CHE 185 – PROCESS
CONTROL AND
DYNAMICS
CONTROL OF BATCH
PROCESSES
STRUCTURE OF BATCH
CONTROL
• THE BATCH RECIPE IS KNOWN AS
STRUCTURED BATCH LOGIC
• IT CAN BE A RECIPE TO PROGRAM AN
ENTIRE CYCLE OF OPERATION
• IT CAN BE A RECIPE FOR A SINGLE STEP
• IT IS NORMALLY IMPLEMENTED USING A
PROGRAMMABLE LOGIC CONTROLLER
STRUCTURE OF BATCH
CONTROL
• THE ACTUAL PROGRAM IS DESIGNED USING A LADDER LOGIC
DIAGRAM
(HTTP://WWW.CONTROLENG.COM/INDEX.PHP?ID=483&CHASH=
081010&TX_TTNEWS[TT_NEWS]=63761)
• A TYPICAL FORMAT IS:
•
HTTP://WWW.GEOCITIES
.COM/RESEARCHTRIANGLE
/1753/PLC.HTML#II
STRUCTURE OF BATCH
CONTROL
• THE SYMBOLS USED IN THIS DIAGRAM INCLUDE:
LADDER LOGIC DIAGRAM
• MOST OF THE TIME THIS DIAGRAM IS
USED
– TO DESIGN THE PROGRAM FOR THE PLC
– AS A BASIS FOR THE ONE-LINE ELECTRICAL
DIAGRAM FOR THE CIRCUIT
LADDER LOGIC DIAGRAM
• DURING A CYCLE (OPERATIONS PHASES),
THE RECIPE NEEDS TO
• CONTROL THE DESIRED ACTION
• IMPLEMENT EACH STEP BY
• RUN THE INITIATION PROGRAM TO CONFIRM THE
PROCESS IS READY FOR THE STEP
• START THE ACTION PROGRAM
• CONFIRM EACH STEP HAS OCCURRED PRIOR TO
STARTING THE NEXT STEP
• TAKE APPROPRIATE ACTION IF THE PROCESS DOES
NOT FOLLOW THE RECIPE (EXCEPTION)
LADDER LOGIC EXAMPLE
• CONSIDER THE SEQUENCE OF SHUTTING
DOWN A CAR
• THE INITIAL STEP IS TO TURN THE KEY TO
THE OFF POSITION, OR PUSH THE BUTTON
FOR A KEYLESS IGNITION
• THIS STEP REQUIRES THE KEY TO BE IN THE
IGNITION OR THE REMOTE TO BE PRESENT
• IT IS POSSIBLE TO DO THIS WHILE THE CAR
IS MOVING, FOR UNANTICIPATED
EMERGENCIES
LADDER LOGIC EXAMPLE
• WHEN THE KEY IS REMOVED OR THE
BUTTON PUSHED
• AN ALARM MAY SOUND IF THE LIGHTS ARE
STILL ON
• THIS ALARM IS ACTIVATED BY THE KEY
BEING REMOVED FROM THE IGNITION AND IS
POWERED BY THE BATTERY
LADDER LOGIC EXAMPLE
• FOR THIS LIGHTS-ON ALARM TO
FUNCTION THE FOLLOWING MUST BE
TRUE:
• THE LIGHTS MUST BE IN AN ON POSITION
• THE KEY MUST BE OUT OF THE IGNITION OR
THE POWER OFF BUTTON PUSHED
• THE BATTERY MUST BE CHARGED
• THE ALARM MUST BE FUNCTIONAL
LADDER LOGIC EXAMPLE
• ONE-LINE DIAGRAM FOR THE PROCESS MIGHT
RESEMBLE:
A
CLOSED
If Lights
Are ON
(Included
in light
switch)
POWER
SOURCE
(Battery,
not
alternator)
CLOSED
If Key is
Not in
the
Ignition
(Included
in
ignition
switch)
ALARM IS
ALWAYS
ON (Can be
audial
and/or
visual)
GROUND
HARD-WIRED VERSION OF CIRCUIT
• THIS ALARM SHOULD NOT GO ON WHEN THE LIGHTS
ARE TURNED ON WITHOUT THE KEY BEING IN THE
IGNITION
LADDER LOGIC EXAMPLE
• EXAMPLE IF THE KEY IS NOT REMOVED
WHEN THE DRIVER EXIT
• AN ALARM WILL SOUND IF THE DRIVER’S
DOOR IS OPENED TO ALERT THE DRIVER
THAT THE KEY IS STILL IN THE IGNITION
• FOR THIS SEQUENCE TO WORK, ALL THE
FOLLOWING MUST BE TRUE:
–
–
–
–
THE KEY MUST BE IN THE IGNITION
THE KEY MUST BE IN THE OFF POSITION
THE DOOR MUST BE OPEN
THE BATTERY MUST BE CHARGED TO POWER
THE ALARM
LADDER LOGIC EXAMPLE
• NOTE - IT IS STILL POSSIBLE TO LOCK THE
KEYS IN THE CAR WITH THE LIGHTS ON
• MANY CARS ALSO HAVE A STEERING
COLUMN LOCK THAT IS ACTIVATED WHEN
THE KEY IS REMOVED FROM THE IGNITION,
BUT THIS IS NORMALLY A MECHANICAL
RATHER THAN ELECTRICAL PROCESS.
