Harvested energy powers industrial automation sensor networks

Ultra-low-power radio frequency transceivers enable a new class of short-range industrial automation sensor networks powered by harvested energy.

05/21/2012


Energy harvesting sensor application. Courtesy: Microsemi Corporation

Short-range sensor networks are used for wireless communication in factories, industrial complexes, and commercial buildings, where they improve manufacturing efficiency, safety, reliability, automation, and security. Applications include ambient/environmental monitoring, industrial building automation and security, access control, structural health monitoring, tire pressure monitoring systems, tank level monitoring, wireless cold chain tracking for pharmaceutical shipments, and flexible smart cards for embedded autonomous sensors, to name a few.

Until recently, almost all sensor networks used costly wired data communications and power connections. Moving to wireless protocols eliminated the data communications wiring, but still required power sources. Batteries such as AA cells have provided a powering solution, but replacing them when they wear out can be expensive, especially when sensors are installed behind walls or in other, similarly unreachable locations.

Now, energy-harvesting sensor nodes are being combined with ultra-low-power radio transceivers to enable a variety of short-range wireless sensor networks for building and industrial automation that do not require battery replacement.

Momentum is building for these solutions, as groups such as ISO/IEC are releasing standards, and system vendors demonstrate the benefits of self-powered wireless monitoring and control systems for sustainable buildings using standards-compliant, interoperable technology and products. Short-range radio transceivers play a pivotal role in enhancing the efficiency of this new class of wireless sensors that operate on harvested energy in the industrial automation environment.

Energy-harvesting wireless sensors

Industrial wireless sensor networks powered by harvested energy can be used to perform a variety of useful functions including factory automation, measurement, and control. Today, several system vendors are developing and promoting self-powered wireless monitoring and control systems for sustainable buildings, using self-powered wireless monitoring and control system standards using energy harvesting technology.

Energy-harvested sensor nodes use low-cost integrated circuits to perform sensing, signal processing, communication, and data collection functions, and also include a low-power wireless communications interface. On the radio link side, sensor designers look at ways to keep the communications very short, infrequent, and mostly unidirectional. In addition, the right choice is required for establishing communication frequencies that provide a good range even at low transmit power and avoid collisions from disturbers. This allows the use of small and low-cost energy harvesters that can compete with similar battery-powered devices.

A basic energy-harvesting wireless sensor consists of the following blocks (see Figure 1):

  • A sensor to detect and quantify any number of environmental parameters required in the application
  • An energy harvesting transducer that converts some form of ambient energy to electricity.
  • A power management module to amplify the energy, regulate voltage supply, and implement energy storage management required by the sensor node
  • A microcontroller to manage the signal from the sensor and communicate with the radio link
  • A radio link with or without an RF wakeup receiver function at the sensor node. 

The output of the sensor is typically connected to a microcontroller, which processes the signal created from measuring the parameter of interest (such as temperature, pressure, acceleration, etc.). The microcontroller usually feeds this information to the radio and controls its activation at some prescribed time interval, or in response to the occurrence of a particular event, which could be generated by an RF wakeup receiver.

Wireless sensors that operate on harvested energy have a special set of needs that are more stringent than traditional wireless sensors. One of the most important is power efficiency. The microcontroller and radio must operate in low power modes whenever possible to maximize the power source lifetime. The drain on the power source can be dominated by steady state or active power consumption depending on the quiescent current of the radio and microcontroller, as well as the transmitter power and duty cycle, and the complexity and duration of any required signal processing. One way to reduce power consumption is through microcontroller firmware algorithms that efficiently manage power-up and power-down sequences, analog-to-digital conversions, and event-driven interrupts. But this isn’t enough.

Ultra-low-power radios

Another key way to optimize power efficiency for energy-harvesting wireless sensors is to use ultra-low-power short-range radio transceivers. There are many factors related to transceiver implementation that, when properly addressed, drive significant improvements in power efficiency. These include the following.

