Category Archives: Mechanics

Ball detector

One sensor that has not been mentioned previously is the “ball detector”. The ball detector helps us to easily detect when a ball has been “caught” by the dribbler and is in a position ready to be hit by the kicker.

The detector is implemented using a classical opto-isolator idea. On one side of the dribbler we put a small infrared LED, that will be constantly emitting infrared light. On the other side of the dribbler, we attach an infrared photodiode (or, to be more precise, we actually use a slightly more sophisticated breakbeam sensor). Now, whenever there is no ball in the dribbler, the sensor will be “seeing” the light from the IR led and produce an “off” (i.e. 0V) signal on one of its pins (note that a conventional photodiode would work the other way around). Whenever a ball comes into the dribbler inbetween the LED and the sensor, it will hide the light from the diode, the signal will go up to “on” (i.e. 5V), and this can be sensed by the controller.

Of course, in theory we could detect the fact that the ball is in the dribbler purely from the camera image, but having a dedicated ball detector sensor is easy and convenient.

Ball detector: IR LED
The black dot under the dribbler shaft is the infrared LED
Ball detector: Photodiode
On the other side, there is a photoelement


A coilgun, is the part responsible for kicking the ball. As you might remember from an earlier post, it consists of a solenoid (coil), a capacitor, a metal bar, a spring and a controller circuit.


The Theory

There are thousands of tiny details that must be decided upon in order to make a coilgun: you need to choose the material of the metal bar, the force of the spring, the parameters of the wire comprising the coil, the diameter and number of turns of the coil, capacity of the capacitor and its operational voltage. Ironically, despite the abundance of those details, in the end there is just one simple thing that matters: the coilgun must kick the ball strongly enough to send it to the opponent’s goal.

Understanding how each of the above parameters influences the strength of the kick requires patience and a solid knowledge of high school physics and beyond. We won’t go into this here, but here’s a brief overview of the main issues that matter:

In order for the kicker bar to jerk strongly enough, the coil must produce a strong magnetic field.  In order to produce a strong field, strong current must flow through the coil, the coil should have many turns and the turns should preferably be small. In order to produce the strong current over a long wound wire we need higher voltage and a large capacitor.

On the other hand, an excessively strong current would melt the wire, and excessively strong voltage would produce sparks, and an excessively large capacitor would be heavy, expensive and dangerous. In the end, there is a kind of balance between “small and light” and “strong and heavy”.

Although in principle it should be possible to come up with a “theoretically perfect set” of parameters which result in the lightest possible, yet functional coilgun, we went along a simpler route of reusing the practical knowledge of our predecessors and intuition:

  • We use a 1500uF capacitor to be charged at 250V. The 1500uF number comes from the fact that such a capacitor was available and the 250V number is related to the coilgun control PCB from the previous year’s Tartu teams, which we could reuse.
  • We have a coil wound of 1000 turns made of 0.4mm copper wire. The wire was chosen fairly arbitrarily and the number of turns was selected so that the total coil resistance would be somewhere around 4 Ohm. At 250V this would result in a discharge current of around 60A (a “good current for a coilgun”, according to the knowledge of local gurus).
  • The 1500uF capacitor charged to 250V can theoretically discharge at this current for at least 6ms, which turns out to be sufficient for the metal bar to make a strong kick.

The resulting coilgun works fine, but it is so strong that it could have been way lighter. If you, dear reader, happen to be in the mood for doing some exciting physics exercises, I dare you to use science and come up with a better set of coilgun parameters.

The Practice

To make the actual coil we used an old electromagnet of unknown origin (perhaps a lock or something) which was kindly provided to us by our fellow Tartu team EKRS.
Old electromagnet, disassembled
Old electromagnet, disassembled
We replaced the wire with our own (on one of the previously posted videos you may actually find a hidden moment, where Reiko is seen in the process of winding the coil using a lathe), added a metal box (which you can see being machine-milled on that same video), and attached everything to the first floor of our robot.
Coilgun in the attachment box (model)
Coilgun in the attachment box (model)
Coilgun, attached
Coilgun, attached

Unfortunately, we do not have a lot of “in progress” photos of the coilgun making. Here is, however, a nice view of the EKRS robot body together with a coilgun control PCB (same as ours), a large capacitor (ours is slightly smaller) and a coil while it was still being developed:

EKRS coilgun development
EKRS coilgun development

 Another Tartu team had their capacitor explode with a loud and scary boom. So, dear reader, please beware: large capacitors can explode easily and violently when connected incorrectly, used with invalid voltage, or damaged:

Exploded capacitor
Exploded capacitor

And finally, here’s a video demonstrating the Telliskivi coilgun in action:



In theory, the dribbler is just a rotating rubber shaft. In practice, making this seemingly simple component is not as trivial as it might seem.

  • First, we need a metal inner shaft. The ends of the metal shaft must fit perfectly into a couple of ball bearings. There should also be a groove along the length of the shaft to keep rubber in place. As those requirements are fairly specific, it means that the metal shaft has to be lathed manually.
  • Secondly, we need to pick a suitable type of rubber coating. It must be easily moldable upon the shaft and “sticky” enough to grip the ball. Following the advice of the course supervisors, we used Dragon Skin silicone.
  • Thirdly, the rubber must be cast onto the metal shaft. This requires preparing three additional components – a properly-sized tube and two caps on it, shaped so that they would hold the shaft right in the middle of the tube. Making the caps meant some more lathe-work for Taavi.
  • Once the mold is done, the silicone (which comes in two separate components) is mixed, cast into the form onto the shaft, given several hours to harden and taken out of the form, hoping no ugly bubbles or unnecessary objects got caught in it during casting.
  • Finally, the shaft must be mounted into the ball bearings and geared to a motor (which, in turn, has to be mounted appropriately and connected to a pin of a microcontroller board).

