ISA-88 (S88) Batch Control Explained

In automation and manufacturing, you will inevitably encounter ISA-88, also called S88.

Published by The International Society of Automation (ISA), the standard defines models and terminology for batch control. Its aim is to standardize batch control so automation suppliers can more easily integrate, communicate and configure batches. Europe has adopted it as IEC 61512-1.

Most manufacturers and automation suppliers working with batch control use the standard today. Food and medicine producers in particular use all or part of it to keep track of batch production.

I will explain S88 and batch control, including why the standard has become so widely used.

What is Batch Control?

The standard opens with definitions of its terminology, including batch process control.

The S88 definition of a batch process is:

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.

If you are new to batch control, the official definition may not explain much on its own. I find it helpful to compare the three manufacturing methods:

• Batch production
• Job production (one-off production)
• Mass production (flow production)

Mass production makes a standardized product in a continuous flow, such as cars moving through an assembly line. Job production makes a custom product, often in very small quantities.

Batch production sits between these two methods.

What is a Batch?

Unlike mass production, batch production has limits on time and quantity. It produces or processes one batch at a time, continuing with the next when the first is finished. Pharmaceutical manufacturing is a good example: medicine is made in batches. Later, we will see why S88 is useful in that industry.

A batch is a limited quantity of goods or material made in one manufacturing run. Producing or processing that batch is a batch process.

S88 describes batch process control through 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

The S88 standard organizes the batch process through a framework called the process model. It divides that process into increasingly specific levels. You can use this standardized hierarchy to describe the whole process first, then work down through the levels until you reach the individual parts of the process.

ISA-88 Process Model

The diagram shows a process divided into process stages. These contain process operations, which in turn contain process actions.

Process Stages

Process stages divide up a batch process. They may run in sequence, at the same time or in a combination of the two. A stage usually works independently and produces a series of physical or chemical changes in the materials.

Examples: Production, Cleaning

Process Operations

Process operations are the major processing activities within a stage. They often cause physical or chemical changes to the material and describe the work more concretely than process stages.

Examples: Mix ingredients, Dry, Drain (under production stage)

Process Actions

Process actions are the tasks required to complete an operation. Each operation can be divided into these actions, the lowest level of the process model.

Examples: Add water, add other ingredients, heat the tank to 55 degrees, hold for 120 mins (under mix ingredients operation)

The Physical Model

The physical model describes all the tangible elements involved in batch production. It organizes the plant into a hierarchy that describes the physical setup needed to perform the processes. In that way, the model connects the hardware available in the plant with the steps described in the process.

The physical model is the foundation for other ISA-88 concepts, including processes and recipe management. Each concept needs a connection to equipment, so you must define that equipment.

Higher levels in the physical model combine lower levels. For example, two equipment modules can form a unit, or three sites can form an enterprise. Each level in the S88 hierarchy either may or must contain one or more sub-levels.

ISA-88 Physical Model

The physical model has 7 levels. Its top three cover business matters, which the standard does not describe further as part of batch processing. The lower four levels concern the batch process.

Enterprise level

The enterprise is the top level, usually a company or department that decides which products to make. It may contain one or more of the lower levels, including all the levels beneath it.

Example: Kellogg’s Company

Site level

The enterprise determines a site as a logical, physical or geographical group. Business criteria set the site’s boundaries.

Example: HQ and factory in Manchester

Area level

The site determines an area as a logical, physical or geographical group. As with the site, organizational or business criteria set its boundaries.

Example: Cereal factory

Process cell level

A process cell contains everything needed to process or produce a batch: a group of logically connected process facilities in one area. It must include units and may also include equipment modules and/or control modules.

Example: Cornflakes production facility

Unit level

Units cover one or more major processing activities and consist of equipment and control modules.

Example: Corn pressing unit

Equipment module level

Equipment modules perform minor processing activities and may contain control modules or other equipment modules.

Example: Draining filter module

Control module level

Control modules occupy the lowest level of the physical model. Typically, a module connects directly to a piece of hardware, such as a sensor or an actuator. However, a control module can also contain several other control modules. You can include multiple sensors and other pieces of hardware within one control module too. A motor and its encoder, for example, can share one control module.

Example: Valve, motor etc.

The Procedural Control Model

This model guides the execution of the batch process. It describes the steps through unit procedures, operations and phases. By setting out the sequence of equipment-based actions, the procedural control model describes what needs to happen to complete the batch efficiently and effectively.

