Energy Harvester Perpetually Powers Wireless Sensors

Energy production requires state-of-the-art monitoring systems. Of course, these systems require energy of their own to operate. For example, GE Energy recently developed a system that monitors machinery conditions for a field trial at the Nyhamna gas plant in Norway. By harvesting the vibration energy from the machines it monitors, this system’s power supply is inexhaustible.

This power supply uses a microgenerator to convert vibrational energy into usable electrical energy. It then stores that energy in a supercapacitor; therefore, enough is available for the higher-power bursts to measure and transmit condition-monitoring data to a basestation.

CONDITION MONITORING Plants and refineries monitor both machines and processes to ensure optimum safety, up-time, and efficiency. Machinery condition monitoring involves measuring the vibrational spectrum of rotating machinery, such as pumps, motors, and turbines, to determine their health. Rotational speed and shaft/bearing construction determine the frequency of vibration. The amplitude of vibration indicates machine health.

For instance, smooth-running machines have low amplitude vibration, while defects in bearing surfaces and/or unbalanced or misaligned shafts increase the amplitude of vibration. As the problems worsen, the amplitude increases. Thus, frequent monitoring of vibration frequency and amplitude reveals problems as they occur to help engineers predict when it’s most economical to take equipment offline for maintenance.

Plants will instrument and do the wiring for turbines, critical high-capacity pumps, motors, and the like when installed. But it isn’t cost-effective to do this for the “balance of plant,” including the less-critical pumps, motors, and compressors that abound in oil refineries, gas plants, and mineral processing plants.

If one of these less-critical pieces of machinery fails unexpectedly, the plant may incur significant costs in lost production and emergency maintenance. Typically, maintenance engineers monitor “balance of plant” by walking around with a vibration transducer and laptop to periodically inspect equipment, the frequency of which is determined by the criticality of the equipment.

It would be far more convenient if a low-cost system consisting of a vibration sensor, microcontroller, and radio transmitter was fitted to the balance of plant, which could periodically report the vibration spectra to a maintenance basestation. The question then becomes how to power these remote sensors.

A PERPETUAL POWER SUPPLY Batteries could power the remote sensors. However, batteries may survive only two to five years in such harsh environments. So, in plants with hundreds or thousands of battery-powered wireless sensor nodes, the cost of monitoring, replacing, and recycling them is significant.

Given we are monitoring rotating machinery, it’s guaranteed that vibrational energy will be available. Hence, the most natural and attractive solution is to capture this vibrational energy to power the remote condition-monitoring sensors, thereby providing a free, perpetual power supply.

This power supply includes two critical components. First, a PMG17 microgenerator from Perpetuum can harvest even very low levels of vibrational energy from a smooth-running piece of equipment (Fig. 1). Second, a supercapacitor from CAP-XX can store this energy and release it in short, high-power bursts to read and transmit the condition monitoring data.

VIBRATION ENERGY HARVESTER The PMG17 is an inductive energy harvester with an optimized magnetic circuit coupled to a magnetic resonator designed for ac motors. It harvests the commonly found “twice the line frequency vibration.” Therefore, the unit is tuned to 100 Hz for a 50-Hz ac supply or to 120 Hz for a 60-Hz ac supply.

Also, the PMG17 is highly efficient. With as little as 25-mg RMS vibration within a 2-Hz bandwidth, it will produce a minimum power output of 0.5 mW (Fig. 2). If a unit is tuned to 120 Hz to harvest energy from a 60-Hz ac motor, then the output power would be 1 mW if there is 25 mg of vibration at 120 Hz, 0.6 mW if there is 25 mg of vibration at 119 Hz, 1 mW if there is 50 mg of vibration at 122 Hz, and so on.

The microgenerator is a high-impedance voltage source (Fig. 3). The power-conditioning and storage block should control the current drawn from the microgenerator to maintain its output voltage at approximately 5 V to maximize the power transferred from it. This current level will be approximately 120 µA.

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POWER CONDITIONING AND STORAGE A dual-cell CAP-XX supercapacitor consisting of a pair of HW109 cells forms the heart of the power-conditioning and storage block. This supercap, which was chosen because it’s small and thin, has two cells, each measuring 28.5 by 17 by 1.1 mm. It provides high energy storage—140 mF at 5.5 V = 2.1 J.

The supercap’s high power delivery is only 120-mO equivalent series resistance (ESR), so its max power transfer is 63 W.  Operation is in the industrial temperature range of –40°C to 85°C. It can be charged with low current down to 50 µA. And its very low leakage current ranges down to about 3 µA with an active balance circuit.

