In modern industrial production, the most dangerous and critical areas requiring protection—specifically, the workpiece processing areas—are equipped with safety doors as passageways between operators and the workpieces. Ensuring these safety doors function effectively as true safety gates is paramount. Different types of safety switches can be installed to achieve the desired safety protection for different opening and closing methods of the safety doors. Due to the high level of danger posed by machine tools, a correspondingly high level of safety protection is required. Therefore, safety switches can be used in conjunction with various safety monitoring devices to achieve a higher level of safety protection. Safety switches are mainly divided into safety door switches and safety limit switches. However, in some hazardous locations, safety light curtains are also necessary. I. Safety Door Switch 1. Functions of Safety Door Switch Safety door switches are mainly installed on the protective doors (or covers) of machine tools and equipment that pose a risk of personal injury. The following functions must be achieved when using safety door switches: (1) Isolation principle (separation principle between people and machines in space): The door is locked before the dangerous movement of the machine stops, and the staff cannot enter the dangerous area; the door is locked before other dangerous factors are eliminated, and the staff cannot enter the dangerous area; the machine cannot be started before the protective door is closed and the hazard source is isolated. (2) Stopping principle (separation principle in time): Closing the movable protective door will not directly start the dangerous action of the machine equipment; the machine must be stopped if the protective door is opened while it is running. 2. Realization of the safety function of safety door switch From a structural point of view, non-safety switches disconnect the contacts by the force applied by the spring, while safety door switches disconnect the contacts by the force applied by the rigid operating lever. In the case of contact welding and spring failure, the contacts that should be disconnected cannot be disconnected, and safety cannot be ensured at this time. The rigid operating lever of the safety door can forcibly disconnect the contacts at this time, thereby ensuring safety. Therefore, in practice, safety switches are often marked with a certified "→" arrow, as shown in Figure 1. By comparing safety door switches and non-safety switches, it can be seen that safety door switches not only achieve their safety function by detecting the opening, closing, and locking of the protective door, but also, with NC contacts linked to a rigid mechanical structure, they possess a guaranteed disconnection function, even if the contacts are fused together. Thus, their safety function ensures that personnel are not harmed by machinery and equipment. 3. International Standard for Safety Door Switches The international standard is IEC 60947-5-1. One section of the general rules concerning low-voltage switches and control devices stipulates: "...guaranteed disconnection ensures that when normally closed contacts are fused, they can be separated by normal operation of the switch, i.e., pressing down the core rod, removing the key, rotating the lever, or pulling the rope. In other words, it guarantees an open circuit even after a circuit fault." 4. Classification of Safety Door Switches Generally, safety door switches are classified into mechanically locked and electromagnetically locked types. Both types of switches have two series: insulated plastic housing and metal housing. The choice of series depends on the environmental conditions, including mechanical load, temperature, and chemical influences. Experience shows that the metal housing series can withstand harsher conditions than the insulated plastic housing series. However, under typical industrial application conditions, the insulated plastic housing series performs better, for example, in various machine tools, industrial robots, printing machinery, papermaking machinery, and packaging machinery industries. Safety electromagnetic switch locks have the following two operating modes. Regardless of the operating mode selected, the maximum tensile force that the safety electromagnetic switch lock may withstand in the locked state must be considered. (1) A safety electromagnetic switch with spring-force locking mechanism locks by means of an internal spring force and unlocks by means of an electromagnetic force generated by energizing an internal electromagnetic coil. If the electromagnetic coil is not energized, the movable protective door will remain locked. In this type of electromagnetic switch lock, the internal spring is a safety spring, and the gap between the spring coils is smaller than the diameter of the spring wire. This avoids damage to the spring and ensures that the spring can achieve a safe locking function. (2) Another working method of electromagnetic switch lock is locking by electromagnetic force. When the electromagnetic coil inside the switch is energized, it generates an electromagnetic force. This electromagnetic force overcomes the spring force and locks the operating part. When the electromagnetic coil is de-energized, the spring will return to its original