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 Post subject: Traction Control
PostPosted: Tue Dec 13, 2016 4:34 pm 
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I'm pondering traction control now.

I have the sensors setup I think, I still need to test on the car.

Form there my thought was just target an allowed % slip (front vs rear) and close the throttle to get it.

But now I'm wondering if a % slip is the right number of if it is more about tire slip rate, so the tire shold slip at say 1m/s for best result meaning its a high % slip at low speed and a low % at high speed?

I need to do some research unless someone here has a good answer?


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 Post subject: Re: Traction Control
PostPosted: Tue Dec 13, 2016 5:19 pm 
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I've pondered but not implemented traction control as well..

For purely launch control (drag racing or off the line), and assuming 2WD, you can make a ton of assumptions. Launch only will always be straight line, so the non-driven wheel speed average (i.e. average of R and L front wheels in a RWD car) should be the ground speed for slip calculation. If you want traction control through turns, you will need to account for the steering changes in the front wheel speeds (probably using a table from steer angle multiplied by the speed to get the 'equivalent' speed).

It seems like % slip is the right way to do it, talking with tire experts (of which I am *not*), although the % slip to target changes based on surface (e.g. snow or gravel, more slip is better, but hard surfaces need very little slip for ideal traction).

My thought:
-TCS provides a throttle limit to ETC - the lower of TCS and Pedal command is always taken
-Slip is controlled with a PI controller which measures the slip ratio (driven speed / ground speed) to the target and controls TPS limit
-When the pedal command goes below the TCS limit, the TCS needs to *slowly* ramp back the PI controller to 100% to prepare for more slip in the future. This allows the I term to stay at the last used limit if the driver is rapidly going on and off pedal or bouncing around the slip limit, where just resetting the PI control or letting it 'unwind' on its own would mean it would take some time to bring the throttle down from 100% all over again when slip is detected next
-If you have driver input of track conditions or tires (e.g. wet/dry switch), you could initialize the throttle limit to a non-100% value (say 60% for 'wet'), and when it is reached, the PI control could raise it if slip is not detected when pedal command > TCS command.

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"Sometimes, the elegant implementation is a function. Not a method. Not a class. Not a framework. Just a function." ~ John Carmack

"Any sufficiently advanced technology is indistinguishable from magic" ~Arthur C. Clarke


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 Post subject: Re: Traction Control
PostPosted: Tue Dec 13, 2016 6:23 pm 
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Well, slip % is pretty easy.....tuning its pretty a blast :)

I don't think TC should ever be able to do anything but close the throttle. Then once the TC position is above the user command it disengages and resets if needed.

Launch control is a little more involved for staging I think but probably a plugin to TC once that's working. I think a launch rpm, then throttle open to a launch setpoint (the driver would need there foot to the floor so it can't open beyond drive intent) when the clutch come up.....then into TC mode. Easy :)


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 Post subject: Re: Traction Control
PostPosted: Tue Dec 13, 2016 7:34 pm 
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The reason I said launch control is simpler is because the conditions to enable it are well-defined and you can make more assumptions on the sequence of events.

In reality, they are both very closely related. TCS handles the post-launch slip, and launch control handles the pre-launch staging.

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"Sometimes, the elegant implementation is a function. Not a method. Not a class. Not a framework. Just a function." ~ John Carmack

"Any sufficiently advanced technology is indistinguishable from magic" ~Arthur C. Clarke


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 Post subject: Re: Traction Control
PostPosted: Tue Dec 13, 2016 8:33 pm 
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I thinkg TS is is just
TS enabled (user setup item)
slip% > slip threshold
Clutch up
Brake up
throttle open

I don't think there is more to it than that.

Launch could be:
Launch enable
TS enable
Clutch down
Brake Down
Maybe a user button
(this sets it)
Then
WOT
Cutch up
brake up
and throttle goes to launch setpoint for a time or until slip threshold
Then system switches to TC


I think....so to me there is a lot more going on in lauch, not hard but 3 steps the last being TC


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 Post subject: Re: Traction Control
PostPosted: Wed Dec 14, 2016 8:21 am 
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The clutch switch is probably optional I'm thinking.....


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 Post subject: Re: Traction Control
PostPosted: Wed Dec 14, 2016 2:15 pm 
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In theory, you can calculate the clutch engagement and disengagement.

Engagement from idle is easy. You look for RPM to dip and/or the slow feedback term to suddenly spike (e.g. gradient of +5% TPS in less than 1sec), this indicates the engine must produce more torque, which is likely required by the driveline because the clutch is engaging.

Disengagement is harder. I would look at N/V ratio (RPM 'N' divided by vehicle speed 'V'). When in-gear it should be a constant number. It will be +inf when stopped, and go down (more V for the same N) as you go up in gears. When the clutch slips but the engine is still making power, the RPM will spike but V will stay the same, so N/V will go up.


One more consideration for TCS if you use the front wheels as the reference, is that they will spin slightly faster during a turn. This will mean the slip % calculation won't be right. You can ignore it (and end up holding a higher slip %), or correct it with at steering wheel position sensor.

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"Sometimes, the elegant implementation is a function. Not a method. Not a class. Not a framework. Just a function." ~ John Carmack

"Any sufficiently advanced technology is indistinguishable from magic" ~Arthur C. Clarke


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 Post subject: Re: Traction Control
PostPosted: Wed Dec 14, 2016 5:05 pm 
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I realized I had a divide by zero error in my model.......so I lost a lot of time finding it/them.

Back to TC now......


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 Post subject: Re: Traction Control
PostPosted: Thu Dec 15, 2016 8:55 am 
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I've pondered the clutch and heres what I've come to.....just require a clutch switch.

I think there are lots of things that could be done to try to determine what the operator is up to or thinking....but why add complexity and end up with a best guess instead of just forcing them to install a switch and actually tell you what they are up to? Why simpler code and probably better too. Simply is good.

On the slip angle...the front ales speed (average or front wheels) goes up on a turn? I would have guessed the opposite thinking ther is the rotaiotns plus the slip so true velocity is hieght than indicated vellocity?

Either way I think having streening angle data is a nice to have but the systems shold be designed to work without it. Maybe just make Allowed_Slip a 1d lookup vs streeing angle? If you don't have steering andle you hide all but 1 cell and enter the slip value?


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 Post subject: Re: Traction Control
PostPosted: Thu Dec 15, 2016 11:54 am 
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Thinking about this for just a minute.....tuning a PID while the wheels are spinning cound be quite a challenge :shock:


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