Estimating clamp force for brake-by-wire systems: Thermal considerations Stephen Saric * , Alireza Bab-Hadiashar, Johannes van der Walt Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, John Street, Hawthorn, Victoria 3122, Australia article info Article history: Received 17 August 2008 Accepted 6 May 2009 Keywords: Brake-by-wire Characteristic curve Estimation Thermal modelling abstract The use of a stiffness curve, otherwise known as the characteristic curve, to estimate clamp force for a brake-by-wire system has been the subject of past research. The requirement to estimate clamp force arises from trying to make the conventional use of a clamp force sensor a redundant component and thereby reduce costs. Previous uses of the characteristic curve do not effectively model parameter vari- ations in response to heating effects. This paper presents foundations for a new approach to model the changes in characteristic curve parameter values under the influence of heating. The new approach cen- ters on predicting pad temperatures and using this data to appropriately adjust characteristic curve parameter values. Two temperature sensors are required to be employed in this new approach. These additional sensors will not have a considerable impact towards the cost savings created by omitting a clamp force sensor. Finally experimental verifications are provided as well as recommendations for fur- ther research. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Following from the aeronautical industry, electro-mechanical systems have begun to replace mechanically actuated systems within automobiles. Otherwise known as drive-by-wire (DBW), such emerging technologies in the automotive industry will pro- vide faster actuation response times as well as the potential to integrate systems to improve performance [1,2]. The development of various DBW technologies is documented in [3–7]. Brake-by- wire (BBW) has already been introduced at a commercial level in vehicles in the form of an electro-hydraulic technology. That is, retaining many of the components used for existing anti-lock brak- ing systems (ABS), this scheme uses a motor driven pump and pro- portioning valves to control the level of braking to each wheel. Researchers and industry experts have since focused on the devel- opment of a completely electro-mechanical approach for vehicle braking. In this scheme hydraulics are omitted, which leads to a lighter and more environmentally friendly design. The electro- mechanical brake (EMB) used for a disc is shown in Fig. 1 as pro- vided by Saric et al. [4]. A motor is employed to drive reduction gearing which then leads to an induced clamp force. The control architecture generally used for an EMB system is shown in Fig. 2 as provided by Saric et al. [4]. Cascaded force, veloc- ity and current control loops are used with feedback sensors re- quired to measure each of these quantities. A resolver sensor is typically used to sense velocity. An alternative control architecture that is less expensive to implement uses Hall effect sensors which replace the resolver sensor. As can be seen from Fig. 2, clamp force is the variable being controlled. The use of a clamp force sensor in an EMB actuator is a challenging task. If a clamp force sensor is placed too close to a pad, it will then have to be able to withstand high temperatures due to friction braking. Also temperature drifts in the sensor may need to be compensated for. To overcome these problems, the sensor can be embedded deep within the caliper. However, this leads to a hysteresis effect which causes a mislead- ing clamp force to be sensed [8]. This hysteresis is caused by fric- tion between the sensed location and inner pad. A clamp force sensor is a relatively expensive component and requires individual online calibration. With the problems involved in using a clamp force sensor in an EMB system, it is highly desirable to eliminate this component from the system. A potential opportunity to achieve this can be realized by using sensory outputs to create a virtual sensor that estimates clamp force. This has been the subject of past research however previous attempts have each had their drawbacks and are detailed ahead in this section. The motor used in an EMB actuator is typically a three phase brushless DC type for the reason of compactness and improved commutation efficiency. Such motors typically require a position sensing device for their shaft so that commutation can be con- trolled. A simple stiffness model to estimate clamp force can be created using the outputs from this position sensing device. The relationship that exists between motor angle and induced clamp force can be viewed in Fig. 3 as provided by Saric et al. [4]. The mo- tor angle was perturbed in a pseudo-static manor, with a line of best fit applied due to hysteresis effects. It can be seen that the relationship, otherwise known as the characteristic curve, is non- linear by nature. The parameters that define this curve are subject 0957-4158/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechatronics.2009.05.001 * Corresponding author. Tel.: +61 3 9367 3950. E-mail address: ssaric@swin.edu.au (S. Saric). Mechatronics 19 (2009) 886–895 Contents lists available at ScienceDirect Mechatronics journal homepage: www.elsevier.com/locate/mechatronics