A false sense of symmetry

Although our initial plan to light the petals was to put waveguides between the petals and LEDs placed on a flat rigid PCB (see “About LEDs“), Alexis recently told us using flexible PCBs might be possible.

Flexible PCBs could be placed directly on the inside of the demi-sphere of the sculpture, thus avoiding the use of waveguides and ensuring a good luminosity. Their drawbacks however includes a high cost and the fact that Alexis hasn’t yet used them for previous projects.

Using flexible PCBs, our idea would be to take advantage of the symmetry of the sculpture and place identical PCBs along each of the 13 spirals. On each PCB, we put LEDs corresponding each to a petal. There is one catch though: the spirals are all slightly different… Indeed, it is an important property of John Edmark’s sculpture to have each petal placed at a unique height and angle on the sphere, with a unique size. While this is great for fluid animations (see Generating 3D Models), it makes the placement of the LEDs on the PCB a bit of a headache…

LEDs placement

We decided to use a smaller version of our sculpture, with only 8 clockwise spirals and 13 anti-clockwise spirals (The previous images we showed you were of a 13 and 21 spiral model)

Ideally, we want each LED to be placed at the center of each petal. But using an identical LED placement on 13 different spirals will obviously introduce off-centered LEDs: our goal is to minimize this. In order to do this, I first aligned all of the spirals in the same orientation, and sorted them according to their lowest petal height:

The petals are getting in our way here, what we want is to just see the quadrilateral surrounding each petal.

We then need to flatten everything, it will makes things even clearer, and the PCB schematics needs to be done in 2D anyway.

The floor you see on this image is the actual floor of the pedestal on which the sculpture lies. You can see some petals are partly buried under the floor: they still need to be lighted for a fluid animation, but some are unfortunately too small to have LEDs placed underneath them. With this additional constraint in mind, I spent some time in blender figuring out the best LED placement, and this is the result:

LEDs are in red, their size is around 5mm on this image

Here is a superposition of the worst case spirals, which was also very helpful to design the placement:

Frustrating, isn’t it ?

Alexis is currently gathering information to assess the feasibility of flexible PCBs for our project, we will make sure to keep you updated on our final choice.

Communicating between Phyllos : Who’s Who ?

In order to display animations that flow back and forth between several Phyllo, the Phyllos need first to be able to know each other’s position. 

For now, we have reduced this problem to this : to display animations in the proper orientation relative to its neighbours, each Phyllo needs to know the direction of each neighboring Phyllo. They have no real need to know exactly how far they are from each other.

Associating detection and communication

It is not enough to merely detect Phyllos if the detection method doesn’t allow us to distinguish one Phyllo from another. We have to be able to both talk to a specific Phyllo AND know where it is physically located.

It’s the same problem as trying to find someone you’ve never met in a crowd, while you’re on the phone with them. At some point you’re going to need a sign to differentiate them from every other person around you.

So far we have discussed several ideas :

The IrDA idea

Our first idea was to place a lot of highly directional IR transceivers facing outwards all the way around the fixed part. Regularly, if a Phyllo isn’t communicating with anyone, it emits its identifier. If one of the transceivers detects an answer, the Phyllo uses this transceiver to speak with the Phyllo who answered. Like that, we communicate directly in the right direction. If three Phyllos are aligned, the middle Phyllo will hide the two others from each other, but it’s not a problem.

However, we do not know if it is feasible. One problem in particular is that, in order to use very directive IrDAs, we would need many of them to cover 360° and the processor probably won’t have enough UARTs.

The magnet sensor idea

Alexis suggested that we use magnetic sensors. It would then be possible to have hardware set ids by playing with north and south poles of magnets in each Phyllo. Alexis also lent us an evaluation board of such a sensor.

However, even with pretty strong magnets, we could not detect anything further than 5cm (which is to be compared with the 15cm diameter of a Phyllo). Moreover, it doesn’t seem easy to discriminate magnetic perturbation from each Phyllos, and even more if each of them hold several magnets. On top of that, we would need to filter out the Phyllo’s own magnets.

The rotating IR emitter idea

Our favorite idea so far is about combining Wifi and IR detection.

The Phyllos start with Wifi to list everyone present. They either already have unique identifiers, or cooperate to give each-other unique identifiers.

Then, in an agreed upon order, each Phyllo emits IR in all directions with several emitters around its fixed base to cover 360°. Each Phyllos also possesses an IR receptor on its rotating part, allowing them to determine the direction of the Phyllo who is currently emitting. Thanks to this protocol, the association between Phyllos’ ids and their location is possible.

