The limit switch plays a key role in automation equipment. Simply put, it tells the control system whether a certain mechanical component is in place through contact or induction. With the increasing level of factory automation, it is particularly important to choose the right model. For example, the equipment suddenly gets stuck, the assembly line stops, and in serious cases, it may even cause industrial accidents. These situations are often related to improper selection.
Our article is mainly to sort out a set of reference standards for engineers when selecting models. Specifically, we will focus on several key factors, such as whether the equipment is linear or rotary, whether there are interference factors such as oil, dust, etc. in the workshop, and how high the detection accuracy is required. These aspects will affect the final type of limit switch to be selected. In particular, we will also involve the problem of how the equipment control system works with the switch, and what safety regulations need to be met, which are often encountered in actual operations.
Select limit switches according to the type of mechanical movement
For equipment that needs to make circular motion (such as machine tool spindles, rotary tables, etc.), commonly used types include cam position switches, non-contact induction switches, angle encoders, etc. Factors to consider when choosing include rotation angle (such as full circle rotation or partial rotation), speed (especially fast rotation requires consideration of whether the switch can respond in time), and mechanical load size (a stronger type is required for situations with high force). For example, in the spindle positioning scenario of CNC machine tools, high-precision encoding devices can achieve very fine position control, which can be as accurate as one percent of a hair.
When the equipment moves in a straight line (for example, the conveyor belt on an automated assembly line), the optional types include mechanical contact travel switches, photoelectric induction switches, or magnetic trigger devices. At this time, it is necessary to focus on the length of the moving distance (short a few millimeters and long may be tens of meters), positioning accuracy requirements (some precision assembly occasions must control the error within 0.01 mm), and whether the working environment has many interference factors such as oil, dirt, and dust. For example, in the assembly line of automobile parts, photoelectric switches do not need to directly contact the measured object, which can avoid the problem of parts loss caused by repeated friction.
In addition, some special working conditions require special treatment: for equipment with particularly strong vibration (such as crushers and stamping machines), it is necessary to choose models with shock-absorbing structures, otherwise the internal parts are easy to loosen and fail; for mechanisms that move back and forth repeatedly (such as the pressure arm of the baler), special attention should be paid to the service life of the switch. Mechanical contact types may have poor contact after tens of thousands of uses. Here is a typical example. When selecting a limit device for equipment such as stamping machine tools, it must not only be able to withstand hundreds of impacts per minute, but also have a special protective design against oil and dust.
Consider the mechanical working environment factors
First of all, we need to pay special attention to the influencing factor of temperature. For example, under high temperature conditions, that is, when the ambient temperature exceeds 80 degrees Celsius, high temperature resistant materials must be used, such as ceramic materials or some special plastics. Such materials need to be able to withstand temperature fluctuations from minus 40 degrees to 150 degrees. For example, when encountering a low temperature environment, special attention should be paid to whether the material will become brittle, and the problem of lubricant freezing. At this time, a lubrication system specially designed for low temperature environments may be required.
Then there are humidity considerations. The most feared situation in a high humidity environment is that the circuit short circuit or the rusting of parts should be selected. At this time, a waterproof and moisture-proof structural design should be selected, such as products that meet the protection standard of IP67. For example, the equipment used in the food processing assembly line should not only use stainless steel materials that will not rust, but also seal the joints.
The main problem caused by the dusty environment is that it is easy to cause equipment jamming or excessive wear. At this time, you need to choose limit switches with better sealing and products with better dustproof ability. For example, mechanical equipment used in mines needs to pay special attention to dustproof design, and usually a special heavy-duty dustproof switch is selected to deal with this situation.
Other issues that need attention include chemical corrosion. At this time, the materials should be corrosion-resistant, such as fluoroplastics or 316 stainless steel, otherwise they are easily damaged by corrosion. For example, in situations where electromagnetic interference is relatively strong, it may be necessary to add a shielding layer to the circuit, or directly use a circuit design solution with strong anti-interference ability.
Determine the limit switch parameters according to the mechanical motion stroke and accuracy requirements
- When determining the stroke range, special attention should be paid to retaining the margin. Generally speaking, it can be understood as adding about 10% to 20% of the redundancy on the basis of the actual required stroke. This part of the margin is mainly used to compensate for the deviations that may occur in the operation of the machine, such as metal expansion caused by temperature changes. For example, equipment such as printing presses need to take into account the possible fluctuations in paper thickness and the position offset caused by wear of mechanical parts after long-term use.
