In very simple terms, the POV globe is a ring of RGB LEDs that is rotated axially at high-speed (300 rpm). Due to this high-speed and the phenomenon of persistence of vision (POV), our brains interprets this moving ring of light as a solid, spherical surface. By changing the colours of the LEDs very quickly, we can display images on this spherical surface. Mounted inside the ring is a Raspberry Pi (RasPi), a small, single-board computer that has become popular in the last couple of years. Its small-size and lightweight design make it ideal for this application. The RasPi has an HDMI output and therefore a custom FPGA-based HDMI decoder was designed and implemented for this project. The decoder takes the HDMI signal and converts it into a form suitable to be displayed on the ring of LEDs.
A system overview of the POV Globe is shown below.
The mechanical structure of the globe is essentially an aluminium ring that has been carefully-designed to be balanced and hence stable when rotated at speed. The ring is mounted within a C-shaped frame at the same angle as the Earth is tilted (23.4°) and can rotate freely due to the heavy-duty thrust bearings at either end.
There is a pulley on the bottom that connects to a stepper motor via a drive belt that drives the ring at the required speed (300 rpm). Since the LEDs are mounted along the entire circumference of the ring, one revolution actually displays two complete frames of the image, giving an effective frame rate of 10 fps.
There are a series of LED driver boards mounted along the circumference of the ring. In total there are 14 boards mounted on the ring (7 on each side). Each board has 24 RGB LEDs giving a total of 168 vertical pixels. The image on the rotating display is updated at such a rate that it gives the illusion of 360 ‘horizontal’ pixels, making an overall resolution of 360 x 168.
The RasPi outputs its display data via an HDMI port and so a FPGA-based HDMI decoder was developed. This runs at sufficiently high clock speed to decode the large amounts of display data and covert it to a form that can drive the LEDs.
At this stage, the POV Glove is essentially a display for the RasPi mounted inside and any applications that run on the RasPi will appear on the globe.
One of the greatest challenges for this type of project is the power supply. The large number of LEDs, RasPi, HDMI decoder, Wi-Fi modules, motor etc. means that 75 W of power is required to run the globe. Getting this amount of power into a ring rotating is achieved using a four-phase commutator.
The final aspect of the project is controlling the globe. This is done through a bespoke iOS application that communicates with the RasPi over Wi-Fi (the University is a member of the Apple University iOS Developer Program and we can thus develop and install apps on iOS devices). The app can be used to control the home screen of the globe as well as individual apps running on the RasPi (e.g. world weather and emulation station).