Illumination and colour control in Flicker free LED lighting
Project ID: AEE-2017-21
Students: Vili Väinölä, Sina Khamehchi, Vishnukumar Murugesan, Tapio Kukkonen, Hassan Rouhi
Project Manager: Vishnukumar Murugesan
Instructor: Paulo Pinho
Other advisors: Max Björkgren, Hannu Vihinen
Starting date: 5.1.2017
Completion date: 15.5.2017
The main purpose of this project is to create two different LED drivers. One of the drivers is focused on the PWM based LED dimming and the other on multicolor LED intensity control. The LED drivers are modified from existing Helvar LED drivers. In the PWM driver, the goal is to drive the LED with a randomized switching frequency to minimize the flicker effect visible on HD cameras when used in LED lighting environment.
The second goal of the project is to design a multicolor LED driver which works in a way that the light intensity is controlled, irrespective of emitted color (additive) output and the color is controlled irrespective of change in intensity (dimming).
Summary and results
The purpose of the first phase of this study was to evaluate the PWM-dimming impact on video, using HD cameras and to see if the effect was corrected with randomized PWM-frequency. The PWM generates undesirable flicker that can be seen by the camera or even under some circumstances with the naked eye. The randomized PWM helps with reducing the flicker, however, this driving and dimming technique is not the solution for flicker-free illumination. Therefore, digital PWM and RPWM is implemented by means of microcontrollers. In addition, the RPWM with the increased frequency is a better solution for diminishing the flicker effect seen through the HD cameras especially with lower shutter speeds, for instance, 1/100 or 1/125. The appropriate frequency for the RPWM tested by various tests is 4300 to 5000 Hz that means increasing the frequency contributes towards minimizing the flicker effect.
The second phase of the project was to produce an RGB led driver for generating different colors along with changing the light intensity separately. The problem with multicolour control is the color chromaticity shift happens while dimming or the intensity changes in the output light while changing the color. A mathematical algorithm was derived to create various colours with the possibility of changing the brightness. Since the control algorithm used a linear estimation of red, green and blue duty cycles, the target colour obtained by the controller was not exactly the wanted colour. Therefore, there were some differences between the target colour coordinates and the actual colour provided by the controller. The other problem seen in the performance of the controller is the blue colour which seemingly has a brighter output in some cases.
In future work, junction temperature based closed loop control can be built to achieve constant output luminous flux for each colour, reducing the colour shift due to varying junction temperature.