Development of Battery-less Wireless Current Sensor Node Utilizing Charging Time of Capacitors with Wide Measurement Range

We report a novel battery-less wireless current sensor node without an analog to digital converter (ADC). If a capacitor is charged using a current transformer (CT) and a rectifying circuit, the charging time depends on the current flowing through a power line. In the case that the node transmits data every time when voltage of the capacitor exceeds a threshold voltage, we can indirectly measure the current by measuring the transmission intervals. In this method, the circuit of the node can be simplified and power consumption for the wireless transmission can be decreased because the measured current data does not need to be included in the transmitted packet. However, the measurable range is about single digit because the transmission interval decreases suddenly as the current increases. In this work, we have expanded the range using one CT, one wireless transmission module, and two charging circuits that include different load resistors connected in series. The results indicated that the measurable range was from 0.5 A to 50 A.


Introduction
Wireless sensor networks are expected to be utilized for health [1] and security applications [2] as well as environmental monitoring [3] and could enable real-time visualization and control of power consumption so that energy efficiency is improved.This is very attractive for the energy management systems in a lot of facilities, such as offices, factories and homes.A wireless current sensor with a current transformer (CT) is useful tool especially to install in existing facilities because the wiring work is not needed [4].However, the nodes must be made battery-less type to be widely used [5].
Our research team has been developing a battery-less wireless current sensor node for an electrical power monitoring system [5].A current value is generally measured by an analog to digital conversion (ADC) of the voltage at a load resistor connected to a CT.This type of node can measure current with high accuracy using an ADC and an amplifier, and transmit data at a constant time interval.However, since this node needs power for an ADC and a timer to operate intermittently, a rechargeable battery and charging circuit with an over-charged protection are required.
Our battery-less wireless current sensor node is composed of simple circuits without an ADC and a rechargeable battery.This node transmits data using the power charged in a chip capacitor with a CT from a power line every time when the power is charged enough for wireless transmission.In this system, the time intervals between the transmissions are depend on the current flowing through a power line.Therefore the current value can be deduced by measuring the transmission intervals.In this node, because an ADC is not needed, a simple circuit can be realized.In addition, since measured current data does not need to be included in the transmitted packet, the power consumption for the wireless transmission can be decreased.However, the node has a demerit that the measurable range is about single digit because the transmission interval decreases suddenly as the current increases.In this paper, we report a method for expanding the current measurable range up to double digits.

