For multipurpose
plants it is not reasonable to create a complete PLC-code for one product. It
is much more effective to define an interface and a recipe description
language, which can be used by the production enginer to configure production
logic. This leads to more intelligent and advanced control applications for
batch plants. Control modules provide a powerfull interface to control the
plant and sequence logic is described in the widely used Sequential Funkction
Chart. All control details are solved within the module and the user has only
to draw a SFC and define the parameters to specify the operation mode of the
modules. A recipe define the parameters values of the control modules for all
production steps. A recipe is therefore a minimal collection of information
that defines the manufacturing requirements for a specific product. This
processing is called a batch process and is ubiquitous. It is most prevalent in
pharmaceutical, food processing and specialty chemical industries.
In order to be
able to realize a recipe controlled plant, it is necessary to well structure
plant software. A commonly used structure, defined in the ISA SP88 norm, will
be described. In order to get a feeling of a control module a short description
of a sample module is provided. Finally further concept for recipe structuring
and handling are provided.
For batch control
a hierarchical software model is preferred. The complete task of controlling a
plant is split into control units as shown in Figure
1. It is reasonable to structure the PLC-software in a
corresponded form. As a consequence, modules can be exchanged with a minimal
effect to the control software. Furthermore, the equipment module defines the
natural interface for recipe control. In the following, the modell in Figure 1 is explainde in detail.
The bottom layers consists of the
control module. The control module software handles the PLC-interface to the
plant and provides, due to ist control logic, smart devices to be used in the
quipment module control. Examples of control modules are motor control or
sophisticated sensors which are controlled from the PLC-Logic. Furthermore,
control module units may contain elements to define display properties on a
SCADA-System, including limit control to create warnings. In intelligent
devices like frequency controlled motors, a control module can also implement
feedback control.
Figure 1 physical Model |
The next level is the equipment module control.
An equipment module is a functional group of
equipment and/or control modules that can carry out a finite number of specific
processing activities. It offers a secure interface for plant control, without
having to know all the details the module. This is also the interface for most
of the recipe control systems. In Figure 2 a batch plant example is shown. The units in a
coloured rectangle are equipment modules, e.g. Feeder-Modules, Temperatur
Control Modules and so on. The equipment module software usually contains most
of the plant control know how. It also the re-used software entities in
industrial automation and are therefore kept in user libraries.
The
next level is the 'unit'. A unit is a collection of Equipment Modules that can
carry out one or more processing activities. Unit control is usually realised
as a recipe control system, which defines control of the Equipment Modules.
Several Units a grouped to a Process Cell. This is the entity capable for
production of a product. Its control logic is teh managenemt of recipes for its
Units.
Structuring of
control software into modules was standardized in various produktion
industries. In Chemical process control the German Norm NAMUR NE33 proposed
control modules similar to the equipment modules in ISA SP88.1.
Corresponding
standardisation occured in the machin, for example in OSACA (Europa) or OMAC (
Figure 2: Structuring of a Batch Plant
Since the
equipment module control provides the user interface for recipe control, a
detailled description of its use is necessary. Today, production is often
transferred from one production site to another. As a consequence, a general
recipe has to be mapped to different site recipes. This process is strongly
simplified if similar modules are parametrized in the same way. For example, it
is reasonable to have a similar interface to all modules in charge of temperature
control. In information sciences, Object Oriented Design with classes and class
inheritance are widely accepted. Similar concepts can be used for equipment
modul control and ist documentation. A class description for all temperature
controlling modules can be build. For individual modules the class description
is extended or specialized. A major focus has to be set on the operation modes
of the modules. The more the operation modes are standardised, the easier it is
to create a recipe, to control the equipment modules on the HMI and to document
the module.
In the following
an example of a class oriented description of a control modul is provided.
The documentation
of a class of equipment modules may consist of the following chapters:
1. General
1.1 General Description
1.2 Abbreviations and Definitions
1.3 Class History
2. Parameters
2.1 Description of Operation Modes
2.2 Parameters Description
2.3 Parameters and Ranges as a function of Operation Modes
3. Safety
3.1 Description of Safety Functions
3.2 Safety Table
3.3 Safety vs. Operation Modes
4. Controls
4.1
Description of SCADA control
4.2
Description of local controls
4.2
manual control
4.3
drawing of local controls
5. Devices
A
simplified, table based description of chapter 2 and 3 für a temperature
control module is proposed in the sequel.
