Archives for category: rotacaster

The large EV3 motor has a different form factor then the NXT motor. I had to redesign the legs of Agilis to make use of these new motors.


EV3HolonomicLeg2The new design is smaller and much prettier to the eye. The smaller size and all the triangular shapes makes the leg very stable. The new leg can be fitted on the same triangular frame that was used for Agilis.

Here are the building instructions for the new leg.

If you only have two large EV3 motors you can easily modify the leg to use the EV3 medium motor. This motor can be attached directly to the drive axis of the wheel and to the underside of the frame. You will have no gearing. But as the medium motor rotates a bit faster you will still have plenty of speed in your robot.


One of the most fundamental problems in mobile robotics is to know the position of the robot.  The question might seem simple, to get an answer is very difficult.  There are two fundamentally different approaches to answer this question. The first approach uses dynamics to keep track of the robots position in respect to its starting position. Most often this technique uses wheel encoders to keep track of wheel movement. This is translated into a position of the robot. This is known as odometry. The second approach uses external references of which the location is known to calculate the position of the robot. Navigation based on stars is the oldest example of this technique, the GPS system is a recent example. As much as we come to rely on GPS nowadays, it is not very useful for our small indoor robots. Indoors the GPS signal is week and the error of a GPS position is in most cases bigger than the range of our robots.

I created a robot that uses the same principles as the GPS system is based on to localize itself. Instead of GPS satellites I use  blinking LEDs (dLights from Dexter Industries) as beacons. The beacons are detected and identified using a standard NXT light sensor. Below you can see the robot in action. The robot is placed on a random location in a random direction in my room. It then has to find out where it is and then drive home.

The robot locates beacons by evaluating the variance in light level while scanning the horizon. Whenever a spot with large variation in light level is found, the robot stops to find out if this variation comes in a frequency that identifies one of the beacons. If so, the current heading of the robot is stored along with the identity of the beacon. If after a full scan three or more beacons are located then the robot has enough information to estimate its position. It does so by triangulation using a Snellius construction. Lejos software for the NXT has a class, lejos.robotics.localization.BeaconTriangle ,that implements all the difficult calculations. If more than tree beacons are located, the robot estimates its position by averaging the position estimated from all unique combinations three beacons. The result is an improved estimation.

I have built a new holonomic robot. It has a smaller wheelbase than Agilis and therefore turns faster. It has a 1:3 gear ratio, it’s maximum controllable speed is about 80 cm/second. It is also very sturdy thanks to all the triangles in its frame. And above all, I think it is prettier than Agilis. However, that is what every father says about his newborn. Below are some pictures






I got some feed-back on my last post with “building instructions” for Agilis. My friend Kirk wondered why I didn’t make proper instructions. He was not impressed by my lame excuses about odd angles or illegal builds and showed me things could be done. He was right, it was about time for me to master some Lego CAD tools. I choose to use LDraw and MLCad for the job. My fears for a steep learning curve proved wrong. The manual is quite good and I was up and running within an hour. The hardest thing to learn was the discipline to work precise immediately. One cannot build a sketch at first and then improve the model. Well, technically you can, but you shouldn’t as every brick you move triggers a chain of new moves that need to be made.

It was fun to create proper building instructions for Agilis. Along the way I also improved the gearbox as was suggested by another reader of my blog, Thimoty. You can freely download the instructions. I do appreciate constructive feedback. This is my first and I am sure there is room for improvement.

Download the building instructions for Agilis

See this post for improved instructions!

By request I created building instructions for Agilis. Well, sort of. It is a series of photos made while taking one of Agilis legs apart. Presented to you in reverse order, so that it looks like a building instruction.

One warning before you start. The gear box can be assambled very easily, but it cannot be disassambled without some tools and, potentially, some damage.

The gear box displayed is for a 1:3.33 gear ratio. Here you find a picture of a 1:2 gear box. I think the 1:2 gear box is a better option.

The part list in the pictures 1 to 5 is for one leg only. Also I did not have the parts to make a nice color scheme so you might end up with a rainbow warrior in black and white if you follow my color scheme.

Please send me a message or picture when you built Agilis.



Today Dexter Industries launched their latest product, an all-color LED called the dLight. I was involved in the development of the dLights so I got them early. I can also give away one set of dLights to one of my readers. More on that later. Let’s take a look at the dLights firs in this video.

I mounted three dLights underneath Agilis, one under each leg pointing to the wheel. I programmed the dLights to give a color that corresponds to the wheel speed of the particular leg. I think the result looks cool.

If you look closely to the sensor ports in the video, you’ll notice that only one is in use. This is one of the great benefits of the dLights, you can daisy chain them. So one free port is all you need to give a robot multiple lights. One set contains four dLights plus the cables to chain them.

As said, I can give away one set of dLights. If you want this set you just have to reply to this message before the first of April.

You’ll find more info about the dLights on the website of Dexter Industries.

In case you might wonder how fast or accurate Agilis is, here are some numbers.


