Investigation of a cantilever structured piezoelectric energy harvester used for wearable devices with random vibration input

TLDR: A good and reliable comparison to those with nonlinear structures is provided, which helps the designers to consider whether to choose a nonlinear structure or not in a particular energy harvester based on different application scenarios.

About: This article is published in Mechanical Systems and Signal Processing.The article was published on 2018-06-01 and is currently open access. It has received 63 citations till now. The article focuses on the topics: Energy harvesting.

Figures

Fig. 4. The output voltage response of the same cantilever clamped with different methods: 1st clamping means being fixed by epoxy glue; 2nd clamping means being fixed by screws joints.

Fig. 3. Side-view schematics of the electroding, poling and attaching methods.

Fig. 10. Dependence of average output power on external resistive load for the cantilever oscillated in a half and a full cycles.

Fig. 5. The pictures of the fabricated piezoelectric energy harvesters: a) two short bimorphs opposite to each other; b) two short bimorphs with a joined tip mass; c) four short bimorphs with a joined tip mass; and d) single long bimorphs with tip masses (silver and gold electrodes).

Table 1: Summary of the average output power generated in different hand-arm motions.

Table 2. Summary of the average output power generated in different head motions.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Investigation of a cantilever structured piezoelectric energy harvester used for wearable devices with random vibration input" ?

The whole loop of the design, simulation, fabrication and test of the energy harvester is presented. This paper provides a good and reliable comparison to those with nonlinear structures. 

Q2. What caused the long cantilever solution to break?

The long cantilever solution induced a considerably low stiffness and thus large displacements and stresses in the piezoelectric layers. 

Q3. What was the thickness of the wet films?

It should be noted that 10 % planar shrinkage and 30 % thickness shrinkage were considered when determining the dimensions of the wet films. 

Q4. What is the effect of the acceleration of the body movement on the piezoelectric layers?

The acceleration of the body movement provided a relative displacement to the tip mass and then inducing a deformation of the piezoelectric layers. 

Q5. How was the second round of sintering done?

2. The second round of sintering was carried out at 1050 °C for 1 hour with a ramp rate of 5 °C/min, in order to obtain a fully sintered ceramic film. 

Q6. What are the characteristics of the piezoelectric energy harvesters?

The output voltage, output power and energy conversion efficiency of the piezoelectric energy harvesters are usually very sensitive to the compositions of the piezoelectric materials and the processing methods used in the fabrication of the piezoelectric components [20]. 

Q7. How much power was generated by the piezoelectric energy harvester?

In average, the smart watch/wristband sized single-cantilever piezoelectric energy harvester was able to generate roughly 50 μW output power in human’s daily activities. 

Q8. How many nW of power can be generated by the harvester?

Even a higher power consumption sensor system requires only 570 nW to operate, more than 80 of which could be powered by the harvester presented in this paper through human’s daily movement. 

Q9. How many kinetic energy harvesters can be powered?

Although in some cases the working frequency range of a kinetic energy harvester can be widened by employing certain nonlinear effect, the design space which can be used for the energy harvester itself in a device to be powered is usually limited (up to 50 % of the whole device) [19]. 

Q10. How many mm was the thickness of the piezoelectric harvester?

Because the harvester was to be operated in a bending mode which requires an addition to the space around the tip mass area, the total thickness of the entire energy harvesting system (considering the space needed for the operation) was limited to 5 mm. 

Q11. Why did the harvester generate a small power?

As mentioned above, due to the degree of freedom of the cantilever did not match the arm’s moving direction, the harvester generated a small power. 

Q12. What was the voltage measured on the piezoelectric layers?

The generated voltage on the piezoelectric layers were measured with an external electric load, so that the output power could be calculated. 

Q13. What was the period of the actuator with a stable voltage?

The period with relatively stable voltage of about 8 V corresponded to when the actuator was running with a constant velocity between the two shocking points. 

Q14. What was the resistance of the probe used to record the data?

The 1MΩ resistive load was near the maximum power point and it was also the resistance of the probe of the oscilloscope used to record the data. 

Q15. What method was used to calculate the average output power?

The average (meaning RMS) output powers were calculated in two methods, one for a longer period (10 s) and the other for a shorter period (the highest peak during the 10 s measurement).