Digital LED light source controller design

In modern automated production processes, machine vision systems are widely used in the field of condition monitoring, finished product inspection and quality control. The machine vision system is mainly composed of illumination system, lens, camera, image acquisition card and image processing system. The image acquisition and processing system is matched by the imaging device such as light source, lens and CCD to check and identify the product. This technology can greatly reduce the technology. Manual inspection of the quality of product quality, effectively improve detection efficiency, and is conducive to production management. Lighting system is an important part of machine vision system, which is the key to ensure the quality of image acquisition and application effect. Light source controller is the core of lighting system. Its main function is to provide different illumination brightness and illumination mode for specific applications. Improve the quality of the lighting source and obtain high quality images. At present, the light source controllers produced by most domestic manufacturers are manually adjusted or communicated through RS232 and Pc machines for communication, which is inconvenient to use, low in efficiency, and high in cost. In this paper, a new type of light source controller is designed. The controller is based on ST's STM32 microcontroller STM32F107VCT6 based on ARM Codex-M3 core. It communicates with the Pc machine through plug-and-play, high-speed USB interface. Real-time setting of the light source controller parameters by the Pc end application software. The STM32 on-chip timer is used to realize PWM digital dimming, as well as pulse frequency, wide adjustment of duty cycle, and precise timing through the cooperation between timers. In addition, the controller implements three lighting modes, normal mode, strobe mode, and external trigger mode. This design can be flexibly adapted to the needs of machine vision inspection systems in different applications.

1 Overall design

Simply put, the main function of the light source controller is to receive various predefined commands and setting parameters sent by the PC host, and process it through the main controller, and then control the LED light source through the driving circuit to realize digital PWM dimming according to the setting.

The light source controller can be divided into a software part and a hardware part according to the architecture, wherein the software part can be further divided into a Pc end user interface application, a USB driver, and a controller hardware firmware. The hardware part can be divided into a main control module and an LED drive module. The main control module is mainly responsible for communication with the PC, command response, and transaction management. It is the core part of the entire controller. The LED driver module mainly provides a suitable driving method for the LED light source. Commonly used driving methods for LED light sources are inductive driving and charge pump driving, wherein the inductive driving circuit is suitable for driving several LEDs connected in series. LED brightness control methods can be divided into analog dimming and digital PWM dimming. Analog dimming adjusts the brightness by changing the LED current. The disadvantage is that the LED will have a color shift phenomenon as the forward current changes, and the accuracy cannot be met. PWM dimming changes the forward current conduction time by turning the LED on and off to achieve brightness adjustment. Since the LEDs are all turned on at a constant current, there is no color shift problem with analog dimming techniques. Therefore, this design uses inductive drive, PWM constant current dimming.