Volt's brake problem with the feel of sudden surge

By the way gen 2 Prius does have accelerometer data flowing on the CAN bus at about 60 Hz. The speed sensor data from each wheel also comes at about 60 Hz. One would think with this kind of data rate the system should be able to add in friction braking fast enough (after going over a bump while braking and reducing regen braking), say in a small fraction of a second, so that the driver won't have to experience the many seconds of sensation of loss of braking, if Toyota or GM bothered to refine the algorithm.

 

Please quantify and reference your measurement of "seconds". Human perception is an inherently flawed measurement device. It has been empirically shown to be between 250ms and 850ms depending on the conditions (actual weather, the actual wheel speed spin differential, etc).

Here is a post that shows almost the worst case sceanario over wet roads, and it is 800ms.

 

Tom in the case of the Volt reports indicated, the vehicle did have loss of traction at a wheel and shut down regen braking and flashed the low traction idiot light. The reports also made clear that the drivers foot was not on the brakes, only the car was in L - low gear - using regen to simulate engine drag. They obvously do not need to add a new piece of hardware to detect what to do.

I did experience the prius brake problem before my software was updated. This is not the same thing, and was the result of a toyota admitted software bug that they have corrected. Brake feel could be better with new hardware and/or software in my gen III, but lack of it definitely did not cause the software bug. BIll wilson just flashed a scenario that he found not the worst case. By the time he had sensors to send out to record, I had my software corrected.

Definitely true, and cars using engine braking will require a driver to use the mechanical brakes too Not all abs systems are equal, and some are even shipped with bugs in the software.

 

Bob did not post the brake peddle signal and we don't know whether the braking recovered itself or maybe he applied more brake peddle pressure to regain braking. Or maybe gen 3 is improved over gen 2.

My measurements on gen 2 shows no recovery of braking after the regen braking is lost. See graph below. Note that after the regen braking is lost around 585 second mark, one has to apply more brake peddle pressure to get the same deceleration as before, and the same brake peddle pressure produced only about half of the deceleration as before. This lasted until the car was stopped around the 600 second mark. That's more than 10 seconds after the lost of the regen braking.

 

Certaintly interesting to see it represented in a graph using accelerometer data. I would trust the accelerometer before my hind end seeing as they are calibrated. It would only make sense in my mind though that if the car decides to cut regen braking that it would take more pedal pressure to engage the friction brakes.. Seems like it would be some kind of safety feature in case you do not want it to continue braking in a dangerous situation, but that situation could go both ways. I see sometimes it takes a large length of time to reproduce, so I will continue trying. I am curious to see it for myself.

Where do you guys get all these cool tools from to analyze all this stuff anyway? The only place I have ever seen accelerometers and stuff like that is at my work for running vibration analysis on helicopters. I suppose I need to update my garage tool and get some of these scanners and other electronic diagnostics tools you guys are using..

 

This is the same as with the Prius. Simulated engine drag is exactly the same as regenerative braking. In the case of the Prius it is fairly light, so it is unlikely that you will experience loss of traction, but it can happen. The Prius also displays the low traction light in this situation.

Tom

 

The accelerometer in the Prius measures yaw. It is an accelerometer, but it measures the wrong axis for braking.

Tom

 

Again, those reporting a problem in the volt do not have their foot on the brakes. They are simply in "L" and anti-lock braking sensors are activated.

Those of us that experienced the bug in the prius brakes, were actively pressing the brake pedal, and the software did not operate properly when traction was reduced. This software has been fixed and updated.

In neither case is an accelerometer needed.

 

There are two accelerometer readings, one for left-right or the yaw as you called it and the other for front-back. The latter is plotted in the chart and should be adequate for braking.

 

Inconsistent brake peddle response is a bad thing in my humble opinion. The engineers are free to cut the regen braking to protect the MGs but an average driver does not need to know about that detail and worse suffer the consequence.

For gen 2 all data can be extracted from the CAN bus. A lot of data can be extracted from gen 3 as well but I don't know all the details. Do a search for Torque app.

