What is the concept behind gyroscopes and accelerometers?By Robolab Technologies In Robotics
Here is an excellent explanation of both Accelerometer & Gyroscope.
Source: A Guide To using IMU (Accelerometer and Gyroscope Devices) in Embedded Applications.
When thinking about accelerometers it is often useful to image a box in shape of a cube with a ball inside it.. You may imagine something else like a cookie or a donut , but I’ll imagine a ball:
If we take this box in a place with no gravitation fields or for that matter with no other fields that might affect the ball’s position – the ball will simply float in the middle of the box.. You can imagine the box is in outer-space far-far away from any cosmic bodies, or if such a place is hard to find imagine at least a space craft orbiting around the planet where everything is in weightless state .. From the picture above you can see that we assign to each axis a pair of walls (we removed the wall Y+ so we can look inside the box).. Imagine that each wall is pressure sensitive.. If we move suddenly the box to the left (we accelerate it with acceleration 1g = 9.8m/s^2), the ball will hit the wall X-.. We then measure the pressure force that the ball applies to the wall and output a value of -1g on the X axis.
Please note that the accelerometer will actually detect a force that is directed in the opposite direction from the acceleration vector.. This force is often calledInertial Force or Fictitious Force .. One thing you should learn from this is that an accelerometer measures acceleration indirectly through a force that is applied to one of it’s walls (according to our model, it might be a spring or something else in real life accelerometers).. This force can be caused by the acceleration , but as we’ll see in the next example it is not always caused by acceleration.
If we take our model and put it on Earth the ball will fall on the Z- wall and will apply a force of 1g on the bottom wall, as shown in the picture below:
In this case the box isn’t moving but we still get a reading of -1g on the Z axis.. The pressure that the ball has applied on the wall was caused by a gravitation force..In theory it could be a different type of force – for example, if you imagine that our ball is metallic, placing a magnet next to the box could move the ball so it hits another wall.. This was said just to prove that in essence accelerometer measures force not acceleration.. It just happens that acceleration causes an inertial force that is captured by the force detection mechanism of the accelerometer.
While this model is not exactly how a MEMS sensor is constructed it is often useful in solving accelerometer related problems.. There are actually similar sensors that have metallic balls inside, they are called tilt switches, however they are more primitive and usually they can only tell if the device is inclined within some range or not, not the extent of inclination.
So far we have analyzed the accelerometer output on a single axis and this is all you’ll get with a single axis accelerometers.. The real value of triaxial accelerometers comes from the fact that they can detect inertial forces on all three axes.. Let’s go back to our box model, and let’s rotate the box 45 degrees to the right.. The ball will touch 2 walls now: Z- and X- as shown in the picture below:
The values of 0.71 are not arbitrary, they are actually an approximation for SQRT(1/2).. This will become more clear as we introduce our next model for the accelerometer.
In the previous model we have fixed the gravitation force and rotated our imaginary box.. In last 2 examples we have analyzed the output in 2 different box positions, while the force vector remained constant.. While this was useful in understanding how the accelerometer interacts with outside forces, it is more practical to perform calculations if we fix the coordinate system to the axes of the accelerometer and imagine that the force vector rotates around us.
Please have a look at the model above, I preserved the colors of the axes so you can make a mental transition from the previous model to the new one.. Just imagine that each axis in the new model is perpendicular to the respective faces of the box in the previous model.. The vector R is the force vector that the accelerometer is measuring (it could be either the gravitation force or the inertial force from the examples above or a combination of both).. Rx, Ry, Rz are projection of the R vector on the X,Y,Z axes.. Please notice the following relation:
R^2 = Rx^2 + Ry^2 + Rz^2 (Eq.. 1)
which is basically the equivalent of the Pythagorean theorem in 3D.
Remember that a little bit earlier I told you that the values of SQRT(1/2) ~ 0.71 are not random.. If you plug them in the formula above, after recalling that our gravitation force was 1 g we can verify that:
1^2 = (-SQRT(1/2) )^2 + 0 ^2 + (-SQRT(1/2))^2
simply by substituting R=1, Rx = -SQRT(1/2), Ry = 0 , Rz = -SQRT(1/2) inEq.1
After a long preamble of theory we’re getting closer to real life accelerometers.. The values Rx, Ry, Rz are actually linearly related to the values that your real-life accelerometer will output and that you can use for performing various calculations.
Before we get there let’s talk a little about the way accelerometers will deliver this information to us.. Most accelerometers will fall in two categories: digital and analog.. Digital accelerometers will give you information using a serial protocol like I2C , SPI or USART, while analog accelerometers will output a voltage level within a predefined range that you have to convert to a digital value using an ADC (analog to digital converter) module.. I will not go into much detail about how ADC works, partly because it is such an extensive topic and partly because it is different from one platform to another.. Some microcontroller will have a built-in ADC modules some of them will need external components in order to perform the ADC conversions.. No matter what type of ADC module you use you’ll end up with a value in a certain range.. For example a 10-bit ADC module will output a value in the range of 0..1023, note that 1023 = 2^10 -1.. A 12-bit ADC module will output a value in the range of 0..4095, note that 4095 = 2^12-1.