BASIC CONTROL AND INTERFACING
TOOLS AND DEVICES
• THE PLC IS A COMPUTER USED TO
PROGRAM RELAYS AND OTHER
COMPONENTS AS SOFT DEVICES
• AN OUTLINE OF THE SKILLS USED FOR
IMPLEMENTING PLC CONTROL IS AT:
HTTP://WWW.PACONTROL.COM/PLC.HTML
• THE PLC PROGRAM REPRESENTS THE
CONNECTION BETWEEN THE LOGIC AND
HARDWARE IN THE SYSTEM
BASIC CONTROL AND INTERFACING
TOOLS AND DEVICES
• CONSIDER THE FOLLOWING EXAMPLE OF A WALK /
DON'T WALK SIGN AT A PEDESTRIAN CROSSING
WITH PLC CONTROL
• ACTION IS INITIATED BY A PEDESTRIAN UPON
ARRIVING AT THE CROSSING PUSHES THE WALK
BUTTON
• THIS STARTS A 15 SECOND DELAY, FOR THE
SEQUENCE TO STOP TRAFFIC.
• AFTER THIS DELAY, THE WALK SIGN WILL BE
DISPLAYED FOR 10 SECONDS. (ALL OTHER TIMES
THE DON'T WALK SIGN IS DISPLAYED.
• THE MNEMONIC CODE LISTING AND LADDER
DIAGRAM ARE ON THE NEXT PAGE
BASIC CONTROL AND INTERFACING
TOOLS AND DEVICES
CROSSING SIGNAL DIAGRAM
BASIC CONTROL AND INTERFACING
TOOLS AND DEVICES
• CROSSING SIGNAL EXAMPLE
• THE FIRST STEP IN THE SEQUENCE
INVOLVES CLOSING A CIRCUIT WITH PUSH
BUTTON 100
• THE SYSTEM RESETS AT THE END OF THE
CYCLE
DEVICE INTERFACES WITH THE PLC
• SEVERAL SPECIFIC TYPES OF UNITS HAVE
BEEN DEVELOPED
• LOAD CELLS ARE USED FOR MEASUREMENT OF
AMOUNTS
• THE PRINCIPLE OF THE LOAD CELL IS A
CHANGE IN A MEASURABLE VARIABLE DUE TO
AN INDUCED LOAD
• LOAD CELL OUTPUT SIGNALS CAN BE INDUCED
MECHANICALLY, PNEUMATICALLY OR
ELECTRONICALLY
• THEY CAN MEASURE WEIGHTS DUE TO
COMPRESSION, SHEAR, TENSION, OR BENDING.
DEVICE INTERFACES WITH THE PLC
• THE LOAD CELL SIGNAL IS READ THROUGH A
WHEATSTONE BRIDGE:
DEVICE INTERFACES WITH THE PLC
• LOAD CELLS CAN WEIGH ITEMS AS SMALL AS A PILL OR AS
LARGE AS A LOADED RAIL CAR
• EXAMPLES OF RANGES FOR UTILIZATION ARE:
DEVICE INTERFACES WITH THE PLC
• EXAMPLES OF RANGES FOR UTILIZATION
ARE:
http://www.omega.com/literature/transactions/volume3/load.
html
DEVICE INTERFACES WITH THE PLC
• EXAMPLES OF RANGES FOR UTILIZATION ARE:
DEVICE INTERFACES WITH THE PLC
• LOAD CELL SPECIFIC APPLICATIONS
FOR BATCHI OPERATIONS
– ADDING AMOUNTS OF MATERIALS TO
BATCH PROCESSES,
– MEASUREMENT OF PRODUCT INTO
FINISHED GOODS CONTAINERS
– DISCRETE DEVICES TWO-STATE DEVICES;
LIMIT SWITCHES; DEVICE STATES
DEVICE INTERFACES WITH THE PLC
• CONTACT POSITION SENSORS (LIMIT
SWITCHES) ARE ELECTROMECHANICAL
DEVICES
– THEY ARE OPEN & CLOSED BY THE
MOVEMENT OF A LEVER ARM THAT IS IN
CONTACT WITH A MATERIAL.
