Using Bluetooth for Data Communications in Industrial Automation


This article looks at the use of Bluetooth® technology in industrial automation applications to provide an alternative wireless data communications capability. It looks at the advantages compared to other wireless technologies and how issues such as interference can be handled. It covers the Stellaris Bluetooth development board from Texas Instruments as well as the Microchip RN-42, ConnectBlue OBS411 and RFM’s WLS1271 modules.

Although better known as a data connection technology in mobile phones, Bluetooth is well suited for wireless integration of automation devices in serial, fieldbus, and industrial Ethernet networks. It has not made dramatic in-roads into industrial automation in Europe, but using the experience and high volume production from the mobile phone business, IEEE 802.15.1 Bluetooth technology can provide small footprint, low power consumption, and cost-effective modules.

Bluetooth provides ready-made modules to simplify the development of a wireless data communications system on the factory floor, but there are some key issues to consider. Bluetooth can be used for two kinds of data network in this area, from providing a flexible user interface for machines to handling data direct from sensors to link to an established wired network such as fieldbus or HART.

For non-mission-critical designs, Bluetooth is increasingly being used to access a built-in user interface based on WEB and WAP technology, mainly for maintenance and configuration. The point-to-point nature of the link is well suited to particular applications such as on a sensor node that is periodically providing data from equipment. This is particularly useful in hard-to-reach areas where the sensor can provide key data on the health of the equipment and flag problems before they become critical.

For the more mission-critical applications, the radio link reliability is key for deterministic behavior and real-time performance. Here, the important issues are the data rates, latency and interference with other Bluetooth nodes, other radio standards such as 802.15.4 ZigBee, and 802.11g and b Wi-Fi, which both operate in the 2.4 GHz band. Other radiating sources like certain types of machinery and microwave ovens operating at 2.4 GHz can also affect the performance of the Bluetooth link.

Another issue is communication error detection and automatic correction. The Bluetooth protocol includes key elements to make this as robust as possible while keeping down the cost and power consumption.

Specification

Bluetooth provides a range of 10 meters, but long-range modules can cover 200 to 400 meters or up to 1 km in free line-of-sight on a campus of buildings. A key element of the Bluetooth standard is Adaptive Frequency Hopping (AFH), Forward Error Correction (FEC), narrow frequency channels, and low sensitivity to reflections and multiple signal paths (also called multi-pathing).

The maximum data throughput for v2.0 of the standard is 780 kbit/s gross and this corresponds to around 700 kbit/s for the data payload. With the latest v2.1+EDR (Enhanced Data Rate), the maximum throughput is increased to 2.1 Mbit/s with a latency of 5 to 10 ms.

To avoid interference, the 2.45 GHz band is divided into seventy-nine 1 MHz wide slots and a new frequency slot is chosen each 625 µS. Each communicating pair of devices has its own frequency-hopping scheme decided when first connected and chosen, in order to avoid conflicts as much as possible. This technique minimizes possibilities for interference within a Bluetooth system and interference with the other radio-based systems.

For robustness, Bluetooth uses both Forward Error Correction (FEC) and packet retransmission. Two FEC codes are used, with a 1/3 rate code for the packet header and a 2/3 rate for the application data. This is a shortened Hamming code and able to automatically correct all one bit errors and detect all two bit errors.

For the packet re-transmission an ARQ packet retransmission scheme is used. Each packet payload contains a CRC checksum to check for errors and each transmitted packet contains an ACK/NAK bit to indicate the status of previous received packet. Retransmission is done if packets are lost or not acknowledged (NAKed).

To get the best data link with minimal power consumption, Bluetooth uses Received Signal Strength Indicator (RSSI) power control. This is mandatory for the long range, high-power radios operating at 20 dBm, but is optional for the low-power radios at 0 dBm. Using the RSSI scheme makes sure that high power is not between a single pair of nodes, and means that independent Bluetooth networks in the same neighborhood will less likely interfere with each other. This helps keep the data rate higher, as the ACK packets will not be lost and have to be retransmitted.

A key difference from other standards is that Bluetooth also demands security features with 128-bit encryption that protects the data.

The complete protocol stack comprises both Bluetooth-specific protocols like LMP and L2CAP, and non Bluetooth-specific protocols like OBEX (Object Exchange Protocol) and UDP (User Datagram Protocol). In designing the protocols and the whole protocol stack, the main principle was to re-use existing protocols for different purposes at the higher layers, instead of inventing new protocols. This allows existing (legacy) applications to work with the Bluetooth technology and to ensure the smooth operation and interoperability of these applications. This means many applications already developed by vendors can take immediate advantage of hardware and software systems that are compliant to the specification.


Figure 1: The Bluetooth protocol stack.

The specification is also open, which makes it possible for vendors to freely implement their own (proprietary) or commonly used application protocols on the top of the Bluetooth-specific protocols.

A key protocol is the cable replacement. RFCOMM is a serial line emulation protocol and is based on ETSI 07.10 specification. This emulates RS-232 control and data signals over Bluetooth baseband, providing both transport capabilities for upper level services such as OBEX that use serial line as transport mechanism. This also makes industrial data transmission systems simple to develop as RS-232 is a well-established and common wired protocol. Using this approach allows a Bluetooth link to simply replace a cable in a design, extending the reach of the data link.

As an example, the OEM SERIAL PORT ADAPTER 411 series from ConnectBlue is a Bluetooth module with support for the Serial Port Profile (SPP) for fast and secure transparent serial data transmissions. The module has a small form factor and low build height, and supports the Bluetooth 2.1+EDR standard for enhanced data rates (see below). As the Bluetooth stack is embedded in the module, it does not require any driver or stack in the host.