The video below illustrates the functioning of a completed dribbler:

Contemporary Manufacturing – CNC milling

Laser cutting is good for making flat sheets, but not enough to produce 3D shapes. CNC (Computer Numerical Control) milling is the way to make those. As with laser cutting, you can order CNC services at a reasonable price nowadays. It was even easier for our team – Tartu University’s Institute of Technology has a mill available and Taavi knew how to use it.

Piece of a G-code program
Piece of a G-code program

The CNC machine is programmed using a special programming language – the G-code, where each instruction corresponds to some movement of the mill. Ironically, despite the obvious practical importance of this language, most programmers or computer scientists have probably never even heard of it. If you are one of them, take a minute to educate yourself and skim through the Wikipedia entry, at least.

Although the CNC code can in principle be generated by CAM systems straight from the 3D CAD-model, it was easier for us to write the program manually, as it allows to account for the specifics of the particular machine in our case.

In the video below, Taavi is running the code he wrote for cutting the coilgun box for the Telliskivi robot:

And if you think this is cool, you might want to take a look at what the expensive industrial CNC mills can do in the hands of expert programmers: (one), (two).

Contemporary Manufacturing – Laser Cutting


The first Robotex competition took place in 2001. Although I did not take part then, I remember observing with interest the progress of Tartu University teams. Looking back 10 years now, it is fascinating to see how much changed since.

One of this changes is the perhaps wide spread of affordable computerized manufacturing. Laser cutters and CNC milling machines have existed for years now, but it seems that in just the last 10 years has their use become accessible to pretty much anyone who needs it. The more recent 3D printing can’t go without mention – one can get a reasonable tabletop 3D printer at the price of a high-end laptop nowadays.

So for those of you who are new to this thing, here’s a brief example of how things work in the marvellous world where one does not need to know anything about saws and hammers in order to create hard metal objects.

Modeling the part

First, we use a CAD system (SolidWorks, in our case), to create a model of the part. This requires some knowledge of the CAD system, but it is not rocket science, really.

CAD model of a part
CAD model of a part

Ordering the part

Next, in order to order an actual part from this model, we need to create two files:

Planar unfolding
Planar unfolding
  •  As in this particular part we would like the manufacturer to help us with the metal sheet bending, we also need to provide the technical drawing of the final part. The CAD system is quite helpful here too: it takes a couple of clicks to generate the drawing from the model:

    Technical drawing
    Technical drawing

Note that depending on the manufacturer, additional files may be required in certain format as a specification, but in our case this was sufficient. You then send the specification to the manufacturer and wait. Most commercial manufacturers would typically produce parts in batches of at least 100 or so, so if you need just one, you would often have to come up with a personal arrangement.

The price range differs depending on the manufacturer (some have cheaper, less precise machines, some have better ones), chosen material (cutting thinner material requires less power) and personal arrangements. In particular, the total price of computerized manufacturing for the single part shown above, including material (1.5mm aluminium sheet), is less than €20, and can be significantly less, if you manage to get a good deal.

Mechanics design

The Robotex course requires participants to present their progress once each week. This automatically imposes an iterative approach to development – instead of working for a long time and coming up with a final design for the robot, we had three iterations – two preliminary drafts and the final one. The design was made using SolidEdge first (because Taavi was more familiar with it) and SolidWorks later (because Reiko was more familiar with it). Both programs are well-known industrial CAD packages. The former provides a student licence to anyone. With the latter, University of Tartu has a special arrangement for student licensing.

Here I must admit that my previous encounters with mechanical drafting was a long time ago, in high school, where we were supposed to manually draw various hypothetical 3D bodies on A4 paper and write labels using a strictly defined font. I recollect we even made one part manually according to that draft. I never enjoyed nor appreciated the course, and in fact never even understood the need for this whole drafting business: building things seemed so much easier without it. Well now I finally got it. It took me 14 years to understand the obvious. You draft something and then let other people manufacture according to it. Duh.

OK, back to the robot. Our first draft was that of a “brick on wheels” (note that this is using the N950, as this was before we got the N9). The dribbler and kicker are not present, but room is left for them. The electronics attachments are not present either, but are already planned to go onto the second floor.

Telliskivi model draft 1
Telliskivi - draft no. 1

In the second iteration, Taavi decided to position the camera higher up, on a tower. This would shift the weight backwards:

Telliskivi - draft no. 2

Finally, the third and the last iteration was completed by the beginning of October.

Telliskivi model
Telliskivi - final model

A couple of simplifications were introduced here. Firstly, the smartphone tower is now connected to the second floor – this makes it easier to assemble the robot. Secondly, the wheels are now directly mounted onto motor shafts. This means that the whole weight of the robot goes on those shafts and if our robot would have been too heavy, this might not have been a good solution. Fortunately, this is not a problem.

Note the pluggable electronics module box, designed by Reiko:

Electronics module
Electronics modules

The small box in the upper right is the Bluetooth module, the black bricks are batteries (why two? stay tuned for a later post). The big box below contains the printed circuit boards for controlling the motors and the coilgun:

Electronics modules
Electronics module inside

Most of the details of the chassis are to be laser-cut from aluminum (first floor, tower, electronic box) and plastic (everything else). The aluminum cutting was ordered in Metec, and plastic cutting – in Tööstusplast.