Our client was a highly innovative company who are pushing the state of the art in their specialised field. They have significant in-house engineering expertise in mechanical engineering and required a development partner to work with them on electronics and software development. Kynesim provided the necessary expertise and flexibility to work with them to develop a system that would meet their changing requirements.
The client needed a bespoke solid-state high power generation and control system to:
- Generate 12V DC power from a 100A multi-phase stator including rectification and current limiting
- Charging a large lead acid battery
- Drive an 80A hydraulic motor
- Control (with millisecond accuracy and torque limiting) a very high torque 20A motor
- Drive numerous other loads totalling around 160A
- Support communications using redundant dual channel NMEA2000 (similar to CAN) and LIN
The system needed to handle an extremely hostile environment with heat, very high vibration, and corrosive liquids present. It also required extensive software for control, measurement, logging, and fault handling.
Development Process
Getting Started
Kynesim and the client’s Principal Systems Engineer worked together over several weeks to agree how the system would interface with the client’s other equipment and systems. Dual channel NMEA2000 (similar to CAN) and LIN were chosen for communications to other devices.
Kynesim created non-form fit prototypes to trial various key aspects of the system including the power generation, motor control and some of the high power outputs and monitoring. A Xilinx FPGA was chosen to control the real-time aspects of the generation where any timing delay would result in immediate permanent damage to the electronics. A high reliability, safety-critical, TI ARM Cortex processor was chosen for all the communications, control, fault handling and other tasks.
The client used in-house expertise to develop a physical form factor for the product. This was a significant challenge given the hostile environment, tight space constraints, the need for liquid cooling, and the numerous connections required.
Throughout the development we maintained a close working relationship with the client including shared documents, system diagrams, flow charts, protocol documentation and both regular and ad-hoc meetings.
Software/FPGA Development
Kynesim developed all of the software for the system in bare metal C. We also developed the VHDL for the FPGA to perform time critical sensing and control of switching components.
Test harnesses were written to simulate other equipment in the system. These implemented all the agreed messaging in both directions over NMEA2000 and LIN and were used to exercise all parts of the system and to simulate failure modes.
Hardware was also simulated using a combination of large bench supplies, synthetic loads, power resistors and lower power motors. Fully automated tests were written for large parts of the system and some of these subsequently formed the basis of the manufacturing testing system. We also created a customised PCB to drive safe power levels onto the stator inputs to allow initial development and testing without the need for a potentially dangerous spinning rotor.
Extensive use was made of oscilloscopes, logic analysers and data loggers throughout this process to ensure that timing constraints were met and that sufficient voltage stability was achieved. Test automation was used for most aspects of the system.
Prototype Testing as part of entire system
The initial prototype versions were then tested by the client in conjunction with their other equipment. This was done in special test cells to ensure that all personal were safe if anything failed during development due to the presence of high speed spinning components. The system was found to be reliable and performant.
Form Fit Designs
Two final product PCBs were created which fit together into a custom made and potted enclosure:
- A power PCB for high power components and heavy connections
- A logic PCB for fine pitch advanced components including the processor and FPGA
Final Testing
Extensive testing was performed by the client on the hardware and software as part of the full product they offer. This proved the overall functionality, reliability, and ruggedness of the entire design. As usual, EMC testing was conducted on the product along with other industry specific testing.
The product has subsequently been sold in many countries and Kynesim has continued to provide support periodically where necessary. A particularly notable example of this was during the Covid pandemic when the price of FPGAs rose around 250-fold due to panic buying by some large industry sectors. We were able to design a variant without the FPGA in favour of a dedicated single purpose microcontroller running customised low level code.
Challenges
This project included significant research and design work especially in terms of real-time system integration with other critical systems. It also had the unusual aspect that if anything failed it would not be acceptable to simply shut the system down entirely so significant effort went into handling exceptions in an appropriate manner.
This product handles the highest currents we have so far put through a PCB with a maximum of around 165 amps. The peak loads could be much higher than this but the software ensures that this value is not exceeded by prioritising certain loads over other ones to avoid excess strain on the battery. We found these pushfit RedCube Terminals from Würth to be extremely effective and our manufacturing partner was able to push them into the PCBs with a special hand operated jig. The PCB itself also has considerably heavier copper layers than would be typical to allow for the high currents. Current measurement is performed using Allegro Hall Sensors which are able to handle the currents used.
The need for high currents also introduces problems with etching of a PCB. It is difficult to use very heavy copper weights and very fine pitch components at the same time. (For example FPGAs often have a pad spacing of around 0.8mm.) Soldering to heavy, very thermally conductive, PCBs also involves more heat which may damage sensitive components. For these reasons two PCBs were used despite the increase connector costs and slightly reduced reliability of having connections.
To achieve adequate vibration and environmental protection the entire unit is potted. This introduces a number of complexities since it prevents the final test systems from having access to test pads which would normally be used. Therefore a JTAG chain is brought out of the unit as part of a multi-way waterproof connector to facilitate testing, debugging, and fault finding on finished units. Potting is also complicated by the physical mass of the PCBs and liquid cooling system. It has proved necessary to heat the entire unit up prior to pouring in potting compound to ensure that it flows adequately. The potting compound is also very good at wicking into supposedly fully sealed inter board connectors which had to be addressed during manufacturing.
Manufacturing
These units are manufactured in the UK by our manufacturing partner AdaptEMS. We worked closely with Adapt to optimise the design for manufacture and to ease assembly (e.g. by ensuring that components did not restrict the flow of potting compound).
Kynesim designed and created factory programming and automated test systems for the units. These were then assembled into suitable metal rigs by a third party.