Sensor Sense: Intelligent inductive sensors

Sept. 1, 2005
The actuator sensor interface (AS-Interface) is a simple two-wire network that replaces traditional sensor wiring architectures.

Actuator Sensor Interface (AS-i) networks use a special two-conductor yellow cable to carry both power and data to network devices such as sensors, analog inputs, pushbuttons, and other devices. The open topology of the network lets sensors attach anywhere along the yellow cable. While normal inductive sensors require input modules, AS-i sensors act as their own module. They may attach either directly to the yellow cable, or via an input module. An auxiliary 24-Vdc power source connects via a black cable to power output devices such as indicator lamps and relays.

Its vendor-independent open topology connects using a single unshielded cable without need of termination and possessing a high degree of noise immunity. The cable carries both data and power for sensors. Typical data signals include discrete I/O, digitized analog signals, encoders, light curtains, safety data, and e-stops. Ordinary sensors may connect to the network through input modules, which convert their on/off electrical signals to the proper data stream for the network. But a new class of sensor incorporates the AS-Interface directly as part of its design.

Smart sensors with built-in AS-Interface chips serve as their own AS-Interface module. One network cable holds up to 62 smart sensors that report their status to a network scanner. The scanner also acts as a gateway, interfacing the sensors to various industrial automation networks such as DeviceNet, Profibus, and Modbus. Each sensor self-monitors its operation offering many programmable and diagnostic features not possible from a standard sensor.

One such feature is a programmable output. Programmers can flip sensor outputs to be either normally open (n.o.) or normally closed (n.c.) as required.

Some AS-Interface sensors provide a prefailure indication. The sensor monitors the sensing range of the target to verify it is within acceptable limits. A target sensed repeatedly from 100 to 120% of the acceptable sensor range generates a warning of impending failure. Technicians can then determine the cause and correct the condition before the target drops out of range.

The sensor can also warn if the target approaches too near the sensor face. Here a precollision warning alerts the user before the target hits the sensor. Technicians may then realign the sensor before it is destroyed.

Intelligent sensors monitor their own health and can, for example, tell if the internal oscillator stops running either from malfunction or damage. The user knows immediately that the sensor has quit working. In contrast, standard sensors require target presence and a missed trigger before the user knows of the failure.

A programmable timer within the sensor acts as an activation filter to eliminate false triggering from noise, weld fields, and other unforeseen disruptions. Technicians can switch this timer in or out as needed.

Pepperl+Fuchs ( provided information for this article.

About the Author

Robert Repas

Robert serves as Associate Editor - 6 years of service. B.S. Electrical Engineering, Cleveland State University.

Work experience: 18 years teaching electronics, industrial controls, and instrumentation systems at the Nord Advanced Technologies Center, Lorain County Community College. 5 years designing control systems for industrial and agricultural equipment. Primary editor for electrical and motion control.

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