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Old 08-07-2002, 03:24 PM   #1
Kha0S
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Talking A walk-through of the VTD-AWD system...

(For details on Torquemada, the box in development to give user-controlled VTD lock, see http://forums.nasioc.com/forums/show...hreadid=227548 )

Well, folks, I finally got it all figured out.

Everyone who has ever looked at the JDM WRX STi v5/v6 TypeRA is familiar with the DCCD, or "Driver Controlled Center Differential" system. There are good and bad sides to this system, particularly in the realm of durability under rally conditions (one of the only places where TypeRA vehicles see regular use).

Well, anyone who has waited for the DCCD system to arrive in the USDM, your wait is over! ... that is, if you have a VTD (variable torque distribution) 4EAT Subaru.

As it turns out, the center differential used by the VTD-AWD system (new in MY2002, for the 4EAT WRX and Outback H6 VDC) is nearly identical to that of the DCCD system in the TypeRA.



It utilizes an epicyclic gearset (or, more simply, a planetary gearset, more on this later) that behaves as an open differential as the base for torque transfer. The system consists of two sun gears, each with its own planets. The planets are bound together rotationally (so when one planet spins, so does the adjacent one).

In the above picture, toward the left side of the photo, is the input shaft. Power coming out of the transmission's planetary shift gears arrives here. This power is applied to the sun gear on the narrow side of the differential. The smaller planet gears interface with this sun gear. As these would rotate, they would force the larger planets at the wider side of the diff to rotate.

These larger planets interface with another sun gear. This sun gear, and the shaft attached to it, represent the rearward output of the differential, and carries power to the rear differential for distribution to the rear wheels.

All of the planets are inside of the planetary carrier, which appears as the sort of "case" for the differential. The gear toward the narrow end of the diff is part of this carrier, and represents the frontward output of the differential. This gear meshes with a reduction gear that then goes out to the front differential for distribution to the front wheels.

Mechanically, the gearset, on its own, operates as an open differential. The default torque bias of this system is 36%F / 64%R. In a worst-case rear wheelspin situation, because it is an open differential, power distribution would be 0%F / 100%R (and, correspondingly, 100%F / 0%R in a front wheelspin situation... remember this, it will become useful later).

Even if all of that went over your head -- make sure to understand this -- the VTD gearset is not torque-sensing. Many of us (including myself) have thought this at various points... but it's simply not the case. Torque-sensing differentials such as the Quaife and Gleason Torsen differentials utilize worm gears (and their inherent properties) to provide an LSD action. There is no such functionality here.

... to be continued ...

/Andrew
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Last edited by Kha0S; 08-09-2002 at 03:00 PM.
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Old 08-07-2002, 03:32 PM   #2
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Moving on, the rear output shaft, just where it exits the differential, is splined.

Also, towards the rear end of the differential, there is that bright silver area, it looks almost like a set of dogs, with a small hole (but it's not). The inside of this, if you were to rotate that image, is hollow, forming a cup with ridges around the edge.

Alternating plates of metal and friction material are layered inside of this cup, with half of them splined to the inside of the cup, the other half splined onto the rear output shaft. A large hydraulic piston fits over the end of the rear output shaft, and nestles into that cup.

As hydraulic pressure is increased on the piston, it will press against the alternating metal/friction layers, causing them to bind. At full pressure, the rear output shaft and the planet carrier become completely locked together, rotating as one unit with the input at the front sun.

So, again, as hydraulic pressure increases on the piston, the clutch pack (made up of those alternating discs) is compressed, which locks the center differential. Since this pressure is variable, the "amount of lock" can be varied.

Realize -- a locked center differential is roughly equivalent to a solid driveshaft connecting the front and rear axles. This would cause excessive binding, massive understeer, poor ABS response (actually, no ABS response), and put a lot of stress on the drivetrain, if used on dry tarmac. On snow, ice, dirt, etc., such a lock is preferable to an LSD that takes time to engage.

... more coming ...

/Andrew
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Old 08-07-2002, 03:40 PM   #3
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The hydraulic pressure on the lockup clutch piston is managed by the "Lockup Duty Solenoid" in the transmission pan area. This solenoid is driven directly by the TCU (transmission control unit) located under, and slightly to the left of, the steering wheel.

