FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION [See section 10, Rule 13] VEHICLE BRAKE CONTROL DEVICE AND VEHICLE BRAKE CONTROL METHOD; MITSUBISHI ELECTRIC CORPORATION, A CORPORATION ORGANISED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS IS 7-3, MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 1008310, JAPAN THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED DESCRIPTION Technical Field [0001] The present disclosure relates to a vehicle brake control device and a vehicle brake control method. 5 Background Art [0002] A brake control device mounted on a rail vehicle, in order to obtain a target deceleration indicated by a brake command, compresses fluid supplied from a fluid source to supply the compressed fluid to a brake cylinder included in a mechanical brake apparatus. An example of this type of a brake control device is disclosed in Patent Literature 1. The rail vehicle brake control device disclosed in Patent Literature 1 calculates a necessary braking force from a brake command, compresses a fluid supplied from a fluid source up to a pressure for obtaining the necessary braking force, and supplies the compressed fluid to a brake cylinder included in a mechanical brake apparatus. The compressed fluid is supplied to the brake cylinder, thereby pressing a brake shoe against a wheel, so that a braking force is obtained. Citation List Patent Literature [0003] Patent Literature 1: Unexamined Japanese Patent Application Publication No. 2003-291797 Summary of Invention Technical Problem [0004] As described above, the brake force is obtained by pressing the brake shoe against the wheel. The braking force is expressed as the product of (i) a friction coefficient of a contact surface between the brake shoe and the wheel and (ii) a pressing force that is a force of pressing the brake shoe against the wheel. The braking force obtained from the same brake command sometimes varies due to a change in the friction coefficient that depends on, for example, a speed of a vehicle at the start of braking, the magnitude of pressing force, and the like. As a result, variation sometimes occurs in the stop position of the vehicle so that the stop position differs from a target position. [0005] In consideration of such circumstances, an object of the present disclosure is to provide a vehicle brake control device and a vehicle brake control method for suppressing the variance in the braking force obtained from the same 5 brake command. Solution to Problem [0006] In order to attain the aforementioned objective, a vehicle brake control device according to the present disclosure is a vehicle brake control device that controls a mechanical brake apparatus that (i) includes a brake cylinder and a friction member to operate in accordance with a pressure of a fluid inside the brake cylinder and (ii) causes generation of a braking force by pressing the friction member against a rotatable body that rotates when a vehicle travels. The vehicle brake control device includes a necessary braking force calculator, an initial speed acquirer, a target pressing force calculator, a target pressure calculator, and an outputter. The necessary braking force calculator acquires a brake command indicating a target deceleration of the vehicle and calculates a necessary braking force that is a braking force necessary for obtaining the target deceleration. The initial speed acquirer acquires a speed of the vehicle in response to acquisition of the brake command by the necessary braking force calculator. Using, as a friction coefficient of a contact surface between the friction member and the rotatable body, an average friction coefficient that varies depending on (i) an initial speed that is the speed of the vehicle acquired by the initial speed acquirer and (ii) a pressing force that is a force for pressing the friction member against the rotatable body, the target pressing force calculator calculates, from the average friction coefficient and the necessary braking force, a target pressing force that is a force for pressing the friction member against the rotatable body in order to obtain the necessary braking force. The target pressure calculator calculates a target pressure indicating a pressure of the fluid inside the brake cylinder that is necessary for obtaining the target pressing force. The outputter compresses, in accordance with the target pressure, fluid supplied from a fluid source to supply the compressed fluid to the mechanical brake apparatus. Advantageous Effects of Invention [0007] According to the present disclosure, from (i) the average friction coefficient varying depending on the initial speed and the pressing force that is the 5 force for pressing the friction member against the rotatable body, and (ii) the necessary braking force, the target pressing force is calculated that is the force for pressing the friction member against rotatable body in order to obtain the necessary braking force. Additionally, the fluid is compressed