INTRODUCTION
I-E96-315B
2 - 1
SECTION 2 - DESCRIPTION AND OPERATION
INTRODUCTION
This section explains the operation of the hydraulic servo slave
(HSS). It gives an overview of the slave and related modules
within a process control system and explains the operation of
key module circuitry.
CONTROL LOOP OPERATION
The hydraulic servo slave, frequency counter slave (FCS) and
multi-function processor (MFP) form the controlling segment of
a closed loop control system. The MFP is the master module
and directs the control process. The HSS and FCS are slave
modules that interface the process to the MFP. Figure
shows a diagram of the turbine speed control loop and the con-
trol signals to and from the process.
Hydraulic Servo Slave
The HSS provides control of throttle valve position in a turbine
speed control system. The MFP sends data to the hydraulic
servo slave, which directs the control of a hydraulic actuator
(via a servo valve). By sending a position demand to the HSS,
the MFP initiates a change in turbine speed. The position
demand travels over the slave expander bus to the HSS. An
on-board microcontroller reads the demand and loads it into a
digital-to-analog converter (DAC). The resulting analog value
drives the servo valve. The servo valve loads hydraulic fluid to
one side of a double acting hydraulic actuator. As the hydrau-
lic actuator opens or closes the throttle valve, the turbine
speed changes.
For the MFP to work with the HSS, it requires function code
150 in its configuration. Function code 150 defines the slave
address, mode of operation (calibration or normal), calibration
cycle time and stores calibration data. The twelve output
blocks include actuator position, LVDT null position and sta-
tus outputs for the module and process equipment.
A linear variable differential transformer measures actuator
position. The HSS supplies either a 1000, 2500 or 10,000 hertz
excitation voltage (switch selectable) to the primary side of the
LVDT. The secondary of the LVDT develops a differential volt-
age proportional to the position of the hydraulic actuator. The
HSS converts the differential voltage to digital data and sends
it to the MFP. Using the data the MFP stores (during valve cali-
bration) in its configuration specifications (S8-S9), it translates
the LVDT secondary differential voltage into actuator position