Working anatomy of a Phyllo

Architecture

Here is a diagram summarizing the architecture of a Phyllo, to allow you to have a global vision : 

Some details

This week, we spend a lot of time trying to choose the components. Here are the solutions we explored and the decisions we took.

Motor and ESC

All the animations we will display are based on two or three basic animations : aging petals, de-aging petals and possibly fixed images. Here are simulations to visualize those animations :

Aging and de-aging petals

In order to display those animations without skipping any petal (have a look at this instructable to remember the “skipping petals” part) while flashing the LEDs at 30Hz (ie 30 frames per seconds), the Phyllo has to rotate at : 

AnimationSpeed (RPS)
Aging petals11.46
De-aging petals18.54
Fixed petals30

Furthermore, we are going to use a brushless motor (less wear-and-tear, less noise) controlled by an ESC. We have chosen not to use the same motor than J. Edmark (KV135) but a motor with a higher KV, in order to consume less power. A higher KV motor has a weaker torque at the beginning, so we will have to choose a precise ESC (some of them are controlled in I2C or SPI). 

For now, we have chosen:

For the ESC, we will first test if the ESC CyLED used last year is able to start our motor. If not, we will use the VESC 6+.

Battery

The LEDs will require a lot of power, but for very short bursts: they will require less than 1W on average. The motor require at most 75W. We would like to have 1h autonomy, so the battery has to provide 80000mWh. The motor we choose requires a 6S battery. Therefore the capacity must be more than 3622mAh. We have chosen this one : Batterie Lipo 6S 4500mAh.

Power transmission

First we wanted to have a wireless transmission to avoid the ugly effect of having a metal bar coming out of the top of the Phyllo, as CyLED did last year. After extensive research, we found that this type of energy transmission is currently used almost exclusively for mobile phone charging. Therefore, output power is usually far lower than our needs (20W against 120W). The few products with better output power are essentially coming out of China without any documentation.

We also considered the possibility of building our own wireless transmission to get an appropriate output power. However, it is too hard to get a satisfactory calibration and we do not have the time required.

The following idea was the slip ring. However it has a big flaw: it wears down quickly. We imagined to have one connection on the ball bearer of the motor. Then, we thought that we could put a second one as a ring on the axis of the motor to completely get rid of the slip ring.

This ball bearer has no other use than electricity transmission. Here the insulating layer enables the ball bearer to stick to the axis. It is also necessary since the axis already bears the current from the ball bearer of the motor.

Regulators

We will need at least a 3.3V regulator and a 5V regulator, to regulate the output voltage of the battery and of the power transmission device. We have considered 2 switching regulators : LT1765 and LM2678.

LEDs and light pipes

We haven’t received all the components yet… While we were working on the components choices, Vlaya has started to design some tests for the LEDs.

LED Drivers

We plan to use a LED driver for each spiral arms. The drivers need to have enough output channels to drive all the RGB LEDs of the spiral without multiplexing. A Phyllo has a different number of clockwise and anticlockwise spirals, so we can take as many drivers as the smaller spiral number, in which case they need to have more channels, or as many drivers as the bigger number, and they can have fewer output channels.

The drivers cannot be interrupted in the middle of a PWM cycle, therefore the clock of the internal PWM of the drivers needs to be fast enough that a PWM cycle is shorter than approximately 100µs. That is the time the LEDs can be flashed, if they stay on for longer periods the image will blur due to the Phyllo’s rotation.

Moreover we need to be able to communicate fast enough with the drivers to send the color values of each LED to the drivers in the time between each frame (1/30 of a second). And we need to be able to simultaneously enable and disable all the drivers.

Finally the output current must be high enough to power our LEDs with an acceptable brightness.

We have chosen these LED drivers : THL3502

  • 24 channels, allowing up to 8 RGB LEDs per driver
  • PWM 10 MHz and 8-bit brightness control, making for 25.6 µs PWM cycles
  • 10 Mbps I2C serial input, allowing us to send colors for 150 RGB LEDs in 240 µs
  • Output current : 100 mA

With the I2C interface, we cannot simultaneously fire and disable all the drivers. If the delay gets too big, a solution could be to separately control the current output to the LEDs with transistors.

Wifi

We will need Wifi to communicate with a Phyllo from a computer, and probably to communicate between Phyllos. At first, we wanted to use an ESP32 because it’s simple to use and easy to find help online. But after discussion with Alexis, we will use a AMW006.

SD

We would like to be able to upload complex animations frames on a Phyllo and maybe allow users to do the same. To provide the necessary storage space, we will use an SD card : JAE_ST1W008S4ER1500.

