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Dry Clutch Micro-Slip Control and Temperature Considerations
Linköping University, Department of Electrical Engineering, Vehicular Systems.
Linköping University, Department of Electrical Engineering, Vehicular Systems.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

With an automatic clutch in the powertrain it is possible to micro-slip the clutch. Micro-slip is a continuous small slip in the clutch used to isolate the driveline from the oscillations in the torque produced by a combustion engine. A control structure has been designed for a micro-slip controller. The basic components are a Linear Quadratic (LQ) controller based on a linearized driveline model and an Extended Kalman Filter (EKF) that can compensate the torque request from the LQ controller for the thermal dynamics of the clutch. To remove some stationary errors, integral action has been added to the LQ controller by adding an extra state. An anti-windup scheme is used, and the additional parameters depend on clutch conditions. The reference slip value is set according to a derived formula for the flywheel-speed-oscillation amplitude together with a dynamic safety margin that can increase during transients. Altogether the controller has a simple structure and there should be no technical problems to implement it in a production vehicle. In simulations with transient torque, unknown road grades, and a mass parameter that has been varied by a factor of 2, the controller is able to follow the slip reference without locking up. The simulations are performed on a non-linear driveline model, previously validated with data, that has been augmented with a model for the oscillative torque produced by the engine in order to more accurately describe micro-slip conditions. The torque model is a sinusoidal model and has been fitted to high resolution data. The oscillation amplitude and frequency agree well with the data. The thermal behavior of a clutch with micro slip is analyzed and the EKF should be used for temperature surveillance together with some suggested counter measures although there were no excessive temperatures during simulations of recorded driving missions. However, the simulations show that the fuel consumption increase might be too large for a heavy-duty-truck application if micro-slip control is to be used at all operating conditions. Further analysis of costs versus benefits are required on a vehicle level but the feasibility of a micro-slip control system for a dry clutch truck has been proven.

National Category
Control Engineering
Identifiers
URN: urn:nbn:se:liu:diva-108837OAI: oai:DiVA.org:liu-108837DiVA: diva2:733163
Projects
LINK-SIC
Funder
Vinnova, LINK-SIC
Available from: 2014-07-08 Created: 2014-07-08 Last updated: 2014-07-11
In thesis
1. Dry Clutch Modeling, Estimation, and Control
Open this publication in new window or tab >>Dry Clutch Modeling, Estimation, and Control
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Increasing demands on comfort, performance, and fuel efficiency in vehicles lead to more complex transmission solutions. One such solution is the Automated Manual Transmission (AMT). It works just like an ordinary manual transmission but the clutch and the gear selection are computer controlled. In this way high efficiency can be accomplished with increased comfort and performance. To be able to control and fully utilize an AMT, it is of great importance to have knowledge about how torque is transmitted in the clutch.The transmitted torque in a slipping dry clutch is therefore studied in a series of experiments with Heavy Duty Trucks (HDT). It is shown that material expansion with temperature can explain torque variations up to 900 Nm for the same clutch actuator position. A dynamic clutch temperature model that can describe the torque variations is developed. The dynamic model is validated in experiments, and shown to reduce the error in transmitted torque from 7 % to 3 % of the maximum engine torque compared to a static model. Since all modeling, parameter estimation, and validation are performed with production HDTs, i.e. production sensors only, it is straightforward to implement the model in a production HDT following the presented methodology.

The clutch model is extended with lock-up/break-a-part dynamics and an extra state describing wear. The former is done using a state machine and the latter uses a slow random walk for a parameter corresponding to the thickness of the clutch disc. Two observability analyses are made: one with production sensors, and one with a torque sensor in addition to the production sensors. The analyses show that, in both cases, the temperature states and the wear state are observable both during slipping of the clutch and when it is fully closed. The latter is possible since a sensor measures the actuator position. The unknown offset in the torque sensor is possible to observe (at all times) if the model is further augmented with engine inertia dynamics. An Extended Kalman Filter (EKF) is developed and evaluated on measurement data for both cases. The estimated states converge from poor initial values, enabling prediction of the translation of the torque transmissibility curve and sensor offset. The computational complexity of the EKF is low and it is thus suitable for real-time applications.

The clutch model is also integrated into a driveline model capable of capturing vehicle shuffle (longitudinal speed oscillations) and engine torque fluctuations. Parameters are estimated to fit an HDT and the complete model shows good agreement with data. It is used to show that the effect of thermal expansion, even for moderate temperatures, is significant in clutch control applications. One such application is micro-slip control. A control structure has been made and the basic components are a reference-slip calculator, an LQ controller and an EKF that can compensate for the thermal dynamics of the clutch. The controller isolates the driveline from the engine oscillations without dissipating more heat than the clutch can handle. An analysis shows a noticeable fuel consumption increase. Nonetheless, the real benefits of micro-slip control will only be evident when combined with other cost-reducing changes in the powertrain. The feasibility of a micro-slip control system for a dry clutch HDT has been proven.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 32 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1612
National Category
Control Engineering
Identifiers
urn:nbn:se:liu:diva-108838 (URN)10.3384/diss.diva-108838 (DOI)978-91-7519-261-1 (ISBN)
Public defence
2014-09-19, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Projects
LINK-SIC
Funder
Vinnova, LINK-SIC
Available from: 2014-07-11 Created: 2014-07-08 Last updated: 2014-08-21Bibliographically approved

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