Geared motors position filters on Mars Rover wide angle camera
2020 is the year of the next Rover mission to Mars. The main payload of the Russian Proton rocket is the ExoMars Rover, developed by the European and Russian Space Agencies (ESA & Roskosmos). The plan is for the vehicle to be equipped with nine measuring instruments, including one that will be mounted on a two-meter mast on the rover.
The Panoramic Camera, which was developed by Mullard Space Science Laboratory (MSSL-UCL), will take stereo images of the planet. The so-called PanCam features two rotating filter wheels which are mounted in front of its wide-angle cameras to enable it to take three- dimensional images of panoramic landscapes. A high resolution camera provides detailed images of landscapes, geological structures, and soil samples. Three stepper motors from Faulhaber, available in the UK from EMS, drive the rotation shaft for the filter change system as well as the focus of the high-resolution camera.
Needless to say, the requirements for equipment planned for use on Mars are stringent. The ambient conditions on Mars require for every piece of equipment to deliver unrivalled performance. For starters, the Rover will be working under an atmospheric pressure of 0.00636 bar, which is equivalent to the pressure found at an altitude of 35 kilometres on Earth. Further, the planet is characterised by temperature fluctuations that go from just under +20 °C down to -120 °C.
In addition to this, the dust kicked up by the Rover is expected to have a negative effect on the operational reliability of its high-precision measuring and analysis instrumentation, which is one of the reasons why the Panoramic Camera will be suspended two meters above the ground on its mast. “This will of course protect the lenses, but there’s another important advantage in that kind of elevated position, and that’s the fact that we’ll be able to get significantly better panoramic images,” explains Jonathan Jones, mechanical and thermal engineer at the Mullard Space Science Laboratory south of London.
With the filters in front of wide angle cameras, MSSL has created a system that will be able to take pictures at various wavelengths during the mission and use them to generate images with varying content. “The plan is to send ten images to Earth every day,” Jones says.
This is actually quite an ambitious target. First, the camera generates three pictures for a single image. These are then sent to Earth and superimposed on top of each other to create the actual image. And then there are the limitations imposed by the low data bandwidth available with radio communications between the two planets, which simply makes it impossible to send more than ten images per day.
With eleven filters per wheel, it is possible for the PanCam WACs to take a wide variety of pictures under various light conditions. These filter wheels rotate in front of the two WACs, and must be brought exactly into position in order to obtain sharp images.
For driving the rotating filter system, MSSL makes use of two stepper motors from the Faulhaber PRECI-step portfolio. These two units have been passing the endurance tests currently being conducted on them with flying colours.
During the development process for the Panoramic Camera, the MSSL engineers looked for motors that would not only be able to deliver reliable and precise positioning performance, but that would also be extremely compact. Stepper motors were the natural choice given these requirements, as they are not only able to precisely position objects with a resolution of 1280 steps per revolution without the need for a separate feedback system, but are also much sturdier and easier to use than conventional servomotors.
The focusing mechanism of the high-resolution camera is driven by a Faulhaber PRECIstep stepper motor. This motor is able to exactly follow an externally applied field without requiring time-consuming and complex adjustments. “It is the perfect solution for optical application as the motors can hold the lens position even without current thanks to their residual torque. Moreover, the control in open loop allows to get rid of jittering effects and therefore obtain very sharp and clear images,” explains Faulhaber’s Sébastien Vaneberg. “In short, it is a simple and robust drive with outstanding capabilities, ideal for the harsh space conditions.”
In each camera of the PanCam, each drive has a diameter of just 10 millimetres. The stepper motor counts 20 steps per revolution, and is combined with a precision gearhead of the same diameter with a gear ratio of 64:1. On top of this, Faulhaber worked closely together with MSSL in order to further customise the engineering behind its two drives so that they would meet the required specification posed by its use on Mars surface. The resulting changes include a dry lubricant and custom sintered bearings. “To put it simply, the motors need to be able to survive on Mars,” Jones says.
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