By Andrew Waugh, Product Manager, Sensor and Safety Products, AutomationDirect.com
When the word “sensor” is used in industrial automation, it usually refers to presence sensing devices such as limit switch, photo eye (photoelectric) and laser sensors. There are many options and technologies to consider when specifying the use of these presence sensors in automated machines.
A quick discussion of the technology used in each of these devices can help in their selection. There is much overlap in functionality of these sensors, and there are also some selection considerations, as each device has its own advantages and disadvantages.
What is a position sensor?
Three common position sensors are mechanical limit switches, photoelectric sensors and laser sensors.
Mechanical limit switches are used in industrial automation systems to detect presence or position. Physical contact with the switches’ actuator, usually adjustable, activates these electromechanically- operated devices.
Photoelectric sensors, also called photo eyes, detect object presence or distance using light. Operation combines a light source and an emitter, and a receiver that detects changes in light intensity reflected by the target object, Figure 1.
Laser sensors operate like photoelectric sensors but with a much more focused light source to detect presence or measure distance, depending on the configuration. Laser photoelectric sensors detect presence. Laser distance sensors measure distance, as the name implies.
The tried and true limit switch
Limit switch actuators are available in many forms including an adjustable arm or a plunger, each mechanically linked to an electrical switch block inside the housing. Rotating the arm or depressing the plunger provides a mechanical action to operate the switch’s electrical contacts. These contacts are available normally open or normally closed in a variety of numbers of poles and throws.
Purely mechanical in operation, most limit switches use linkages and movement of the sensed object to operate the switch contacts. Heavy-duty contacts are common in limit switches, enabling switching of higher currents than other presence and position sensing devices.
Screw terminals inside the switch housing provide convenient electrical connections. A common configuration of the housing includes threaded openings for conduit or cord grip installation. A removable cover provides environmental protection for the switch and its electrical connections.
Diffuse, retro-reflective and through-beam photoelectric sensors
The typical light source used in the emitter, or sender, of a photoelectric sensor is visible red or infrared. The light is collected by a receiver where the intensity is measured. These emitter and receiver elements can be installed in the same housing or in different housings.
Diffuse and retro-reflective configurations are available when an emitter and receiver are integrated together. Diffuse sensing bounces emitted light directly off an object and back to the receiver in the same housing.
A retro-reflective, also called reflective, configuration uses a single housing as well, but bounces emitted light off a fixed reflector, not an object. The reflector must be attached to a fixed surface and can be polarized to improve sensing by eliminating unwanted reflections.
A through-beam photoelectric sensor uses a configuration where the emitter and receiver are in different housings. Through-beam photo eyes provide the longest operating distances since the emitter shines light directly to the receiver.
Laser sensor operating principles
Laser distance sensors are available in diffuse, background suppression and retroreflective types. These sensors provide accurate distance measurements using CMOS (complementary metal-oxide-semiconductor) or transit time technologies, Figure 2.
Laser light is focused, keeping it in a narrow beam over long distances. The light color is also held to a narrow spectrum. This well-controlled light source can be triangulated or pulsed, with each emission or pulse measured to enable distance readings.
Short-range, high-precision laser sensors often use CMOS technology. Optical triangulation is the principle of operation. Diffuse triangulation transmits laser light through a lens, and then bounces it off a target and back to a separate receiving lens that focuses the light onto a CMOS linear imager. Distance to the target changes the angle of where the focused point of reflected light is received on the linear imager.
Long-range, precision laser distance sensors use time-of-flight technology and are often called rangefinder laser sensors. A laser diode in the transmitter generates a very precise, short pulse of visible red or infrared light. Accurate and high-speed electronics measure the light transmit time from transmission to reflection off the target object and back to the sensitive, laser energy detector — the receiver diode. Using the light transit time and the constant for the speed of light, the object’s distance from the sensor can be calculated.
With any presence or position sensing application, it’s important to understand the devices specifications. These specifications along with specific application requirements drives the selection considerations, Table 1.
Limit switches must touch an object to detect it, which limits sensing distance to the travel distance of the switches’ rotary lever arm or plunger type actuator. However, this short sensing range provides reliable detection of an object regardless of its color, shape or size. Care must be taken to ensure the object touching and activating the actuator roller or plunger does not damage it.
Both photoelectric and laser sensors have a sensing range from close to far. Adjustable light intensity and sensing thresholds help tune this range. This wide and adjustable detection makes the photo eye a popular choice for presence detection, and it has an excellent sensing range-to-size ratio. As the sensing distance increases or a more accurate or a smaller spot size is needed, laser becomes a popular choice.
Mechanical limit switches have moving parts that can wear out, so the speed of actuation must also be limited, making it unsuitable for high-speed applications. The electronics in both photoelectric and laser sensors enable fast switching operation. With operation speeds — from 25 Hz on the low end to thousands of times per second on the high end — high-speed applications are possible, and the number of actuations is not a concern.
When looking at presence sensors’ repeatability, or repeat accuracy, it is important to understand when it’s needed, because many sensors do not excel in this area. For applications where a high degree of repeatability is needed, precision touch limit switches are available with an accuracy of 5 microns or less.
Photoelectric sensors can be very repeatable, but it depends on the type and how it is installed. Its repeatability also varies due to fluctuation in reflectivity. Laser sensors are typically more repeatable than photoelectric sensors. Those designed for measurement have a measurement accuracy around 10 microns or less, depending on the range, but repeatability can be affected by fluctuations in reflectivity.
Environment, installation and construction
Because a limit switch touches a product, flag, carrier, trollie or traveler on a machine or automated process to operate, it works well in rugged environments. However, guards are often needed to protect the switch from accidental operator actuation, and the switch should be mounted to keep water or debris from accumulating around the actuator arm, seals and bearings.
Although a photo eye’s housing is rugged, a dusty, wet or harsh environment can affect operation by dispersing the light reaching the target and its reflection. Many laser sensors are protected against harsh environments. Dirt and debris may be a problem, but the light intensity can often be increased to burn through these airborne interferences. However, it is important to understand the Laser Class to avoid eye contact.
When it comes to installation, limit switches must be mounted to touch the object when actuated, and the actuation must operate the switch properly, perpendicular to the arm for example, and possibly through use of cam action or a ramp.
Photo eyes easily mount using off-the-shelf brackets, but many times need “teaching” to set switching thresholds. Lasers sensors often mount like photo eyes, but if measurement accuracy is required, both the sensor’s and the object’s mounting point must be rigid and stiff, so vibration or other movement does not cause measurement errors.
Construction of presences sensors are similar among all devices, but there are differences such as rectangular or tubular housings, and materials such as plastic or metal. While mechanical limit switches are the most rugged, photo eyes and laser sensors are also built to survive harsh industrial environments.
Applications touch, light and laser
There is much overlap in the application of limit switches, photoelectric sensors and laser sensors, so carefully check the device’s specifications; how it will fit into an application; and how it will be used. Limit switches are just one of the many solutions to use in automated applications, but they are a simple and rugged choice for presence sensing, providing reliable detection of many components or parts, Figure 3.
Photo eyes are probably the best general-purpose position sensor. They have a longer sensing range than other presence sensing options such as inductive, magnetic, capacitive and ultrasonic. As they are available in a wide range of configurations and housing sizes, they fit well in many applications.
Lasers are best for precise measurement of distance in applications where the part or component can’t withstand contact with a mechanical switch. Close, far, dusty or clean — they work well in most environments. They also work well for small part detection, much better than the other two technologies.