Help with black box RL analysis?

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I have been trying to reverse engineer a 2 terminal device and I'm stumped. Its a coil in a car strut. Reads 2.8 ohms and 4 mH with a meter. Neither goes to the metal strut body. The car tests to see if its there right after engine start. I set the scope to capture one trace and got: 12V exponential decay to 5V in 240ms. Scope is on terminal 1. Terminal 2 doesnt show anything, assume its a gnd? The test signal from the controller might switch in a cap or something and measure the decay time as a way of seeing if the R and L is in range. I can plug in a custom wound coil of 2.8 ohms and 4 mH and the darn ecu can tell it aint the factory strut and says 'Adaptive Dynamics Fault'. Like its got eyes or something. Right after this decaying exponential wave form, it gives a 12V 10% 500Hz PWM for a couple hundred ms. During normal operation with the factory strut installed, the PWM is active when the ride height is going up and down, presumably so the pump doesnt have to fight the strut. If you were tasked with designing a network that gave this 240 ms decaying exponential from 12V to 5V, what would be in the parts list? Like I said, I'm stumped. Thanks in advance. Man this is a great forum.

 

Imagecraft compiler user

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Hi Bob,

   Just a guess as I'm not an automotive engineer, sounds like the ECU can control the damping factor dynamically via the PCM signal, the electronics in the strut gets its power from the movement of the strut during normal driving, not unlike those shaker flashlights, but at engine startup the auto has not moved yet so it charges up the electronics by doing the 12 volt boot strap.  Is this on a domestic or import model? 

 

JC

 

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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Its a hi end luxury car with dynamic suspension. I have the guts of the strut at my desk. Its got a valve in it and I can hear it click closed if I apply 5V on and off. I can energize the valve with 3V or greater and it opens. I can blow in one end and open and close it with the variable power supply. I managed to capture my decaying exponential waveform in the first seconds after engine on. I assume this is the magic 'detect the strut is present' test that determines whether or not the 'adaptive dynamics fault' message is presented when the ride height controls are actuated. My hypothesis is if I make a network that exhibits this waveform, I'm golden. I want the car to think the factory struts are in and working. (Aftermarket item. No help from the factory. Maybe some help from the AVRfreaks!)

 

Imagecraft compiler user

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On the original struts, do they only adjust the stiffness of the strut, or does it also have a pressure input, i.e. self-leveling/roll control system?   If self-leveling, then the feed back maybe from the level sensors, as in, start car, try to adjust level, no movement detected, so AD fault. 

 

JC

 

 

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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Bob,

 

did you check what is happening on the lines when the black box is not connected?

If there is other behavior it might also explain.

 

as said the 12V might be a way to get things going, after that the voltage drops to an operating 5V or a 5V holding voltage

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Bob, it sounds to me like its more than just an RL circuit

 

If you are measuring an exponential decay then that says to me you are measuring the voltage across the inductor, it starts at max and decreases which should obey

 

VL=VDC*e^-(t/Tau)

 

with 2.8 ohms and 4mH thats a Tau (L/R) of 1.42ms

 

0.24 seconds is more rthan five time constants so the voltage would be in steady state

 

Working backwards with your, 12V decays to 5V in .24 seconds

 

5=12*e^-(.24/Tau)

 

Tau=-0.24/Ln(5/12)

 

Tau = 0.274

Check

 

VL@.24 seconds

 

VL=12e^-(.24/.274)

 

VL=4.99  V

 

This is assuming it is simply an RL circuit i.e its first order and Tau is defined as L/R

 

As you have measured the values and the Tau does not match the measurements then I suspect its much more complicated than just a first order system

 

I suppose you could fudge an RL circuit to have the same decay to see if that works

 

But looking at

 

L/R=0.274

 

a 1ohm resistor needs 274mH inductor!, thats a canny size inductor and increasing R to realistic values means the inductor hets bigger so its not very practical

 

If we added a C in there then I am sure we can achieve the response you require are you sure there is no capacitance in there?, have you measured the capacitance?

 

 

 

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Good analysis! The car can tell the difference between the real strut that measures 2.8 ohms and 4 mH and an inductor of the same ohms and mH. I know the strut is a valve or relay, because I can hear it click with 5V on and off, but the v/I is always 2.8 above and below the relay on voltage of about 3V, so it doesnt seem to be switching in another coil. If it was a solenoid, the L would change as the plunger pulled in. I have tried to measure the L by resonating the coil against a 10uF cap and a signal generator and a dc power supply both applied to the coil thru 22 ohm resistors. Peak the sine wave, note freq. The Rs get hot, but I didnt see the resonant freq change with the coil energized or not. During the test, the strut is just sitting on the air vent by the winshield wipes, nothing attached except the 2 wires from the controller. So it really is a 2 terminal black box. Its a Range Rover. Any AVRfreaks rich enough to drive a Supercharged V8 Range Rover? Feel like sticking your R and L meter on the front strut?

Imagecraft compiler user

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Is there another sensor that is not seeing something happen when you use the external coil?

If it's a solenoid is it releasing pressure that the cars computer isn't seeing change?

 

Keith Vasilakes

Firmware engineer

Minnesota

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Nope, strut is loose, sitting on the hood. Only connection to car is the 2 wires into it. No height sensor, pressure sensor, air pressure, nothing. Strut reads 2.8 ohms and 4mH. If I plug in a coil of 2.8 ohms and 4 mH, the car can tell. Its like Supercar with Xray eyes. I know these car companies dont want aftermarket little guys reverse engineering their stuff, but this is a real good job of hiding how the 'strut is present' detection is working.

 

Imagecraft compiler user

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Bignoob wrote:
Bob, it sounds to me like its more than just an RL circuit

Yeah - everything has a microcontroller (or microcontrollers) in it these days!

 

It could well be signalling back via some clever modulation on the supply wires.

 

There could even be an RF link - so what happens if you use some really long extension wires and take the genuine article far away from the car...?

 

 

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There is a 2 wire connector. Seems like something with active electronics in it would have at least a 3 pin connector for juice, ground, and io. This thing is the guts of a strut. There is a valve in it that clicks back out on a spring when 5V is removed. My hypothesis was this changed the L somehow, but I cant discover any change to the R or L by measuring at the wires. Keep those ideas coming gents. 

 

Imagecraft compiler user

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Keep those ideas coming gents

Did you measure the capacitance bob?

 

is there any C between the terminals?

 

 

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bobgardner wrote:
Seems like something with active electronics in it would have at least a 3 pin connector for juice, ground, and io

Not necessarily!

 

As I said, it could be signalling by modulating the supply somehow. Look at the Maxim (formerly Dallas) 1-Wire for an example of this...

 

Also, there could be an RF link...

 

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Or the feedback is provided by another sensor elsewhere on the car, i.e. a gyro, or some other level sensor.

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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But the strut isnt installed on the wheel. Its just sitting on the hood, connected by 2 wires. The car can tell if its there or not one second after start. No stimulation of gyros or level sensors. Electrical testing.

 

Imagecraft compiler user

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That still leaves the possibilities of RF and/or feedback via the supply wires...

Top Tips:

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  2. "Garbage" characters on a serial terminal are (almost?) invariably due to wrong baud rate - see: https://learn.sparkfun.com/tutorials/serial-communication
  3. Wrong baud rate is usually due to not running at the speed you thought; check by blinking a LED to see if you get the speed you expected
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