While the cost of fuel increases, government subsidies for fuel decrease, and pressure to clean up bus emissions mounts. Bus operators are facing huge regulations to clean up their fleets. Some of the largest fleet owners in the United Kingdom run more than 7,500 buses, each with a lifetime of more than 25 years. One of the only viable solutions to quickly make a meaningful difference to fleet fuel consumption is to retrofit existing vehicles with cleaner propulsion methods.
Using National Instruments CompactRIO and LabVIEW system-design software Vantage Power Ltd is developing a hybrid powertrain that can be retrofitted into existing double-decker buses and can demonstrate a 40% fuel savings in real-world operating conditions.
The Main components
A typical hybrid bus powertrain consists of an electric drive motor; a power converter; a lithium-ion battery; a generator; and an engine. In the layout shown in Figure 1, a series hybrid creates energy in series with the engine whilst being mechanically decoupled from the wheels. In this scenario, each of the main components in the energy conversion chain needs to be controlled so that their power output matches the requirement of the following component.
Figure 1. Schematic of Series Hybrid Powertrain System using a Lithium-Ion Battery
Due to the complexity in such a control system, we needed an architecture capable of very high-speed repetitive calculations with a high level of determinism to analyse and log the large sets of acquired data. In addition, the driver needs to be updated and information fed back to the other powertrain system components and the prototyping dashboard. CompactRIO is the ideal platform for our control system, because it delivers all of the benefits of a real-time processor and a user-programmable FPGA, perfectly integrated into one piece of hardware. The FPGA provides the best platform on which to implement highly deterministic and safety-critical functions, such as processing accelerator pedal inputs into commands to control acceleration and braking, as well as fault management. Functions that are not safety-critical and require less determinism, such as high-level power and thermal management, data logging, and providing data to the prototyping dashboard, run on the real-time processor.
Figure 2. B320 Hybrid Retrofit System for Double-Decker Buses
Powertrain Control System
CompactRIO is the core of the hybrid vehicle control system. It can maintain control over each power delivery component via a controller area network bus interface. Through input and output modules, it acquires analogue sensor signals such as accelerator and brake pedal position and fuel flow rate.
In a hybrid vehicle, the electric drive motor not only provides acceleration, but also acts as a brake by regenerating the vehicle kinetic energy and storing it in the battery. One of the tasks fulfilled by the CompactRIO device is to interpret the inputs given by the driver through the pedals and convert these to smooth acceleration and deceleration commands to the drive motor. Whilst the powertrain controller will always attempt to satisfy the driver’s request for acceleration and deceleration, enough power must always be available for the driver’s demands to be met. This is where the powertrain controller will constantly be estimating the available power from the battery and the engine or generator. If there is not enough stored energy available in the battery, the engine or generator must increase its output. This increase in performance must happen fast enough to make sure the driver does not experience any delay, whilst making sure that operational limits are not exceeded, which would result in stalling the engine.
Unlike in a conventional vehicle, where engine speed is coupled to vehicle road speed, the choice of speed and torque operating point in a hybrid vehicle comes down to control strategy. The powertrain controller is configured to use a mapped set of speed and torque points, depending on power required, but more importantly, it can calculate the optimal power delivery split between battery and engine in real time, in order to optimise vehicle fuel consumption.
Thermal Management System
Whilst the powertrain controller must ensure satisfactory driving performance, the CompactRIO device provides a thermal management control system. Using input and output modules, it measures temperature via thermistors and controls variable-speed fans and pumps with pulse-width modulation signals. With more than seven different components that need liquid cooling, each with its own temperature information, it is paramount that the CompactRIO device scales to add input modules, aiding rapid development.
In order to estimate component thermal dynamic behaviour, we used parameter estimation functions to build a nonlinear dynamic battery model. Utilising the built-in function in LabVIEW, with minimal effort, we implemented an advanced algorithm called the extended Kalman filter to provide crucial on-board estimation functions for battery charge state, health, and thermal behaviour.
Using CompactRIO as a Development Tool
Built-in LabVIEW modules provide tools for every phase of development. The LabVIEW Control Design and Simulation Module played a significant role in simulating driving behaviour. Using simulation, we could gain knowledge about expected energy demands in different traffic and environmental conditions, test and tune control strategies and their impacts on efficiency, and correctly size components. Being able to use the same software platform for both development and the deployed control system greatly reduced time and cost.
Thanks to the graphical representation and the built-in debugging tool, the localisation of the problem and finding out the necessary action is much faster compared to a text-based language. The seamless integration between LabVIEW and the CompactRIO allowed the team to focus their time on the algorithms rather than spending time on the hardware-level implementation. As an inexperienced user it allowed the team to use the benefits and performance of FPGA from the first week, without having to know anything about VHDL programming. Deploying the code onto hardware is very simple thus allowing testing and deployment as early as possible which greatly reduces development times.
Not only does the CompactRIO perform well as a high performance and accuracy embedded controller, but through the graphical programming language, LabVIEW, it provides the ability to develop custom development dashboards. This played an absolutely vital role for the engineers when the hybrid system and vehicle was first tested. Quantities could be graphed and viewed in real time whilst changes to control system parameters could be made on the fly. This significantly shortened development times and costs by reducing the number of costly visits needed to a specialist testing facility or track.
Using the 4 GB of on-board storage and external USB hard drives, the team could log data at high frequencies and quickly and easily analyse using purpose-built virtual instruments. In addition, CompactRIO mechanical ruggedness makes it the perfect tool for prototype-vehicle development, as it provides a quick solution for mounting directly to the vehicle chassis.
Sponsorship and Training
The NI Small Enterprise Start-Up Programme sponsored Vantage Power by providing licenses for the LabVIEW software package, access to training courses, and engineering support from NI applications engineers. With support and training, an engineer without previous CompactRIO hardware or LabVIEW software experience could become a proficient expert user within months. Learning how to use such an important and valuable tool within such a short period of time was a key part of the success to designing, building, and testing the most efficient hybrid double-decker bus within 12 months.
Results and Outlook
The control system has now been operational in the hybrid vehicle for some time. During both on-track and stationary testing, the team has made continual updates and improvements to the software, and will continue doing so as the vehicle goes through more testing. The bus has achieved road-worthiness after passing all required homologation testing, and is carrying out trials in a public service fleet in the UK. These trials, run on a real bus route to prove the new hybrid powertrain’s reliability and fuel consumption, this began with one vehicle, followed shortly by a second prototype with further-developed and more enhanced hardware.
Toby Schulz – Vantage Power Ltd