Logix 520MD Digital Positioner Technical Bulletin - Flowserve 522MD+37-W2D2F0-GM3-

Logix 520MD Digital Positioner Technical Bulletin - Flowserve 522MD+37-W2D2F0-GM3-

Introduction
The Logix 520MD Series combines superior positioning and tuning functions with convenient setup and powerful, easy to use diagnostic tools unlike other positioners that have only one gain. The Logix 520MD positioner uses a multi-variable variable gain tuning algorithm. This allows the positioner to make large step changes with minimal overshoot, while achieving the resolution to respond to very small step changes.The Auto-tune procedure cycles the actuator to produce a measured response and selects gain values that provide appropriate actuator performance. The Auto-tune function includes a gain modifier selector that can be used to increase or decrease the calculated gain in order to achieve optimal performance. By setting the Auto-tune on/off DIP switch, the tuning mode can be changed from auto to manual. The Logix 520MD Series positioners provide several preset gain settings with a locally adjustable gain set selector directly from the user interface on the positioner. If custom settings are desired, tuning sets can be modified with a handheld or with ValveSight™ software, to accommodate a wide range of actuator sizes and types.

Logix 520MD Digital Positioner Technical Bulletin - Flowserve 522MD+37-W2D2F0-GM3-

The 520MD, 521MD and 522MD models
• Three versions:
 Basic
 Advanced (Advanced includes pressure sensors)
 Pro (Pro includes pressure sensors and full featured valve diagnostics)
• HART Command 1, 3, 9, 33 & 48
• Burst Mode available for continuous data transmission
• Position command with analog 4-20 mA loop current
• Final value of command after characterization
• Supply pressure (Advanced +Pro)
• Stem position in percent
• Onboard temperature sensor to measure local positioner ambient temperature
• Stroke speed limiter (configurable through HART or ValveSight™)
• Step test, friction test, HRL, data logger
• DTM Available
Logix 520MD Digital Positioner Technical Bulletin - Flowserve 522MD+37-W2D2F0-GM3-

Principle of Operation
The Logix 520MD positioner is a digital positioner with various options. The positioner consists of three main modules:
 1. The microprocessor based electronic control module includes direct local user 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 microprocessor, where it is
compared to the measured valve stem position. The control algorithm in the processor 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
position and the measured position 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.
Logix 520MD Digital Positioner Technical Bulletin - Flowserve 522MD+37-W2D2F0-GM3-