Dredging Industry Beginning to Automate Control and Monitoring Systems
Many U.S. dredges still utilize pneumatic and hydraulic technology for process control and information feedback. Production data when it exists is usually written down on a piece of paper. More often actual production information isn’t obtained until a survey is done and compared to the original site survey. Real-time information showing where and how much the dredge is digging is often unavailable. Determining the depth that a dredge is digging at is usually determined with a calibrated stick or length of line hung overboard.
The dredging industry now finds itself playing catch-up to the manufacturing world. Dredge users and manufacturers are being bombarded with an array of new and confusing terminology. Acronyms such as PLC HMI IO GPS RTK DPGS and OIT are completely foreign. Terminology such as nuclear densitometer mass flow meter digital interface serial protocol and wireless network can be as confusing as a foreign language.
Breaking through the confusion requires an understanding of the various pieces that make up a control and monitoring system. Kruse Controls Inc. applies concepts to the dredging industry that were learned during their years servicing the manufacturing world. Working with us can help clear up the automation confusion and provide a clear understanding of how to apply automation technology in a way that best serves your purposes.
Looking At The Basics
Perhaps the simplest place to start is with a monitoring system. This is a system whose sole purpose is to automatically gather process information on the dredge.
This information includes operating conditions such as oil temperature engine speed dredge pump discharge velocity slurry density tons per hours (TPH) etc. The purpose of monitoring this data is to provide the operator with an accurate indication of how his dredge is running and to allow him to make informed adjustments to maximize his production. If this basic data is massaged additional information can be derived. Examples of this are run-time hours total production tonnage and quality data.
Operating parameters are monitored using sensors and transmitters. The sensors convert physical conditions to low-level electrical signals and the transmitters convert the low level electrical signals into standard electrical formats (e.g. magnetic signals from a flow sensor to a millivolt signal and then to a 4-20 mA current signal). The standard signals can be wired directly to digital displays that will provide a calibrated reading that an operator can use (e.g. 4-20mA flow rate to 0-1000 GPM display).
If it is desired to derive additional information from the data the instrument signals must be sent to an intelligent device. In these cases Kruse Controls Inc. uses programmable logic controllers (PLC). The PLC is the heart of a monitoring or control system. It is an industrially hardened device consisting of input and output modules (I/O) a microprocessor running a control program and if required communications modules to allow it to send and receive data from other devices such as computers and graphical displays.
For example the PLC can display an instantaneous density value a velocity rate and through calculations in the control program a tons per hour production rate. In a monitoring system the PLC might do little more than collect signals and send them to a single display for use by an operator. In control applications as we’ll see later the PLC’s program will monitor its inputs and automatically control on-board devices.
There are several options for displaying monitored information. At the lowest level individual hardwired displays can be used for each signal. A more efficient solution is an operator interface terminal (OIT). An OIT is a graphic monitor that displays information on customizable screens. The highest level solution is a human machine interface (HMI) system which is a sophisticated software package that runs on a computer workstation. Similar to the OIT the HMI displays data from the PLC but unlike the OIT can run other software packages and store data in databases for future evaluation. When faced with the question of which display to use the integrators at Kruse Controls Inc. will evaluate the customer’s needs and budget to determine the best display solution. Also factored in are the performance improvements that have been seen by simply providing the operator with real-time operating information.
Controlling The Dredge
Taking advantage of the capabilities of a PLC the next step beyond merely displaying process information is controlling the process. This is referred to as |!|closed loop control|!|. These are references to control theory in which a condition is monitored and compared to a setpoint to determine an error. A response to the error is automatically made in an attempt to reduce the error to zero. In a very basic example an engine temperature switch could be monitored. If the switch closes (providing an error) a cooling pump might turn on (trying to correct the |!|error|!|). A more complex example might be varying the position of a cooling valve to maintain the engine temperature at a specific setpoint. In both of these examples the PLC monitors the process variable (PV) through its input determines how to respond based on its program and controls the control variable (CV) through its output. In the discrete on/off example the control program uses basic logic algorithms. In the more complex example the control program might use a |!|PID|!| algorithm in which the response to the error is programmed via three tuning constants known as proportional response (P) integral response (I) and derivative response (D). More complex applications might use a combination of these and other control techniques.
