June 23 2008
Shifter modification for regenerative braking.
While driving my Honda ACRX for several years I tried several ways to implement regenerative braking - using stock brake pedal, using accelerator pedal (off-throttle regen) and using hand-operated one. My personal preference is to use right hand to engage regen. Turn out, all 3 methods [electrically] work, but:
- if I use stock pedal which first engages regen and further depressing brake pedal activated mechanical brakes, it has a shortcoming on the wet and slippery surfaces - since Honda is FWD, the regen naturally applied to the front wheels only as well. So as I brake, it's like rear brake lines are cut off and not there. Every time I want all 4 wheels to brake I have to reach the switch and disable regen for duration of that braking and then re-enable it again. Audi is all wheel drive car and this will not be an issue, however this set up requires either modification of existing braking system, since independent ABS and regen will not coexist very well (Honda ACRX does not have ABS).
- Off-throttle regen, where you'd set middle of throttle pedal position as "neutral" (coasting) and from that position fully depressed is 100% acceleration demand and fully released - 100% regen demand (so one pedal operation) almost got me into big trouble. IT may be OK for someone just learning to drive, but such setup is very different from conventional, and if your hands and feet in emergency operate vehicle quicker than brain thinking about it, you won't like such change. It is relearning to drive. Once I drove on a freeway evaluating such set up. First, I noticed I have to freeze my foot in strictly one position as I drive, no stretches, no movements. I had to take something from behind my seat while cruising on a freeway, and to reach that far corner I had to stretch and take my foot off the accelerator pedal for 2-3 seconds - action I never think about as it would not be noticeable for "normal" setup. Well, it was equivalent to slamming brakes 100% while in the middle of freeway and no one in front of me, so no one behind would expect such a stunt. And, I wasn't consciously braking, the effect was as if some one slams on the brakes for you without any warning. The rear of the car breaks loose, I'm thrown toward wind shield and intuitively as in any emergency situation, before my brain assess the situation, my right foot goes from already 100% braking accelerator to real brake pedal, compounding effect. Now the car really grips the road, scarring me out of my pants! Mind you, I had about 450kg (1,000 lb.) of lead in the car at that time (I think around 2002). It would take thinking twice to realize that to rectify situation I had to slam on "gas". Anyway, I'm coming to a complete sketching stop in the middle lane of I84 freeway. I was LUCKY - no one was following me on my lane, and I noticed couple of by passers looked at me as if I was total moron (and I probably was!). This was first and last time I tried off-throttle regen. You may like it, but it would take a lot of money to make me do it again...
Anyway, to make long story short, I will leave normal brakes operating as usual - for better or worse my brain is wired to use them certain and consistent way that never failed me yet, and I'm not going to change that. Off-throttle - you know. So I decided to use shifter lever to engage regen - the arm rest is helping to keep relative position of lever in check. Instead of leaving the lever in fixed position and installing the sliding pot on the stick (as in Honda ACRX), I will use lever itself to gradually engage regen. That way I still can brake independently with hydraulic brakes, don't have to take foot off the throttle pedal for braking (just release it) and based on Honda
experience it promises most convenience and pleasure to drive. So, I had to modify shifter mechanism making it smooth and installing dual regen potentiometer (controlling both front and rear inverters simultaneously). Here is what I did:
To reach the mechanism, rear of central console has to be removed. I had to start from the end of the cover, accessing the bolts holding parking brake between front seats backs.
Removing the arm rest.
Now the cover over center tunnel can come off. Coca-Cola spills again...
Removing side button and face cover. The guts reveal printed circuit board with LEDs illuminating shifter position.
Shifter knob comes off, PCB disconnected. The power coming to this connector will be reused for something.
Overview of the shifter mechanism.
After unbolting 4 nuts in the corners and taking off plastic plate, I can see that only locking cable to the ignition lock holding it in place. The cable is unlinked. I should not forget to modify the other end of cable and the lock, or I won't be able to pull out the key!
Another small connector off (park position lock solenoid) and the mechanism can be dropped off through the body cutout.
Enough disassembly, time to create something! Here is modification of the shifter converting it from the speed selector to the regen control.
Wabash dual TPS (throttle position sensor) which will be used as dual regen potentiometer. Each has 2.5k resistance.
These two little wires were made to try do initial fit and determine travel of the pot lever arm and its limits - the mechanical stops must be on the lever itself, not the potentiometer. This means it will not be complete stop to stop travel and electrically initial and end resistance in extreme lever positions (+/- some margins) will be calibrated in the software to correspond to 0% and 100% regen demand
The trial swing arm and linkage to the stock shifter arm was made this way.
The pot was attached in the corner on the bottom of shifter assembly. All unnecessary plastic parts were discarded, indexing wheel and stopper were removed as well and this provided one smooth swing of the lever.
The trial setup is ready for test
This is how it will work. (Quick Time movie, 3.6MB file)
After determining dimensions (bending wires I can shorten or lengthen the lever arm as well as the linkage length), real arm was fabricated from a piece of aluminum. Small ball bearing was press fit in the end of the arm.
Completed arm and link. For the link, the stock cable with plastic eye molded over the metal rod was reuses. The rod was drilled from its end for about 10mm depth, thinner wire was pressed in it and bent 90 degree. The bent part went through the arm's ball bearing.
Overview of assembled linkage. Return spring was attached (inside, not visible).
Another photo from the bottom of completed assembly.
Top view of the assembly.
Overall view, ready to get installed back. Much less parts than in original shifter. Believe it or not, it took me whole day to modify this assembly. Well, how does it work?
Beautifully! (Quick Movie 4.8MB file).