SCADA system tasks and components
A SCADA system would typically manage an entire manufacturing plant or a large portion of it. A SCADA system is implemented with specific goals in mind. In order to better understand the system implementation process, here are the main tasks of a SCADA:
- Control of manufacturing equipment on the plant floor
- Control and view of plant floor devices: Programmable Logic Controllers, sensors, valves, variable frequency drives, temperature probes, etc
- Display of real-time critical process information
- Acquisition, storage, and display of historical data
The architecture of a SCADA system is made possible through a direct connection to the levels below and above within the manufacturing stack. Let’s examine each component and how it is tied into a SCADA system
Instrumentation Layer
The instrumentation layer contains all the plant floor equipment that is responsible for acquiring information and controlling the process directly. This includes photo eyes, temperature probes, pneumatic valves, variable frequency drives, motors, flow meters and more. In other words, every device that either sends or receives a digital or analog signal directly is considered to reside within this layer. This layer would not talk to a SCADA directly. However, a SCADA system receives information from the PLC that will communicate with this layer.
Imagine that the same grinding machine is capable of faulting out. It may stop due to a lack of maintenance (low oil level), unforeseen breakdown (jam at the infeed), or an operator mistake (safety door opened). Each one of these fault states is being displayed by an LED directly on the electrical panel. Since we have this information available, we may choose to add it to an HMI to provide a more granular feedback mechanism to the operator who will eliminate the inherent fault.
PLC and HMI Layer
The devices from the instrumentation layer interface a PLC. A PLC understands the current state of the process by receiving information through inputs and makes decisions and controls the process through the use of outputs. A Human Machine Interface (HMI) would be a local screen that would allow an operator of the process to observe the status of the process and control certain portions. A typical HMI system would display the current status of the system, alarms associated with the asset as well as a control screen used to make adjustments. An HMI would send the information to the PLC and vice versa; it would not interact with the instrumentation directly.
SCADA Layer
The SCADA system would directly communicate to multiple PLCs on the manufacturing floor. Furthermore, many SCADA systems require control systems engineers to create a communication layer that would be instantiated within every PLC in order to pass data accordingly. An important infrastructure within this layer is the network. Although the PLC and HMI layers will require a network for data, the SCADA system would create an additional strain on the plant network due to the volume of data it will consume.
A SCADA system is capable of providing an HMI service in addition to the floor HMIs. The main difference is that the SCADA based HMI solutions would be networked to a server while the ones at the PLC & HMI layer would communicate with the local PLC only.A SCADA system would implement a database to store the data it collects from the plant floor. This database may be dedicated to the system or shared with the layers above. Typically, a new installation during which it is possible to revamp the MES system would centralize the database into a single location backed up with redundancy.
MES - Manufacturing Execution Systems Layer
The MES layer will gather information about the manufacturing process and provide a high-level overview of the raw ingredients to final goods information. In other words, this system would typically track the amount of raw materials entering the plant and the outcome of production activities in terms of final goods produced.
These systems include several sub-systems of various types. OEE, or Overall Equipment Effectiveness, allows the manufacturing facility to track the reliability of equipment on the manufacturing floor. Furthermore, OEE is used as a critical manufacturing metric that will give management information about the production rate of each asset. MES Systems are used in manufacturing, distribution, supply chain operations and more.
ERP - Enterprise Resource Planning Layer
ERP systems extend beyond the manufacturing facility. They facilitate the tracking of assets, raw materials and finished goods to third parties. These may include suppliers, vendors, distribution centers and more. The goal of these systems is to provide the data as well as accountability of the flow of operations. The ERP will create necessary information to manage shipping, receiving, transportation, purchasing and more.
SCADA system components
A SCADA system would typically refer to the underlying software component of the operation. However, as mentioned above, a SCADA system will rely on multiple hardware components within a manufacturing setting. In this section, we’ll explore these components and their role within the system.
Server Infrastructure
SCADA software runs on a server that is located within the facility or on the cloud. An appropriate implementation of this layer is highly critical and will play an impact on how reliable and accurate the data captured by the SCADA system will be. A proper implementation of the server infrastructure would be reliable through dual-redundancy, scalable through virtual machines, and properly networked through adequate networking infrastructure.
Networks
Due to the fact that a lot of data flows to the SCADA system, properly established network infrastructure is required for optimal operation. Furthermore, a vast variety of industrial network protocols makes it difficult to establish this in practice. In terms of best practices, an industrial manufacturing plant should have an established network prior to deploying a SCADA. However, both integrations may happen in parallel. A network that is build to support a growing SCADA system would include switch redundancy, routing tables and VLANs that segment the plant and redundant hardware that would eliminate failure points.
Visualization and HMIs
The SCADA system will collect data from the manufacturing floor and deliver it through dedicated visualization mediums. These may be dedicated Human Machine Interfaces (HMIs) across the floor, computers within control rooms, web interfaces and mobile applications. Regardless of the medium, the presentation of data plays an important role in the system. Therefore, it’s important to be aware of who will be using the SCADA system and how this information will be delivered to these devices. The simplest SCADA implementation will only deliver the data to the plant management.
A SCADA can leverage a distributed topology for the HMI systems deployed in the field. In other words, the data and control systems would be serviced from the server to the floor applications in the form of nodes. This integration requires the components mentioned thus far and provides substantial savings for the manufacturing plant due to the low cost of the terminals that would be required through this distribution.
Programmable Logic Controllers | PLCs
PLCs are the devices with which the SCADA will interact with the most; data will continuously flow between the two. Due to this fact, a robust implementation must be created on both ends. The data that is sent upwards from a PLC should be accurate, buffered and made available despite a disconnect. This requirement is typically met on the PLC side through efficient code that would store the data for a certain period of time before a connection to the SCADA database is established.
