How to Improve the Common Mode Rejection Ratio (CMRR) of a Hot Runner System

The main methods for figuring out whether a hot runner system's CMRR is adequate are to assess signal noise, observe temperature control stability, and perform anti-interference tests. In order to achieve precision molding standards, make sure that temperature changes under electromagnetic interference conditions are ≤±2°C.

1. Direct Judgment Standards (on-site)

An indicator

Reasonable Standards

Justification

Variations in the Temperature Display

≤±2°C (Heating Stage)

Inadequate CMRR or incorrect wiring are indicated if fluctuations are more than ±5°C.

Burrs in PID Output

A temperature control curve that is smooth and has no sharp edges

Frequent PID adjustments brought on by high-frequency noise might quickly result in overheated or cold material.

Effect of Motor Start-Stop

When starting and stopping high-power equipment, there are no temperature swings.

Strong anti-interference is indicated if there is no variation when the pump starts or the injection molding machine closes the mold.

Practical Case: CMRR≥80dB is needed for a certain automotive components injection molding line. Temperature variations on the site were only ±1.5°C, indicating that the system's anti-interference is up to par.

 

2. Methods of Instrument-Assisted Testing

Method of Oscilloscope Testing:

Attach the oscilloscope probe to the thermocouple input in parallel;

Check to see if high-frequency glitches, like 50/60Hz harmonics, are superimposed on the signal;

Inadequate CMRR is indicated if the peak-to-peak noise is greater than 1 mV and the accompanying temperature deviation is greater than 7 °C (K type).

AC Voltage Range Testing with a Multimeter:

Determine the AC voltage between the thermocouple's positive and negative terminals;

Strong common-mode interference is indicated if the reading is greater than 1V, and the CMRR needs to be raised.

Advice: To better replicate real-world operating conditions, the test should be conducted when the equipment is working at maximum load.

 

3. Other Verification Techniques (No Specialized Equipment)

Shielding On/Off Comparison: Turn on or off shielding grounding and see if temperature stability improves noticeably;

Replace temporarily with a high CMRR module by connecting a temperature controller with a CMRR of at least 100 dB and comparing the fluctuations before and after;

Historical Fault Analysis: A subpar initial CMRR is indicated if interference has resulted in short-circuiting, flash, or carbonization.

Safety Rule: "Investigate fluctuations immediately, correct noise immediately, and verify before use." System dependability is ensured by combining these three ideas.