Design
The developed node transmits data using power charged in a capacitor every time when a voltage of the capacitor exceeds a threshold voltage V th .The charged power is consumed, then the voltage of the capacitor drops to the minimum voltage V min .Figure 1(a) shows the basic model of this circuit.Since the current obtained from a CT is rectified, the module composed of the CT and the rectifying circuit represent DC power source as the equivalent circuit in this figure.The power control circuit detects enough power charged in the capacitor C and provides the power to the wireless transmission module by the switch.Time interval t between the transmissions is expressed by following equation derived from the equation of capacitor charging.
where C is the capacitor to charge the power, R lim is a current limiting resistor, and E is a rectified and smoothed voltage generated from a CT.E is expressed as: where K is coupling coefficient of a CT, I 0 is current flowing through a power line, R L is a load resistor, and n is a turn ratio of a CT. Figure 1 (b) shows transmission intervals at the case that C is 100 μF, R lim is 1 kΩ, V th is 2.1 V, and V min is 1.8 V. Since the time interval t drops rapidly as E increases, the resolution at high current becomes low.The maximum measurable range mainly depends on variability of the time interval caused by ripples generated from a rectification circuit and noises.On the other hand, the minimum measurable range depends on the power consumption of the power control circuit and on the power obtained from the CT.The measurable range does not expand dramatically even if the minimum and maximum limit can be improved.
Although the easiest method to increase the measurable range is to use multiple nodes with different load resistors, the cost of the manufacturing and installation increases.In this research, we have developed a method to expand the range using one CT, one wireless transmis- sion module, and two charging circuits that include different load resistors connected in series.
The challenge is how this type of node detects measuring current I 0 by itself to switch two charging circuits.If E can be measured, I 0 can be calculated using Equation (1).In this node, however, E cannot be measured if E is higher than V th .Because if the voltage of C exceeds V th , the power charged in C is consumed for wireless transmission.In this development, we have proposed a method to detect increasing rate of voltage of C.
Figure 2 shows a proposed circuit without bypass capacitors, current-limiting resistors, damping resistors, and voltage reference generators.The resistors R 0 and R 11 are load resistors for the CT.The circuit A including R 0 and B including R 11 are for measuring low and high current flow, respectively.The devices U 0-4,10,11 are comparators and U 5-9,12 are analog switches.The Cockcroft-Walton (CW) circuits [6] are used as stepup converter in both circuit A and B. The power sources of the devices U 0-9 and the others U 10-12 are charged capacitor C 0 and C 4 , respectively.The all voltage references V ref are same value and generated by bandgap reference generators in both circuits.The circuit A has the circuit A1 and A2 to change the power source for the wireless transmission module depending on current flowing through a power line.The circuit A1 switches the charging circuit from the circuit A to B if the measuring current exceeds a threshold value.The circuit A2 turns the charging circuit back.
Figure 3 shows the relationship between measuring current I 0 and the charging circuits.I A and I B represent I 0 when power charged by the circuit A and B is used for wireless transmission, respectively.I th_up and I th_down are threshold current for switching the charging circuit from the circuit A to B and from the circuit B to A, respectively.These threshold values are set different values to make behaviors of this circuit stable.
Figure 4 shows the voltage changes at the points V 0-6 of the circuit in Figure 2. Figures 4(a) and (b) indicate the case of low and high current I 0 , respectively.V T1 , V T2 , V T3 , and V T4 represent the threshold voltages for V 0 at U 0 , U 1 , U 2 , and U 10 , respectively.V Z shows the zener voltage of DZ 0 and DZ 1 .
First, behaviors of the circuit in the case of Figure 4(a) are explained below.When current I A flows through a measured power line, voltage V 0 exceeds V T1 , but V 5 does not increase up to V T1 .When V 0 increases up to V T1 , the output of comparator U 0 changes from low to high level.The resistor R 7 and the capacitor C 2 compose a delay circuit.The voltage V 2 increases up to the voltage produced by a voltage divider (R 7 and R 9 ) according to the time constant (R 7 × C 2 ).When V 2 increases up to V ref , the output of the comparator U 3 becomes high level.Using this delay circuit and the comparator U 1 , whether the increasing In Figure 4(b), V 0 exceeds V T2 during the above mentioned delay, then the output of comparator U 1 becomes high level.The analog switches U 8 and U 9 change the power source from C 0 to C 4 and the output of analog switch U 7 changes the connection of a micro controller unit (MCU) from V AGND to the analog switch U 12 .At this time, since the power charged in C 0 is not used for the wireless transmission, V 0 increases up to the voltage which depends on the power consumption of circuit A and the power obtained from the CT.In this case, although V 0 can exceed withstand voltages of the devices used in the circuit A, the zener diode DZ 0 is used to prevent breakdowns of the devices.In the circuit B, wireless transmission is performed like the circuit A every time when the voltage V 5 increases up to V T4 .At the connection between U 12 and the circuit A, influence of pulsating voltage must be prevented.Although V 5 is DC voltage if V BGND is the reference voltage, V 5 is pulsating voltage if V AGND is the reference voltage.Since behavior of U 12 can be unstable by the pulsating voltage, the effect is decreased using the diode D 5 .rate of V 0 exceeds a threshold voltage can be detected.In other words, if V 0 increases up to V 2 during the delay, this circuit can detect that I 0 exceeds I th_up .In the case of Figure 4(a), I 0 is less than I th_up .When the output of the comparator U 3 becomes high level, the output of the comparatorU 4 also changes to high level immediately.The analog switch U 6 connects C 0 to the wireless transmission module, then the module starts wireless transmission.At this time, V 0 drops without delay, then output of U 0 becomes low level.However, the output of U 4 keeps high level by the delay circuits composed of C 2 , R 9 , C 3 , and R 10 until the wireless transmission finishes.U 8 and U 9 are analog switches for switching the power sources charged by the circuit A or B. The output of analog switch U 7 indicates which power source is used.Low and high level of output of U 7 shows that the wireless transmission is performed using the power charged by the circuit A and B, respectively.The transmitted packet includes an identification number of the wireless sensor node and one bit data that indicates which power source is used, but does not includes current data.
When I 0 decreases from I B to I th_down , V 0 does not decrease to V T2 .Because the power consumption of the circuit A except for wireless transmission is small.Actually, V 0 does not decrease to V T2 unless I 0 decreases to around minimum of the measurable range.The current I A0 in Figure 3 indicates I 0 at which V 0 decreases to V T2 .In this case, the power charged by the circuit B cannot be charged enough for the wireless transmission if I 0 is lower than I th_down .Thus, the power charged by the circuit A must be used for the wireless transmission at less than The resistance values of R 1-6,12,13 which al reduce the power consumption.CTL-10-CLS (UR_D), VICES) and LTC1540 (LINER TECHNOLOGY) were U 1 to high level at the I th_up .In this case, V T2 was 2.29 V. nd I th_down .To change the power source, V need to be de-MCP644x (MICROCHIP), ADG849 (ANALOG DEc event at which I 0 decreases to I th_down and decrease V 0 .The output of the comparator U 2 becomes high level when V 0 decreases to the voltage V T3 at which I 0 is I th_down .This high level signal must be changed to low level immediately after V 0 decrease to V T2 .Because U 5 continues to connect V 0 and V AGND if the output of U 2 is high level.In this case, power cannot be charged in C 0 .To make one shot pulse signal, this high level signal is differentiated by a differentiator composed of the capacitor C 1 and the resistor R 8 .The analog switch U 5 connects V AGND with V 0 through a current limiting resistor only for the time of the pulse width, then V 0 drops to less than V T2 .The diode D 3 is intended for preventing a signal which is generated when the output of U 2 changes from high to low level.If D 3 is not in this circuit, the circuit B cannot be used to charge the power for wireless transmission.Because the power is consumed every time when V 0 exceeds V T3 .