state, thereby unlocking the operating part. The EN 1088:1996 standard clearly states that electromagnetic switch locks with spring-force locking can be used as safety switches to protect personal safety; while electromagnetic switch locks with electromagnetic force locking can only be applied to a few situations where the machine equipment has been analyzed and confirmed not to cause personal safety, but protection is needed for the machine equipment itself or the material processing process. The reason is obvious: when using an electromagnetic switch lock that locks by electromagnetic force, if the electromagnetic coil fails to be energized due to a problem with the external wiring, the operating element cannot be locked, allowing the movable safety door to be easily opened, and the electromagnetic switch lock loses its safety locking function. Omron's safety door switches (including the D4NS standard safety door switch, D4GS-N slim safety door switch, D4BS safety door switch, D4GL miniature electromagnetic locking safety door switch, D4JL electromagnetic locking safety door switch, D4NL miniature electromagnetic locking safety door switch, D4BL electromagnetic locking safety door switch, D4NH miniature safety hinge door switch, and D40B miniature non-contact door switch) all meet the above requirements or regulations. 5. Characteristics of Safety Door Switches A safety door switch consists of two independent parts: the switch body and the switch operating element. When the safety door is opened, the switch operating element is pulled out from the switch body, at which point the normally closed contact inside the switch will be opened by a mechanical rebound device. When the safety door is closed, the switch actuator inserts into the switch body, at which point the normally closed contact closes, allowing the machine's drive or control system to operate. Under no circumstances should a safety switch with a separate, independent actuator be used as a mechanical stop switch. A safety door switch is designed to be difficult to operate with common tools such as keys, coins, screwdrivers, pliers, or knives, except with specialized devices (keys, etc.). This prevents safety issues caused by misoperation or malicious operation. 6. The application of safety door switches is illustrated by the D4JL electromagnetic locking safety switch. Under normal equipment operation, with the electromagnetic switch ON, the safety door is locked by the locking switch. Even if an operator inserts a key, they cannot open the safety door to enter the danger zone, thus ensuring safety. (D4JL, locking force 3000N). Only when the electromagnetic switch is OFF and a key is inserted can the door be opened, at which point the equipment inside stops operating. If an employee enters the danger zone for equipment inspection or maintenance, even if the door is closed and the person has a key, the electromagnetic switch remains OFF, so the door is not locked. If the electromagnetic switch is accidentally turned ON and the door is locked, it can be unlocked using a special back-release key, thus preventing mechanical damage and ensuring safety. II. Safety Limit Switches 1. Safety limit switches are widely used in industries such as escalators, plastic machinery, automated storage and retrieval systems, printing, and packaging. A safety limit switch is one where the normally closed contact of the switch will definitely open when the operating part is moved to the operating position, providing a safety function. IEC and European standards have corresponding requirements and standards for this type of switch, indicated by symbols (e.g., International Electrotechnical Commission, Germany). Using safety limit switches can prevent malfunctions and accidental operation of ordinary switches, ensuring that when the switch is moved to the open position, the normally closed contact opens, disconnecting the power supply to the equipment and preventing malfunctions and accidents. Our company's safety limit switches D4NA, D4N, D4F, D4B, and D4N-□R all contain a normally closed contact that will definitely open, providing a safety function. Therefore, they are marked and have European CE certification. 2. Classification and characteristics of limit switches (1) Omron safety limit switches are often classified into plunger type (such as D4NA4131), adjustable ball rocker type (such as D4NA-12G), ball plunger type (such as D4NA4132), etc. (2) If the screw is changed, the position of the rocker can be set at any position of 360° in 7.5° units. Since the grooves of the rocker and the rotating shaft are designed respectively, the rocker can be effectively prevented from sliding out of the rotating shaft. In addition, the screw can also be loosened to change the length of the rocker of the adjustable ball rocker type. When installing the rocker in reverse (forward-backward), please remove the screw on the front of the rocker first. At this time, the action range is set within the horizontal 180° range. The appearance of our company's safety limit switch is shown in Figure 3. III. Safety light curtain In modern industrial production, on the one hand, the production process is becoming more and more complex. There are two reasons for this: (1) The diversification and differentiation of customer needs have led to the transformation of factories from