PSI - Issue 17

Pedro J. Sousa et al. / Procedia Structural Integrity 17 (2019) 828–834 Sousa et. al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

High-speed digital image correlation requires image acquisition with very short shutter times. This means that there is less time to integrate the incoming light on the camera sensor and, as such, it is necessary to have a significant light flux in order to acquire quality images. Illumination is usually the major concern in such high-speed image applications. One option is to use a continuous light source, permanently on, which supplies the necessary light. For continuous acquisitions, this may be a good option. However, long-term constant lighting can cause heat buildup on the specimen, which may alter its mechanical properties and thus affect the measurements. The alternative is to use strobing light, which outputs light during the exposure periods only. This allows for a cooling period for the specimen, dependent on the duty cycle. Additionally, by using light pulses shorter than the exposure time, it is also possible to effectively reduce the exposure and acquire unblurred images that would not be otherwise possible with that camera. Among strobe lighting devices, LED-based ones are gaining market share, in part due to their efficiency, size and power consumption. There are several commercial solutions for LED-based illumination, such as those presented in [1]. These products are often capable of pulsed operation in the microsecond range and usually feature multiple LEDs. However, they are usually expensive, large and with limited user control. Academic devices for the application of pulsed LEDs to several different areas have been developed since at least 2003, when Buttsworth and Ahfock [2] developed a system intended for use in schlieren flow. More recently, Willert et. al. [3,4] developed a system for imaging flow velocimetry and Nasibov et. al. [5] developed another for application in particle image velocimetry. These systems use somewhat similar power control, using a MOSFET to control the flow of current through the LED. The present work reports the development of a pulsed lighting system for use in high-speed digital image correlation (DIC). The main application for this system is the monitoring capability of high speed rotating targets for which, from previous measurements [6 – 8], it is possible to ascertain that the strobe illumination system should be capable of pulsing in the microsecond range with low duty cycle and illuminating an area of at least 200 mm diameter. The objective maximum repetition rate for this system is approximately 1 kHz (period of 1 ms), which corresponds to one pulse per rotation at 60.000 rpm, or sixty pulses at 1.000 rpm, for example. Another requirement of the illumination source is compactness, in order to create a beam using a single reflector. If more illumination is necessary in a measurement situation, the system can be replicated and more light sources can be used. It was deemed necessary to select an LED to fulfill the requirements above. The final choice was between Luminus Devices’ CXM32 series and Cree’s CMA3090 series, which feature a similar price range, illuminance and size. Nonetheless, the slightly larger CXM32 is also slightly more powerful. Both of these LEDs are Chip on Board (COB) solutions, which include a matrix of smaller LEDs in a single chip, and usually operate around 50V. Therefore, the electronics were designed to be compatible with both. In order to be able to adjust the pulse intensity, it was necessary to regulate the current flowing through the LED. In a simplified approach, this can be done by controlling the LED supply voltage. Additionally, as this system is not intended for continuous wave operation, it is also possible to use currents that are significantly higher than nominal if the pulse duration is short enough. The designed electronics includes a common power switching configuration, similar to those used in [2 – 5] but for higher voltages, shown in Fig. 1. 2. Power control