Power supply

The transceiver’s power supply requirement is key a factor in the design and application of energy-harvesting wireless sensors for industrial applications. To run sensors from energy harvesting sources, sub-2-V supply voltages are preferable. This means that short-range radio transceivers must be designed for low-voltage operation—ideally, down to 1.1 V in order to optimize design flexibility and reduce power management constraints. In contrast, radios that operate at 2.5 V consume twice as much power as those with the same current consumption operating at 1.25 V. Operating at higher voltage is only required when output power in excess of 5 dBm is needed. In short-range applications, output power rarely exceeds 0 dBm. Other key power supply considerations include the ability to maintain transceiver and receiver performance, and the use of a current profile without excessive peaks to fit supply impedance.

Peak current

Peak current is another key transceiver parameter. Almost all wireless-based sensor networks rely on duty-cycling to save power and restrict the usage of radio space. This generates peaks in the current consumption profile of the sensor. Low peak current consumption in the radio transceiver reduces constraints on the wireless sensor’s power supply. These constraints are even more important for wireless sensors that run from harvested energy sources. Often, energy harvester transducers have higher output impedance than batteries. The micro-power management layer between the transducer and the sensor converts the supply characteristics, including source impedance. Therefore, the low peak current consumption in the radio transceiver reduces constraints on the power supply of the wireless sensor.

Power amplifier (PA)

A third issue to consider is the radio transmitter’s PA, whose power consumption can be very large. Many IEEE 802.15.4 or Bluetooth radios consume 25-40 mW for a 25-meter free-space range, and waste more than 95% of it. The three main factors impacting power consumption are the transmitter PA’s receiver sensitivity, output impedance, and carrier frequency. They are additive, and together can represent over two orders of magnitude in PA power consumption variation for an identical range.

Among these PA factors, the PA receiver sensitivity is particularly important, and defines, for a given range, how much power must be radiated. Most radios fall into the -85 dBm to -95 dBm sensitivity range, resulting in a factor 10 in PA power consumption. Output impedance also affects PA power consumption, and most radios have output impedance below 100 Ohms. Low impedance is only required for high output power in long-range applications, and results in up to five times higher current consumption than higher output impedance options that are more suited for short-reach wireless interconnect applications. Overall, assuming a similar receiver sensitivity and PA efficiency, a high-impedance 900 MHz radio would use only 1 mW in its PA to achieve the same range as a 50 Ohm 2.4 GHz radio using 25 mW to 40 mW.

The choice of carrier frequency also influences PA power consumption. The two available options within the industrial, scientific, and medical (ISM) radio bands are 2.4 GHz or sub-GHz frequencies. The most prevalent 2.4 GHz protocols in the industrial control, process automation, and public utility markets are Bluetooth (owned by Bluetooth SIG Inc.) and ZigBee (ZigBee Alliance), which offer highly sophisticated link and network layers, and enable large and complex networks. They also enable messages to travel long distances, hopping from node to node, but their frequency is usually very low, from every few minutes to hours or even days, which is at least an order of magnitude lower than typical Bluetooth Low Energy (BLE) and ANT Alliance protocol stack alternatives.

In low-power and lower-data-rate industrial monitoring applications, however, sub-GHz wireless systems offer several advantages, including reduced power consumption, as well as longer range for given power. The Friis Equation quantifies the superior propagation characteristics of a sub-GHz radio, showing that path loss at 2.4 GHz is 8.5 dB higher than at 900 MHz. This translates into 2.67 times longer range for a 900 MHz radio since range approximately doubles with every 6 dB increase in power. To match the range of a 900 MHz radio, a 2.4 GHz solution would need greater than 8.5 dB additional power. Another benefit of sub-GHz carrier frequencies is that they reduce the risk of interference from airways that are crowded with colliding 2.4 GHz Wi-Fi (Wi-Fi Alliance), Bluetooth, and ZigBee signals used in wireless hubs, computers, and cell phones. Sub-GHz ISM bands are mostly used for proprietary low-duty-cycle links and are not as likely to interfere with each other. The quieter spectrum means easier transmissions and fewer retries, which is more efficient.

