In this section you will learn
- Equipment and control modules
- The differences between the module types
- How modules combine to form functional equipment
In the last section, we introduced the physical model and discussed the top five level (Enterprise, Site, Area, Process Cell, Unit). This section is dedicated to the last two levels, equipment and control modules, as well as talking about how modules combine to form functional equipment.
There is actually one level “below” the control module level, as shown in Figure 11. A Final Control Element is a physical component of the process or equipment. These are actuators and sensors, such as valves, pumps, proximity switches, and instrumentation.
Figure 11: The Physical Model with Final Control Element
A module is a piece or group of equipment with a clearly defined function that includes embedded control to make the equipment behave appropriately to accomplish its function.
A Control Module is the most basic element of the physical model. The standard defines a control module to be
typically a collection of sensors, actuators, other control modules, and associated process equipment that, from the point of view of control, is operated as a single entity.
Each control module provides a direct “connection” to the process through actuators and sensors. Control modules typically:
- are given commands (such as open, close, start, stop, or pump at 40 GPM)
- drive equipment to a state (such as opened, closed, running, stopped, or at set speed)
- can provide information (such as current state or alarm conditions)
A module can be as simple or as complex as needed. The following are examples of modules:
- A valve
- A group of valves arranged in a header
- A pump
- A variable-speed pump with flow transmitter that runs at a defined flow rate
Some of the examples successively built upon earlier examples to show how modules can be made up of simpler modules. This is why equipment and control modules in Figure 11 show links back to themselves.
For example, let’s say you want to control the rate of flow of some liquid. You use a pump driven by a variable-speed motor and a flowmeter. A proportional-integral-derivative (PID) controller reads the flow rate and sets the pump speed appropriately. The procedural part (i.e., high-level sequencing logic) of a control system wants only to control the flow rate and considers the combination pump, flowmeter and PID loop as a single control module. Figure 12 shows what we mean.
Figure 12: A Variable-Speed Pump Control Module
Keep in mind that even though the control module exists in the physical model, not all elements need be physical. In our example, the PID controller can be a PLC instruction or DCS object and not a stand-alone device that physically links the flowmeter to the pump.
As a second example of a control module, consider a group of valves arranged in a header to select a material to transfer. The procedural part of a control system wants only to select a material to transfer and considers the multiple valves as a single control module. Figure 13 shows what we mean.
Figure 13: A Valve Header Control Module
It’s not uncommon to have control modules embedded in control modules. For example, in Figure 13, each valve could have its own control module, then the Material Selection Control Module encapsulates the four valve control modules.
In the simplest form, control modules can just be device drivers, but they can do so much more for you. Control modules should provide a robust method of device control, including these functions:
- Modes — Three common modes include automatic control, where some sequencing logic controls the device state, manual mode, where an operator controls the device state, and simulation mode, allowing an operator or engineer to test software without actually manipulating equipment.
- Permissives — These prevent a device from actuating if constraints or permissions are not true. For example, a tank outlet valve cannot open if the adjoining header is being cleaned.
- State transition logic — While a control module drives to a state, how it gets there may not be as simple as a binary (on/off, open/closed) sequence. For example, perhaps several valves need to open or close to create a flow path.
- Error detection and failure logic (alarms) — These provide feedback on a device’s operation. For example, an alarm is set if a valve is commanded open, but a limit switch does not confirm that it has opened.
Are you interested in learning about interlocks or reading more detail about modes, permissives, or exception logic?
If so, then take a look at Applying S88, the only S88.01 tutorial available on the market today.
We consider two issues especially important with regard to control modules:
- Instrumentation is “global” (can serve more than one module) – For example, one flowmeter can be used by a control module that charges water into a batch tank, and another control module that charges cream.
- A device is controlled by one (and only one) control module – There may be several logical events that trigger the control module to manipulate the equipment. For example, a pump can be started by production logic or by cleaning logic.
S88 defines an Equipment Module to be
a functional group of equipment that can carry out a finite number of specific minor processing activities.
In other words, equipment modules group physical devices for performing one or more specific functions. An equipment module may be made up of control modules or other equipment modules.
Unlike control modules that drive equipment to a state, equipment modules always have some procedural control. That is, an equipment module has a fairly complex set of logic that carries out sequencing. Figure 14 shows an equipment module that selects material and pumps a certain quantity of it. Notice the control modules embedded within the equipment module.
Figure 14: A Material Charging Equipment Module
In Figure 14, it looks like a poor valve is hanging out all by itself. The Material Charging Equipment Module could control it directly, but the best answer is to have a that valve managed by a 2-state valve control module. The equipment module then commands that 2-state valve control module.
Equipment modules are generally the workhorse of automatic control in a process. They usually perform one (and only one) process task that is not product dependent. But, they’re complex enough that operators attempting to perform the same function manually will often need to follow a standard operating procedure (SOP).
Equipment modules can operate stand-alone, but are most often associated with providing process functionality for units. When associated with units, equipment modules can be permanently attached or temporarily attached to a unit. If an equipment module can be active only with one unit at a time, it is called an exclusive-use resource. If it can be active with several units simultaneously, it is called a shared-use resource.
For example, a plant might have a recirculation loop that is common to two batch tanks. The recirc loop can be active while a recipe is running or the operators can run it “manually” as a stand-alone function. As the plant only wants to “recirc” the contents of one unit at a time (the operators wouldn‘t want to mix the contents of two batch tanks), the recirc loop equipment module is an exclusive-use resource.
The table below compares features and behaviors of equipment modules and control modules.
|Equipment Module||Control Module|
|Runs procedural (sequencing) logic||Provides basic control only (drives to a state)|
|Connects to equipment via control modules||Connects directly to equipment|
|Commonly used as shared equipment||Not often used as shared equipment|
|Can contain control modules or other equipment modules||Can only contain other control modules|
Putting it all together
Control and equipment modules combine together with other equipment to form units to provide the functionality needed to execute a process. Figure 15 shows one simple example.
Figure 15: A Process Cell
The tank can be considered the unit in this process cell. However, if we think of all of the shown equipment and control modules as permanently attached, the boundaries of the unit really extend to the entire figure. The table below provides more information about units and equipment modules by comparing the functionality of both.
|Centered on major equipment||Centered on auxiliary equipment|
|Can run multiple procedures (execute phases) simultaneously||Generally has only one procedure (phase)|
|Can get a different recipe for each product||Same commands regardless of product|
|Intended to stand alone||Usually bound to units|
|Can communicate with other units and common resources||Receives requests and commands, only responds|
Now you have a good overview of equipment.
But are you interested in learning more about how to segment your process to be S88-aware?
You can find detail like this in Applying S88.
Remember the three important elements to define a batch control system?
- How to make the product (recipes)
- What physical tools are needed to make the product (equipment)
- How to run that equipment (control activities)
We have learned about recipes and equipment. Now, it’s time learn about the third element: control activities. We’ll do this by showing how recipes link to equipment control.