Setting the sampling frequency for 3-wire sensors requires strict adherence to the Nyquist sampling theorem and must be tailored to the specific sensor type and the dynamic characteristics of the signal. The core principle is this: the sampling frequency must be greater than twice the highest frequency component of the signal; in practical applications, a factor of 3 to 10 times or more is typically adopted to ensure signal integrity.
I. Basic Principles for Setting Sampling Frequency
Nyquist Sampling Theorem
To prevent signal aliasing, the sampling frequency ($f_s$) must satisfy the condition: $f_s > 2 \times f_{max}$, where $f_{max}$ represents the highest frequency component of the signal being measured.
Engineering Safety Margin
In practical applications, a multiplier of 5 to 10 times is often selected to enhance waveform reconstruction accuracy, particularly for sinusoidal signals.
Anti-Aliasing Filtering
In high-precision systems, a low-pass filter should be installed prior to the Analog-to-Digital Converter (ADC), with a cutoff frequency set slightly below $f_s/2$, to prevent high-frequency interference from contaminating the signal.
II. Recommended Sampling Frequencies Based on Sensor Type
|
Sensor Type |
Signal Characteristics |
Recommended Sampling Frequency |
Notes |
|
PT100/PT1000 RTD |
Slowly varying signal (response time in seconds) |
1–10 Hz |
Temperature changes slowly; excessively high sampling rates are unnecessary and may instead increase signal noise. |
|
3-Wire Hall-Effect Speed Sensor |
Pulsed square wave; frequency is directly proportional to rotational speed |
≥ 5 times the maximum output frequency |
E.g., for a measurement of 3000 rpm (50 Hz), a sampling rate of ≥ 250 Hz is suggested; ≥ 500 Hz is recommended. |
|
3-Wire Vibration Sensor (Accelerometer) |
High-frequency vibration (tens to thousands of Hz) |
≥ 10 times the highest frequency |
E.g., for measuring 2 kHz vibration, the sampling rate should be ≥ 20 kHz to ensure accurate FFT analysis. |
|
3-Wire Pressure Transmitter (4–20 mA Output) |
Slowly varying or low-amplitude fluctuating signal |
10–100 Hz |
Suitable for dynamic monitoring of hydraulic and pneumatic systems, balancing both response speed and stability. |
Calibration Recommendations: For static calibration, set the sampling rate to 1–5 Hz and calculate the average value; for dynamic calibration, use an oscilloscope or data acquisition card to ensure the capture of complete signal waveforms.
