Hope exists, but the road to hope must be paved inch by inch with sweat and brain cells.
Making your own inductive vibration probe sounds simple, but it's full of pitfalls in practice. The limited equipment in the lab is simply not enough for handling precision sensing.
Sun Mei was in charge of the circuit, which was the most crucial part. She rummaged through drawers and found some old electronic components—resistors, capacitors, transistors, and a small roll of thin enameled wire. Since there was no ready-made signal generator, she used a few transistors and resistors/capacitors to build a simple high-frequency oscillation circuit, the output of which was so unstable it was frustrating.
“The signal is too weak, and the signal-to-noise ratio is pitifully low,” Sun Mei stared at the faint waveform on the oscilloscope that was almost invisible due to noise, her brows furrowed. “We must design multi-stage amplification and find a way to filter out the interference.”
With higher amplifier gain, inherent noise and drift became new problems. She repeatedly adjusted component parameters and tried different filter networks, often spending most of the day at a time.
Liu Li was responsible for the probe's mechanical structure and coil winding. Lacking a dedicated frame, she found a short piece of nylon rod with good insulation and carefully machined a coil core herself on a lathe. Using a roll of thin enameled wire, she held her breath and manually wound the measuring coil, turn by turn. Even a slight tremor of her hand could cause the coil to become uneven, affecting its sensitivity.
"How many circles have we gone around?" Zhou Wei asked as he came over.
"I've wound almost a thousand turns, my hands are about to cramp up." Liu Li didn't look up, focusing intently on the work in her hands. "Wrap it tighter, increase the inductance, and the signal will be stronger."
The wound coil needs to be fixed on the bracket and aligned with the position of the analog spindle. Without a precision adjustment mechanism, Liu Li used the most basic method—shielding thin copper sheets and adjusting it little by little by hand, using a multimeter to monitor the minute changes in inductance and find the optimal sensing distance.
Zhou Wei wasn't idle either. Based on the preliminary circuit parameters provided by Sun Mei and the coil inductance value measured by Liu Li, he continuously revised his control algorithm model. "The upper limit of the response frequency of your probe is probably not too high. I need to adjust the frequency adaptation range in the algorithm. Don't expect it to handle too high frequency vibrations."
The first joint debugging session was a complete disaster. With the power on, the oscilloscope displayed nothing but noise; there was almost no visible signal related to vibration. The slightest breeze or someone walking by would cause the waveform on the oscilloscope to jump wildly.
"The electromagnetic interference is too severe!" Sun Mei pointed to a section of burr that had suddenly appeared. "There are so many motors and relays in the lab; they're all sources of interference!"
“The mechanical installation isn’t stable enough; it’s a bit shaky, and the baseline is drifting.” Liu Li was checking the probe bracket.
Problems kept piling up. The three of them huddled around the rudimentary device, checking the circuitry, tightening screws, adding shielding, changing the grounding... When something failed, they analyzed the cause, adjusted the solution, and tried again.
Countless failures and countless sleepless nights passed. Finally, late one night, after Sun Mei adjusted the bias resistor of the last stage of the amplifier circuit, the previously chaotic baseline on the oscilloscope screen began to rise and fall clearly and regularly as the analog spindle (the device with the eccentric wheel) rotated!
"I've got it! Look!" Sun Mei's voice trembled.
Liu Li and Zhou Wei immediately came over. Although the signal amplitude was not large and the waveform was somewhat distorted and mixed with noise, the clear periodic changes synchronized with the rotation speed undoubtedly proved that their makeshift inductive probe could indeed "sense" the vibration!
"We did it! We got it working!" Zhou Wei slapped his thigh excitedly.
Liu Li breathed a long sigh of relief, looking at the pulsating waveform, a relieved smile appeared on her face, even though her eyes were bloodshot.
This rudimentary probe, cobbled together from enameled wire, nylon rods, discarded transistors, and countless hours of painstaking work, may not be as powerful as a professional sensor, but it is like a crude yet crucial key that has forcibly opened a crack leading to the next step.
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