  • The gear ratio of the motors to the wheels is 1:2, making the wheels rotate twice as fast as the motors. (It is possible to change the gear ratio to 1:3).
  • Prototype orange Rotacaster wheels. This is the hard compound. There isalso a medium compound (gray) and soft compound (black) available.
  • The batteries were rechargeable NiMH, 2500 mAh batteries. These were not fully charged.
  • The test surface was a clean linoleum floor.


  • Reliable top speed is about 60 cm/sec, equivalent to 2.16 km/h or 1.35 mph. At this speed the robot is still accurate as there is ample margin for the PID controllers of the motors.
  • Unreliable top speed is about 75 cm/sec, equivalent to 2.7 kmh or 1.68 mph. At this speed the robot is not very accurate, especially the heading.


  • The test track is a square with sides of one meter each. During each run the test track is traveled 4 times. Making the total distance of the test track 16 meters.
  • The robot finishes the test track on average within 10 cm of its starting position. Expressed as a percentage of the total distance the error is about 0.6%.
  • The movement error is systematic. The robot always ends up above and to the right of the starting position.
  • The robot is more accurate at slower speed and acceleration settings.

The images shows the result of the accuracy testing. For each test the robot was placed exactly on the origin (bottom left in the picture). It then traveled a square with sides of one meter for four times, making the total distance traveled 16 meters. The finish location of the robot was then marked on the floor. This test was repeated three times for a single set of settings of speed and acceleration. Three different dynamic sets were used,  speed: 50 cm/sec and  acceleration at 100 cm/sec^2, speed 50 cm/sec and acceleration at 750 cm/sec^2 and speed 30 cm/sec and acceleration 60 cm/sec^2.

foto (7)

I want to repeat the tests with a 1:3 gear ratio and also with the black Rotacaster wheels.

Remember my plan to make a ball balancing robot? Last year I set myself the goal to make a ball balancing robot. I even build the robot. Since then I wondered off my original goal and made a guardbot, Koios, from this platform. Now I am having another shot at making a balancing robot.

Programming a balancing robot is easy in theory. You just need a sensor that tells you how much the robot is tilted, most often people use a gyro for this. I use my IMU for this, so that I do not suffer from gyro drift. The tilt angle is then feeded to a PID-controller that transformes tilt to motor speed. The hard part is to tune the PID controller, it has to translate tilt into just the right amount of motor speed, too little and the robot falls of the ball, too much and the robot goes over the top and falls of on the other side of the ball. Falling of the ball damages the robot. So I had a problem, how to tune the PID controller without damaging the robot?

To be able to tune the PID-controller without damaging the robot I made a special platform. It is a large disk with a small pole in the middle pointing down Due to the pole the disk will always be tilted when lying on the ground, only when it balances perfectly on the pole it is level. Therefore this disk can be used to tune the controller.  The robot can ride off the disk, but it doesn’t fall then, it just comes on the floor with one or two wheels.  Afbeelding

When I tested this setup I discovered that the disk whas too smooth, the wheels didn’t have enough grip and slipped. To increase the friction I coated the surface of the disk with sillicon rubber, It is the light blue surface you see in the picture. Now I have a very “slick” surface.I only hope it lasts under the forces the NXT motors generate.But for the moment this problem is solved.

But there are other problems. One is the fact that these holonomic wheels make the robot vibrate, this affects the IMU filter, there is still some drift although it stays within certain limits. I do have prototype rotacaster wheels. The manufacturer told me that the production wheels are more round and generate less vibrations. If you are ever going to by these wheels, and they are a lot of fun, I advice you to take the black ones. They have the best grip. Anyway, I will have to tune the IMU as well.

Tuning PID controllers is difficult and very, very time consuming. There is some theory around tuning PID controllers but in the end it is mostly trial and error. Everytime I want to try a new set of parameters I’ll have to modify the program, download it to the brick, run the program and evaluate the results by watching the robot. It is hard to understand what goes wrong when you see the robot ride of the disk and make a run for the door to the staircase.

But not anymore. Kirk, one of the developers of Lejos made a very nice program that allows you to tune a running PID controller during over bluetooth. The tool is still under development so you won’t find it in Lejos 0.9.1 yet. This program is an add-on to the charting logger I often use to evaluate internals of the robot on the PC. So basicly, this program shows me what is going on in my robot and allows me to modify PID parameters on the fly. I think this is a great tool. Below is a screen shot of it.


So, now I have the robot, a test platform and a efficient tuning tool. That must mean immediate succes! Well, to be honest I don´t think so. I´m still not sure if I can get this robot to work as there are problem with weight and inertia as well. The robot weigths 998 grams. This is quite heavy, even for three powerful NXT motors. The robot is quite stiff, but there it still bends a bit under weight. This affects the IMU sensor. And I´m working on other projects as well. So in the end I think there is a bigger chance to fail than to succeed.

To be continued.

This time I want to introduce you to Koios the guard bot. Koios guards my house, it detects intruders and scares them away.

To perform this complicated task I have split Koios’ behavior into different subtasks and put these in a subsumption architecture.

