BrickEngineer: LEGO Design

LEGO Engineering for LEGO NXT and Robot Enthusiasts

Category Archives: Robotics

MATLAB Packages for the NXT


There are now several MATLAB packages for robotics, and specifically for the NXT.  One paradigm is to run the code on a PC and have it communicate direct commands to the NXT Brick via Bluetooth or USB.  I have found this paradigm to be a bit dangerous since in the event of a MATLAB crash or a miscommunication, the NXT Brick will continue with its last command until ordered to stop.  This has the potential to destroy your robot.  The paradigm that I prefer to use is to write several programs that run on the brick.  These programs take commands from files on the brick that can be uploaded rapidly from the PC.  The MATLAB code then is in charge of sending the command files and starting and stopping programs.  In the event of a MATLAB crash or communication failure, the software running on the NXT Brick can be designed to terminate gracefully.

Here are the MATLAB packages that I know of.  The first two are specifically geared toward the NXT; whereas the last is a general robotics package.

Little Rover with Instructions and Code

 

I have finally compiled building instructions for my Little Rover, which can be seen above in a 3D Rendering courtesy of POVRay.  An earlier version of this rover can be seen in this YouTube video:

Little Rover Prototype Video

Rover Design

The complete detailed building instructions can be found here in this 94-page pdf file.
Warning: it is about 9MB in size.  The design is not entirely compatible with the standard NXT Mindstorms Kit.  This design relies on two touch sensors, several 1×9 bent liftarms, and as far as I can tell from Peeron, the NXT Kit has only two.  This may require a little redesign.  Other compatibility issues and their solutions can be found in the Parts List in the instructions.

Remember to download the software DriveSmart here as well.
Installation instructions can be found in the zip file.

DriveSmart Code

The main file is called DriveSmart.rbt.  Drive Smart runs four threads:

Drive Thread
The Drive Thread (lowest one of the four) drives until a warning flag is set by one of the other
threads. It then waits until it gets an all clear message via the Wait Until Free block, and then
it starts driving again.

Bumper Threads
There are two threads that monitor the bumpers.
The reaction is only activated if nothing else is currently commanding the robot.  In this case the
bumper has been pressed and the robot will veer away from the hazard.

Ultrasound Thread
This thread monitors the ultrasound rangefinder.
The reaction is only activated if nothing else is currently commanding the robot.  When the robot
comes too close to a hazard, the robot is commanded to stop.  It then looks both ways and then turns
in the direction with more room.  If the robot is within 10 cm of a hazard on both sides, it then
backs up.

The robot can roam about a wide variety of rooms and not get stuck.
He does not detect stairs though!  So be careful.

Download: instructions and code.

Enjoy!
Kevin Knuth

LEGO NXT Motor Wiring

After posting Hacking the LEGO Mindstorms NXT Standard Motor, I received several requests for more information regarding the wiring of the motor.

The NXT cable has six wires. Below I list a table with the wires and their colors:

Color Name
White Motor 1
Black Motor 2
Red GND
Green 4.3 Volts
Yellow Tach01
Blue Tach02

The WHITE and BLACK wires (Motor 1 and Motor 2) deliver power to the motor.
If standard batteries are used, the potential difference will be 9 volts, otherwise the NiMH rechargeable batteries provide 7.2 volts. If the white wire is positive and black is negative, the motor will turn one way. If you reverse the polarity, the motor will turn the other way.

The RED wire is connected to the ground (GND). Note that in the sensors, RED and BLACK are connected to one another. This is not the case in the motors.

The GREEN wire is connected to the +4.3 NXT power supply.

The YELLOW and BLUE wires are connected to the quadrature encoder, also called an incremental rotary encoder.

Basic Rotary Encoder

Square waves from quadrature encoder

As shown in the figure from Wikipedia above, (http://en.wikipedia.org/wiki/Quadrature_encoder) the wires return square wave pulses that are 90 degrees out of phase. If the rising pulse on TACH00 leads the rising pulse of TACH01 by 90 degrees, then the motor is going forward. If it instead lags by 90 degrees, the motor is rotating backwards. One complete square wave cycle corresponds to 2 degrees of rotation. In the diagram above, if TACH00 refers to A and TACH01 refers to B, we can see that the motor is going backwards as TACH00 is lagging TACH 01.

