Supposing you have a 3-axis accelerometer, I would point the accelerometer with the X axis to the front of the car and the Y axis to the left of the car so that the Z axis of the accelerometer is perfectly normal to the ground and car plane. This way, gravity (you know, an accelerometer is also subject to static accelerations, not only dynamic accelerations - for an explaination on this see my thesis) won't impact on anything but the Z axis.
Then, everything > that 1G acceleration on the X axis means a braking.
Just hope not to get 1G acceleration on the Y axis, as that would mean you have been hit by a car or something
So you will not have a deceleration greater than g.
QuoteSo you will not have a deceleration greater than g.Brick wall?
Sorry, I forgot the smiley after "brick wall"Anyway, you never heard of "the other way of stopping"?
Your scenario isn't nearly as simple as it's being portrayed, and your end goal is going to dictate the best method of reading/detecting braking to accomplish that.
1.0-1.2g braking is likely max braking for most vehicles under ideal conditions. Dirt roads, dirt on the road, rain, snow, will all have a significant impact on max braking forces.
Most normal driving will also never see anywhere near 1.0-1.2g of braking. More likely to be around 0.5g for normal daily driving (maybe even less for some people). So are you looking to detect any normal braking event, or are you looking to detect some sort of emergency stop situation (in which it is assumed that you are braking as hard as possible) Are you going to be doing anything when this is detected that could be problematic if it's just a case of the person stopping really hard just for the fun of it?
According to simple law of physics, the static friction is only a small portion of gravity of an object, the factor is called static frictional coefficient. Unless you have some tank tracks or sticky surfaces (spider man), you will not have a static friction that exceeds gravity. So you will not have a deceleration greater than g.Somebody has something to say about this coefficient for tire on dry road:http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/frictire.html
As for the braking thing, I think you should ride with an accelerometer and record the entire process of a number of hard breaks, soft breaks and what not so you have data to look at. Without data, I could argue 0.5g is trigger and needs to be at 0.5g+ for 0.1 second and you say different and neither has proof this is what actually happens.
Two options that I can think of. 1)Put a pressure sensor on the brake master cylinder to detect brake pressure and send to Arduino. They make a simple on-off style switch or you could have a gauge pressure sensor if you wanted precision.
2)If you still have your brake light switch that opens and closes by the movement of the brake pedal, you can send that signal through a voltage divider to the Arduino letting it know that you have applied the brakes. If the Arduino knows when the brakes are applied then, you will know when you have data that is related to braking.