Do accelerometers know when they are moving straight up or straight down?

Question

I need a sensor that I can only place on top of an object, to count the number of times the object moves up and down.

Would placing an accelerometer on top of the object accomplish this or can it track the transition between moving up and down?

If not what type of sensor(s) would accomplish this?

---------------- EDIT: -------------------------

Ideally I would like a sensor I can set on top of the box outlined below and count the times it goes up and down. It has multiple guide rails that prevent it from twisting. I plan on using an arduino as a microcontroller that does the math, just need a sensor capable of sensing the up/down motion. All suggestions welcome. thanks.

   +    +
   +    +
   +    +
------------
|   box    | 
| moves up |
| and down |
------------
   +    +
   +    +

Answer 1

I think an accelerometer and arduino is a great way to count the number of times the box moves up and down. It's a compact, reliable, non-contact way of detecting movement.

It could work for movement as gentle as the rise and fall of your chest, when breathing, lying down, and certainly anything faster than that.

You might get away with a simple threshold detector with some hysteresis - count one when the acceleration exceeds +x, then set the threshold to -x, until it crosses -x, then set threshold to +x.

If the acceleration is more gentle, or there is background vibration which you don't want to count (even though it is actually movement up and down), then the signal might be lost in the noise.
Then you will have to implement a simple digital filter on the arduino, that in effect "searches" for movement at a particular rate. This could be a simple as just adding up the last 100 acceleration samples (at 100 Hz) and putting the average through the threshold detector, or you could design a more elaborate filter.

If you're planning to use an accelerometer for anything, try it out first with your smartphone.

Look on your app store for an accelerometer monitor app, something that records the sensor to a file, and do some experiments. If it makes a CSV file, you can open it in excel and plot graphs etc.

The accelerometer in your phone is quite basic and noisy, but is a good substitute for any other sub-$100 accelerometer, and might give you a rough idea of what a $1000 one could do.

Here's an example of what I've done with the phone's sensor.

Good luck! Post your results here (answer your own question) if it works out for you.

Answer 2

In general, no. Imagine throwing a ball straight up (and ignore wind resistance): once it's moving, the only force on it is gravity, which produces a constant acceleration. There's no way to know from the acceleration alone, when the ball has reached its highest height and started back down – the acceleration is the same throughout its path until it hits the ground.

If you know that the object is starting at rest and you measure its acceleration you can figure out its velocity by integrating acceleration over time. If it slows down you'll see a negative acceleration, and the integral will go to zero when it stops. Then when it starts moving again you'll see a change in acceleration and you can start integrating all over again. However, this depends strongly on the accuracy of the measurements and the calculations. If either one is off by a little bit the error will gradually increase until you no longer know what's really happening.

Answer 3

Firstly it's important to understand how an accelerometer behaves in a gravitational field, such as we experience here on earth. An accelerometer does not measure acceleration, in the sense that it does not measure the change in velocity alone. It measures change in velocity plus gravity - which means that you need to take the measurement of gravity out if you plan to attempt what you outline.

Also, the gravity will be measured as an upwards acceleration - which is somewhat counter-intuitive but makes sense if you imagine an accelerometer as being built something like this:

^{simulate this circuit – Schematic created using CircuitLab}

I've abused the schematic editor to show a ball bearing suspended in a tube by two perfectly damped springs. Imagine that the accelerometer makes its measurement by telling you how far from the resting position the ball bearing currently is. Now imagine turning this device on its end, and you can see how the ball bearing sags down under gravity, and how this is indistinguishable from acceleration. I think relativity has something to say on this topic, but anyway.

So - if you have a vertical accelerometer, and you remove the effect of gravity, and the movements of your device are sufficiently larger than the noise floor of your accelerometer, and moreover the movements are sufficiently smooth to not confuse your algorithm - then it might be possible. There's a answer above that mentions freefall, which would be a problem, but I'm assuming from your description that your device will not be in freefall for any length of time. This is a guess though.

If you really do want to use an accelerometer your best approach to a problem like this is to record the accelerometer's output somehow (I prefer to output it over SPI and record it using a USB logic analyser) and write down exactly what you do (or even video it, if you're really keen). Then you can play around with algorithms offline, without the overhead of constantly wondering if your Arduino is really doing what you think it is.

As others have mentioned, other types of sensor may be more appropriate. We'd need more detail about exactly why you're attempting what you're attempting to comment I think.

Answer 4

If the object moves up and down on fixed guides, I think the easiest way to count its movements would be to use a limit switch that would be closed when the object is at rest, and open when away from its rest position.

Answer 5

Careful, a reversal of acceleration doesn't mean the box turned around. The box starts up--you see upward acceleration. It's now moving up at a steady pace--you see no acceleration. It slows to half speed--you see downward acceleration. It continues at half speed--you see no acceleration. It speeds back up--you see upward acceleration. It stops, you see downward acceleration.

You saw up, down, up, down but the box really only moved up. You will have to integrate over time and have enough accuracy in your accelerometer to figure out what way you really are going--watch you for cumulative error, it could make this problem very hard!

Have you investigated using a laser range finder to measure where the box is?

Answer 6

Accelerometers are measuring accelerations. If the movement is performed with acceleration, it will be detected, given the accelerometer is sensitive enough on this axis. The direction of the acceleration will be indicated by the sign of the read values. Please note, that when measuring accelerations on the vertical axis, the measurement will be biased by the gravitation acceleration constant g.

Answer 7

If you don't want physical contact (such as provided by a typical limit switch or microswitch), you could mount a magnet and use a reed switch or a hall effect switch. Or mount something that sticks out slightly and is opaque, and travels through an optical detector (the sort with a small slot in it).

An accelerometer is complicated, more expensive than any of the above, and for the reason eloquently explained by Pete Becker (and now also by Dave Branton), is unlikely to work.

Answer 8

A lot of great info here, but I haven't seen anyone suggest a linear potentiometer ...

Simple, apply voltage and count how many times the GPIO reads a HI. Cheapest linear pot? Volume slider from an old radio, trim adjustment from R/C toy controllers, etc

Answer 9

If its following rails, what about using an ultrasonic range finder? The arduino can do the math to determine acceleration, and it will be a lot simpler implementation than attaching wires to something that moves. This will certainly work best if the track is straight. Here is one. https://www.sparkfun.com/products/639

Answer 10

I would recommend using two thin metal plates to form a capacitor. One plate would be placed on top of the box and the other fixed at some height from the box. As the box moves up and down, the capacitance (C) will vary due to the change in the separation (d) of the plates (C = kA/d). This change can be "converted" by appropriate means to a change in voltage, frequency, etc. and then to a form appropriate for the arduino's interface (amplification might be necessary).