PT700 Electro Pneumatic Transducer

PT700 Electro Pneumatic Transducer-Data Sheet (PDF Download)

The electro-pneumatic transducer PT 700 series provides the proportional conversion from a 4...20 mA input signal to a 0...100% supply pressure output signal. This device provides the interface between the electro-pneumatic positioner and a pneumatic actuator of process industrial applications (chemical, petro-chemical, refineries etc.).
They are easy to adapt and improve the performance of the control loop with optimized dynamic behaviour. The transducer PT 700 series is the first which uses piezoelectric microvalves with low power consumption allowing connection to standard two-wire circuits. No extra power supply is needed and makes the device very compact and robust.

Features
• Accurate conversion of the electrical input signal by using a silicon pressure sensor for feedback.
• Robust modular design
• Pressure regulator eliminates variations in supply air pressure.
• Innovative, reliable transducer technology (piezo electric microvalve)
• Input signal: 4 — 20mA
• Split range start 4 mA or 12 mA
• High resistance against shock and vibration
• Two wire system for signal and power supply line
• Easy maintenance due to modular design
• Universal mounting kit according to NAMUR specifications in stainless steel
• Precise control performance and high dynamic behaviour with PI control characteristics in the internal control loop behaviour with PI control characteristics in the internal control loop

The PT 700 positioner is an analog positioner with variousoptions. The positioner consists of three main modules:
1. The electronic control module including direct localuser interface switches
2. The piezo valve-based electro-pneumatic converter module
3. The infinite resolution valve position sensor.
The basic positioner operation is best understood by referring to figure 1. The complete control circuit is powered by the two-wire, 4-20 mA command signal. The analog 4-20 mA command is passed to the electronic,where it is compared to the measured valve stem position.
The control algorithm performs control calculations and produces an output command to the piezo valve, which drives the pneumatic amplifier. The position of the pilot valve in the pneumatic amplifier is measured and relayed to the inner loop control circuit. This two-stage control provides for more responsive and tighter control than is possible with a single stage control algorithm. The pneumatic amplifier controls the airflow to the actuator.
The change of pressure and volume of the air in the actuator causes the valve to stroke. As the valve approaches the desired position, the difference between the commanded pressure and the measured pressure becomes smaller and the output to the piezo is decreased.
This, in turn, causes the pilot valve to close and the resulting flow to decrease, which slows the actuator movement as it approaches, the new commanded position.When the valve actuator is at the desired position.the pneumatic amplifier output is held at zero, which holds the valve in a constant position.