Seeking Expertise: Challenges in Acquiring Voltage Data from Piezoelectric Films

[Alioune]

I hope you are doing well. I am reaching out to you as a student working on a project involving the characterization of piezoelectric materials, and I humbly seek your expertise to help me overcome significant challenges that I am facing in the execution of this project.

My main objective is to simultaneously acquire both the alternating voltage and alternating current from piezoelectric films. To achieve this, I am using a machine capable of applying mechanical deformation to my piezoelectric films at various frequencies. This deformation generates a certain voltage in the films, which I acquire using a DAC acquisition device, the DAC MC USB-205. Additionally, I measure the current associated with this voltage using a KEITHLEY 6485 picoammeter. My intention is to visualize this data directly on my computer through LabVIEW.

However, I am encountering significant obstacles in the realization of this project. Initially, I observe a voltage of 1.75 volts when the film is connected to the GND and an analog input pin of the DAQ MC USB-205. This value remains constant regardless of whether the DAQ is in an open or short-circuited circuit. Furthermore, when I introduce a 10 kΩ resistor, the voltage drops considerably. For instance, with a PVDF film that typically produces voltage peaks of around 60 volts, these peaks are reduced to only 2 volts.

It is also important to note that I do not yet have knowledge of the amount of current supplied by my films, although it is very low (in the range of nano to a few hundred microamperes). Another challenge I am facing is that the measured voltages exceed 10 volts. To address this, I have considered using a voltage divider to reduce the input analog voltage by a factor of 10, and then compensating for this factor in LabVIEW during the acquisition of the analog input voltage from the DAQ.

Being relatively inexperienced in electronics, these problems appear complex to solve on my own. This is why I am turning to you, hoping that you could provide your expertise to assist me in overcoming these challenges. Any form of guidance, technical explanations, or practical advice that you could offer would be greatly appreciated.

Time is of the essence, as I have less than a month to complete this project within the specified deadlines. Your contribution would be crucial to the success of my project. I sincerely thank you for your consideration of my situation and for the valuable help you can provide.

I extend my warm regards and thank you once again for your support.

[JRys]

1.75 volts is a typical value that is returned by the USB-205 when nothing is connected to the input. Read the input with it connected to GND. If the new value is not close to zero and remains at 1.75 volts, the USB-205 is malfunctioning. 

Regarding your application, piezoelectric crystals tend to have very high impedance and because of this, you need a measurement device with a much higher input impedance.  The USB-205 input impedance is only 1M ohm, whereas a device like the USB-1808 is 1G ohm. This could be why your attempt at a voltage divider did not work. 

[Alioune]

I would like to sincerely thank JRys for their previous message. 

However, I would need some clarification regarding a part of the content. Specifically, I would like to understand the meaning of the following statement: '1.75 volts is a typical value that is returned by the USB-205 when nothing is connected to the input. Read the input with it connected to GND. If the new value is not close to zero and remains at 1.75 volts, the USB-205 is malfunctioning.'

Regarding my situation, I attempted to perform a measurement by connecting channel CH1 to the piezoelectric film in order to measure the piezo voltage, while I connected AGND to the ground of the piezo. However, despite these connections, I continue to observe a constant voltage of 1.75 volts.

Furthermore, I would also like to discuss the impedance of my piezo. According to my measurements, the resonance impedance of my piezo is 3500 ohms.

If anyone could provide further insight into these points or offer suggestions to resolve my issue of unexpected voltage despite correct connections, I would greatly appreciate it. Your knowledge and assistance would be highly valued.

[JRys]

Regarding the 1.75-volt measurement, there are two possible scenarios. The piezoelectric film puts out 1.75 volts, or the USB-205 believes the inputs have no signal. The USB-205 is designed to work with low-impedance signals (less than 100 ohms), which the piezoelectric film is not. You can test the USB-205 by reading the input with the piezoelectric film disconnected. Then, add a wire from the USB-205 input to GND and take a reading. The two readings should be approximately 1.75 and zero.