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HMI vs SCADA, The difference between HMI and SCADA

HMI vs SCADA

HMI and SCADA systems perform a similar task, monitoring and controlling some processes, but the main difference between HMI and SCADA is that:

  • HMIs are limited to one or two processes/systems whose monitoring and control are performed locally.
  • SCADA is commonly used to control and monitor more complex systems located in a broader area and requires advanced control tools and techniques

Table of Contents

Introduction

Modern control and automation systems become predominantly interactive systems where two-way communication between the user (operator) and the system plays a crucial role. For this purpose, to simplify and to establish the most efficient way of user-system communication, different interfaces are developed.

The rapid development of electronic components, microcontrollers, and industrial computers also affected the automation technology with various new devices utilized in different fields.

Keeping track of the cutting-edge technologies inevitably confused the users, operators, investors, and even engineers. From that point of view, the primary meanings of HMI (Human Machine Interface) and SCADA (System Control and Data Acquisition) are often equaled or even assimilated. 

In the list below and the following paragraphs, we will try to present a brief explanation that will help us to distinguish those two terms HMI vs. SCADA. Furthermore, based on these differences, we will try to help the users to match their needs to different control system functionalities, dimensions, and topologies.

Topic HMI SCADA

Usage and Applications

Monitoring and controlling - Small applications

Monitoring and controlling - Large applications

System Composition

One Device

Servers and clients PC, Visualization, Reports, Historian

Cost Effectiveness

Cheap

Expensive

Engineering and Deployment

Relatively Easy

Complex

Location

In the field near the process

Server Room

Scalability

Limited

Almost unlimited

Redundancy

Not Exist

Exist and usually used

Complexity

Low

High

Flexibilty

Limited

High

Human Machine Interface - HMI

Human Machine Interface (HMI), as its name is indicating represents the first (commonly basic) level of interactions between the user (operator) and the controlled process.

HMI

These kinds of devices are mainly used to display the critical parameters of the controlled system (most often, it is only the part of the system) and to enable the basic commands to be sent to the actuators.

With the development of computers and electronics, HMIs have also developed. But, if we go through the history, the first HMIs were combinations of the buttons and measuring scales (gauges) placed on the cubicle’s doors and used to track some significant measurements and turn some actuators on or off.

Nowadays, modern HMIs are implemented in different technologies, providing various visualization, installation, and utilization possibilities.
Modern HMIs are primarily implemented as displays with touch screens and/or additional equipment like a keyboard, touchpad mouse, etc.

HMI+PLC System

Besides the essential functions of interface and commands to the specific process (or part of the process), HMIs also enable limited data capture, different graphs, and reports.

The storage capacity used to store data locally depends on the type and manufacturer of the specific HMI. Usually, it enables some limited data history for a few days or weeks.

Another critical function of HMI is local alarming. This functionality is utilized to create specific alarm signals displayed locally on HMI and, depending on the system’s architecture, could be transferred to the superior control center.

Local alarming functionality is essential in industry and other fields where the operator’s monitoring is inevitable. With an adequate signalization of alarms and emergency states, you could avoid a wide range of faults and damages.

Except for the significant savings achieved by appropriate and timely warnings displayed on HMI, the alarming functionality also appears as a vital part of operator protection, especially in high-risk areas such as explosive environments and/or sensitive chemical processes.

System Control and Data Acquisition - SCADA

Unlike the HMI, which is commonly a single device, the SCADA is implemented as a comprehensive system based on different subsystems, devices, and components. 

The fundamental guideline in SCADA is to achieve separated control hierarchy levels which will encompass all necessary control, data acquisition, visualization, communication, and data storing functionalities. 

Modern SCADA systems are large systems implemented using a variety of hardware and software. Some of the main SCADA subsystems are:

  • Communication – based on different communication protocols, adapters are used to integrate a wide range of communication mediums and interfaces into a unified format understandable to the other SCADA’s subsystems. Usually, modern SCADA has an independent communication processor (or Servers) which is only in charge of communications and networking tasks.
  • Data acquisition – which in charge of collecting data from all connected nodes, testing their validity, and transferring them to other subsystems;
  • Visualization – which is in charge of displaying all necessary info (statuses, measurements, commands, alarms, events, etc.) retrieved from different plants/substations usually placed in various locations;
  • Databases and data storage – used to archive all defined data for some more extended period – for example, for purposes such as Electrical Power transmission systems, all data are commonly archived and stored for ten years or more.
  • Reporting and alarming – Compared to the HMI’s local alarms, this subsystem encompasses all alarms from all connected nodes (substations/plants) displayed separately or together to get a broad picture of some extensive system. This functionality also covers the internal alarming and diagnostic functions which track the status and availability of all SCADA components;
  • Additional subsystems – used in simulations, predictions, and analytics purposes.
SCADA System

From the hardware point of view, SCADA consists of different devices such as:

  • Main processors – servers
  • Communication processors
  • Peripheral devices – monitors, keyboards, video walls, printers, storage, etc.
  • Network equipment – switches, routers, antennas, etc.
  • Auxiliary equipment – UPSs, LANs, etc.

On the other hand, SCADA software consists of different modules integrated into one or more functional units. Depending on the controlled process, complexity, and dimensions, these modules could also be very complex.  

Different modules are commonly implemented for all subsystems mentioned above. Their full functionality and cooperation make the unique SCADA software utilized to monitor and control some specific processes/systems.

Compared to the single HMI or group of connected HMIs, the implementation of SCADA requires much more engineering, equipment, financial, and computing resources.

Conclusions

To summarize the story, HMI and SCADA are two connected but completely different terms. The applicability of HMIs is limited to some limited processes/systems whose monitoring and control are performed locally.

SCADA commonly controls and monitors more complex systems in a broader area and requires advanced control tools and techniques. But on the other hand, these benefits are accompanied by the increased implementation complexity and, finally, increased price.

We hope now it’s clear what the difference is between HMI and SCADA.

FAQ

HMI can and usually perform as a standalone system. In some cases, the HMI can be one component of a distributed SCADA system.

Usually, HMI devices will communicate with at least one PLC. But since the HMI is a separate device, the answer is: Yes, it is possible to work with HMI without a PLC.

Since a DCS is a complete control system, it can have few HMIs as system components.

Since a DCS is a complete control system, it usually has a built-in SCADA system.

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