in accordance with the target pressure indicating the pressure of the fluid inside the brake cylinder that is necessary for obtaining the target pressing force, and then the compressed fluid is supplied to the mechanical brake apparatus. As a result, a vehicle brake control device and a vehicle brake control method can be provided that suppress the variance in the braking force obtained from the same brake command. Brief Description of Drawings [0008] FIG. 1 is a block diagram illustrating a configuration of a vehicle brake system according to an embodiment of the present disclosure; FIG. 2 is a block diagram illustrating a configuration of a vehicle brake control device according to the embodiment; FIG. 3 is a flow chart illustrating one example of operation of brake control performed by the vehicle brake control device according to the embodiment; FIG. 4 is a view illustrating an example of an average friction coefficient table of the embodiment for identifying an average friction coefficient from a speed of a vehicle and a pressing force; FIG. 5 is a view illustrating an example of a method of the embodiment for calculating the average friction coefficient for each pressing force at an initial speed; FIG. 6 is a view illustrating an example of a relational expression of the embodiment for identifying a pressing force using the average friction coefficient as a variable; and FIG. 7 is a diagram illustrating an example of a hardware configuration of the vehicle brake control device according to the embodiment. Description 5 of Embodiments [0009] A vehicle brake control device and a vehicle brake control method according to an embodiment of the present disclosure are described below in detail with reference to drawings. Components that are the same or equivalent are assigned the same reference signs throughout the drawings. [0010] FIG. 1 illustrates a vehicle brake system 1 mounted on a rail vehicle as an example of a vehicle. The vehicle brake system 1 includes a brake setter 2 that outputs a brake command in accordance with an operation of a driver, a variable load detector 3 that detects a weight of the vehicle, a speed sensor 4 that detects a speed of the vehicle, a fluid source 5, a mechanical brake apparatus 6 that generates a braking force for the vehicle, and a brake control device 10 that compresses, in accordance with a brake command, a fluid supplied from the fluid source 5 and supplies the compressed fluid to the mechanical brake apparatus 6. In FIG. 1, an electric signal is indicated by a solid line, and a flow of the fluid is indicated by a dotted line. [0011] The brake setter 2 includes a master controller provided in a cab and sends, to the brake control device 10, a brake command corresponding to an operation of the master controller by the driver. The brake command includes a brake notch indicating a target deceleration of the vehicle. The variable load detector 3 is mounted on the vehicle, detects the weight of the vehicle including the weights of passengers in the vehicle, the weights of devices mounted on the vehicle, and the weight of luggage, and sends the detected value to the brake control device 10. The speed sensor 4 includes a pulse generator (PG) attached to an axle and sends, to the brake control device 10, a signal indicating a rotational frequency of the axle obtained from a pulse signal output by the PG. The fluid source 5 supplies, to the brake control device 10, air as an example of the fluid. [0012] The brake control device 10 (i) compresses, based on the brake command output by the brake setter 2, the detection value of the variable load detector 3, and the detection value of the speed sensor 4, the fluid supplied from the fluid 5 source 5 and (ii) supplies the compressed fluid to the mechanical brake apparatus 6. The mechanical brake apparatus 6 includes a brake cylinder and a friction member that operates in accordance with a pressure of the fluid inside the brake cylinder. When the pressure in the brake cylinder is increased by supplying, to the brake cylinder, the air that is supplied from the fluid source 5 and compressed by the brake control device 10, the friction member is pressed against a rotatable body that rotates during traveling of the vehicle, thereby generating a braking force. The mechanical brake apparatus 6 includes a brake shoe as an example of the friction member. The brake shoe is pressed against a wheel that is an example of the rotatable body, thereby generating the braking force. The braking force is expressed as a product of (i) a pressing force that is a force for pressing the brake shoe against the wheel and (ii) a friction coefficient of a contact surface between the brake shoe and the wheel. [0013] Although the details are described later, the brake control device 10 calculates a pressing force for obtaining the target deceleration indicated by the brake command using, as the friction coefficient of the contact surface between the friction member and the