One option would involve the use of a higher gain or directional antenna to improve signal strength. Another is the ability to use a meshing system. Meshing is the ability to use devices known as routers or repeaters to essentially extend wireless signals and make communication paths more reliable. When a wireless sensor network exists with meshing capabilities, various communication paths can be created, as seen in Figure 1. In partial mesh topology, nodes are only connected to certain other nodes, but in full mesh topology, every node is connected to one another. This way, if a device loses communication, perhaps by loss of battery power, an alternate communication path is automatically created, greatly reducing the chance of data loss. While Bluetooth operates primarily on a master/slave relationship, ZigBee and WirelessHART support meshing. In Figure 2, we can see a common configuration of a ZigBee mesh network consisting of end devices, routers and coordinators. Coordinators and routers can communicate with any other device, but end devices can only communicate with routers and coordinators.
Figure 2: ZigBee Wireless Networks |
A typical WirelessHART network configuration is an example of a full mesh network topology. In addition, ZigBee has a retry metric that offers the ability to resend data if the original message was not properly sent and received the first time. In ZigBee, there are proper sequences that need to take place, such as acknowledgments between the wireless device and the gateway when a data packet is transmitted. If an acknowledgment does not occur on both ends, it is assumed that the data was lost and then attempted to be resent. This is a parameter that can be configured similar to other ZigBee parameters.
Figure 3: Battery Life Chart |
It should be noted that many of the features mentioned above have an effect on the battery life of the wireless sensing devices. One example of this is turning on 128-bit encryption, which tends to consume slightly more battery life than if it were off. Similarly, the latency and/or response time of wireless sensors also have a large effect on expected battery life, as seen in Figure 3.
Many wireless sensors have programmable transmission frequencies; in other words, it is easily adjustable to ask the device to transmit a reading once every minute or 10 times per second. Considering the actual radio frequency transmission consumes the most battery power, we can expect to see a shorter battery life from a device that transmits 10 times per second. For this reason, a consumer may choose a ZigBee-enabled device. ZigBee is known for its ultra-low power requirements. The transmission rates are programmable, along with the actual transmit power. ZigBee devices also resort to a very low power mode when the device is not transmitting in an effort to conserve power.
After discussing the sensing side of the wireless equation, it is also imperative to understand the receiving portion, also known as the gateway device. Gateways are used to receive wireless signals, but more importantly, process the data to allow interface with a company’s existing control equipment. These gateways offer numerous hardware interfaces, such as serial RS-232 or USB and digital I/Os or relays, as well as software interfaces, such as MODBUS-RTU or TCP/IP, OLE for Process Control (OPC) or FTP and Telnet. Common applications involve connecting a gateway device to a CNC machine’s controller through RS-232.
Wireless pressure sensors can monitor clamping pressures on tooling pallets inside of machines to verify that parts are being machined properly. These measurements would be provided to the machine’s controller through MODBUS-RTU. If the clamping pressure is too low, the machine’s cycle would automatically cease, preventing the destruction of accompanying machine parts. Another option would be for temperature measurements to be wirelessly transmitted to a gateway device. This data would be processed and published to a company’s network or existing historian software over an Ethernet connection having an OPC interface.
Figure 4: ZigBee Message over IP |
Recently, many companies have shown great interest in having the ability to control all of their sensing needs via the Internet. ZigBee, for example, is one of the first wireless standards to adopt a functional IP protocol and set of IP-connected devices that can interoperate natively with other IP-connected devices. ZigBee’s proprietary IP protocol, implemented in the application layer, is designed to easily integrate existing ZigBee devices. The standard ZigBee data packet structure is combined with a conventional IP data packet, as seen in Figure 4.
The goal is to define a compact, low-traffic message format that can support embedded systems that are attached to low-bandwidth, low-power networks. Devices are allowed to communicate with one another by implementing manufacturer-specific application profiles. These profiles are a set of parameters that define communication channels and other vital settings to assure proper communication and operation between devices. ZigBee/IP profiles also allow for larger sensor networks and increased network security.
Another main focus of this protocol is to fit the needs of extremely resource-constrained devices. Unlike HTTP or XML, which require more resources to decode and process, the binary messages created by ZigBee are simpler to implement and are much better suited for low-bandwidth networks.
Aaron LaJoie works for Electrochem Solutions Inc. For more information, please visit www.electrochemsolutions.com.
