Using Force Transducers for a Mobility System in a Micro-Rover

Canada’s Carleton University has chosen Sherborn Sensors’ miniature force transductors for its innovate mobility system, which will help improve traction and combat slippage when micro-rovers traverse a Martian surface. The force transducers were selected for their range and reliability, accuracy and to provide a path to flight testing.

There are a growing number of space mission programmes that involve lunar and planetary exploration using rovers. These vehicles are semi-autonomous and can either be manned or controlled remotely to drive across a Martian surface to explore for resources and to perform scientific experiments.

Two well-known examples are Opportunity and Spirit, which landed on Mars in 2004 and made important discoveries about pre-historic wet environments that may have been favorable for supporting microbial life.

Both Opportunity and Spirit were solar-powered rovers and were built for a mission intended to only last 90 days. However, Spirit continued for more than six years and drove over 7.7 kilometers from its initial landing point at the Gusev Crater (the possible site of a pre-historic lakebed).

It returned over 124,000 images before getting stuck in sand in 2009 and ceasing communications in 2010. Opportunity has driven 33.5 kilometers since landing on the other side of Mars at Meridiani Planum and continues to conduct experiments.

Spirit’s fate highlights a major challenge that has been a problem for previous planetary rovers: they are prone to getting stuck in the fine-grained soil and other topographic features of Martian surfaces. Furthermore, they are expensive, susceptible to damage and large. Consequently, 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 micro-rover flew in 1997 and was only 12 kg in mass, but was very limited.

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.

Alex Ellery, Professor of Mechanical and Aerospace Engineering at Carleton University and Canada Research Chair in Space, Robotics and Space Technology.

Breaking New Ground

With the CSA searching for a variety of different-sized prototypes to support the ESM program, Professor Ellery and his colleague Dr Ala’ Qadi, a Post-doctoral Researcher at Carleton University, created a co-operative partnership in order to develop a new micro-rover design.

Members include MPB Communications; University of Toronto Institute for Aerospace Studies; MacDonald, Dettwiler and Associates (MDA); Xiphos Technologies; Ryerson University; and University of Winnipeg.

The partnership was awarded a $1.8 million (CAD) contract after submitting their design to CSA. This contract was to develop a smart and all terrain micro-rover based on a modular architecture to allow for optimal reconfiguration for Mars and Moon science and exploration. The micro-rover, named ‘Kapvik’ after the Canadian Wolverine, employs a number of innovations in sensors, robotics, real-time intelligent software and microstats to enable high functionality within a mass budget of 30 kg.

This represents a significant reduction in mass on larger rover designs such as Opportunity (174 kg) and Spirit, as well as NASA’s 900 kg ‘Curiosity Mars’ rover which is due to touch down on the Red Planet 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).

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).

Dr Ala’ Qadi, Carleton University.

The chassis and frame for Kapvik was built, according to Dr Qadi, ‘from the ground up’ using a rocker-bogie design, which is proven for negotiating obstacles up to 15 cm in height and where speed is not a concern (Kapvik has a top speed of 80 meters per hour). “However, we recognised that is 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

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

“Without load cells you cannot obtain the critical information necessary to perform traction analysis,” explains Tim Setterfield, who designed Kapvik’s mobility system while studying for a Masters’ of Aerospace Engineering at Carleton University and who currently works at the European Space Agency. After evaluating a range of available options, Tim chose Sherborne Sensors’ SS4000M miniature force transducers because of their small size and wide range and also due to the fact the company is AS9100B certified and has experience 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 thermomechanical interactions between the wheels and the soil,” says Tim. “For example, if you have more weight over the wheel you can develop more traction – it’s like having the engine over the drive in wheels in cars, as this enables you to generate more traction on those wheels.”

Tim created a proof-of-concept net traction algorithm for estimating normal load, drawbar pull, resistive torque, wheel-terrain contact angles and slip using conventional on-board rover sensors, a velocimeter and the force transducers. “The SS4000M force transducers were the perfect size and range for the application and working with Sherborne Sensors has been great,” Tim continues. “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

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

Carleton University had a number of challenging requirements, but the specification of our SS400M force transducer proved the perfect it,

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.

Jesse Bonfeld, Sherborne Sensors.

The data provided by Sherborne Sensors’ SS400M force transducers has been critical in increasing the agility and capability of the Kapvik micro-rover while keeping its mass budget low compared to other planetary rovers. Kapvik’s rocker-bogie mobility system can equilibrate ground pressure between all six wheels and negotiate obstacles of up to one wheel diameter in height (15 cm).

The micro-rover is designed to operate either on its own for low-cost planetary exploration or as a collaborative assistant to manned or large rover missions, thereby lowering the chances of losing more expensive and elaborate rovers to inhospitable terrain. Furthermore, the technologies being developed for the rover might also be useful for Earth-based application in areas such as security, transportation and mining industries.

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

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

This information has been sourced, reviewed and adapted from materials provided by Sherborne Sensors.

For more information on this source, please visit Sherborne Sensors.

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