Automation Mimicking Manual Controls
Kruse Controls Inc. has applied closed loop control techniques to a number of on-board dredge applications. Although these techniques and applications are unique at their core is the same basic closed loop concept. Many of the algorithms were developed to mimic the manual controls that an operator would normally do. To serve as examples of what can be done on a dredge here are some of the control applications developed by Kruse Controls Inc.:
* Automatically control ladder swing speed to maintain an optimum pump suction pressure
* Automatically detect a wall and raise the ladder if the swing pressure is high the cutter pressure is low and the density is low; restore the ladder once the process conditions return to normal
* Automatically control slurry concentration by monitoring density and varying the opening of the suction side relief valve
* Automatically detect a line plug by monitoring discharge velocity and clear the plug by automatically raising the ladder; automatically lower the ladder once the discharge velocity is restored
* Determine when a cave in occurs by monitoring for high suction vacuum and automatically move the dredge back to clear the ladder
* Monitor discharge velocity and automatically control dredge pump speed to maintain on optimum velocity
* Automatically control the winches (and/or spuds) to move the dredge backwards and forwards
* Monitor winch cable tension and adjust winch drives to keep a constant safe tension
Global Positioning Systems
At its most basic a global positioning system (GPS) utilizes a receiver to satellite link to calculate its coordinates on the earth’s surface. A basic GPS system uses a single receiver and three satellites to determine position coordinates to within 10 feet. This system is what is typically found in car and personal GPS systems.
A more advanced GPS system is differential GPS (DGPS) which uses four satellites. Using geometric calculations and a technique known as trilateration the DGPS can provide surface coordinate accuracy to within 3 feet. The highest commercially available GPS system is the real-time kinematic GPS (RTK GPS) system. The RTK GPS system expands on the DGPS system by adding a base station set at a fixed location with known coordinates. It also provides more sophisticated error handling technology. This system provides an accuracy of less than an inch. It also has the ability to provide the elevation relative to sea level.
Determining which GPS system to use requires an analysis of needs versus budget. As may be expected the more accurate systems cost significantly more than the base GPS. Often times a general understanding of where the dredge is located is all that’s required. In this case a relatively inexpensive GPS with a local position display will suffice. If a more accurate understanding of location is required DGPS or RTK GPS might make sense.
In the context of an automation system the GPS can be viewed as a sophisticated sensor. By interfacing it to a PLC and HMI Kruse Controls Inc. can use it to display position coordinates and digging elevation alarm operators when digging outside of plan lines or below permit levels and even use it to control the dredge.
Contour dredging is a great example to look at as it ties together the various automation concepts previously discussed. The need for contour dredging is often seen in the dredging of reservoir ponds where cutting a tiered sloped side wall is necessary. Other applications for contour control are in environment dredging channel dredging and marina dredging. In these cases a specific amount of material must be removed to a pre-determined depth. In both of these cases cutting above or below the desired depth is not acceptable and can lead to rework cost overruns and financial penalties.
Contour dredging requires knowing where the cutter head is at all times. Kruse Controls Inc. accomplishes this by interfacing various sensors and a GPS system to a PLC. The GPS provides the PLC with the exact coordinates and elevation of a point on the dredge. As this point is offset from the cutter head sensors are utilized to provide position information for the ladder.
Inclinometers (sensors that monitor an object’s angle) and encoders (smart sensors that provide rotational position) provide information to the PLC about the position of the ladder. Using a collection of trigonometric algorithms in the PLC and knowing the position of the dredge from the GPS Kruse Controls Inc. can calculate the position of the cutter head to within one inch of accuracy.
As the ladder is swinging along a cut the PLC continually compares where the cutter head is to where it should be along the desired cut line. If there is an error the PLC can raise lower and swing the ladder as appropriate to reduce the error. The sophisticated control algorithms in the PLC program minimize the cutter position error causing the resulting cut to be smooth and at an accurate depth.
Automation applied to dredging
This article has attempted to provide a basic overview of modern automation systems and how they can be applied to the dredging industry. But this is only the beginning of what can be accomplished. The PLC and HMI can serve as the foundation for additional levels of automation.
Automatically storing process information in databases for future use is commonplace. Capturing and storing downtime information can help identify and solve maintenance problems. Wireless networking technology allows on-shore plants and dredges to communicate and coordinate their activities. Creating and automatically emailing production and downtime reports is now possible. With an understanding of current capabilities and a little imagination the potential to utilize automation has never been greater.
Example of production trend screen:
About the author:
Mike Sassano has been in the process control industry for 22 years. As one of the partners at Kruse Controls Inc. he has helped implement numerous automation projects in the dredging and aggregate industries. He graduated with a BSEE from Virginia Tech in 1985. For questions or comments he can be reached at 800-810-8966 or at firstname.lastname@example.org