rotatable body, an average friction coefficient that changes depending on (i) an initial speed that is a speed of the vehicle acquired in response to the acquisition of the brake command and (ii) the pressing force. Additionally, the brake control device 10 (i) compresses the air supplied from the fluid source 5 in accordance with the pressure of the fluid inside the brake cylinder that is necessary for obtaining the calculated pressing force, and (ii) supplies the compressed air to the mechanical brake apparatus 6. The brake control device 10 supplies, to the mechanical brake apparatus 6, the air compressed based on the pressing force calculated as described above, thereby enabling suppression of the variation in the brake force obtained from the same brake command. [0014] The brake control device 10 includes (i) a brake controller 11 that calculates a target pressure indicating the pressure of the brake cylinder necessary for obtaining the target deceleration indicated by the brake command, (ii) an outputter 5 12 that compresses the air supplied from the fluid source 5 in accordance with the target pressure to output the compressed air to the mechanical brake apparatus 6, and (iii) a pressure sensor 15 that detects the pressure of the air output by the outputter 12 to send a feedback signal to the brake controller 11. [0015] As illustrated in FIG. 2, the brake controller 11 includes (i) a necessary braking force calculator 21 that calculates a necessary braking force that is a braking force generated by the mechanical brake apparatus 6 in order to obtain the deceleration indicated by the brake command, (ii) an initial speed acquirer 22 that acquires the initial speed, (iii) a target pressing force calculator 23 that calculates a target pressing force that is a force for pressing the brake shoe against the wheel to obtain the necessary braking force, and (iv) a target pressure calculator 26 that calculates a target pressure indicating the pressure of the fluid inside the brake cylinder that is necessary for obtaining the target pressing force and performs feedback control to adjust the target pressure based on a feedback signal acquired from the pressure sensor 15. The target pressing force calculator 23 includes (i) an average friction coefficient calculator 24 that calculates an average friction coefficient of the contact surface between the brake shoe and the wheel for each pressing force at the initial speed, and (ii) an operator 25 that calculates the target pressing force from the average friction coefficient calculated by the average friction coefficient calculator 24 and the necessary braking force. The brake controller 11 having the above-described configuration (i) calculates the target pressure from the brake command acquired from the brake setter 2, the detection value of the variable load detector 3, and the signal indicating the rotational frequency of the axle acquired from the speed sensor 4, and (ii) sends, to the outputter 12, an electrical command indicating the target pressure. [0016] As illustrated in FIG. 1, the outputter 12 includes (i) an electro-pneumatic conversion valve 13 that converts the electrical command sent from the brake controller 11 into a pneumatic command and (ii) a relay valve 14 that compresses, 5 in accordance with an output of the electro-pneumatic conversion valve 13, the air supplied from the fluid source 5 and outputs the compressed air to the mechanical brake apparatus 6. The electro-pneumatic conversion valve 13 adjusts, in accordance with the electrical command sent from the brake controller 11, the pressure of the air supplied from the fluid 10 source 5 and outputs the pressure-adjusted air to the relay valve 14. The relay valve 14 (i) compresses, in accordance with the pressure of the air output by the electro-pneumatic conversion valve 13, the air supplied from the fluid source 5 and (ii) supplies the compressed air to the mechanical brake apparatus 6. [0017] A summary of the operation of the brake control device 10 having the above-described configuration is described with reference to FIG. 3. During a period in which the brake command is not acquired (No in step S11), the necessary braking force calculator 21 repeats a process of step S11. Upon acquiring the brake command (Yes in step S11), the necessary braking force calculator 21 calculates the necessary braking force from (i) the target deceleration indicated by the brake command and (ii) the detection value of the variable load detector 3 (step S12). The initial speed acquirer 22 (i) acquires, in response to the acquisition of the brake command by the necessary braking force calculator 21, a pulse signal from the speed sensor 4 and (ii) calculates the initial speed from the pulse signal (step S13). Using the average friction coefficient that changes depending on the initial speed and the pressing force, the target pressing force 25 calculator 23 calculates, from the average friction coefficient and the necessary braking