Modding brake pedal signal to match rubber bung mods

[Outspacer]

With a G29, or other modded pedal using a rubber piece, the output matches travel rather than pressure. Obviously a load cell solves this, but at a cost!

My first thought was to swap the linear pot for a log pot, or similar passive mod, but I think that wouldn't work well because the pedal only uses (at most) about 2/3 of the pot's travel. (I haven't checked, but I don't think it would be easy to make the used 2/3 go right to one end of the pot; it uses the middle).

So the next level would be a small active circuit, either with opamps or a microcontroller, to change the output within the used range so it matches the pressure applied to the pedal.

Curious to know if this has been tried already, before I get in deep!

 

[mrbasher]

I have something that does this. Never released it... Maybe I will some day.

You can also do something similar with diview I believe and I know you can in iRacing. Though both are only on PC.

The reason I did it was because one of the biggest differences between the "feel" of using a pressure sensitive brake vs. a position based brake. The biggest issue is modulation at the very beginning of travel. It's too "sharp" when using a position based solution.

The setup I have does quite a few things which I'm not going to mention here, but the one relating to this actually allows a person to set exactly what that curve looks like. Where it starts, stops and in-between. Hell you can almost make as "S" curve with it.

I used it for a long time with my T500 pedals and brake mod. It helped considerably in my opinion. Though it does not equate directly to a pressure sensitive brake. However, I believe you are correct in assuming that a person could reasonably approximate one. If the medium being "squished" does so in a fashion you could measure, that curve could then be applied to the position of the pedal (pot).

It is possible to turn the output of a linear pot from linear to what amounts to exponential (or log, depedending on the direction of rotation), though it does affect the "range" and you'd have to do some calculations to get the right values for the pot and resistor. It's super cheap though and requires only a single resistor.

Here's how it's done:

(have to try and explain this without a picture...)

Connect what used to be the "output" of the pot to the positive terminal on the pot.
Place a resistor between ground and what used to be the negative terminal on the pot.
Your output is now that junction between the negative terminal on the pot and the resistor.

Edit: Just to add, you could technically just connect your positive voltage to what used to be the output on the pot and leave the other leg disconnected. What you're doing here is creating a voltage divider where only 1 side is being changed. In a pot, both sides change at the same rate, making it linear output.

Now... That's is only going to work for one direction of rotation of the pot. Other orientations can be used depending on what you need.

 

[mrbasher]

Another thing to note, unless they changed something. I believe Logitech, like Thrustmaster, uses pots that only use about 60 degrees of the rotation. This is to try and get the most voltage swing they can out of such limited range.

Edit:

Just adding to this, and trying not to make new posts haha.

One major "hurdle" comes at the end of travel, where you are pressing the hardest. On most of the higher end, and even the sets like the ClubSport pedals, this portion of travel is very small and yet can yield a wide range of output. It would seem that the same can be done with the solution we are speaking of here and it technically can, for the most part. However, resolution will start to become an issue very quickly in this portion of travel. Essentially, we are increasing the dynamic range at the beginning of travel and decreasing it at the end.

Just wanted to throw that out there. On a pressure sensitive brake, you don't have this issue. That's why you can use things like rubber that don't compress in a linear fashion. A pressure sensitive solution "decouples" the travel from the output. If you stuck a standard, linear spring in place of whatever rubber, foam, etc. that was originally used, the output would have a linear relationship to the pedal travel. At least until the spring bound up.

Anyhow, this stuff is very interesting.

 

[Outspacer]

Ha! Knew I wouldn't be the first, thanks for posting

Did you use a micro for your mods? I have some ATmega32U4 boards around.

Here's some diagrams of modding a linear pot with resistor(s) that I saw:
http://www.learnabout-electronics.org/Resistors/resistors_09a.php
https://www.maximintegrated.com/en/app-notes/index.mvp/id/838

I don't think any of them would work well (on a G29, anyway) because the used part of the travel barely reaches the point at which the log slope is greater than the linear, so it wouldn't increase sensitivity enough over the range of the rubber buffer. If the pedal used the whole travel, or at least went to the end when fully pressed, some of those would be pretty good approximations - e.g. just the right hand half of this:

(Possibly inverted/reversed as needed).

Yes, resolution would prevent a true mimic of pressure. If I can push it as far as, say, the last 20% of the (used) travel producing 50% or 60% of the braking I'd be fairly happy I think. Better if it could manage more. Even though it would still be far from an accurate brake simulation, it would be a lot closer than the linear pot. Actually, I'm not averse to fitting a higher quality pot, as long as it's still cheap relative to a load cell.

 

[mrbasher]

So, you are correct about the voltage swing and what really happens. You'd likely loose part of the beginning and end given the limited swing. Exactly what would depend a lot on where that swing falls on the graph of course. I do remember testing a set of G27 pedals that I have and they used a surprising range considering. Still, not 100%.

The spreadsheet on the Maxim site is good to mess with for testing this out, and you can add some stuff to it to test what I'm about to say.

Something to remember that this is further complicated by the fact the all of the wheels out there right now automatically calibrate the minimum and maximum range. So even though it starts late and cuts out early, you get the full range of values. Albeit, missing steps in the middle and reducing actual resolution.

So, these intersections, translated might go from this:

To this:

Effectively...

You can adjust that some by using a different value resistor. Also, that blue line likely isn't the right fit... I just put a spline over the black line and scaled it to fit. Seemed reasonable haha

I have a couple different processors I use. Microchip, Cypress, and some different ARM procs. Ideally, you'd do this with a micro controller and some assorted components.

On the notion of pot resolution. Inevitably, the subject of actual pot resolution and/or accuracy comes up at some point when discussing pedals and resolution. I can tell you that you'd be amazed at what even a crappy pot can do with regard to resolution. They are not the limiting factor... Unless of course they are worn out.

 

[Orayani]

 

[Outspacer]

Something to remember that this is further complicated by the fact the all of the wheels out there right now automatically calibrate the minimum and maximum range. So even though it starts late and cuts out early, you get the full range of values. Albeit, missing steps in the middle and reducing actual resolution.

Yes, so I'm assuming they do that in firmware, after sampling the ADC, and even stock they don't use the full range of ADC values. Reducing that further and relying on the auto-calibration wouldn't be good, but nothing that can't be solved with a bit of gain - making the wheel see that blue line as its input.

You can adjust that some by using a different value resistor. Also, that blue line likely isn't the right fit... I just put a spline over the black line and scaled it to fit. Seemed reasonable haha

Hmm, that is reasonable Needs a smaller resistor to heighten the effect, but I think it could work. Something like 470R for R1 with 3x gain centred on about 4.5V looks somewhat right, considering fully pressed on the G29 is toward the LHS of the graph:

Unfortunately, now I can't find my opamp stash