Monitoring and Control Systems: 12 Things to Consider

By Karl Lambert

Automated monitoring and control systems have been around since the late ‘70s and are now used in a variety of industries. These systems have become increasingly important in the water industry where water quality, levels, flow rates, and equipment must be monitored and controlled to conserve, protect, and manage one of our most valuable resources.

The range of measurement instrumentation available for monitoring and control systems can be overwhelming, but regardless of the names or acronyms used to refer to the varying pieces of instrumentation, these systems typically consist of a few common components: sensors, a measurement and control device, and management software. Sensors convert physical parameters, such as water level or flow rate, to electronic signals. A measurement and control device, such as a PLC, RTU, or datalogger, reads the sensors and, based on internal programming, controls equipment, triggers alarms, and/or records the measurement data. The software provides data access and control capability. A variety of other factors determine additional system peripherals, such as power supplies, telemetry equipment, and communication protocols.

There are many considerations to make when configuring a measurement and control system. In this article we’ll take a brief look at 12 of the most important.

1. Sensor accuracy

Your monitoring and control system will only be as good as the accuracy of your measurements. Measurement accuracy starts with your sensors. Once you have determined the parameters your system will measure, consider the accuracy required for those measurements. Usually the cost of a sensor increases with its accuracy. If a flow rate only needs to be controlled at ±30 gpm, you may be spending more than you need on a sensor that has an accuracy of ±5 gpm.

2. RTU and sensor compatibility

The type of signal that a sensor outputs to the RTU depends on the sensor’s technology. Most RTUs support a variety of sensor outputs (e.g., 0 to 5 vDC, 4 to 20 mA, pulse/frequency, or serial communication). Obviously, the signal output of the sensors has to be supported by the RTU’s input types. Verify that the RTU supports the type and number of sensor inputs that are needed.

3. Accuracy of the RTU

For those sensors that output analog signals, the accuracy of the RTU, or lack of it, may add to the error of the sensor. The error introduced by the RTU is due to internal offsets and the analog to digital conversion. If the measurements matter, it is best to use an RTU that is much more accurate than the sensor, since the total measurement error is a combination of the sensor’s and the RTU’s error. You will likely want to select an RTU with enough accuracy that it never decreases the accuracy of any sensor you may use, now or in the future. The analog accuracy of the RTU should be listed in the manufacturer’s specification sheets. Look for the offset, A/D conversion, and temperature error.

Sensors that output a pulse, frequency, or send serial messages (e.g., Modbus, SDI-12) do not lose accuracy. These signals are either counted or read directly by the RTU with no internal conversion or measurement error.

4. Networking

Measurement sites can be stand alone, or multiple sites can be linked together via a network. Networks use communication protocols to manage how data is packaged and sent from one site to another. Both open and proprietary protocols exist. Examples of open protocols are Modbus, DNP3, IEC 61850, and TCP/IP. The rules and specifications for open protocols are published and available to anyone. Proprietary protocols are not open and are owned by a specific manufacturer.

Often manufacturers support both proprietary and open protocols allowing you to choose between the benefits of the unique functions of their proprietary protocol or using a common open protocol. If you adopt an open protocol, any compatible product can be added to your network. When choosing a protocol, consider the data collection capabilities needed (real time and/or stored values), the need for the RTUs to be able to store data, the speed of data collection, and the need for information to be sent from one RTU to another (machine to machine, M2M).

5. Data retrieval

There are a variety of methods available for retrieving data from a measurement site. Options include both wired and wireless methods. Considerations for wired systems include cable, installation, and repair costs. Wireless options include satellite, cellphone, licensed and unlicensed RF. Wireless considerations include frequency interference and licensing costs.

6. Historical data storage

Depending on your application, data storage capability may or may not be a primary consideration–it depends on the importance of the data or measurements. Historical data can be useful for verifying that measured parameters are meeting requirements, identifying when events occurred, and identifying trends over the long term. If you do not need historical data, having a measurement device that only sends out the latest readings is sufficient.

