What makes smartphones tilt-sensitive? Will they retain this ability in zero-gravity conditions?

Question

Most smartphones are tilt-sensitive, but what device makes this possible? Additionally, how does it (and the sensors associated with it) work?

Also, since the working of these sensors seem, almost certainly, based on the presence of an external gravitational field (for instance, the earth's), this begs the second question: Do smartphones retain their tilt-sensitivity under zero-gravity (hypothetical) conditions?

(Recently played an aircraft simulator game on my phone...the fact that the plane responded so well to tilting took me aback; hence the urge to ask this question)

---

Extras:

I put some thought into this myself, so I'll be putting that up here too. For all intents and purposes, my question ended after the second paragraph, but what I've added after this might help tailor an answer that fits my current understanding of physics.

I'm currently in high-school, and if I recall correctly, there are six degrees of freedom for a particle in a 3D Cartesian system. From my experience with the aircraft simulator app, smartphones seem to detect motion in only three degrees of freedom: pitch, roll and yaw

Speaking of tilt-sensitive sensors: The way I assume these sensors/transducers work, is by detecting the minute changes in gravitational potential energy (which may manifest itself as small-scale motion of some tiny components of the sensor) that is associated with the phone's change in spatial orientation.

The way I see it, such a sensor would require moving parts, and cannot simply be another chip on a circuit board.

Under these circumstances, if I were tasked with building a tilt-sensitive device that perceives minute changes in gravitational potential energy, I would probably require at least 3 pairs of sensors (a pair in each of the three coordinate axes). Also, seeing how very sensitive my smartphone appears to be to tilting, I'd have to build a ridiculously large device, with each sensor in a pair placed several meters apart to achieve tilt-sensitivity comparable to that of my phone.

However, smartphones have dimensions smaller than that of a typical sandwich, so having "sensors in a pair placed several meters apart", apart from being impractical, is clearly not the case.

^ I went ranting about this, so that you can get a feel of my genuine perplexity in the sub-question that follows:

How come these sensors are so sensitive, despite their small size?

Answer 1

You are right, in a sense. These sensors do need moving components. However, they are a chip on your board.

Tiltsensors (actually, accelerometers), and gyroscopes (and pressuresensors, ...) are part of a family called MEMS: Micro-electromechanical systems.

Using similar techniques as already common in integrated circuit fabrication, we can make amazing little devices. We use the same processes of etching away things, depositing new layers, growing structures, etc.

These are incredibly tiny devices. this is an example of a gyroscope:

link to the original website.

Most of these work by sensing changes in capacitance. A gyro would sense the changes due to rotation (the big thing in the picture would twist around the center axis. This will bring the tiny teeth that are interleaved closer together, and increase capacitance. Accelerometers work under a similar principle. These teeth can be spotted in the rightbottom corner of the second image.

What about zero-gravity?

It would not change much in terms of the functioning of the devices. You see, accelerometers work by sensing acceleration. The key however is that gravity is the same to them - it just feels like you are being accelerated up at 1G, all the time. They use this "constant" to get an idea where "down" is. This also means that while the chips will function just fine in micro gravity, your phone would not - it will be confused as there seems to be no "down".

Quick addition to address a (very good) point that user GreenAsJade brings up: When you look at the common definitions of gyroscopes on sources like wikipedia they are often described as something along the lines of a spinning disk. The pictures above don't seem to have any spinning parts. What's up with that?

The way they solve this is by replacing the rotation with vibration. The disk shaped object in the pictures here are only connected with very thin and flexible structures to the center axis. This disk is then made to vibrate around it's axis at high frequency. When you move the entire structure along an angle, this will cause the disk to try and continuously resist this - similar to a classic gyroscope. This effect is called the Coriolis effect. By sensing the amount of tilt of the disk compared to the surrounding solid material, it can measure how fast it is spinning.

Answer 2

The sensory device is a weight on a spring. It is indeed "small-scale motion of some tiny components of the sensor", and it's also "another chip on a circuit board".

