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Micro matters

Micro matters

By TA News Bureau:

Professor (Dr) Luc G. Frechette of Quebec’s Universite de Sherbrooke’s Department of Mechanical Engineering and Director of Research at the Centre in Nanofabrication and Nanosystems (CRN2) is a renowned researcher in microengineering. His expertise covers miniature systems for energy conversion, such as microfabricated heat engines, fuel cells and vibration energy harvesting. His activities range from work on integrated device development to microfluidics, heat and mass transfer studies. He is also involved in developing Micro-Electro-Mechanical Systems (MEMS) sensors and actuators for aerospace applications and microelectronics. In this interview to Tyre Asia he explains in detail the practical application of his work in intelligent tyres to robots

Work on intelligent tyres is progressing among many tyre manufacturers, especially in the light of the need to have speciality tyres for electric vehicles. In this context can you explain the details of the dynamic load sensors that you have developed?

As a vehicle travels down the road, all the forces required to guide its motion are transmitted through the tyres, such as during braking, accelerating, and manoeuvring. We proposed to monitor the forces imparted on the tyres by measuring their deformation. When a force is applied at the contact patch of the tyre with the road, there is not only a local deformation, but the entire tyre is affected. We noticed that the sidewall and tread all around the tyre also deform as a force is applied at the contact patch. This implies that with only one (or a few) sensors located around the circumference, we can continuously monitor the forces seen by the tyre. We, therefore, first studied the deformation field over the tyre to find the locations that are the most sensitive to road excitation.
The tyre deformation can be measured by multiple ways, such as piezoresistive strain gauges, piezoelectric patches or capacitive sensors; the last two having been evaluated in our work. The capacitive strain sensor simply consists of two electrodes separated by a small gap. Since the capacitance between the electrodes is inversely proportional to the gap, it will vary when stretched. By fixing the sensor onto the rubber, we can therefore measure the tyre’s deformation. This requires the sensor to be highly flexible, so the electrodes were fabricated using a silver nanoparticle ink stamped onto a flexible polymer film. We also tested the piezoelectric approach by using Micro Fibre Composite patches, which consist of thin piezoelectric fibres held between two polyimide films with metallised electrodes. Both were found to sense the loads from the road, validating the proposed approach.

How will the Capacitive Strain Transducer and the Micro Fibre Composite Transducer be incorporated into tyres?

Currently, both types of sensors are fabricated on their thin polymer films, and are then mounted on the tyre with an adhesive. To protect the sensor from harsh environmental conditions, it should be mounted on the inside of the tyre, with careful attention to the adhesion. Alternatively, the electrodes that form the capacitive sensor could be stamped directly onto the tyre inner wall, if this improves adhesion and eases manufacturing.

How can the introduction of sensors to monitor the vehicle dynamics actually work to ensure safety and comfort?

Active control strategies for traction control, braking and manoeuvring could use the load information to gain precious fractions of a second to correct a dangerous situation. For example, loss of lateral traction when turning would be sensed as a change inside deformation of the tyre, prompting the active braking or active suspension system to react and avoid a loss of control. In addition, noise coming from non-uniformities of the road can also be cancelled before they are transmitted to the cabin and perceived by the occupants. The road-induced vibrations travel through the tyre, the suspension, the car’s body and into the cabin as acoustic noise. In this case, a feed forward algorithm can use the measured load fluctuations on the tyre to generate an opposite vibration further up in the suspension that will cancel the road-induced vibrations. This will reduce the noise generated in the cabin, similar to a noise cancelling principle found in some headphones.
Tracking the forces in the tyre, therefore, provides unique knowledge of the vehicle’s interaction with the road, even before the vehicle’s motion is affected.

You bonded on the tyre CST using printing electrodes. In the tyre manufacturing process, when and where can these sensors are embedded?

Adding the sensors after the tyre manufacturing process appears appropriate from a performance perspective, but it could be interesting to integrate them during the tyre fabrication process for manufacturing or reliability issues. Embedding the sensors within the rubber itself can be envisioned, but the materials should be selected to withstand the curing temperatures. The capacitive sensors only require conductive electrodes, which open many possibilities in terms of material used. The piezoelectric sensors are however more limited. The most common piezoelectric material (PZT) can lose its properties when heated above 200ºC. Embedding piezoelectric sensors would therefore require the use of more exotic materials that remain poled above 280ºC.

What are the cost factors of your process to manufacture intelligent tyres?

Although we have not evaluated the cost of integrating such sensors, we expect it to be marginal for the capacitive approach. The capacitive transducer fabrication itself is well adapted to batch manufacturing, similar to printing. The Macro Fibre Composite patches are currently much more expensive, which was a motivation to develop the capacitive sensors. A complete sensor for intelligent tyres also requires the electronics to measure the change in capacitance and transmit the information, in addition to the transducer. The cost of high volume microchips is very low (<1$), but their packaging and integration to the tyre or wheel can dominate the cost if not well designed. Finally, an autonomous intelligent tyre also requires power to drive the electronics. Although it could be powered by a battery, such as in common TPMS units, the continuous measurements and transmission of data in an intelligent tyre would drain the battery much quicker than in TPMS. Energy harvesting is therefore an attractive option to power the electronics from the vibrations, which we have also been developing. Appropriate harvesting systems are however not commercially available so their impact on cost is not clear at this time.
For those developing advanced vehicle controls, how could you research be used while designing intelligent tyres?
Measuring the loads directly in the tyre provides the most comprehensive and rapid information about the interaction between the road and the vehicle. This opens a new paradigm in active control strategies, where the tyre plays a central role, as opposed to inertial sensors on the vehicle. A complete system must however combine measurements in the tyre with active control in the vehicle, so this required the tyre manufacturers to work closely with automotive OEMs, which is a hurdle for innovation in this area. A standard protocol for communication and shared specifications could overcome this barrier. Further research and technological demonstrations are however required to converge towards the appropriate standards. Early players in the field may also impose their protocols and therefore establish the industry standards. Our work provides a basis to use tyre deformations as a measurement of loads and defines the range for some of the requirements, such as frequency and load amplitude. More work is however needed to define other specifications and communication standards.

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