Figure 1: Rig Design
The Challenge
When it comes to efficiency in crossing the oceans, bigger is always better. Container ships today are exceeding 400m in length and weighing approx. 200,000 tonnes! Moving these giants requires a power plant the size of a medium 3 story office building, driving gearing and propellers of suitably huge proportions. To ensure this drive train will not fail, an equally colossal test rig is required to apply forces and torques to meet real use demands.
The Solution
Computer Controlled Solutions worked with Comar Hydraulics to design and build the rig. We created a control and acquisition system based on National Instruments LabVIEW software and a range of the Compact RIO devices. The control method and safety is critical when handling a 30 tonne test unit with better than millimeter accuracy. This is where a Compact RIO solution with FPGA excels in ways no current programmable logic controller (PLC) can.
Figure 2: Rig Build
The Problem of finding the best Electronic Solution solved with Compact RIO
With a rig of these proportions and vast power capabilities our key is to use an approachable
Development platform which can keep all aspects of control, acquisition and safety in one.
Typically one may design such a rig with a logger from one supplier, multiple PID control systems from another supplier, logic handling from yet another. The downside with this approach is that it can easily end up separating skills and spreading suppliers where different engineers have to be called upon from different companies. When a problem occurs the blame game then gets into full swing with delays in identifying any route cause problems.
Computer Controlled Solutions solve this problem with our mantra of ‘Minimize the Brains’. We use Compact RIO with its modular I/O and it brings with it the following benefits:
1. Modular: off the shelf hardware available quickly and worldwide
2. Parallel: All logic efficiently coded in FPGA firmware with 25nS response. FPGA use is a critical decision, it means all the critical control and acquisition is handled AT THE SAME TIME and therefore no reliance on micro-processors which have to execute all logic in series where one process can hang all the others.
3. Single development environment: The whole project is self-contained in a LabVIEW project file, so no 3rd party future forgettable add-ons to upset maintenance in future years.
4. Expandable: The modular nature and rack mounting of the cRIO product means the system can be expanded easily in the future. Here, we used 3 Compact RIO systems all synchronized and handling 100’s of I/O with high bandwidth acquisition of data.
Figure 3: Operator Control Desk alongside our Electronics Cabinets
The Problem moving 25 tonnes and applying large load
The test rig has a 25 tonne platform which we have to raise and lower with sub-millimeter accuracy to engage splines. We then apply up to 400kN of tensile or compressive load.
In addition, we have to accurately apply 300kNm torque to the unit whilst rotating at constant speed. Considering the average sports car can typically produce 1/2kNm force we were dealing with a considerable torque. The load was applied using two large actuators precisely synchronized and able to swap between a displacement and load control mode. Our solution was to use a custom written closed loop control code on the FPGA. Based on standard PID control loops, the cRIO platform allowed us to design more complex algorithms to account for precise dual control of load and displacement. In addition we used digital devices wherever possible, in torque measurement and displacement measurement to ensure total calibration accuracy, 100% linearity and high speed measurement for critical feedback channels.
Flexible Software Design Criteria and how Diadem TDMS Format Helped
As this was a new machine for testing units at the end of production there were still questions arising as to how to apply the high loads and torques, rather than just a simple test, analyze, report situation. The engineers needed ultimate flexibility in using this rig for research, quality testing and finally in a production test process.
The solution was to design the software with a clear status ribbon along the main screen. This ribbon indicated the full range of loading, installing and testing the unit in such a way that the operator could step forwards and backwards at any time along the process. If required the operator could then go once through a whole test process or skip parts and carry out retests at will. The issue with this approach is; How do you acquire all this data and keep it in a tidy format, preferably one file and analyze it with any popular package?
This is where saving in a TDMS format comes into its own. Here we open up a new file when the unit is loaded and can then save separate data blocks at will, with full calibration information, varying channel count and frequency as required. This data is in a compact single file which can easily be loaded into Excel, Diadem, Matlab etc. with very clear data identification for analysis and reporting.
Figure 4: Main Control Screen showing Status Ribbon and Quickstart guide
Fast fault detection
With a test rig of this power and size, detecting a transducer fault quickly is critical. By writing algorithms in LabVIEW on the FPGA all critical transducers can be constantly monitored, ie: the torque cell frequency and displacement bus clock. Any failure will instantly put the rig into a controlled safe shutdown and clearly indicate to the operator the nature of the fault and its location.
Results
The rig is fully completed and operational. Shipping, building and commissioning this giant test rig to a remote location in Europe was aided using secure remote access to directly assist the operators and carry out any modifications to software.
The Future
As a company CCS has learned a lot about upscaling to these huge forces and best control methods to suit.
We also have high resolution cameras to assist in the unit loading and in future we can incorporate these into LabVIEW with machine vision software to help automate the measurement of gear contact patterns.
The LabVIEW software and National Instruments hardware approach proved to us we had the best solution and one we can take advantage of in building more ‘big engineering’ products such as this.
