There are many choices for detecting presence and position in machine automation applications, but limit switches are often the best option when high precision is required.
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By Tim Wheeler, Technical Applications Engineer, AutomationDirect
Many types of sensors are available for detecting presence and position in industrial automation applications, from course to fine precision. Photo eyes and inductive switches are commonly used, but when a high degree of accuracy is needed, precision limit switches are often a better option because they provide repeatable accuracy down to the sub-micron level.
Industry workhorse
Industrial limit switches are the battleships of the sensor world and have been used in manual, semi-automatic and automated machinery applications for more than 100 years to detect presence and position of parts and mechanisms. These sensors come in several configurations including NEMA, heavy-duty IEC, double-insulated IEC, compact, miniature and precision, Figure 1.
The major difference between NEMA and IEC limit switch is the robustness, a common distinguisher in these comparisons. The NEMA is designed for demanding applications such as in heavy machinery, foundries and mining, and tends to be more expensive. In applications such as material handling and gantry equipment, a similar but more cost-effective heavy-duty IEC limit switch is usually suitable. Both the NEMA and heavy-duty IEC housings are typically made of metal, die-cast zinc alloy or aluminum depending on the device.
Double-insulated IEC limit switches are typically made of plastic, and the compact IEC switches are made of plastic or metal. Many of the precision touch limit switches are made of stainless steel. Precision limit switches have a barrel design, similar to a very small (M5 x 0.5) proximity switch. These stainless steel, plunger actuation switches have a high degree of repeatable accuracy.
It is worth mentioning that safety switches are used to protect personnel and equipment from hazards by monitoring the position of movable guarding. They are often used on hinged guard doors where an actuator key engages the switch body when the guard is closed. Cable-operated safety switches are also used to activate an emergency stop when the cable is pulled.
Key selection considerations
Limit switches are suitable for use in a wide range of applications and harsh environments on the factory floor due to their ease of installation, reliable operation and rugged design, Figure 2. Limit switches are typically used in physical contact applications that cause wear and tear on the switch actuator and electrical contacts, so exceeding two operations per second should be avoided. When selecting a limit switch, consider the application and actuation method first, as often they are the clearer determining factors.
Other sensing options include inductive proximity switches and photo eyes, both of which are a touch-free method to sense position and presence. An inductive proximity (prox) switch detects ferrous and non-ferrous metals with no mechanical contact needed. A prox switch typically has a solid-state output, instead of mechanical contacts as with a limit switch. Its non-contact sensing method and solid-state output make it a good choice for high cycle-rate applications, and when debris may interfere with limit switch actuation.
However, there are many quality and long-lasting limit switches available. With a properly specified limit switch, it's not unusual for it to have a mechanical life of 30 million actuations, and an electrical life of 5 million operations. The limiting factor is often electrical contact life, but replacement contact blocks are available for quick replacement at a low cost with some limit switches.
NEMA and heavy-duty IEC limit switches are both suitable for used in harsh environments. Foundries, shipping and dockside operations are common applications. They are also found in machines in industries such as automotive, food and beverage, and pulp and paper, and power. These switches also work well on large conveyors and welding equipment.
Limit switches are also used in more medium-duty applications such as earth moving equipment, agricultural machinery, and farm and tractor implements. Other medium-duty applications include CNC machine tools, overhead hoists, large cranes, and textile and printing machinery, Figure 3.
Limit switches are often used in consumer grade machines and equipment such as escalators and elevators, industrial automatic doors, aircraft access platforms, point-of-sale dispensing kiosks, scissor lifts and slot machines. With careful design, limit switches work well in most of these applications, and are easily interfaced to programmable logic controllers.
Limit Switch Selection Criteria
The leading limit switch selection criteria are listed in Table 1 and explained below.
Table 1, Limit switch selection criteria
Limit switches are actuated several ways including side rotary, top and side push, and wobble stick. These actuators are often mounted onto 90° adjustable heads. Travel to operate contacts and the amount of force needed varies, and should be determined with a high degree of certainty before selecting a switch.
Levers are adjustable to any angle on the operating shaft and need around 5° rotation, from a total of 90° travel or more, in either direction to operate contacts. Typical actuator types include various length stainless steel levers with nylon or metal rollers. Spring stainless steel rods (whiskers), and loops of nylon or metal cable, are available as well.
Push-operated (plunger type) limit switches are top or side actuation in a pushbutton or roller style to operate contacts, and require 2 mm or less travel. Wobble head limit switches are typically top operated using different rod-type operators such as a nylon rod, stainless steel rod or spring steel'and will operate with about 10° of actuation, perpendicular to the rod, from any direction.