Angular position sensor

To be able to display neat and clear animations in a precise direction, we must be able to detect the alignment of the motor with its original position with an accuracy of less than half a degree (to have distinct images like J. Edmark). To do so, we will use a Hall effect sensor (TLE4964) and we planned an optical sensor (the same than Cyled : TCUT1600X01) just in case, even if theoretically, the Hall sensor is precise enough. The idea is to get a pulse every rotation and use these to deduce motor speed, and then integrate the speed, coupled with the once-a-rotation pulse, to deduce the angular position.

IrDA

To communicate inside a Phyllo between the fixed part and the rotating one, we will use IrDA. We have chosen these transceivers : TFDU4101-TT3.

The IrDA Design Guide requires a transmission distance of 0cm to at least 1m and an emission angle of at least 30 °. Thus, if we place the transceivers close to the axis of rotation, they will be able to communicate during most of a turn, so no worries about the transmission speed.

If we can not place them close enough to the axis of rotation (because of the wide motor diameter), we can reduce their directivity by putting a diffuser so that the LED illuminates the entire interior of the phyllo (or at least most of it) and the two transceivers can communicate all the time. It will not be necessary that it affects the communication of the other Phyllos.

If we cannot place them close to the axis of rotation nor diffuse enough light, and we are obliged to place one at the periphery of the PCB: we place one near the center facing outwards, the other on the periphery oriented towards the center. They will be able to communicate during 2.7ms minimum. In this case, we must take elbow transceivers like TFDU4101-TR3, probably faster ones…

To be continued…

The following post will detail our ideas about detection of other Phyllos and communication between them. And Monday, we have a meeting with Alexis to decide on a solution. We will keep you updated 🙂

About LEDs

First ideas

Our original idea was to place LEDs on the inner sphere of the sculpture, either with flex PCB, or by drilling the sphere, placing the LED in the holes and connecting them with wires to a rotating PCB contained into the sphere. To facilitate the positioning of the LEDs, we could have modified the design so that we can pin the petals one by one on the inner sphere rather than print everything in one block.

But these designs are not easily achievable. First, Alexis does not know how to design flex PCBs. Second, to have a satisfactory visual impression, we would like to have at least 100 petals. To justify this number, here is a model with 60 petals, and another with 100 petals, and a third with 150 petals.

With 60, 100 and 150 petals respectively.

In the above images, there will also be petals all the way to the center, but there will be only one light for all of them. In the above images, only the petals that would have their individual LED are shown. There will of course be petals all the way to the center, but the center ones will all share the same light.

With 100 petals and a single LED per petal, it would make 400 solder wire, which is unthinkable (especially given that the sculpture will turn).

Flat PCB

After discussion with Alexis, we agreed to place the LEDs on a flat rotating PCB with the same phyllotactic arrangement. Here is a simulation example to imagine how we will place the LEDs:  

The idea of abandoning 3D sculptures without struggle did not please us. That’s why we will try to give a 3D impression while leaving the LEDs on the flat PCB. To do this, our best idea so far is to use light pipes to guide LEDs light to the petals, like in the figure below:

Ideally, we would be able to keep flower-shaped sculptures similar to those of John Edmark. But in case we cannot have full-blown 3D sculptures, we could still add a little bit of  relief. Here’s what it could look like:

Today, Alexis has ordered a selection of compatible light pipes and LEDs, so that next week, we can do tests to measure the loss of brightness per cm inside the pipes and the influence of a LED on its neighbors (loss of brightness at the entrance of the guide).

LEDs overheating

Another problem that we are likely to encounter is the overheating of the LEDs. For 100 RGB LEDs (which need to be switched on at the same time), with a maximum of 100 mA per color, a current of 30A is required. So if we power the LEDs with 4V, we need a power of 120W.
We started to think about solutions to cool the LEDs: drill holes in the sculpture to ventilate the LEDs, use a Metal Core PCB (MCPCB, such as below) instead of a FR4-PCB, use heat sinks, LEDs with an electrically isolated thermal path (such as Cree XLamp LEDs).

Layers of a MCPCB

But our LEDs will be under light pipes so they will not be well ventilated by the holes in the sculpture. The MCPCB will not allow us to put all the components that we need, in particular we can not solder components on the back of the PCB. The mask solder is only on the top layer, on which the LEDs are placed, and the bottom layer ends with aluminum or copper, and it is not possible to add FR4 and copper after the layer in aluminium. And the heat sinks all seem to have a disproportionate size for our use.

Before continuing research on the overheating of LEDs, we will use LEDs already ordered which I mentioned above to test their heating.

  If you have any idea or suggestion, all comments are much welcome !

Generating 3D Models

TL;DR

  • We need to generate 3D models of phyllotactic patterns.
  • We give an explanation of how to generate phyllotactic patterns on a sphere.
  • We present an issue we encountered, and the solution we found.