The first task of Koios is to map its surrounding using two range sensors. These are an ultrasonic sensor for long range (<200 cm) and a Mindsensors dist sensor for accuracy on the short range (<80 cm). To map the surrounding Koios makes a full turn in place while scanning. The resulting map is what I call a circular range map. This means that the map stores the distance to the nearest obstacle for all directions (I recognize 24 directions, each 15 degrees wide). The map looks like a radar scan when plotted. This map is not permanent, it will be thrown away when Koios moves. As Koios does not try to build and maintain a more permanent map of its surrounding I did not have to deal with uncertainties about its absolute position. Therefore the mapping algorithm could be kept simple.

The second task of Koios is to find a safe spot on the map and then to move to this spot. A safe spot for Koios is a spot where Koios is surrounded by objects. A place next to a wall is good, a place in a corner is better. Koios finds a safe spot by examining the map for a place between obstacles. When the safest spot on the map is found Koios travels to this place in a straight line.
Once arrived at the new location Koios again makes a map of the surrounding. If, at second glance, the current location is safe enough then Koios will stay there. If not, it will try to find an even safer spot. This process is repeated until a location is found that is safe enough for Koios.
The video below shows Koios scanning the area and finding a safe spot. I actually had some trouble shooting the video. At first I had placed my webcam on a chair to shoot the video. But to my surprise Koios hid itself under the chair. This indeed is a very safe spot, but it was outside the frame of the video. In the end I placed the camero on my desk to shoot the clip.

When Koios has found a really safe spot it will start guarding the area. It will slowly turn in place while scanning the area again. If an obstacle is detected its location will be compared to the map. When the obstacle is already on the map it will be regarded as a part of the surrounding and ignored. If on the other hand it isn’t on the map then it must be something new and it will be treated as an intruder.
The task of guarding is rather complicated as there is always some uncertainty in both the map and the range sensor data. Suppose the map tells Koios that there is an obstacle at 150 cm and the range sensor detects an object at 145. Is this an intruder or part of the surrounding? The choice Koios makes is based on statistics. To support this kind of decision making a range map stores more information than just the range to the nearest object. It also stores statistical quantities like the number of measurements taken at the direction and the variance in measured ranges. This makes it possible to calculate the probability of an obstacle being new, an intruder, or old, part of the surrounding. If part of the surrounding the measurement is used to improve the map even further.

But if the object is an intruder Koios will home in on it! Koios will quickly run to the intruder until it is very close. I haven’t written this part of Koios’ behavior yet. So everything you read from now on is just on the drawing board.
For one thing I have not decided yet if Koios just runs blindly to the location where the intruder was detected or that it tries to maintain a lock on the intruder while homing in on it. It would be nice if Koios was able to maintain a lock. But it will also be complicated. Mayby I could use a heat sensor like some air-to-air missiles do to maintain a lock on the intruder.

Anyway, once close to the intruder Koios will scare it away using both light and sound. First it will mimic some kind of scary animal with eyes that glow in the dark while making scary noises. Then it will mimic the police using flash lights and and a siren. Then it will map its surrounding again to find a save spot and make a hasty retreat.

Last week I spent my evenings redesigning my holonomic platform. It had te be wider in order to get the center of gravity (relatively) lower. I also wanted to gear it up to make it more agile. And I wanted the brick to be better accessible, especially the USB port. Two other demands remained from Sidbot, the wheel angle should be adjustable so that the robot can be used both on a flat surface and on top of a large ball. It also had to be sturdy.


As it had to be sturdy I decided that the motors should be fixed to the frame. Sidbot had its motors adjustable to make the wheel angles adjustable. The wheels have to be adjustable on the new robot as well, this meant that the hinge point had to be between motor and wheels somewhere in the gear train. I tried different designs and gears but I always ended up with grinding gears. At last I ended up using a 5×7 liftarm to house part of the gear train. This effectively stopped the grinding but resulted in a very wide wheel housing as well. This is not so pretty so I’m still trying to improve this part. However, I now got a 2:1 gear ratio. With a motor speed of 120 RPM and a wheel diameter of 48 mm this gives the robot a top speed of 30 cm per second.

The frame of the robot consists of two triangles stacked vertically. The triangles are made of three 11 stud liftarms connected at the ends with 3 and 4 stud liftarms. This makes a very rigid frame, the brick lies on top of it making it easy accessible. The motors are mounted horizontally with the flat side down. This gives the robot width and also the ground space that is needed when riding on a ball. To prevent torsion between motor and frame I made a housing with L-shaped for the motors.
I used light bluish gray and dark bluish gray for the color scheme as these are the colors the motors and brick are made from. The result is a bit dull but still rather nice looking. It resembles a Starwars like vehicle. Maybe I should mount some laser guns on top.

The resulting robot does meet all my design specifications. But I have not been able to test it yet as I’m one 40 teeth gear short. I hope to get it this week.

The robot still needs a name. If you have a good suggestion you can post it in the remarks. There is a nice price for the winner a second hand Hitechnic gyro sensor. Submit your entry before November 2011.