onboard speed sensing

Hi folks ... I am utterly new to this but would like my device to know when it is moving above 5mph in any direction and sound an alarm and keep the alarm going until it slows down again. I've thought of accelerometers but don't they just show tilt as with gyroscopes .. i know i know they measure acceleration .. but not for extended periods ? .. think of a falling rc plane ... surely that object would just measure the instance when it starts falling and then would just hit the ground having measured a series of tilt angles ? ... how would it distinguish that from say flying down under control ... ? .. i am a little confused as to how you would put that 'plane' into an alert state of 'i am falling ' ?? any help generally please :D

  1. It appears that your using a plane, but I'm not sure. If your using a car I would use motor encoders.

think of a falling rc plane....... how would it distinguish that from say flying down under control

A falling plane will likely be in a spiral which would be easy to pick up with gyro + accelerometer. Add in the fact that there will also be other signs of a problem... fuel/battery running low, motor issues, ect. and it would be dead easy.

I... would like my device to know when it is moving above 5mph

It would be possible, not accurate, to do that with an accelerometer. I would buy or build a Air speed indicator (assuming this is on a plane).

Thanks - -actualy thats a good point about the asi -- ill try that ... it will be in a plane eventually .. i was just gonna get it working on the ground first so wasn't really trying to pigeon hole it as i wanted to be able to come from any angle .. but in fairness ill give the air speed indicator a try ... :)

An accelerometer measures acceleration.

You can "integrate" the acceleration to obtain speed, but without some other reference the error and noise will accumulate, so it is not a particularly useful method.

An accelerometer is only useful as a "tilt sensor", if the device it is attached to is NOT accelerating.

You can determine speed with a GPS.

You can determine speed with wheel encoders.

You can also determine speed by measuring the speed relative to surroundings using various laser, lidar and radar methods.

You can also determine speed using a pitot tube arrangement like an aircraft does.