Chris Newill, the Control Engineer who designed the system for Blackrow is a fan of automation hardware from Mitsubishi Electric, and said: “I’ve used Mitsubishi Electric controllers for more years than I care to remember. It is the ease of use, combined with the reliability that usually draws me to them, however, the redundancy requirement on this job also called for the use of a back-up PLC that used a different central processor to the main unit – and Mitsubishi Electric happened to offer just that with the iQ Series PLC and the FX Series models.”
Newill continued: “A lot of automation equipment uses the same internals, especially ICs, so the fact that Mitsubishi Electric is one of the few companies that manufactures its own chips meant I knew there was a good chance of success. I called Gary Hatfield who looks after my area from a technical sales point of view and he was able to confirm that there were indeed two fully compatible Mitsubishi Electric controllers with enough speed, flexibility and functionality to do what we needed – that also employed two different processors.”
The application, which is a solid rocket fuel press, is both simple and complex at the same time. The process is simple in that a pre-mixed solid fuel which has the consistency of play dough is inserted into the press and squeezed through a die. As the rocket fuel ‘spaghetti’ emerges it is cut into tiny beads. The beads are then further processed to create the fuel. The finer the bead, the more surface area the fuel has and the faster it will burn, in turn providing more boost to the rocket engine.
The fuel is used for a variety of applications from military ordinance to civil and space applications, although those two terms are becoming merged with the onset of commercial space travel. Different fuel mixes can also be used to control speed and range in a projectile with a fixed volume fuel chamber. The size of the bead is then used to further control burn time and boost levels. After this point it ‘is’ rocket science.
Since the rocket fuel is reasonably safe to handle at ambient temperatures and pressures (obviously with a great deal of safety training and precautions in place) the mix becomes more volatile when pressure and heat are applied, which is exactly what happens when you squeeze something in a large mechanical press.
The press is a large vertical tube with a hydraulically driven plunger at the top and the die at the bottom. The die is held up against the bottom of the tube using hydraulic pressure, the pressure keeping the die in place is kept just above the pressure being used to push the fuel mix down the tube and through the die. Very specialised and precise proportional hydraulic control valves from Moog are used for the hydraulic control. Moog famously make valves for F1 and aerospace applications and Newill rated them highly too.
The main failsafe is that if the pressure differential changes based on influences other than the pressure that is being applied by the hydraulics, then the Mitsubishi Electric iQ Series PLC control system – which runs the control algorithms and links to the many pressure, position and temperature sensors on the system – recognises this and stops the process.
The Mitsubishi FX series controller, which is technically further down the price and functionality model hierarchy, actually matches the functionality of the iQ Series in this application and duplicates everything the iQ unit does, including dual sets of sensors with their own separate physical control loops.
The FX controller is used as a real-time reference point for the main operating controller, checking that all signals and values correspond across the system, again if anything is out of sync or not within carefully calibrated differential tolerances, the process is brought to a swift and safe stop.
This effectively creates the next layer of redundancy in terms of the sensor system; if the sensors detect an unusual rise in pressure or temperature, then this is being caused by an outside influence such as an inconsistency in the fuel mix, or even a metallic inclusion such as a stray nut, bolt or metal shaving which, while highly unlikely, has to be accounted for.
If there were ever to be an issue with the main PLC, its power supply or its other network and I/O rack components, then the overall control would instantly and seamlessly switch over to the FX controller that is working in parallel.
Newill explained: “Reaction time is obviously crucial, so the fact that the clock speeds of the two units are close enough for them to be compatible and for us to use them on the same application was great.”
“The fact that we could use the same MELSEC software suite to programme both PLC units and the Mitsubishi Electric GOT2000 colour touch screens we employed for the control station was very helpful. The fact that the operational software modules we created were also transferable between devices made a huge difference to bringing down the cost and complexity of the job.”
Chris Newill puts the safety aspect of the system in perspective: “When I was first made aware of this application, which was when I first automated it (this is the second time he has upgraded the machine) we looked at the die end of the press and it was in a separate room that looked like a concrete bunker from a 70’s James Bond baddies’ lair. The doorways and corridors were solid and acted like baffles. The roof is also held on with flexible cords so it wouldn’t fly off, or break into pieces if there was a big pressure wave and air movement from an explosion.”
He concluded: “Performance was essential in this application, but more of a prerequisite; as was the fact that the primary controller and the back-up had to employ different processors. (To avoid systemic component failure if there was ever an issue with a particular design or batch of chips). The really important aspects were the reliability, which is something I have built up trust in personally over the years through my direct experience of using Mitsubishi Electric automation hardware, and the compatibility of the two control units, which meant communication and parallel running was seamless and enabled us to use the same code and development environment.“