New LED surgical shadowless lamp

Shadowless lamps are an indispensable and important device for illuminating surgical sites. They are required to be able to optimally observe objects of different depths, sizes, and low contrast in incisions and body cavities. Therefore, in addition to being "shadowless", they also require uniform illumination and good light quality, and the ability to distinguish the color difference between blood and other tissues and organs in the human body. In addition, shadowless lamps must be able to work continuously for a long time without emitting excessive heat, because overheating can cause discomfort to the surgeon and dry the tissues in the surgical area [1].
Currently, surgical lamps generally use ring-shaped energy-saving lamps or halogen lamps. However, with the continuous development of light-emitting diode (LED) technology, especially the development of high-brightness white light LEDs, LED shadowless lamps have completely solved the inherent defects of ring-shaped energy-saving lamps and are an upgraded product of ring-shaped energy-saving lamps. At present, the superiority of LED shadowless lamps has gradually been understood and accepted by microscope users. The cost of use is also lower than that of ring-shaped energy-saving lamps, reducing the trouble of replacing ring-shaped lamp tubes every 1 to 2 months.
At present, the technology has successfully solved the problems of high-power LED packaging, constant current driving, heat dissipation and illumination control, and can realize multi-level regulation of the illumination of shadowless lamps, with flexible and convenient dimming. The advantages of LED shadowless lamps are [1-2]:
(1) Excellent cold light effect: The new LED cold light source is used as surgical lighting, which is a real cold light source. There is almost no temperature rise in the doctor's head and wound area.
(2) Good light quality: White light LED has a color quality characteristic that is different from that of ordinary surgical shadowless lamps. It can increase the color difference between blood and other tissues and organs of the human body, making the doctor's vision clearer during surgery. The various tissues and organs of the human body can be more easily distinguished in the flowing and infiltrating blood. This is not available in ordinary surgical shadowless lamps.
(3) Stepless brightness adjustment: The brightness of the LED is steplessly adjusted by digital means. The operator can adjust the brightness at will according to his or her own adaptability to the brightness, so that it can achieve the most ideal comfort and prevent the eyes from getting tired after working for a long time.
(4) No flicker: Because the LED shadowless lamp is powered by pure DC, it has no flicker and is not easy to cause eye fatigue. It also does not produce harmonic interference to other equipment in the work area.
(5) Uniform illumination: Using a special optical system, the object being observed is evenly illuminated at 360°, without any shadows, and with high clarity.
(6) Long life: The average life of the LED shadowless lamp is long (35,000 h), which is much longer than that of the circular energy-saving lamp (1,500~2,500 h), and the life is more than ten times that of the energy-saving lamp.
(7) Energy saving and environmental protection: LED has high luminous efficiency, is impact-resistant, not easy to break, has no mercury pollution, and the light it emits does not contain infrared and ultraviolet radiation pollution.
1 System structure
1.1 Overall structure
Since the luminous efficiency of a single LED cannot meet the requirements of the illumination intensity and uniformity of the surgical shadowless lamp, it is necessary to use multiple LEDs to form an array, and the arrangement and distribution must be reasonable and uniform so that the illumination of each target plane meets the requirements, thereby determining the number of LEDs and the distribution rules. Although LEDs have high luminous efficiency and generate less heat, so many LEDs in a sealed box will still generate a lot of heat, and the temperature may even reach 60℃~80℃, so the heat sink and heat dissipation channel must be reasonably designed to effectively reduce the thermal resistance of the system.
The LED surgical shadowless lamp is composed of multiple lamp heads in the shape of petals, fixed on the balance arm suspension system, with stable positioning and vertical or circular movement, which can meet the needs of different heights and angles during surgery. The entire shadowless lamp has 144 high-brightness white LEDs, 8 of which are connected in series in a group, which is called a high-brightness light-emitting diode string HBLEDs (High Brightness Lighting Emitting Diode), and 18 groups are connected in parallel. Each group is independent of each other. If one group is damaged, the others can continue to work, so the impact on the operation is small. Each group is driven by a LM3402 for constant current, and according to user needs, it is controlled by a microprocessor (CMU) P89LPC932 for PWM pulse width modulation, which can be adjusted steplessly. The current flowing through each LED is about 120~320 mA. The electrical schematic diagram of the LED shadowless lamp is shown in Figure 1.