large-scale single production to small-batch multi-variety production; (2) The production equipment used to produce products is becoming more and more precise and complex, and is also becoming more and more highly automated, which has led to the complexity of the production process. On the other hand, with the progress and development of society, the humanistic spirit of "people-oriented" has become increasingly popular, which will objectively lead to increasingly strict requirements for safety in laws and regulations. The complexity of the production process has made it increasingly difficult for people to control mechanical safety in conventional safety measures, while improving mechanical safety is an inevitable and urgent requirement of social development. The contradiction between the two is becoming more and more prominent, so people have raised the question: Is it possible to achieve both production efficiency and mechanical safety in production? In practical applications, most safety products have solved this problem well. For example, the safety limit switch in the safety switch plays the role of a conventional limit switch. When connected to the relevant controller, it can quickly understand its technical status during maintenance and repair, which is conducive to shortening the maintenance time and thus improving production efficiency. On the other hand, it must also ensure the personal safety of the operator when the switch fails. Among safety products, safety light curtains are the most representative and typical example of applications that balance safety and production. 1. What is a safety light curtain? When processing parts are continuously fed in and out of high-speed, potentially dangerous machines, openings are necessary. If protective doors are installed at these openings, production will be severely affected, and it will be impossible to balance production and safety. Safety light curtains are ideal for these openings, providing protection through a beam of light. This is a sensing device consisting of rows of through-beam photoelectric switches mounted from top to bottom. It monitors a certain range; when a person enters the machine, the light is cut off, stopping the machine and protecting personnel. However, when processing parts are continuously fed in and out of the light curtain, the machine will not stop. Thus, without affecting production, it effectively protects operators from injury, as shown in Figure 4. 2. Differences between safety light curtains and ordinary photoelectric switches, area sensors, and ordinary light curtains: Ordinary photoelectric switches, area sensors, and ordinary light curtains typically have blind spots. This is mainly because they lack self-diagnostic functions and cannot guarantee safety in the event of a malfunction. Safety light curtains have no blind spots and, in the event of a malfunction, can ensure safety due to their self-diagnostic function. 3. International standards for safety light curtains include IEC 61496-1 (Safety of machinery—Photoelectric protective devices—Part 1) and IEC 61496-2 (Safety of machinery—Photoelectric protective devices—Part 2), as well as other international standards that provide detailed specifications for photoelectric protective devices such as safety light curtains. For example, IEC 61496-2 specifies special requirements for the design, manufacture, and testing of photoelectric protective devices. EN 61496-1, corresponding to IEC 61496-1, explicitly states that its standard applies to area sensors (safety light curtains are a special type of area sensor) that detect the presence of operators through designed circuitry, outputting a control signal for personal protection. It also specifies the following: fault detection performance, software design specifications, heat resistance, EMC performance, vibration resistance and electric shock protection, indicator light colors, label details, and even operation manuals. The corresponding standard prEN61496-2, which corresponds to IEC61496-2, also specifies requirements for the minimum detection object and anti-interference. Therefore, when using light curtains for personal protection, only light curtains that meet this safety standard can be used; others cannot be used arbitrarily. For this reason, manufacturers of safety light curtains often need to undergo third-party safety certification to meet these standards, rather than simply claiming that they produce products that "can" meet the relevant standards. 4. How do safety light curtains balance safety and production efficiency? a. The role of safety light curtains in production: From an installation perspective, installation is convenient (our company has freely positioning fixing components), flexible (several light curtains can be connected in series without blind spots), and saves on wiring, making installation very easy. From a usage perspective, due to the complexity of the production process, the product models produced on-site often change, resulting in different requirements for the safety light curtain settings. Safety light curtains are equipped with flexible and adaptable software, which can easily change the shielding and blanking effects according to various on-site needs of customers, greatly reducing the time spent on re-layout and rewiring. From a maintenance perspective, safety