1. OBJECTIVE

The purpose of this document is to present a simple tutorial for measuring the gyroscope scale factor of an Inertial Measurement Unit (IMU).  

2. BACKGROUND

A gyroscope measures angular rate (how fast an object is rotating) and is measured in degrees/second.  The three gyroscopes in the MEMS IMU produces angular rate measurements and also measures linear acceleration around x, y, z axes.  The nominal scale factors, for the gyroscopes in the ADIS16467-3 IMU for 16-bit: 0.1 dps/LSB and for 32-bit: 0.1/*2^16) dps/LSB. For the purpose of this tutorial we will only be looking at Z-axis.  The following image shows the axes of rotation, which are mutually orthogonal with respect to each other. 

What is scale factor and why is it needed ?

Scale factor is also known as "sensitivity" is the digital response to rotation. It is the ratio between the output error (deviated from the fitted straight line slope) over input and is usually expressed in percentage or ppm (parts per million). For example, if a device makes one complete rotation, its equivalent angular displacement is 360 degrees, If the device makes that complete rotation, in one second, the average rate of rotation is 360 degree/second. The most common method for measuring scale factor, in gyroscopes is to spin them at a known rate of rotation, while observing their outputs. It has a direct impact to check on how well a system can track the angular motion. For example: if the scale error is 0.5% and the device rotates 100 degrees, the resultant angle estimation will be 100.5 degrees.  

3. EQUIPMENT

3.1. Hardware Tools:

Item	Model	Description	Picture
BREAKOUT BOARD	ADIS16IMU4/PCBZ	The Breakout Board provides a simple connector translation, between the MEMS IMU and the PC-based evaluation system
EVALUATION SYSTEM	EVAL-ADIS-FX3Z	The Evaluation System provides a platform to perform communication between MEMS IMU and the PC.
MEMS IMU	ADIS16467-3BMLZ	The MEMS IMU measures both acceleration and angular rate.
PC	MS Windows platform

3.2. Software tools:

• Please follow the steps provided in the following links below:
  • iSensor Eval GUI User Guide to set up the iSensor Eval software on your system.
  • EVAL-ADIS-FX3 Setup and Troubleshooting Guide [Analog Devices Wiki] for connections and troubleshooting.

4. SETUP

Listed below is step by step, for everything that is required to make this work. 

4.1. Hardware Setup

4.1.1. Install the IMU onto the breakout board

Using the J2 silkscreen outline, on the breakout board, as an approximate alignment guide, press the IMU's connector into the 14-pin mating connector (J2 on the breakout board). 

Make sure that the pins are properly aligned, before pressing the IMU into a "fully seated" position.  

4.1.2. Secure IMU to breakout boards

Use three (3) M2 machines screws, washers and nuts to secure (provided with ADIS16IMU4/PCBZ)

4.1.3. Secure breakout board to the spacer, using M2 machines screws (4x), with washers and nuts (provided with ADIS16IMU4/PCBZ)

          

4.1.4. Install ribbon cable onto breakout board

Install one side of the ribbon connector, onto the J1 connector (on the breakout board). 

Make sure that the pins are properly aligned, before pressing the IMU into a "fully seated" position.  

4.1.5. Install ribbon cable onto the Eval board

connect the EVAL-SYSTEM with the other end of the ribbon cable. 

4.1.6.  IMU and the Eval-Board are connected using ribbon cable. 

4.1.7. Connecting the EVAL-BOARD to the PC

The FX3 is connected to the PC using USB Type-C port

4.2. Software Setup

4.2.1. Download USB Driver

Go to  EVAL-ADIS-FX3 Setup and Troubleshooting and  to the Downloading and Installing the FX3 Drivers and Software section and download the latest version of Evaluation GUI and device drivers. Follow steps to finish installation on your PC.

4.2.2. Install USB driver 

After downloading the latest version, proceed to install the software on your desktop. 

4.2.3. Launch Application

4.2.4. Connect to FX3

4.2.5. Select DUT in iSensor GUI Application 

If the DUT Type in the previous section is not correct, follow the steps below to select the correct DUT for the exercise.