ISA-88 Procedural Control Model

This model describes how the equipment will make our batch. The three models work together: procedures in the procedural model relate to equipment in the physical model and processes in the process model.

Procedures

The procedure is the highest level and sets the strategy for making a batch, leaving out the details. It consists of unit procedures. Across the models, a procedure and a process cell together carry out a process.

Examples: make ice cream, make beer

Unit Procedures

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

Operations

Below unit procedures are operations, defined in the standard as ordered sets of phases. A physical or chemical change to the material can be an operation. An operation and a unit together carry out a process operation.

Examples: mix ingredients, heat while mixing

Phases

Phases are the lowest level in the procedural control model. If necessary, you can divide a phase into smaller pieces. A phase can issue commands and cause actions such as enabling or disabling states. It can also set or reset alarms and change controller constants, among other actions. I will explain states later in the article. To carry out a process action, you can combine a phase with a unit or an equipment module.

Examples: heat, add water, add yeast

ISA-88 Model Relationship

The diagram below shows how the three models connect physical equipment to 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.

Relationship between the three models from ISA-88

Batch Control Concepts

ISA-88 defines batch control concepts as well as the three models. Connecting equipment to procedures does not mean you always want procedural control. Some functions need manual or basic control instead.

ISA-88 therefore describes three types of control.

Basic Control

Basic control establishes an equipment or process state and keeps it in that state. We will look at those states shortly. For now, consider what you can control through basic control and how it responds when particular conditions arise. The standard defines basic control as follows:

• 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

Establishing or maintaining a state can still involve control and monitoring. Elements under procedural or coordination control can also activate or deactivate basic control elements.

Procedural Control

Procedural control carries out the procedures described in the procedural control model.

Batch production needs procedural control to perform a batch process with the equipment.

The procedural elements that control the equipment-oriented actions come from the model. They are procedures and unit procedures, followed by operations and phases.

Coordination Control

Coordination control manages and allocates resources and equipment so they work together. It puts the right processes in place and selects the resources they need. It also resolves conflicts to keep operations running smoothly.

Modes

Modes describe how procedural and equipment elements operate and respond to commands. For a procedural element, the mode also determines how the procedure continues and who can change its flow.

The standard proposes three modes:

• Automatic
• Semi-automatic
• Manual

Procedural elements can use all three modes. Equipment elements can use only automatic or manual mode.

Semi-automatic mode governs transitions in a procedure. After the transition conditions become true, a procedural element in this mode needs manual approval to continue.

The table below sets out each mode’s behavior and permitted commands. A mode’s behavior also depends on the type of control in use.

Changing one element’s mode may change other elements too. For example, changing an operation’s mode may put the phases beneath it into that mode.

States

Procedural and equipment elements also have states. A mode describes behavior; a state describes the element’s current condition. A motor, for example, may be 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.

The standard defines states including:

• Idle
• Running
• Complete
• Pausing
• Paused
• Holding
• Held
• Restarting
• Stopping
• Stopped
• Aborting
• Aborted

Several of the state names look very similar, but they describe different conditions. The categories include final states and quiescent states, along with transient states.

States whose names end in “ing” are transient, meaning temporary. Once that state is complete, the element moves to a quiescent or final state. For example, a procedural element passes through stopping before it reaches stopped.

To understand state transitions, we first need to look at commands.

Commands

Commands initiate state transitions. Giving the stop command to a procedural element in the running state changes its state to stopping. The start command may similarly change its state from idle to running.

There is a defined order to the states: an element can move from one state only to certain other states. For example, changing from idle to stopping would not make sense. You can show the permitted changes in a state transition matrix or diagram. Your team can use that diagram or matrix to understand how the states connect to commands and transitions.

The example below distinguishes final states from quiescent and transient states.

State transition diagram with states and commands

Exception Handling

Exception handling is the last batch control concept I want to cover. Problems can arise in any state or mode. An exception occurs when batch control does not behave as expected or desired; exception handling describes how we respond.

Exceptions can arise at different procedure or equipment levels. They occur most often at the hardware level of the equipment model, but a shortage of input material or a process problem can also cause an exception.

The response to an exception matters most. For example, a malfunction might lead you to change a valve’s state to idle or faulty. As that may affect the whole batch process, you must decide whether the batch can be completed.

For more detail, read about exception handling and S88 here.