How Much Energy Needs for Running Energy Harvesting Powered Wireless Sensor Node?

Introduction

Development on consumer electronics, personal computing and wireless communications makes people become more interested in connecting different devices to form a network (Riddle 2009). A Personal Area Network (PAN) can now easily be formed by using cables to connect devices in order to achieve communications between them. However, in the cable-connected networks mobility, flexibility and scalability of the connected devices are very low since the devices are required to be connected at particular positions with special interface plugs (Riddle 2009). Wireless Personal Area Network (WPAN)s solve these problem in extent. Cost-effective WPANs have the unique potential to implement wireless connectivity in many end products where this functionality was not considered previously. This resultantly improves the mobility, flexibility and scalability of the system. Furthermore, they are also useful for sensing, monitoring, and controlling applications. These advantages of WPAN have led to so many applications and different standards. Examples include health and wellness monitoring devices like FitBit (Montgomery-Downs, Insana, and Bond 2012), WiThings (ASMAR 2011), Nike+ (Kane et al. 2010) or mobile accessories such as Pebble Watch.

Further to strengthen the ideas and studies, a new group 15 was added on the Institute of Electrical and Electronics Engineers (IEEE), 802 local and metro access network Local Area Network (LAN)/Metropolitan Area Network (MAN) standard. The main task of this group is to solve the wireless personal area network (WPAN) problems. Depending upon the end user application, this group was further broken down into seven task groups at the moment where 802.15.1 deals with Bluetooth devices and standard, 802.15.2 arranges the coexistence between WPAN and Wireless Local Area Network (WLAN) devices when operating in the same frequency, 802.15.3 caters for systems which require high data rate and 802.15.4 provides applications for system which requires low energy consumption rate. 802.15.5 foster for mesh networking, 802.15.6 incorporates all the applications where body area networks are involved and 802.15.7 deals with visible light communications. Depending upon the user specification and requirements, same task can be carried out by different task groups.

IEEE 802.15.4 Zigbee

The main motivation of having a IEEE-802.15.4 standardwas to cater for low power WPAN applications (Li et al. 2009). The origin of ZigBee came from the behaviour of honeybees. After zigging and zagging around the fields, the bee returns to the hive and performs a Waggle dance to communicate the distance, direction and type of food to the other bees in the hive. After coining of the name, an alliance was also formed known as the ZigBee Alliance. The main aim of the alliance was to work together for reliable, cost-effective, low-power, wireless networked monitoring and control products based on an open global standard (Zigbee 2015).

The main applications of ZigBee are to provide wireless connectivity which leads to application in building automation, personal health care (Jovanov 2006; Istepanian, Jovanov, and Zhang 2004), industrial control (Verdone et al. 2010), residential and commercial control, personal computers and consumer electronics. ZigBee can be used to build applications related to human life. We can control lights and switches, thermostats, furnaces, hotel-room air-conditioners, the front desk, central command posts and other appliances. Although ZigBees underlying radio-communication technology is not revolutionary, it can support data rate till 250 kbps (Akyildiz et al. 2002; Tubaishat and Madria 2003). This data rate enables to increase the battery life, which can be enhanced up to 10 years. It has been shown that when using ZigBee protocol for just opening and closing a gate can run a AA battery for up to five years (Zigbee 2015). Furthermore, it provides two-way communications, which makes the utility more applicable than the previous standards, which only provided one-way communications, such as garage door openers, and clapper that turns light on or off. It gives greater access to many sensors that can be linked to perform different tasks. All these advantages make the wireless community be interested to add ZigBee to mobile devices so that a mobile can also be turned into a remote controller.

There are three types of ZigBee nodes in the network topology (Lu, Krishnamachari, and Raghavendra 2004).

• Coordinator: This is the most capable node in the system. It plays a fundamental role when initializing the system. It assists in starting the network, selecting the radio channel for transmission and enabling different nodes to join the network. Therefore, there is exactly one ZigBee coordinator in each network. It is able to store information about the network, including acting as the trust centre and repository for security keys. A ZigBee network is incomplete without a network coordinator.

• Router or Full Function Device (FFD): As well as running an application function, a router relays messages from one node to another. It also helps to increase the network size by enabling the nodes to connect to the network (Bougard et al. 2008). The router has to remain active and cannot sleep. As the router supports all the functions, it is sometimes referred to as a full function device.

• End Device or Reduced Function Device (RFD): Contains just enough functionality to communicate to the parent node. Therefore, either the coordinator or a router it cannot route data from other devices because of limited memory (Petrova et al. 2006). This functionality enables the end device to sleep for a significant amount of the time, which results in a longer battery life. The end device is less expensive as compared to a router or a coordinator. They can be though as nodes that only serves to interact with the physical world.

Zigbee Operation

There are three types of ZigBee nodes in the network topology (Lu, Krishnamachari, and Raghavendra 2004). Coordinator: This is the most capable node in the system. It plays a fundamental role when initializing the system. It assists in starting the network, selecting the radio channel for transmission and enabling different nodes to join the network. Therefore, there is exactly one ZigBee coordinator in each network. It is able to store information about the network, including acting as the trust centre and repository for security keys. A ZigBee network is incomplete without a network coordinator.