– THE POSITION SWITCH HAS TWO
POSITIONS WHICH RELATE TO A BINARY
SIGNAL, SO THEY WORK WELL WITH THE
PLC TYPE OF PROGRAM.
DEVICE INTERFACES WITH THE PLC
• CONTACT POSITION SENSORS TYPICAL
APPLICATIONS
– VALVE POSITION - WHEN MOUNTED ON
THE ACTUATOR OR VALVE SHAFT
– POSITIONING WHEN A UNIT IS LOCATED
CORRECTLY
– ON A CONVEYOR BELT, TO ASSURE THE
BELT IS ON TRACK
DEVICE INTERFACES WITH THE PLC
• NON-CONTACT POSITION SENSORS
• PHOTOELECTRIC DEVICES – THE LEVER ARM IN THE LIMIT SWITCH IS
REPLACED WITH A LIGHT BEAM (FROM AN
LED) AND A PHOTOCELL
– THESE CAN BE VERY SENSITIVE BUT THAT
ALSO CAN LIMIT THEIR APPLICATION TO
VERY CLEAN SYSTEMS
DEVICE INTERFACES WITH THE PLC
• NON-CONTACT POSITION SENSORS
• ULTRASONIC DEVICES – THESE CAN BE EITHER ANALOG OR
DIGITAL
– THEY HAVE A WIDE RANGE OF
APPLICATION THOUGH THEY WILL NOT
WORK WITH MATERIALS THAT ARE SOUND
ABSORBING
DEVICE INTERFACES WITH THE PLC
• NON-CONTACT POSITION SENSORS
• CAPACITANCE DEVICES
– MEASURE THE PRESENCE OF A MATERIAL
IN A FIELD
• SENSITIVITY DEPENDS ON DIELECTRIC
CHARACTERISTICS OF THE TARGET
• SENSITIVITY INCREASES WITH DIELECTRIC
CONSTANT
• MATERIALS WITH RELATIVELY HIGH
DIELECTRIC CONSTANTS CAN BE DETECTED
INSIDE CONTAINERS WITH LOW DIELECTRICS
CONSTANTS, E.G., WATER IN A GLASS
CONTAINER
DEVICE INTERFACES WITH THE PLC
• NON-CONTACT POSITION SENSORS
• INDUCTION DEVICES
• DEVICES THAT USE
ELECTROMAGNETIC EDDY FIELDS
GENERATED BY METALLIC
COMPONENTS IN MOTION
• USED FOR NON-CONTACT
OPERATIONS SUCH AS VIBRATION
SENSORS ON ROTATING SHAFTS
DEVICE INTERFACES WITH THE PLC
• NON-CONTACT POSITION SENSORS
• INDUCTION DEVICES
• SENSITIVITY IS HIGH FOR METALS AND
LOW FOR NON-METALS, SO THESES
ARE NOT AFFECTED BY OIL AND
GREASE
• THESE DEVICES CAN BE ANALOG OR
DIGITAL IN CONFIGURATION
DEVICE INTERFACES WITH THE PLC
• FLOW METERING SYSTEMS
• FLOW CAN BE CONTROLLED BY
DIRECT MEASUREMENT OR LOSS-OFWEIGHT METHODS
• DIRECT MEASUREMENT USES SOME
TYPE OF DIRECT MEASUREMENT OF
THE FLOW
• THESE ARE THE IN-PIPE DEVICES
DEVICE INTERFACES WITH THE PLC
• FLOW METERING SYSTEMS
• BELT SCALES USE THE COMBINATION OF LOAD
CELLS WITH SPEED SENSORS TO MEASURE
MATERIAL FLOW ON A BELT. THIS SHOWS THE LOAD
CELLS INSTALLED FOR A CONVEYOR BELT IDLER.
