For the periodic fluctuations in the hot runner temperature control system, the specific PID parameter adjustment suggestions are: prioritize reducing the proportional gain Kp to suppress system over-response; match the integral time Ti to 3-10 times the dominant fluctuation period to eliminate steady-state error but avoid saturation; and cautiously enable the derivative time Td (recommended to be 1/8 of the oscillation period) to smooth the response. In particular, combining FFT spectrum analysis to identify the dominant frequency and then adjusting the parameters accordingly can achieve precise control from "uncontrolled oscillation" to "rock-solid stability."
I. Causes of Periodic Fluctuations and Their Relationship with PID Control
Periodic fluctuations in hot runner systems are typically caused by the following factors, which are closely related to PID parameters:
Fluctuation Characteristics | Possible Causes | Corresponding PID Problems
High-Frequency Small Amplitude (<20-second period) | Signal noise, frequent relay operation | Excessive Kd, unfiltered, excessive Td
Medium-Frequency Oscillation (20-100 seconds) | Excessive Kp, excessive Ti | Excessive proportional gain, integral saturation
Low-Frequency Large Fluctuations (>100 seconds) | External disturbances (mold opening/closing, material change) | Kp mismatch, excessive Ti, slow response
Synchronized with Mold Opening/Closing Cycle | Control rhythm not matching process cycle | PID response lag, segmented control required
Core Principle: Prioritize stability over speed. Temperature systems have high inertia; stability takes precedence over response speed.
II. PID Adjustment Strategy Based on Spectrum Analysis
If the dominant frequency has been obtained through FFT analysis, the following strategies can be used for precise adjustment:
1. Dominant Frequency < 0.01 Hz (Period > 100 seconds) – Large Low-Frequency Fluctuations
Phenomenon: Temperature drifts slowly and then rebounds sharply, with the period matching the production cycle.
Adjustment Suggestions:
Reduce Kp: Reduce to 1.5~2.0 to decrease the system's sensitivity to slow-changing disturbances;
Increase Ti: Set to 3~10 times the period (e.g., period 120 seconds → Ti=360~1200 seconds) to avoid excessively rapid integral accumulation;
Turn off Td: Differentiation is ineffective at low frequencies and amplifies noise instead.
Applicable Scenarios: Large multi-cavity molds, needle valve systems.
2. Dominant Frequency 0.01~0.05 Hz (Period 20~100 seconds) – Mid-Frequency Oscillation
Phenomenon: Typical "4:1 decay" failure, resulting in continuous oscillation.
Adjustment Suggestions:
Kp = 0.45 × Ku (Ku is the critical gain);
Ti = 0.5 × Pu (Pu is the oscillation period);
Td = 0.125 × Pu (e.g., Pu = 60 seconds → Td = 7.5 seconds);
Operation: First stabilize the PI, then add Td to suppress overshoot.
3. Main frequency > 0.05 Hz (period < 20 seconds) – High-frequency jitter
Phenomenon: The temperature curve is "sawtooth," and the output fluctuates frequently.
Adjustment Suggestions:
Check signal quality: Confirm good thermocouple contact and shield interference;
Add digital filtering: Set a filter time constant of 0.5~1 second in the controller;
Decrease Td or turn off the D term: Prevent high-frequency noise from the differential amplification;
Kp can be appropriately increased: Due to the fast system response, it can be set to 2.5~3.0.
Reference Formula:
Recommended Ti value = (3~10) × Dominant Cycle
Recommended Td value = 0.125 × Pu (Pu is the measured oscillation cycle)
III. Adjustment Examples for Different System Types
System Type | Problem Phenomenon | Parameters Before Adjustment (Kp/Ti/Td) | Recommended Parameters After Adjustment | Adjustment Points
8-Cavity Hot Runner | 60-second oscillation cycle | 3.0 / 60s / 15s | 2.0 / 180s / 0 | Reduce Kp, increase Ti, turn off D
16-Cavity Needle Valve System | Synchronized with mold opening cycle (90 seconds) | 2.5 / 120s / 30s | 1.8 / 450s / 0 | Ti set to 5 times the cycle
Internal Heating Hot Nozzle | High-frequency vibration (15-second cycle) | 3.5 / 90s / 20s | 3.0 / 90s / 0 | + 1s Filtering: Turn off D, add filter
IV. On-site Operation Suggestions and Risk Avoidance
1. Step-by-step debugging, avoid one-step adjustment: First adjust P → then I → finally try D; Adjust only one parameter at a time, observe for at least two complete cycles.
2. Enable Anti-Windup: When the output reaches the limit (e.g., heating to 100%), pause integral accumulation; Most intelligent temperature controllers (e.g., Siemens, Omron) support this function.
3. Segmented PID control: Heating stage: Kp=3.0, rapid heating; Constant temperature stage: Kp=1.8, Ti=300s, stable temperature control; Automatic switching via PLC program.
4. Verification method: Apply a ±10℃ step disturbance and observe the recovery process; Run continuously for 1 hour, record steady-state fluctuations ≤±3℃; Check for periodic defects in the product (e.g., alternating odd and even modulus weights). Engineering recommendation: After commissioning, use the SCADA system to perform SPC analysis to ensure that Cpk ≥ 1.33 and that the process capability is sufficient.