If access to past data is critical to your application, you will want to evaluate available methods for storing data, and in some cases you may want to consider redundancy. Data can be archived in several ways including data storage on the RTU, redundant servers, and copying the data to multiple locations.

7. Software capabilities

The data is important, but almost as important is how you are going to view the data and interface with the equipment (e.g., turn pumps and motors on and off). Usually the interface is a graphical software package, often called HMI (human machine interface) software. Examples include, but are certainly not limited, to Genesis64, VTScada, and Intouch. Most software companies can interface to a wide variety of equipment from multiple RTU vendors. Things to consider when selecting a software package (HMI) are the need for redundant servers, speed of data access, and the clarity and readability of the graphics.

8. Remote monitoring and equipment ruggedness

The location of the monitoring site can have a big impact on a monitoring system’s configuration. If the site is away from the grid, you must consider equipment power consumption, data collection options, and the ruggedness of the equipment. Equipment that is designed for remote monitoring will be capable of running on DC power supplies (batteries) charged with solar panels. It will also be designed to withstand extreme temperatures, wide relative humidity ranges, vibration, and resistance to nearby lightning strikes. Check into the telemetry methods that are available for transmitting data from the remote site.

9. Equipment reliability

Whether the measurement site is near or far, failure of any part of the measurement and control system will disrupt operations and could cause loss of sensitive data. Identify which manufacturers show their trust in their products by offering long warranty periods. Talk with others in the industry to find out what their experience has been. Finally, consider how long the RTU and sensors are designed to last. Ask manufacturers to provide mean time between failures (MTBF) or failure rate data for their products–both of which can give you a good idea of equipment reliability. You may want to have spares on hand to be able to continue operations immediately if one fails.

10. Level of support

Although a good manual is indispensable, it is not uncommon to need expert help from the manufacturer or manufacturer’s representative–especially if you are planning to do any of the installation, testing, or maintenance yourself. Find out (1) what kind of support will be available, and what it costs, if anything, (2) how long will you have to wait for someone to answer your call, (3) if the support reps are capable of answering technical questions, and (4) the hours of operation of the help desk. An evaluation of the support or customer service section of the website may give some indication of a company’s level of commitment to helping its customers.

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11. Maintenance

Equipment maintenance is sometimes overlooked at first, but it’s important for long-term operation and data reliability of both the RTU and sensors. Typical maintenance includes maintaining the correct environment (temperature and humidity levels), recalibration, and cleaning. Equipment that requires more frequent calibration may cost less initially, but requires a longer-term investment.

For critical operations, you may need to show NIST traceability, which provides validity to the calibrated instrument’s measurements. If this is important, find out how often the RTUs and sensors must be calibrated to maintain the certification.

Sensors will usually require more frequent service and checks and will usually need a regular maintenance and calibration schedule. Find out what is required, how often, if special tools are needed, and if it must it be sent back to the manufacturer. Remember to include calibration and cleaning costs in your estimates. They will usually not be included in first quotes.

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12. Value/cost benefits

Determining value is a personal decision. While cost is a factor, the real determination is getting a satisfactory combination of quality and features for a reasonable price. In the context of measurement and control systems, value will depend on many of the items mentioned above, such as accuracy, reliability, and compatibility. A careful consideration of many of these items can help you find good value in a system based on features that fit your specific application.

A more in depth guide to some of the above sections could cover many pages, and there are certainly other considerations that should be made. The above 12 items, carefully considered, will provide a good introduction to the issues to keep in mind when considering your measurement and control system. WW

About the Author:
Karl Lambert is a Senior Applications Engineer at Campbell Scientific, where he has worked for the last 12 years as technical and project support in the Water group. He has an MCE, Civil Engineering, from Auburn University and a BS in Chemical Engineering, from Brigham Young University.