by Steward-Warner and intended for NB sensors. Just program one analog output to the range the display instrument uses.
Because we display a lot of textual information as well on the display for diagnostics we chose to use a 2x16 display. The large versions of these displays are expensive and would not easily fit in a hand-held unit.

5. Pressure compensation
The measurement method we use and have applied for a patent indeed does allow to theoretically compensate for exhaust pressure. Unfortunately the Bosch sensors have a temperature dependent parasitic resistance in the virtual ground of the pump/Nernst cell. This interferes with our pressure compensation by simulating exhaust pressure changes when there is in actuality only a change in sensor housing temp. We are working on a software way around that so that you can download it later to the meter. This is a try, not a promise.

6. Pricing
In my original post 7 month ago on DIY-EFI I mentioned a target price of $300.-. This was a little optimistic (mea culpa). At our current production quantities we cannot reach that price and still have decent margins to allow further development. We think at $349.- we still are very competitive given the features we were able to pack in the product. We are committed to continuously improve and enhance the product through software updates which can be downloaded for free from our web-site.

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Klaus is offline   Reply With Quote