Limit Switches Information
Image credit: Assured Automation | AA Electric | Esterline Power Systems / Leach International
Limit switches are switches that detect and are controlled by the motion or presence of an object.
Understanding Limit Switches
Switches are electrical devices used to break electrical circuits. Limit switches are essentially switches that can be actuated by an object other than a human operator. Mechanical limit switches use physical contact to detect the presence of an object, while solid state devices use proximity sensors, light sensors, or electric switches. They can be used as control devices for normal machine operation or as emergency switches.
Limit switches are extremely common devices used to solve a multitude of problems in packaging, manufacturing, motor control, safety, and consumer applications. Within these applications, they can be used to detect presence/absence, speed, position, range of motion, and travel limit, as well as to count discrete objects or events and break a circuit. Typical uses include:
Stopping a length of rod or web at the correct cutting point
Slowing a fast-closing containment door before it crashes to the floor
Switching a loading system when the loaded pallet reaches a set level
Stopping a washing machine when the load becomes unbalanced
Limit switches are primarily classified by the type of motion they limit or control.
Rotary limit switches operate by means of a rotating shaft. After the shaft reaches a set number of revolutions or a set angle, the switch activates. In the image below, the shaft can be seen extending through the front and rear of the device, while the gears on top are responsible for controlling attached machinery upon switching.
Image credit: Industrial Power & Control
Linear limit switches detect and are triggered by linear motion. They are often used in conjunction with linear actuators.
A linear limit switch. Image credit: Baumer Ltd.
While most limit switches use electromechanical means to detect an object, some may use solid state components to accomplish this.
Electromechanical switches use arms, levers, knobs, plungers, or other actuators that make physical contact with another object. When the object contacts the actuator, the switch's contacts either make or break a connection depending on the switch's contact orientation.
A pin-actuated mechanical switch. Image credit: Haydon Switch and Instrument, Inc.
Because of the physical nature of operation, mechanical limit switches are subject to wear but are often manufactured as extremely rugged devices to compensate. Other advantages include high current switching capability (typically up to 10 A), excellent environmental resilience, and low cost. However, mechanical limit switches may be limited by the need to make physical contact with an object.
Solid State Switches
Solid state limit switches contain no moving parts. They detect objects using an optical sensor or other electronic device, and switch using solid state components such as resistors, capacitors, and transistors. While solid state devices usually have longer operating lives than electromechanical types, they are also more expensive.
A solid state limit switch module. Image credit: Hellotrade
The GlobalSpec SpecSearch database contains information about important limit switch specifications, including contacts, switches, approvals, and features. The video tutorial below presents an overview of most important mechanical switch specifications, such as normal state, poles, throws, and switch applications.
Video credit: Ongytenes
When discussing mechanical switches, the term "contacts" refers to the conductive pieces of metal that touch to complete a circuit or separate to break it. Contact specifications include contact type and state.
A limit switch may be normally open (NO) or normally closed (NC) in its normal resting position. A NO device, when actuated, switches to close (or "make") the circuit, whereas an NC switch would open and break the circuit when actuated.
A limit switch may feature one of three different types of contact type.
Momentary contact switches remain open or closed only during actuation. For example, a NO switch would close at the first instance of the actuation condition; when this condition ceases, the switch would reopen. Momentary contact is sometimes referred to as spring return.
Maintained contact devices maintain their triggered position even after the actuation condition ceases. The contacts must be reset by mechanical action.
Positive opening (or positive breaking) contacts remain open in the activated position even in the event of mechanical failure. Positive opening switches are often used in critical safety applications because of their reliability.
When selecting a limit switch it is important to understand the switch's poles and throws specifications.
The term "pole" describes the number of separate circuits controlled by a switch. The number of circuits controlled by the limit switch determines the number of switch contacts, which in turn determines the poles needed to make or break the contacts. Switches typically have between one and four poles.
The image series below illustrates, from left to right, a single pole (SP) and double pole (DP) switch.
Image credit: Enasco | Skycraft Surplus
It is also important to consider a limit switch's throws, or the number of distinct positions a switch is capable of.
Single throw (ST) switches are open in one position and closed in another. For example, a single pole single throw (SPST) switch is a simple on-off switch, such as a light switch. A double pole single throw (DPST) switch is an on-off switch that opens and closes two contacts with a single motion.
Double throw (DT) switches are two-way devices. Double throw relays have three contacts and two positions: in the first position, Contacts 1 and 2 are in contact, but the third remains open. In the second position, this connection is reversed to Contacts 2 and 3.
Certifications / Approvals
A limit switch may be certified or approved by a standards body or other organization.
A Conformité Européenne (CE) mark ensures that a product complies with European Union (EU) legislation and may be placed on the European market. A CE mark is not legally required to certify a product's quality, but often ensures quality in specialty applications such as medical use.
A CE mark. Image credit: MatthiasDD
UL / CSA
Underwriters Laboratory (UL) is a U.S. product safety certification organization that develops standards and test procedures for products, materials, components, and tools.
The Canadian Standards Association (CSA) is a comparable standards organization for products manufactured in Canada.
The registered mark of the UL or CSA indicates that a product has been tested and certified to meet the organization's recognized standards.
UL (left) and CSA marks. Image source: ul.com | intertest.net
Stephen L. Herman, Industrial Motor Control (USA: Thomson Delmar Learning, 2005), 153-155.