51-4
Flowserve Corporation, Valtek Control Products, Tel. USA 801 489 8611
Figure 1: Logix 520 Principle of Operation
Principle of Operation
The Logix 520 positioner is a digital positioner that in-
corporates HART protocol for communication. The po-
sitioner consists of three main modules:
1. The microprocessor-based electronic control
module includes HART communications and direct
local user interface switches
2. The piezo value-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 HART module sends and receives the superim-
posed FSK HART digital communications over the
4-20 mA signal wires providing two-way remote digital
communications to the microprocessor. The analog 4-20
mA command is passed to the microprocessor, where
it is compared to the measured valve stem pos-
ition. 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 ampli-
fier is measured and relayed to the inner loop control
circuit. This two-stage control provides for more respon-
sive and tighter control than is possible with a single
stage control algorithm. The pneumatic amplifier con-
trols the airflow to the actuator. The change of pres-
sure 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 posi-
tion 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.
MOUNTING AND INSTALLATION
Mounting
Before starting installation, inspect the digital positioner
for any transit damages. The Logix 500 series positioner
is installed with a mounting kit (according to NAMUR
specification) to the left-hand actuator support rod.
Generally, the unit can be installed in any mounting
position. The stroke feedback is realized by a follower
arm and stem clamps. For shipment, the follower arm
(maximum stroke of 2.5 inch / 65 mm) is detached from
the positioner to save space. A follower arm for a 4.0
inch (100 mm) stroke must be ordered separately.
Local
User
Interface
4 – 20 mA
+ HART
HART
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
22 – 87 psi (1.5 – 6.0 bar)
Supply Air
∑
-
Micro
Processor
Gain
Pressure Regulator
Piezo Valve
Pneumatic
Amplifier
Control
Valve
+
