So that everything runs smoothly on the red planet

The Swiss machine industry has an excellent international reputation - as it always has. When it comes to precision and niche applications, products from Swiss manufacturers are used time and again. In this respect, the company maxon in Sachseln OW, for example, has acquired a worldwide reputation. This company manufactures small electric motors, which have repeatedly been supplied to NASA. DC motors from maxon have been used in almost all robot missions on Mars - according to the company, there are over 100 drives to date. And now there are more to come: A new rover called "Perseverance" will - as soon as it arrives on the Red Planet in February 2021 - collect soil samples that will later be examined in more detail on Earth. A helicopter drone called "Ingenuity" will also be a first. It will be the first "real" flying object on Mars.

The long road to "marketable" engines

maxon's motors play a key role in the successful mission to Mars. On the one hand, they are responsible for the movement of the rover's robot arm. The sealing mechanisms for the sample containers are also driven by brushless DC motors from this manufacturer. For their development and modification, a team from maxon worked together with specialists from the Jet Propulsion Laboratory (JPL). Motors from maxon are also used in the helicopter, but these required less modification.

The motors used in the Rover are the EC 20 flat and EC 32 flat types from the "flat motor" product family, as used in many industrial applications. They have a cost-effective overall design that delivers high torque and - thanks to the external rotor - allows excellent convection cooling and thus high continuous power under the conditions on Earth. The conditions on Mars, however, are quite different. So what requirements do the motors used have to meet and what has been done to ensure that their functionality remains guaranteed even there, in the hostile environment on the Red Planet with its high temperature fluctuations and violent sandstorms? This article provides a small insight into the extensive development and testing work, during which many details had to be taken into account.

Modifications to the series model

Fortunately, maxon was able to draw on some experience from previous Mars missions. Nevertheless, various modifications had to be made once again, especially as the rover was venturing into new areas of application. These modifications included both design adjustments and the use of alternative materials. For example, in contrast to the series models, FR4 was used as the circuit board material, a material that is also commonly used in other space missions. It outgasses less and is robust enough to support the Hall sensors required for the control system in a highly vibrating environment. It also meets the requirements of the IPC-A-600 Class 3 technical guideline for printed circuit boards and the tests required by JPL. The entire architecture of the motor has been modified so that the whole system does not change even in the event of a hard impact, i.e. individual parts do not shift in the event of shock or vibration. A more robust spring with washer ends ensures that the forces are better distributed.

Pyroshock tests show limits of resilience

The engines must also meet the highest mechanical requirements and have been tested accordingly. A test commonly used in space technology is the so-called pyroshock test. This involves testing components to see whether they can withstand the stresses that occur, for example, during pyrotechnic ignitions (such as the separation of a rocket stage; similar pyrotechnic ignitions also occur, for example, when airbags are triggered). The test object is excited to oscillate by a short and very strong impulse. These impulses are generated by bolt setting devices or drop or pendulum hammers. Of course, these pyroshock tests are validated and include the standards ECSS-EST-10-03C Space Engineering - Testing, MIL- STD-810 G Environmental Test Methods as well as NASA-STD-7003 Pyroshock Test Criteria.

The EC 32 flat motor from maxon was also tested by JPL according to these criteria. It was subjected to a force of 3000 g (3000 times the acceleration due to gravity) at an oscillation frequency of 1600 Hz. After the test, the motor was examined. During operation, a coffee grinder-like noise was audible. From a purely external perspective, however, the motor appeared to be undamaged, and a computer tomographic examination also showed no discernible damage to the structure. Further testing, including a frequency analysis of the motor noise and an examination of the rotating shaft, revealed that the motor bearings had slight damage. So what to do? Redesigning the engine again would have taken too much time. It was therefore decided to install it anyway, at a location where the load from pyroshock was present, but where the "coffee mill effect" did not prove to be detrimental.

The question of temperature monitoring

Another challenge with electric motors is heat generation, triggered by the electrical resistance in the windings. In contrast to previous motors used on Mars missions, this time it was possible to dispense with the installation of a platinum resistance thermometer, which would have triggered an emergency shutdown in the event of imminent overheating. Thanks to the design with an external rotor, the danger of overheating of the housing could be minimized. The installation of a thermocouple directly in the windings could be considered for later. Coupled to the control board, this would also allow a temperature alarm to be triggered and an emergency shutdown.

The language of research - and industry

maxon was finally able to deliver 99 EC 32 flat motors and 40 EC 20 flat motors to JPL - in each case more than were actually needed for installation in the rover. JPL was thus able to select the most powerful motors and store the surplus for later missions. The development of the motors for the new Mars mission was once again an example of how private-sector industrial companies can work together with government-funded, purely scientific organizations like JPL. After all, questions about technical intricacies, while of interest to researchers because they can determine the success or failure of a costly space mission, are often of little relevance in a commercial setting. However, as JPL and maxon both state, they have once again been able to learn and benefit a great deal from each other in this prestigious project. We can only hope that the joint work will also pay off in the success of the Mars mission.