Achieving precision with limit switches

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.

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.

Figure 1: These limit switches are available in a variety of shapes and sizes to fit a wide range of applications.

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.

Figure 2: Prewired limit switches provide easy integration into most industrial environments.

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.

Figure 3: Detecting position of a conveyor lift gate is one of many industrial limit switch applications.

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

  • Actuation method
  • Repeatability
  • Travel to operate and reset
  • Force to operate
  • Contact configuration
  • Environment requirements

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
  • Sensor impact
  • Switching frequency
  • Maximum actuation speed

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.

Figure 4: Precision limit switches can be used to home motion stages, often providing better repeatable accuracy than photo electric and inductive sensors.

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.

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, 1/2000 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

 

 

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