Furthermore, the narrower sub-GHz bandwidth creates higher receiver sensitivity and allows efficient operation at lower transmission rates. For example, at 300 MHz, if the transmitter and receiver crystal errors (XTAL inaccuracies) are both 10 ppm (parts per million), the error is 3 kHz for each. For the application to efficiently transmit and receive, the minimum channel bandwidth is two times the error rate, or 6 kHz, which is ideal for narrowband applications. The same scenario at 2.4 GHz requires a minimum channel bandwidth of 48 kHz, which wastes bandwidth for narrowband applications and requires substantially more operating power.

Payload transport time

Finally, overall power consumption of a wireless sensor is influenced by the amount of time the radio needs to run to transport the payload data over the air. This is dependent on the data rate requirement, the protocol overhead to establish and maintain the communication link, and the latency requirement of the network.

The data rate is a particularly important for duty-cycled wireless links. The average power is almost inversely proportional to the link data rate. For instance, a 100 kbps radio will consume almost half the power of a 50 kbps radio for the same payload. For any given payload, a higher data rate can be seen as a way to improve energy efficiency. For this reason, “energy per bit” is a better indicator when evaluating RF transceivers than current consumption. But high data rate radios are often those with the higher peak currents, and these are highly undesirable for most small batteries or energy harvesters as they result in large, leaky, storage capacitors, generally a few 100 microfarads.

Some protocol stacks generate more overhead than others during payload transport. Standards like ZigBee and Bluetooth offer highly sophisticated link and network layers, but also have larger overheads. For ultra-low-power systems, the “one size fits all” standardized option is rarely the optimum solution. Instead, designers developing solutions for ultra-low-power applications should consider using the protocol best suited for the need.

The latency requirement of the network also has a significant impact on payload transport time and associated power consumption, including the amount of time nodes spend listening, or “sniffing,” which is a function of latency. Low latency means continuous or frequent sniffing. In highly duty-cycled systems, the receiver power due to sniffing is the largest portion of the power budget. For example, in IEEE 802.15.4 mesh networks, about 9% of the system power is used for receive functions. In higher payload systems, sniffing may not be as dominant, but receive power will still be more than 50% of the RF budget. The lowest possible receiver power consumption is often essential to achieving ultra-low-power RF telemetry.

Block diagram of a typical wireless sensor based on the Microsemi ZL70250. Courtesy: Microsemi Corporation

A transceiver can address factors driving sensor efficiency in energy-harvesting industrial applications. The design can be optimized for supply voltage and peak power consumption and housed in a chip-scale package approximately 2 mm x 3 mm. It can have standard 2-wire and SPI interfaces for control and data transfer using any standard microcontroller. An analog-to-digital converter (ADC) in the microcontroller connects to the ultra-low-power analog front-end device. See figure 2.

Arrival of energy-harvesting power technology, combined with advances in ultra-low-power transceivers, is making it possible to build smart, flexible, and smart wireless sensors for a variety of industrial control and building automation applications. Proper transceiver selection is critical for addressing the key design issues that drive optimal power efficiency, so that short-range wireless sensor networks can perform duty-cycled spot measurement and other functions without having to change batteries.

- Reghu Rajan, technical marketing, Microsemi Corporation, communications and medical products group (CMPG); Edited by Mark T. Hoske, content manager CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer. Hoske can be reached at mhoske(at)cfemedia.com.

www.microsemi.com 

www.bluetooth.org 

www.wi-fi.org 

www.zigbee.org 

http://controleng.com/wireless

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Transceiver for sensor efficiency in energy-harvesting industrial applications

One example of a transceiver solution that addresses factors driving sensor efficiency in energy-harvesting industrial applications is the ZL70250 device from Microsemi, which is optimized for supply voltage and peak power consumption. Housed in an approximately 2mm x 3mm chip-scale package, it has standard 2-wire and SPI interfaces for control and data transfer using any standard microcontroller. The microcontroller’s analog-to-digital converter (ADC) connects to the ultra-low-power analog front-end device as shown in the second diagram.