Saturday, February 9, 2008

Smoothing out the DMOC acceleration/braking

One of the annoying behaviors of the motor system in my AC 914 kit is that the default DMOC445 parameters causes the acceleration and braking to have oscillations. In physics speak, the application of the positive or negative torque happens so quickly that the system is underdamped. The step function of the torque perturbs the mass of the car and it oscillates for a bit before settling down, primarily in first gear where the motor has the most influence on the acceleration and deceleration of the car. These oscillations can put extra stress on the drivetrain and cause general annoyances with the passengers.

So, I took the car out into the neighborhood with the laptop hooked up and started playing around with the EEX???TorqueSlew parameters listed in the DMOC445. The original parameters are:

EEXTorqueSlew: 985 Nm/s
EEXUnloadTorqueSlew: 985 Nm/s
EEXBrakeTorqueSlew: 477 Nm/s

When I floor the accelerator in first gear, the car oscillates (EEXTorqueSlew) and then oscillates again when I quickly release the accelerator (EEXUnloadTorqueSlew). Similarly, when I apply the regen brakes in first gear, the car lurches forward and oscillates (EEXBrakeTorqueSlew). When I release the regen brakes, the car oscillates as well(EEXUnloadTorqueSlew again).

After some messing around, I finally settled on the following values:

EEXTorqueSlew: 209 Nm/s
EEXUnloadTorqueSlew: 388 Nm/s
EEXBrakeTorqueSlew: 239 Nm/s

These new values slow down the rate that torque is applied or removed in the system. With the new values, I can accelerate and regen-brake in first gear without any oscillations but still get relatively quick response from the pedals. In fact, the reduced EEXBrakeTorqueSlew allows me to up the MaxBrakeTorque variable to 60 Nm/s for better braking without having the passengers lurch forward. This is very important due to the weakness of the 914 manual brakes combined with the additional 800 pounds of battery weight.

At this point, the last major thing to fix on the car is upgrading the front brake calipers to BMW320i parts to improve braking. Beyond that, I'm just trying to get out as many thoughts as I can before just commuting with the vehicle and enjoying it.

I've started an 914 Electric Conversion Wiki at the URL below. Please contribute to the Wiki as you can. The Wiki is a shared resource as opposed to an individual blog, so I'm hoping we can save people time by putting our collective brains together and helping people move forward in converting their 914s to electric. I'm hoping this will also save us all some time in that we don't have to answer the same questions over and over to new people considering 914 conversions.

Thanks to everyone for their support on this project.


1 comment:

Ross Cunniff said...

Hey, Tim,

Interesting solution. Mine was different - see here - basically I just tweaked the voltage ramp. It would be interesting to compare our results...