Router or Full Function Device (FFD): As well as running an application function, a router relays messages from one node to another. It also helps to increase the network size by enabling the nodes to connect to the network (Bougard et al. 2008). The router has to remain active and cannot sleep. As the router supports all the functions, it is sometimes referred to as a full function device.

End Device or Reduced Function Device (RFD): Contains just enough functionality to communicate to the parent node. Therefore, either the coordinator or a router it cannot route data from other devices because of limited memory (Petrova et al. 2006). This functionality enables the end device to sleep for a significant amount of the time, which results in a longer battery life. The end device is less expensive as compared to a router or a coordinator. They can be though as nodes that only serves to interact with the physical world.

ZigBee operates in either non-beacon or beacon mode. In the following, these two modes are described in more details.

Beacon-Mode

Beacon-Mode is a fully coordinated technique where all devices know when to coordinate with each other (Pollin et al. 2008). In this mode, the network coordinator will periodically wake- up and send out a beacon to the devices within its network. This beacon subsequently wakes up each device, which must determine if it has any message to receive. If not, the device returns to sleep. When using beacon mode for transmission following ways are adopted.

Transmitting from a Device to Coordinator

Figure 1 shows the transmission of data from the node to the coordinator. In this technique, the device or nodes first listens to the beacon. On finding the beacon, it synchronizes first to the super frame structure and the start and the end time of the contention access period is determined by this process (Bianchi 2000). The device will now compete with its peers to share the channel. On its turn, it will transmit the data to the coordinator. The coordinator may reply back with an acknowledgement, if it is not optional.

Figure 1:

Beacon-Mode: Device communicates to a coordinator.

Transmitting from a Coordinator to a Device

Figure 1 shows the transmission of the data from the coordinator to the end device. As the coordinator has data to be transmitted to the device, it indicates this in the pending address fields of its beacon. Devices tracking the beacons, decode the pending address fields. If a device finds its address listed among the pending address fields, it realizes it has data to be received from the coordinator. It then issues a Data-Request Command to the coordinator which replies back with an acknowledgement. If there is data to be sent to the device, it then transmits the data. Figure 2 shows the communication flow between coordinator and device in beacon-mode technique.

Figure 2:

Beacon-Mode coordinator communicates with device.

Non-Beacon-Mode

Non-beacon mode is less coordinated, as in this technique, any device can communicate with the coordinator at will (Buratti and Verdone 2009). However, this operation can cause different devices within the network to interfere with one another, and the coordinator must always be awake to listen for signals, thus requiring more power. For non-beacon mode the transmission remains the same, except we do not have the beacon signal as it is in Beacon-Mode.

Energy Consumption Model

Energy consumption models are compared by study (Liu 2000) that shows the components that consume energy in Wireless Sensor Network (WSN)s. In this research, it is assumed that the power energy that is consumed is mostly derived by the Radio Frequency (RF) module for transmission signals that are involved in sending and receiving packets in wireless sensor nodes. Figure 3 shows energy model for WSNs that are based on packet size and distance between transponder and receiver.

Figure 3:

Energy model system.

Following research in (Li, 2013) (Lindsey, Raghavendra, and Sivalingam 2002) (Heinzelman, Chandrakasan, and Balakrishnan 2000), the mathematical model for energy consumption by transmitting and receiving packets per bits of each sensor node are calculated as follows. Energy consumption in RF module in the receiver is given as:

[1]ERxk=Eelec×k

Where E_Rx is energy consumption in the receiver node, E_elec is the energy required to process one bit in the electronic modules and k is the length of message (bit) and energy consumption in transmitter RF module is given as:

[2]ETxk,d=Eelec×k+Eamp×k×d2

Where E_Tx is energy consumption in transmitter node, E_amp is the energy required to transmit one bit in the RF module and k is the length of message (bit) and d denotes the distance between transmitter and receiver measured in metres.

Evaluation System Model

This section of the paper describes the methodology used to calculate the current consumption of the model in active and sleep mode. A detailed explanation of both the modes will be carried out and then we look into the low powered wireless sensor nodes followed by the new design approach of the implemented system.

Figure 4:

Schematics of Jennic microcontroller JN5148.

Figure 4 shows the schematics of JN5148 microcontroller and microcontroller’s pins connections with sensors and com port for serial communication. This schematic has been used for this system model.

Performance Evaluation

Current Consumption in Transmission Mode

In order to design the system, IEEE 802.15 standards has been selected. This standard is ZigBee. The details of these standards can be found in (Lu, Krishnamachari, and Raghavendra 2004). JN5148 as a ZigBee wireless microcontroller is chosen. The transmission/receiving modes are explained in the next sections. The current consumption is the chosen criteria instead of power as the voltage the system is constant to 3.3 Volts. At the moment the current consumption was measured with this assumption that the power to the microcontroller is always available. The current was measured with the Keithley 2612-B (Figure 5) as a power supply and measurement device.

