Background
I am working on a project where I am gathering a lot of data. One of the sources of my data is my 9Dof sensor. Since I am logging at sub-second intervals, I wanted to know how long (the cost of time) each sensor takes to return data, so I make sure to get as many data points, from as many data sources as possible.
I set up this project with the sole intention of determining the cost in milliseconds of the standard example, then try to figure out how to make it faster.
Setup
I grabbed my trusty LinkIt ONE and my 9Dof sensor (MPU9150, connecting over I2C), and looked at the example library. I wanted to get data from each of the sensors as fast as possible, and I will do post-capture processing for heading and orientation. I set my sights on these three programs:
- getAccel_Data
- getGyro_Data
- getCompass_Data
I then made blocks of C to run in a loop:
Serial.print(millis() % 1000);
Serial.print(",Acc1,");
getAccel_Data();
Serial.print(Axyz[0]);
Serial.print(",");
Serial.print(Axyz[1]);
Serial.print(",");
Serial.print(Axyz[2]);
Serial.print(",");
Serial.println(millis() % 1000);
This is my code for checking a single Accelerometer data gathering. I figured all I had to do is try running the same block, with additional runs of “getAccel_Data;” and notations of “”Acc_” so I can to comparisons afterwards. I then take the time difference between the start and end and map it to the sensor read type, and the sensor read count. After that, I subtract the time of the single run to the double and triple run to find how much longer it took for the one or two additional runs.
Accelerometer Tests
Let’s have a look at one set of my results (do note, time is in milliseconds, 1/1000th of a second):
| Row Labels |
Average of Runtime |
StdDev of Runtime |
| Acc1 |
23.307 |
2.230 |
| Acc2 |
45.701 |
3.985 |
| Acc3 |
68.123 |
6.254 |
First thing I see here is that 23.3 + 45.7 does not equal 68.1. This is good, this is what I expect, and am glad to see. My formula for calculating the actual average runtime of getAccel_Data is as follows:
( (Acc2 – Acc1) + (Acc3 – Acc1) ) / 3
This is set up to give a double weight to the difference between Acc3 and Acc1, since there was an additional run compared to Acc2 to Acc1.
Testing All Sensors
I created a few sets of functions besides the stock ones to test out.
- getAccelGyro_Data: Combined getAccel_Data and getGyro_data, eliminating duplicate runs of accelgyro.getMotion9
- getDof_Data: Combines getAccelGyro_Data and getCompass_Data
- getRawDof_Data: getDof_Data without the calculations, just raw values
Ultimately, I let my computer run, and I got 4600 records for each set of sensors. Here’s my results:
|
|
Runtime |
| Function |
Runs/cycle |
Average |
StdDev |
| getAccel |
1 |
23.307 |
2.230 |
| getAccel |
2 |
45.701 |
3.985 |
| getAccel |
3 |
68.123 |
6.254 |
| getAccelGyro |
1 |
23.364 |
1.998 |
| getAccelGyro |
2 |
45.844 |
3.899 |
| getAccelGyro |
3 |
68.235 |
6.166 |
| getGyro |
1 |
23.303 |
2.005 |
| getGyro |
2 |
45.729 |
3.924 |
| getGyro |
3 |
68.192 |
6.200 |
| getCompass |
1 |
11.231 |
2.411 |
| getCompass |
2 |
21.774 |
4.139 |
| getCompass |
3 |
32.368 |
6.095 |
| getDof |
1 |
33.926 |
3.493 |
| getDof |
2 |
66.875 |
7.062 |
| getDof |
3 |
99.803 |
10.856 |
| getRawDof |
1 |
33.945 |
3.418 |
| getRawDof |
2 |
66.967 |
7.083 |
| getRawDof |
3 |
99.951 |
10.899 |
From this, I calculated the following:
| Function |
Actual Cycle Cost |
Overhead Cost |
| getAccel |
22.4038 |
0.8983 |
| getGyro |
22.4380 |
0.8591 |
| getCompass |
10.5597 |
0.6631 |
| getAccelGyro |
22.4504 |
0.9287 |
| getDof |
32.9420 |
0.9872 |
| getRawDof |
33.0094 |
0.9418 |
Conclusion
- The library should not have “getAccel_data” and “getGyro_data”, rather just combine the two together for optimal performance.
- Also, when we look at the ‘getDof” vs. “getRawDof”, by saving the larger values instead of reducing their size before saving, it actually costs time. About 0.0674 milliseconds per cycle.
- You save 22.4596 milliseconds per cycle by combining gathering the sensor data into one procedure.
- You save 0.0681 milliseconds by running “getDof_data” instead of “getAccelGyro_data” and then “getCompass”.
