ISA-88 (S88) Batch Control Explained
When it comes to automation and manufacturing ISA-88 or just S88 is inevitable.
It is a standard for published by The International Society of Automation (ISA) that sets out models and terminology addressing batch control. The standard was published in an attempt to standardize and thereby making it easier for automation suppliers to integrate, communicate and configure batches. It is adopted by Europe as IEC 61512-1.
Today, the standard is used by most manufacturers and automation suppliers that deal with batch control. Especially productions of food and medicine utilizes if not the whole, then at least parts of the standard to keep track of their batch production.
In this article I will try to explain what S88 and batch control is and why this particular standard has become so widely used.
What is Batch Control?
One of the first things you will find in the standard is the definitions of terminology. Among them, also a definition of what batch process control is.
S88 defines a batch process as follows:
A process that leads to the production of finite quantities of material by subjecting quantities of input materials to an ordered set of processing activities over a finite period of time using one or more pieces of equipment.
Of course this is just the official definition, and it might not seem very descriptive if you’re new to batch control. I like to explain batch processes by looking at the three different ways of manufacturing:
- Batch production
- Job production (one-off production)
- Mass production (flow production)
Mass production is a continuous production of a standardized product. Take a car producer for example, where cars are produced in a constant flow from an assembly line. On the other hand is job production which is the production of a custom product, often in very small amounts.
Right in between these two is the batch production method.
What is a Batch?
On the contrary to mass production, batch production is limited by time and quantity. The product is produced or processed in batches. First one batch is produced, then another, then another and so on. A good example of batch production is medicine production in the pharmaceutical industry. Medicine is produced in batches, and you will see later in this article why S88 is so useful for that industry.
A batch is simply a limited quantity of goods or material produced in a single manufacturing run. A batch process is the production or processing of a batch.
In order to describe the batch process control S88 puts out three models:
- Process model: chemical and physical changes to the materials (product)
- Physical model: looking at batch control in terms of hardware
- Procedural control model: looking at batch control in terms of software
The Process Model
In the S88 standard the batch process is organized in something called the process model. It is to divide the batch process into a hierarchy of smaller and smaller subdivisions. Doing so gives us a standardized way to describe not only the whole process, but also the subdivisions of it.
As you can see above the process consists of process stages, that consists of process operations, that at last consists of process actions.
Every batch process can be divided into process stages. Process are in series (after each other), in parallel (simultaneously) or a combination. Each process stage typically operates independently from other process stages. A sequence of physical or chemical changes to the material is usually the result of a process stage.
Examples: Production, Cleaning
Each process stage consists of process operations. They represent major processing activities, where the result is often physical or chemical changes to the material. Process operations are a bit more concrete than process stages.
Examples: Mix ingredients, Dry, Drain (under production stage)
Every process operation can be divided into process actions. They are the lowest level of the process model. The actions needed to complete the operations.
Examples: Add water, add other ingredients, heat the tank to 55 degrees, hold for 120 mins (under mix ingredients operation)
The Physical Model
All the physical facilities for a batch production are described with the physical model. It is by means of this model that the plant is divided into a hierarchy of smaller sections. When the processes has been described the physical assets (hardware) to accomplish those are described.
In fact, the physical model is the foundation for all the other ISA-88 batch control concepts like processes, recipe management etc. All these other concepts has to be linked to some equipment, and that requires you to define them.
The physical model is a hierarchy of different levels. Lower levels are combined to form higher levels. Two equipment modules together are a unit or three sites are an enterprise as examples. Each level in the S88 physical model either may or must contain one or more sub-levels.
There are 7 levels in the physical model. The top three levels are considered as business and not further described as part of the batch processing in the standard. On the other hand, the lower four levels are relevant to the batch process.
At the very top of the model is the enterprise level. It may contain one or more of the lower levels. Here we’re usually talking about a company or a department that decides which products will be manufactured. It may contain all of the underlying levels.
Example: Kellogg’s Company
A logical, physical or geographical group determined by the enterprise. The boundaries of a site is decided by business criteria.
Example: HQ and factory in Manchester
A logical, physical or geographical group determined by the site. Just like the site level, the boundaries of the area level are decided by organizational or business criteria.
Example: Cereal factory
Process cell level
A process cell is everything required to process or produce a batch. It is defined as a group of logical connected process facilities in on area. Each process cell must include units and may include equipment modules and/or control modules.
Example: Cornflakes production facility
Units are one or more major processing activities. They are made up of equipment and control module.
Example: Corn pressing unit
Equipment module level
Equipment modules carry out minor processing activities. They may include control modules and other equipment modules.
Example: Draining filter module
Control module level
Control modules are the lowest level. These modules are typically tied directly to one piece of hardware like sensors and actuators. One control module can also be a collection of multiple control modules. You can also have multiple sensors and pieces of hardware in one control module. Like a motor and encoder in one control module.
Example: Valve, motor etc.
The Procedural Control Model
The procedural control model describes how the batch process should be carried out. In the model you will find that the procedure has three subordinates, unit procedures, operations and phases. The procedure describes all the equipment-oriented actions which in an ordered sequence will carry out the batch.
This is the model that describes how we’re going to make our batch with our equipment. As you will see later these three models are tied together. So when you’re describing procedures with the procedural model, it is in relation to your equipment described with the physical model and your processes described in the process model.
The highest level in this model is the procedure. The procedure defines the strategy for making the batch. Since this is the highest level in the hierarchy all details are left out. The procedure consists of unit procedures. In terms of relation the the other models, the procedure combined with a process cell is what carries out a process.
