# A Vibrational Energy Harvesting Sensor Based on Linear and Rotational Electromechanical Effects
## 5 Experimental Design and Testing  
A computer controlled and calibrated experimental vibration testbed was developed for analyzing the prototype sensor designs. Figure 4 depicts the testbed and its fundamental components. The input excitation for the sensor is generated by an electrodynamic shaker, designed to operate within a bandwidth ranging from 1 Hz to 5 kHz. The computer (not shown for clarity) interfaces with the lock-in amplifier, digital oscilloscope and a laser vibrometer. The lock-in amplifier provides a sinusoidal drive signal to the shaker which is controlled by a MATLAB-GPIB [25] interface on the computer. The generated drive signal can be swept across the experimental bandwidth with very fine resolution. The drive signal from the lock-in amplifier can be amplified 40 dB as needed to boost the output acceleration of the shaker and drive the VEH sensor into a nonlinear operational regime. A 20 cm aluminum angle bar is bolted to the shaker on one end. The VEH sensor and the reference accelerometer are mounted on the free end of the aluminum bar. The primary need for the aluminum bar is to minimize the effects of the strong magnetic field from the electrodynamic shaker on the VEH sensor prototypes. Measurements with a laser vibrometer on the free and clamped ends of the stiff aluminum beam over the frequency range (1 Hz through 200 Hz) indicate very little variation for modest acceleration levels (\(\ddot{y}(t)<0.5\) g). The experimental measurement setup incorporates three independent methods for characterizing the sensor response. The first method utilizes an optical laser vibrometer which can be focused on the inertial mass, the sensor case of the VEH sensor or on the aluminum beam. This non-contact measurement enables precise velocity measurement of the inertial mass without influencing the dynamic response of the sensor. Output from the laser vibrometer is recorded onto the scope and saved via internal storage to a thumb drive. The nature of this sensor design makes this measurement challenging as the inertial mass is designed to wobble, which reduces the signal to noise of the vibrometer and ultimately limits its utility for this situation. The second measurement is performed by an accelerometer mounted near the VEH sensor location on the rigid aluminum beam. This accelerometer captures the input acceleration, \(\ddot{y}(t)\), experienced by the sensor base. The accelerometer provides an accurate standard reference measurement of the magnitude and transient behavior (e.g., frequency content) of the input signal \(\ddot{y}(t)\) to the VEH sensor. Also, since the accelerometer is calibrated, it is also utilized to determine the relative g-force on the sensor for different input voltages to the shaker. Finally, the third measurement is the actual output of the VEH sensor. Here, either a transient response can be recorded on the oscilloscope or a forced response function can be measured using the in-phase and quadrature capabilities of the lock-in amplifier [26]. The lock-in amplifier provides an alternative approach to measuring the response of a system by not relying upon calculating the instantaneous spectrum in a short time window, as is the case for spectrum analyzers. The nature of the phased locked loop in the lock-in amplifier allows for very accurate narrow-band measurements of the order of a few Hertz to be recorded over the bandwidth of the sweep. In addition, the time constants and filter settings associated with the dwell time can be adjusted to improve the sensitivity. Figure 4: Diagram of the experimental testing setup. The system can simultaneously measure the coil current, \(i(t)\), inertial mass velocity, \(\dot{x}(t)\), input acceleration, \(\ddot{y}(t)\) and the harvesting performance (volts per time). The input excitation can be amplified to push operation of the VEH sensor into a nonlinear regime as needed. The computer-controlled lock-in amplifier is utilized as both a signal source and as a tool to record the response of the sensor.