Experimental Result
In this research, we designe node with measurable range fr the transmission intervals are less than 0.5 s.The 3 stage CW circuit was used in the circuit A and B. We have not examined how many stages of the CW and how much capacitors used in the CW circuit are suitable for this current sensor.
Table 1 shows the designed values of the resistors and the capacitors.
ways consume the current were more than 10 MΩ to used as a CT, comparators, analog switches, voltage reference generators, respectively.A wireless sensor module was mainly composed of C8051F930 (Silicon labs) used as MCU and nRF24L01 (Nordic) used as RF IC.
The voltage reference was 1.182 V [7].The conditions [5] of the wireless transmission were same with except for the packet structure.Since the lower limit voltage at which both two ICs work was 1.8 V [8,9], V T1 must be more than 1.8 V.In this research, we determined V T1 was 2.15 V.In this case, since the required charge for the wireless transmission was about 4.18 μAs [5], the capacitance values of C 0 and C 4 must be more than 11.9 μF.On the other hand, these capacitors have a function to smooth ripples.In this research, we determined the capacitance values of C 0 and C 4 were 0.8 mF and 1.3 mF, respectively.The reason that the capacitance of C 4 is larger than C 0 is that I B is higher than I A .The resistance values of R 10,14 and the capacitance values C 3,5 were determined according to [5].The output voltage of the CW circuit depends on the load.First, the resistance values of R 1-6 were temporarily set to 10 MΩ in order to fix the load.Next, resistance value of R 0 by which V 0 exceeds the V T1 when I 0 is 0.5 A was determined.Not to become the transmission interval (t trans ) short, small resistance is better.In this research, we determined the resistance value of R 0 was 6.2 kΩ.In this case, the transmission intervals were almost not changed if I 0 exceeded about 6 A. Thus, we determined I th_up and I th_down were 6.5 A and 6 A, respectively.The resistance values of R  Figure 5 shows the experimental setup.In this measurement, current calibrating apparatus (FLUKE 5080 A), the maximum current of which is 20.5 A, was used.When we measured transmission interval at more than 20.5 A, the power line was wrapped one time or two times an quency of AC current was 50 Hz.The transmission interval was measured by a receiver system that can save times when the receiver receives packets.
Figure 6 shows the results.The solid and dotted line indicates that the transmissions were performed using the power charged by the circuit A and circuit B, respectively.The each point is the average of ten measured intervals and the error bars are also shown.This graph shows this wireless sensor node can m om 0.5 A to 50 A. Table 2 shows the differences between this work and [5].

Conclusion
In this research, the battery-less wireless current sensor   ode using that the char is the cu pe of sensor, there is a nce the cha d current inc ng time b ases.In th omes short as the measu p tors were used, and demonstrates that the node can measure current flowing thorough a power line from 0.5 A to 50 A. If the step-up converter and the resistors of R 1-6,12,13 are optimized or lower power consumed devices than the devices used in this research are utilized, the measurable range can be expanded.

Figure 1 .
Figure 1.(a) Basic model of the circuit for the developed node.(b) Transmission interval at the case that C is 100 μF, Rlim is 1 kΩ, V th is 2.1 V and V min is 1.8 V.

Figure 2 .
Figure 2. Schematic circuit diagram for battery-less wireless current sensor node using two different load resister.

Figure 3 .
Figure 3. Relationship between measuring current I 0 and charging circuits.

Figure 4 .
Figure 4. Voltage changes at the points V 0-6 of the circuit in Figure 2. (a) and (b) indicate the case of low a high current I 0 , respectively.
V at I th_down .In the circuit B, the wi issions were someti perf r ed if V T o V T1 .The reason has not clarified but we th ripp s gen ated by the step up converter can the stability.Thus, we determined T4 was 2.26 V.If the resistance values of R 12,13 were set d clamped the double or triple power lines.and 11 MΩ, respectively, it was found that V 5 exceeded V T4 at 220 Ω of the resistance R 11 .The resistance values of R 5,6 were set to change the output of U 2 from low to high level at I th_down .In this case, V T3 was 3.61 V.

Figure 6 .
Figure 6.Relationship between current flowing through a power line and transmission interval.

Table 2 .
Differences between this work and [5].flowing through a power line was developed.In this ty problem that the measurable range is about single digit si rgi ec re re is aper, we proposed the circuit in which two load resiscce, 26-29 October 2008, pp.625-628.

Table 1 . Values of the resistors and capacitors in Figure 2.
3,4,7,9 and capacitance value of C 2 were set to change the output of