- The selection of the parameter of contact accuracy needs to be adjusted according to the specific application scenario. For use environments with higher requirements, such as occasions such as semiconductor chip manufacturing equipment that require extremely high precision, precision components such as micro switches are usually required, and their accuracy can reach about plus or minus 0.005 mm. In actual applications, it is found that if it involves nano-level positioning requirements, non-contact detection devices may be required to meet the requirements.
- When it comes to the problem of repeated positioning accuracy, this parameter can be understood as the stability of the switch after repeated use. At this time, a life test is needed to check its attenuation. For example, after the switch is operated continuously for 100,000 times, its positioning deviation is tested to see if it exceeds the allowable range. Based on experience, it is recommended to choose models that have been verified through long-term testing, especially products with a nominal mean time between failures of more than 100,000 times, which are more reliable. This indicator is particularly important in the application scenario of welding robots on automobile production lines.
Evaluate the compatibility of limit switches with mechanical control systems
Electrical interface matching
For example, it is necessary to ensure that the voltage parameters (such as the common 24V DC or 220V AC) and current values are consistent with the equipment control system. Here, attention should be paid to the selection of signal output type, such as NPN or PNP. For example, in a PLC control system, you may need to choose a model that complies with the International Electrotechnical Commission standard interface.
Communication protocol adaptation
Give priority to those types that support industrial common protocols, which can reduce the trouble of system integration. For example, in the factory Internet of Things scenario, you may need to use an intelligent limit device that can connect to the OPC UA communication framework. One thing that needs to be noted here is that there may be protocol conversion problems between devices of different brands.
Software-level coordination
Pay special attention to whether the switch product provides a driver module that matches the existing system, or opens an API docking channel. For example, in a monitoring system such as SCADA, you may have to choose products that are compatible with OPC DA/UA communication specifications. The experience here is that software version compatibility is often easily overlooked but has a great actual impact.
Select limit switches according to mechanical safety requirements and reliability requirements
During the mechanical safety configuration process, the selection of limit switches needs to focus on two dimensions: safety level and reliability index. When making specific choices, you should refer to the relevant regulations on mechanical safety, such as international standards such as ISO 13849 or EN 954-1. In short, you should choose models that have passed safety level certifications such as PLe or SIL3. Taking common industrial robots as an example, they are usually equipped with limit switches with dual-channel structures. This design is mainly to achieve the protection effect of double insurance.
Reliability evaluation mainly depends on three factors: brand qualification, test data and environmental adaptability. Generally speaking, you need to choose brands that have passed common certifications such as TÜV and UL, and ask the supplier to provide a complete MTBF test report. For example, in the equipment of nuclear power plants, it is usually specified to use limit switches with an MTBF value of more than 500,000 times.
Certification compliance needs to pay attention to two levels: international access certification and environmental regulations. It is necessary to check whether the product has undergone basic certifications such as CE and UL, and at the same time confirm whether it meets environmental protection requirements such as RoHS. For example, mechanical equipment exported to the European market must be equipped with limit switch components with CE marks according to local regulations.
Conclusion
In the specific selection process, the dimensions that need to be focused on actually involve the synergistic relationship between multiple system parameters. For example, it is necessary to consider the basic characteristics of the movement trajectory of the equipment itself, such as whether it is linear or rotary, whether there are common situations such as dust accumulation or oil erosion in the working environment, and whether the allowable range of detection error is within the specific numerical range of ±1mm. In addition, there is the matching degree of the control system signal type and whether it meets the requirements of industry specifications such as GB/T 14048. By systematically evaluating each parameter index in this way, the stability and safety factor of equipment operation can be improved.
From the perspective of development trends, with the advancement of Industry 4.0 and intelligent manufacturing, such products may become smarter and more network-connected. For example, limit switches with IoT functions can now remotely view the status and discover potential faults in advance. For users, you can pay more attention to new products that combine wireless transmission modules with fault self-checking functions, just like we can install an APP on our mobile phones to watch the camera at home, which will significantly help reduce the unexpected downtime of equipment.
