Monthly Archives: June 2013

Netduino Plus 2 Interrupt Performance

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I had been considering using the Quadcopter IMU to drive the execution of the orientation and stabilisation algorithms. The MPU 6050 has an interrupt output which can be configured to trigger when there is data available to be read etc.

I had read discussions about the maximum frequency which the Netduino & the NetMF could cope with and just wanted to check for myself. For this test I assumed that the PWM outputs (once initialised) consume little or no CPU and that with only an InterlockedIncrement in the interrupt handler that these would be the maximum possible values.

Netduino Plus 2 Interrupt Testing

I used the onboard PWM (D3) to drive an interrupt (D1) and then had a background thread display the number calls to the interrupt handler in the last second. The button (D4) in the picture above allowed me to increase the frequency of the PWM output in 100Hz steps. The desired count and actual count where displayed using Debug.Print and the .Net Microframework deployment tool

100 101
200 107
700 453
1000 867
1500 1225
1800 1629
2100 1943
2900 2470
3200 3093
3600 3432
4000 3799
4000 4020
4000 4021
4000 4020

The counts seemed to be reasonably accurate until roughly 13KHz then there would be major memory allocation issues and the device would crash.

 

MPU 6050 I2C read rates using Netduino plus 2

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The quadcopter will need to determine it’s orientation then update the thrust to be delivered by each motor many times a second. So I was interested to see how fast my Nedtduino plus 2 could read data from the MPU 6050 accelerometer & gyroscope. I stumbled across this blog and it appears that they are building a Quadcopter as well(Google translation was hard work to read).

There was also some very useful C# NetMF code which I used as the basis for my performance test harness.

10,000 readings of 16bit values from the Accelerometer (X,Y,Z) Gyroscope (X,Y,Z) and temperature sensor. The initial run didn’t look to positive

21.78, 21.77, 21.77, 21.77, 21.77, 21.77, 21.77,21.77,21.77,21.77 sec Avg 21.77

Which is roughly 460/sec

I then had a look at the I2C interface code and noticed that the bus speed was set to 100KHz so I increased it to 400KHz

10.33, 10.23, 10.23,10.23,10.23,10.23,10.23,10.23,10.72,10.23 Avg 10.29

Which is roughly 970/sec

I then had a look at the I2C interface code and changed the way that the register values were read so the array of registers to read didn’t have to be loaded.

8.55,8.57,8.55,8.57,8.55,8.57,8.57,8.57,8.55,8.57 Avg 8.56

I think 1170/sec should be sufficient, but I’m going to have a look at the Digital Motion Processor (DMP) capabilities of the MPU6050. This will require significant effort as there currently (June 2013) doesn’t appear to be any NetMF support for this approach.

Quadcopter MPU 6050 IMU mounting board ordered

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For testing I’m going to need a shield to mount my MPU 6050 breakout board and connectors for the four motor speed controllers so a protoshield looked like an ideal solution. The netduino plus 2 I am planning to use for the Quadcopter controller has a slightly different I2C setup to the original Netduino & Netduino plus. After some research it looks like the Netduino plus 2 arrangement is the same as the Arduino Uno & Arduino Leonardo with the I2C SDA & SCL pins next to the AREF pin. I did consider using a software based I2C implementation but discounted this because I was worried about performance and additional complexity.

Many of the Arduino/Netduino protoshields currently on sale (June 2013) don’t have the necessary SDA & SCL pins e.g. Sparkfun, Freetronics, Adafruit, Makershed, ladyada etc.

I have sourced 2 x Leoshield from GorillaBuilderz in Australia which look suitable.

Quadcopter frame and motors ordered

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Using the suggested parts list for a quadcopter on Hovership. I have ordered

1x Turnigy Battery Strap 330mm
1x On-Board Lipoly Low Voltage Alarm
2x Turnigy nano-tech 1800mah 3S 25~50C Lipo Pack
1x Hobby King Quadcopter Power Distribution Board
5x Turnigy Multistar 10 Amp Multi-rotor Brushless
5x Turnigy Aerodrive SK3 – 2822-1090kv Brushless Outrunner Motor
5x 8045 SF Props 2pc Standard Rotation/2 pc RH Rotation
1x Hobbyking X525 V3 Glass Fiber Quadcopter Frame 600mm
1x Hobbyking X525 V3 Glass Fiber Main Frame Bottom Plate
1x Hobbyking X525 V3 Aluminum Square Booms (4pcs/bag)
2x Hobbyking X525 V3 Glass Fiber Motor Mount (4pcs/bag)
2x Hobbyking X525 Glass Fiber Landing Gear (4pcs/bag)
1x Hobbyking X525 V3 Glass Fiber Main Frame Upper Plate (1pc/bag)
1x CNC Alloy Prop Balancer for Propeller, EDF & Heli Shaft/Blade

I have ordered plenty of spare propellers, motor mounts and frame bits as based on my experience with my Crazyflie Nano Quadcopter there will be many crashes while I debug the software and learn how to fly. I will purchase a Lipoly charger and battery bag locally.

MPU 6050 Quadcopter IMU has arrived

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The  3 x MPU 6050 breakout boards (GY521) I ordered on Alibaba arrived from China yesterday. I’m looking at using a device which has both an accelerometer & gyroscope in one package rather than an IMU assembled from several discrete devices to make the software simpler (only one I2C bus address).

To make the Microsoft I2C implementation work with multiple discrete devices usually requires a layer of abstraction like commonly used AbstractI2CDevice class which I was wanting to avoid in this project.

The test rig for my light weight high performance driver, a netduino 2 plus & breakout board.

Image

If I need a compass to make the Quadcopter controller work I’ll upgrade to an MPU 9150.

I2C read rates on Netduino Plus vs Netduino plus 2

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As part of my quadcopter project I purchased a Netduino Plus 2 for the flight controller. The increased performance 48MHz vs. 168MHz CPU and other improvements looked like it could make it possible to implement most or all of the control algorithms in C#.

So I could compare the performance of the I2C interfaces I setup two test rigs which polled an ADXL345 Accelerometer (using the Love Electronics sample code) 10,000 times for X,Y & Z acceleration values.

NetduinoPlusAccelerometer

The different I2C pins on the Netduino plus 2 required some jumper cables

Netduino2PlusAccelerometer

Both devices were running NetMF 4.2 and I monitored the output of the test harness using MF Deploy.

Netduino Plus

22.2797,22.3126,22.3313,22.3129,22.3590,22.3689,22.3497,22.3049,22.3649,22.3836

Average 22.3 seconds roughly 450/sec

Netduino Plus 2

6.8312,6.8059,6.8003,6.8051,6.8319,6.7999,6.8104,6.8194,6.8233,6.8096

Average 6.8 seconds roughly 1470/sec