New Applications of Wireless Digital Sensors

Even in their infancy, digital sensors are expected to be a major driver of the electronics market in the near future. Creating interfaces for digital sensors and supporting the diverse communication protocols used in digital sensor networks are both huge challenges to the technology. The heterogeneous characteristics of sensors and the diversity of their operating conditions also pose huge challenges to the technology. There are currently more than 3,000 sensor manufacturers operating worldwide, and Intechno Consulting estimates that their total sales will exceed $50 billion in 2008.

Emerging Standards for Digital Sensors

Today, sensor networks use a variety of technologies to serve different industries. The number of proprietary protocols used in sensor networks has increased dramatically. This requires the introduction of comprehensive translation paths in sensor networks, which is a complex and expensive system. Therefore, people need a greater degree of standardization.

The Internet Engineering Task Force (IETF) is developing a standard for wireless sensor networks based on Bluetooth, Wi-Fi and 802.15.4 networks to connect sensor nodes to the wider Internet. The goal of the IETF is to establish a way for sensors to communicate without the need for proprietary translation paths. The protocol will be finalized in June 2009. The standard, developed by the 1451 committee of the IEEE (Institute of Electrical and Electronics Engineers), was introduced in the mid-1990s and provides a common and fully transparent interface between transducers (sensors and actuators) connected in a network. This standard system includes different subsystems: IEEE P1451.0 defines the necessary conditions, communication protocols and basic functions at the physical level; IEEE1451.1 controls the acquisition and transmission of information on the network; IEEE1451.2 deals with the interface of wired networks and is compatible with standards such as RS-233, ES-485 and USB; IEEE1451.3 enables the operation of sensors in multi-branch networks; IEEE P1451.4, adopted in 2003, stipulates that data related to digital sensors (such as sensor type, model, operating parameters and location) must be able to be read from the EEPROM (electrically erasable read-only memory) memory integrated in the sensor in the format of an electronic data sheet, which is called a transducer electronic data sheet (TEDS-Transducer Electronic Data Sheet). The IEEE P1451.4 standard greatly simplifies the wiring of sensors and the replacement of faulty sensors. It also eliminates manual intervention in the setting and calibration of sensors, which is time-consuming and error-prone. In this way, sensors become plug-and-play devices. The National Instruments website contains a database of valid TEDS, which includes specific information on various older or non-EEPROM sensors that can be used in IEEE P1451 networks. The IEEE 1451.5 standard is still under review and specifies the necessary conditions for connecting TEDS to wireless networks, such as 802.11, Bluetooth, or ZigBee. Recently, the IEEE 1451.6 standard proposal was proposed, which defines the interface between TEDS and the CAN bus.

Figure 1 The IEEE P1451 standard was developed to provide a common and fully transparent interface between transducers (sensors and actuators) connected to the network.
Wireless Digital Sensors

Billions of wired sensors are widely used in electronic systems, ranging from simple thermocouples to more complex application-specific systems. They are used to measure and monitor physical parameters. Many engineers are considering switching to wireless sensors because they have some significant advantages, including low price, good adaptability, and easy installation and use even in adverse environments. According to market analysis agency Cahners Instat, by 2010, more than 160 billion wireless sensor network nodes will be sold.

