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Sensing solutions for advanced clutch control systems

29 May 2007

Over recent years, clutch control has gone electronic, creating a need for sensors to deliver position, speed and temperature measurements to advanced control systems. Electricfil Automotive masters all the leading sensor technologies to provide optimum solutions, opening doors to a wide range of clutch-related functions that enhance driving comfort, safety and fuel economy.

Today clutches are operated by electric or hydraulic actuators. These actuators are either controlled automatically in dual clutch (DCT) and automated manual (AMT) transmissions or by a clutch pedal in manual transmissions (MT). Whatever the system, electronic clutch control and driver assistance functions greatly enhance driving comfort and the service life of clutch components. For example, starting on even the steepest hill is now easy for the driver – and easy on the clutch. Safety lock-outs prevent shifting into wrong gear, starting the engine with the clutch engaged or downshifting on slippery roads. The parking brake is automatically released when the clutch is engaged. Other clutch-related functions include torque limiting, cruise-control switch-off, anti-stall systems, clutch-riding alarms and clutch position indication. And the list is growing continuously.

Measurement needs
To implement these driver assistance and clutch control functions, sensors are required to determine the clutch position or status and send the information to the electronic control system. Depending on the type of transmission, this information is generally monitored via the position of the master and/or slave cylinder, the clutch release bearing or the clutch pedal. With electronic transmissions such as AMTs and increasingly popular DCTs, the driver no longer needs to press a pedal and manually shift gears. The electronic control system determines the right time to operate the actuators for optimum clutch control. This decreases fuel consumption and increases driving comfort through smoother shifting without any driver intervention, explaining the bright future for these solutions, particularly in Europe. To determine the optimum points for engaging or disengaging the clutch, the electronic control system needs information such as the speed of the input and output shafts, in addition to the clutch position. The oil temperature can be measured to determine the viscosity of the lubricant, used to fine-adjust the clutch actuation control strategy. Other information such as current and target transmission ratio and gas pedal position may also be of use.

Sensor requirements
The sensors that deliver this information must provide high accuracy and fast response, as well as diagnostic capabilities and high reliability. And all this in a small package at the lowest possible cost. Depending on where they are installed, they must also withstand severe environmental conditions including high temperatures and rapid temperature fluctuations as well as electromagnetic disturbances, vibrations and corrosive liquids. The problem is obviously complex and only an expert can provide the best solution to meet all these demands and the short deadlines often required.

Electricfil solutions
Electricfil Automotive has the necessary expertise in all the leading sensor technologies to produce optimum sensing solutions for every need. An Electricfil solution is much more than just a sensor, and the company's global solution path is based on more than 25 years of experience in the field. For a given project, Electricfil's engineers begin by analyzing the customer's needs to determine the most suitable sensing technology:

Electricfil sensing technologies for clutch control measurements

Clutch position measurements
To ensure low wear and high reliability, only non-contact sensor technologies can be considered. The best solution is often based on an analog LVDT linear position sensor, capable of accurately measuring strokes up to 100 mm through walls made of non-magnetic materials and with airgaps up to 4.5 mm. For these sensors, Electricfil has developed an application-specific integrated circuit (ASIC), providing the necessary intelligence to achieve high accuracy, reliability and diagnostic capabilities in either PWM or analog mode. For less demanding applications, Analog Hall linear position sensors can be used and Eddy-current solutions will soon be available for applications requiring total insensitivity to external magnetic fields. All these solutions are fully programmable. For applications requiring only clutch status (engaged/disengaged), Electricfil uses analog Hall linear position switches to provide the necessary binary output signal.

Shaft speed measurements
Electricfil developed effective VR solutions in the early 90s, today often replaced by Hall speed sensors. Hall technology offers higher accuracy and airgaps up to 3 mm for metal targets and 5 mm for magnetic targets, all in a small package. Hall speed sensors are also extremely robust in relation to vibrations and wobble. GMR solutions will soon be available to satisfy needs for even larger airgaps and high measurement repeatability.

Oil temperature measurements
Electricfil solution-ready technologies include NTC thermistors, suitable for most needs, and PT100 and PT1000 for applications requiring faster response times or higher resolution and accuracy. Then the degree of integration is determined, leading to single sensor, sensor cluster or mechatronic solutions. After designing the circuits and ASICs if required, the electronic components and assembly technology are selected, with surface mounted devices or discrete components depending on the application. The package is designed in parallel, with selection of the best suited material (PA, PPS, PBT, etc.) and assembly process (laser, thermal or vibration welding) along with the necessary potting, overmolding or encapsulation techniques. Then come the interconnections (leadframe, flex foil, cable harness, etc.) and the design and sizing of the magnetic circuit, including the magnet materials and pole pieces. All these phases include FMEA and reliability studies, computer simulations, prototyping and a host of other analyses and tests leading up to the initial samples and the final product.

Electricfil knowhow
Sensor solutions currently represent 75% of Electricfil’s sales, with applications in transmission systems, engine management and vehicle systems. These solutions help customers enhance engine performance, reduce fuel consumption and emissions and improve passenger comfort and safety while at the same time cutting costs. More than 100 engineers work in the new Technopole R&D center or on customer sites around the world, applying proven core technologies to produce sensor solutions that fit customer needs for individual sensors, sensor clusters or mechatronic modules. Innovation is an ongoing process at Electricfil, and car manufacturers and automotive suppliers alike can count on them to keep pace with their sensor needs.

Electricfil Automotive has unequalled expertise in the field of optimized engine performance and a capacity for innovation that places the group among the key players in automotive electronic and electromagnetic systems. With 1200 employees, Electricfil Automotive produces 30 million sensors and develops more than 25 new solutions every year. More than 150 million engine speed sensors designed and produced by Electricfil are currently on the road worldwide.

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