The repeatability of an assembled limit switch is determined by the type of operating head used. The switches with the best repeatability are the direct-operated pushbutton or plunger type, typically 0.003-in. or less repeatable accuracy. The addition of rollers and operating levers adds tolerance stickup due to concentricity. The tolerance can easily double when rollers are used.
Limit switch operation starts at an initial, normal position at rest. When operated they move through a pretravel range to an operating point, where the electrical contacts close, and then into the overtravel range. The travel to operate and the travel to reset are different. There is hysteresis in limit switch actuation. It may take 1 to 2 mm, or 5° to 10° of pretravel, to operate a limit switch's contacts, but only half of that to reset upon return, but precision limit switches often have zero pretravel.
The force to operate a limit switch varies widely depending on the device. Many heavy-duty switches require 4 lb or more to operate the contacts. Small, precision limit switches may only take 0.1 lb to operate.
Many limit switches have single-pole, single-throw electrical contact configuration, either normally open or closed. Some have more contact configurations available. Snap-action contacts are available where the contact motion is independent of the speed of the actuator. The contacts will still close quickly even with very slow-moving actuators. Alternatively, slow-make/slow-break contacts are available where contact motion is dependent on actuator speed.
A limit switch will operate reliably in extreme environments; at high or low temperatures; and in moist, wet or contaminated areas. Degree of protection varies from IP40 to IP67/NEMA/UL. Common enclosure Types are 1, 3, 3S, 4, 4X, 6 and 6P.
Design considerations
Limit switch design considerations listed in Table 2 and can affect the operation and reliability, but careful design can mitigate potential problems.
Table 2, Limit switch design considerations
Mechanical life of a limit switch can be extended by adhering to these guidelines. Limiting the severity of the impact of the target material on the limit switch operator is important, as is limiting the overtravel of the switch during operation as much as possible. There is no need to rotate or push the switch actuator more than necessary for reliable actuation, and levers are adjustable to any angle on the operating shaft to minimize impact and overtravel. Pressing a limit switch plunger to the stroke end may cause malfunction due to the impact, but this can be mitigated by installing a hard stop.
Some limit switches allow right-angle operation to the plunger with an appropriate lead in cam angle, often 30°. However, many limit switches require contact with the object at a right angle or within a few degrees. Action is limited between the tip end and the edge of the internal bearing, but side loads can cause damage to the switch over time. The end of the housing may deform if impacted, causing failure in the return.
Shock and vibration specifications should be carefully considered during design. When severe, it could operate the switch and cause a fault. Mounting a limit switch out in the open where it can be accidently impacted, placing a side load on the axis of rotation, or causing overtravel of the plunger should be avoided.
As discussed previously, switching frequency must be less than 2 cycles per second as mechanical and electrical activations are limited. The actuation speed of a limit switch can be too fast or too slow as well. If activated too fast, the switch could bounce or wear quickly. Slow activation, less than 50 mm/minute, can cause repeatability errors, especially in precision limit switches.
Precision Limit Switches
Traditional limit switches, proximity sensors and photo eyes are somewhat limited in terms of accuracy and repeatability, typically in the 25- to 100-µm range depending on the device. However, there are some cost-effective and ultra-precise mechanical limit switches with repeatability in the 0.5- to 10-µm range, Figure 4. Considering a human hair is typically 50- to 60-µm in diameter, that's precise sensing.
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These precision limit switches are often used to replace fiber sensors used for positioning parts. When using a photo eye, uneven surfaces, water droplets and even fingerprints on the part can make precision position sensing less accurate. However, since the precision limit switch contacts the part, it minimizes the effects of an uneven surface, and can push through a water drop or fingerprint.
In a sheet thickness application, such as detecting a double-fed label, designers may consider limit switches unsuitable for detection of an extra label due to low repeatability. However, newer high precision touch and tool setter switches are capable of 0.5 µm repeatability, and can reliably detect 50 µm or thinner sheets. Take-up material can be checked to ensure the label was removed.
These precision limit switches can be used in harsh environments including CNC machine tools with coolant spray and metal shavings, and in motion control and robotic automation. In these applications, the switches are typically used to find a home position, an edge of the tooling, or an edge on a part. Applications include finding the soldering tool tip to adjust the z-axis on a robot, homing x-y tables, and detecting the edge(s) of a grinding wheel in a CNC grinding application
On automated machines and robots, there are many sensors such as vision systems, lasers and photo eyes. Finding the position of the tooling or part is often critical to operation. Precision limit switches are an excellent option to find a repeatable home or start position. Finding a position within 0.5 micron, 1/ of a millimeter, with a mechanical switch adds the word precision in front of machining and assembly.
