# Accelerometer polarity question

Will an accelerometer detect when an object moving it in the -ve x direction and +ve x direction? or will they both produce positive values? (assuming gravity is acting perpendicular to movement, in z direction)

to clarify - I need my divice to produce negative values when moving in one driection and positive values when moved in the other.

Kind regards,

Dunc

• Which accelerometer? what does the data sheet say? how many wires does it have? – Solar Mike Jun 3 at 13:38
• I think of getting the MPU6050, 4 wires, VCC,GND, SDA and SCL! – Duncan O'Hara Jun 3 at 14:02

For starters, as Solar Mike mentions, to some extent it should depend on your accelerometer and what it is capable of. That said, it's important to understand what you are measuring, and how that effects your question.

Acceleration is the change in velocity over time. This means that whether or not the object has negative or positive acceleration wont just depend on if it has a positive or negative velocity.

Picture an object moving in the $$+x$$ direction. Now imagine that an acceleration acts on the object that is decreasing it's velocity, while still moving in the $$+x$$ direction. The velocity is still moving in the $$+x$$ direction; but the acceleration is acting in the $$-x$$ direction; which causes an increase in negative velocity (or equivalently a decrease in positive velocity).

Basically, your accelerometer will not tell you directly which direction it is moving just based on the acceleration it is experiencing. You would need other information about starting conditions at least, to determine what your current velocity is given accelerations and times.

The accelerometer can tell you if the object is starting to gain or lose velocity in any direction; but acceleration alone cannot tell you what the current velocity is; unless you know initial velocity and account for all the accelerations leading up to the time you want to know about.

• that makes alot of sense! yeah my analogy was poor, but yeah I need to be able to read postive and negative accelerations regardless of what direction the object is moving in! – Duncan O'Hara Jun 3 at 14:05
• @DuncanO'Hara You'll be able to read positive and negative accelerations no problem. If you want to convert them into the current velocity of the object, you will need some way to process the accelerometer data and calculate the current velocity based on previous velocity and acceleration. I'm not sure how easy it would be to calibrate it for accurate readings. – JMac Jun 3 at 14:08
• No need for me to change into velocity for my project thank goodness! thank you for your help! – Duncan O'Hara Jun 3 at 14:11
• for fun, look up the definition and history of murphy's law. Murphy was supposedly a human subject for acceleration and deceleration experiments done on a rocket sled by the air force in the 1950's. in the first experiment, all the accelerometers attached to his body were wired backwards, and no data was collected from them- or so goes the legend. – niels nielsen Jun 3 at 17:12