Figure 5:

Lab.

ZigBee Life Cycle Power Consumption

The current across the microcontroller JN5148 is measured with Keithley 2612-B. We have tried to measure the current required to carry out one transmission with the sensors on board. Then the current consumed by the sensors and other wattage has been removed that we can compare how much is the actual required current to carry out a single transmission. Figure 6 shows the current consumption of a JN5148 microcontroller in a life cycle.

Figure 6:

Current consumption in Jennic 5148.

The current consumption of a ZigBee node when using JN5148 is shown in the Figure 6. From the figure it can be observed that the current consumption can be broken down into following steps:

1. As the device utilizes beacon mode, the first step is to listen for the beacon which consumes an average current consumption of 7.1 mA for a duration of 4.1 ms duration.

2. After listening to the beacon there is a wakeup time for the microcontroller which consumes 1.1 mA current for a duration of 3.5 ms duration.

3. Mode-3 is when a current of 4 mA is drawn to power the microcontroller to start measurement. The duration is 0.8 ms duration.

4. Mode-4 is when the microcontroller measures the temperature. It should be observed that ADC uses 0.655 mA current while the temperature sensor only utilizes 6 μA current. The average current consumed is 5.5 mA with a duration of 0.7 ms duration.

5. Mode-5 helps us measure the current consumption when using accelerometer. The current consumed is 7.7 mA which results in 1.3 ms duration.

6. Mode-6 is in between Model-5 and 7 and it measures the current the light sensor requires. The current consumed is 8.8 mA with a duration of 0.8 ms duration.

7. Mode-7 is a peak when the microcontroller is ready for transmission. The current consumed is 10.5 mA for a short duration of 0.2 ms duration.

8. Mode-8 is when the microcontroller searches for a vacant channel. The current consumed is 15.6 mA in 1.6 ms duration.

9. Mode-9 is when the microcontroller carries out the data transmission. The current consumption is about 15.6 mA for 1.6 ms duration.

10. Mode-10 is waiting for an acknowledgement and keeping the receiver on. The current is 18.1 mA and the duration is 1.3 ms duration.

11. Finally, in mode –11 it is a peak to tell end of transmission. The current consumption is 5.7 mA and duration is 2 ms duration.

12. Idle. In this mode the on time is 25 ms and the average current consumption is 5.9 mA.

Table 1:

Current consumption in Jennic 5148.

S/No	Mode	Duration	Current (Avg)	Watt	Energy
1	Beacon	4.1 ms	7.1 mA	23.43 mW	96.06 MicroJ
2	Wake Up	3.5 ms	1.1 mA	3.63 mW	12.71 MicroJ
3	Start Sensing	0.8 ms	4.0 mA	13.2 mW	10.56 MicroJ
4	Temperature	0.7 ms	5.5 mA	18.15 mW	12.71 MicroJ
5	Accelerometer	1.3 ms	7.7 mA	25.41 mW	33.03 MicroJ
6	Light Sensor	0.8 ms	8.8 mA	29.04 mW	23.23 MicroJ
7	Radio On	0.2 ms	10.5 mA	34.65 mW	6.93 MicroJ
8	Channel Scan	1.6 ms	15.6 mA	51.48 mW	82.37 MicroJ
9	Transmit	1.6 ms	15.6 mA	51.48 mW	82.37 MicroJ
10	Acknowledgment	1.3 ms	18.1 mA	59.73 mW	77.65 MicroJ
11	End	2 ms	5.7 mA	18.81 mW	37.62 MicroJ
12	Idle	25 ms	5.9 mA	19.47 mW	486.75 MicroJ
	Total	42.9 ms	6.78 mA	22.37 mW	959.84 MicroJ

The total current consumption can be calculated as = 42.9 × 6.78 equal to 22.37 mWatt for one life cycle for one transmission by JN5148 microcontroller and related sensor circuits. The Table 1 summarises the above steps.

Conclusion

Energy harvesting systems can harvest very limited energy from the ambient environment and designing an energy harvesting powered wireless sensors is a big challenge. To find the requirements of a wireless sensor system, the specification of each element in this system have to be defined. This paper investigates the energy needs to run a wireless microcontroller. The investigation has discovered the energy need in different phases of a life cycle of a wireless sensor node. In this research the power consumption in the life cycle of wireless communication sensors, consisting of JN5148 microcontroller and custom built sensors: a 3-axial accelerometer, a temperature sensor and a light sensor, has been studied. All measurements are based on a custom-built test bed. The power required carrying out a life cycle of wireless sensing and transmission is analysed. The results show for one transmission, the system needs in average 6.78 mA during the 42.9 ms equal to 0.873 mWatt. The most power hungry phase is radio transmission phase and especially in Acknowledgment phase by 18.1 mA in 1.3 ms. The results show the energy usage and time period in 12 phases of one transmission in wireless sensor node.