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Road simulator on rotating rollers for real fatigue testing of mechanical components in buses, trucks and trailers
"A highly efficient solution was created for deterministic performance, redundant security and low installation and maintenance costs."
- Stefano Vianelli, EURINS
The challenge:
Realize the entire automation and data acquisition system for a test system capable of subjecting heavy vehicles to real heavy work situations to study their mechanical problems.
The solution:
Develop an architecture based on an NI PXI Real-Time node, three NI CompactRIO nodes and a computer with OS Windows to distribute measurements, calculations and implementations at strategic points of the system, minimizing cabling and locating intelligence.
System features
A heavy vehicle can be subjected to fatigue testing by simulating the road positioning the wheels of an axle over a pair of rollers emerging from the floor that, rotating up to the speed of 100 km / h driven by independent electric motors, drag or brake the whole mechanics of a trailer or the entire "driveline" (from the wheels to the engine) of a tractor or bus. The vehicle can also move transversely with respect to the longitudinal axis of the plant, using different portions of the roller surfaces, whose 3-meter diameter provides a contact surface and inertia suitable for road simulation.
Thanks to the independent management of the speed of the single rollers and thanks to the mounting of the metal plates on part of their surface with which the roughness of the road is simulated when the wheels pass over, it is possible to carry out a multiplicity of tests summarized in the following four main applications:
• Simulation of the road load
The vehicle is active on the rollers, so it is a truck or a bus, dragging them with its own driven wheels driven by its own engine. The rollers, braked by the electric motors connected to the power drives, oppose a resistance consistent with that which the vehicle would have if it were on the road, so the workload required for the mechanics of the vehicle is controllable and repeatable.
• Speed profiles
The vehicle is passive on the rollers which, driven by electric motors, drag the wheels of an axis at predefined speeds into a profile where the two sides (wheels and mechanics connected to them) can be stressed asynchronously both as speed and as asperities present on the surface of the roller, traveled according to the transverse position of the vehicle.
• Temperature testsMaintaining the vehicle in the pre-established times and in specific working conditions, it is possible to analyze the thermal behaviors of the tires, brakes and differentials, and to understand any critical issues of overheating, the consequences of which could lead to failures and fires.
• Consumer tests
Repeatable tests may be carried out for motor vehicles in which the fuel consumption is measured to analyze the effect of the modifications in order to improve the efficiency of the mechanical components and reduce the polluting emissions.
The management of this system requires the setting of fully automatic tests to be performed at Real-Time level to ensure determinism and repeatability. At the same time, the distributed nodes are responsible for measuring, transmitting and using locally the dynamic parameters that control the cooling temperatures, movement positions, speeds and torques of stress, and system safety.
Below are some examples of integrated integration, through the management of:
• the asynchronous speed and load generated by the rollers deriving from the result of the parameters continuously exchanged in Profibus by the PXI with the two independent power drives. Their internal closed loop control algorithms are backed up with the analogues executed in the Real-Time nodes connected in ethernet network to each other
• the cooling of electric motors on the basis of temperatures, environmental parameters of pressure and humidity detected in several points of the system to avoid thermal shock and condensation, according to variable laws during the longest tests
• the transversal movement of the vehicle by shifting at a variable speed a support that reproduces the system for connecting a trailer to its tractor, so as to use the right portions of the rollers, with or without the mounted plates, during controlled periods of time
• a network of laser sensors whose pulses must be modulated in PWM with strategies that guarantee the birth of interference between the various emitters and receivers
• the two independent power drives (one for each electric motor) through Profibus communication, with PXI Real-Time as primary master and one of the CompactRIOs as a redundant master in case of PXI problems, simultaneously with the Ethernet network connecting the distributed nodes the Windows computer and then forms the backbone of the system, providing and implementing local and global security strategies in the event of communications failures
• measurements inside the vehicle at the same time as the system. One of the CompactRIO can be mounted and connected as needed by adding analog sensors and interfacing with the on-board CAN network, communicating with the rest of the system via an Ethernet network
• the activation of the hydraulic brakes whose pincers are redundant as they are partly managed by PXI Real-Time and partly by one of the CompactRIOs in order to guarantee complete braking.
If traditional programming techniques had been adopted, by abusing the available memory and computing resources, they would have run out with the risk of not adequately dimensioning the "brain" of the system.
The focus is mainly on the level of programming and the flow of communication.Some details of the technical solutions adopted:
• CompactRIO Real-Time controllers are connected to their FPGA chips using 64-bit FIFOs DMA to optimize volumes and access times
• Real-Time applications in each of the four nodes are structured with a massive use of multithreading and different execution priorities, according to criteria of hierarchy that minimize jitter, even with variable workloads over time
• Ethernet communication between distributed nodes is based on an incremental and intrinsically safe TCP / IP protocol where the flows are compacted and reconstructed bi-directionally to the advantage of the continuous visibility of each measure and process by all the involved levels. Locally, security strategies act if there is a lack of consistency between measures taken independently and redundantly
• telemetry is able to propagate multiple levels of information to and from the network, so the Windows computer is the configuration interface for distributed systems and continuously shows the status of the measures and implementations managed, and the timing within the individual threads of the various processes.
The tests can be performed manually and automatically, having a total configurability of the alarm criteria to be considered and the strategies to be adopted depending on the situation
• the saved data are automatically loaded into the National Instruments NI DIAdem program which is the analysis environment and report generation to complete the software chain, allowing the automation of the presentation of the results which are useful for the technicians to make future decisions
• three 24-inch monitors, uninterruptible power supplies and power supplies for subsystems, RAID harddisk in the computer, independent Ethernet networks, and redundant Profibus network are used.
About the author:
Stefano Vianelli
EURINS