INTRODUCTION
I-E96-322A
2 - 1
SECTION 2 - DESCRIPTION AND OPERATION
INTRODUCTION
This section explains the operation of the IMHSS03 Hydraulic
Servo Module. It gives an overview of the hydraulic servo (HSS)
module and related modules within a process control system
and explains the operation of key module circuitry.
CONTROL LOOP OPERATION
The HSS module, frequency counter (FCS) module and
multi-function processor (MFP) module form the controlling
segment of a closed loop control system. The MFP module
directs the control process. The HSS and FCS modules inter-
face the process to the MFP module. Figure
shows a dia-
gram of the turbine speed control loop and the control signals
to and from the process.
HSS Module
The HSS module provides control of throttle valve position in a
turbine speed control system. The MFP module sends data to
the HSS module, which directs the control of a hydraulic actu-
ator (via a servo valve or I/H converter). By sending a position
demand to the HSS module, the MFP module initiates a change
in turbine speed. The position demand travels over the I/O
expander bus to the HSS module. An on-board microprocessor
reads the demand and loads it into a digital-to-analog (D/A)
converter. 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 hydraulic actuator opens or
closes the throttle valve, the turbine speed changes. If the
hydraulic actuator is being driven by an I/H converter, then
the HSS module scales the signal sent by the MFP module to a
four to 20 milliamp signal or 20 to 160 milliamp signal and the
PID controller circuitry is bypassed. This four to 20 milliamp
signal linearly drives the I/H converter to load hydraulic fluid
to one side of the double acting actuator.
The MFP module requires function code 55 or 150 in its config-
uration to direct the operation of the IMHSS03 module. Func-
tion code 55 defines the module address, mode of operation
(calibration or normal), calibration cycle time and stores cali-
bration data. The ten output blocks include actuator position,
digital-to-analog converter output, servo valve coil output cur-
rent, and status outputs for the module and process
equipment.