To improve the accuracy of industrial temperature control, we can start from two aspects: PID parameter optimization and sensor calibration. The following are specific techniques:
PID parameter optimization techniques
Manual adjustment method
Proportional gain (Kp): gradually increases from zero until the system begins to oscillate. Proportional gain is responsible for providing control output based on the size of the current error. Gradually increasing Kp will make the system more sensitive to errors, causing oscillations. Then gradually reduce Kp until the oscillation decreases, and find the appropriate proportional gain that can quickly respond to the system while maintaining stability.
Integral time (Ti): Gradually increase Ti and observe the response speed and stability of the system. Ensure that the system does not have integral saturation, that is, the integral term does not cause the system to overreact, which can improve the stability and anti-interference ability of the system.
Differential time (Td): Gradually increase Td and observe the suppression oscillation and stability of the system. Ensure that the differential term does not introduce additional noise or cause system instability. However, excessive Td may introduce noise, leading to system instability. Therefore, when adjusting Td, attention should be paid to balancing the suppression of oscillations and stability.
Ziegler Nichols method
Set initial conditions: Set the integral term Ki and differential term Kd to zero, while retaining only the proportional term Kp.
Determine the critical gain and period: Start from zero and gradually increase the proportional gain Kp until the system experiences sustained oscillations (periodic output fluctuations). Record the proportional gain Kpc and oscillation period Tpc during sustained oscillations, which are the critical gain and critical period of the system.
Calculate Ki and Kd: Ki can be calculated using the formula Ki=0.5 · Kpc/Tpc, and Kd can be calculated using the formula Kd=0.125 · Kpc · Tpc.
Adjust parameters: Further adjust the values of Kp, Ki, and Kd according to actual needs. This method is mainly applicable to first-order or second-order systems, and may not be precise enough for higher-order systems. Caution should be exercised when using it, as instability may be introduced in actual systems.
Frequency response method
Generate frequency sweep signal: In the frequency response method, a sine wave signal is usually used as the input signal.
Measurement system characteristics: Input sine wave signals into the system and measure the output amplitude and phase at the corresponding frequency of the system.
Analysis curve: Observe the frequency response curve of the system to identify key characteristics such as cutoff frequency and phase margin.
Adjusting PID parameters: Based on the analysis results of the frequency response curve, adjust the PID parameters to make the system's frequency response more in line with performance requirements. It is usually necessary to balance the stability, response speed, and anti-interference ability of the system.
Sensor calibration techniques
Preparation before calibration
Determine calibration cycle: Determine the calibration cycle based on the type of sensor, accuracy requirements, usage environment, and importance. The calibration cycle for high-precision and critical process sensors is relatively short, possibly every few months; The calibration cycle for low precision, non critical position sensors can be relatively long, possibly once a year or longer.
Prepare tools and equipment: Prepare corresponding standard calibration devices or calibrators according to the type of sensor, ensuring that their accuracy is higher than that of the calibrated sensor. At the same time, prepare suitable cables or connecting wires to ensure that the sensor is securely connected to the calibration equipment and that signal transmission is interference free.
Check the sensor: Before calibration, the sensor should be visually inspected to ensure that there are no signs of damage, deformation, or corrosion. Check whether the sensor model is consistent with the system requirements to ensure that the replaced sensor meets the usage requirements.
Calibration steps
Environmental condition control: Place the sensor in the calibration room to ensure that the indoor temperature, humidity, and other environmental parameters are stable and meet the calibration requirements.
Connection and setup: Connect the sensor to the calibration device through a connecting wire to ensure a secure connection. According to the operation manual of the calibration equipment, set the corresponding calibration parameters, such as range, accuracy level, etc.
Zero point calibration: For certain sensors, such as displacement sensors or weight sensors, the theoretical zero point is first confirmed. By calibrating the equipment, place the sensor at zero point (such as no pressure, no displacement, etc.), adjust the internal parameters of the sensor, and make the output signal consistent with the theoretical zero point.
Full range calibration: Place the sensor in the full range state (such as maximum pressure, maximum displacement, etc.), observe and record the readings of the calibration equipment. According to the instructions of the calibration equipment, adjust the internal parameters of the sensor to make the output signal consistent with the standard value or within the allowable range of error.
Evaluate calibration results: Compare the calculated error with the error limit specified in the sensor manual or system requirements to assess whether the calibration results are qualified. Record and archive all data, charts, and evaluation results during the calibration process in detail for future reference or traceability.
Special calibration method
Comparative calibration: Compare the sensor to be calibrated with a standard sensor of known accuracy for measurement. Measure the same physical quantity under the same conditions, compare the output differences between the two, and adjust the calibrated sensor accordingly.
Absolute calibration: Direct calibration of sensors using standard substances or instruments with known accurate values. For example, for temperature sensors, standard thermometers can be used for calibration; For pressure sensors, a standard pressure source can be used for calibration.
Online calibration: Calibrate the sensor using specific equipment and methods while it is working normally. This method can reduce the disassembly and installation of sensors and improve calibration efficiency, but it requires professional online calibration equipment and technology.
Maintenance after calibration
Regular cleaning: Regularly clean the surface of the sensor to remove dust, oil, and other pollutants. You can use a clean soft cloth or specialized cleaning agent for cleaning, but be careful not to damage the sensitive components of the sensor.
Protective measures: For sensors used in harsh environments such as high temperature, high humidity, corrosive environments, corresponding protective measures should be taken, such as installing protective covers and using corrosion-resistant materials.
Regular inspection: Regularly check the appearance of the sensor for damage, deformation, or looseness. Check if the connection lines are normal, whether there are any issues such as broken wires, short circuits, or poor contacts.
Functional testing: Use testing equipment or analog signal sources to test sensors and verify their measurement accuracy and stability.