Author Topic: Integral ABS - CAN (servo brakes) "Pressure in rear/front circuit too low"  (Read 97661 times)

tpdnexus

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Sorry for the long silence. The COVID19 situation did basically stop any work on this
project for the past months :'(.
So far we found:
- The sensor I received from Jughead, after we sorted out some FEDEX problems,
  arrived in good conditions, one sensor marked as defect
- we carefully removed the gray ceramic plate of the defect side
  and found the following:
  > Schuppi (Wolfgang) was correct in his post. The sensor is in reality an active sensor (MEMS)
     with its own integrated, micro Controller circuit on the back side of the ceramic.
     The ceramic has electronic parts and circuitry printed on the backside which appears to
     be a thick film technology
  > The sensor chip is  an MLX 90308 with an integrated µC and
     still available (US$21.-) . The iABS computer communicates over a 1 line serial
     interface with the µC to program the function and receives an analog signal back.
     The other two wire of a total of four for each sensor are Vdd and GND.
  > The pressure sensor appears to be a resistive wheatstone bridge
     printed on the ceramic plate, a quite common configuration for pressure sensors.
  > the round circle you can see through the ceramic in above picture
     is the part which presses on the four resistors (the bridge) in a defined manner (area) on the ceramic plate.
     So this is the real pressure sensor, and the signal is then amplified, digitized and the data
     adjusted (program) for the application and the output to the iABS computer.
     The rest of the ceramic real estate is just used for the electronics.
 
Next steps:
- reverse engineer the sensor chip firmware and the algorithm
  which generates the analog output signal for the
  ABS unit computer and compare it with the "good Sensor".
- reverse engineer the electric circuit on the ceramic plate

In general the resistive pressure sensors (the printed resistors)
of the defective sensor look and measure okay.  The chip on the sensor
from Jughead has a date code of 2006, so a real possibility
it died or, what I was noticing in my iABS, becomes erratic at high
environmental temps. Not unheard that the probably older EEPROM
technology used for this design has developed errors over time.
So there is a chance that this type of failure can be
repaired by exchanging the chip only (assuming the iABS computer
downloads the firmware during the initialization). It would require
a SMD solder station with a microscope and is worth  a shot.

A fault like a broken ceramic plate I have seen in another picture
might also be fixable: As long as the crack is not near the "round
sensor" area and under the IC itself, simply bridging the cracked printed wires on the backside
might do the job  :o

I have decided to take out my own ABS unit in exchange for a Second Life
unit to have a test bed for the sensors and the further reverse engineering  ;).
In principle  8) I only have to catch the initialization data sequence coming from
the iABS with a logic analyzer for getting all the parameters....
but of course it will be more difficult than that.
So waiting for the 2nd Life kit now.

TBC...

Jughead

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Thanks for the update.  Look forward to a positive result.  ;D

Conley11

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thanks for this post!
hi there

brag

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When diag says "pressure to high" or/and the output from the sensor is about 5v try to wash the sensor unit thoroughly in isopropyl alcohol. Often there is an unwashed flux between the analog output and Vdd (f**ing FTE :) Don't try to remove the ceramic plate cuz it will damage the thick film strain sensor resistors.

MLX90308 has nonstandard custom firmware so standard commands from datasheet do not work. But this chip responds with ASCII encoded hex data to commands 0x52 0x53 and 0x54. I guess 0x52 is some sort of read-RAM command, 0x53 is RESET command and 0x54 is read-EEPROM command.

Here is an example of responses:
Code: [Select]
54: 3e 31 32 33 34 34 33 30 33 38 33 34 44 36 34 30 33 30 30 41 35 30 43 38 43 30 41 38 45 38 37 45 42 30 31 44 46 38 35 36 36 30 36 33 34 30 31 30 30 30 30 41 32 30 30 30 30 30 30 30 30 45 31 30 30 30 30 30 36 30 30 30 30 30 30 30 41 30 42 30 43 30 30 37 35 42 33 30 36 30 30 30 30 30 30 30 30 45 43 33 36 3c 0d
53: 23
52: 25 30 31 30 34 33 32 30 31 38 43 30 41 41 35 30 43 38 43 30 41 41 35 30 43 30 31 43 41 30 30 32 36 44 46 38 35 36 36 30 36 30 30 30 30 35 32 36 34 38 43 30 41 30 30 30 30 38 43 30 41 30 30 30 30 39 34 30 45 30 30 30 30 46 45 46 46 42 34 30 30 30 30 30 30 30 30 30 30 32 41 30 30 30 30 30 30 30 30 30 30 30 30 30 30 32 30 32 30 30 31 34 44 30 31 33 31 42 35 30 31 33 34 30 32 30 42 30 37 30 37 30 33 38 33 45 34 33 39 26 0d

knappen

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Great post! Thanks for the advice  :)