DC Current Beat: Wireless and Non-Invasive DC Current Sensing Scheme ^†

Abstract

This paper presents a wireless and Non-invasive DC Current (DCC) sensing scheme as an IoT sensors. A RF module transmits only ID codes to a receiver, and the ID transmissions are called as “DCC Beat”. The interval time of DCC Beats depend on the inductance of ferrite clamp which is non-invasively installed at the wire of the DC current to be measured, so that the interval time corresponds to DC Current. The ID data transmission range reaches up to 50 m with 1.2 mW operating power using a 2.4 GHz RF module. DC current from 0.2 to 4 A can be measured within error of 5.7%.

1. Introduction

For non-invasive DC current measurement, Hall Sensor with ferrite clamp is often used [1]. We have proposed Power Beat Sensor [2] and Temperature Beat Sensor [3] as energy harvesting wireless sensors for IoT applications. This paper first demonstrates DC Current Beat Sensors, in which wireless and non-invasive DC Current (DCC) sensing can be successfully realized, so that it is used as IoT sensors at such various kinds of applications as HEMS, BEMS, DC motor control, Solar Cell control, charge control on secondary batteries, and smart grid using DC power supply. In the DCC Beat Sensors, only ID signal is transmitted by an RF transmitter, the interval time of the ID signal corresponds to the DC current to be measured.

2. Structure of the DCC Beat Sensor

The simple circuit for the DCC beat generations consists of DCDC converter, storage capacitor, Voltage Detector (VD), Low Dropout Regulator (LDO), and MCU and a 2.4 GHz RF module [4], and no ADC is required as shown in Figure 1. As shown in Figure 2, voltage of 1.2 V is leveled up to higher voltages by a switching circuit with an inductances of L₁ in the DCDC converter, and the storage capacitor C_ST is charged up. When Schmitt trigger VD detects the predetermined high value V_H of 2.6 V, the LDO is turned on and it supplies LDO out of 1.9 V, which corresponds to the supply voltage of the MCU and the RF module. Then ID transmission is carried out by the RF module. After the ID transmission is finished, the LDO is turned off. Then the storage capacitor is charged up again followed by the next ID transmission. We use two inductances, where L₁ is fixed value, and L₂ is depending on DC current (Io) to be measured.

When the nMOS switch at the DCDC converter is on, the current I₁ flows on the constant inductance L₁, so that energy stored in the inductance L₁ becomes E₁ as shown Equation (1). When the nMOS switch turns off, the E₁ is transferred to the energy on L₂ and the storage capacitor C_ST as described in Equation (2). So the storage capacitor is charged up as nMOS is switching by the 100 KHz pulses. Interval time (ti) of the ID transmissions becomes short when the inductance L₂ decreases, where L₂ decreases as DC current to be measured increases.

We used soft magnetic ferrite core clamp for L₂ where coil for inductance is 11 turns. The inductance value is measured by LCR meter at a frequency of 100 KHz as shown in Figure 3. The maximum inductance value is 147 uH, and it decreases as DC current to be measured (Io) decreases, and little hysteresis was observed. Therefore, as the DC current increases in the circuit, the interval time ti is shortened. The interval time of the beats depends on the DC current to be measured.

$${\mathrm{E}}_1=\frac{1}{2}·{\mathrm{L}}_1·{{\mathrm{i}}_1}^2,$$

(1)

$${\mathrm{E}}_1=\frac{1}{2} ·{{\mathrm{L}}_2}_{·}·{{\mathrm{i}}_2}^2+\frac{1}{2} ·\mathrm{C}\{{{\mathrm{V}}_{\mathrm{ST}}}^2(\mathrm{n})-{{\mathrm{V}}_{\mathrm{ST}}}^2(\mathrm{n}-1)\}$$

(2)

The photos of the measurement are shown in Figure 4. L₁ inductance and L₂ inductance are respectively made with coils around two cores of the two clamps. L₁ of 18 uH, and L₂ of 147 uH were respectively obtained at the Io of 0 A. The inductance of L2 depends on the DC current to be measured. A 2.4 GHz RF module (Nordic nPF24L01) with a output power of 0 dBm [4] is used to transmit the ID code for the DCC beats. Measured communication range in line of sight was up to 50 m for the RF module. The switching frequency was 100 KHz. We have successfully obtained ID signals, the interval time of which corresponds to the DC current to be measured as shown in Figure 5. The power consumption of the circuit to generate the ID signals is typically 1.2 mW at the switching frequency of 100 KHz.

As shown in Figure 6a, the Io and ti is correlated by the 4th order of correlation formula for calibration. Measured DC current obtained by the interval time corresponds to Io, and the error is max 5.7% at DC current of 0.2 A after calibration as shown in Figure 6b.

The specifications of the DCC Beat sensors are summarized and compared to commercially available hall type DC current clamp sensor as indicated in

Table 1. Comparison Table of the DC current sensor.

	This Work	Hall Sensor (LAD-240) (1)
Wired/Wireless	Wireless, up to 50 m comm. range	Wired
Non invasive	Yes	Yes
DC current range	0.2 to 4 A	1 to 200 A
Error	5.7% after calibration	2.5%
Resolution	0.1 A	1 A
Size	40 × 50 × 20 (TX), 19 × 19 × 25 mm (Clamp)	44 × 148 × 21 mm
Consuming Power	1.2 mW	Coin Battery LR-44 × 2

.