Let’s move on by considering a simple example, suppose our 10bit ADC module gave us the following values for the three accelerometer channels (axes):
AdcRx = 586
AdcRy = 630
AdcRz = 561
Each ADC module will have a reference voltage, let’s assume in our example it is 3.3V.. To convert a 10bit adc value to voltage we use the following formula:
VoltsRx = AdcRx * Vref / 1023
A quick note here: that for 8bit ADC the last divider would be 255 = 2 ^ 8 -1 , and for 12bit ADC last divider would be 4095 = 2^12 -1.
Applying this formula to all 3 channels we get:
VoltsRx = 586 * 3.3V / 1023 =~ 1.89V (we round all results to 2 decimal points)
VoltsRy = 630 * 3.3V / 1023 =~ 2.03V
VoltsRz = 561 * 3.3V / 1023 =~ 1.81V
Each accelerometer has a zero-g voltage level, you can find it in specs, this is the voltage that corresponds to 0g.. To get a signed voltage value we need to calculate the shift from this level.. Let’s say our 0g voltage level is VzeroG = 1.65V.. We calculate the voltage shifts from zero-g voltage as follows::
DeltaVoltsRx = 1.89V – 1.65V = 0.24V
DeltaVoltsRy = 2.03V – 1.65V = 0.38V
DeltaVoltsRz = 1.81V – 1.65V = 0.16V
We now have our accelerometer readings in Volts , it’s still not in g (9.8 m/s^2), to do the final conversion we apply the accelerometer sensitivity, usually expressed in mV/g.. Lets say our Sensitivity = 478.5mV/g = 0.4785V/g.. Sensitivity values can be found in accelerometer specifications.. To get the final force values expressed in g we use the following formula:
Rx = DeltaVoltsRx / Sensitivity
Rx = 0.24V / 0.4785V/g =~ 0.5g
Ry = 0.38V / 0.4785V/g =~ 0.79g
Rz = 0.16V / 0.4785V/g =~ 0.33g
We could of course combine all steps in one formula, but I went through all the steps to make it clear how you go from ADC readings to a force vector component expressed in g.
Rx = (AdcRx * Vref / 1023 – VzeroG) / Sensitivity (Eq.2)
Ry = (AdcRy * Vref / 1023 – VzeroG) / Sensitivity
Rz = (AdcRz * Vref / 1023 – VzeroG) / Sensitivity
We now have all 3 components that define our inertial force vector, if the device is not subject to other forces other than gravitation, we can assume this is the direction of our gravitation force vector.. If you want to calculate inclination of device relative to the ground you can calculate the angle between this vector and Z axis.. If you are also interested in per-axis direction of inclination you can split this result into 2 components: inclination on the X and Y axis that can be calculated as the angle between gravitation vector and X / Y axes.. Calculating these angles is more simple than you might think, now that we have calculated the values for Rx,Ry and Rz.. Let’s go back to our last accelerometer model and do some additional notations:
The angles that we are interested in are the angles between X,Y,Z axes and the force vector R.. We’ll define these angles as Axr, Ayr, Azr.. You can notice from the right-angle triangle formed by R and Rx that:
cos(Axr) = Rx / R , and similarly :
cos(Ayr) = Ry / R
cos(Azr) = Rz / R
We can deduct from Eq.1 that R = SQRT( Rx^2 + Ry^2 + Rz^2).
We can find now our angles by using arccos() function (the inverse cos() function ):
Axr = arccos(Rx/R)
Ayr = arccos(Ry/R)
Azr = arccos(Rz/R)
We’ve gone a long way to explain the accelerometer model, just to come up to these formulas.. Depending on your applications you might want to use any intermediate formulas that we have derived.. We’ll also introduce the gyroscope model soon, and we’ll see how accelerometer and gyroscope data can be combined to provide even more accurate inclination estimations.
But before we do that let’s do some more useful notations:
cosX = cos(Axr) = Rx / R
cosY = cos(Ayr) = Ry / R
cosZ = cos(Azr) = Rz / R
This triplet is often called Direction Cosine , and it basically represents the unit vector (vector with length 1) that has same direction as our R vector.. You can easily verify that:
SQRT(cosX^2 + cosY^2 + cosZ^2) = 1
This is a nice property since it absolve us from monitoring the modulus(length) of R vector.. Often times if we’re just interested in direction of our inertial vector, it makes sense to normalize it’s modulus in order to simplify other calculations.
Part 2.. Gyroscope
We’re not going to introduce any equivalent box model for the gyroscope like we did for accelerometer, instead we’re going to jump straight to the second accelerometer model and we’ll show what does the gyroscope measure according to this model.
