Are wireless sensors ready for me?

A reader wants to know what his best options are for reliable, and easily connectable wireless sensors for vibration monitoring, temperature and presence sensing, and other monitoring and control applications.

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The AnswerWe need wireless sensors for vibration monitoring, temperature sensing, presence sensing, and other machine monitoring and control applications. These sensors need to be inexpensive, reliable, and easily connectable to our PLC and PC-based control systems. Do wireless sensors that meet our qualifications exist, or are we asking too much too soon?

---From March 2007 Control Design


I’m Still Waiting
I have not seen any wireless sensors for these applications, but I can see how they would be useful for equipment preventative maintenance reasons. I have been advocating that motion control vendors consider making vibration monitoring at least an option as part of their motor/controller design to make it easier for OEMs to provide this functionality without adding separate sensors, I/O, and the logic to go with it. Particularly these days with more and more equipment going shaftless, this feature would be a benefit. This concept wouldn't eliminate the need for devices the machine builder is asking for in areas that need to be monitored and which are not independently driven.

Chris Cote, manager – R&D Electrical Engineering, Goss International Americas



Tough Sell for Users
In my experience in the bulk material-handling industry, moving our clients into using networked fieldbus I/O is sometimes a hard sell. So, using wireless sensors as I/O might be asking a little too much. If the wireless sensors are used for monitoring and not for critical control, I can see a growing need for reliable and inexpensive wireless sensors. These sensors must be easy to maintain and configure by maintenance personnel. Wireless sensors will have different needs and requirements depending on what industry is being served. While these sensors might not take hold in one industry, another industry could have an immediate need, allowing wireless sensors to fill a needed role in process control.

Norm Seidel, Electrical Design & Controls, Smoot--Division of Magnum Systems



How We Do It
The practicality of wireless sensor technology has made great leaps forward in the past couple of years, and wireless sensor networks that are affordable, reliable, and easy to connect to PLCs and PCs now are on the market.

The following relates to a wireless network—such as the SureCross from Banner Engineering—that consists of one or more sensors connected to a node that collects data from the sensors and transmits it to a gateway. It’s important to note that multiple sensors of different types can be attached to one node, so you could monitor temperature, vibration, presence, and numerous other traits at the same time from one node. The gateway, which can receive data from multiple nodes, can control a machine directly or be attached to a control device (other than a safety device), PLC, or PC. This network architecture enables complete bidirectional signal verification, so all devices acknowledge that they’ve received communication from the sending device.

The method a network uses to sustain a signal affects its reliability. Among the most reliable yet cost-effective methods is frequency-hopping, spread-spectrum (FHSS). The information the sensor gathers is transmitted in small packets across numerous radio frequencies in a unique pattern, called the hop-code pattern. If interference on one frequency prevents a data packet from being received, the transmitter moves to the next frequency in the pattern and resends the data packet.

In the rare instance that the signal between a node and gateway is interrupted, devices attached to the network can perform a user-specified action. For example, if the gateway loses communication with an ultrasonic sensor that monitors the level of fluid being pumped into a tank, the pump can shut off until the signal is restored.

Part of the beauty of this type of wireless network is that it can be retrofitted into existing industrial systems, it can operate independently to manage a machine, or it can be used in conjunction with a PLC and/or PC software. Supported communication protocols include EtherNet/IP, Modbus RTU (RS485/RS232) and Modbus TCP/IP, and RS485/RS232.

The cost for a simple wireless network with a gateway and one node is about $1500—far more cost-effective than wiring and easily justified in many applications.

Ty Fayfield, president, Sensonix



Connect Them to Fieldbus
If each sensor must be wireless, then the solution is not a fieldbus. But if the physical arrangement of the sensors permits connection to a fieldbus or fieldbus-compatible I/O, with wireless connection from that point, then wireless Profibus or Profinet could be used. Such situations might include rotating machinery where slip rings are not practical, machines with linear motion where festooned cables are not practical, or autonomous machinery such as automatic guided vehicles. These kinds of applications are common and there are many suppliers of wireless connectivity for Profibus or Profinet. In fact with Profinet, standard IEEE 802.11 wireless components can be used. An additional benefit from this approach is that safety functionality can be integrated. Profisafe integrates safety with standard fieldbus functionality seamlessly over Profibus and Profinet. A number of applications using wireless and Profisafe have been installed.

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