This solenoid takes a 50Hz PWM (pulse-width modulated) signal (although, this is from memory, it might be higher frequency). As duty cycle goes up, more pressure is applied to the piston, and more differential lockup occurs.

Normally, the VTD-AWD system uses wheelspin sensors, ABS sensors, and other items in its CAN (controller area network) to calculate an estimation of surface friction (u) and current yaw angle (a) of the vehicle. The TCU then takes this estimated u, estimated a, steering angle inputs, and throttle and braking inputs and calculates the desired amount of center differential lockup to maintain stability of the vehicle (usually characterised as minimum yaw angle and minimum wheelspin).

Now, this is really annoying if you've ever tried to rallycross, ice race, or even autocross a VTD-AWD WRX. It makes the understeer/oversteer characteristics of the vehicle unpredictable, but, luckily, the system errs on the side of caution. This makes it great when operating on public roads, but in racing situations, is suboptimal.

... more on the way ...

/Andrew
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Old 08-07-2002, 03:53 PM   #4
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Now, here comes the cool bit.

The DCCD system employed by the TypeRA uses a near duplicate of this system, but lacks the TCU, and all of its ungainly calculations. As far as I can tell, it also utilizes an electromagnetic, instead of electrohydraulic, clutch pack, as the 5MT doesn't have a freely available source of hydraulic power (the 4EAT, on the other hand, has tons of hydraulic bits just hanging around).

Hence, the DCCD system allows the user to select a range from "FREE" to "100% LOCK" depending on operating conditions. There is also an interlock for the emergency brake that automatically releases the diff to "FREE" mode when it is pulled. I'll go into why this is cool in a little bit.

Here's a page (thanks to an earlier post about DCCD from John Felstead) from the English translation of the STi TypeRA manual:



This illustrates the way the DCCD system works -- oddly enough, exactly like we'd expect from VTD!

... so ...

I am currently in the process of designing a little black box to permit multiple operation modes of the VTD center differential.

Much like the DCCD system, it will have "DIFF FREE," "DIFF LOCK," and an analog knob for anything in between.

There will be a switch that can be toggled for VTD-pass-through mode for use on public highways, so you can get the advantage of TCU calculation when you're not in a racing situation.

It will also have the ebrake interlock.

... interesting bonus on the ebrake interlock. Recall earlier, when discussing the gearset, that when the clutch pack is fully disengaged (that is, "DIFF FREE" mode), the diff behaves as an open differential. Well, what happens when one side of an open differential stops? The other side gets all the power, but it causes little/no wear on the diff itself (only the engagement/disengagement stress on the clutch packs, which is minor).

What does this mean? Well, with the ebrake interlock, if you have a VTD transmission with this little black box, when the ebrake is pulled, you can continue to apply power, and all of it will be routed to the front wheels only, permitting FWD-style powered ebrake maneuvers! This is a key feature of DCCD, and I look forward to exploiting it on a VTD differential as well.

... final notes shortly ...

/Andrew
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Old 08-07-2002, 03:58 PM   #5
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Dude, you rock

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Old 08-07-2002, 04:03 PM   #6
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So! Pretty cool stuff, huh?

Effectively, with no wheelspin, and no turning, there's no difference between DIFF FREE and DIFF LOCK. As soon as cornering occurs, or wheelspin occurs, the diff's behavior becomes relevant.

As far as the stock behavior of the diff, I don't have the details right in front of me... but my old research with the oscilloscope tied to the transfer duty solenoid showed that the diff normally runs at a nearly 50% duty cycle when cruising, indicating a partially locked diff most of the time. As such, it becomes clear that the VTD clutch pack is designed for near-constant partial engagement, and that this little black box won't push the diff outside of its normal operating parameters.

This little black box is going to be a pet project, and I'm still unsure whether I'll be taking a simple, discrete op-amp approach, or whether I'll invest in a PIC system in order to make a more programmable and reliable unit. It'll have the benefit of reducing stress on the differential, the clutch packs, and the rest of the driveline. It'll allow more precise control of the vehicle dynamics to the driver, and make the car more predictable at the limits of its handling characteristics.

I'll keep you all posted as development continues.


...