The wireless network of sensors connects various sensors distributed in a certain area and combines them with intelligent circuits for signal processing and data transmission. Its possible application areas include military, environmental and road traffic monitoring, security and alert, home and industrial automation, medical and health systems and automobiles. In industrial environments, 80% of the cost of building a sensor network is spent on wiring, and building such a network is sometimes unrealistic or even impossible. In the automotive field, an important application of sensors is for remote keyless entry systems (RKE-Remote Keyless Entry Systems), which replaces the CAN bus with wireless, which greatly saves cost and space and removes expensive and thick cables. According to a recent forecast by Cahners Instat, more than 160 billion sensor network nodes will be sold by 2010; and it is very possible to build a network containing 10,000 to 100,000 sensor nodes. In this way, scalability becomes a very critical factor. In addition, the sensors must be programmable and self-configurable on site; the network must be able to operate normally in the presence of one or more faulty sensors. Another key parameter is energy consumption, because wireless sensors may also be set up in remote areas. In terms of wireless sensor networks, a very promising wireless technology is ZigBee, which is developed according to the IEEE 802.15.4 standard and can ensure low energy consumption and low cost. The founding companies of the ZigBee Alliance are Huawei, Ember, Freescale, Philips, STMicroelectronics, Texas Instruments, Samsung and Siemens. West Technology, a company that conducts market research on radio technology, predicts that the sales volume of sensor chipsets made with ZigBee technology and compliant with the IEEE 802.15.4 standard will increase from 31 million sets this year to 312 million sets in 2012, with a compound growth rate of slightly more than 100%. Other wireless technologies used in wireless sensor networks include Wireless-USB launched by Cypress Semiconductor; and z-Wave, a technology developed by Zensys, which has more than 160 companies forming a z-Wave Alliance to jointly promote this technology.

Wireless Sensors: Emerging Uses

Sensors for residential applications have a potential market of 6 billion nodes worldwide, and wireless sensor-based monitoring systems are an important example of wireless sensor applications. By 2012, wireless sensor networks for smart homes will have a global market value of $2.8 billion; by comparison, the market was only $470 million in 2007. The most promising uses for sensors are lighting control, energy-saving systems, security modules, entertainment systems, and telemedicine. Wireless sensors are easier to install than wired sensors and eliminate expensive and bulky cables. This advantage is particularly important in the industrial sector, where cables account for 80% of the installation costs of sensor networks. Wireless sensors have advantages in harsh environments, where sensors must withstand vibration, high temperatures, electronic noise, and even explosive gases. Installing wired sensors in such harsh environments is impractical, expensive, or even impossible. Security issues and possible interference between different radio networks in a factory have hindered the use of wireless sensors in the industrial sector in the past. Experience with wireless sensors in other fields can be used as a reference to solve these problems in industrial environments. For example, decoding techniques are used to protect wireless sensor networks from unauthorized access.

Another strong area of ​​wireless sensor applications is generated by tire pressure sensors in automotive pneumatics. Under a law passed in 2001, the United States requires that every new car must be equipped with such tire pressure sensors. The Yole Développement, a French market analysis company, predicts that the market for such sensors will reach $183 billion by 2012, compared with $168 million in 2007, with a compound growth rate of only 2% due to falling prices. In terms of sales volume, Germany's Wicht Technologie predicts that the number of such wireless sensor nodes on the market will increase from 100,000 in 2011 to more than 60 million in 2015. If tire pressure sensor laws similar to those in the United States are passed in Europe and Asia, the market may exceed $300 million by 2012.

Necessary conditions for wireless sensor networks

The main component of wireless sensor networks is digital sensors. The main requirements for wireless sensors are a reduced form factor, low power consumption, and effective operating range, which is proportional to the transmitted radio signal strength and power consumption. Wireless sensor networks must be efficient, reliable, and scalable, and they should be able to support a large number of sensor nodes (10,000 to 100,000). Another requirement is extremely low maintenance requirements. Wireless sensors are often installed in remote areas, where human intervention, even for simple tasks such as changing batteries, is difficult and expensive. Therefore, wireless sensors must have extremely low power consumption so that they can operate for months or even years between battery replacement cycles. For these reasons, several battery-free solutions have been developed. They are based on energy harvesting technology, which can extract the energy required for operation from the environment. They can usually use energy from solar energy, thermal energy, or vibration. In addition, the sensors must be able to be programmed and self-configured in the field; the network can operate even in the presence of one or more faulty sensors. Signal transmission rate is usually not an issue because in most applications, the amount of data generated by the sensors is not large and can be sent in a discontinuous manner. Liu Lili (translator)