1.  Field of the Invention
The present invention relates to a pump drive motor control apparatus for controlling the rotational speed of a motor for driving a pump, and more particularly to a pump drive motor control apparatus which controls the rotational speed of a motor through on-off control performed such that supply of electricity to the motor is resumed on the basis of a result of comparison between a predetermined threshold and a voltage which the motor generates during a period in which supply of electricity to the motor is stopped, and such that the supply of electricity is stopped after the supply of electricity is continued for a predetermined period of time. 
2.  Description of the Related Art
A conventional pump drive motor control apparatus of such a type is disclosed in, for example, Japanese kohyo (PCT) Patent Publication No.  2002-506406.  The disclosed apparatus is applied to a motor for driving a hydraulic pump which pumps brake fluid having discharged to a reservoir as a result of operation of an antilock brake system and supplies the pumped brake fluid to a hydraulic circuit of the antilock brake system.  In principle, the apparatus controls the rotational speed of the motor through on-off control performed such that supply of electricity to the motor is resumed when a voltage which the motor generates in accordance with its rotational speed during a period in which supply of electricity to the motor is stopped (i.e., a voltage generated by an induced electromotive force which the motor generates as a result of acting as a generator (hereinafter may be simply referred to as “generated voltage”)) becomes equal to or less than a predetermined first threshold, and such that the supply of electricity is stopped after the supply of electricity is continued for a predetermined period of time. 
If the reservoir, to which brake fluid is discharged as a result of operation of the antilock brake system, is filled with brake fluid, further discharge of brake fluid from the hydraulic circuit of the antilock brake system to the reservoir becomes impossible, resulting in failure of the antilock brake system to attain brake fluid pressure control (hereinafter, referred to as “ABS control”).  Accordingly, by use of the above-mentioned hydraulic pump, brake fluid must be pumped out of the reservoir in order to prevent the brake fluid from filling the reservoir. 
The time-average of flow rate at which the hydraulic pump pumps brake fluid out of the reservoir and discharges the brake fluid (hereinafter, may be referred to as “average discharge flow rate” or simply “discharge flow rate”) is proportional to the time-average of rotational speed of a motor for driving the hydraulic pump (hereinafter, may be referred to as “average rotational speed” or simply “rotational speed”). 
Meanwhile, the brake fluid pressure within the hydraulic circuit of the system (accordingly, the discharge pressure or load of the hydraulic pump) serves as a force that lowers the rotational speed of the hydraulic pump (accordingly, the motor).  Therefore, the rotational speed of the motor (in particular, the rotational speed during a period in which supply of electricity to the motor is stopped) decreases to a degree that increases with the load imposed on the hydraulic pump. 
In other words, as the load imposed on the hydraulic pump increases, the discharge flow rate of the hydraulic pump decreases, thereby making the ABS control easily susceptible to failure.  Further, in the case where a large load is imposed on the hydraulic pump, the rotational speed of the motor tends to drop drastically immediately after the resumed supply of electricity to the motor is stopped. 
In view of the above, the aforementioned conventional apparatus is designed such that, only during a predetermined short period after the resumed supply of electricity to the motor is stopped, instead of comparing the generated voltage with the first threshold, the apparatus compares the generated voltage with a predetermined second threshold lower than the first threshold, and determines that a large load is imposed on the pump, when the generated voltage is lower than the second threshold.  In this case, the conventional apparatus immediately resumes the supply of electricity to the motor, and increases the continuation time during which the supply of electricity is continued.  As a result of this operation, the rotational speed (average rotational speed) of the motor having temporarily decreased because of the large load increases, thereby avoiding failure of the ABS control. 
However, even in the case where a load is continuously imposed on the hydraulic pump at a level such that the generated voltage does not drop below the second threshold within the predetermined short period after the resumed supply of electricity to the motor is stopped, the quantity of brake fluid stored in the reservoir gradually increases as a result of a continuous decrease in the average rotational speed of the hydraulic pump resulting from the load.  As a result, failure of the ABS control may occur.  In such a case, the conventional apparatus cannot avoid the failure of the ABS control.