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Improving Safety in Machine Control Systems with Limit Switches

Technology Overview

Control Systems Imagine a product being moved on a conveyor belt that somehow gets out of place. Not having a limit switch at the edge of the belt to stop the process could cause damage to the product and equipment or pose a safety hazard to the operator and facility. That is the risk for machine control systems with moving mechanical parts. We will discuss different types of limit switches and how they are critical components that serve many purposes including safety and mechanical.

The need for limit switches was identified over 100 years ago. The first “Automatic Transformer cut-out” was patented in 1906 and is essentially the same as many modern limit switches that are still in use today. There is a wide range of necessary uses for limit switches. Limit switches can be found in power tools and consumer products. A chainsaw has a limit switch that stops the blade from moving as soon as the operator’s hand is no longer holding the on switch in place. Similarly, a limit switch on a power garage door will stop the motor when the door reaches the fully open or closed position.

Limit Switch Components Will Vary Primarily by 6 Specifications:
  1. Actuator type
  2. Dimensions
  3. Environmental requirements
  4. Materials
  5. Electrical specs
  6. Receptacle / mounting configuration

Actuator Type

Control Systems All mechanical limit switches have some type of physical mechanism that responds to motion to cause the electrical instruction to stop the machine. The simplest type is a plunger actuator, whereby depressing the protruding plunger by a specified travel distance separates an electrical contact to turn the machine off (conversely, that plunger motion could also make an electrical connection that activates a safety step). There are many types of motion to detect for safety purposes, so there are many actuator types apart from the common plunger type. These include rotary, hinge, rolling, and lever types. Other actuator mechanisms apply to any conceivable motion type, and it is important to match the type of motion to the actuator type. A thorough understanding of all possible motion on the machine is necessary to make the best actuator selection.

Dimensions

The size of a limit switch is normally driven by the application, mainly the space available, the size of the machinery, and the actuator travel distance. For complex machinery, the space available for a limit switch might be small. The applications could include electronics assembly, printing, industrial valves, and chemical processing. In these cases, miniature sized limit switches could be used. Larger form factor limit switches are possible in applications with larger scale machinery, such as mining, agriculture, energy, construction, and transportation. Often a larger limit switch is required for mechanical robustness in these applications in response to the motion of larger, heavier parts. Another important dimension is the actuator travel distance. This depends on the spatial range of motion being detected or the variability in part size. The precision of the limit switch integration into the machine control system is another factor in defining travel distance specifications.

Environmental Requirements

A key consideration in limit switch design is the environment in which it operates. If there is significant contamination (dirt, sand, particulate matter, etc.), then a tight seal around the switch mechanism is required. Other environmental considerations include the operating temperature range, storage temperature range, humidity, shock, and vibration. A high-tech factory would have different requirements for these factors compared to an oil rig, for example. There are typically families of limit switches designed for different sets of environment application requirements, with features that mitigate the challenges of specific applications. Options include a more robust design, higher strength materials, special surface finishes in corrosive environments, or reinforced mounting.

Materials

The material specifications for limit switches are also typically influenced by the application. Considerations include corrosion resistance and operating temperature range. To determine the correct limit switch, you will need to consider the materials of three main components: the metal contact inside of a switch, the movable parts of the mechanisms, and the housing and cases.

The most common materials used in the metal contact inside of a limit switch are:
  1. Gold: often used due to its excellent corrosion resistance compared to other metal contact options. One drawback is that gold is relatively soft, so it can be deformed or adhere under large loads.
  2. Gold Alloys: gold-silver or platinum-gold-silver can be used for larger loads.
  3. Silver: can be an appealing option because of its higher electrical and thermal conductivity.
  4. Silver alloys: with nickel, indium, or tin are used where resistance to arcing and welding is needed.
For the movable parts of actuation mechanisms, copper beryllium is a common alloy because of its conductivity and spring behavior, and stainless steel is also used when corrosion resistance is important. The housings and cases for limit switches can be plastic or metal, depending on the external mechanical and environmental requirements of the application.

Electrical Specifications

Limit switches are electromechanical components, so electrical specifications are required. The primary specification is a current limit, which is driven by the materials and dimensions of the current-carrying contacts. This limit defines how the limit switch is implemented into the electrical system of the machine.

Receptacle / Mounting Configuration

There are many types of actuator styles and nearly an unlimited variety of receptacles and mounting configurations for limit switches. They can be surface mounted (i.e., flush on a surface of the machine) or side mounted depending on the geometry of the system. The actuator can also be vertical or horizontal compared to the mounting. The mounting holes for screw attachment must be compatible with the machine to which it is being attached. The receptacle that houses the limit switch can be opened so that it can be wired by the user, or it can be “pre-wired” and filled with epoxy to ensure stability of the wiring.

Summary

The basic mechanical limit switch is used in common tools and consumer products we use everyday. They are also an essential safety feature in machine control systems. When a part of the machine reaches an unsafe point, the machines motion is stopped. First you need to determine the type of limit switch you need for your application. What type of motion and action do you want the switch to influence? Once you have that figured out, determine what other specifications are required to get the right limit switch for your application. Our degreed engineers can also help you with your selection process.

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