Investigation of an alternative method of measuring the vibrations of plants, using a Laser Doppler Vibrometer

Post authored by Dr Letitia Schoeman

Ms Frances Steyl, from the Department of Mechanical and Mechatronic Engineering at Stellenbosch University, recently completed her Mechatronic Project 478 where she investigated an alternative method of plant vibration measurements.

During drought conditions water loss through a plant’s stomata can create a negative pressure gradient within the xylem vessels. If this gradient becomes too negative, air bubbles or embolisms can form within the vessels, blocking the water movement through the plant. This phenomenon is also known as cavitation. During the cavitation process the plant emits an acoustic emission, or sound, which can be measured and used as an indication of drought stress.
This project investigated the use of a Laser Doppler Vibrometer (LDV) to measure the vibrations of a plant. A LDV works by sending a light pulse towards an object and measuring the time it takes the pulse to return to the sensor (Figure 1). When combining this measurement with the Doppler effect, the velocity of the vibrations of the target can be determined.

Figure 1: Schematic of a Laser Doppler Vibrometer.

The objective of this project was to determine if a LDV can be used to measure cavitations in the xylem vessels of a plant. The cavitations must be detectable in the data recorded by the vibrometer in order to be counted and used to determine the water level of the plant. Ultrasonic measurement devices are currently used to measure cavitation in plants. However, these sensing devices are expensive and invasive. Optical and hydraulic measurement methods can also be used to detect cavitations but makes use of expensive laboratory equipment. In contrast, LDV provides a more affordable and non-invasive solution to drought stress monitoring, by measuring the vibrations caused by the audible acoustic emissions of the cavitations. It also does not need a laboratory set-up as required for hydraulic and optical methods, making it more suitable for outdoor use.
The desired outcome of this experiment was a filtered signal in the time domain, where the presence of cavitations could be detected and recorded. The number of cavitations per time period can be used to determine the hydration level of the plant and monitor its drought stress in a non-invasive manner.
The laser sensor head was focused on the target by using the pointer LED. A photo of the experimental set-up can be seen in Figure 2. The target was a Eucalyptus plant, with varying stem and bark thickness to help determine the ideal measurement location. The three testing locations were marked along the stem of the plant. Location 1 was at the base of the stem where the bark is the thickest, while Location 2 was on the younger section of the plant where no bark is present. Location 3 was also on a stem section with no bark but much closer to the top of the plant. The experiment took place over a period of 10 days to enable the plant to dry out between measurements.

Figure 2: Experimental set-up for the measurements on the plant.

The plot in Figure 3, shows a peak immediately after 0.225 s, which is followed by smaller peaks that slowly die out. This response is consistent with what is expected from the decaying signal produced by a cavitation in the xylem vessel.

Figure 3: Voltage vs. time plot at Location 2 on Day 2.

The results from the experiment demonstrated the detection of cavitations in the time domain. Distinct peaks could be identified in the measurements taken on Day 2, and when isolating these peaks a decaying response was observed, as expected from the cavitations.

Laser Doppler Vibrometry proved to be a viable solution for measuring the vibrations caused by cavitations in young plants with thin bark. The exact effect of the stem and bark thickness on the measurements is still unclear and should be further investigated.

The LDV demonstrated to be a feasible alternative method to measure cavitation in plants – a new and exciting frontier is awaiting!



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