The key word here is MEMS. It's possible to build small silicon structures and then etch away underneath them, leaving a free-floating piece. If the piece is long and thin, it will deform under gravity (or any acceleration) by an amount proportional to its Young's modulus. The change in position affects the capacitance between the moving part and stationary parts around it, which can be measured electronically.

Generally they have only one three-axis accelerometer. Better precision can be achieved by adding gyros or another accelerometer separated by a distance; Nintendo did this with Wiimote add-ons.

Many phones also contain a magnetometer, which tells you vaguely where magnetic north is relative to the phone, although the calibration tends to be bad on these.

Addressing specific parts of the question:

• What makes smartphones tilt-sensitive?

MEMS accelerometers. Few mm square chip package, $0.50 or less in quantity.

• Will they retain this ability in zero-gravity conditions?

Not exactly. They no longer have a convenient reference vector. However, they can still detect acceleration, so if you have one of those "lightsaber" apps and wave it around it will still work on the ISS. But neither you nor the phone have a clear idea of "up".

(The Raspberry Pi kit sent up there has an accelerometer and a bunch of programs written by schoolchildren, so there's almost certainly a video demonstrating this somewhere)

The raw output of a 3-axis accelerometer is a vector of 3 values measured in m/s^2. The magnitude of this vector will usually be about 1g, but the direction varies. For a stationary phone it will point downwards. If you move it then the acceleration vector will change direction. If you drop the phone, i.e. it goes into freefall the same as a phone on an orbiting craft would be, then the magnitude goes to near-zero. This makes the direction of the vector swing wildly and turn to noise.

The use of accelerometers as drop detectors for hard disk safety was popularised about a decade ago by Macbooks. People found other uses for them.

• how does it work?

Answered in more detail by other answers.

Answer 3

Theoretically, yes, a phone or tablet could work just as well in say the International Space Station (ISS) as it does here on the ground.

Let's break this down a bit.

There are two types of motion a device needs to detect.

Linear Motion

Self-contained accelerometers use the deviation of a spring-coupled mass from a normal rest point as a measure of acceleration force in that axis. Obviously you need three of these to detect motion in any axis.

Knowing, and tracking those forces you can "dead-reckon" the speed and direction of travel of the device from its original "power-on" location. Factor in an accurate clock, and you can also figure the current position.

That sounds simple, but the math is actually quite complex and errors in the system cause a drift over time.

Rotation

Rotation is obviously spin about any axis.

Spin Sensors

Rotation can be measured using a gyroscope or a spin sensor. These devices again have a loosely coupled mass that is free to rotate, or is driven, in a particular axis. When the body of your device rotates, the difference between rotations tells you how much the device is rotating.

Spin sensors and gyroscopes don't care about gravity, other that perhaps some frictional differences.

Gravity Referenced Accelerometer Rotation

Since accelerometers measure the force acting on a loosely suspended mass, when that sensor is vertical relative to the earth, there will of course be a deflection in the spring due to the weight of the mass due to gravity. This offset is mathematically removed by the software in order to extract the acceleration part.

However, since the three axis accelerometers will produce different offsets depending on their orientation, it is possible to mathematically detect spin from the difference in the offsets.

However, though this method works, it is subject to variances in G. It would not work in space. It would also be significantly less functional in a maneuvering aircraft. Even a car going round a tight bend at speed could be problematic.

Accelerometer Spin Detection

It is possible, with two sets of sufficiently sensitive accelerometers, to detect spin from the difference in acceleration between accelerometers.

Since each accelerometer has to move relative to the other, there will be a difference in acceleration in that axis between each. Those values can again be used mathematically to predict the spin.

Simply put, if you can tell from the accelerometers centered at one end of the phone that the centre point has moved to \$X_1,Y_1, Z_1\$, and the other end is now at \$X_2,Y_2, Z_2\$, calculating the three angles is trivial.

This method is NOT affected by gravity.

Will YOUR Phone or Tablet Work On The ISS

As you can see from the above it really depends on which methods your device uses.

