Older vehicle brake systems are mostly hydraulic brake systems, in which the friction pads are driven by hydraulic cylinders. But with the recent introduction of sophisticated brake control arrangements, including anti-lock brake systems (ABS's), electric brake systems that use no hydraulics are gathering attention.
Electric brake systems include a linear motion mechanism in which the rotary motion of a rotary shaft driven by an electric motor is converted to a linear motion of a linear motion member, thereby pressing a friction pad against the brake disk with the linear motion member. Many of such electric brake systems include a sensor for detecting the pressing force with which the friction pad is pressed against the brake disk in order to control the braking force to a desired value.
Each of JP Patent Publication 2003-287063A, JP Patent Publication 2003-014018A, and JP Patent Publication 2004-204990A discloses an electric brake system including a sensor for detecting the pressing force of the friction pad.
The electric brake system disclosed in JP Patent Publication 2003-287063A includes a caliper body having opposed pieces facing each other on opposite sides of the brake disk and connected together by a bridge, and a linear motion mechanism mounted in the caliper body. One of the friction pads is pressed against the brake disk by a linear motion member of the linear motion mechanism. A strain sensor is mounted in the caliper body at its portion where the bridge is joined to one of the opposed pieces to detect the pressing force of the friction pad.
The electric brake system disclosed in JP Patent Publication 2003-014018A also includes a caliper body having opposed pieces facing each other on opposite sides of the brake disk and connected together by a bridge, and a linear motion mechanism mounted in the caliper body. A pair of electrodes are embedded in the linear motion member of the linear motion mechanism. By measuring the electrical resistance between the electrodes, which changes with strain, it is possible to detect the pressing force of the friction pad.
The electric brake system disclosed in JP Patent Publication 2004-204990A also includes a caliper body having opposed pieces facing each other on opposite sides of the brake disk and connected together by a bridge, and a linear motion mechanism mounted in the caliper body. A hydraulic pressure chamber is defined in the linear motion member of the linear motion mechanism. A hydraulic pressure sensor is used to measure the pressure in the hydraulic pressure chamber, which changes with strain, thereby detecting the pressing force of the friction pad.
Generally speaking, the area surrounding the friction pads is heated to high temperature due to frictional heat generated between the friction pads and the brake disk.
In this regard, with the electric brake system disclosed in JP Patent Publication 2003-287063A, since the strain sensor for detecting the pressing force of the friction pad is provided in the caliper body at its portion where the bridge is joined to one of the opposed pieces, the strain sensor tends to be heated to high temperature. Thus, it is impossible to provide a circuit for processing the sensor signal sufficiently close to the strain sensor. Thus, the sensor signal tends to pick up noise. Also, when the caliper body is heated to high temperature, the caliper body tends to suffer from heat strain due to uneven temperature distribution. Heat strain of the caliper body makes it difficult to measure only the strain due to the pressing force of the friction pad with high accuracy.
In the electric brake system disclosed in JP Patent Publication 2003-014018A, too, since the electrodes for detecting the pressing force of the friction pad are provided in the linear motion member of the linear motion mechanism for pressing the friction pads, the electrodes tend to be heated to high temperature, which makes it impossible to position the signal processing circuit sufficiently close to the electrodes. This increases the possibility of the detection signal picking up noise. Also, since the linear motion member, in which the electrodes are embedded, moves, it is necessary to movably arrange the cables leading to the electrodes, which pushes up cost.
With the electric brake system disclosed in JP Patent Publication 2004-204990A, if hydraulic fluid in the hydraulic pressure chamber leaks and air mixes into the hydraulic pressure chamber instead, the output of the hydraulic pressure sensor will not change corresponding to the pressing force of the friction pad. Thus this system has a problem in long-term reliability. To improve the long-term reliability of this system, it is necessary to improve the liquid tightness of the hydraulic pressure chamber. But this solution incurs additional cost.