Digital hydraulic control enhances plating plant productivity

A new generation of Bosch Rexroth HACD digital controllers have been used to upgrade a plating line hydraulic power pack and control system at linear slide manufacturer Accuride's Northampton site, resulting in improved productivity, enhanced reliability, reduced chemical consumption and a much faster system start-up. The system was designed and manufactured by Bosch Rexroth's sales partner Kanflu.

To sustain its competitive advantage, Accuride maintains close control of its manufacturing processes by designing and manufacturing its own bespoke production systems, including the plating lines.

The plating line features seven key stages: caustic cleaning, water rinse, zinc plating, passivation (if necessary), hot water rinse, cold water rinse and hot air drying. The entire machine features 130 process stations and 30 individual chemical baths. Due to the wide range of component shapes and sizes, manual jigs offer the quickest and most cost-effective method of loading the line. Fully loaded, the plating line could be processing 10,000 components simultaneously. The line operates five-days week on a two-shift (6am to 3am) basis.

Plating quality and uniformity are critical to the performance of the finished linear slides. In addition to providing a corrosion-resistant and aesthetically pleasing surface, the plating has a direct impact on the slide's mechanical performance. Fine rectification and process control is required to provide a smooth, precise plated bearing running surface. Any unevenness would lead to a rough, inferior bearing movement.

In operation, three pairs of hydraulic cylinders are used to 'step' the jigs through the plating process, while a seventh cylinder vertically lowers the jigs into the baths/stations, holds them for a set period, and then raises them. The single lift cylinder powers a series of linear chain drives that lift the individual jigs.

Reliability and accuracy

Accuride's plant engineering manager, Mick Jeffs, explained why the reliability and accuracy of this process are so critical to quality and productivity: "Reliability is paramount. Stopping the line with the jigs in the chemical baths would be a disaster because components in the caustic cleaning stage would be damaged, components in the plating bath would be over-plated and components in the heating oven would overheat. Likewise, the accuracy and repeatability of the motion control is also vital.

"For example, if the line experiences unexpected acceleration, deceleration, shock or judder, there is a danger that components will jump off the jigs and fall into the chemical tanks. The jigs would then have to be raised, the line stopped and the components fished out of the tanks.

"On a similar point, as the jigs are pulled through the zinc bath, the fluid resistance tries to drag the components off their hooks, thus it is important to accurately control the acceleration, velocity and deceleration.

"The final issue is productivity. To reduce its overall length, the line is split into two synchronised sections running side-by-side. Thus, productivity is reliant on both sides of the line starting and finishing their motion cycles simultaneously."

Fully loaded, the 20m lifting beam and 40 jigs weigh 3.5tonnes, making power and safety additional considerations.

At the heart of the new system lie seven Rexroth Eurocard-format HACD digital controllers. Six are used to control the three pairs of transfer cylinders via proportional valves, while the seventh is used to control the lift cylinder.

Key reasons for choosing the HACD for this application included accuracy and ease-of-use. No programming knowledge is required because applications are built using up to 32 pre-defined function blocks. Each block contains command and control parameters, with blocks triggered by a variety of conditions such as digital inputs, analogue signal comparison and dwell time. In this application the HACD controllers were configured using a laptop PC via BODAC (Bosch Rexroth Operation Interface for Digital Axes Controller) software, which can be freely downloaded at www.boschrexroth.com/hacd.

Another important feature of this controller is its optimisation for non-linear valve characteristics. Additional hydraulic features include overlap jump and creep/residual velocity. Accuride designed and manufactured the enclosure housing the seven HACD controllers. The cabinet provides access to the front mounted RS232 ports.

Productivity improvements

The new HACD controllers, proportional valves, high-flow power-pack and transfer cylinders had an immediate effect on productivity. Prior to the upgrade, the original cycle time was 20 seconds. By improving the synchronisation of the transfer steps, the cycle time has been reduced to 17 seconds, which is an 18 per cent improvement. The reason the original system had difficulty synchronising the transfer steps was load variation.

For example, because each jig can carry varying numbers of differently shaped components, the force required to pull the jigs through the baths changes at every process step. Therefore the side of the line with the lightest loaded jigs would complete its cycle first and then have to wait for the opposite side to catch up.

Now, using digital encoder feedback, the HACD controllers can sense and compensate for any imbalance between the transfer cylinders, thereby ensuring each step starts and finishes in synchronisation. Lift cylinder positional feedback is provided by a wire encoder, while transverse cylinder sensing is managed by a non-contact magnetic transducer.

Lower running costs

Interestingly, Accuride has decided to sacrifice an element of this cycle time advantage to improve the plating process, reduce chemical consumption and lower the environmental impact of the process. During each process step, the component dwell time above the chemical tanks has been slightly increased. This provides more time for the liquid chemicals to drip back into their respective tanks, rather than dripping into the following tank and gradually degrading the chemical composition. This reduces the cost of analysing and replenishing the various process chemicals.

With the controllers compensating for varying load conditions and offering digital positioning accuracy, the likelihood of the cylinders juddering or touching their end stops has also been eliminated. This has resulted in smoother motion, reduced maintenance and less likelihood of components jumping of the jigs. If an emergency stop is activated, the system comes to an immediate halt.

Faster warm-up

Another unexpected productivity improvement relates to warm-up time. With the previous control system that was sensitive to load variations, Accuride often needed to wait up to 30 minutes on cold mornings before the oil warmed sufficiently for the motion to synchronise. Using the HACD controllers, the system now compensates for oil temperature and viscosity, meaning the line is immediately ready for full production.

Mick Jeffs concludes: "Accuride prides itself on developing technical solutions to design, manufacturing, legislative and standards issues such as the company's drive for the TS16949 automotive industry standard. This direct involvement often calls for in-house CE marking, which, in turn, means we need to deal with distributors that can fully document the equipment they provide. This was an important reason for choosing Bosch Rexroth and Kanflu.

"After the power-pack and control gear was installed and commissioned it proved highly reliable. Regarding reliability, the HACD controllers have also eased maintenance procedures thanks to improved fault-finding. Due to the embedded intelligence, engineers can access the controllers via their laptop computers and then follow a diagnostic procedure."

Versatile digital control

The versatile and secure HACD digital controller for hydraulic and electro-hydraulic drives is suitable for a full range of pressure, flow and positional functions; it can be used as a control card for one closed-loop circuit or two circuits, as well as a command value card for generating, processing and scaling signals.

Presented in a standard Eurocard format, complete with integral fieldbus communication facilities, the HACD can be used for open-loop controlled traversing and closed-loop position control, speed control and pressure/force control, as well as alternating closed-loop control. Offering both analogue (voltage/current) and digital (SSI) feedback, it features control algorithms that are optimised for hydraulic axes and can be configured for a variety of servo and proportional valves. Configuration is performed very easily on a PC, running Windows-based BODAC (Bosch Rexroth Operation interface for Digital Axes Controller) software, which can also used for diagnostic purposes.

The HACD is a user-friendly, simple-to-commission digital amplifier card, that covers the majority of closed-loop control applications, including machine tools, plastics processing, general hydraulic machinery and production plant. Pitched - in price and functionality - between standard Rexroth electronic controllers and the sophisticated, freely programmable HNC100 digital series, the HACD is available for around 800 and needs no specialist knowledge of control technology. Rexroth can even provide technical support and fault analysis via email.