8 ms·

So a big reason why this probably wouldn't fly is that it has a common mode failure which is loss of power. Most EVs don't use permanent magnetic motors so it n

So a big reason why this probably wouldn't fly is that it has a common mode failure which is loss of power. Most EVs don't use permanent magnetic motors so it needs power to be able to use the motors for regen. So lets say the relay fails from the main battery pack suddenly you have no power, and no way to energize the coils in the motors to use them to regen. This could be mitigated by using permanent magnet motors but you still have the issue if control power is lost the vehicle cannot stop. The beauty of hydraulic brakes is that they work really well with power, but still function without it.

crgwbr 4y ago
Interesting. Is there no way to design the circuitry so that when given no input, it defaults to a fully brakes-engaged mode? I’ve only worked with PM motors, so I know how that sort of h-bridge could be designed, but my knowledge of non-PM motors is fuzzy at best.
- HeyLaughingBoy 4y ago
  Hmmm. So you're travelling along the interstate at night, you lose power and your car screeches to a stop in the middle of high-speed traffic and your lights are off. I would love to be present at the DFMEA (Design Failure Modes Effects Analysis) meeting when this came up!
- Bilal_io 4y ago
  If you're driving 75 mph and one or more motors suddenly lose power and it fully brakes, that's a certain death in my opinion.
  - notatoad 4y ago
    are people under the impression that when you're driving down the highway at 75mph, there isn't already a hundred ways you could die?
    - progman32 4y ago
      I mean, now we have 101 ways, and not all those ways are equally likely...
  - mellavora 4y ago
    Well, it is certainly a very shaky proposition, and the car will probably be a total loss. But there are ways you could walk away from this. You see some pretty crazy crash survivals in race driving, for example. And I remember stories from a few years ago, when Ford had a recall on F150 tires because they tended to blow out at highway speeds, with an effect similar to full wheel lockup-- yes, people died, but others didn't. life == hope
- steveBK123 4y ago
  I'd imagine this was introduce adverse failure modes like - slamming to a stop without warning within seconds on the highway if power is lost, rather than coasting to a halt... and then being untowwable as the car can't be put into neutral. Tesla's that lose battery are already a challenge to tow as the parking brake cannot be disengaged without power.
- sbradford26 4y ago
  So looking into it a bit there are what is called Eddy current brakes. In the section of disk eddy current brakes it talks about the power tool implementation where there is just a metal disk and the magnetics are pulled away from the disk when the device is powered on and passively return to the disk when power is removed. Something like that could work, but I am unsure of the size of magnets or disks necessary to adequately stop a vehicle. Most likely you could also add a normally closed relay hooking the motor to a resistor load. So when power is applied the resistor load is disconnected, and in a loss of power event the motor is directly tied to the resistor load. https://en.wikipedia.org/wiki/Eddy_current_brake https://en.wikipedia.org/wiki/Eddy_current_brake
  - hoosieree 4y ago
    Eddy currents develop in proportion to the speed of the moving magnetic field. So if you used eddy currents for braking, it would be most effective at high speed and gradually have no stopping power at low speed.
    - sbradford26 4y ago
      That is mostly true about regenerative braking as well. Once you hit a certain speed the vehicle has to use some energy to get the motors to actually stop the vehicle. Usually EVs and such will just rely on the friction brakes for the last bit but some cars like the Bolt will automatically use some energy to slow the car to a stop so you can use do one pedal driving.
      - hoosieree 4y ago
        Good point, and a good reason to have friction brakes also installed.
bilsbie 4y ago
Maybe dedicated capacitors it’s enough power for one stop? Or a mechanical fail safe? Just a big pad that drops down on top of your tires.
- sbradford26 4y ago
  Hold up capacitors definitely could extend the length of time the brakes would work but it would depend on what the power failure was and how long you need to brake. A mechanical fail safe just seems like a worse version of hydraulic brakes.
  - p1mrx 4y ago
    Maybe the capacitors could store enough energy to start braking, and then the braking energy could recharge the capacitors.
extrapickles 4y ago
That is easy to accommodate as you can design the electronics such that when there is no power they cause maximum braking force and dump energy into the braking resistors. This can be done completely with power taken from the motor itself and if it’s not turning to generate power, then I would expect the parking brake system to engage, eg: a spring defaults it to “On” and power is used to keep it disengaged.
- sbradford26 4y ago
  This can only be done with permanent magnetic motors which is not the norm for EVs for many reasons. In a non permanent magnetic motor on loss of power there is no energy to dump into braking resistors since the motor is not causing any resistance because the coils have been de-energized.
  - extrapickles 4y ago
