The Tesla LDU traction control thread ?.

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alexsunny123
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The Tesla LDU traction control thread ?.

Post by alexsunny123 »

Hello,

Some background as to why: (1) I haven't seen this done yet. (2) Human in the loop traction control seems like a recipe for disaster when a LDU produces enough torque to break traction at almost any speed, particularly when used on the front axle.

The LDU setups which retain the Tesla logic board rely on either the driver skill or the inbuilt torque limiting which is tuned for the OEM car that utilises an open differential and brake-assisted wheelspin control. A comment I have heard from several places is that people are preferring to use a SDU or reduce the power output because the car is 'less fun to drive' with more power. We can and should have both if we optimise the axle torque throughput to match the tire capabilities. https://ometv.onl https://chatroulette.top https://omegle.wtf https://bazoocam.cam

This is all new to me so there will be a bit of learning and getting a few/all facts wrong to begin with. Please jump in with helpful info anytime.

traction control therefore needs to modulate applied torque in a similar way to antilock brakes modulate brake pressure in order to maintain a stable state under varying conditions and speed.

We know the axle speed and therefore angular acceleration from the motor encoder and we know the applied torque from the phase current. We don't however know the reference (zero slip) wheel speed or any other forces applied to the tires such as cornering so calculating a simple ratio is probably not feasible.

A simple approach could be to determine the maximum acceleration of the vehicle by trial and error and implement a lookup table of allowable phase current values for a range of axle speeds and PID that to the commanded power/speed. This would prevent gross wheelspin during acceleration but may not optimise the acceleration or be particularly useful at preventing a spinout if accelerating while cornering.

A more complex approach could be to determine the point at which phase amps begin to roll off and angular velocity begins to climb based on a product of these two variables. I'm guessing it will be prone to noise but this could give an active monitoring of energy transfer through the tire (ultimately what we want to achieve) that is independent of the available traction at any point.

ABS brake systems modulate brake pressure relatively slowly, around 5-10Hz so the processing load will not be that great if the same can be true for traction control. I believe field weakening starts at around 150 rev/sec and above that the motor is less likely to produce enough torque to break traction so that would be a logical maximum speed range (approx 120 kph, 9000 rpm motor, 16 r/sec axle). If the traction loop updated at 10Hz that would not be many updates per axle revolution but is similar to the apply-cycle rate of ABS.

thanks
alexsunny
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Re: The Tesla LDU traction control thread ?.

Post by P.S.Mangelsdorf »

alexsunny123 wrote: Sat Jul 24, 2021 4:58 pm
We know the axle speed and therefore angular acceleration from the motor encoder and we know the applied torque from the phase current. We don't however know the reference (zero slip) wheel speed or any other forces applied to the tires such as cornering so calculating a simple ratio is probably not feasible.
At least some people have considered/implemented traction control based on comparing front and rear wheel speeds using either factory or aftermarket wheel speed sensors.

I haven't looked into it that closely as I don't want traction control for my current build.
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muehlpower
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Re: The Tesla LDU traction control thread ?.

Post by muehlpower »

I have made a few thoughts and attempts on this subject.

The concept:
Rear drive with LDU, speed detection on the front axle, controller that reduces the torque when accelerating or decelerating if necessary.

The hardware:
ABS sensors on the front axle, Arduino with 2 MOSFEDs, which can pull the accelerator pedal signal and the brake pedal travel signal to ground. The two sensors have 1kOhm resistance in series, which limits the current in the MOSFEDs. In that way my hardware and my program can only reduce power, but never accidentally accelerate! The ABS signals are evaluated by a MAX 9926, which delivers a nice 5V square wave signal.

The software:
is pretty easy. I measure the time between two positive edges from the wheels and read the speed from the LDU via CAN. From the measured time I calculate the frequency and scale it to revolutions per minute of the LDU. Up to a certain speed difference nothing happens, then I start with a PWM signal and increase to 100% with increasing speed difference. Positive or negative speed differences indicate whether the accelerator pedal or the brake pedal signal is being cut.

Corresponding diagrams indicate a stable area of ​​3% -6% slip and a maximum traction value of 10% -15%. That means I will use 3% -6% for decelerating to keep the rear stable and 10% -15% when accelerating, at least on the strip, and a little less on the road.
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Re: The Tesla LDU traction control thread ?.

Post by collin80 »

What about the easiest approach? A limited slip differential is available for the LDU. If you put one of those in you get traction on both sides and thus your ability to run it into a tree or concrete barrier is diminished without having to do any fancy software tricks.

But, yes there is some worry about putting such a powerful drivetrain into vehicles.

The idea to monitor RPM and current is pretty decent. That would allow for backing off on regen or forward power if things seem to be getting out of hand. It's not super precise but it doesn't require anything to be added except some software smarts. It's the cheapest option and could be implemented right in software.

Having wheel speed sensors is likely the better option. Naturally the wheels will all track at different rates while steering. But, the rates can be calculated based on the size of the vehicle's wheelbase and the angle of the steering. So, you can get within a very tight window of what the wheel speeds should be. From there it is necessary to compare all the wheel speeds and see if any are out of spec. MuehlPower's solution would work - just modulate brake or accelerator to limit the slip.

However, this creates an upper limit to power where more power could be put to the road if a different solution were used. Obviously the most powerful solution is to do independent wheel braking like Tesla does. With the Tesla brake modules one could do this. So, I wonder if anyone has attempted to use the Tesla iBooster plus their brake distribution block to do custom braking? This seems like still an untapped area of research.
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Re: The Tesla LDU traction control thread ?.

Post by jon volk »

collin80 wrote: Sun Nov 21, 2021 2:55 pm , I wonder if anyone has attempted to use the Tesla iBooster plus their brake distribution block to do custom braking? This seems like still an untapped area of research.
Im running an ibooster with eventual intentions of exploring this. Are you aware of any CAN logs that might be available from the "CAN Private" bus that presumably outputs all the good stuff during an autonomous braking event?
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