Complete skip drive solution offers superior control

Increasing the safety and reliability of quarrying operations...

Motion control is paramount in hazardous industries like quarrying. If a skip laden with a ton and a half of product were to go hurtling off its rails at 80mph, the consequences could be devastating. Here's how industrial drive specialist, Technidrive along with motor and drive manufacturer, WEG, developed a new skip drive with a brake control solution to upgrade TH Moore (Contracts) Ltd’s quarry equipment to improve reliability and safety on site.

TH Moore, a GibsonBros Ltd associated company, is a supplier of quarry materials, high grade aggregates and CE marked bituminous products. Before the project, the quarry in question used an outdated skip drive system on a set of rails powered by a motor to move product on site. The system saw a skip drive up the rails to sit below a mixer or above three different bin positions — one used for loading straight into a lorry and two for storage bins.

The system’s usage was dependent on the product, it was operational intermittently and had no set running hours. Plagued with a wide range of electrical problems, and equipment breakdowns leading to multiple safety concerns, these faults were made harder to identify and rectify, as the system was only operational for two to three months each year.

Technidrive was brought on board to design and commission a new skip drive solution and control system with a programmable logic controller (PLC) and full touch screen integration. The company turned to its long-term partner, WEG, to supply a motor and a variable speed drive (VSD) for the skip upgrade project.

Reduced mechanical wear

One of the main problems associated with the old system was the excessive brake wear on the skip motor, as it wasn’t used with a VSD. If an operator selected bin one and pressed start, the skip would firstly drive down to the mixer. As soon as it would hit the limit switch, it would slam the brake on and disengage the motor, resulting in a harsh stopping time. Once the system had sent the message that the deposit was complete, it would then engage the motor and run harsh acceleration to drive up to bin one’s position.

For the upgrade, Technidrive worked with WEG who provided a 22kW 4 pole B5 cast iron brake motor with an integrated 1024 pulse encoder. With superior efficiency levels, this motor was selected as it offers excellent cost effectiveness, greater productivity and the durability required for quarrying applications. For the skip drive, WEG recommended the CFW700 VSD with integrated high torque brake control. This integrated brake control, along with some intelligent programming of the on board soft PLC, means Technidrive has been able to virtually eliminate brake wear and significantly reduce motor maintenance.

The new skip system features an auto slow down function, which allows the motor to stop on the VSD before the brake is applied. Now the brake is only used as a holding mechanism and the system is no longer driving against an engaged brake. Furthermore, virtual positions programmed for each location, allow for a slow ramp up using the VSD. Before the skip arrives at its final position, the new system leaves space for slow down. This allows the skip to stop from a slower speed in the same position every time. This has further reduced wear and tear on the mechanical system.

Replacing an obsolete system: control, visibility and reliability

Another glaring issue with the old system was that it used an outdated, unreliable and obsolete PLC system. This meant that if there was a digital input or card that went faulty, replacements were not readily available.

The original PLC system worked off a series of limit switches used as part of the control system as a safety interlock. Technidrive designed a new PLC system that was able to read the encoder. The new system features a 10-inch Hitachi human machine interface (HMI) instead of a series of push buttons and controls on a busy panel.

Due to the encoder feedback and the live skip position function, the operator knows exactly where the skip is at any point. This results in easier and safer control between the four positions, visible on the touch screen display. Monitoring can also be conducted from a tablet or mobile, so problems can be diagnosed remotely and quickly.

If for any reason the encoder fails, the operator also has full manual control. For example, if the encoder slips on the shaft of the motor and the skip is moving up, the encoder will not recognise it, a belt and braces system is in place to prevent the skip driving over the end. To further increase safety, anti-free wheel protection was built into the system to stop the skip motor if freewheeling.

“The project was completed in April 2021 and ever since, our plant has been operating at full efficiency,” explained Mark Poots, quarry manager at GibsonBros. “We used to be marred with electrical problems, but Technidrive provided a cost-effective way of upgrading the obsolete control system to an industry leading alternative. This has not only reduced downtime, but if component failure does occur, replacements are more readily available. No one wants to see a skip rocket across the quarry and our new system is the safest way of operating this system.”

“This project is an excellent example of Technidrive’s expertise as a systems integrator,” added Gary Kernaghan, automation manager at Technidrive. “With our extensive engineering knowledge, plus support from our trusted manufacturing suppliers, like WEG, meant we could develop a state-of-the-art system for TH Moore.”

Safety was the primary concern when developing this control system and Technidrivewanted to give TH Moore as much control and reliability as possible. This new system has essentially reduced the number of failure points from eight to one.