By measuring the frequency of the square wave oscillation, one can compute the rotational velocity. Since one cycle corresponds to 2 degrees of rotation, one cycle per second (1 Hz) corresponds to 2 degrees/sec. If you record a frequency of X Hz, then the rotation rate is 2X cycles/sec.

Note also that by tracking both square waves, you can identify quarter cycles, which gives you a resolution of 1/4 of 2 degrees, which is 0.5 degrees.

The motor speed is controlled by pulse-width modulation (pwm), which works by driving the motor with a variable duty cycle square wave. This effectively turns the motor on and off, fast. The longer it is on, the more torque it will generate and the faster it will go.

These details and more can be found in the excellent book: Extreme: NXT with a sneak peak here.

Additional details can be found in the excellent book Extreme NXT: Extending the LEGO MINDSTORMS NXT to the Next Level (Technology in Action) by Michael Gasperi, Philippe E. Hurbain, and Isabelle L. Hurbain.

Philo uploaded a comment, and reminded me that “Note that there are some internal photos of the NXT motor here: http://philohome.com/nxtmotor/nxtmotor.htm and schematics here: http://www.brickshelf.com/cgi-bin/gallery.cgi?i=1846577”

Happy Hacking!

Basic Electronics Supplies for Beginners

I am getting interested in more general robotics projects, but will still be relying on LEGOs for their construction.  The LEGO brick is a bit too limited with its specialized programming languages and limited sensor and motor ports.

So for those interested in some LEGO electronics hacking, here is a list of supplies that will get you up and running fast for about $275… just a but more than the cost of a single Mindstorms kit.  Plus you’ll now get to learn electronics!

First, check out the book:
Making Things Talk: Practical Methods for Connecting Physical Objects

This book explains how to wire, program and interconnect various microcontrollers, some of which are very closely related to those used by the NXT Brick.

Supply List

Item Number Description Quantity Unit Price Total
Amazon.com
  Making Things Talk 1  $19.79 $19.79
Jameco.com
19166 Desoldering Pump 1 $4.95 $4.95
159291 Wire Stripper 1 $10.15 $10.15
161411 Diagonal Cutter 1 $7.49 $7.49
35474 Needlenose Pliers 1 $5.49 $5.49
127271 Mini Screwdriver 1 $1.89 $1.89
681002 Helping Hands 1 $8.75 $8.75
159611 Power Connector 2 $1.79 $3.58
10444 Alligator Test Clip Leads 2 $4.39 $8.78
103377 Header Pins 10 $0.16 $1.60
119011 Push Button (PCB Type) 10 $0.27 $2.70
29082 Potentiometer 2 $1.05 $2.10
242115 LM1117T-3.3 Voltage Regulator 3 $1.39 $4.17
51262 7805T 5v Voltage regulator 3 $0.32 $0.96
38236 2N2222A Transistor NPN 5 $0.41 $2.05
32993 TIP120 Power Transistor 5 $0.45 $2.25
643488 3.3V Zener Diode 5 $0.03 $0.16
35991 1N4004 Diode 5 $0.04 $0.20
152792 LED Yellow 10 $0.17 $1.70
152805 LED Red 10 $0.21 $2.10
153139 LED Orange 10 $0.35 $3.50
156962 LED Green (567 nm) 10 $0.20 $2.00
334529 LED Bargraph Red 1 $1.31 $1.31
334537 LED Bargraph Yellow 1 $1.23 $1.23
334511 LED Bargraph Green 1 $1.28 $1.28
17187 7-segment LED Display 3 $0.88 $2.64
38818 4-switch DIP 4 $0.48 $1.92
38842 8-switch DIP 2 $0.89 $1.78
103166 Resistor Refill 1 $12.95 $12.95
15270 0.1 uF 10 $0.15 $1.53
94161 1 uF 10 $0.12 $1.20
29891 10 uF 10 $0.06 $0.60
158394 100 uF 10 $0.11 $1.08
MPJA
4443 TE Solderless Breadboard 1 $4.95 $4.95
4447 TE Large Solderless Breadboard 1 $22.95 $22.95
7027 TE Jumpers 2 $3.95 $7.90
14213 TE Digital Multimeter 1 $14.95 $14.95
15860 TL Mini Soldering Station 1 $14.95 $14.95
Sparkfun
Wiring Platform DEV-00744 1 $84.95 $84.95
Radio Shack
64-025 Lead Free Solder 1 $3.89 $3.89

Note that the light gray items are optional, and not necessary.