Technically it could be built, and programmed, to do so. You may need to shut it down and power it up again to recalibrate it, but with the right systems in place it should work fine. At least for playing that "aircraft simulation game".

Drift may be a larger issue on the ISS though. Since phones in normal G have the ability to known which way "down" is at that particular moment, they can re-adjust over time. A space-based unit would need an occasional manual reset to indicate the "normal" direction.

Answer 4

All the comments and answers are great to help you understand how it is possible. But, here's something that will help you understand how it's actualized in real products.

(image source)

This is a tiny IC (3x3x1 mm) by InvenSense. It has a three-axis accelerometer (for lateral movement), a three-axis gyroscope (for rotation), and a three-axis magnetometer (like a compass needle). It has internal code which will do all the complicated math. It consumes very little power.

This is only an example. There are several companies making similar products. Some are more accurate than others, some are cheaper, most don't have the magnetometer, etc...

Answer 5

This is a rare case on the Electronics site where, none of the answers clearly and crisply answered the question!

Do cellphones retain the ability to detect tilt in zero-gravity conditions?

The answer is:

They retain (at a hardware level) the ability to detecting tilting, but they can no longer detect tilt.

Further,

At the level of app software, in fact, almost all (very likely "all") app-software-writers would not allow for the corner case of zero-gravity, so very likely the gyro-accel functions would act whackily overall, in most/all actual apps.

Regarding how gyros/accels work in phones, you can easily google the APIs for these on the two platforms (example).

Note however that all OSs as of writing, in practice wrap the lower-level gyro/accel functions in some sort of convenient higher-level motion manager:

Accels/gyros, are in fact wrapped together at the OS level

So in fact...

in practice, for any fairly newly-written app (remembering that, let's say, about 25% of apps in the store are decayed / not updated regularly), it would come down to how the team at Apple which wrote (in their case) "Coremotion" handled (if at all!) the zero gravity environment case. (There's a similar situation for Android).

And further, for games as such...

Today almost any game you pick up and play on a phone was created in Unity3D, rather than as a native app. (And as a rule, if you look at the set of "apps which use the accel/gyros", 90% (more?) of them are just games.) So in fact (on all platforms) the software-writers are actually using's Unity's level of software wrappers.

Hence, the actual behavior in the extreme corner case of earth orbit, would depend on what those folks did when writing that.

One confusing point...

that hasn't been clarified. When you're writing software for phones, it's totally commonplace to have to deal with "zero gravity" ... for short periods of time: that is, when the phone is in free-fall. So if you're making one of the (100s of) apps for skateboarders, skiers or the like which measures hang-time and so on, you deal with this as a matter of course.

Gyros were introduced to phones about 2010; accels were in them from the start.

A French/Italian company called STMicroelectronics pretty much makes most of the gyros for both apple and samsung.

Regarding accelerometers, most phones now have a couple of them since it works better that way. I have heard that there is more variety of suppliers of accelerometers (Bosch, etc).

You can literally buy MEMS gyros or accels, if for example you are making an electronic toy that includes such a feature.

• How MEMS accelerometers actually work at a substrate level
• How MEMS gyros work

Just to repeat, the fundamental quick answer to the question posed is

In "zero g", they retain (at a hardware level) the ability to detecting tilting, but they can no longer detect tilt.

In terms of the software,

1. it would, almost certainly, "totally fail!" in the whacky "you're in orbit" case. Since no gane or app engineer (I know) would be so OCD as to cover that case, but don't forget...

2. it's totally commonplace to have "zero gravity" .. during short periods of freefall (this applies as a commonplace matter if you're making one of those "action sports apps").

Answer 6

I think they might use a sagnac interferometer in smartphones. A Sagnac Interferometer is a device which produces a constant interference pattern while at rest and its pattern varies when the setup is rotated.

So when 3 such interferometers are placed we can measure the rotation about all 3 axis.

Sagnac interferometers come in very small sizes and it comprises of optical fibers to channel light , a source of light(coherant), and an detector.

Of course it should be calibrated before use.