Examples: make ice cream, make beer
Unit procedures consists of an ordered set of operations. They describe which operations will happen within each unit to carry out a process stage. Only one operation can be activated at a time in a unit. You should also know that a unit procedure combined with a unit is what carries out a process stage.
Examples: freeze ice cream mixture, start fermentation
At the level below you will find the operations. They are by the standard defined as ordered sets of phases. When a change (physical or chemical) happens to the material you can call that an operation. At last, an operation combined with a unit is what will carry out a process operation.
Examples: mix ingredients, heat while mixing
The lowest levels in the procedural control model are the phases. You can even divide them into smaller pieces if necessary. A phase can give command and cause actions like enabling/disabling states, setting/resetting and changing alarms, controller constants etc. I’ll explain what states are later in this article. You can combine a phase with both a unit or an equipment module to carry out a process action.
Examples: heat, add water, add yeast
ISA-88 Model Relationship
This relationship between the three models is illustrated below. It is crucial to know about these relationships, because this is how you connect the physical equipment with the processes. Largely speaking, the procedural control model is what you want to do, the procedural model is how you do it, the physical model is with what you do it.
Batch Control Concepts
Beside the three models ISA-88 also defines some batch control concepts. Although you have tied your equipment to your procedures, you do not always want procedural control of the equipment. Sometimes you just need some manual or basic control of the functions.
For this reason ISA-88 introduces three different types of control.
Basic control is all about establishing and maintaining a state of the equipment and process. More on the states soon. But what you should know here is what you can control and how basic control responds to certain conditions. This is what the standard defines basic control:
- includes regulatory control, interlocking, monitoring, exception handling and repetitive-discrete or sequential control
- may respond to process conditions that could influence the control outputs or trigger corrective actions
- may be activated, deactivated or modified by operator commends or by procedural or coordination control
So, even though you’re only establishing or maintaining a state, you can still do certain types of control, monitoring etc. Basic control of elements can also be activated/deactivated by elements in procedural or coordination control.
Next type of control type is the procedural control. As the name indicates, procedural control carry out the procedures from the procedural control model.
This type of control is essential to batch production. You will use procedural control to perform a batch process with your equipment.
The procedural elements that controls the equipment-oriented actions comes from the model – procedures, unit procedures, operations and phases.
If you have ever heard the term coordination before, then you might have a feeling of what coordination control is. Coordination, allocation, selecting and arbitration are some of the typical things coordination control can be used for.
Coordination and allocation of resources and equipment to batches are common algorithms to use here.
Both procedural and equipment elements can have modes. Modes defines how the elements operate and respond to different commands. With procedural elements they also define how the procedure will continue and who can affect the flow if it.
In the standard three modes are proposed:
Procedural elements can be in any of the three modes, while equipment elements can only be in either automatic or manual.
Semi-automatic defines the way transitions in the procedure are handled. When a procedural element is in semi-automatic mode the procedure needs a manual approval, after the transition conditions are true.
The behavior and allowed commands of the different modes are defined as below. You should also notice that depending on what control type you’re using, the modes have different behaviors.
|Automatic (procedural)||The transitions are carried out without interruptions.||Operators can pause progression but not force transitions|
|Automatic (basic)||Equipment are manipulated by their control algorithm.||The equipment can not be manipulated by the operator|
|Semi-automatic||When transition conditions are true they are carried out
by manual commands.
|Operators can pause the progression or redirect to an appropriate point.
Transitions can not be forced.
|Manual (procedural)||The procedural elements are executed in the order chosen
by the operator.
|Operators can pause progression or force transitions.|
|Manual (basic)||Equipment are not manipulated by their control algorithm.||Equipment can be directly manipulated by the operator.|
One last important thing is when you change the mode of one element others can change too. If you for example change the mode of an operation its lower levels (phases) may also change to that mode.
Just like modes, procedural and equipment elements can have states. While the modes are defined by behavior, states are defined by the current condition of the element. A motor for example may be in the state of e.g. running or stopped.
Many states are defined in the standard and some of them are more specific for components. A valve can have the state of 30% open, while a motor can’t. On the other hand, a motor may have the state 50% speed, which is generally not possible for a valve.
Here are some of states as defined by the standard:
As you can see some of the states look awfully a lot like each other. Some states are final states, some are quiescent states and some are transient states.
All the states that ends with “ing” are transient states. A state is transient means that the state is temporary. As soon as the state is completed the state will change to a quiescent or final state. Think of stopping as an example. Before the procedural element is actually stopped it is in the state of stopping.
Before we dig deeper into the transitions of states we have to look at commands.
The commands are what initiates transition of states. An example of a command is the stop command. When that command is giving to a procedural element in running state, the state will change to stopping. Same goes with the start command which may initiate the change of state from idle to running.
There’s a certain order to the states or to be more precise, which states can change to which. In some way it also makes sense, that you can’t change state from idle to stopping. An illustration of this is called a state transition matrix or diagram. You and your team can use the diagram or matrix to find the relation between your states, commands and transitions.
Below is an example of such a diagram. Notice the difference between final states, quiescent states and transient states.
The last thing I want to cover within the batch control concepts is exception handling. You should know that no matter which state or mode things can always go wrong. The way we handle things after they go wrong is called exception handling. An exception occurs when the normal or desired behavior of the batch control doesn’t happen.
Exceptions can occur in different levels of the procedure or equipment. Most common are the exceptions that happen on the lower levels of the equipment model – hardware. But exceptions can also occur because of lack of input material, problems with the process etc.
Most important here is how you handle these exceptions. One exception handling could be to put change the state of a valve to idle or faulty if a malfunction happens. This might affect the whole batch process and it should be decided, whether the batch can be finished or not.
You can read more detailed about exception handling and S88 here.