Figure 2: The Serial port adaptor 411 from ConnectBlue.

Bluetooth low energy technology

Bluetooth v4.0 with low energy technology entered the market in 2011. It is well suited for sensors, actuators, and other devices that need very low power consumption, and supports higher numbers of nodes in a network and lower power, allowing nodes to run for years from a single coin cell battery. It introduces a new communication method, called the attribute protocol (ATT), which is optimized for small packet sizes used in Bluetooth low energy.

The ATT allows an attribute server to expose a set of attributes and their associated values to an attribute client. These attributes can be discovered, read, and written by peer devices.

The generic attribute profile (GATT) provides a framework for discovering services and for reading and writing characteristic values on a peer device. It interfaces with the application through the application’s profiles.

This may only be relevant for a small number of sensor links to provide a wireless link into a hard-to-reach area of equipment.

Connecting a valve

A good example of Bluetooth used in industrial application is controlling a valve with a built-in control system using the Modbus protocol. The control system provides the status and control variable to the bus interface, and using a Bluetooth adaptor, the data can be carried back to a central access point (AP) that contains a Web server. Via the server, the valve can be monitored and updated. This can be done either via the local area network or locally by monitoring staff.

While this can be done with one valve to start with, once the access point is in place it can be used to easily connect up other devices on the factory floor to create a cluster. The user hooks into the AP and, using a built-in web interface in the AP, sees the list of all the connected Bluetooth devices. The user chooses the desired device establishing a data connection to the device with the AP acting as a router.

Implementation

A key advantage of Bluetooth is that all the certified devices are interoperable at the same level (i.e. v2.0, v2.1 or v4.0). This gives the system designer significant flexibility in choosing the right device or module for different sensors on the factory floor. Some may be interoperable across the different specifications.

The DK-EM2-2560B Stellaris 2.4 GHz CC2560 Bluetooth Wireless Kit from Texas Instruments provides an easy way to evaluate the capabilities of the TI CC2560 Bluetooth Transceiver and Bluetopia Bluetooth Protocol Stack. It includes an expansion board and a Panasonic PAN1323 ETU Bluetooth Module featuring Texas Instruments’ CC2560 Bluetooth Transceiver as well as the eZ430-RF2560 Bluetooth Evaluation Tool. This provides the capability to set up the Serial Port Profile (SPP) to provide a data communication link from the module.

The Microchip RN-42 is a small form factor, low power, class 2 Bluetooth radio for designers who want to add wireless capability to their products. The RN-42 supports multiple interface protocols, is simple to design in, and is fully certified, making it a complete embedded Bluetooth solution. With an on-chip antenna and support for the higher data rate Bluetooth EDR specification, the RN-42 supports links up to 3 Mbps over distances up to 20 meters.

The RN-42 is also available in a package without an antenna that is shorter in length and can be used for longer-range applications or more space constrained designs.

While not required, the SPI bus interface is very useful for configuring the Bluetooth modules’ advanced parameters. The bus is required when upgrading the module’s firmware and uses a 6-pin header to gain access to this bus. A minimal version might simply use the SPI signals (4 pins) and obtain ground and VCC from elsewhere in the design.

Figure 3: The Microchip RN-42 small form factor Bluetooth module.

Figure 4: The RN-42 block diagram.

Coexistence in a module

Other 2.4 GHz wireless technologies are not mutually exclusive with Bluetooth. The WLS1271L from RFM provides 2.4 GHz WLAN plus Bluetooth functionality in an ultra-small module. It is designed to fit into small spaces, requiring a minimum of external components to operate.

The module is based on TI’s WL1271L SOC and has optimized RF performance including a high-efficiency RF front-end circuit. A key advantage of using the module approach is that an integrated DC-DC converter allows the module to operate from a single input voltage.

Figure 5: The RFM WLS1271L module block diagram showing the Wi-Fi and Bluetooth coexistence.

It includes an IEEE 802.11b/g + IEEE 802.11n WLAN MAC baseband processor that supports higher data rate Wi-Fi links, as well as the Bluetooth point-to-points using a single-ended digital radio processor (DRP) RF implementation with internal LNA.

On the Bluetooth side it supports v4.0 with the low power modes including the 2 and 3 Mbps enhanced data rates and has a high rate 4-wire UART HCI (H4) and 3-wire UART HCI (H5) interface to connect to sensors or other equipment. It is optimized for low current consumption in all operating modes at 35 mA and has a low current sleep mode to reduce the power consumption with a long duty cycle in an industrial application.

On the software side, it supports lower Bluetooth layers up to HCI included and software drivers are available for Linux®, Android™, and WinCE operating systems for embedded designs.

Conclusion

Using Bluetooth in industrial designs for data transmission is becoming an increasingly popular option. The low cost and availability of devices and modules that are interoperable is giving system architects more options for providing data links across the factory floor. Adding an access point that is connected to the Fieldbus network in the factory and using low cost Bluetooth modules to connect to the equipment around it is a scalable, incremental way of adding more control and monitoring. This provides more information that allows the factory operator to replace equipment before it fails, maintaining availability and enhancing reliability.

The new generation of Bluetooth v4.0 low energy devices is also providing more options. By integrating alongside Wi-Fi, there are more wireless options for transmitting data without worrying about interference, and the low energy profiles allow sensor modules to run for years from a small battery, almost avoiding the high costs of replacing batteries in hard-to-reach places. As time goes on, so the costs of modules falls further, leveraging the high volumes of chips in the mobile market, making Bluetooth even more attractive to the industrial equipment designer.
Supplier