Each gyroscope channel measures the rotation around one of the axes.. For instance a 2-axes gyroscope will measure the rotation around (or some may say “about”) the X and Y axes.. To express this rotation in numbers let’s do some notations.. First let’s define:
Rxz – is the projection of the inertial force vector R on the XZ plane
Ryz – is the projection of the inertial force vector R on the YZ plane
From the right-angle triangle formed by Rxz and Rz, using Pythagorean theorem we get:
Rxz^2 = Rx^2 + Rz^2 , and similarly:
Ryz^2 = Ry^2 + Rz^2
also note that:
R^2 = Rxz^2 + Ry^2 , this can be derived from Eq.1 and above equations, or it can be derived from right-angle triangle formed by R and Ryz
R^2 = Ryz^2 + Rx^2
We’re not going to use these formulas in this article but it is useful to note the relation between all the values in our model.
Instead we’re going to define the angle between the Z axis and Rxz, Ryz vectors as follows:
Axz – is the angle between the Rxz (projection of R on XZ plane) and Z axis
Ayz – is the angle between the Ryz (projection of R on YZ plane) and Z axis
Now we’re getting closer to what the gyroscope measures.. Gyroscope measures the rate of changes of the angles defined above.. In other words it will output a value that is linearly related to the rate of change of these angles.. To explain this let’s assume that we have measured the rotation angle around axis Y (that would be Axz angle) at time t0, and we define it as Axz0, next we measured this angle at a later time t1 and it was Axz1.. The rate of change will be calculated as follows:
RateAxz = (Axz1 – Axz0) / (t1 – t0).
If we express Axz in degrees, and time in seconds , then this value will be expressed in deg/s .. This is what a gyroscope measures.
In practice a gyroscope(unless it is a special digital gyroscope) will rarely give you a value expressed in deg/s.. Same as for accelerometer you’ll get an ADC value that you’ll need to convert to deg/s using a formula similar toEq.. 2 that we have defined for accelerometer.. Let’s introduce the ADC to deg/s conversion formula for gyroscope (we assume we’re using a 10bit ADC module , for 8bit ADC replace 1023 with 255, for 12bit ADC replace 1023 with 4095).
RateAxz = (AdcGyroXZ * Vref / 1023 – VzeroRate) / Sensitivity Eq.3
RateAyz = (AdcGyroYZ * Vref / 1023 – VzeroRate) / Sensitivity
AdcGyroXZ, AdcGyroYZ – are obtained from our adc module and they represent the channels that measure the rotation of projection of R vector in XZ respectively in YZ planes, which is the equivalent to saying rotation was done around Y and X axes respectively.
Vref – is the ADC reference voltage we’ll use 3.3V in the example below
VzeroRate – is the zero-rate voltage, in other words the voltage that the gyroscope outputs when it is not subject to any rotation, for the Acc_Gyroboard it is for example 1.23V (you can find this values in the specs – but don’t trust the specs most gyros will suffer slight offset after being soldered so measure VzeroRate for each axis output using a voltmeter, usually this value will not change over time once the gyro was soldered, if it variates – write a calibration routine to measure it before device start-up, user must be instructed to keep device in still position upon start-up for gyros to calibrate).
Sensitivity – is the sensitivity of your gyroscope it is expressed in mV / (deg / s) often written as mV/deg/s , it basically tells you how many mV will the gyroscope output increase , if you increase the rotation speed by one deg/s.. The sensitivity of Acc_Gyro board is for example 2mV/deg/s or 0.002V/deg/s
Let’s take an example, suppose our ADC module returned following values:
AdcGyroXZ = 571
AdcGyroXZ = 323
Using the above formula, and using the specs parameters of Acc_Gyro board we’ll get:
RateAxz = (571 * 3.3V / 1023 – 1.23V) / ( 0.002V/deg/s) =~ 306 deg/s
RateAyz = (323 * 3.3V / 1023 – 1.23V) / ( 0.002V/deg/s) =~ -94 deg/s
In other words the device rotates around the Y axis (or we can say it rotates in XZ plane) with a speed of 306 deg/s and around the X axis (or we can say it rotates in YZ plane) with a speed of -94 deg/s.. Please note that the negative sign means that the device rotates in the opposite direction from the conventional positive direction.. By convention one direction of rotation is positive.. A good gyroscope specification sheet will show you which direction is positive, otherwise you’ll have to find it by experimenting with the device and noting which direction of rotation results in increasing voltage on the output pin.. This is best done using an oscilloscope since as soon as you stop the rotation the voltage will drop back to the zero-rate level.. If you’re using a multimeter you’d have to maintain a constant rotation rate for at least few seconds and note the voltage during this rotation, then compare it with the zero-rate voltage.. If it is greater than the zero-rate voltage it means that direction of rotation is positive.