How Limit Switches Can be Used as an Alternative to Sensors

Automated control systems use actuators and sensors along with controllers to interact with the physical environment around it. Sensors detect environmental parameters ranging from temperature to physical motion, and actuators translate electrical signals to physical work. In an industrial environment or factories, sensors are also responsible for safety measures. But a limit switch can also sense a physical input and provide an output. In this article, we discuss how a limit switch can operate as a replacement for a sensor.

Review: Limit Switches

A limit switch is an electromechanically operated switch. It changes state when an object comes into physical contact with its actuator. It makes or breaks the electrical connection to indicate the state of the switch contacts. The switching action generates a signal which helps to control electric supply to the machine. Figure 1 shows the architecture of a limit switch and its components.

Figure 1: Limit Switch Architecture

• Enclosure Case:  This is a metal or plastic enclosure for protecting the limit switch components.
• Terminals:  Limit switches have three terminals, Normally Closed, Normally open and Common terminal.
• Spring: This mechanical arrangement is used to reset the switch position.
• Actuator: This is a combination of plunger and lever. The actuator physically comes in contact with objects. The actuator translates a linear or rotary action into an electrical signal. Different types of actuators  are available to have proper mounting and orientation with the moving parts.

Limit switches as an Alternative for Sensors

Contactless sensors work on the principles of electromagnetic radiation for sensing moving parts of a machine. Photoelectric, proximity, ultrasonic, and reed switches are the most common types of contactless sensors. But contactless sensors can be affected by environmental factors; faulty sensing can lead to disastrous events. Let’s quickly go through how a limit switch can be used to overcome these threats.

The photoelectric sensor detects objects by using a modulated infra-red light wave. It has a transmitter which will transmit the wave, and the receiver waits to receive the transmitted light wave. When objects moving through the light beam obstruct it, the receiver detects the object. The sensor accuracy can be affected by dust and other polluting particles that cover the transmitter and receiver. Poor reception of the transmitted wave can indicate the presence of an object even when there are none. Ambient light and water vapor content can also affect the accuracy of the system. Limit switches encapsulated in weather-resistant enclosures can be a viable replacement.

Reed switches are used to detect magnetic objects. The operability of the reed switch is influenced by the alignment of the magnetic material to the sensor. Reed switches can be replaced with limit switches as it is not necessary to use magnetic materials, providing more flexibility.

Ultrasonic sensors consist of a transmitter and receiver where the transmitter emits a sound wave and the receiver receives the reflected wave. The object surface should be sonic reflectable, and the proximity of the object is calculated using the time of flight. Particle impurities present in the air, like fog or dust, can affect the time of flight and can lead to inaccurate sensing. Since limit switches are not susceptible to such noises, they are a viable alternative to ultrasonic sensors. They also do not depend on the object surface.

Contact sensors operate by using variable resistors that translate a linear motion to a varying electrical signal. However, these type of sensors do not have an instantaneous output. Limit switches with a lever type actuator can be used instead of contact sensors to have near real-time object detection.

Applications

A wide range of applications uses limit switches to control the movement of devices performing on a production line. The limit switches also widely used in safety-related controls. Let us discuss some application examples.

• Elevators: The elevator consists of sensors, electric motor, and controller. Elevator uses the infrared sensors, level sensors, hall-effect sensors, and encoders for safety and position detection. The level sensor detects the position of the car and floor number to activate the brake mechanism. Limit switches can replace level sensors to detect the position of an elevator car. Safety limit switches are used to detect the upper and lower limit of an elevator. When the elevator car comes in contact with the limit switch, it transmits a signal to the controller and activates the braking system. The controller gets the input from the limit switch and displays the floor number along with the direction of motion.

• Rail doors: Sliding doors or rail doors are used for high-temperature rooms in industry, locomotive, and residential applications. Rail doors perform a sliding motion while opening and closing, driven by an electric motor. The rail doors include object detection sensors, placed at both edges of the door tracks to avoid crashes into endpoints. We can replace the object detection sensor by limit switches. These switches must be operated in low and high-temperature range and housed with sealed enclosures to prevent a spark or electrical discharge that could ignite an explosion. When the limit switches experience pressure on its actuator, it sends a signal to the controller. The controller drives the motor either in a forward or reverse direction and slows down the door just before it is fully opened or closed.

• Conveyor: Conveyors are used in the material handling, agricultural, and transport industries. Conveyors consist of an electric motor, controller, belt, object detection sensors, and safety sensors. Limit switches can be used in a conveyor as object detection and safety sensors. They are used to count objects and ensure that materials on the conveyor are correctly positioned. Safety Limit switches are used to monitor cable-alignment. On the instance of misalignment, the limit switch lever, switches and sends an emergency stop signal to the controller. It provides a safe-to-use environment for operators working near conveyor systems.

Figure 2: Limit Switches Used in Conveyor Applications