In recent years, from ecological and health standpoints, a bicycle equipped with an electric assist is drawing attention among those who have been using cars for transportation. Among such electric assist bicycles, an increased focus has been on bicycles that are capable of driving a longer distance with a single charge, and charging a battery with the regenerative energy in particular. Against this background, bicycles that are configured to start the regenerative charging in a braking operation after the braking action is actuated are generally available. However, when the regenerative charging is started after the braking action is activated, the energy utilization efficiency becomes low. Therefore, it is more convenient if the regenerative charging can be initiated prior to the actuation of the braking action when a user of the bicycle starts squeezing a brake lever so as to apply brakes. In order to do so, it is necessary to provide an instrument that is capable of detecting a point where the user starts squeezing the brake lever before the braking action is activated, which is when the brake wire is under tension, and that is capable of measuring a very small amount of movement that is proportional to the tension in the brake wire, i.e., a displacement amount.
FIGS. 17(A) and 17(B) show a relationship between a brake lever operation amount and a braking force of an electric assist vehicle. In the above-mentioned electric assist bicycle and the like, when the user starts squeezing the brake lever, the brake lever operation amount in a play stage where the manual brake has not yet engaged shown in FIG. 17(A) needs to be measured in accordance with the amount of movement of the brake wire, and then, a mechanical brake actuation point P1, which is a point where the brake is applied by brake pads to slow or stop the spinning wheel, needs to be detected based on a stretched amount of the brake wire. This operation is necessary because if the control of the regenerative brake and the control of the mechanical brake are not coordinated smoothly before and after the brake actuation point, a driver including a passenger may feel strangeness as if the brake was applied abruptly, or may feel a lack of the braking force.
In particular, in a brake of the electric assist bicycle and the like, it is very likely that the amount of play is changed when the driver replaces the wire or makes adjustment for the wire tension, which causes the brake lever operation amount required to actuate the mechanical brake to change from the point shown as the mechanical brake actuation point P1 to a point P2 in FIG. 17(B). Conventionally, only the brake lever operation amount has been detected, and the point where the operation amount reaches a prescribed required operation amount to start the mechanical brake has been determined as the start of the mechanical brake. This configuration has a problem in that if the user makes the adjustment and the mechanical brake actuation point is shifted to the point P2 as described above, the mechanical brake actuation point P2 cannot be detected, and as a result, the control between the regenerative brake and the mechanical brake cannot be conducted smoothly. Thus, in order to maximize the efficiency of the regenerative charging, it is desirable to employ a system structure that is capable of directly detecting the mechanical brake start point by measuring both the brake wire movement amount and the brake wire stretched amount simultaneously or chronologically.
As a method of measuring a very small displacement such as the amount of movement or the stretched amount of the brake wire, an optical interferometer has been conventionally used. A Michelson interferometer 300 shown in FIG. 18(A) includes a laser light source 302, a collimator lens 304 that turns laser light into parallel light, a splitter 306 that divides a beam into two beams, one of which is reflected toward a stationary mirror 308, and the other is reflected toward a moveable mirror 310, and that makes the two reflected light rays interfered, and an optical sensor 312. In the Michelson interferometer 300, when the moveable minor 310 moves relative to the stationary unit 314 in the direction of the beam by one wavelength, bright and dark lines appear on a detector twice. These bright and dark lines are observed as an interference pattern 316 as shown in FIG. 18(B), and a displacement that is equal to or smaller than one wavelength can be detected by reading the voltage values of the respective bright and dark lines. A displacement that is equal to or greater than one wavelength can be measured by determining the number of occurrence of the light and dark lines (interference patterns). That is, with respect to a movement of the minor, the path difference is doubled in a round-trip of the light, the displacement (travel distance) can be determined by “one wavelength×the number of light and dark lines×2” as shown in FIG. 18(C) (a mechanism for detecting a direction of the movement is also required). Such techniques utilizing the optical interference include the phase difference detector and the phase difference detecting method described in Patent Document 1 below, for example.