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 to Eq. 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_Gyro board it is for example 1.23V (you can find this values in the specs)
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.