Figure 2: In addition to providing basic on / off signals, many modern electronics-based devices, such as photoelectric sensors, are IO-Link compatible to provide advanced data and connectivity.
While some industrial facilities are still implemented with basic control circuits, modern systems are usually automated by microcontrollers, programmable logic controllers (PLCs) or other digital platforms. Similarly, although traditional electromechanical limit switches are quite common in simple equipment applications, today equipment is more likely to be heavily instrumented, taking advantage of advanced sensor devices and communication methods.
It is advantageous for designers to know the many technologies available for discrete object detection and connectivity methods in order to be able to determine high-performance and cost-effective products for their applications.
Detection of discrete objects
The term “discrete object detection” refers to any method of detecting the presence or alternatively the absence of a physical object, including a machine component, or a product or material being handled. The result is a yes / no signal. For example, a discrete object sensor may be configured to detect whether the drive is in the correct position to receive a part, and a second sensor may be used to sense whether the part is loaded.
Some sensors, especially electromechanical limit switches, require direct physical contact, but this wears them out and can damage the target. In many cases, it is preferable to use modern electronics-based technologies that can sense closeness from a greater distance without contact. Some of the most easily deployed techniques for detecting discrete objects (Figure 1) include:
- Limit switches: They use a mechanical lever, piston or other device to control electrical contacts.
- Magnetic proximity: They can sense magnetic targets over relatively long distances, but are only suitable if a magnet can be mounted.
- Inductive proximity: They use an electromagnetic field to detect various metal objects. Ferrous metals are the easiest to detect.
- Capacitive proximity: They use an electrostatic field to detect all types of objects and materials.
- Ultrasonic sensors: They transmit and receive sound waves reflected from the target and are useful for detecting clear or obstructed roads.
- Photoelectric sensors: They use light – visible red, infrared or laser wavelength – to detect targets and come in many variations.
Each sensor selection includes an evaluation of a series of trade-offs to find an acceptable application and a price / performance ratio. Costs should include installation and operating costs in addition to the base price of the sensor. And performance includes compliance with physical form factor requirements, environmental compatibility, sensor range and accuracy.
Making the connection
Sometimes sensor technology is associated with a certain form factor and connection methods. Form factor considerations include the efficient installation of sensors and the selection of building materials and sensor methods that can withstand the environment.
For example, electromechanical limit switches are available with many types of mechanical operators and usually have threaded pipe connections for use with traditional wiring methods. However, a wide variety of sensors are available in more compact barrel-type arrangements and can be supplied with attached wiring cables or quick shut-off options. Quick shut-off options are preferred in many cases for simple installation and maintenance.
Sensors used for industrial purposes usually have to be rated by NEMA or IEC, making them suitable for use in dusty – and sometimes wet or flushing – environments. Exposure to shock and vibration is another factor to consider, and solid-state electronic sensors can perform better in this type of environment than mechanical versions.
Traditional discrete object sensors are simply connected to a 2-wire on / off signal for an electrical circuit or digital PLC input using 24 VDC or sometimes 120 VAC. Electromechanical contacts are marked as normally open (NO) or normally closed (NC) and designers must ensure that a sensor provides sufficient quantities of these NO and NC contacts to meet the needs of the application.
In recent years, many original equipment manufacturers (OEMs) have switched to using 24VDC 3-wire connection methods, which are typically applicable to more advanced types of solid state output sensors. Solid state outputs are electronic in nature, so they have a service life that far exceeds electromechanical contacts. These outputs typically use DC, although some are available in AC versions. When working with DC sensors, designers must coordinate NPN / sink and PNP / source arrangements with the entire electrical circuit design.
The connection methods discussed so far are “read-only”, but some of today’s electronic sensors can support two-way communication with additional advanced information and configuration options. This can be achieved using a protocol such as IO-Link, which is a standardized input / output technology to provide intelligent communication between a sensor and a connected monitoring device such as a PLC (Figure 2). Many OEMs take advantage of these capabilities to deliver optimized automated equipment.
There is no one size fits all
There is no one-size-fits-all sensor technology, and in fact many of them overlap in their application support capabilities. With the basics in mind, the designers are well served, working with vendors offering a wide portfolio of sensor types and connectivity options to get the most appropriate industrial automation products.
About the author
Bill Sonental is a technical marketing engineer at AutomationDirect. He has been with AutomationDirect since 2009 in technical and marketing roles and holds a BSEE degree from the Georgia Institute of Technology. Prior to joining the company, Bill spent 15 years designing, programming and commissioning control systems in the newspaper and printing industries.