    Most EVs have permanent magnet motors of some flavor right now (some have both a PM motor and a pure induction). I would assume that if you were having the motor be the primary braking system, and it was induction, there would be a tiny cheap magnet installed in it to generate bootstrap power to prevent exactly this scenario.
    - londons_explore 4y ago
      An even simpler design is to assume that the car can only start moving when there is power. If there is a loss of power while moving, then you can use the energy in the HV capacitors in the inverters to make a bootstrap field. That can be maintained till the car stops (and the user still has control of when/how much to brake). When the car comes to a halt, the parking brake is applied, and it won't move again till repaired. All of that can be done with software-only - no hardware changes needed.
- spaetzleesser 4y ago
  Parking brakes are pretty useless to slow down a car from any decent speed.
spaetzleesser 4y ago
Good point. Feels a little like airplanes where at some point it was decided that mechanical backup is not needed for fly by wire.
- sbradford26 4y ago
  So a fly by wire aircraft might not have physical cable moving surfaces, but most of them have a backup mode that relies basically entirely off of physical switches and direct communication with the surfaces. Those modes are only used in extreme emergencies but they are still designed into the system. Also what works in airplanes doesn't always translate to cars. Aircraft have rigorous maintenance requirements, while cars leave the manufacturer and might not have any sort of inspection or maintenance done for 20 years.
Verdex 4y ago
Interestingly enough, I've had hydraulic brakes fail on me for two different cars (thankfully it was 20 years apart, but still I would have preferred that the first time was the last time). iirc both times it was some sort of brake fluid leak. So, while I totally agree that having motorized breaks is really scary, hydraulic brakes can fail as well.
- sbradford26 4y ago
  I don't deny that there can be physical failures to the systems. And I also drive a hybrid currently that most of the time only uses the electric motor for slowing down, I don't find using the motor for braking scary. I personally see having both regen and hydraulic brakes on a vehicle a great step forward in redundancy and safety.
  - _0w8t 4y ago
    Electrical slowdown in hybrids does not provide redundancy for emergency braking. The battery has to absorb like 500kWt during 1s, this is well outside what a small battery can do.
    - sbradford26 4y ago
      Redundancy doesn't mean it will bring the car to a stop as quickly as hydraulic brakes, it is just another means to bring the car to a stop safely. From personal experience I could safely get my car to a stop in many situations without using the friction brakes at all.
- drewzero1 4y ago
  I've also had two cars lose brake pressure. I'm very thankful that the cable-operated parking/emergency brake was functional on both cars; it saved one of them from going through the back wall of the garage. The cable-operated E-brake definitely doesn't stop the car anywhere near as well as the power-assisted hydraulic brakes, but having two different braking methods can be a literal lifesaver when one system fails. (And all systems fail! I'd previously had two other cars where the E-brake didn't work.)
snowwrestler 4y ago
“It still kind of works without power” was an idea that kept hydraulic actuators in planes for a long time. But now most new planes are fly-by-wire (fully electric). Reliably stopping an electric car in an emergency should be a solvable problem.
- NegativeLatency 4y ago
  It could be solved but it might not be worth it. Planes have a whole different set of expectations on their reliability and upkeep. For example I don’t think people would be excited about an electric car with an aux power unit.
  - HPsquared 4y ago
    They already have an auxiliary power unit of sorts: the 12V battery.
  - Kon-Peki 4y ago
    The air brakes used on heavy vehicles (semi trucks, etc) have a very simple fail-safe. You need power to not brake. I.E. Air pressure is used to disengage the brakes. If you lose power/air pressure, the brakes engage and are stuck engaged.
- sbradford26 4y ago
  So the fly by wire just means it is digital control to the hydraulic actuator instead of a physical cable running from the yoke or something to the actuator. They have not gotten rid of hydraulic actuators yet, they are working do that in the future but that is still a long ways out. Aircraft power systems also have significantly more redundancy so loss of control power is not a common mode failure due to have multiple dissimilar power sources.
peteradio 4y ago
I see a lot of good ideas for how to get around this failure mode, I'll throw one more in the ring: "giant spring loaded spike to jab into the road"
- londons_explore 4y ago
  I actually don't think that's a bad idea... Specifically, most cars can only brake at about 1 G, and if that's insufficient, they use crumple zones and airbags which brake at far more than 1 G. If emergency braking could use an explosive-spike-in-ground deceleration at say 5 G, then far more crashes would be survivable. I think the reason it hasn't been done are human reasons - users don't like the idea that jabbing the brake pedal hard will mean they have to pay tens of thousands of dollars to repave the road and replace their spike.

crgwbr 4y ago

Interesting. Is there no way to design the circuitry so that when given no input, it defaults to a fully brakes-engaged mode? I’ve only worked with PM motors, so I know how that sort of h-bridge could be designed, but my knowledge of non-PM motors is fuzzy at best.

