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Discussion Starter · #1 · (Edited)
TL DR: How do you design a 2 channel controller that smoothly transitions between drive, coast, regen brake, and allows coasting EMF from one motor channel to drive the other motor channel in a controlled way.

All the rest is about motivation I guess.

I've been noodling an odd small hybrid differential drive off road EV today and it would require precise torque and low speed control <30MPH of left and right wheels driven by simple 2 wire PMDC brushed motors.

It needs the usual independent L/R(Left/Right) speed up/slow down(regen braking) that I can get with something like a Sabertooth 2X60 regenerative dual motor driver which I have experience with and understand fine in that realm. In fact that may be exactly what I need if it can handle a mixed mode of operation I'll call differential static torque for lack of a better term. What I need from the community is help to better understand what I am asking for.

I need help understanding what the controller should do in smooth transitions between driving, coasting, and braking modes of operation, the circuit topology necessary to achieve it, a bit of the program pattern to control it, and any vocabulary I need to ask better questions.

So, have you ever
1. shorted the leads of a motor and observed how it acts as a brake? Motor dissipates brake energy as heat.
2. connected wires of two motors together and spin the shaft of one, the other will rotate also.

I need to take advantage of those properties and smoothly vary the ratio of 1. (non-regen braking) to 2. (differential torque) in cases where a large difference of torque between left and right exists, or must be created.

In cases where L/R torque differential (either needed or existing) is less than some threshold, then traditional speed control and regenerative braking should be used instead.
How do I get both?

Why am I asking? Here is a bit about the potential vehicle.

Converting my gas motorcycle to a hybrid trike that can slowly drive around my hilly wet grass covered yard with electric motor traction control so I don't tear up the yard and piss off my wife.

The rear end (differential) operating principle is like a tank triple differential.
But it isn't quite that as the center is a spool or sprocket and the side differentals are planetary gear sets, and there is no jackshaft. But the fundamentals are similar to driving a tank except my electric motors substitute for tank brakes actuating steering.

Electric motors are a completely redundant means of drive and steering. Either electric differential drive or prime mover can fail and the vehicle still has steering and drive ability. Now that I mention it, that sounds valuable. I don't know if the idea is novel or not. But in case it is, this post publishes it in the public domain making it unpatentable, I think. Let me know if you use it just for funsies.

In the center of the rear axle is any standard drive method, electric, diesel, steam, nuke, or gas, via pinion & ring gear or belt or chain. Chain sprocket is compact and easiest to imagine with a sun gear protruding from each side. The planet carrier on each side is connected to the wheel. For fabrication I was going to use front wheel drive splined wheel hubs driven by 2 flipped solid axles with the splines mating and the lug studs facing center serving as planet axles. The ring gear on each side would also have external sprocket teeth, or gear teeth, or belt teeth connected to that side's electric motor.

If you are having a hard time understanding the purpose or mechanical power flow, imagine a standard open differential(rear end) where the spider gears can be driven in opposite directions by electric motor. In that case, the electric motor cannot drive the vehicle, but it can steer the vehicle. All the extra stuff I added is mostly just packaging so the motors aren't continuously spinning end over end inside a differential and to allow contributing to vehicle drive.

My first idea for this bike was in this thread Ebike regen controller at highway speed
For some reason, the forum never accepted my last post to that thread(below) which described my intention for the build. It still holds for this variation as well. I don't know if it will end up a tadpole, a trike, or maybe a weird 4 wheel motorcycle despite the problem registering it. For now it serves as a nice mental distraction. If anyone is near Memphis, I welcome a hand.
I don't think you get it yet. High speed downhill is definitely not the point.

The point is to take a 2000 MotoGuzzi Jackal which I have enjoyed riding for years and modify it in a way that I can continue enjoying it with my spine injury.

I can no longer sit upright on it. I require a prone position. That requires sacrificing the area where the 12v battery is for seating. I also now need a little help keeping it upright at slow speed thus 3 wheels. Moving battery weight over front wheels. Even with prone position I can't endure long rides anymore so I wanted to use it to ride around my beautiful yard which is mostly off limits to me because I cannot walk it without pain and my wheelchair gets stuck. To not get stuck in grass on the bike turned tadpole, it needs slow, coordinated all wheel drive traction control. I also want to retain the experience I remember of fast acceleration from the gas engine for quick trips on the road.

So high speed mode is about making it as close to the old motorcycle experience as I can manage. Low speed mode is about riding down to the pond for some fishing with my son. I didn't think those details were applicable to a wider audience, but solving some of the technical problems might be.

I hope that clears up any confusion. The whole electric propulsion system is a traction and stability enhancing add on, not the core of the vehicle.
Edit: After posting, I started getting different search engine results for the same search terms, including which has been helpful with gear ratio math and terminology. Apparently this is a differential planetary set. On that page, it mostly resembles Fig.7: Differential Planetary arrangement 2 except the prime mover input is to to the common sun gears, L/R electric motor input is to the periphery of each L/R ring, gear, and L/R output is via each L/R planet carrier.

While contemplating a proof of concept build with relatively weak electric motors, the problem of backdriving came up. Obvious solution, worm and wheel, each L/R electric motor has a worm shaft and the wheel is each L/R ring gear periphery. Well that makes a nice package and negates the need to manage motor coast/brake because the system will be mechanically braked whenever the electric motors aren't running. This eliminates a few use cases, but greatly simplifies the problem, rendering this thread academic. I am still interested in the academic answer however for the sake of those abandoned use cases.

I wanted to see if this works sooner than I could build it myself, so I submitted a project idea to Grind Hard Plumbing Co If you think this idea has merit and would like to see how it performs in the real world, maybe send them some encouragement. Please point out any problems you notice. This is purely a mental exercise so far, there are likely key issues I missed.

Electric hybrid differential planetary set and associated controls have these advantages

  • intuitive steer/throttle driver interface with L/R hand throttles. Roll on power to hand pushing handlebar away, Roll off power to the hand drawing toward your body. Works for twist, thumb, or paddle throttles.
  • prime mover throttle is an algorithmic tunable mix of L/R
  • select different throttle algorithms based on eco, race, launch modes on loose(snow, sand, dirt track) or tight(pavement, slick rock crawling, studded tire ice racing) surfaces. Better algorithms integrated with steering angle sensor.
  • (loose surface) brake dig or reverse inside wheel and spin outside for fast pivot around sharp curves
  • (pavement) control inside vs outside wheel speed to optimize cornering tire grip at the limit
  • compensate for inherent oversteer or understeer
  • force feedback into L/R hand throttles signalling approaching limits of individual tire grip could significantly improve lap times.
  • redundant. Get home from the trail with broken driveline as long as you have a functional alternator or solar panel
  • electric reverse (engine off)
  • engine off maneuvering. very tight turn radius w/front wheel on dolly.
  • slower low gear(electric reverse at idle)
  • faster top gear(electric forward)
  • hybrid charging
  • plug in charging
  • redundant. Get home with failed electrical drive system because you still have a prime mover and traditional steering.
  • all added weight is sprung weight
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