2.1 Operation Modes
|
Processparameter |
||
Operation Mode A |
Description |
active Devices |
GoOn Condition |
0: Off |
Not active |
no |
TRUE |
1: Wall Temperature Control |
The
temperature feedback controller regulates wall temperature Tx.2 to
setpoint (Par 3) |
Temperature
Control Valve TV x.5.1 - 4 |
TRUE if
temperature is inside range for 5 min |
2: Reactor Temperature Control |
The
temperature feedback controller regulates reactor temperature Tx.1 to
setpoint (Par 3) using a cascaded
control structure with wall temperatur control. |
Temperature
Control Valve TV x.5.1 - 4 |
TRUE if
temperature is inside range for 5 min |
3: Reactor Temperature Control with
Temperature difference limitation |
The temperature feedback controller regulates reactor temperature Tx.1 to setpoint (Par 3) using a cascaded control structure with wall temperatur control. Setpoint of the wall temperature controller is limited to [Tx.1 - DT ... Tx.2 + DT]. (DT: Par 4) |
Temperature
Control Valve TV x.5.1 - 4 |
TRUE if
temperature is inside range for 5 min |
4: Ramped Reactor Temperature Control |
The
temperature feedback controller regulates reactor temperature Tx.1 to
setpoint (Par 3) using a cascaded
control structure with wall temperatur control. Setpoint of the reaktor
temperature controller is ramped with Ramp velocity(Par 5) |
Temperature
Control Valve TV x.5.1 - 4 |
TRUE if
temperature is inside range for 5 min and final setpoint value is attained |
2.2 Parameter description
Nr. |
Name |
Unit |
Description |
1 |
manual control enabled |
Bool |
enables manual control |
2 |
operation mode A |
Enum |
Temperature control mode |
3 |
Setpoint |
°C |
Setpoint for
Temperature Control, Controller selected according to operation mode |
4 |
Delta T |
°C |
allowed Temperature difference |
5 |
Ramp velocity |
°C/min |
maximal rate
of change of a setpoin |
6 |
T-limit high |
°C |
Low Limit for
Temperature Monitor |
7 |
T-limit low |
°C |
High Limit
for Temperature Monitor |
2.3 Parameters
and Ranges as a function of Operation Modes
Parameter |
Parameter and
Range (X: no special limits) |
||||||||||||||
Operation mode |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
11 |
12 |
13 |
14 |
15 |
1: Wall Temperature Control |
X |
X |
X |
|
|
X |
X |
|
|
|
|
|
|
|
|
2: Reactor Temperature Control |
X |
X |
X |
|
|
X |
X |
|
|
|
|
|
|
|
|
3: Reactor Temperature Control with Temperature
difference limitation |
X |
X |
X |
X |
|
X |
X |
|
|
|
|
|
|
|
|
4: Ramped Reactor Temperature Control |
X |
X |
X |
|
X |
X |
X |
|
|
|
|
|
|
|
|
3. Safety
3.1 Description of Safety Functions
Name |
Description |
TSL |
- Softswitch,
can be parameterised (Par 6) - creates
Alarm 1, acknowledge not necessary. - cooling
valve is forced to close |
TSH |
- Softswitch,
can be parameterised (Par 7) - creates
Alarm 1, acknowledge not necessary. - heating
valve is forced to close |
TSHH |
- Temperature
limit switch, Device - leads to
fast stop - acknowledge
necessary. - emergency
cooling is started |
3.2 Safety Table
In a safety table
all safety related actions of the control software are summarized.
3.3 Safety vs.
Operation Modes
Functionality of safety action may depend on the operation mode. This is documented in the following table.
|
Operation Mode |
|||||
Safety Switch |
0 |
1 |
2 |
3 |
4 |
5 |
TSL |
|
X |
X |
X |
X |
|
TSH |
|
X |
X |
X |
X |
|
TSHH |
|
X |
X |
X |
X |
|
A sequential
function chart with parameterized action is called a recipe. The PLC-Norm
IEC61131-3 does not foresee to use actions with parameters. Nevertheless, based
on the former GRAPHCET-Norm SFC-programmable recipe control in process
industrie was available since 1994.
Recipe control
provide the tools and formalism for a production engineer to configure unit
control without changing code and without help of a control engineer. Today, it
is desirable that a production recipe can be formulated in a way that it is
independet off the production plant. A plant independent description can be
transferred from one production site to another. Obviously a plant independet
recipe cannot be used directly to control a plant. The recipe has to be adapted
to a specific plant. In order to do this in a standardised was, the ISA SP88.01
Norm was borne in a rather lengthy and cumbersome way. In the following, the
recipe modell will be described.
Recipe model:
The recipe model
of the Norm is shown in Figure 3.
The „General Recipe“ is a plant independent
description, how a product has to be manufactured. The „Site Recipe“ will be
created from the „General Recipe“ and takes account of the local materials,
language and also for locale product variants (e.g. 220V power supply ). The
„Master Recipe“ ist a map of the 'Site Recipe' to a plant specific recipe. Here,
the capabilities of the plant, i.e. the available variants of equipment modules
have to be considered. Finally the recipe for one product batch is called the
'Control Recipe'. Mapping a recipe from a Master recipe to a control recipe is
not an easy task. Several research project were done to investigate, how this
map can be supported. Whenever a step has to be refined into substeps, it is a
benefit to include hierarchical concepts into SFC.
Figure 3: Rezeptstruktur
A hierarchical structure results from the
procedural structuring of a recipe, as shown in Figure 4. A
General Recipe and a Site Recipe are a manufacturing description at the
Procedure and Unit Procedure level. As
soon as it is clear, on which type of plant a product will be produced, it is
reasonable to specify the operation and phase level. For a specific plant with a know set of
equipment modules, it is possible to specify the step level. A Control Recipe
may contain step sequences, but it is worthwhile to handle step sequences
within the equipment module control software.
Figure 4: recipe hierarchy |