Wireless Industrial Switch/Sensor FAQs

  • May 02, 2016
  • Feature

By Leonard Patti, Application Specialist, STEUTE Industrial Controls

In the past, the majority of wireless industrial control devices available have been for the monitoring, transmitting, receiving and processing of continuously variable (analog) parameters such as temperature, pressure, flow, humidity and speed.  This is now changing with the development and introduction of a number of robust, reliable digital switches, sensors and related accessories that generate and receive simple “on-off” signals for start/stop control, presence/position sensing, counting, alarm signaling, high/low level detection,  and other desired digital inputs.

With their availability, interested users have posed a number of questions as they strive to become familiar with the design, performance and application of these new control solutions.

The following represent a compilation of those most frequently asked questions as they relate to the range of currently available industrial-grade wireless digital switches, sensors and accessories (e.g. receivers, range extenders, antennas, power supplies, field strength monitors, et al) . 

Figure 1

Question 1: What types of functional wireless devices are currently available?

Answer: The following are among the types of available wireless industrial-grade control components:

  • Limit switches*
  • Miniature position switches*
  • Non-contact magnetic sensors
  • Non-contact inductive sensors
  • Pull-wire switches*
  • Push button command stations*
  • Photo-optical switches
  • Selector switch command stations*
  • Key-operated switch command stations*
  • Foot switches*
  • Door handles with integrated push buttons
  • Universal wireless transmitters
  • Receivers with relay outputs
  • Receivers with bus-compatible outputs
  • Range extenders
  • Antennas

Question 2: What is “energy harvesting” ?

Answer: Energy harvesting refers to the capturing of available sources of energy (such as wind, light, mechanical motion, thermal energy, et al) and storing/converting it into other forms of energy (such as electrical) for use as needed.  Well-known energy harvesting devices include solar cells, wind turbines, and hydroelectric generators.

Question 3: How do selected wireless switches operate without a battery power source?

Answer: Those wireless switches designed for battery-less operation use “energy harvesting” techniques for powering the devices’ transmitter electronics.  These techniques typically include the use of a solar cell or an “electrodynamic energy generator” to respectively convert light energy or mechanical energy into electrical energy.

Question 4: What is an “Electrodynamic Energy Generator” and how does it work?

Answer: Electrodynamic energy generators are typically a design element of those wireless switches characterized by mechanical actuation, such as limit switches, push button switches, foot switches, or pull-wire switches.  The “generator” consists of a permanent magnet and an electrical coil.  The mechanical actuation of the switch moves the magnet through the coil, generating an electrical current in the coil.  This electrical energy is sufficient to allow the transmitter to send wireless “telegrams” (control signals) to the receiver and, depending upon the design, may also be sufficient to process confirmation of receipt of the signal by the receiver and resend the signal if its receipt has not been confirmed.

Figure 2

Question 5: What is the typical mechanical life an "electro dynamic generator”?

Answer: Electro dynamic generator designs for use in industrial control applications typically have a mechanical life expectancy in excess of 1 million actuations. 

Question 6:  What is the battery life for those transmitter devices that are battery powered?

Answer: Battery life is a function of several factors. These include battery cell size, battery energy density, and the frequency of switch/sensor actuation.  Depending upon the specific switch or sensor, the battery life can range from several months to several years.  Typical battery life for various duty cycles are generally provided with manufacturers’ wireless device specifications.

Question 7: Do these components have the required third-party certifications?

Answer: Those industrial-grade products requiring third-party certification (e.g. line-powered devices such as receivers and repeaters) typically have the necessary certifications for the intended countries of use (e.g. cCSAus, FCC, IC, et al.)

Question 8: Why don’t the wireless transmitters have UL or CSA certification?

Answer: Most of the available wireless transmitters (switches & sensors) are of such low energy/low power that they do not represent a safety hazard and do not require UL/CSA safety certification.

Question 9: Are these wireless industrial components suitable for use in machine guarding/safety applications?

Answer: Such applications typically require independent, redundant control signals to meet the safety standards.  Since most of these devices are designed for single-frequency operation (e.g. 868 MHz or 915 MHz), they are not intended nor suitable for use in safety applications. 

Question 10: What is the typical transmission distance for these wireless devices?

Answer: Due to the diversity of installation environments and conditions, wireless device manufacturers cannot state an absolute transmission distance for a given transmitter set.  However, for evaluation and comparison, they typically provide nominal transmission distances indoors and outdoors (in “free air”). Depending upon the model/type of transmitter, nominal indoor transmission distance is in the range of 30 to 60 meters and nominal outdoor transmission distance is in the range of 150 to 700 meters.  Actual transmission distances will be affected by structures and/or equipment in the wireless signal’s path. If in doubt of the efficacy of the application, it is suggested that the user conduct an in-situ evaluation of the transmitter(s) and receiver(s) in their specific situation.

Question 11:  Can these transmission distances be increased?

Answer: Yes. Some manufacturers offer “repeaters” for this purpose with the number of such repeaters that can be used to extend the transmission range between a transmitter and the receiver to which it has been paired specified by the manufacturer.

Question 12:  What is the effect of various construction materials in the transmission path of the wireless telegram?

Answer: Since the wireless signals are electromagnetic waves, some signal attenuation is not uncommon from the transmitter to its receiver.  In addition to the signal strength being inversely proportional to the square of the distance between the transmitter and its receiver, sensing range can be affected by walls/objects in the transmission path.

Question 13:  What is a “repeater" and what is its benefit?

Answer: A “repeater” is a signal amplifier that can be located at some point in the transmitter to receiver signal path.  It extends the operating range between the rransmitter and its receiver. 