Also, this list does not include some sort of power supply. Pulling one out of an old computer is an easy option. Or rechargeable batteries work well too (in which case you will need battery holders).

Last, there are special items in the book Making Things Talk that you may decide to purchase separately, such as flex sensors, or bluetooth boards, etc.

You can store your electronics in much the same way you store your small LEGO parts. Check out the article on Storage.

Enjoy Hacking!

Center of Mass of LEGO NXT Motors

I am working on designing a walking machine, but I needed to know the Center of Mass of the LEGO NXT Motors.  When using Newton’s Laws to compute the forces on the system, we can treat the motor as if all of its mass is located at a single point.  The Center of Mass is the location of this point.

Hanging an NXT motor to find its center of mass.

There are several ways to find the Center of Mass of the motor.  The most straightforward way is to hang the motor from an axle placed in one of the holes.  The motor will orient itself so that the Center of Mass lies directly below the axle.  By hanging a mass on a string from the axle, the Center of Mass must lie somewhere along the line defined by the string. 

The Center of Mass Lies along the line defined by the vertical string

After performing this experiment, I placed a small piece of Scotch tape over the string so that I can keep track of where that line is.  I then cut the string off of the axle.

A piece of Scotch tape holds the string in place

Now to find the precise point, we simply perform the experiment again, but place the axle through a different hole.  This gives us a second line.  Since the Center of Mass must be on both the first line and the second line, it is located at the intersection of these two lines.

The intersection of the two strings indicates the position of the Center of Mass

The Center of Mass is very close to being aligned with the holes on the motor.  Below is an MLCAD image of the NXT motor (from Philo).  I have overlayed a Cartesian coordinate system that corresponds to that used to define the 3-D image file.  The origin of this system is at the center of the axle hole on the motors drive axis.  This is perfect for me since I will be rotating the motor and trying to compute the position of the Center of Mass after the motor has rotated through some arbitrary angle.

The dimesions of the LEGO NXT Motor

This image not only helps with identifying the Center of Mass of the NXT motor, but also in understanding the dimensions of the NXT motor overall.

Kevin Knuth
Albany NY

Intelligent Instruments

Intelligent Robotic Arm

The LEGO Mindstorm NXT robotics system is an excellent testbed for research in machine learning and artificial intelligence.  At Knuthlab Robotics at the University at Albany, we are developing intelligent instruments using LEGOs.

Our first instrument is a robotic arm that is designed to locate a characterize a white circle on a black background using the LEGO light sensor.  It relies on Bayesian inference, which is implemented using a technique called Nested Sampling, which was developed by John Skilling.  This software allows the robot to learn the characteristics of the circle using the light sensor data that it has collected.  The real advance here is the inquiry engine, which uses Bayesian adaptive exploration to decide which measurements to take next.  It does this by considering all the possible measurements that it could take, and computes the expected gain in information from each possible measurement.  It then chooses to take the measurement with the greatest expected information gain.  The process then repeats as the robot learns about the circle.

The system is easily generalized to solving other problems, such as exploring rooms, interpreting people’s emotions, and doing real science.

We recently presented our research at the MaxEnt 2007 workshop in Saratoga Springs NY.  Below are links to a video of the talk, my slides, and our research paper.

Video: Designing Intelligent Instruments, K.H. Knuth

Slides: Designing Intelligent Instruments, K.H. Knuth

Research Paper:
Knuth K.H., Erner P.M., Frasso S. 2007. Designing intelligent instruments. K.H. Knuth, A. Caticha, J.L. Center, A. Giffin, C.C. Rodriguez (eds.), Bayesian Inference and Maximum Entropy Methods in Science and Engineering, Saratoga Springs, NY, USA, 2007, AIP Conference Proceedings 954, American Institute of Physics, Melville NY, In press.

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