Optimising draglines with PLC coding

Unlike most equipment used in earthmoving, the design and control systems for draglines have for the most part remained unchanged since they were first invented in the early 1900’s. However, since the early 2000’s some advances in systems and methodologies have occurred, with universal dig dump (UDD) technology representing the first major change in decades. The key to maximising the productivity gains provided by UDD is through optimised PLC code.

As one of the biggest pieces of machinery in the world, draglines are highly productive, offer the lowest material cost per ton, and have an operating life of 40 years. They are utilised similar to cranes, and are primarily used for mining operations to dismantle and transport materials, and to remove overburden (topsoil) to a spoil pile.

Draglines are designed with four controlling motions:

• Hoist - This motion controls the bucket’s height
• Drag - This motion controls the bucket’s horizontal position
• Swing - This motion enables the dragline to spin within a fixed 360° circle
• Propel - This motion allows the dragline to move to its required position for operation

UDD technology turns draglines into highly sophisticated machines which are managed by control systems that are able to monitor and manipulate all facets of their operation, including the three-dimensional position of the bucket.

Furthermore, UDD machines use three ropes (two hoist and one drag) to manipulate the bucket instead of standard draglines which use two ropes (one hoist and one drag). The additional rope provides a more precise hoisting system, enabling better control of the bucket angle by varying the lengths of the two hoist ropes.

While UDD machines generally have higher productivity than a standard dragline (an estimated 13-21 per cent), they also often have greater mechanical issues.

The challenges of achieving optimal productivity

Dragline operations are not static, and over time the requirements of each dragline will change depending on a variety of factors including location, type of overburden, and type of coal. Therefore, it is impossible to take into account all of these factors when first designing each dragline.

This means the dragline will need to be “tuned” to meet its specific requirements at that particular time. The main challenge with this is that the machine is expected to remain in operation without any downtime, leading to “quick-fixes” when problems arise to ensure the machine is back up and running as soon as possible. However, these “quick-fixes” may not necessarily be what is the best solution, resulting in an inefficient system, a decrease in productivity and unnecessary wear on its parts.

Improving efficiency by optimising PLC codes

One way to improve efficiencies on UDD draglines is by optimising the PLC code.

There are a number of different approaches engineers can take to improve efficiencies.

One approach is to examine the PLC code, and delete or modify any code that has been made redundant due to it being written to control legacy hardware which has since been upgraded or removed. This redundant code can impact on scan time and may complicate fault finding.

Additionally, code developed for the earliest versions of PLC controllers had to utilise limited memory capacity. This necessitated the re-use of memory registers for the numerous calculations required to control the dragline and were referred to as working registers. As PLC’s have been upgraded and code migrated to suit, often the logic is left unmodified resulting in the continuing use of working registers. While working registers were necessary in earlier times, debugging software became quite difficult as values in working registers changed depending on which calculation was being performed at the time. With modern PLC’s, memory is no longer an issue and the conversion of working registers to dedicated memory areas for the respective calculations allow for faster fault finding and easier code development.

As code migrates toward a more structured approach, logic can then move over to an object-oriented style with reusable functional blocks. Object oriented programming focuses on the data to be manipulated rather than the actions required to manipulate them, allowing more accurate data analysis, faster development time and easier modification. Additionally, these function blocks are documented and the functionality verified before use in control logic. This allows the deployment of function blocks which have a known predictable response such that commissioning can focus on the process itself rather that the underlying individual logic blocks.

Another approach is the removal of all unnecessary points of failure in the system including the removal of all third-party hardware devices from the network and incorporating their functionality into the PLC code. Given the power and memory capacity of modern PLC processors, this added functionality typically does not adversely impact on the logic performance.

An example of this approach is the UDD Blackbox. This Blackbox is a separate unit to the PLC which controls the angle of the bucket using the two separate hoist ropes. By rewriting the unit’s functionality using C binary code and importing it into a function block in the PLC code, the physical module can be removed from the control network. The net result is a simplification of the control system and allowing fault finding to occur in a single system rather than trying to diagnose two separate logic controllers and the communication network between them.

An innovative approach to PLC coding

Automation IT specialises in control systems engineering and software development and is able to provide tailored solutions to meet the exact needs of each individual client, no matter how common or unique the job is.

We have worked across numerous projects in the mining industry including working with UDD draglines so we have extensive experience with the various machines and processes on site.