I. Core Calculation Logic
The core function of an RC low-pass filter is to retain the effective temperature measurement signal and filter out high-frequency interference. The cutoff frequency needs to be set between the highest frequency of the effective temperature measurement signal and the lowest frequency of the interference. The effective temperature measurement signal frequency of a hot runner thermocouple is typically ≤10Hz, which can fully cover the sampling requirements of normal temperature changes.
II. Step-by-Step Parameter Calculation Method
Determine the target cutoff frequency fc. First, measure the main frequency f_interf of the interference signal at the input end using an oscilloscope. Set the filter cutoff frequency fc to 1/5 to 1/10 of f_interf to ensure that the attenuation of the interference signal is ≥20dB.
In a typical hot runner scenario, if the interference frequency has not been measured, fc can be directly preset to 50~100Hz, fully covering the 50Hz power frequency interference and kHz-level motor drive spike interference commonly seen in industrial scenarios.
Selecting the Standard Capacitor Value C: For the input of the hot runner temperature controller, a 10nF~100nF ceramic capacitor is typically used. A 100nF (0.1μF) universal capacitor is preferred due to its low cost, ease of procurement, and high-frequency characteristics suitable for industrial interference scenarios.
Calculating the Matching Resistance R: Substituting into the core RC filter formula:
R = 1/2πfcC R = 2πfcC Example: When fc is set to 100Hz and C is 100nF, the calculated R≈15.9kΩ. In engineering applications, a 16kΩ standard resistor can be directly used.
III. Hot Runner Scenario-Specific Adaptation Rules
When the main interference frequency is 50Hz (power frequency): When fc is set to 10Hz and C is 1μF, the calculated R≈16kΩ, which can achieve attenuation of more than 30dB for power frequency interference.
When the main interference frequency is 10kHz (motor peak): fc is set to 1kHz, C is selected as 10nF, and R≈16kΩ is calculated. While filtering high-frequency interference, the temperature measurement signal delay is ≤1ms, which does not affect the temperature sampling response speed at all.
When strictly controlling the temperature measurement response delay to ≤200ms: the RC time constant τ=R×C must be ≤0.2s to avoid signal lag introduced by filtering leading to overshoot and runaway temperature rise.
IV. Verification Method After Calculation
After substituting the parameters, the filtered output signal is measured, confirming that the attenuation of the target interference signal is ≥20dB, while the thermocouple temperature measurement signal shows no significant amplitude attenuation.
Field Operation Verification: The temperature controller reading shows no jumps, there is no significant sampling lag during the heating process, and the temperature control accuracy meets the process requirements.