The PMG17 produces ac, which is full-wave-rectified by the diode bridge D1-D4 (Fig. 4). To maximize efficiency, the diodes should have low forward voltage and low reverse leakage current. The BAS16 diodes from ON Semiconductor provide good characteristics from –40°C to 85°C.

Based on Figure 3, the PMG17 operating voltage should remain in the range of 4 to 6 V. There’s approximately a 1-V drop across the diode bridge, so VCAP will be in the 3- to 5-V range. VCAP must be greater than 3.2 V to supply a buck converter with a 3.0-V output that drives the data-gathering and transmission circuits.

D5 has a reverse voltage of 2 V at a reverse leakage current of 3 µA. R2 and D5 ensure that Q1 doesn’t turn on until VCC approaches 5 V. As Q1 turns on, VCC drops as charge current flows to the supercapacitor, turning Q1 off again. The PMG17 then charges C9, C11, and C12 until VCC is sufficient so that reverse current flows through D5 and the voltage across R2 reaches VGS of Q1 to turn it on again.

With a few tens of microamps, the combined capacitance of C9, C11, and C12 = 66 µF can be charged to 5 V in less than 30 seconds. In this manner, R2, D5, and Q1 regulate VCC to approximately 5 V, ensuring maximum power transfer from the PMG17 microgenerator to the supercapacitor. At 120 µA, it will take about an hour and a half to charge the supercapacitor to 4 V, where it can support a data-gathering and transmit cycle.

The two supercapacitor cells need to be balanced so that neither goes over voltage. The balancing circuit connects to the node labelled “Active balance” in Figure 4. The balancing circuit current + supercapacitor leakage current must be much smaller than PMG17 output current ˜ 100 to 200 µA. A high-impedance, low-power operational amplifier, as shown in the balancing circuit of Figure 5, will only draw approximately 3 µA, including the supercapacitor leakage current.

The operational amplifier chosen needs to be rail-to-rail. It only draws less than 1-µA supply current and can source or sink up to 11 mA to bring cells quickly into balance. Once the supercapacitor is charged, the op amp only supplies or sinks the difference in leakage current between the two supercapacitor cells in series to maintain voltage balance.

DETERMINING SUPERCAPACITOR SIZEFigure 6 shows the load supported by the supercapacitor, and the table details the energy required. With dc-dc converters that are 75% efficient, the supercapacitor must deliver 83/0.75 = 111 mJ. The supercapacitor energy delivered equals:

1/2 × C × (V²_INITIAL – V²_FINAL)

Therefore, C required equals:

2 × energy/(V²_INITIAL – V²_FINAL)

2 × 111/(52 – 3.22) = 15 mF + 20% tolerance = 18 mF

Allow for loss of capacitance due to supercapacitor aging, so start with double the capacitance. Therefore, initial C > 36 mF. Select the smallest CAP-XX part that operates over the industrial temperature range, which is the HW209 with C = 140 mF and ESR at room temperature range = 120 mΩ.

ESR at –40°C is approximately 3 × room temperature ESR. Check the suitability of the HW209: peak current = (28 mA × 3/3.2 V + 3 mA × 18/3.2 V)/0.75 = 58 mA. As a result, voltage drop due to ESR at –40°C = 0.36 Ω × 58 mA = 21 mV

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SUCCESSFUL FIELD TRIAL Shell conducted a 12-month condition-monitoring field trial of this energy-harvesting system in the harsh environment of its Norwegian Nyhamna gas plant. Six motors were monitored, and the trial was a complete success—no failures occurred with the PMG17 or power-conditioning circuitry.

As stated in “Successful trial of wireless monitoring at Nyhamna gas plant” in the January 2008 special edition of Technology: Shell EPE Technology Learning Publication: “The system means that much greater numbers of monitoring points—many in hazardous areas—can be regularly monitored and so help the plant maintenance engineers identify potential system breakdowns in advance.”

In the same article, Sicco Dwars, Shell Global Solutions R&D Engineer, says, “A self-generating power supply is important because batteries have a limited life, particularly when they are required to work outdoors, with temperatures spanning from tropical to arctic conditions.”

To summarize, combining the PMG17 energy harvester and power-conditioning circuit with a CAP-XX supercapacitor has proven to be an ideal solution to power remote sensors where vibration energy from machinery rotating at ac line frequencies is available (Fig. 7).

For more about supercapacitors, see “Ultracapacitors Branch Out Into Wider Markets” by Components Editor Mat Dirjish at www.electronicdesign.com, ED Online 20034.