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User Instructions Logix 520si - LGENIM2520-00 09/09
Figure 1: Logix 520si Principle of Operation
nOTE: The air supply must conform to ISA 7.0.01or IEC 770
(a dew point at least 10 ˚C / 18 ˚F below ambient temperature,
particle size below five microns – one micron recommended
– and oil content not to exceed one part per million).
5 PRinCiPLE OF OPERATiOn
The Logix 520si positioner is a digital positioner that incor-
porates HART protocol for communication. The positioner
consists of three main modules:
1.The microprocessor-based electronic control module includes
HART communications and 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 refer-
ring to Figure 1. The complete control circuit is powered by
the two-wire, 4-20 mA command signal. The HART module
sends and receives the superimposed FSK HART digital
communications over the 4-20 mA signal wires providing
two-way remote digital communications to the microproces-
sor. The analog 4-20 mA command is passed to the micro-
processor, where it is compared to the measured valve stem
position. The control algorithm in the processor performs
Local
User
Interface
Inner Loop
Piezo Control
Stroke
Inner Loop
Position Feedback
1 Digital Control Circuit
2 Electro-pneumatic
Converter Module
3 Valve Position
Sensor
Filter / Regulator
for Supply Air
1.5 – 6.0 bar (22 – 87 psi)
Air Supply
-
Micro-
Processor
Gain
Pressure Regulator
Piezo Valve
Pneumatic
Amplifier
Control Valve
+
4 – 20 mA
+ HART
HART
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.
6 TUbing POSiTiOnER TO ACTUATOR
After mounting has been completed, tube the positioner
to the actuator using the appropriate compression fitting
connectors:
Air connections: 1/4” NPT (standard air connection)
Auxiliary power: Pressurized air or permissible