 

And again, it is exactly the same sort of braking with or without having a foot on the brake. In the case of the Volt, the simulated drag can be higher given the gearing. It is likely that the issue with the Volt can be improved with software, just as with the Prius Gen 3, but the software only reduces the duration of the event.

Thanks. I didn't know about the longitudinal accelerometer. That is exactly what is needed, assuming enough sensitivity and speed.

Left-right and yaw are related but not the same. A left-right accelerometer, or lateral accelerometer, measures any acceleration in a sideways direction. A yaw accelerometer only measures acceleration around the vertical axis.

For example, if a car were blown sideways by a gust of wind, a left-right accelerometer would register the acceleration. A yaw sensor would not, since the car is still pointing in the same direction.

On the other hand, if a car spins out on a sheet of ice, the yaw sensor will register the rotation. In this situation the reaction of a left-right accelerometer is entirely dependent on its location relative to the center of rotation. If the left-right sensor is in the front of the car, it will recognize movement in one direction. If it is in the back of the car, it will recognize movement in the opposite direction. If it is located in the center it will see no acceleration, since the center will continue down the same path.

Yaw sensors generally contain two accelerometers measuring in opposite directions. Common signals from the two cancel, eliminating left-right readings. Differential signals from the two indicate yaw, since a differential means one accelerometer is accelerating faster than the other, or rotating.

Tom

 

The front-back accelerometer signal is easy to verify and I have done that by casually observing it during driving and also when stopped on slopes. To tell whether the other signal is left-right or yaw I need to find a slope and park the car sideways. Parking lots are usually built to be level around here. Maybe I will find a gentle curb somewhere.

 

I believe that regen systems are inherently worse in this respect because they can't respond as quickly as a good-old-fashioned hydraulic ABS system.

When driving inappropriately and trail-braking into a corner, where precisely timed inputs are important, I have experienced a tiny bit of extra braking coming on line at exactly the instant where I was easing off pressure. Various engineers have commented to me that it is typical behavior with hybrids using regen. I think they just can't respond quickly enough.

OTOH, I cannot detect the blending in of hydraulic brakes in normal operation. The regen is pretty strong, contributing most braking up to 0.3g of decel (even at 3750lbs, 60kW will really slow you down) so it really doesn't happen that often. Just driving around normally, my front brakes are usually at ambient temp when I get out of the car.

Blending out the regen is noticeable at 7mph or so. My mind has filtered it out well enough that now I come to a stop with a slight jerk in a normal car.

 

It appears another bit of data that perhaps hasn't been correlated... It looks like you had light brake pressure, then went over a bump, and it took a much longer time to "recover". All the testing I did with my accelerometer data (external, not over the CAN bus) was with moderate braking and over potholes, not speedbumps. Perhaps the response is different based on these variables.

I think it is time to break out the ol' recorder and find some patchy roads...

 

The Volt's regen brakes are extremely powerful. Providing the individual was in L mode you can not use the brakes at all except to bring the car to a stop from 10mph or so. Providing there is no one behind me I allow the car to coast to a stop. Even going downhill the regen in L is strong. I have been trying to duplicate it in D and using the actual foot brake though. If I can get it to reproduce on my car can at least call the Volt Advisor line and open up a support ticket so we can get to the bottom of it.

 

Certainly this is true. First, regenerative braking works with only the drive wheels, and even then through a differential. All it takes is the loss of traction of a single tire to cause a loss of all regenerative braking. With friction brakes all four wheels are braked independently.

Furthermore, it takes a finite amount of time to switch to friction braking from regenerative braking. As you point out, this exacerbate the problem

With heavier braking it is likely that some of the braking is already friction. This would lessen the feel of losing regenerative braking.

Tom

 

Will try it in L on the way home as I located some areas where I live where there are storm drains. Really what I need is a good day in Florida where it rains as it will be easier for me to break traction.

To reply to what Tom said.. To me if I am not in L it feels like the Volt is primarily using the friction brakes. The only sure fire way I have found to get good regen braking in is to put it in L. In D even if I lightly push on the brake the green spinning leaf ball (don't know what else to call it) will immediately go down showing me I am using the friction brakes. In L mode though all you have to do is life off the gas and the spinning ball stays centered telling you that you are getting maximum regen.