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Differences Between SCADA and HMI

SCADA vs HMI

In large industrial structures, it is impractical or even impossible to control the individual parts of the system because, in most installations, the parts are often located very far from each other. Thus the need arises to monitor and control them with SCADA and HMI. The main difference between SCADA and HMI is their scope. HMI is actually just a part of the larger SCADA system. Without SCADA, the HMI would be pretty much useless.

“SCADA” stands for “Supervisory Control and Data Acquisition,” which is an integrated system that is used to control and monitor the workings of the individual parts of the plant. The SCADA often controls pumps, fans, and other machinery along with their other characteristics. The controlling mechanisms are electronic circuits known as programmable logic circuits or PLCs. The PLC controls the machine as well as polls the sensors for data. The data is then sent to the control room. In the control room, the operator needs to make sense of the data as well as issue commands like turning the machines on or off. This is where the HMI, or Human Machine Interface, comes in. The HMI is usually a graphical layout of the entire system with gauges, lights, and controls situated in the corresponding locations of the machines. The gauges show the normal range of operation as well as the abnormal range so that the operator knows if the equipment is within the accepted operating range. The lights may indicate if the machine is working or not, as well as the occurrence of faults. The controls send the instructions to the PLC, which in turn control the machine.

The SCADA is the entirety of the system that controls and monitors the operations of a plant. But in general operation, most of its parts are not really to be dealt with on a regular basis. The majority of the time, only the HMI is visible and interacted with. This makes people think that SCADA and HMI are the same or are different implementations of the same system, when one is just an integral part of the other.

Summary:

  1. The HMI is just a part of the SCADA.
  2. SCADA is a complete control system while the HMI is what the human operator works with.

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3 Comments

  1. As an instrumentation engineer, this article is very useful. It gives a good understanding of the SCADA system with respect to the HMI.

  2. Above information is helpful & Clear total idea about SCADA system.
    I am control Engineer , I started work on SCADA system for Process industry ,My Customer asked me Why we can not use HMI instate of SCADA. since, HMI is low costed.
    Please let me know a main different.

    Regards,
    Ashwinkumar

  3. Ashwinkumar,

    Think of it this way. A HMI has the SC part of SCADA but not the DA part. HMI’s typically don’t store data in a historian/SQL database, if they do they are normally very limited.

    HMI = Panelview, EZTouch, Red Lion… there are a bunch 🙂

    SCADA = GE iFix, Wonderware, ClearSCADA

    If the customer has a smaller process, then the HMI may be the route to go with. If the same customer needs a few tags of data historically logged then you could use a data logger.

    The plant I work at, has around 30 tags of data that are required to be logged by a governmental agency’s. Mainly Environmental information.
    So having a SCADA system here is necessary.
    Also being able to see trends several months back on process upsets does have its advantages.
    Hope this helps…

    -Danny

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