light curtains offer convenience for on-site maintenance and debugging. They feature various indicator lights to show their working and fault status. Several red LED fault cause indicators, through different combinations, quickly identify the cause of the fault and provide solutions. b. Safety characteristics of safety light curtains: From a safety circuit perspective, fault detection capability is the most important criterion for classifying safety light curtains; therefore, light curtain levels are divided into 1, 2, 3, and 4 based on their fault detection capabilities. In addition to a reset function, safety light curtains also have an interlocking function: when the power is on or the light is blocked, the control output is turned off and maintained in this state until a reset signal is input. Some safety light curtains, besides having external testing functions, also have external relay EDM monitoring functions to detect malfunctions in external relays (or contactors) controlling dangerous parts of the machine, such as contact welding. From a standards perspective, safety light curtains are manufactured according to international standards and must obtain third-party certification, not just claim to meet international standards. c. Safety functions of safety light curtains On the one hand, when the AGV (Automated Guided Vehicle) sends the workpiece into the dangerous area for processing, the robot's manipulator continues to work as usual and does not affect its efficiency. On the other hand, when the operator enters, the safety light curtain should be able to detect the operator and control the manipulator to stop its operation, thereby ensuring the operator's safety. Therefore, according to the different ways the AGV enters the dangerous area, the safety functions of the safety light curtain are divided into: (1) Shielding: The shielding function is to temporarily disable the safety function of the safety light curtain, and even in the light-blocking state, the control output can remain ON. This solves the problem of not being able to set up the light curtain because the AGV needs to pass through, and can take into account both safety and productivity. Of course, there are also specific situations such as partial shielding and position shielding. During the shielding process, in order to let people around know that the safety function is temporarily disabled, shielding indicator lights (external indicator lights) can also be used in places where all operating positions can be confirmed to warn. (2) Fixed blanking: disables a specific optical axis of the safety light curtain. In this case, regardless of whether there is an object in the failure area, the control output always remains ON. (3) Floating blanking: When there is a moving object of a certain amplitude that is not to be detected in the detection area, the detection function is disabled. 5. Precautions for using safety light curtains (1) Pay attention to the correct calculation of the safety distance. When setting the position of the safety light curtain, the detection capability of the sensor, the machine stopping time, the speed of human intrusion, etc. should be considered in order to calculate the distance between the danger zone and the setting location using the formula. Formula for calculating the safety distance: (ISO13852～5) Safety distance S=KXT+C Where: K——human intrusion time T——response time of the entire device system C——additional distance calculated from the smallest detected object Insufficient safety distance calculation will lead to safety accidents. (2) When installing and setting multiple sets of safety light curtains, the mutual interference between the light curtains should also be considered. Light curtains with automatic anti-interference capabilities can be used. Our company's safety light curtain has a built-in interference light avoidance algorithm. Even if there are 3 sets of light entering the device, including its own projector, it can prevent malfunctions. It is very suitable for safety light curtains on mobile devices such as AGVs and safety light curtains on fixed devices that cannot prevent interference through wiring. However, when too many safety light curtains are installed, measures must still be taken to prevent mutual interference. These measures include using light shields, changing the direction of incoming light, using setting tools, and maintaining sufficient distance. Of course, regardless of the number of safety light curtains, other anti-interference measures are necessary. For example, when using reflectors, do not set the safety light curtain as a reflective structure (reflectors can be used to change the light path). When installing on highly reflective surfaces such as metal walls, floors, ceilings, and workpieces, the distance from these surfaces should be considered in the user manual. If interference cannot be avoided after taking the above measures, consult the safety light curtain manufacturer or strictly follow the relevant manual. IV. Conclusion By understanding the relevant content of safety switches and safety light curtains, it is foreseeable that as China's status as the "world's manufacturing factory" further strengthens and mechanical safety awareness increases, safety products will play an increasingly important role in China's mechanical safety efforts.