THEY MEASURE THE LOAD ON THE IDLERS
http://www.processauto.com/loadcell_b
elt_scale.htm
DEVICE INTERFACES WITH THE PLC
• FLOW METERING SYSTEMS
• BELT SCALES
• THE SPEED MEASUREMENT DEVICE IS MOUNTED SO
THAT IT CONTACTS THE INNER SURFACE OF THE
RETURN BELT.
DEVICE INTERFACES WITH THE PLC
• FLOW METERING
SYSTEMS
• LOSS-OF-WEIGHT CAN BE USED TO
MEASURE MATERIAL FLOWS FROM A
DEVICE MOUNTED ON A WEIGH
SCALE AND USING LOAD CELLS, THE
RATE OF DISCHARGE IS CONTROLLED
BY THE ROTATION RATE FOR THE
DISCHARGE AUGER (SCREW). THE
SCREW IS LOCATED AT THE BASED
OF THE HOPPER OR BIN. LOAD CELLS
ARE USED TO DETERMINE THE RATE
OF WEIGHT (LOSS.HTTP://WWW.WEBTECH.COM.AU/LOSS2045.HTM)
• THESE UNITS CAN ALSO BE USED AS
VOLUMETRIC FEEDER SYSTEMS
DEVICE INTERFACES WITH THE PLC
• FLOW METERING
SYSTEMS
• DRIBBLE FLOW FEEDERS ARE
ACTUALLY USED FOR LOADING
INTO PACKAGES
• FIXED AMOUNTS OF MATERIAL
ARE LOADED INTO WEIGH
HOPPERS AND THEN LOADED
INTO CONTAINERS
http://www.fischbein.com/americas/products.php?cat=1332
DEVICE INTERFACES WITH THE PLC
• FLOW METERING SYSTEMS
• DRIBBLE FLOW FEEDERS EXAMPLE
SEQUENCE CONTROL
• THE NATURE OF BATCH CONTROL
• BATCHES REQUIRE DISCRETE QUANTITIES OF RAW
MATERIALS
• BATCH PROCESSES NEED TO HAVE TRACKING OF
PRODUCTS IN PROCESS
• SOME BATCH PROCESSES HAVE MORE THAN ONE
PRODUCT IN PROCESS SIMULTANEOUSLY
SEQUENCE CONTROL
• THE NATURE OF BATCH CONTROL
• INTERMEDIATE BATCH PRODUCTS ARE MOVED
FROM ONE OPERATION TO THE NEXT IN DISCRETE
AMOUNTS
• SOME PROCESSES INCLUDE CONTINUOUS
PROCESSING INTEGRATED WITH BATCH
OPERATIONS
• THE NATURE OF BATCH PROCESSES MAKES THEM
MORE COMPLEX THAN CONTINUOUS OPERATIONS
SEQUENCE CONTROL
• THE NATURE OF BATCH CONTROL
• EACH STEP IN A BATCH PROCESS HAS A POTENTIAL
FOR FAILURE, SO FAILURE MODES NEED TO BE
MORE COMPLEX THAN FOR CONTINUOUS
OPERATIONS
• EACH STEP IN A BATCH PROCESS NEEDS TO BE
CONFIRMED FOR COMPLETION
• MOST STEPS IN BATCH PROCESSES NEED TO GO
TO COMPLETION ONCE THEY ARE STARTED.
SEQUENCE CONTROL
• INPUT/OUTPUT FOR BATCH SYSTEM
CONTROL
• I/O MUST PROVIDE CONTINUOUS STATUS OF
A BATCH PROCESS
• CONTROL SYSTEM SHOULD ALLOW
CONTINUOUS MONITORS AND CONTROLS
FOR SOME PARTS OF THE PROCESS.
PROGRAMMING FOR BATCH
OPERATIONS
• EACH OPERATION IN BATCH SYSTEMS
NEEDS TO BE PROGRAMMED
INDEPENDENTLY, BUT STILL BE COMPATIBLE
WITH THE OVERALL PROCESS CONTROL
• THE CONTROL SYSTEM MUST
AUTOMATICALLY DETECT PROCESS UPSETS
AND EQUIPMENT FAILURE
PROGRAMMING FOR BATCH
OPERATIONS
• THE BATCH PROCESS MAY BE USED FOR
MORE THAN ONE TYPE OF PRODUCT, SO
NEEDS TO BE EASILY REPROGRAMMED.
• SOME PROCESSES, ESPECIALLY BIOTECH,
MAY USE THE SAME EQUIPMENT FOR MORE
THAN ONE STEP, WHICH LEADS TO MORE
COMPLEX CONTROL SYSTEMS.