Expanded Equipment Sensor Options | Manufacturing enterprise technology

Figure 2: In addition to providing basic on/off signals, many modern electronic devices such as photoelectric sensors are compatible with IO-Link to provide advanced data and connectivity.

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While some industrial equipment is still implemented with basic hard-wired control circuits, modern systems today are typically automated by microcontrollers, programmable logic controllers (PLCs), or other digital platforms. Likewise, although traditional electromechanical limit switches are quite common in simple equipment applications, today’s equipment is more likely to be heavily instrumented, taking advantage of advanced sensing devices and methods. Communication.

It is beneficial for designers to be aware of the many discrete object detection technologies and connectivity methods available so that they can specify high-performance, cost-effective products for their applications.

Discrete object detection

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 of a product or material manipulated. The result is a “yes/no” signal. For example, a discrete object sensor can be configured to detect if an actuator is in the correct position to receive a coin, and a second sensor can be used to detect if the coin has been loaded.

Some sensors, especially electromechanical limit switches, require direct physical contact, but this subjects them to wear and can damage the target. In many cases, it is preferable to use advanced electronic technologies, which can detect proximity at a greater distance without contact. Some of the more easily deployed discrete object detection techniques (Figure 1) include:

  • Limit Switches: These use a mechanical lever, piston, or other device to actuate electrical contacts.
  • Magnetic proximity: These can detect magnetic targets at relatively long distances, but are only suitable if a magnet can be fitted.
  • Inductive proximity: These 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: These transmit and receive sound waves bouncing off a target and are useful for detecting clear or obstructed paths.
  • Photoelectric sensors: These use light (visible red, infrared or laser wavelengths) to detect targets and come in many variations.

Figure 1: No discrete object sensor is the optimal selection for all industrial applications.  Designers are therefore best served when they can choose from a broad portfolio of devices from a vendor like AutomationDirect.Figure 1: No discrete object sensor is the optimal selection for all industrial applications. Designers are therefore best served when they can choose from a broad portfolio of devices from a vendor like AutomationDirect.AutomationDirect

Each sensor selection involves evaluating a series of trade-offs to find the acceptable fit for the application and price/performance ratio. Cost considerations should include installation and running costs in addition to the base price of a sensor. And performance means meeting requirements for physical form factor, environmental compatibility, detection range and accuracy.

Establish connection

Sometimes sensor technology is associated with certain form factors and connectivity methods. Form factor considerations include mounting the sensor efficiently and choosing materials of construction and sensing methods that can withstand the environment.

For example, electromechanical limit switches are available with many types of mechanical operators and typically have threaded conduit connections for use with traditional wiring methods. However, a wide variety of sensors are available in more compact barrel type arrangements and may come with attached leads for wiring or quick disconnect options. Quick disconnect options are preferred in many cases for simplified installation and maintenance.

Sensors used for industrial purposes generally must be NEMA or IEC rated, making them suitable for use in dusty and sometimes wet or wash down environments. Exposure to shock and vibration is another factor to consider, and solid-state electronic sensors may perform better in these types of environments than mechanical versions.

Traditional discrete object sensors are simply wired with a 2-wire on/off signal to an electrical circuit or PLC digital input, using 24 VDC or sometimes 120 VAC. Electromechanical contacts are designated as normally open (NO) or normally closed (NC), and designers should ensure that a given sensor provides sufficient amounts of these NO and NC contacts to meet the application’s needs.

In recent years, many original equipment manufacturers (OEMs) have moved to using 24VDC 3-wire connection methods, which are generally applicable to more advanced types of sensors that generate solid-state output. Solid-state outputs are electronic in nature, so they have a lifespan that far exceeds electromechanical contacts. These outlets generally use direct current, although some are available in AC versions. When working with DC sensors, designers must coordinate sink/sink and source/source arrangements with the electrical design of the entire circuit.

The connectivity 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 accomplished using a protocol such as IO-Link, which is a standardized input/output technology for providing intelligent communication between a sensor and an associated monitoring device such as a PLC (Figure 2 ). Many OEMs are leveraging these capabilities to deliver optimized automated equipment.

Figure 2: In addition to providing basic on/off signals, many modern electronic devices such as photoelectric sensors are compatible with IO-Link to provide advanced data and connectivity.Figure 2: In addition to providing basic on/off signals, many modern electronic devices such as photoelectric sensors are compatible with IO-Link to provide advanced data and connectivity.AutomationDirect

No one size fits all

There is no single best universal sensor technology, and in fact many capabilities overlap to support applications. With the basics in mind, designers are well served by working with vendors offering broad portfolios of sensor types and connectivity options so they can get the products best suited for automating industrial equipment.

About the Author

Bill Sonnenthal is a Technical Marketing Engineer at AutomationDirect. He has worked at 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 industry.

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