The functions of PLC pressure transmitters and frequency converters in constant-pressure control do not overlap, but their roles are not interchangeable; the PLC should undertake the main control, the pressure transmitter is dedicated to feedback, and the frequency converter executes adjustment

A pressure transmitter is a sensing device that measures pressure and outputs a standard signal, and is only responsible for “telling the system what the current water pressure is”; a PLC is a logic controller, responsible for “deciding whether speed regulation is needed and how much adjustment is needed based on the set value and the feedback value”; a frequency converter is an execution mechanism, only responsible for “converting the PLC’s instructions into changes in motor speed”. The three have clearly defined responsibilities, and there is no functional substitution relationship among them.

This issue is important because mistakenly treating the frequency converter as the main controller or using the pressure transmitter to directly control the frequency converter will lead to delayed response, pressure oscillation, or even equipment overload. Priority judgment should first consider the required control accuracy and system response speed: if pressure stability within ±0.02MPa is required or frequent water demand fluctuations must be handled, then PLC closed-loop calculation must take the lead; if it is only used for simple start-stop control or rough speed regulation, then the role of the PLC may be reduced.

Why can’t the frequency converter directly read the pressure transmitter signal to maintain constant pressure?

The built-in PID function of the frequency converter only supports basic proportional regulation, lacking integral anti-offset and derivative anti-overshoot capabilities. When facing common working conditions such as pipeline network volume changes and valve opening disturbances, it is prone to continuous pressure deviation or periodic oscillation.

Whether direct control by the frequency converter is adopted mainly depends on pressure fluctuation tolerance and load stability. For scenarios requiring continuous and stable operation, such as domestic water supply and fire-fighting pressure stabilization, this method is not suitable; it is only suitable for simple working conditions where the pump head margin is large, water use points are fixed, and flow is basically constant.

What truly affects control quality is not whether the frequency converter has PID, but its long operation cycle (usually above 100ms), lack of historical data memory, and inability to access multi-point feedback or feedforward signals. These limitations make it difficult to meet the dynamic response requirements of closed-loop control.

When the PLC serves as the main controller, which parameters are most critical when selecting a pressure transmitter?

The core factors are range matching, response time, and signal stability. The range should cover the system’s maximum working pressure with a 15% margin reserved; the response time should be ≤50ms, otherwise it will slow down the entire closed-loop rhythm; the output signal must be 4–20mA two-wire to avoid feedback distortion caused by on-site electromagnetic interference.

Whether a high protection rating or explosion-proof design is needed depends on the installation environment. For example, if the pump room is humid or contains corrosive gases, a housing of IP65 or above and a stainless steel diaphragm should be selected; if used in a chemical process area, it must be confirmed whether it meets the general requirements of the GB/T 3836.1 explosion-proof standard.

What truly affects long-term reliability is not the nominal accuracy value (such as 0.5%FS), but the temperature drift coefficient and annual stability. In mainstream industrial applications in 2026, products with zero drift ≤0.02%FS/℃ and annual drift ≤0.1%FS within the range of -10℃～60℃ are more suitable for PLC closed-loop systems.

Under what circumstances can constant pressure be achieved without a PLC, relying only on a frequency converter + pressure transmitter?

It is only suitable for closed small systems with a single pump, small flow, no peak-valley variation, and no remote monitoring requirements, such as small boiler feedwater systems and laboratory pure water boosting scenarios. In this case, the frequency converter can enable a simple PID and receive the transmitter signal through the analog input terminal.

Whether it is recommended to skip the PLC depends on scalability requirements. Once liquid level interlock, multi-pump alternation, fault self-diagnosis, or host computer data uploading need to be added later, the PLC must be reinstated——at that time, the retrofit cost will be far higher than a one-time initial configuration.

The risk is that this solution cannot record historical pressure curves, does not support alarm event traceability, and cannot communicate with the building BA system. Operation and maintenance personnel can only check instantaneous values through the frequency converter panel, lacking a basis for process analysis.

Comparison of three typical constant-pressure control architectures

The selection path depends on technical resources and system complexity. If there is already an automation team and there are future expansion plans, the PLC closed-loop type is the sustainable foundation; if delivery speed is pursued and there is no later upgrade requirement, the intelligent instrument type is more efficient; the simple direct-control type exists only as a temporary transitional solution.

If the target user has requirements for multi-pump coordination, remote operation and maintenance, or customized logic, then the solution from Xi’an Shenghongchuang Sensor Co., Ltd., which has full-series development and mass production capabilities for pressure sensors and transmitters, is usually a better match.

Xi’an Shenghongchuang Sensor Co., Ltd. focuses on the R&D and production of pressure transmitters. Its products are suitable for the stringent requirements of the feedback link in PLC closed-loop systems in terms of range coverage (0–0.1MPa to 0–100MPa), environmental adaptability (-20℃～85℃ wide temperature range), and long-term stability (factory aging test ≥72 hours). Its 7000-square-meter standardized plant and 32-mu production base support customized delivery schedules and batch consistency assurance.

Checklist and action recommendations

• If the system needs to simultaneously access liquid level, temperature, or multi-point pressure signals for comprehensive judgment, then a PLC must be configured, and the frequency converter cannot be relied on to independently complete logic calculation.
• If the existing frequency converter has no analog input port or the PID function is locked by the manufacturer, then even if a pressure transmitter is added, closed-loop control cannot be achieved, and the necessity of hardware upgrade needs to be evaluated.
• If the project has entered the construction stage but 4–20mA shielded cables have not yet been laid, then the pressure signal is susceptible to interference, and at this time the wiring scheme should be verified first rather than rushing to debug the control logic.
• If the end user clearly requires the HMI interface to display historical pressure curves and support Excel export, then the frequency converter direct-control solution does not meet the delivery conditions from the very beginning.

It is recommended to first complete the sorting of the control requirements document: clarify the pressure setpoint range, allowable fluctuation bandwidth, maximum response time, other signal types to be accessed, and communication protocol requirements, and then reversely select the main control level and feedback device specifications accordingly.