Data on "brake loss" (aka "sudden acceleration") after bump

Discussion in 'Gen 2 Prius Technical Discussion' started by 2009Prius, Apr 27, 2011.

  1. 2009Prius

    2009Prius A Wimpy DIYer

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    Data on "brake loss" (aka "sudden acceleration") after bump

    The other day when I experienced the "brake loss" after hitting a small bump on the road, I happened to glance at my PriiDash(TM) display and saw the regenerative braking current dropped significantly, coinciding with the sensation of brake loss.

    Here is the data collected and plotted:
    [​IMG]

    If we trust the somewhat noisy accelerometer reading (red curve), then the rate of deceleration had a sudden but not large (less than 50%) reduction after hitting the bump (dotted line at ~ 585 sec mark). It's interesting to note that such moderate change in deceleration would give the sensation of "brake loss" or even "sudden acceleration".

    Also seen clearly in the plot is that the reduction of deceleration rate seems to come at least partially from the decrease in regenerative braking. After hitting the bump the regen current (yellow curve) dropped quickly from 60 ~ 70 A down to less than 10 A - as if the brake pedal was not applied. For comparison a normal braking event at a later time (~635 sec mark) in the graph showed regen current of 30 ~ 40 A at similar speed (~ 20 MPH) and brake pedal application (~20/127).

    The bump was not big; no traction loss was recorded.
     
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  2. uart

    uart Senior Member

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    That's good data 2009, you've captured the problem of regen to ABS transition really clearly there.

    One question. Is the purple trace roughly proportional to the force on the brake pedal, or is it more related to it's position rather than force?
     
  3. 2009Prius

    2009Prius A Wimpy DIYer

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    Thanks! I have a vague impression that the brake sensor is sensing position not force but I will wait for experts to give a definitive answer.
     
  4. uart

    uart Senior Member

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    I notice that about 2 seconds after the (partial) loss of deceleration that the brake signal increases about 50% and then the original deceleration rate is restored. Was that a conscious decision to push harder or was that the brake pedal just sinking a bit lower with not much extra force.

    The one time I experienced something like this I was descending a long hill with a full (8 green) SOC and near the end it cut out the regenerative braking (because of the high SOC). I wasn't braking hard, just gently as a approached a red traffic light, but I felt the braking effort suddenly reduce a bit and I had to let the brake pedal sink just a little bit further down before I got back to the original deceleration rate.
     
  5. Hal W

    Hal W New Member

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    Yes , a person has to be ready for the transition from regen to no regen! By the way how did you capture that data? Was it printed from the black box? Hal
     
  6. cwerdna

    cwerdna Senior Member

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    Maybe he's got an accelerometer and is the new bwilson4web? ;)
     
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  7. dogfriend

    dogfriend Human - Animal Hybrid

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    I wanted to thank you for this post, but I'm so sick of this subject from previous unending threads that I can't do it. :madgrin:
     
  8. 2k1Toaster

    2k1Toaster HID Guru

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    All this info is available over the CAN bus.
     
  9. qbee42

    qbee42 My other car is a boat

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    Humans are very sensitive to changes in acceleration, but not to the value of acceleration. In other words, we can sense small changes, but we can't tell whether acceleration is positive or negative. A drop in deceleration (negative acceleration) feels like acceleration (positive acceleration).

    If you want to demonstrate this, have someone make a constant turn in a parking lot while you keep your eyes shut. After a couple of loops around the circuit, have them ease off on the turn, still with your eyes shut. You will swear that they just turned in the opposite direction.

    The brake pedal transducers are pressure sensors. They sense pedal pressure and send it to the brake ECU.

    Tom
     
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  10. xs650

    xs650 Senior Member

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    Jerk:D
     
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  11. qbee42

    qbee42 My other car is a boat

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    Exactly. Most humans are sensitive to jerks. ;)

    (apologies to the non-physicists in the audience)

    Tom
     
  12. uart

    uart Senior Member

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    Yes it's interesting how many people we've seen describe this as a "surge forward" and have been totally adamant about that no matter how many people tried to tell them otherwise.

    Allowing for the DC offset in the accelerometer and the fact that the brake signal is decreasing slightly during the interval I'd estimate it to between 25% and 30% loss of deceleration.
     
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  13. cwerdna

    cwerdna Senior Member

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    The "surge forward" and "lurches forward" seemed to mostly be coming from newbie 3rd gen owners. Bwilson4web had accelerometer data on his 3rd gen (prior to the application of SSC-A0B that showed the brake pause was just that.
     
  14. skilbovia

    skilbovia Member

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    Nice to see a troll free thread regarding this issue with actual data. To me it feels like my Prius is coming out from under me a little bit. I have a spot on my ride home where this happens repeatably. Sort of a washboard of bumps just before I take a corner. It never feels dangerous. I wonder if now that this has been documented if there will be a recall.
     
  15. cwerdna

    cwerdna Senior Member

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    Are you talking about the 2nd or 3rd gen? Besides Bwilson4web's measurements on the 3rd gen, there was a running thread of known repro locations. He seemed to like collecting them. I contributed one spot myself.
     
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  16. qbee42

    qbee42 My other car is a boat

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    There was a TSB to update the braking software. It reduced the sensation by shortening the time to apply ABS.

    Other than that, it's not likely that there will be a recall, as you would need to recall the roads, not the car.

    Tom
     
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  17. dogfriend

    dogfriend Human - Animal Hybrid

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    I vote for recalling the potholes. :madgrin:
     
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  18. jayman

    jayman Senior Member

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    If one of the wheels hits a manhole cover or you are braking on a washboardy surface, the FJ Cruiser does the same thing

    No regenerative braking here, just ABS that gets easily confused
     
  19. 2009Prius

    2009Prius A Wimpy DIYer

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    Soon after the sensation of brake loss I must have subconsciously thought it was me that wanted to let go the brake, so my foot actually eased the brake pedal a little for a short time until I consciously reminded myself that it was caused by the bump, and then pressed harder on the brake. Thus the recorded brake pressure was decreased slightly at first and then increased.

    The SOC was nowhere near full in my case.

    See below.

    See below.

    Bingo!

    +1
     
  20. xs650

    xs650 Senior Member

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    I'm an engineer. you insensitive clod.:D

    (Apologies to non /. ers in the audience)
     
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