New piezoelectric device simplifies vibration energy harvesting

Many low-power industrial sensors and controllers are moving toward alternative energy sources as a primary or secondary power source. Ideally, this harvested energy will eliminate the need for additional wired power supplies or batteries. Transducers that generate power from readily available physical energy sources (such as temperature difference devices "thermoelectric generators or thermopiles", mechanical vibrations "piezoelectric or electromechanical devices" and light "photovoltaic devices") are becoming practical power sources for many applications. Many wireless sensors, remote monitors and other low-power applications are evolving into near "zero" power devices, using only harvested energy (some people often call it "nanopower").

Although “energy harvesting” has been around since the early 2000s, it is only recent technological developments that have propelled it to the commercial stage. In short, we are at a turning point in 2010 and will enter its “growth” phase. Building automation sensor applications using energy harvesting technology have been promoted in Europe, indicating that the growth phase may have begun.

Commercial viability of energy harvesting

Although the concept of energy harvesting has been known for many years, implementing such a system in a practical environment is cumbersome, complex and expensive. However, market examples that have adopted energy harvesting methods include transportation infrastructure, wireless medical devices, tire pressure monitoring, and by far the largest market is building automation. In building automation, systems such as occupancy sensors, thermostats and light switches are able to eliminate the power or control wiring that is usually required and replace it with a mechanical or energy harvesting system.

Likewise, wireless networks using energy harvesting technology can connect any number of sensors within a building to reduce heating, ventilation and air conditioning and lighting costs by cutting power to non-critical areas when no one is on duty. In addition, the cost of energy harvesting electronics is often lower than the cost of operating the power lines, so the choice of energy harvesting technology can clearly bring economic benefits.

Figure 1: The four main blocks of a typical energy harvesting system.

A typical energy harvesting configuration or system (see Figure 1) usually consists of a free energy source, such as a piezoelectric transducer connected to a vibrating mechanical source, such as an air conditioning duct or window glass. These small piezoelectric devices are able to convert small vibrations or strain differences into electrical energy. This electrical energy can then be converted by an energy harvesting circuit and changed into a usable form for powering downstream circuits. These downstream electronics usually include some type of sensor, analog-to-digital converter, and an ultra-low power microcontroller. These components can take this harvested energy (in the form of current) and wake up a sensor to obtain a reading or measurement, which can then be transmitted through an ultra-low power wireless transceiver (represented by the fourth block in the circuit chain shown in Figure 1).

Each circuit system block in the chain (except perhaps the energy source itself) has a unique set of constraints that have hampered its commercial viability to date. Low-cost and low-power sensors and microcontrollers have been available for several years; however, ultra-low-power transceivers have only recently become commercially available. However, the laggard in the chain has been energy harvesters.

Existing implementations of power manager modules often use low-performance discrete structures, often including 30 or more components. Such designs have low conversion efficiency and high quiescent current. Both of these deficiencies result in performance losses in the end system. Low conversion efficiency increases the time required to power up the system, which in turn increases the time interval from taking a sensor reading to transmitting that data. High quiescent current limits how low the energy harvesting source can go, because it must first exceed the current level required for its own operation before it can provide any excess power to the output.

New piezoelectric energy harvester

Until now, what has been lacking is a highly integrated, high efficiency DC/DC step-down converter with a low loss full-wave bridge rectifier that can harvest and manage piezoelectric energy from vibration or strain sources. Recently, Linear Technology's new LTC3588-1 piezoelectric energy harvesting power supply has greatly simplified the task of harvesting surplus energy from such sources.

Figure 2: The LTC3588-1 circuit converts a vibration or strain source into an electric current.

The circuit shown in Figure 2 uses a small piezoelectric transducer to convert mechanical vibrations into an AC voltage supply to feed the internal bridge rectifier of the LTC3588-1. It can harvest small vibration energy sources and generate system power without using traditional battery power.

The LTC3588-1 is an ultralow quiescent current power supply designed for energy harvesting and/or low current step-down applications. The device connects directly to a piezoelectric source or AC source, corrects the voltage waveform and stores the harvested energy on an external capacitor, bleeds off any excess power through an internal shunt regulator and maintains a regulated output voltage with the help of a nanopower high efficiency step-down regulator.

The LTC3588-1's internal full-wave bridge rectifier is accessible via two differential inputs, PZ1 and PZ2, which rectify the AC input. The rectified output is then stored on a capacitor at the VIN pin and can be used as an energy reservoir for the buck converter. The low-loss bridge rectifier has a total voltage drop of about 400mV and a typical piezoelectric generation current of about 10μA. The bridge is capable of delivering up to 50mA. When there is sufficient voltage at the VIN pin, the buck converter is enabled to produce a regulated output.

The buck regulator uses a hysteretic voltage algorithm to control the output through internal feedback from the VOUT sense pin. The buck converter charges an output capacitor through an inductor to a value slightly above the regulation point. It accomplishes this task by ramping the inductor current up to 260mA with an internal PMOS switch and then ramping the inductor current down to zero with an internal NMOS switch, effectively delivering energy to the output capacitor. Its hysteretic method of providing a regulated output reduces losses caused by FET switching and maintains an output under light load conditions. The buck converter provides a minimum average load current of 100mA while it is switching.

Conclusion

Designing efficient energy harvesting systems such as the one shown in Figure 1 has been difficult due to a global shortage of people with expertise in analog switch-mode power supply design. However, this is about to change with the introduction of the LTC3588-1, a piezoelectric energy harvesting power supply with an integrated low-loss full-wave bridge rectifier. This revolutionary device is able to extract energy from virtually any source of mechanical vibration or strain. In addition, its comprehensive feature set and ease of design greatly simplify the difficult power conversion design in the energy harvesting chain. This is good news for system designers because these "useful vibrations" can be used to power their energy harvesting systems without having to struggle with traditional configuration challenges.