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Real Time Remote Monitoring over Cellular Networks (GPRS/CDMA) using IoT technology with SCADA Systems

Real Time Remote Monitoring over Cellular Networks (GPRS/CDMA) using IoT technology with SCADA Systems

Polling of remote devices in SCADA systems has always been a challenge. It is not always feasible to lay a optical or copper wire the site. Traditionally, radio communication was used but this suffers from issues like slow data transmission speeds, data loss and security. With the reduction in cost of cellular data plans it is easy and economical to deploy IoT based remote monitoring solution. IoT based solution minimizes site visits for configuration and maintenance, and can reliably work in high bandwidth applications like video surveillance and critical low-bandwidth process data applications.

This blog discusses the installation of a IoT based SCADA system by our system integration partner Ztech Instruments for one of its customer.

Project

The project required to replace an ailing system where two GPRS modems were used to create a VPN tunnel to poll field devices at a site that was 30 Km from the SCADA control room. The project required getting real time data every 30 second for the following number of IOs:

  1. DI signal : 150 Nos. (Potential free contact NO/NC)
  2. Energy Meters : 30 Nos.(Conserve make) RS485 port
  3. Flow transmitter :4 Nos. (E+H : 4-20mA Hart)
  4. Pressure Transmitter : 12 Nos.(E+H:4-20 mA Hart)
  5. Level Transmitter : 4 Nos. (E+H :4-20mA Hart)
  6. Battery Charger : 2 Nos (4-20mA)
  7. IP Camera : 5 Nos. (RTP)

Challenges in Current Approach

The current system suffered from the following challenges:

  1. Fixed IP addresses
  2. Polling
  3. Timeouts

1. Fixed IP addresses

For the central SCADA server to directly query the field devices both the cellular modems at the remote sites and the SCADA server required static public IPs. However, cellular carriers charge higher fees for static IPs, while some carriers do not provide fixed IP address. This setup is extremely expensive. If VPN is not used this is less secure as fixed IP of both the devices and server make them more prone to an hacker attack.

2. Polling Field Devices

Polling field devices is normally used technique in SCADA systems. This polling architecture is not suited for cellular networks because in some scenarios cellular connectivity can be flaky and intermittent. You might lose a lot of important data due to network issue.

3. Timeouts

Cellular networks can have varying latency and this leads to timeouts of many pollings. In case of flaky GPRS connection it was difficult to synchronize the polling with the time window a device link was live. This together lead to 30% polling failing in current system.

Alternate Solutions with InstaMsg

There are three approaches that can be used:

  1. Polling
  2. Push with Catching
  3. Push

Polling

This solution can be used in SCADA system that cannot handle push data. Most SCADA that Ztech came across for their water monitoring application were designed to poll the field devices over fixed IP.

Push

In push architecture the data is pushed by the IoT gateway to the Scada client (e.g. an OPC client).

Unlike the polling architecture, push architecture makes bi-directional communication possible mobile GPRS networks that are using a dynamic IP address. Push architecture solves the Fixed IP address issues and reduces data loss and bandwidth consumption. There are two strategy that you can use:

  1. Push with Catching
  2. Push with OPC

Push with Catching

In this case the field devices push data to the InstaMsg client which catches it locally. When the server issues the command to the local interface (where InstaMsg client is listening) the response to the poll command is received from the local cache. The cache has an expiry which you can set so that stale data is not visible on the SCADA. This solution is efficient then the polling solution.

Push with OPC

In this case the field cellular device pushes data to the InstaMsg SCADA side client and the data is pushed to an OPC server. The OPC server sends a configured point's data to the SCADA immediately whenever the point changes value. This is the most efficient option, and has the least latency. But this solution is only possible with systems that can support tag update via push.

Advantages of InstaMsg based Push Solution

  1. No Fixed IP on client or server side
  2. Supports both Polling and Pushing
  3. Network Diagnostics
  4. Remote Configuration

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