Energizing low power WSN using Microstrip Antennas at GSM-900 band

Abstract-

The global demand for "green" Technology  is driving a new generation of low-power Wireless sensor networks. Wireless sensor networks (WSN) are being developed for use in remote sensor-based systems, for both industrial and control applications. This article presents an RF energy system that can harvest energy from the ambient surroundings at the downlink radio frequency range(935.2MHz – 959.8MHz) of GSM-900 band. The harvesting system provides an alternative source of energy for energizing low sensor networks. The system design consists of three modules: two E-shaped patch antennas, a pi-matching network and a 7-stage voltage doubler circuit. All the three modules can be fabricated on a single printed circuit board. Previous harvesting systems constitute a single antenna, so the output power is considerably low. In this system, two Microstrip antennas are used to receive the RF signal and therefore the combined power of the two antennas is much higher than the previous system. The harvester nearly produces a voltage of 6V for a received signal of -27dBm. This voltage is enough to power a wireless sensor system.          
  1.Antenna:

The antenna applied here was an E-shaped single patch from the conventional wide band microstrip antennas. The antenna is designed and optimized to capture the energy from the ambient at a downlink radio frequency range of GSM-900 band. In order to expand its bandwidth, two parallel slots are incorporated into this patch. The pi matching network is designed and optimized to provide an impedance matching for the antenna. The feed line is appropriately positioned on the upper leg of the E-shaped patch antenna as shown in Figure 1. The slot length, width, and position are important parameters in controlling the achievable bandwidth. Traditionally, the property of the patch antenna is suitable for narrow bandwidth applications. The challenge here is to make the patch antenna for the wideband energy harvesting environment. The antenna design required to look into the permittivity or dielectric constant of the substrate, width, length of the patch antenna and the ground plane. The permittivity of the substrate plays a major role in the overall performance of the antenna.

2. Matching Network:

            The matching network in the geometry was designed to provide a good impedance match for complex load (RF-DC convertor) impedance 63-j117 Ω to source (antenna) impedances 377 Ω to transform maximum power from the source to the load. The output of the pi matching network is directly connected to the input of RF-DC converter circuit.

3. RF-DC Conversion Module:

The An energy conversion module is a voltage doubler circuit used to convert the harvested energy from ambient radio frequency into DC voltage. The design contains stages of the Villard voltage doubler circuit. A 7-stage Schottky diode voltage doubler circuit is designed, modeled, simulated, fabricated and tested. Multisim is used for the modeling and simulation. Simulation and measurement were carried out for various input power levels at the specified frequency band. The voltage multiplier circuit in this design uses zero bias Schottky diode HSMS-2850 from Agilent.

Each independent stage with its dedicated voltage doubler circuit can be seen as a single battery with open circuit output voltage V₀, internal resistance R₀.The number of stages in the system has the greatest e®ect on the output voltage. The capacitors, both in the stages and at the final stage of the circuit, affect the speed of the transient response and the stability of the output signal. The number of stages is essentially directly proportional to the amount of voltage obtained at the output of the system.

The simulation and practical implementation was done with fixed RF at 945 MHz ± 100 MHz, which is close to the down link center  radio  frequency  (947.5 MHz)  of GSM-900 band.    The  voltage obtained  at  the final node (VDC 7) of the  doubler  circuit  was recorded for various  input  power levels from −40 dBm  to +5 dBm  with  power level interval  (spacing)  of  5 dBm.   The  input  impedance  63-j117 Ω of the  voltage  doubler  is obtained  using the  network  analyzer.   This  63- j117 Ω was tested  from  900 MHz to  1000 MHz.   as the  antenna was designed for the down link radio frequency  range of GSM-900 band.

4. Step-up Circuit:

            A Booster converter (Step-up converter) is a DC-DC power converter with its output voltage greater than its input voltage.  It is a class of

switched-mode power supply (SMPS) containing at least two semiconductor switches (a diode and a transistor) and at least one energy storage element, a capacitor, inductor, or the two in combination. It is applied between the Energy conversion system and Load to successfully accomplish the load requirement.

Operating Principle:

(a) When the switch is closed, current flows through the inductor in clockwise direction and the inductor stores the energy. The polarity of the left side of the inductor is positive.

 (b) When the switch is opened, current will be reduced as the impedance is higher. Therefore, change or reduction in current will be opposed by the inductor. Thus the polarity will be reversed (means left side of inductor will be negative now). As a result two sources will be in series causing a higher voltage to charge the capacitor through the diode D.

·         The step-up circuit is used depending on Voltage requirement.

5. Load:

A wireless sensor network (WSN) consists of spatially distributed autonomous sensors  to monitor physical or environmental conditions such as temperature, sound, pressure etc. and pass the data to a corresponding control center.

A WSN is built of ‘nodes’- each having a transceiver, an electronic circuit for interfacing with the energy source usually a battery or from an energy harvester. Each node can be equipped with a power of less than 0.5- 2 amp-hour and 1-3V.

An example of such sensor node is a STLM20 temperature sensor. All the nodes in the network are connected to a master node through which the data is communicated and controlled     bidirectionally.

Conclusion:

A novel 900MHz RF energy harvesting system for powering low power sensors has been analyzed and discussed.

            Firstly, two 377Ω E-shaped patch antenna with compacted size were implemented.

            Subsequently the pi matching network located in between the antennas and the RF-DC conversion module is designed to simulate and

implemented to provide a good match from the load to the source.

