Miniature force transducers help deliver new mobility system for concept micro-rover

Sherborne Sensors’ miniature force transducers have been chosen by Canada’s Carleton University for its innovative mobility system, which will help improve traction and combat slippage when micro-rovers traverse a Martian surface. The force transducers were selected for their accuracy, range and reliability, as well as providing a path to flight testing.

A growing number of space mission programs are involving lunar and planetary exploration using rovers. These semi-autonomous vehicles can be manned or controlled remotely and drive across a Martian surface to explore for resources and perform scientific experiments. Spirit and Opportunity are two well-known examples, having landed on Mars in 2004 and made important discoveries about pre-historic wet environments that may have been favourable for supporting microbial life.

Both Spirit and Opportunity were solar-powered rovers built for a mission intended to last 90 days. However, Spirit kept on going for more than six years, driving over 7.7 kilometres from where it landed at the Gusev Crater (the possible site of a prehistoric lakebed) and returning over 124,000 images before getting stuck in sand in 2009.  Spirit eventually ceased communications in 2010, but Opportunity continues to conduct experiments and has driven 33.5 kilometres since landing at Meridiani Planum on the other side of Mars.

The fate of Spirit highlights a major challenge that has dogged previous planetary rovers in that they are prone to getting stuck in the fine-grained soil and other topographic features of Martian surfaces. They are also large, expensive and susceptible to damage. This is why a new micro-rover prototype is being developed as part of the Exploration Surface Mobility (ESM) program co-ordinated by the Canadian Space Agency (CSA).

“The first micro-rover flew in 1997 and was only 12 kg in mass, but was very limited,” says Alex Ellery, professor of mechanical and aerospace engineering at Carleton University and Canada Research Chair in Space, Robotics and Space Technology. “Since then, rovers have been getting larger and more capable. However, this neglects the fact that sensors and instruments have been getting smaller, so we haven’t yet explored fully the capabilities of the micro-rover design.”

Breaking new ground

With the CSA looking for a variety of different-sized prototypes to support its ESM program, Professor Ellery and his colleague Dr Ala’ Qadi, a Post-doctoral Researcher at Carleton University, established a co-operative partnership to develop a new micro-rover design. Members include MPB Communications; Ryerson University; University of Toronto Institute for Aerospace Studies; University of Winnipeg; Xiphos Technologies; and MacDonald, Dettwiler and Associates (MDA).

Having submitted their design to the CSA, the partnership was awarded a $1.8-million (CAD) contract to develop a smart and all terrain micro-rover based on a modular architecture to allow optimal reconfiguration for Moon and Mars exploration and science. Employing a number of innovations in robotics, sensors, microsats and real-time intelligent software to enable high functionality within its 30 kg mass budget, the micro-rover – named ‘Kapvik’ after the Canadian Wolverine – represents a significant reduction in mass on larger rover designs such as Spirit and Opportunity (174 kg), as well as NASA’s 900 kg ‘Curiosity Mars’ rover, which is currently making its way to the Red Planet aboard an Atlas V rocket (due to touch down in August 2012).

“We had to develop the chassis, the frame and the avionics box for the micro-rover – including all the controls for the motors, instrumentation and the control algorithm (electrical and mechanical),” says Carleton University’s Dr Ala’ Qadi. “We also had to contribute to the navigation sensor integration, which includes all of the algorithms necessary to collect and process data from sensors in the pan-tilt unit incorporating the camera, the laser scanner, a sun sensor and instrumentation and measurement unit (IMU).”

According to Dr Qadi, the chassis and frame for Kapvik was built ‘from the ground up’ using a rocker-bogie design, which is proven for negotiating obstacles of up to 15 cm in height and where speed is not a concern (Kapvik’s top speed is 80 metres per hour). “However, we recognised that it would need to obtain sensor readings from over the chassis and combine these with the actual load power ratings in order to enable dynamic traction control. Looking at the problem, I suggested we might have to use load cells (force transducers), which I had employed successfully before on tethered rover designs.”

Sensor innovations

Carleton University’s development team decided to use force transducers situated over each of Kapvik’s six wheel hubs and integrated into the mechanical system of the chassis to provide the critical data needed to help improve traction and combat slippage.