HeyLaughingBoy 4y ago
Hmmm. So you're travelling along the interstate at night, you lose power and your car screeches to a stop in the middle of high-speed traffic and your lights are off. I would love to be present at the DFMEA (Design Failure Modes Effects Analysis) meeting when this came up!
Bilal_io 4y ago
If you're driving 75 mph and one or more motors suddenly lose power and it fully brakes, that's a certain death in my opinion.
- notatoad 4y ago
  are people under the impression that when you're driving down the highway at 75mph, there isn't already a hundred ways you could die?
  - progman32 4y ago
    I mean, now we have 101 ways, and not all those ways are equally likely...
- mellavora 4y ago
  Well, it is certainly a very shaky proposition, and the car will probably be a total loss. But there are ways you could walk away from this. You see some pretty crazy crash survivals in race driving, for example. And I remember stories from a few years ago, when Ford had a recall on F150 tires because they tended to blow out at highway speeds, with an effect similar to full wheel lockup-- yes, people died, but others didn't. life == hope
steveBK123 4y ago
I'd imagine this was introduce adverse failure modes like - slamming to a stop without warning within seconds on the highway if power is lost, rather than coasting to a halt... and then being untowwable as the car can't be put into neutral. Tesla's that lose battery are already a challenge to tow as the parking brake cannot be disengaged without power.
sbradford26 4y ago
So looking into it a bit there are what is called Eddy current brakes. In the section of disk eddy current brakes it talks about the power tool implementation where there is just a metal disk and the magnetics are pulled away from the disk when the device is powered on and passively return to the disk when power is removed. Something like that could work, but I am unsure of the size of magnets or disks necessary to adequately stop a vehicle. Most likely you could also add a normally closed relay hooking the motor to a resistor load. So when power is applied the resistor load is disconnected, and in a loss of power event the motor is directly tied to the resistor load. https://en.wikipedia.org/wiki/Eddy_current_brake https://en.wikipedia.org/wiki/Eddy_current_brake
- hoosieree 4y ago
  Eddy currents develop in proportion to the speed of the moving magnetic field. So if you used eddy currents for braking, it would be most effective at high speed and gradually have no stopping power at low speed.
  - sbradford26 4y ago
    That is mostly true about regenerative braking as well. Once you hit a certain speed the vehicle has to use some energy to get the motors to actually stop the vehicle. Usually EVs and such will just rely on the friction brakes for the last bit but some cars like the Bolt will automatically use some energy to slow the car to a stop so you can use do one pedal driving.
    - hoosieree 4y ago
      Good point, and a good reason to have friction brakes also installed.

bilsbie 4y ago

Maybe dedicated capacitors it’s enough power for one stop? Or a mechanical fail safe? Just a big pad that drops down on top of your tires.

sbradford26 4y ago
Hold up capacitors definitely could extend the length of time the brakes would work but it would depend on what the power failure was and how long you need to brake. A mechanical fail safe just seems like a worse version of hydraulic brakes.
- p1mrx 4y ago
  Maybe the capacitors could store enough energy to start braking, and then the braking energy could recharge the capacitors.

extrapickles 4y ago

That is easy to accommodate as you can design the electronics such that when there is no power they cause maximum braking force and dump energy into the braking resistors. This can be done completely with power taken from the motor itself and if it’s not turning to generate power, then I would expect the parking brake system to engage, eg: a spring defaults it to “On” and power is used to keep it disengaged.