A lot of thought and research went into this, and it took me a good bit of time to fully understand the system to the extent that I could consider myself an authority on it.

I'd like to thank:

Exeter Subaru: for the old broken parts from when I blew up my center diff
Sully @ Manchester Subaru: for helping me find assembly diagrams for the 4EAT VTD extension case
North Ursalia: for info on DCCD and other transmission info
mdntrdr: letting me borrow the tranny manual foreverago
mad-dog999: for listening to me ramble, and convincing me to keep researching
RiftsWRX: for being a sounding board and pointing out the obvious
webkris: for letting me use his Legos to figure it all out


... coming, URLs that this was all based on ...

/Andrew
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Old 08-07-2002, 04:08 PM   #7
ciper
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So now we can say that

AUTO'S RULE, 5 SPEED DROOL
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Old 08-07-2002, 04:10 PM   #8
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Useful URLs:

http://www.northursalia.com -- Has all kinds of miscellaneous service, technical, and assembly documentation. There's a few 4EAT articles in his archives.

http://www.subaru-global.com/about/parts/06.html -- a somewhat misleading, but useful description of VTD. There are a lot of errors in this mini-article, but it's still quite useful.

http://forums.nasioc.com/forums/show...hreadid=209370 -- John Felstead's attempts at translating the DCCD documentation.

http://210.101.116.115/fisita/pdf/G347.pdf -- a useful document on the technical specifications of the software side of VTD, and how it works (and how it interacts with VDC in the Outback H6). Also has a neat, but hard-to-read diagram at the top of the whole transmission assembly.


There are also a number of articles on electronically locking center differentials and some of their applications, though, none of them give any real specifics. The VTD and DCCD systems are very similar to the system in the Porsche 964 Carrera 4, and the Lancia Delta.

Cheers,

/Andrew
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Old 08-07-2002, 04:13 PM   #9
Kha0S
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Oh -- I'm also taking suggestions on what I should call the little black box.

I'll be releasing the circuit/software (depending on the route I take) to the public domain as soon as I get a prototype working.

/Andrew
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Old 08-07-2002, 04:20 PM   #10
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I bet you would get better sales if you had two different sets of labels on the box. Here me out..


The non VTD center diff works in a similar fashion, except that its either full front or locked (never an open diff). This box should function correctly on this diff as well.

So it could have an FREE or LOCK for WRX and H6 VDC or FWD LOCK or 4WD LOCK for any other auto subaru 90+ !!

Personally I wouldnt mind being a tester and apply this to my car. Im running a 95 imprezza transmission on a 90 legacy TCU
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Old 08-07-2002, 04:24 PM   #11
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ciper -- I'm not going to be selling this, or manufacturing it. Too much stress.

You're right, however -- FWD/50:50 scale for MPT-AWD 4EATs would certainly make people happy. I'm not sure, however, how well the duty clutch would put up with being engaged to 100% all the time, but it probably wouldn't hurt anything.

I'll keep everyone posted on the development of the box here.

/Andrew
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Old 08-07-2002, 04:29 PM   #12
ciper
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I think I have confused myself.

Im fairly sure that when no power or 0% is aplied to the solenoid its 4wd full and when its run at 100% you get FWD, thats the function of the FWD fuse.
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Old 08-07-2002, 04:33 PM   #13
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Yeah, I might have my duty cycles backwards as well. I need to plug the oscilloscope back in and go for a drive. I've got it documented somewhere around my apartment in a notepad. Just gotta' find it.

/Andrew
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Old 08-07-2002, 06:16 PM   #14
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I wish my auto could do this
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Old 08-08-2002, 11:15 AM   #15
Geek Guy
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Quote:
Originally posted by Kha0S
Oh -- I'm also taking suggestions on what I should call the little black box.

/Andrew
TorqueMonkey
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Old 08-08-2002, 01:29 PM   #16
ciper
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As long as the owners of the boost monkey dont mind I think its a great name!

DoctorNick: why couldnt your transmission do this?
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Old 08-08-2002, 03:30 PM   #17
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Quote:
Originally posted by ciper
As long as the owners of the boost monkey dont mind I think its a great name!
Well, Kha0S helped research, prototype and design the Boostmonkey, so I doubt it will be an issue. On top of that Kha0S is my navigator and Kris, the guy selling Boostmonkeys, is the other driver in Trunkmonkey Racing.