1. click on "Select DUT Type"
2. A new window will open, select the Product Family: ADIS1646x
3. Select model : ADIS16467-3
4. Click on Reboot FX3, this will reset all the settings back to original
5. After Reboot is finished, "Connect to FX3" to complete connecting to the right IMU.

5.  PROCEDURE 

5.1. Configure DUT 

5.1.1. Once connected the FX3 status will change to Connected. (If facing any issues with the software. Go to iSensor Eval GUI User Guide for troubleshoot and issues.)

Before we start the test, there are some settings that needs to be set using register map on iSensor Eval GUI.

5.1.2. Setting Decimation Rate 

DEC_RATE = 0x0063, sets the IMU's sample rate to ~20 SPS (2000/100)

In the Register Access page, find DEC_RATE and change the rate to 0x0063 (or just 63).  0x0063 is hexadecimal value for 99 in  decimal. When the IMU is operating in internal clock mode, the nominal output data rate is : 2000/(DEC_RATE+1) = 2000/(99+1) = 2000/100 = 20 SPS. 

5.1.3. Verify if the new DEC_RATE is updated

To check if the value has been written onto the register. Click on Read Page(1). Now to Check the Output Data Rate and confirm that we are getting ODR at 20 Hz, Click on Measure Data Ready (2) 

   

5.1.4. Setting the NULL_CNFG Register

NULL_CFG = 0x070A, kept default, sets the average record size to ~32 seconds.

The NULL_CNFG register establishes the total average time and provides on/off controls for each register. The factory default configuration for the NULL_CNFG register enables the bias null command for the gyroscopes, disables the bias null command for the accelerometers, and sets the average null time to ~32 sec. 

5.1.5. Setting the GLOB_CMD

GLOB_CMD = 0x0001, executes the Bias Correction Update.  Following same steps as 5.1.2, write 0x0001 in GLOB_CMD register. 

Note: GLOB_CMD is Write only register

5.2. Collecting Data

5.2.1. Going to register logging

After the device configuration is done. Data collection is done from using Register Logging. After clicking on Data Logging, select Register Logging from the two options below (refer to image below)

Another window opens after clicking on Register Logging. 

5.2.2. settings in Register Logging

• follow the settings as shown in the images below to select the axis and number of samples.

             

5.2.3. Capturing Data

• collect the data points by moving the IMU around 180 degrees, then back.

6. RESULT

Once the Capture Progress is completed, the Raw Angular Data of Z_GYRO will stored in the .csv format

• For processing the data, simply divide each gyroscope data point by the sample rate, then add them all together to produce a real-time angle estimate. As we can see from the plot, the integrated result shows a 180 degrees and angular displacement and the device returns to zero. The processed data can be found here. 
  
  Steps for post processing:
  • Converting the Raw Angular Data from Z-axis to degrees/second using 2's complement.
  • Dividing the data with the Sample rate to get angular displacement of each point
  • Adding all the points to get real time angle estimate

Scale Factor Error Calculation:

ADIS16467-3 has two nominal scale factors:

1. 16-bit scale factor = 10 LSB/degree/second
2. 32-bit scale factor = 2^16 x 10 LSB/degree/second  

We know scale factor is the ratio between  the scale of original output and the new output.  The original output for the IMU rotation is 180 degrees as we rotate the IMU to 180 degrees and back to zero. The new output could be more or less than 180 degrees

• For the following tutorial, let us take the average of the output when the IMU is rotated to 180 degrees and taking average of the readings.

• The scale factor is calculated by taking the ratio of original output and the average output.

Scale Factor (tutorial) = ABS(180/-178.86) = 1.00638323

• As mentioned above, the nominal scale factor for 16-bit scale factor is 10

• The new scale factor for this tutorial :

7. ADDITIONAL REFERENCES

Topic	Links
Datasheet	Datasheet
Frequently asked questions (FAQ)	Engineer Zone
Product Page	ADIS16467 Product Page
Publications	A Simple Calibration for MEMS Gyros (analog.com)