sbradford26 4y ago
This can only be done with permanent magnetic motors which is not the norm for EVs for many reasons. In a non permanent magnetic motor on loss of power there is no energy to dump into braking resistors since the motor is not causing any resistance because the coils have been de-energized.
- extrapickles 4y ago
  Most EVs have permanent magnet motors of some flavor right now (some have both a PM motor and a pure induction). I would assume that if you were having the motor be the primary braking system, and it was induction, there would be a tiny cheap magnet installed in it to generate bootstrap power to prevent exactly this scenario.
  - londons_explore 4y ago
    An even simpler design is to assume that the car can only start moving when there is power. If there is a loss of power while moving, then you can use the energy in the HV capacitors in the inverters to make a bootstrap field. That can be maintained till the car stops (and the user still has control of when/how much to brake). When the car comes to a halt, the parking brake is applied, and it won't move again till repaired. All of that can be done with software-only - no hardware changes needed.
spaetzleesser 4y ago
Parking brakes are pretty useless to slow down a car from any decent speed.

spaetzleesser 4y ago

Good point. Feels a little like airplanes where at some point it was decided that mechanical backup is not needed for fly by wire.

sbradford26 4y ago
So a fly by wire aircraft might not have physical cable moving surfaces, but most of them have a backup mode that relies basically entirely off of physical switches and direct communication with the surfaces. Those modes are only used in extreme emergencies but they are still designed into the system. Also what works in airplanes doesn't always translate to cars. Aircraft have rigorous maintenance requirements, while cars leave the manufacturer and might not have any sort of inspection or maintenance done for 20 years.

Verdex 4y ago

Interestingly enough, I've had hydraulic brakes fail on me for two different cars (thankfully it was 20 years apart, but still I would have preferred that the first time was the last time). iirc both times it was some sort of brake fluid leak. So, while I totally agree that having motorized breaks is really scary, hydraulic brakes can fail as well.

sbradford26 4y ago
I don't deny that there can be physical failures to the systems. And I also drive a hybrid currently that most of the time only uses the electric motor for slowing down, I don't find using the motor for braking scary. I personally see having both regen and hydraulic brakes on a vehicle a great step forward in redundancy and safety.
- _0w8t 4y ago
  Electrical slowdown in hybrids does not provide redundancy for emergency braking. The battery has to absorb like 500kWt during 1s, this is well outside what a small battery can do.
  - sbradford26 4y ago
    Redundancy doesn't mean it will bring the car to a stop as quickly as hydraulic brakes, it is just another means to bring the car to a stop safely. From personal experience I could safely get my car to a stop in many situations without using the friction brakes at all.
drewzero1 4y ago
I've also had two cars lose brake pressure. I'm very thankful that the cable-operated parking/emergency brake was functional on both cars; it saved one of them from going through the back wall of the garage. The cable-operated E-brake definitely doesn't stop the car anywhere near as well as the power-assisted hydraulic brakes, but having two different braking methods can be a literal lifesaver when one system fails. (And all systems fail! I'd previously had two other cars where the E-brake didn't work.)

snowwrestler 4y ago

“It still kind of works without power” was an idea that kept hydraulic actuators in planes for a long time. But now most new planes are fly-by-wire (fully electric). Reliably stopping an electric car in an emergency should be a solvable problem.

NegativeLatency 4y ago
It could be solved but it might not be worth it. Planes have a whole different set of expectations on their reliability and upkeep. For example I don’t think people would be excited about an electric car with an aux power unit.
- HPsquared 4y ago
  They already have an auxiliary power unit of sorts: the 12V battery.
- Kon-Peki 4y ago
  The air brakes used on heavy vehicles (semi trucks, etc) have a very simple fail-safe. You need power to not brake. I.E. Air pressure is used to disengage the brakes. If you lose power/air pressure, the brakes engage and are stuck engaged.
sbradford26 4y ago
So the fly by wire just means it is digital control to the hydraulic actuator instead of a physical cable running from the yoke or something to the actuator. They have not gotten rid of hydraulic actuators yet, they are working do that in the future but that is still a long ways out. Aircraft power systems also have significantly more redundancy so loss of control power is not a common mode failure due to have multiple dissimilar power sources.

peteradio 4y ago

I see a lot of good ideas for how to get around this failure mode, I'll throw one more in the ring: "giant spring loaded spike to jab into the road"

londons_explore 4y ago
I actually don't think that's a bad idea... Specifically, most cars can only brake at about 1 G, and if that's insufficient, they use crumple zones and airbags which brake at far more than 1 G. If emergency braking could use an explosive-spike-in-ground deceleration at say 5 G, then far more crashes would be survivable. I think the reason it hasn't been done are human reasons - users don't like the idea that jabbing the brake pedal hard will mean they have to pay tens of thousands of dollars to repave the road and replace their spike.