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4. Choosing a Control System and Communications Protocol Stack

Communication between the application’s CPU and the FOC hardware is via the serial peripheral interface (SPI). This was chosen because SPI is a simple interface provided by almost any processor. In general, a communications protocol stack may be better handled by software within a processor. From the software point of view, the FOC hardware is a register set with an SPI communication interface for register access.

5. Deciding Mechanical Integration Form Factor/Size/Housing

This was an especially important decision, since form factor and size were critical to the application. Normally, FOC implementations are built for industrial uses, which means an optional rack mount is mandatory, requiring a big box. Furthermore, the drivers are usually designed to drive several amps, which results in significant power losses in the driver stage. Therefore, an external heatsink is also mandatory, and most manufacturers use the housing as a heatsink. This would clearly not be possible with a prosthetic that needed to be so small.

The challenge was therefore to shrink the size of the motor drive module’s printed circuit board (PCB), which was reduced to 65 × 40 mm, between 1/10 and 1/3 the size of typical FOC boxes or servo vector controllers. This was done by implementing the FOC algorithm in a very hardware-efficient form, based on prior experience in the implementation of real-time critical algorithms in hardware, together with prior experience in the implementation of motion control algorithms in software.

Connectors are standard screw terminal connectors, defined by Össur so that the whole knee can be easily assembled in production. The power supply consists of 12 Li-Po cells, with a nominal voltage of 44.4 V. Although the maximum voltage is 50.4 Vdc (charging circuit voltage), the MOSFETs are designed for 60 V in order to recuperate as much energy as possible in e-caps. Recuperation into the battery is not possible since this would significantly shorten battery life.

6. Determining Electrical and Thermal Ratings of the Power Stage

This was also an especially important decision. Since the knee is battery powered, the battery has to last a long time and the electronics need to stay cool. The MOSFETs and other parts of the motor drive that heat up the most had to be carefully placed so that heat is spread out and distributed around the motor drive module’s PCB. Also, the mechanics had to be designed so the Power Knee’s frame is connected to the electronic parts on the PCB that heat up the most, so heat gets out of the PCB and onto the frame where it can dissipate.

Overcoming the size restriction challenges and not surpassing the basic geometry of the Power Knee required fitting a motor controller with a peak output of 500 W into an area of no more than 65 × 40 mm. Managing power dissipation in such a small space requires a good thermal connection to the aluminum structure, so several designs including active-multilayer topologies were developed in parallel, and compared with functional models. These experiments resulted in the selection of a design with surface-mounted circuit breakers with optimal thermal connection to the housing.

7. Determining Motor Type and Ratings

Determining motor type and ratings is usually an important decision during development of a motor drive, especially when engineers are developing it from scratch. In this case, however, the Power Knee’s motor was custom made, and its current ratings had already been determined by Össur.

8. Determining Motion Control Planning

The challenge for Össur was how to control the knee with higher-level algorithms so that it moves with a natural motion. Össur did the complete motion control planning. Trinamic’s challenge was to transfer the digital information generated by Össur’s electric prosthesis control system into physical motion, while meeting the requirements of Össur’s existing solution.

This article was written by David Langlois, Technical Product Lead – Knees, Össur, Reykjavik, Iceland, and Mario Nolten, Field Application Engineer, Trinamic Motion Control, and Dr. Lars Larsson, IC Designer and Chief Operating Officer, Trinamic Motion Control, Hamburg, Germany. For more information, Click Here.

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