Question 14:  Are tools or instruments available that can aid in the evaluation/ installation/maintenance of a wireless system?

Answer: Some manufacturers offer “field strength measuring” instruments for this purpose.  Depending upon their design, they enable a user to identify the operating transmitter, count telegrams received, see the relative strength of the signal at various points in the transmission path, and/or confirm receipt of the transmitted signal at the wireless receiver’s location prior to actual installation of the receiver.

Question 15: At what frequencies do the currently available wireless industrial control components operate and why were these frequencies selected?

Answer: Most of the currently available wireless transmitters (switches & sensors) for industrial, non-safety applications are available for 868 MHz and 915 MHz operation.  These are the preferred frequencies for industrial control in Europe and the USA-Canada respectively.

Question 16:  What output signal options are available from the currently available wireless receivers?

Answer: The following are among the more popular receiver output protocols currently available … discrete relay, PNP, TCP/IP, USB and RS232 Serial outputs.  Other output protocols are reportedly in development and are expected to be available soon.

Question 17: How does one establish a communications link (e.g. “pair”) between a specific transmitter and one of its’ compatible wireless receivers?

Answer: “Pairing” is typically accomplished through a brief series of sequential steps using the readily accessible and easily-operable programming buttons and built-in status LEDs on the receiver.  For example, the steps to pair one manufacturer’s transmitter (switch/sensor) with a single-channel receiver with discrete relay outputs include:

(1) Press programming button on the receiver for 1 second. Green LED flashes slowly (2Hz). (2) Actuate wireless switch/sensor being “paired” (taught in). Orange LED flashes. (3) Press programming button again for 1 second to exit “pairing” (teach in) mode.  Green LED stays lit.

Figure 3

Question 18:   How many transmitters (switches/sensors) can be monitored by a single receiver?

Answer: This varies depending upon the manufacturer and receiver model number. Models are available that monitor a single transmitter per channel, while others can accept multiple inputs per channel.   Bus-compatible receivers offer the possibility of monitoring many discrete transmitters and interfacing to a PLC or other master controller.

Question 19:  How does one remove a transmitter from a receiver with which it has been previously “paired”?

Answer: Removing a previously “paired” transmitter is typically accomplished through a brief series of sequential steps using the readily accessible and easily-operable programming buttons and LEDs built into the receiver.  For example, the steps to remove one manufacturer’s paired transmitter (switch/sensor) with a single-channel receiver with discrete relay outputs include:

(1) Press programming button on the receiver for 5 seconds.  Green LED flashes quickly (5HZ). (2) Actuate the wireless switch’/sensor to be deleted. Orange LED flashes. (3) Press programming button again for 1 second to exit “delete” mode.  Green LED stays lit.

Question 20: What is the response time from switch/sensor actuation until the receiver output changes state?

Answer: Typical response time, of course, will vary depending upon the specific model and manufacturer.  However, they are generally well within the required response time required in most industrial applications.

Question 21: What is meant by “bi-directional’ communication?

Answer: Some manufacturers have designed many of their transmitters and receivers to communicate in both directions … e.g. from the transmitter to the receiver and from the receiver to the transmitter.  This capability enhances system reliability.  For example, it enables the system to be designed such that receivers can send an “acknowledgement signal” to the transmitter upon receipt of its transmitted signal.  Should the transmitter not receive this acknowledgement signal within some predetermined time frame from its original transmission, it can resend the telegram a second time.

Figure 4

Question 22: What steps have manufacturers taken to optimize the reliability of their wireless industrial automation components and systems?

Answer: Depending upon the manufacturer, the following are among the design features utilized to optimize reliability:

  • Use of bi-directional communications between transmitters and receivers (e.g. acknowledgement signals, redundant transmissions, et al.)
  • Use of short telegrams to minimize in-air time.
  • Use of battery charge status monitoring.
  • Use of transmitter “status” signals.
  • Use of frequency-hopping (e.g. devices operating in the 2.4. GHz band).
  • Use of uniquely-coded telegrams from each transmitter.
  • Use of listen-before-talk (LBT) technology.

Question 23: What is the listen-before-talk (LBT) technology, and what are its benefits?

Answer: Listen-before-talk technology has been developed for those applications involving a large number of transmitters where the probability of simultaneous transmission from one or more transmitters (operating at the same frequency) is high.  This feature enables the transmitter to repeatedly listen to determine if there is another telegram at the same frequency in the air, prior to sending its telegram.  If it detects the presence of another telegram, it waits a programmed interval, and listens again reducing the probability of sending its telegram when another telegram at the same frequency is present.   In so doing it greatly reduces the possibility of losing a telegram due to a “collision”  enhancing system reliability in highly populated applications.

Question 24: What is the minimum number of components required to realize a wireless control function?

Answer: A typical wireless “system” requires a transmitter, a receiver, a receiver antenna, and line power for the receiver.

Question 25: What are some of the applications that have benefitted from the use of these wireless digital components?

Answer: Since these devices are relatively new, many potential applications have yet to be identified and realized.  However, the following are among the applications already experiencing their benefits:

  • Valve position monitoring
  • Remote crane control
  • Remote point presence or position sensing
  • Assembly station inventory management systems
  • Fire vent position monitoring
  • Tank level monitoring
  • Hatch and access port monitoring
  • Hopper flap/diverter monitoring
  • Safety shower/eyewash station alarm monitoring
  • Rotary machinery
  • Automatic door control
  • Adjustable cut-to-length control
  • Door monitoring
  • Presence/position sensing in explosive environments 

Figure 5

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