force, the target pressing force that is the force for pressing the brake shoe against the wheel in order to obtain the necessary braking force (step S14). The target pressure calculator 26 (i) calculates the target pressure indicating the pressure of the fluid inside the brake cylinder that is necessary for obtaining the pressing force calculated by the target pressing force calculator 23 and (ii) adjusts the target pressure in accordance with the feedback signal acquired from the pressure sensor 15 (step S15). The outputter 12 (i) compresses the air until the pressure of the air supplied from the fluid 5 source 5 reaches the target pressure and (ii) supplies the compressed air to the mechanical brake apparatus 6 (step S16). When there is no change in the brake notch indicated by the brake command (Yes in step S17), the processing returns to step S15 and the above-described processes are repeated. Specifically, the following processes are repeated: the target pressure is calculated using the target pressing force calculated in step S14; the target pressure is adjusted in accordance with the feedback signal acquired from the pressure sensor 15; the air supplied from the fluid source 5 is compressed until the pressure of the air reaches the target pressure; and the compressed air to the mechanical brake apparatus 6 is supplied. A variable used for calculating the target pressure may be different from a variable used for previously calculating the target pressure. In a case in which the same brake command is not continuously acquired, that is, in a case in which the brake notch indicated by the brake command is changed (No in step S17), the processing returns to step S11 and the above-described processes are repeated. [0018] The operation of each component of the brake control device 10 that performs the above-described processing is described in detail. Upon acquiring the brake command from the brake setter 2, the necessary braking force calculator 21 calculates the necessary braking force F1 from the below-described equation (1) based on a target deceleration α indicated by the brake command and a weight W1 of the vehicle detected by the variable load detector 3. The necessary braking force calculator 21 sends the necessary braking force F1 to the average friction coefficient calculator 24. The below-described equation (1) represents the necessary braking force for each mechanical brake apparatus 6 in a case in which the vehicle is supported by two bogies, each of the bogies includes four wheels, and each of the wheels is provided with the mechanical brake apparatus 6. Additionally, the necessary braking force calculator 21 sends the target deceleration α to the initial speed acquirer 22. [0019] The initial speed acquirer 22 acquires the speed of the vehicle 5 in response to the acquisition of the brake command by the necessary braking force calculator 21. Specifically, the initial speed acquirer 22 detects a change in the target deceleration α acquired from the necessary braking force calculator 21, that is, a change in the brake command. Upon detecting the change in the brake command, the initial speed acquirer 22 calculates the speed of the vehicle from the pulse signal acquired from the speed sensor 4. The initial speed acquirer 22 sends the calculated speed of the vehicle to the average friction coefficient calculator 24. In the following description, the speed of the vehicle calculated by the initial speed acquirer 22 is referred to as the initial speed Vint. The change in the brake command includes (i) a case in which the brake command is input from a state in which the brake command is not input and (ii) a case in which the number of notches of the brake notch included in the brake command changes, that is, a case in which the target deceleration α changes. [0020] The average friction coefficient calculator 24 (i) calculates, from the average friction coefficient of the contact surface between the friction member and the rotatable body, an average friction coefficient for each pressing force at an initial speed, the average friction coefficient of the contact surface between the friction member and the rotatable body being predetermined for each pressing force at each of mutually-different predetermined vehicle speeds, and (ii) sends the calculated average friction coefficient to the operator 25. Specifically, the average friction coefficient calculator 24 stores an average friction coefficient table for identifying the average friction coefficient from the speed of the vehicle and the pressing force. The average friction coefficient table illustrated in FIG. 4 is a table for identifying the average friction coefficient by combining vehicle speeds V1, V2, ..., and Vk and pressing forces N1, N2, ..., and Nm. For example, the average friction coefficient μ for the vehicle speed V1 and the pressing force N1 is μ11. In the following description, the following relationships are assumed to be satisfied: V1 < V2 < ...< Vk, and N1