Torquemonkey rocks as a name, by the way.

There is no cone | PGT Impreza L Rally Car | P Neon Rally Car
Trunkmonkey Racing | sean@trunkmonkey.com | What is a Trunkmonkey?
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Old 08-08-2002, 04:40 PM   #18
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If this keeps up we will have a car full of monkeys!

Id like to help in any way possible, in order to make this applicable to non VTD subaru.
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Old 08-08-2002, 10:34 PM   #19
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I agree

TorqueMonkey

Andrew, please oh please keep us up to date on this. Yes, I would like to see the circuit you come up with.
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Old 08-09-2002, 02:46 AM   #20
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Fantastic Tutorial! Thanks
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Old 08-09-2002, 09:57 AM   #21
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An update --

I've decided on a name. After discussing the topic with North Ursalia, webkris, Sean, and everyone else at Denny's tonight, I've decided to go with a suggestion that North Ursalia kicked out...

Torquemada -- known for his ruthless commanding of the variable clutch packs in Subaru transmissions, ruthless efficiency, and, yes! Fear, surprise, and ruthless efficiency!

The monkeys are cool, but "TorqueMonkey" sounds a bit too much like the infamous "Torque Chip." I also want to minimize webkris getting phone calls from people looking for a TorqueMonkey, and thinking that Rumblesoft sells them.

The current plan is a little more daring than the beginning ... I was originally pondering a simple op-amp based solenoid driver, but I've come to realize that there's a lot of logic that I'd like to integrate. Also, when dealing with a transmission, reliability is of the absolute utmost importance -- unlike a boost solenoid where you have a boost cut as protection, a wrong move with this unit could potentially blow up gears in your transmission. As such, a microcontroller is a much safer, and more extensible, even if more expensive, way to do it.

I'm currently evaluating the M68HC12 microcontrollers (particularly the model with onboard A/D and multi-channel PWM output). A full development kit for this series of controllers, and a development/carrier board (seeing as the M68HC12 is a QFP surface mount package) is about US$130... with only a very small number of external components, this will be able to reliably drive the transfer duty clutch, with extensibility to other transmission functions (smooth ramped-duty manual torque converter lockup, for instance, is planned).

The final result may be a few months off, but the sooner I can pull together the cash to order the development kit, the sooner I'll get coding.

Anyone with M68HC11/HC12 experience is more than welcome to give me reccomendations, tips, or hardware donations.

/Andrew
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Old 08-09-2002, 01:53 PM   #22
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Good luck, dude!
Are you going to make it a rheostat-type dial thingy like the 22B/STi RA???

-Dennis
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Old 08-09-2002, 02:16 PM   #23
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Quote:
Originally posted by bluesubie
Good luck, dude!
Are you going to make it a rheostat-type dial thingy like the 22B/STi RA???
That's the exact goal of v1.0 of Torquemada. The dial will come in on an 8-bit A/D input into the M68HC912B32, and the solenoid will be driven by one of the 8-bit PWM outputs through either an H-bridge or a power MOSFET (IRF510 or equivalent).

/Andrew
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Old 08-09-2002, 02:38 PM   #24
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Quote:
Originally posted by Kha0S


That's the exact goal of v1.0 of Torquemada. The dial will come in on an 8-bit A/D input into the M68HC912B32, and the solenoid will be driven by one of the 8-bit PWM outputs through either an H-bridge or a power MOSFET (IRF510 or equivalent).

/Andrew
Do not use analog dial. Go full digital and it will be cheaper (no A/D required) and more durable. You only need a few stages. I think 8-stage or 3-bit is enough. 8-bit is over kill.

Also, you can take extra input from original TCU output so that you can have an position on the dial called 'automatic' that keep the original function of the car.
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Old 08-09-2002, 02:39 PM   #25
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Andrew,

Very cool stuff here. I didn't know you had the ability to create circuits and what not. I actuall have an interest in doing such, but my work path took more of a software development rather then hardware dev path.

Anyways keep up the good work. Also if your ever bored and would want to work on a proto type idea I have, let me know.

MadMax
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