The 101 on various sensors for stamping operations

Jun 02, 2023

Inductive proximity sensors detect the presence of metallic objects at close range. Shown is a Contrinex inductive sensor.

Editor's Note: This article comprises excerpts from AutomationDirect's "Practical Guide to Discrete Sensors for Industrial Applications."

Many types of object-detection sensors used in stamping are available on the market. Some of them, such as a limit switch, require direct contact with the objects. Others, such as proximity sensors, need to be in close physical range but do not make contact with the object. Still others can detect objects from much farther distances; examples include photoelectric, laser, and ultrasonic sensors.

Limit Switch. A limit switch is one of the most common object-detection sensors used in industrial controls. It is energized when an object makes physical contact with the actuator. It normally provides an electromechanical contact closure, but some also move a pneumatic valve.

Inductive Proximity Sensor. Inductive proximity sensors detect the presence of metallic objects at close range (under 1.5 in., typically). Ferrous metals allow the greatest distances to the sensor; other metals may reduce detection range.

Magnetic Proximity Sensor. A magnetic proximity sensor is used for noncontact object detection beyond the normal limits of inductive sensors. Used with a separate damping magnet, they offer very long sensing ranges in a small package and can detect magnets through walls of nonferrous metal, stainless steel, aluminum, and plastic overmolding.

Capacitive Proximity Sensor. A capacitive proximity sensor uses a dielectric plate that generates an electrostatic field to detect both metal and nonmetal objects. It often is used to detect fill levels of parts, liquids, pellets, and powders through container walls. It typically senses distances up to 40 mm.

Photoelectric Sensors. A photoelectric sensor uses reflected or detected light waves to detect an object's presence. It typically contains all required optics and electronics in a single unit, requiring only power to provide an output based on its specifications and object-detection criteria. Photoelectric sensors can detect a variety of materials and have extended sensing ranges.

Specialty Sensors. Other object-detection sensors are available for specialized applications. Some specialty sensors that work similarly to a photoelectric sensor are laser sensors, fiber-optic sensors, fork sensors, and area sensors.

A limit switch is energized when an object makes physical contact with the actuator. This is an Eaton limit switch.

Sensors may offer a threaded conduit fitting, an attached (embedded) cable, or a quick-disconnect fitting to which a mating cable can be attached.

Threaded Conduit Connection. On many limit switches, the connection is a threaded conduit connection opening in which individual wires can be terminated. Usually, this is either a female NPT or female PG threaded connection.

Attached Cables. Attached cables (embedded) typically are 6 ft. in length and are molded into the body of the sensor at the attachment point. They typically are the less expensive option. The cable can be cut to length or may not be long enough to reach all the way to the termination point controller. Field junction boxes may be required, and numerous sensor signals often are combined into multiconductor cables at these junction points. Lastly, a cable exiting from the end of the sensor body may not fit (physically) in all applications.

Quick Disconnects. Sensors with quick-disconnect fittings require the use of a separate cable to complete the installation. These cables typically include industry-standard M8, M12, or micro-AC-style connections on one end and a pigtail (flying lead) on the other end to complete the connection. Quick disconnects offer several advantages. It's easy to replace a damaged sensor without any rewiring, and the cables can be obtained in longer lengths with axial or 90-degree connections at the sensor.

Many factors determine which sensor is optimal to use for a given application. By answering a few questions, you can narrow down your selection to a sensor type.

Close Proximity. Is the part, slug, or object you are trying to detect very close, possibly touching or within 40 mm? If the object or part will be touching the sensor actuator or sensing face, then most likely a limit switch will be the best selection. If the object is metal, you should use an inductive proximity sensor, though you may also want to consider a capacitive proximity sensor.

At a Distance. Are the materials you are trying to detect at a distance in a bin or tank, and do you want to measure distance or detect fullness or emptiness? If so, use an ultrasonic sensor.

If your part, slug, or object is farther away than 40 mm from the sensor, an inductive proximity sensor is out of consideration. More needs to be known, such as the object's size, the exact distance from the object to the sensing face, and the object's color (dark, light, reflective).

If your item is large, at a distance greater than 40 mm and up to 6,000 mm, and the application does not require high accuracy (such as a forklift near a machine rather than a small box on a specific location on a conveyor), an ultrasonic sensor would work.

A magnetic proximity sensor is used for noncontact object detection beyond the normal limits of inductive sensors. Shown is an AutomationDirect magnetic proximity sensor.

If your item is small, is farther than 6,000 mm away, or if high accuracy is required, use a photoelectric or laser sensor.

Can you mount components on both sides of your sensing application? If so, you have many more options available. You can use retroreflective or through-beam photoelectric sensors. Again, for long-distance sensing applications, a laser sensor will be best.

If not, you must use a diffuse photoelectric or diffuse laser sensor. Keep in mind that with diffuse photoelectric sensors, the sensing distance will be much shorter, and you might get false-negative reads from darker objects and false-positive reads from ambient light pollution. Diffuse laser sensors are exempt from these limitations but are more costly than photoelectric sensors.

Are you trying to detect a magnet from up to 70 mm away? If so, a magnetic proximity sensor is best--even through nonmagnetizable materials.