            The energy conversion module that comprises of 7-stage voltage doubler circuit with zero bias Schottky diodes was successfully implemented and found to be efficient in converting the RF signals captured by the antenna to the required DC output voltage for powering the WSN.

            Finally, a booster converter which can be applied based on the voltage requirement is discussed.

Light Activated SCR

Photothyristors are light-activated Thyristors. 
Two common photothyristors include the Light Activated Silicon Controlled Rectifier (LASCR) and the light-activated triac.

• A Light Activated SCR (LASCR) acts like a switch that changes states whenever it is exposed to a pulse of light. Even when the light is removed, the LASCR remains ON until the anode and cathode polarities are reversed or the power is removed.
• A light-active TRIAC is similar to a LASCR but is designed to handle AC currents.

LASCR:

• The LASCR is also known as Light Triggered Thyristor(LTT).
• It may be triggered with a light source or with a gate signal. Sometimes a combination of both light source and gate signal is used to trigger an SCR.
• In this, the gate is biased with voltage or current slightly less than that required to turn it on, now a beam of light directed at the inner p-layer junction turns on the SCR.
• The light intensity required to turn-on the SCR depends upon the voltage bias given to the gate. Higher the voltage(current) bias, lower the light intensity required.
• These devices are available up to the rating of 6kV and 3.5ka, with on-state voltage drop of about 2V and with light-triggering the requirements of 5mW.
• The symbol for a LASCR is shown below.
  

Basic Operation:

• When no light is present, the LASCR is OFF; No current will flow through the load.
• However, when the Light Activated SCR (LASCR) is illuminated, it turns ON, allowing current to flow through the load.
• The resistor in this circuit is used to set the triggering level of the LASCR.

How LASCRs Work
The equivalent circuit shown here helps explain how a LASCR works.

• When photons from a light source collide with electrons within the p-type semiconductor, they gain enough energy to jump across the pn-junction energy barrier—provided the photons are of the right frequency/energy.
• When a number of photons liberate a number of electrons across the junction, a large enough current at the base is generated to turn the transistors ON. The net result is a current that flows from the anode to the cathode.
• Even when the photons are eliminated, the LASCR will remain ON until the polarities of the anode and cathode are reversed or the power is cut.

LASCR Applications:

• The primary use of light triggered Thyristors is in high-voltage high- current applications, Static reactive-power compensation etc.
• The Light activated SCRs have complete electrical isolation between the light-triggering source and the high-voltage anode-cathode circuit.
• High Voltage Direct Current (HVDC) transmission systems, several SCRs are connected in series-parallel combination and their light-triggering has the advantage of electrical isolation between power and control circuits.

Voltage Regulation in SMPS using PWM technique

SMPS

Switched Mode Power Supply (SMPS):

It  is an electronic power supply that incorporates a switching regulator to convert electrical power efficiently. Like other power supplies, an SMPS transfers power from a source, like mains power, to a load, such as a personal computer, while converting voltage and current characteristics. An SMPS is usually employed to efficiently provide a regulated output voltage, typically at a level different from the input voltage                            

Unlike a linear power supply, the pass transistor of a switching-mode supply continually switches between low-dissipation, full-on and full-off states, and spends very little time in the high dissipation transitions (which minimizes wasted energy). Ideally, a switched-mode power supply dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time.   

They are, however, more complicated; their switching currents can cause electrical noise problems if not carefully suppressed, and simple designs may have a poor power factor.

SMPS can be operated using a Error Amplifier, but latest development is done using Pulse Width Modulation.1

PULSE WIDTH MODULATION (PWM)

PWM is a powerful technique for controlling analog circuits with a processor’s digital outputs. PWM is employed in a wide variety of applications ranging from measurement and communications to power control and conversion. PWM uses a square wave whose  duty  cycle is modulated resulting in the variation of the average value of the waveform. PWM can be used to reduce the total amount of power delivered to a load without losses normally incurred when a power source is limited by resistive means. This is because the average power delivered is proportional to the modulation duty cycle. With a sufficiently high modulation rate, passive electronic filters can be used to smooth the pulse train and recover an average analog waveform.

High frequency PWM power control systems are easily realizable with semiconductor switch. The discrete on/off states of the modulation are used to control the state of the switch which correspondingly controls the voltage across or current through the load. The major advantage of this system is the switch are either off and not conducting any current, or on and have (ideally) no voltage drop across them. The product of the current and the voltage at any given time defines the power dissipated by the switch, thus (ideally) no power is dissipated by the switch. Realistically, semiconductor switches such as

MOSFETs or BJTs are non-ideal switches, but high efficiency controllers can still be built. 

Operation:

            Output of the transistor Q1 is sensed back to the Error Amplifier(EA) through sampling resistors. The other input to the EA is Reference voltage(Vref). The output voltage of EA is fed to the Pulse Width Modulator(PWM), other input to PWM is oscillator signal which can be Sawtooth waveform or Triangular waveform.

            Output of the PWM is a rectangular waveform. The width of the rectangular waveform is dictated by the output voltage of EA. This pulse can be used to drive the transistor Q1 through Driver.

            When the width of the pulse is varied, the ON time of transistor Q1 will also vary and consequently the amount of energy taken from the input voltage. So, by controlling Duty cycle, one can stabilize the output voltage.

            Let consider, the output is less than the required voltage then the output of EA will be more, therefore the ON time will be more and transistor Q1 will feed more power to the load .

·        In real-time scenario this total control is accomplished in an IC

Ø But, the transistor in the IC cannot drive the output. So, a driver is used to drive the pulse needed for the output.