1.2 Operation
panel There are 4 buttons (ambient lighting, off, brighten and dim buttons) and 8 LED indicators on the operation panel. Pressing the ambient lighting button means that only the laparoscope ambient lighting is needed, leaving only 3 LEDs in one channel for lighting, and turning off the remaining LED light-emitting diode strings; pressing the off button will extinguish all 8 LED light-emitting diode strings. Pressing this button again will return to the original illumination display state, and it can also return to the set illumination state after power failure or restart; the brighten and dim buttons are used to change the illumination of the shadowless lamp, corresponding to 8 indicators, indicating 8 levels of brightness. From left to right, an additional indicator LED lights up for each additional level. When the shadowless lamp is in the off state, the 2 indicator LEDs on the far right light up. All operation information is transmitted to the driver board MCU through the RS485 bus.
1.3 Driver board
The driver board is the core of the entire shadowless lamp controller, mainly composed of an MCU P89LPC932 and 18 LM2402 constant current voltage regulator circuits, and also includes RS485 communication circuits.
The P89LPC932 is a low-power MCU produced by Philips. It has a power supply voltage of 3.3 V and can run at low power. It is suitable for many occasions that require high integration and low cost, and can meet various performance requirements. The P89LPC932 uses a high-performance processor structure, and the instruction execution time is only 2 to 4 clock cycles, which is 6 times that of the standard 80C51 device. In addition, the P89LPC932 also integrates many system-level functions, which can greatly reduce the number of components, circuit board area and system cost. It has two internal timers and can be used as a PWM generator with 256 timer clock cycles [3].
The LM3402 is a step-down regulator derived from a controllable current source. It can drive a series of high-power, high-brightness light-emitting diodes and can accept an input voltage range of 6 V to 42 V. When using the pin-compatible LM3402HV, the upper limit of the input voltage can reach 75 V. According to the needs of surgical illumination, the output voltage of the converter is adjusted to maintain a constant current level through the LED array. Figure 2 is a typical application circuit diagram of LM3402, where RSNS is the current setting resistor, the average current IF≈0.2/RSNS, and the RON value is related to the number of LEDs in the LED string. When there are 8 LEDs, the value can be 300 kΩ. After testing, when the constant current nominal value is 250 mA (RSNS=0.8 Ω), the current fluctuation is within ±20 mA, which can fully meet the light sensitivity requirements of surgery [4].

1.4 Safety measures
Considering the particularity of medical instrument safety requirements, corresponding safety measures should be taken in every link of the system. First of all, the operating room is an environment with strong electromagnetic interference. It is very important to prevent the MCU from crashing. Therefore, the following measures must be taken: (1) The hardware reset circuit design and internal reset program must be carefully handled; (2) The error interference signal must be eliminated, so the entire system adopts complete electrical isolation to prevent the mutual influence of various parts of the circuit. In addition, the Modbus redundancy check method is also used; (3) The price of high-brightness white LEDs is relatively high. In order to avoid damage, the influence of power grid and power supply damage on the system must be eliminated. Therefore, this paper adopts overvoltage and overcurrent automatic protection circuits. When the voltage or current exceeds 20% of the set value, the system automatically cuts off the power supply to ensure the safety of the system circuit and high-brightness LED.
2 Program design
2.1 Program structure
The program mainly includes two independent programs: the panel controller and the driver board. The panel controller program sends switch or illumination adjustment commands to the driver board according to the input status of the 4 buttons, and displays the command status on the 8 LEDs. After the driver board program is initialized, it mainly receives operation information through the serial port interrupt, performs Modbus data redundancy check, and then sends different PWM signals according to the command instructions. The driver board program flow chart is shown in Figure 3.

2.2 PWM Generation
The current of the high-brightness LED string is mainly regulated by PWM on the DIM port of the LM3402. The proportion of the actual current to the set current value depends on the duty cycle of the PWM. The PWM output of the internal timer T0/T1 of the P89LPC932 occupies the same pin as the count input and timer trigger output, and the port output is automatically triggered when a timer overflow occurs. This function enables timers 0 and 1 respectively through the control bits ENT0 and ENT1 in the AUXR1 register. When this mode is turned on, the output of the port is logic 1 before the first timer overflow. In order for this mode to take effect, the C/T bit must be cleared to select PCLK as the clock source for the timer. The reference program for timer initialization setting is as follows:
void Timer1_init(void)
{
TMOD|=0x20; //Timer works in mode 6
TAMOD|=0x10;
TH1=256-n; //n is the duty cycle
AUXR1|=0x20; //Timer enable
TR1=1; //Start timer
}
Among them, the duty cycle duty cycle=256-TH1, the overflow of timer 1 will cause the P1.2 or P0.7 port to flip, so the output frequency is 1/2 of the overflow rate of timer 1.
2.3 Energy saving mode
Energy consumption control is of great significance in the entire shadowless lamp control system. Most of the time, HBLEDs are off and the control system is in standby mode, reducing power consumption to a minimum. When the power control register PCONA is set to 0xFF, the external function module is powered off; when the power control register PCON is set to 03H, the MCU can be completely powered off. It can only be awakened when an interrupt is triggered, and then the external function module is powered on and the MCU starts working. The MCU on the driver board is awakened by the serial port receiving interrupt, and the MCU on the panel is awakened by the keyboard interrupt. The keyboard interrupt reference program is as follows:
void KEY_ISR() interrupt 7
{
key_push=1; //Key pressed flag
PCONA=0x00; //External function module power-on
KBCON=0x00; //Clear keyboard interrupt flag
}
The annual market demand for shadowless lamps in China is more than 20,000 units. At the same time, due to the demand for low-carbon economic development, energy-saving transformation is required, so the market promotion prospects of LED shadowless lamps are very broad. The LED shadowless lamp technology introduced in this article has been put into mass production in a well-known medical device company in Jiangsu, and has achieved good social benefits.
References
[1] Zhu Gendi, Liu Minjuan. Research on the illumination characteristics of a new type of surgical shadowless lamp [J]. China Medical Device Journal, 2009(3):183-187.
[2] Zhu Yue. Quality requirements and market forecast of surgical shadowless lamps [J]. China Medical Device Information, 1999(4):44-47.
[3] Guangzhou Zhou Ligong Microcontroller Development Co., Ltd. P89LPC932A1 Flash microcontroller user guide [Z]. 2001.
[4] RICHARDSON C. LM3402/02HV demo board [Z]. National Semiconductor Corporation, June, 2007.