Why 3D Models ?

When we first started thinking about the project, we quickly realized we would need to generate 3D models of the sculpture ourselves.

First because we need to have full control on the model, to try various configurations for the future 3D printed sculpture. And second because it will greatly help us visualize all the kinds of animations we are imagining.

Accordingly, Vlaya wrote a script using the blender python API to generate a 3D model, following John Edmark’s online explanation.

Phyllotactic Patterns on a Sphere

We place the first point on the “equator”. Then to place each following point, we rotate 137.5° and move a little closer to the top of the sphere.

The big question is : how much closer is “a little closer to the top of the sphere” ?

137.5° is the golden angle, derived from the golden ratio. It has the nice mathematical property of being as hard as possible to approximate with rational numbers. This allows it to waste as little space as possible, which is why plants use it. And it looks nicer. You can try the simulation on this page to get a better grasp of this.

A First Attempt

The first thing we tried was to actually place the points on a cylinder, rotating them 137.5° and going up by a fixed amount each time, and then projecting those points onto the sphere :

Projection from the cylinder onto the sphere
The projected points
Connecting the dots

But we soon noticed an annoying discrepancy between what we got, and what John Edmark’s Blooms look like :

A Bloom by John Edmark on the left, our attempt on the right

Our petals get more and more squashed as we near the center.

A consequence of that is, if we follow a spiral from the center outwards, at some point we’ll realise that what we’re following isn’t a spiral anymore.

All this doesn’t happen with John Edmark’s Bloom.

Follow a clockwise spiral from the center outwards and see what happens

The same thing actually happens in sunflowers:

Spirals “breaking up” in a sunflower

But in our case, it happens because the points step too slowly towards the center, whereas in a sunflower, they step too fast.

This paper on phyllotactic spirals gives a good explanation.

A Better Solution

Let’s formalise things a bit:

Spherical coordinates

A point on the sphere can be located by two arcs of a circle : parallel arcs (“horizontal”) and meridian arcs (“vertical”). To rephrase the previous explanation, we place each following point by moving along a parallel for 137.5°, and then moving up along a meridian for some arc length.

A face formed by intersecting spirals

As we get closer to the top, the parallel arcs of circle become smaller.

The meridian arcs also need to get smaller, otherwise we would overshoot the top.

We want the meridian arc lengths to decrease so that the faces keep roughly the same square shape.

To do that, they need to decrease at the same rate as the parallel arc lengths, to remain in proportion.

A bit of geometry yields this differential equation, where a(x) is the “vertical” angle between point x and the horizontal plane, and k is a coefficient:

a'(x) = k cos(a(x))
a(0) = 0

The solution is:

a(x) = Arctan(sinh(kx))

We had to tweak the value of k a little bit to get nice results.

Here’s what it looks like:

3D rendering of our latest design

Flat Circular Phyllotactic Patterns

We can do the same thing with flat circular patterns : place each following point by rotating 137° and moving closer to the center. The question is still “how much closer ?” and the geometry is merely a bit different.

We get this differential equation, where r(x) is the distance of point x from the center, k is a coefficient and R is the radius of the circle:

r'(x) = - k r(x) 
r(0) = R

The solution is:

r(x) = R exp(-kx)

This gives us flat patterns like that:

Flat phyllotactic pattern

Stay tuned for upcoming 3D animations !

Phyllo

Description

Ever heard of phyllotactic spirals ? John Edmark was inspired by these naturally occurring leaf patterns to create a series of sculptures he called “Blooms”.

This is the starting point for our project, Phyllo.

As John Edmark explains in this instructable, he spins a Bloom under a stroboscopic light to create the illusion of a flower blooming.

In our project, we will build upon this and:

  • Color each petal individually from the inside with stroboscopic LEDs.
  • Play colorful animated patterns on the sculpture (such as falling or rising petals, lines, spirals, rings…).
  • Display animation patterns across neighbouring phyllos: when two sculptures are close enough, they can synchronize and the animations will propagate from one to the other, creating globally coherent phyllotactic patterns.

Here’s a simple 3D simulation as a basic example of a revolving spiral:

More on phyllotaxy

The Golden Angle and packing leaves: https://www.mathsisfun.com/numbers/golden-ratio.html

A mathematical analysis of phyllotactic spirals: http://pzacad.pitzer.edu/~dbachman/Papers/FibonacciSpirals.pdf

Some history

These spinning sculptures are an updated version of the zoetropes and phenakistoscopes of the early 19th century. They are the precursors of modern Motion Picture.

Take a look at this modern 3D zoetrope:

We will keep you posted as the project moves forward. Stay tuned !