At present, surgical lights generally use annular 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 annular energy-saving lamps and are an upgraded product of annular energy-saving lamps. At present, the advantages of LED shadowless lamps are gradually understood and accepted by microscope users. The cost of use is also lower than that of circular energy-saving lamps, reducing the trouble of replacing circular lamps every 1 to 2 months.
At present, problems such as packaging, constant current drive, heat dissipation and illumination control of high-power LEDs have been successfully solved technically, and multi-level control of shadowless lamp illumination can be achieved, with flexible and convenient dimming. The advantages of LED shadowless lamps are:
(1) Excellent cold light effect: Using a new type of LED cold light source as surgical lighting, it is a true cold light source, and there is almost no temperature rise in the doctor's head and wound area.
(2) Good light quality: White LED has color characteristics that are different from ordinary surgical shadowless light sources. 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. Various tissues and organs of the human body are easier to distinguish in the flowing and penetrating blood. This is something that ordinary surgical shadowless lamps do not have.
(3) Stepless adjustment of brightness: Digital method is used to steplessly adjust the brightness of the LED. The operator can adjust the brightness at will according to his own adaptability to the brightness, so that it can achieve the most ideal comfort level, so that eyes that work for a long time are less likely to feel tired.
(4) No flicker: Because the LED shadowless lamp is powered by pure DC, there is no flicker, which will not cause eye fatigue and will not cause harmonic interference to other equipment in the work area.
(5) Uniform illumination: using a special optical system, 360° uniform illumination on the object being observed, no ghost shadows, and high definition.
(6) Long life: The average life of LED shadowless lamps is long (35 000 h), which is much longer than that of ring energy-saving lamps (1 500~2 500 h). The life span is more than ten times that of energy-saving lamps.
(7) Energy saving and environmental protection: LED has high luminous efficiency, is impact resistant, not easily broken, has no mercury pollution, and the light it emits does not contain radiation pollution from infrared and ultraviolet components.
1 System structure
1.1 Overall structure
Since the luminous efficiency of a single LED cannot meet the requirements of illumination intensity and uniformity of surgical shadowless lamps, multiple LEDs need to be used to form an array, and the arrangement must be reasonable and evenly distributed so that the illumination of each target plane meets the requirements. This determines the number of LEDs and the distribution rules. Although LEDs have high luminous efficiency and generate little heat, so many LEDs in a sealed box will still generate a lot of heat, and the temperature may even be as high as 60°C to 80°C. Therefore, the radiator and heat dissipation channels must be reasonably designed to effectively reduce the system thermal resistance.
The LED surgical shadowless lamp is composed of multiple lamp heads in the shape of petals, fixed on a flat-arm suspension system, with stable positioning, capable of vertical or circular movement, and 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 to form 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 it will have less impact on the operation. Each group is independently driven by a LM3402 constant current, and accepts the PWM pulse width adjustment control of a microprocessor (CMU) P89LPC932 according to user needs. It can be adjusted steplessly. The current flowing through each LED is about 120~320 mA.
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 cavity mirror is required for ambient lighting, leaving only 3 LEDs in one channel for lighting, and turning off the remaining LED strings; pressing the off button will turn off all 8 LED strings, and pressing the button again will return to the original illumination display state. state, power failure or restart can also return to the set illumination state; the brightening and dimming buttons are used to change the illumination of the shadowless lamp, corresponding to 8 indicator lights, indicating 8 levels of brightness. Each additional level from left to right will add an indicator LED to light up. When the shadowless lamp is in the off state, the two rightmost indicator LEDs will light up. All operating 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. It mainly consists of an MCU P89LPC932 and 18-channel LM2402 constant current voltage stabilizing circuit. It also contains RS485 communication circuit.
P89LPC932 is a low-power MCU produced by Philips. The power supply voltage is 3.3 V. It can operate with low power consumption. It is suitable for many occasions that require high integration and low cost, and can meet various performance requirements. P89LPC932 adopts a high-performance processor structure, and the instruction execution time only takes 2 to 4 clock cycles, which is 6 times that of the standard 80C51 device. In addition, P89LPC932 also integrates many system-level functions, which can greatly reduce the number of components, circuit board area and system cost. There are 2 timers inside and can be used as a PWM generator with 256 timer clock cycles.
LM3402 is a step-down voltage regulator derived from a controllable current source. It can drive series-connected high-power, high-brightness LED strings and can accept input voltages ranging from 6 V to 42 V. When using the pin-compatible LM3402HV, the input voltage can be capped at 75 V. Adjustment of the converter's output voltage maintains a constant current level through the LED array based on the surgical lighting needs. Figure 2 is a schematic diagram of a typical application circuit of LM3402, where RSNS is the current setting resistor, the average current IF≈0.2/RSNS, and the value of RON is related to the number of LEDs in the light-emitting diode string. When there are 8 LEDs, the value can be 300 kΩ. After testing, when the nominal constant current value is 250 mA (RSNS=0.8 Ω), the current fluctuation is within ±20 mA, which can fully meet the surgical light sensing requirements.
1.4 Security Measures
Considering the particularity of the safety requirements of medical instruments, corresponding safety measures should be considered in every aspect 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) False interference signals must be eliminated, so the entire system uses complete electrical isolation to prevent the mutual influence of various parts of the circuit. In addition, the Modbus redundancy check method is used; (3) High-brightness white LEDs are more expensive. In order to avoid damage, the impact of grid and power supply damage on the system must be eliminated. Therefore, this article uses over-voltage and over-current 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 LEDs.
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