Limit switches are tried and true in machine automation applications, and you have probably used them before. Don't run a switch too hard or fast, and use the correct activation method. And don't forget about the precision limit switch. With repeatability starting well under one thousandth of an inch (10 µm), and down to sub-micron repeatable accuracy, finding precise position has never been easier.
Automation Direct
Automationdirect.com
All figures courtesy of AutomationDirect
Limit switches are electromechanical devices that play a crucial role in regulating machinery. They are essential for ensuring operational safety and efficiency in industrial environments. Limit switches are a contact proximity sensor device that consists of an actuator mechanically linked to a set of output contacts. When an object comes into contact with the actuator, the limit switch sensor operates the contacts to make or break an electrical connection. Limit sensor switches are vital in preventing machinery malfunctions and accidents, thus maintaining smooth and safe operations.
Limit switches are made up of three key components:
Roller Lever Limit Switch: The roller lever limit switch features a lever with a roller at the end, designed to detect objects in motion. Common applications include conveyor belts and automated systems where the limit switch types must withstand frequent contact.
Plunger Limit Switch: The plunger limit switch is ideal for space-constrained environments due to its compact, straightforward design. It is commonly used in machinery where direct, linear contact is required.
Whisker (or Cat Whisker) Limit Switch: The cat whisker limit switch is highly sensitive, making it suitable for detecting light objects or precise movements. It is often used in delicate applications such as robotics or small-scale automation.
Forked Limit Switch: The forked limit switch operates by detecting objects passing through its forked opening, making it perfect for conveyor systems and assembly lines where precise positioning is crucial.
Rotary Limit Switch: The rotary limit switch converts rotational motion into electrical signals, ideal for applications involving rotating machinery, such as wind turbines and cranes.
Heavy Duty Limit Switch: The heavy duty limit switch is robust and designed for harsh industrial environments, such as mining and heavy manufacturing, where durability and reliability are essential.
Adjustable Limit Switch: The adjustable limit switch allows for precise calibration and positioning, making it suitable for applications requiring exact control, such as CNC machines and automated assembly lines.
Safety Limit Switch: The safety limit switch enhances machinery safety by ensuring critical operations stop immediately upon detecting unsafe conditions, crucial in protecting workers and equipment.
Miniature Limit Switch: The miniature limit switch is compact and ideal for use in electronics and light industries, where space is limited, and precision is required.
Push Button Limit Switch: The push button limit switch is convenient and easy to use, often employed in control panels and user interfaces where manual operation is frequent.
Limit switches are available with a wide range of actuator types. The type of actuator used can be determined by the mounting position, force required and objects to be detected.
A Single Pole Double Throw (SPDT limit switch) has one input and two outputs, allowing it to switch between two circuits. This configuration is useful in applications where a single input needs to alternate between two outputs, such as in basic light control or toggling between two states. For instance, an SPDT limit switch is ideal for a conveyor system where a single switch needs to alternate the direction of the conveyor belt by switching the motor's power supply between forward and reverse circuits.
In contrast, a Double Pole Double Throw (DPDT limit switch) has two inputs and four outputs, functioning like two SPDT switches operated by a single actuator. It can control two separate circuits simultaneously, making it suitable for more complex operations requiring simultaneous switching of multiple connections. A DPDT limit switch is preferable in situations where a machine needs to simultaneously change the direction of two motors, such as in a robotic arm that requires precise coordinated movement of multiple joints.
Limit switches are critical in various industries, including manufacturing lines, packaging plants, and robotics. In manufacturing lines, limit switches ensure machinery operates safely and efficiently by detecting the presence or absence of objects, thereby preventing collisions and maintaining the workflow.
For example, they can stop a conveyor belt when an item reaches the end of the line or control the positioning of machine tools. In packaging plants, limit switches play a crucial role in automating packaging processes. They can detect the position of packages, ensuring accurate labeling, filling, and sealing operations. This automation improves precision and reduces manual labor, enhancing overall productivity.
In the realm of automation, limit switches perform specific tasks such as stopping a conveyor belt when an object is detected at a certain point, controlling the movement of a machine arm to pick and place items accurately, and ensuring the correct alignment of components in assembly lines. These applications highlight the importance of limit switches in maintaining safety, efficiency, and reliability in industrial operations.
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