6 ms·
And this is where I take a minor quabble with TC. He thought that GM “over engineered” the volt’s brake light system because it used an accelerometer. But hones
by spookthesunset 3y ago
And this is where I take a minor quabble with TC. He thought that GM “over engineered” the volt’s brake light system because it used an accelerometer. But honestly I think it is the easiest way. The only “trick” is to make it stay on once stopped (I’m sure there are conditions when you wouldn’t want them on when stopped though).
In fact it looks like the new standards are going to be defined by the rate if deceleration regardless of the means used to do so. Which means basing the lights on an accelerometer is the most reliable way to meet the spec. It shouldn’t matter how you are slowing down, just that you are.
- jakemoshenko 3y agoWhy do we need an accelerator when we know how fast the wheels are turning directly? The first derivative of wheel speed is almost always car acceleration. Is this to handle some edge cases like not turning the brake lights on when we're actually sliding on ice with the wheels locked? Seems unnecessary.
- ilyt 3y agoCould be as simple as "the controller doesn't have wheel speed on its can bus"
- willis936 3y agoWhy is it unnecessary? Isn't the primary goal of brake lights to inform people of deceleration and the secondary goal to indicate when a vehicle is locked stationary? Showing a speeding vehicle behind you that you've lost control and should slow down is well within the conops of brake lights.
- NikolaNovak 3y agoI think there's an agreement on the goal. I believe the poster is asking why do we need another sensor to achieve that goal? We already have a speedometer based on wheel motion. A trivial computation gives acceleration.
- willis936 3y agoWhat do you save by not having that sensor? Nothing. What do you lose by not having that sensor? The ability for the system to perform its requirements. I'm assuming you read the context of loss of traction being a requirement.
- WorldMaker 3y agoRelatedly, having a dedicated accelerometer sensor is a baseline dependency of many of other modern features like cross-comparing compass headings and GPS for map heading information (much less any of the Level 2+ "self-driving" mechanics). Most cars likely want one, anyway, even in base models. It's one of the cheapest sensors in a suite of increasingly standard sensors (in almost any form factor of device, not just cars, but phones/watches/toasters/etc).
- oceanplexian 3y ago> What do you save by not having that sensor? A $9000 repair bill in 6 years when your mechanic tells you “Sorry your car is immobilized and won’t start, the deceleration brake light sensor went and we need to remove the motor and 3/4 of the wiring harness to replace it”. And for anyone who thinks I’m being sarcastic, try owning a BMW or an Audi and you’ll know it’s the truth.
- Turskarama 3y agoThe problem there is owning an Audi or BMW, not the sensor itself.
- willis936 3y agoA MEMS accelerometer is pennies. If it is critical then add redundancy and don't accept single fault tolerance. If it is optional or the signal can be estimated from other sources with acceptable error then fail gracefully. This is honestly simple stuff. Accepting less from manufacturers is a bad deal.
- 3y ago
- tetromino_ 3y ago> Why do we need an accelerator when we know how fast the wheels are turning directly? Because the car doesn't precisely know the wheel's radius. The radius depends on the rim, tire, and tire pressure - and the car's operator may have accidentally or deliberately chosen something unexpected for any one of those 3 parameters.
- mikestew 3y agoABS relies on the fact that the computer knows how fast the wheels are turning in relation to each other. Wouldn't seem to be much of a leap to leverage that system to determine if the car is slowing down.
- bentcorner 3y agoYou're not wrong but how much does not precisely knowing the wheel's radius change things? The operator would also experience an incorrect speedometer. For the purposes of a brake light it'd be off (either lighting up too soon or too late) by some amount but I imagine it'd be "close enough" except for the most pathological cases of tire sizes.
- oh_sigh 3y agoYou don't need to know the wheel radius, you just need to the rate of change of the wheels RPM. Then the brake light could be set to some conservative value where regardless of any realistic tire size/inflation scenario the brakes will activate at a reasonable deceleration. In any case, with standard gas/brake pedals on ICEs, I can barely tap the brakes and have my brake lights turn on without any deceleration on my part, or I can do that and still be pressing the gas and so actually be accelerating with my brake lights on.
- yjftsjthsd-h 3y agoIf the car doesn't know the size and speed of the tires it also can't accurately display the current speed, which seems like at least as pressing a problem. Edit: Or is detecting deceleration more sensitive? I guess I don't know how precise it needs to be relative to how precise a speedometer needs to be.
- 3y ago
- chandlerswift 3y agoOne more possible reason might be when driving down a hill: When descending a sufficiently steep incline, I may be braking just to maintain my speed! That said, I'm still _feeling_ negative acceleration to maintain a constant speed. (That said, if I'm doing this for more than a handful of seconds, I'll generally downshift, and _that_ doesn't trigger brake lights, despite also causing increased negative acceleration. So maybe regenerative braking is analagous, and fine without brake lights?)
- nonfamous 3y agoI don’t think it’s such an edge case at all. I imagine it would be disconcerting to the driver behind you if you slammed on the brakes on a wet road, locked the wheels into a skid, and the brake lights went out. (Shouldn’t happen with modern braking systems, but still…)
- Turskarama 3y ago>The first derivative of wheel speed is *almost* always car acceleration. There you go, you answered your own question. When it comes to safety features, "almost" always working isn't really good enough.