“Without load cells you cannot obtain the critical information necessary to perform traction analysis,” says Tim Setterfield, who designed Kapvik’s mobility system while studying for a Master’s of Aerospace Engineering at Carleton University and who now works at the European Space Agency. Having evaluated a range of available options, Tim specified Sherborne Sensors’ SS4000M miniature force transducers, due to their small size and wide range, as well as the fact the company is AS9100B certified and has experience in working with space qualified systems.

“The force transducers output a voltage corresponding to the magnitude of the force. Applying a scaling factor means you then can obtain values for the normal loads acting above the wheel hubs; used in the right way, these can tell you something about the terramechanical interactions between the wheels and the soil,” Tim continues. “For example, if you have more weight over the wheel you can develop more traction – it’s like having the engine over the drive wheels in cars, as this enables you to generate more traction on those wheels.”

Using conventional on-board rover sensors, a velocimeter and the force transducers, Tim created a proof-of-concept net traction algorithm for estimating normal load, slip, drawbar pull, resistive torque and wheel-terrain contact angles. “The SS4000M force transducers were the perfect size and range for the application and working with Sherborne Sensors has been great,” says Tim. “They provided a considerable amount of additional information that proved extremely helpful with our modelling and analysis, while their previous experience in flight qualification will be important moving forward.”

Ensuring a path to flight

The CSA stipulated that all components employed by Kapvik be ‘flight representative’, which ensures a path to flight qualification should a mission be confirmed. Sherborne Sensors was recently awarded AS9100:2009 Rev C, the international standard that specifies requirements for a quality management system for Aviation, Space and Defense Organizations.

“Carleton University had a number of challenging requirements, but the specification of our SS4000M force transducer proved the perfect fit,” states Sherborne Sensors’ Jesse Bonfeld. “The development team also had myriad technical questions, so the fact we were able to address these quickly and have previous experience in space applications ensured we were the supplier of choice.”

The data provided by Sherborn Sensors’ SS4000M force transducers has proved critical in driving up the agility and capability of the Kapvik micro-rover while keeping its mass budget low in comparison to other planetary rovers. Kapvik’s rocker-bogie mobility system is able to negotiate obstacles of up to one wheel diameter in height (15 cm) and to equilibrate ground pressure between all six wheels.

The micro-rover is designed to operate independently for low-cost planetary exploration or as a collaborative assistant to manned or large rover missions, therefore lowering the chances of losing more elaborate, expensive rovers to inhospitable terrain. However, the technologies being developed for the rover could also be used for Earth-based applications in areas such as mining, transportation and security industries.

One of the aims of the Kapvik project is to position Canada as a potential partner in international space exploration, with a number of flight opportunities still being considered. The prototype has been submitted to the CSA by Carleton University and MPB Technologies for further terrestrial field tests that will reproduce key conditions of space missions.

“At the moment, Kapvik is a prototype looking for a flight opportunity, but it is sufficiently versatile that it adapts itself to a multitude of different possibilities,” concludes Professor Ellery.

About Carleton University

Carleton University is a dynamic, interdisciplinary research institution located in Ottawa-Canada’s capital. It has innovative programs in sciences, engineering, arts, and public administration and has realised partnerships with numerous public and private sector organisations. Its strengths have led to international recognition for its faculty, as well as an ability to attract outstanding students.

Meanwhile, Carleton University professor Alex Ellery is writing a book entitled ‘Planetary Rovers: Tools for space exploration’. This will be the first text book written on the topic of planetary rovers and will detail the complete history of their development, including applications and enabling technology innovations. Published by Springer Praxis Books, it should be available by the end of this year.

About Sherborne Sensors

Sherborne Sensors is a global leader in the design, development, manufacture and supply of high‐precision inclinometers, accelerometers, force transducers and load cells, instrumentation and accessories for industrial, military and aerospace customers. Products are supplied under the AS9100B Quality Accreditation and are renowned for their ultra‐reliability and long‐life precision within critical applications. The acquisition of synergistic technologies by Sherborne Sensors within its product portfolio has allowed customers to benefit from expanded product lines, with added benefits of engineering support, global sales presence, repair, refurbishment and calibration services, stocking programmes and continuous product improvement. For further information go to www.sherbornesensors.com  

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