Flowserve 3200MD User Manual download pdf (Page 7)

User Instructions - Digital Positioner 3200MD LGENIM0059-09 12/13

The piezo valve pressure modulator controls the air pressure under

a diaphragm by means of a piezo beam bender. The piezo beam

deﬂ ects in response to an applied voltage from the inner-loop elec-

tronics. As the voltage to the piezo valve increases, the piezo beam

bends, closing off against a nozzle causing the pressure under

the diaphragm to increase. As the pressure under the diaphragm

increases or decreases, the spool valve moves up or down respec-

tively. The hall effect sensor transmits the position of the spool back

to the inner-loop electronics for control purposes.

4.3 Detailed Sequence

of Positioner Operations

A more detailed example explains the control function. Assume the

unit is conﬁ gured as follows:

• Unit is in Analog command source.

• Custom characterization is disabled (therefore characterization is

Linear).

• No soft limits enabled. No MPC set.

• Valve has zero deviation with a present input signal of 12 mA.

• Loop calibration: 4 mA = 0% command, 20 mA = 100% com-

mand.

• Actuator is tubed and positioner is conﬁ gured air-to-open.

Given these conditions, 12 mA represents a Command source of

50 percent. Custom characterization is disabled so the Command

source is passed 1:1 to the Control Command. Since zero deviation

exists, the Stem Position is also at 50 percent. With the stem at the

desired position, the spool valve will be at a middle position that bal-

ances the pressures above and below the piston in the actuator. This

is commonly called the null or balanced spool position.

Assume the input signal changes from 12 mA to 16 mA. The posi-

tioner sees this as a Command source of 75 percent. With Linear

characterization, the Control Command becomes 75 percent. Devia-

tion is the difference between Control Command and Stem Position :

Deviation = 75% - 50% = +25%, where 50 percent is the present

stem position. With this positive deviation, the control algorithm

sends a signal to move the spool up from its present position. As the

spool moves up, the supply air is applied to the bottom of the actua-

tor and air is exhausted from the top of the actuator. This new pres-

sure differential causes the stem to start moving towards the desired

position of 75 percent. As the stem moves, the Deviation begins to

decrease. The control algorithm begins to reduce the spool opening.

This process continues until the Deviation goes to zero. At this point,

the spool will be back in its null or balanced position. Stem move-

ment will stop and the desired stem position is now achieved.

One important parameter has not been discussed to this point: Inner

loop offset. Referring to Figure 2, a number called Inner loop offset

is added to the output of the control algorithm. In order for the spool

to remain in its null or balanced position, the control algorithm must

output a non-zero spool command. This is the purpose of the Inner

loop offset. The value of this number is equivalent to the signal that

must be sent to the spool position control to bring it to a null posi-

tion with zero deviation. This parameter is important for proper con-

trol and is optimized and set automatically during stroke calibration.

Figure 2: System Positioning Algorithm

Sensor

Summer

Integration

Offset

Loop

Inner

Gmult

Pmin

Pmax

Deviation

Mode)

(Analog

4-20

Mode)

(Digital

Command

Signal

Input

Digital

Analog

CONTROL

COMMAND

Output

D/A

Percentage

Algorithm

Control

Supply

Air

Control

Spool

Loop

Inner

ATO

Tubed

Sensor

Position

Stem

Voltage

Valve

Piezo

MPC

Soft Limits

Characterization

Linear Mode

Output

Inner-Loop

1 2 3 4 5 6 7 8 9 10 11 12 ... 39 40

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Flowserve 3200MD User Manual Page 7

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