Wednesday, May 1, 2013

The vibration problem

If you want a steady quadcopter (or multi-copter), you need an accurate reading of angles (pitch and roll).

The reading is made through a combination of calculous (which I find really hard to understand) that mixes the information of a 3-axis accelerometer, 3-axis gyroscope and a 3-axis magnetometer.
When you have 4 motors spinning at 10,000 RPM each anywhere near your inertial sensor, you will probably sense all the vibration. I can bet you will.


When the vibration is very hard, no software correction or filter is possible. So a mechanical solution is needed.

This is how we solved the problem:

IMU anti-vibration mount
The IMU (9 DOF) is attached on an independent board, which is "floating" on rubber mounts. This absorbs most of the vibration generated by the motors, and the rest of the vibration is filtered out by software.


Wednesday, February 20, 2013

First 4-motor test

The four motors will lowly increase RPM until a lift is detected with the sonar located on the bottom. Then, everything will stop running.

Saturday, February 9, 2013

The frame (version 0.1)

CAD model

First frame layout finished

Building motor mounts

Finished motor mount

Finished motor mount 

All motors and props in place :-)


Tuesday, January 1, 2013

First PID tests

From Wikipediaproportional-integral-derivative controller (PID controller) is a control loop feedback mechanism (controller) widely used inindustrial control systems (Programmable Logic ControllersSCADA systemsRTU's etc). A PID controller calculates an "error" value as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error in outputs by adjusting the process control inputs.

A quadcopter uses a PID mechanism to stabilize itself. It measures the inclination angle with an IMU (Inertial measurement unit) and the corrects the error applying power to each motor.

This is our first low budget test we made with a PID model using a Servo,  a sonar to measure distance, a potentiometer to establish the desired set point, and a piece of paper :-)



Then, it got more complex. Same test but using a real motor, ESC and IMU:


In order to do this test, we had to create a plotting library for Processing. You can download it for free.



Monday, December 10, 2012

Parts of a Quadcopter: making your shopping list

Too many acronyms and technical terms.... What the heck is all this?

So many parts, so many new stuff to learn, its time to explain how everything works:

  • Flight commands
  • Radio control (TX, RX)
  • Brushless Motors
  • ESC (Electronic Speed Control)
  • Propellers
  • LIPO batteries



Flight commands:

  • Roll: will move the quad left and right.
  • Pitch: will move the quad forward and backward.
  • Yaw: will rotate the quad over its own axis.
  • Throttle: will send power to all motors and make the quad go up and down.



Radio control:

There are many different radio models and types out there. I'm not an expert on the subject, but I can tell you a few things so you can chose yours.

Modes: There are mode 1 and mode 2 radio transmitters (TX). This determines on which hand you will use to control each command on the radio. Here's an excellent article from Spektrum that explains how it works. Summing up "Mode II is by far the most popular, but your final decision will likely be based on the recommendation of a friend or instructor pilot."

# of channels: each channel controls one command on the plane / quad / heli. A quad needs at least 4 channels (throttle, pitch, roll and yaw) but you may need more channels to control your GPS mode, flight mode, altitud hold mode, etc. So the more channels you have, the better. Also, the more channels you have, more expensive will be your radio.

Frequency: You will find radios that transmit on different frequencies such as 900 MHz. 1.2 GHz, 2.4 GHz, etc. different frequencies will give you different ranges but also more

Receivers (RX): some radios come together with its receiver. It looks like this.



Brushless motors:

There are many articles on internet about brushless motors so I encourage you to search on Google how it works. The most important part is that brushless motors spin freely with nearly no friction and optimal torque, which make it ideal for electrical flight. 
Brushless motors are connected through 3 wires, so you will need to sequence the power over the wires to make it spin at different speeds.
that's why you need an Electronic Speed Control device (or ESC). Read on to see what it is.
Each motor-propeller combination generate a thrust. In general, motor makers will tell you the resulting thrust they got with different prop sizes.


ESCs:

As told, Electronic Speed Controls will receive a digital signal from the Radio receiver (RX) and transform it into electrical power in a proper sequence through the motor coils to make it spin at diferent speeds. The digital signal that tells the ESC what to do is the same signal that would tell a digital Servo what to do. So, instead of telling a servo to stay in a 45 degree angle, it will tell the motor to spin at a certain speed.

Propellers:

Propellers will push the air down to make your quad fly. The more air they push, the more lift you'll get. There are several ways to push more air: 
1) Increase spinning speed (you will need more RPM)
2) Increase propeller size (you will need more space and motors with higher torque)
3) Increase propeller blade pitch (you will need more torque)

Each of these options will have implications in other parameters such as RPM, power consumption, torque, weight, so keep all this in mind when choosing the right propeller.


LIPO batteries:

Again, there are lots of articles out there about LIPO batteries you can read. LIPO stands for Lithium Polymer which is the material the batteries are built with.
All you need to know for now is that this technology gives you more power storage in a compact and lightweight enclosure. But there's always a downside. These batteries are dangerous and need special care when you charge and store them. So read this article and do some more research before using them.